Forced Dispersion of Heavy Gas Clouds by Water Curtains

Transcription

THESE
présentée devant
L’ECOLE DOCTORALE DE SAINT-ETIENNE
par
Karin HALD
Forced Dispersion of Heavy Gas Clouds by Water Curtains
-Experimental and Numerical ApproachesDispersion Forcée de Nuages de Gaz Lourds par Rideau d’Eau
-Approches Expérimentales et Numériques-
Soutenue le 11 juillet 2005 devant la commission d’examen composée de :
GRAILLOT Didier
DELVOSALLE Christian
CASAL Joachim
DUSSERRE Gilles
BUCHLIN Jean-Marie
BONY-DANDRIEUX Aurélia
Professeur, Ecole des Mines de Saint-Etienne, France
Professeur, Faculté Polytechnique de Mons, Belgique
Professeur, Université Polytechnique de Catalogne, Espagne
Chargé de Recherche, Ecole des Mines d’Alès, France
Professeur, Institut von Karman, Belgique
Enseignant-Chercheur, Ecole des Mines d’Alès, France
Invités:
DUVAL Denis
SERRES Isabelle
Direction de la Sécurité Industrielle du Groupe TOTAL
Ingénieur de recherche, Gaz de France
Travaux menés conjointement
Au Laboratoire Génie de l’Environnement Industriel, Ecole des Mines d’Alès,
France
A l’Institut von Karman, Belgique
Contents
Summary
vii
Acknowledgements
ix
List of Figures
xv
List of Tables
xvii
List of Symbols
xix
Résumé (in french)
I
1
Introduction
31
II Literature survey: Atmospheric dispersion and
mitigation of heavy gas clouds
37
Introduction
39
1 Dispersion and mitigation of heavy gas
1.1 The release . . . . . . . . . . . . . . .
1.1.1 Liquid phase . . . . . . . . . . .
1.1.2 Two-phase . . . . . . . . . . . .
1.1.3 Gaseous phase . . . . . . . . . .
1.2 Heavy gas cloud formation . . . . . . .
1.3 Natural dispersion . . . . . . . . . . .
1.3.1 Experimental investigation . . .
i
clouds
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
41
41
42
42
43
43
44
44
1.3.2 Description of the phenomena
1.3.3 Modelling . . . . . . . . . . .
1.4 Different means of mitigation . . . .
1.4.1 Foam . . . . . . . . . . . . . .
1.4.2 Greenbelts . . . . . . . . . . .
1.4.3 Air fans . . . . . . . . . . . .
1.4.4 Fire curtain . . . . . . . . . .
1.4.5 Water & Steam curtains . . .
1.5 Water curtain definition . . . . . . .
1.6 Conclusion . . . . . . . . . . . . . . .
2 Experimental approach
2.1 Air entrainment . . . . . .
2.1.1 Single spray at rest
2.1.2 Wind effect . . . .
2.2 Thermal effect . . . . . . .
2.3 Absorption . . . . . . . . .
2.4 Conclusions . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
3 Modelling of transport phenomena in
3.1 Multi-dimensional approach . . . . .
3.1.1 The gaseous phase . . . . . .
3.1.2 The droplet phase . . . . . . .
3.1.3 Numerical models . . . . . . .
3.2 One-dimensional approach . . . . . .
3.3 Typical results . . . . . . . . . . . .
3.3.1 Air entrainement . . . . . . .
3.3.2 Thermal effect . . . . . . . . .
3.3.3 Absorption . . . . . . . . . .
3.3.4 Wind effect . . . . . . . . . .
3.4 Semi-empirical approach . . . . . . .
3.5 Conclusions . . . . . . . . . . . . . .
4 Conclusions
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
liquid
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
sprays
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
45
46
47
48
49
50
50
51
51
57
.
.
.
.
.
.
59
60
60
61
64
65
68
.
.
.
.
.
.
.
.
.
.
.
.
71
71
72
73
75
75
77
77
82
84
85
87
90
93
ii
III
Field tests
97
Introduction
99
5 Description of the set-up
5.1 Objectives of the different campaigns . . . . . . . .
5.2 Gas source . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Chlorine gas . . . . . . . . . . . . . . . . . .
5.2.2 Carbon dioxide gas . . . . . . . . . . . . . .
5.3 Water-curtain & Nozzles . . . . . . . . . . . . . . .
5.3.1 Nozzle characteristics . . . . . . . . . . . . .
5.3.2 The water curtain . . . . . . . . . . . . . . .
5.4 Measurement points & technique . . . . . . . . . .
5.4.1 Instantaneous concentration measurements .
5.4.2 Mean chlorine concentrations . . . . . . . .
5.4.3 Temperature measurements in the gas cloud
5.5 Meteorological conditions . . . . . . . . . . . . . . .
5.5.1 The vane propeller anemometer . . . . . . .
5.5.2 The ultrasonic anemometer . . . . . . . . .
5.6 Experimental procedure . . . . . . . . . . . . . . .
5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
101
101
103
103
104
107
107
108
109
110
111
114
114
114
115
115
119
6 Results
121
6.1 Temperature measurements . . . . . . . . . . . . . . . . . . . 121
6.1.1 Temperature measurements in the near field of the source121
6.1.2 Temperature measurements far from the source . . . . 123
6.2 Free dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.2.1 Gaussian distribution . . . . . . . . . . . . . . . . . . . 125
6.2.2 Concentration with the distance to the source . . . . . 128
6.2.3 Cloud width as a function of the distance to the source 129
6.3 Forced dispersion . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.3.1 Influence of the RM ratio . . . . . . . . . . . . . . . . 129
6.3.2 Concentration decrease with distance to the source . . 133
6.3.3 Influence of the water curtain on the width of the cloud 134
6.4 Dilution factor . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.4.1 Different definitions . . . . . . . . . . . . . . . . . . . . 134
6.4.2 Concentration distribution . . . . . . . . . . . . . . . . 137
6.5 Water curtain response time . . . . . . . . . . . . . . . . . . . 139
iii
6.6 Comparisons of various field tests
6.6.1 Operating conditions . . .
6.6.2 Results . . . . . . . . . . .
6.7 Conclusions . . . . . . . . . . . .
IV
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Laboratory experiments
141
141
143
146
149
Introduction
151
7 Description & Preparation
7.1 The Water Spray Facility . . . . . . . . .
7.2 The Wind Gallery . . . . . . . . . . . . .
7.2.1 Similarity criterion . . . . . . . .
7.2.2 Description of the Wind Gallery .
7.2.3 Gas source . . . . . . . . . . . . .
7.2.4 Water curtain . . . . . . . . . . .
7.2.5 Measurement points & technique
7.2.6 Experimental procedure . . . . .
7.3 Conclusions . . . . . . . . . . . . . . . .
8 Laboratory results
8.1 Spray characteristics . . . . . .
8.2 Wind Gallery visualisations . .
8.3 Concentration profiles . . . . .
8.3.1 Free dispersion . . . . .
8.3.2 Forced dispersion . . . .
8.4 Dilution factor . . . . . . . . .
8.5 Influence of height ratio Hwc /Hc
8.6 Instantaneous measurements . .
8.7 Conclusions . . . . . . . . . . .
V
.
.
.
.
Numerical Simulations
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
153
. 153
. 154
. 155
. 156
. 157
. 158
. 159
. 162
. 162
.
.
.
.
.
.
.
.
.
163
. 163
. 166
. 167
. 167
. 168
. 170
. 171
. 174
. 175
177
Introduction
179
iv
9 Model description
9.1 Governing equations . . . . . . . . . . . . .
9.1.1 The discrete phase . . . . . . . . . .
9.1.2 The gaseous phase . . . . . . . . . .
9.1.3 The coupling of the phases . . . . . .
9.1.4 The new “Euler-Source” method . .
9.2 Domain . . . . . . . . . . . . . . . . . . . .
9.2.1 Single spray at rest . . . . . . . . . .
9.2.2 Two dimensional approach with wind
9.3 Gas source . . . . . . . . . . . . . . . . . . .
9.4 Water curtain . . . . . . . . . . . . . . . . .
9.5 Wind profile . . . . . . . . . . . . . . . . . .
9.6 Operating conditions . . . . . . . . . . . . .
9.7 Conclusions . . . . . . . . . . . . . . . . . .
10 Simulations
10.1 Air entrainment in a single
10.1.1 Spray envelope . .
10.1.2 Gas phase velocity
10.2 Wind effect . . . . . . . .
10.2.1 Free dispersion . .
10.2.2 Forced dispersion .
10.2.3 Dilution factor . .
10.3 Conclusions . . . . . . . .
VI
spray
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
effect
. . . .
. . . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
181
. 181
. 181
. 183
. 183
. 185
. 186
. 186
. 187
. 188
. 189
. 189
. 190
. 190
.
.
.
.
.
.
.
.
191
. 191
. 192
. 193
. 195
. 196
. 196
. 201
. 202
Synthesis
203
Introduction
11 The
11.1
11.2
11.3
11.4
205
wind effect
Recirculation bubble . . . . . . .
Dilution factor F D and efficiency
Modelling the wind effect . . . . .
Illustrative exercise . . . . . . . .
v
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
207
. 207
. 211
. 216
. 219
VII
General conclusions
223
Bibliography
231
vi
Summary
The present thesis is a part of a contract collaboration between the von
Karman Institute and Ecole des Mines d’Alès, TOTAL and Gaz de France.
Water curtains efficiency in dispersing a heavy gas cloud is investigated as a
mitigation tool typically applicable in petro-chemical and gas industries.
A water curtains action on a gas cloud may be threefold: the mechanical
dispersion due to the air entrainment within the sprays, the dilution by
chemical absorption and the buoyancy effects by heat exchange. This thesis
emphasize on the mechanical dispersion induced by the sprays in terms of
air entrainment.
The objectives are to evaluate the water curtain efficiency with respect to
the operating conditions, and define the critical parameters required. For
this purpose, three different approaches are undertaken. First, field tests are
performed to simulate under real conditions, gas cloud behaviour in front
of a water curtain. In this manner three dimensional effects are accounted
for. Secondly, Wind Gallery experiments simulate the same problematic in
two dimensions. This is needed to perform some parametrical investigations
that are to complex to carry out on the field. At last, numerical methods
are tested in order to evaluate their capability to simulate the gas cloud behaviour in front of a water curtain.
In this manner, a comparative investigation leads to a clear understanding
of the gas cloud behaviour under forced dispersion. First, it is highly related
to the water-to-wind momentum ratio. The higher the ratio, the more the
water curtain will behave as a moving obstacle in front of the cloud. The
cloud is then blocked and affected by an air flow that obliges it to recirculate
upwind the water curtain. In this manner, the forced dispersion of the cloud
vii
is enhanced with low wind velocities and high water flow rates in the water
curtain. This effect is observed in the three approaches. In fact, efficiencies
of 90% are achievable for water-to-wind momentum ratios close to 10.
The water curtain to gas cloud height ratio is found as an essential parameter for an optimal effect. The dispersion is enhanced if the entrained gas by
the water curtain also consists of fresh air and not only pollutant gas. As a
practical rule, water curtains more than twice the height of the gas cloud are
recommended.
By numerical simulations, the evolution of the dilution factor with respect
to the distance to the source demonstrate that the peak value actually takes
place between the source and the water curtain. Next, it constantly decreases
with the distance. Again, the higher the water-to-wind momentum ratio, the
more efficient is the water curtain; the protected area also increases with the
same ratio.
At last, a simple wind model is introduced and fitted to field test results.
It demonstrates that high efficiencies are difficult to reach for wind speeds
larger than 6 m/s.
viii
Acknowledgments
This thesis has been carried out in two places, and with three completely
different approaches. It is therefore clear, that the number of supporting,
helpful, and understanding people is large!
I am really grateful to Prof. Buchlin, Gilles Dusserre and Aurélia BonyDandrieux to promote this thesis that has given me so many different experiences in the field, the laboratory and in front of the computer. During the
thesis, you have given me responsibility, trust as well as suggestions, motivation and enthusiasm. Sincere thanks also, to the other members of the jury;
Prof. Graillot, Prof. Delvosalle, Prof. Casal, M. DUVAL and finally Isabelle
Serres.
At the Ecole des Mines d’Alès, I was mostly working on field tests, organizing, preparing and analysing. And field tests are definitely a team work
and the results in this thesis have depended on all of you! I has been fun to
share whole days on the field working together and creating a team. A warm
thought goes to the irreplaceable men, Michel Alcon and Roro. Beside, living
in what we called the prison, so dense and so tight, have also resulted in really close friendships. I don’t give names, you know if I am talking about you!
At the von Karman Institute, supervising laboratory tests and making numerical simulations demonstrated different needs than for the field tests. But
collaboration always gains. A special attention goes to Patrick and Diederick
for the numerous constructive discussions.
To all the Professors, technicians, students, and every one of you that had a
hand in the game, I am grateful.
ix
To my family in Norway, present and supporting through kilometres, by
touching letters (pappa), or by more modern technologies (maman et son
Mac), or even by visits (det har vært gøy!): tusen takk; A ma grand-mère
chérie que j’ai mieux connu et admiré pendant mon séjour en France et en
Belgique, merci pour ton exemple de courage; and finally, MAlpaRIDO ♥
Beto, you have a mammoth patience . . .
x
List of Figures
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1.1
1.2
1.3
1.4
Vue d’entrainement d’air dans un spray . . . . . . . . . . . . .
Schéma du montage de la source de carbone de dioxide . . . .
La buse tangentielle et la distribution de diamètre correspondante pour D0 =5.1 mm . . . . . . . . . . . . . . . . . . . . .
Vue d’essais de dispersion libre et forcée par rideau d’eau . . .
La buse tangentielle et la distribution de diamètre correspondante pour D0 =5.1 mm . . . . . . . . . . . . . . . . . . . . .
Distribution latérale de concentration au sol derrière le rideau
d’eau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facteur de dilution en fonction du temps . . . . . . . . . . . .
Schéma de principe de la Galerie à Vent . . . . . . . . . . . .
Visualisation de la bulle de recirculation pour différentes valeurs
de RM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Profil vertical de concentration et l’effet du rapport des hauteurs Hwc /Hc . . . . . . . . . . . . . . . . . . . . . . . . . . .
Champs de vitesse autour du spray. Comparaison entre les 2
techniques CFD . . . . . . . . . . . . . . . . . . . . . . . . . .
Profils radiaux de la composante vertical de vitesse du gaz . .
Streamlines in forced dispersion cases for various RM values .
Evolution du facteur de dispersion en fonction de la distance
de la source . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Facteur de dispersion par rapport au rapport de quantité de
mouvement . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modèle de l’effet du vent . . . . . . . . . . . . . . . . . . . . .
Gas storage tank with various accidental release options .
Greenbelts [31] . . . . . . . . . . . . . . . . . . . . . . .
Sketch of water curtains . . . . . . . . . . . . . . . . . .
Flownumuber versus orifice diameter FN ∝ D02 [15] . . .
xi
.
.
.
.
.
.
.
.
.
.
.
.
4
7
9
10
11
11
12
15
16
17
20
21
22
23
25
26
42
49
52
53
1.5 Different types of nozzles . . . . . . . . . . . . . . . . . . . . . 54
1.6 Different nozzle types . . . . . . . . . . . . . . . . . . . . . . . 55
1.7 Droplet size distribution . . . . . . . . . . . . . . . . . . . . . 56
2.1 Air entrainment visualization in a single spray [15] . . . . . . . 60
2.2 Air entrainment in upward and downward water spray curtain
for no or low wind speeds . . . . . . . . . . . . . . . . . . . . 62
2.3 Typical results of the thermal behaviour in Wind Gallery [51] 66
3.1 Single spray entrainment . . . . . . . . . . . . . . . . . . . . .
3.2 CFD simulations of droplet and air velocity in a single spray
[48] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Comparisons between MARRS and experimental results in the
Water-Spray-Facility for the gas flow rate in a spray [71] . . .
3.4 Synthesis of experimental data and McQuaids air entrainment
correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 The spray angle α and diameter D . . . . . . . . . . . . . . .
3.6 One-dimensional simulation of vertical thermal behaviour . . .
3.7 One-dimensional simulation of vertical thermal behaviour . . .
3.8 CFD simulations from NEWSPRAY with wind effect [18] . . .
3.9 NEWSPRAY simulations compared with experimental results
[18] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.10 CFD simulations [14] . . . . . . . . . . . . . . . . . . . . . . .
3.11 Dilution factor with respect to the distance to the water curtain
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
The field sites . . . . . . . . . . . . . . . . . . . . .
Disposition of chlorine bottles . . . . . . . . . . . .
Carbone dioxide gas source system . . . . . . . . .
Evolution of the carbon dioxide release . . . . . . .
Sketch of the water-curtain . . . . . . . . . . . . . .
Full cone tangential nozzle . . . . . . . . . . . . . .
Instantaneous concentration measurement positions
Instantaneous concentration measurement positions
Mean concentration measurement set-up . . . . . .
Mean chlorine measurement positions . . . . . . . .
Experimental procedure in the first campaign . . .
Experimental procedure for the second campaign .
xii
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
77
78
79
80
81
83
85
86
86
87
89
102
104
105
106
107
108
111
112
112
113
116
118
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
6.15
6.16
6.17
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.1
Ground temperature in the centre axis of a chlorine gas cloud
at 1 m from the release under free dispersion . . . . . . . . . .
Ground temperature in the center axis of the gas cloud 10 m
downwind from the source for free and forced dispersion . . .
Gaussian distribution, influence of wind speed . . . . . . . . .
Gaussian distribution, influence of gas release rate . . . . . . .
Gaussian distribution with respect to the source . . . . . . . .
Concentration reduction with the distance to the source . . . .
The recirculation bubble visualisation for different RM values
Concentration distribution, influence of RM . . . . . . . . . .
Instantaneous chlorine captors position and measurements . .
Concentration decrease for different RM values . . . . . . . .
Evolution of dilution factor with respect to the RM . . . . . .
Various F D definitions used on the field test results . . . . . .
Instantaneous measurements of carbon dioxide concentrations
Histogram of carbon dioxide concentration in four positions
downwind the water curtain in free and forced dispersion cases
Dilution factor with function of time . . . . . . . . . . . . . .
Experimental setup for the presented study, the Buxton test
series and the field tests of Moore & Rees . . . . . . . . . . . .
An example of instantaneous concentration measurement from
Moodie [60] . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PDA system in the VKI-Water-Spray facility . . . . . . . . .
PDA measurements of droplet and air velocity in a single spray
[3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VKI Wind Gallery . . . . . . . . . . . . . . . . . . . . . . .
Photography of source injection (smoke for visualisation) . .
Water curtain in the Wind Gallery . . . . . . . . . . . . . .
Measurement point location in the Wind Gallery . . . . . . .
Schematic of the measurement acquisition . . . . . . . . . .
Schematic representation of the hot wire probe . . . . . . . .
Experimental calibration curves for air-forane and air-carbon
dioxide mixtures . . . . . . . . . . . . . . . . . . . . . . . .
122
124
126
127
127
128
130
131
132
133
135
137
138
139
140
143
144
. 154
.
.
.
.
.
.
.
155
156
157
158
159
160
161
. 161
Sauter diameters in the radial direction for various nozzles,
∆P =10 kPa . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
xiii
8.2 Droplet and gas phase velocities at 0.5 m from the nozzle
(∆P =10 kPa) . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3 Visualisation of various RM experiments . . . . . . . . . . .
8.4 Vertical concentration profiles in free dispersion . . . . . . .
8.5 Vertical concentration profiles for various RM values 2 m
downwind the water curtain . . . . . . . . . . . . . . . . . .
8.6 Dilution factor FD as a function of the water-to-wind momentum ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7 Sketch of cloud height variations . . . . . . . . . . . . . . . .
8.8 Concentration profiles for different water curtain to gas cloud
height ratio . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.9 Dilution factor with respect to RM . . . . . . . . . . . . . .
8.10 Instantaneous pollutant concentrations with IR-captor . . .
8.11 Comparsion between hot wire probe and IR-captor for different RM values . . . . . . . . . . . . . . . . . . . . . . . . .
9.1
9.2
9.3
9.4
Calculation steps in FLUENT . . . . . . . .
The computational domain for a single spray
The computational domain with wind effect
The computational domain with wind effect
. . . . . . .
simulation .
. . . . . . .
. . . . . . .
.
.
.
.
.
.
.
.
. 165
. 166
. 167
. 169
. 171
. 172
. 172
. 173
. 174
. 175
.
.
.
.
184
186
187
188
10.1
10.2
10.3
10.4
10.5
10.6
10.7
Spray envelope at 1000 kPa . . . . . . . . . . . . . . . . . . . 192
Gas-phase velocity in the radial position . . . . . . . . . . . . 194
Radial velocity [m/s] comparisons for the two CFD approaches 195
Chlorine mass fraction in the computational domain . . . . . . 196
Streamlines in forced dispersion cases for various RM values . 197
Mass fraction of chlorine for different RM values . . . . . . . . 199
Vertical concentration profiles at 3.5 m downwind the water
curtain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
10.8 Concentrations & dilution factor at ground level with respect
to the distance to the source . . . . . . . . . . . . . . . . . . . 201
11.1 The recirculation bubble in the different approaches . . . . .
11.2 The recirculation bubble in the different approaches . . . . .
11.3 Dilution factor with respect to the water-to-wind momentum
ratio RM . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.4 Schematic of the modelling . . . . . . . . . . . . . . . . . . .
xiv
. 209
. 210
. 211
. 213
11.5 Corrected dilution factor with respect to the water-to-wind
momentum ratio RM, all . . . . . . . . . . . . . . . . . . . .
11.6 Dilution factor with respect to the water-to-wind momentum
ratio RM, all . . . . . . . . . . . . . . . . . . . . . . . . . .
11.7 Water curtain efficiency η with respect to the water-to-wind
momentum ratio RM, all . . . . . . . . . . . . . . . . . . . .
11.8 Model of wind effect on dilution factor . . . . . . . . . . . .
xv
. 214
. 215
. 217
. 218
xvi
List of Tables
2.1
Additives in water curtains to enhance absorption . . . . . . . 68
3.1
3.2
3.3
Terms of the gas-phase equation [18] . . . . . . . . . . . . . . 72
Terms of the droplet-phase equation [18] . . . . . . . . . . . . 74
Water curtain characteristics . . . . . . . . . . . . . . . . . . . 82
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
Chlorine gas characteristics . . . . . . . . . . . . . . .
Carbon dioxide characteristics . . . . . . . . . . . . . .
Gas flow rate for the various campaigns . . . . . . . . .
Field tests nozzle characteristics . . . . . . . . . . . . .
Water curtain characteristics for the various campaigns
The first campaign test characteristics . . . . . . . . .
The second campaign test characteristics . . . . . . . .
The third campaign test characteristics . . . . . . . . .
The different field test campaign characteristics . . . .
6.1
6.2
6.3
6.4
6.5
6.6
6.7
Mean temperatures 1 m from the gas source . . . . . . . . . . 123
Mean temperatures 10 m from the gas source . . . . . . . . . 124
Cloud width at 7.5 m from the source in the second campaign 129
Various dilution factors from instantaneous measurements . . 139
Operating conditions of comparative field tests . . . . . . . . . 142
Characteristics of the Buxton tests series . . . . . . . . . . . . 144
Characteristics of the field tests described by Moore & Rees [63]145
8.1
8.2
Mean Sauter diameter at different nozzle scales . . . . . . . . 165
Heights of water curtain and gas cloud in the wind gallery . . 168
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
103
105
106
108
109
117
118
119
119
11.1 Example of efficiency η by wind; L=15 m, Hwc =3 m . . . . . . 220
11.2 Effect of nozzle diameter D0 ; L=15 m and Hwc =3 m . . . . . 220
11.3 Effect of water curtain height Hwc ; L=15 m . . . . . . . . . . 221
xvii
xviii
List of Symbols
D0
Ds
FD
FN
FR
Hc
Hwc
L
ṁl
ṁl,u
Ns
Qa
Qw
RM
U0
Ug
V
Xc
Nozzle orifice diameter
Spray diameter
Dilution factor
Nozzle flownumber
Heating factor
Gas cloud height
Water curtain height
Source to water curtain distance
Liquid flow rate
Liquid flow rate per unit length
Nozzle spacing
Air flow rate
Water flow rate
Dimensionless water-to-wind momentum ratio
Initial droplet velocity in nozzle
Gas phase velocity
Wind velocity
Water curtain to concentration measurement position
[mm]
[m]
[-]
√
[kg/s/ Pa]
[-]
[m]
[m]
[m]
[kg/s]
[kg/s/m]
[m]
[kg/s]
[kg/s]
[-]
[m/s]
[m/s]
[m/s]
[m]
∆P
η
ρw
ρc
Operating pressure
Efficiency
Water density
Cloud density
[kPa]
[%]
[kg/m3 ]
[kg/m3 ]
xix
xx
Résumé
Introduction
L’atténuation des risques associés à la formation d’un nuage de gaz lourd
toxique et/ou inflammable est une préoccupation majeure dans le monde industriel de la pétro-chimie et du gaz.
La mitigation par rideau d’eau sur les sites industriels est une technique
prometteuse compte tenu de sa simplicité et de sa flexibilité de mise en œuvre. Le principe consiste à placer dans le chemin du nuage dangereux une
barrière d’eau pulvérisée (rideau d’eau) qui va favoriser la dilution du polluant dans l’atmosphère et ainsi diminuer sa concentration, ce qui a pour
conséquences de réduire les distances d’effet en cas d’accident.
Pour répondre à la question de ses performances en tant que moyen dispersif et mettre à jour les paramètres de contrôle, un programme de recherche
appliquée a été mené dans la cadre de cette thèse. Elle s’est effectée conjointement à l’Ecole des Mines d’Alès et à l’Institut von Karman avec le
support de Gaz de France et de TOTAL. La méthodologie suivie se focalise
essentiellement sur la dispersion mécanique ou forcée par rideaux d’eau. Elle
comporte trois grands volets:
• Des essais “terrain” avec des buses d’aspersion de type et de dimension
industriels. Ces essais assurent une approche en grande echelle. Les
résultats concernent la caractérisation du nuage de gaz par des concentrations au sol. La comparaison entre des essais sans et avec rideau
1
Résumé
d’eau est effectuée.
• “Approche laboratoire” conduite dans une galerie à vent permet de
vérifier l’influence de paramètres non controllables dans les essais “terrain”. Les influences de la vitesses du vent et du rapport de la hauteur
entre cell du rideau d’eau et celle du nuage de gaz est particulièrement
étudiée.
• Des simulations numériques confirment les visualisations expérimentales
sur l’effet du vent sur le rideau d’eau.
Dispersion de gaz lourd et la mitigation de ses
conséquences
Formation et dispersion d’un nuage lourd
La dispersion d’un nuage de gaz est très fortement dépendante de nombreux
paramètres dont notamment les conditions de rejet. Sur site industriel, les
gaz sont souvent stockés sous forme liquéfiée à haute pression. Les rejets
issus de ces réservoirs peuvent alors se faire sous trois formes.
Pour une fuite en phase liquide, le rejet crée alors une flaque à partir de
laquelle par évaporation et vaporisation, le nuage de gaz se développe. Cette
forme de rejet est celle générant le débit massique le plus important. Lorsque
la fuite a lieu dans l’espace gazeux du réservoir, le nuage est directement
formé à la brèche. La fuite peut également générer un rejet diphasique.
La dispersion d’un nuage de gaz, est dépendante, entre autres, de la nature du nuage fonction par ailleurs des conditions de rejet. Le nuage formé
peut être dit lourd (de masse volumique supérieure à celle de l’air), neutre
(de masse volumique voisine de celle de l’air) ou léger (de masse volumique
inférieure à celle de l’air). La masse volumique d’un nuage de gaz peut être
associée au fait que la masse moléculaire est supérieure à l’air (comme le
chlore, le dioxyde de carbone, le propane, le butane ...), soit pour des gaz
initiallement plus légers que l’air par le fait que la vapeur est plus lourde
2
Résumé
que l’air à basse temperature (GNL) ou par la présence d’aérosols qui sont
de fines gouttelettes de gaz en phase liquide (cas typique de l’ammoniac
[38], [26]). Dans le cas d’un rejet accidentel, les nuages de gaz lourds sont
problématiques parcequ’ils concernent souvent des gaz dangereux, et qu’ils
possèdent la particularité en se dispersant bien plus lentement que les gaz
légers ou neutres.
De nombreuses études (expérimentales et modèlisations) ont décrit la dispersion des gaz lourds dans l’atmosphère. Dans une première étape, l’effet
de la gravité est dominant. Le nuage va se propager au niveau du sol, la
largeur va augmenter et la hauteur diminuer jusqu’à ce que le nuage devienne passif (avec une densité proche de celle de l’air). A ce moment là, la
turbulence atmosphérique va contrôler la dispersion du nuage de gaz, lui permettant une dispersion verticale jusqu’alors freinée, amplifiant le processus
assuré par la dispersion longitudinale et latérale.
Ainsi, pour diminuer la dangerosité de ces nuages, il est essentiel de mettre en place des dispositifs de protection et de réduction des conséquences.
Différentes méthodes de mitigation ont été proposé dans la littérature (mousse
neutralisante [19], [53], [30], [73], barrière végétale [47], [32], ventilateur [39],
rideau de feu [30] ou d’eau [60], [63], [14], [26]). Celle du rideau d’eau est
intéressante pour l’industrie car cette technique offre trois principales actions
que sont la dispersion forcée mécanique, le réchauffement du nuage et/ou le
transfert de masse par absorption.
La mitigation par rideau d’eau
Le rideau d’eau est constitué par un alignement de buses de pulvérisations
sur une tuyère. Différentes configurations de rideaux d’eau existent et permettent pour les plus utilisées d’entre elles de générer une projection d’eau
dans le plan vertical en mode ascendant ou descendant. Le choix de buses,
d’espacement Ns en [m] et de la pression d’alimentation ∆P en [Pa] définit
le flux d’eau ṁl,u par mètre de rideau:
ṁl,u =
FN
· ∆P
Ns
(1)
3
Résumé
√
où FN est le nombre d’écoulement de la buse en [kg/s/ Pa] qui relie le débit
à la pression. Différents types de buses existent. Ils se classent généralement
en trois familles; le cône plein, le cône creux et les jet plat.
Dispersion mécanique
Un spray d’eau génère un entraı̂nement d’air comme illustré par la visualisation et la simulation numérique proposées à la figure 1.
(a) Visualisation
(b) Modélisation
Figure 1: Vue d’entrainement d’air dans un spray
Une modélisation de l’entraı̂nement d’air relie le débit d’eau Qw au nombre d’écoulement FN de la buse, au diamètre du spray Ds par la relation
[54]:
√
ρw FN
Qa
∝ f(
).
(2)
Qw
Ds2
4
Résumé
où D est le diamètre du spray.
Il faut néanmoins noter que lorsque plusieurs buses sont montées sur une
rampe pour former le rideau d’eau, l’interaction des sprays diminue l’entraı̂nement d’air prédit par l’équation 2 [45].
De plus, l’entraı̂nement d’air, au delà des paramètres intrinsèques du rideau
d’eau, est également influencée par les conditions atmosphériques. Cet effet a
été mesuré et modélisé en introduisant le rapport de quantité de mouvement
eau-vent, défini par la relation
RM =
ṁl,u · U0
ρc V 2 Hwc
(3)
où U0 est la vitesse initiale d’éjéction de l’eau à la sortie des buses, ρc la
masse volumique du nuage, V la vitesse du vent et Hwc la hauteur du rideau
d’eau.
Ainsi, le facteur de dispersion F D qui est défini par le rapport de concentration sans et avec rideau d’eau:
FD =
Concentrationdispersion libre
Concentrationdispersion f orcee
(4)
peut être relié au RM.
La diution d’un nuage de gaz par effet mécanique est essentiellement locale.
Effet thermique
Comme les gaz sont souvent stockés sous pression, les températures sont
généralement très basses. C’est pourquoi, l’entraı̂nement d’air par le rideau
d’eau peut aussi réchauffer le nuage de gaz et ainsi abaisser sa masse volumique. Le nuage pourra alors se diluer plus facilement dans l’atmosphère.
5
Résumé
Absorption chimique
Le rideau d’eau peut également se comporter comme un réacteur physicochimique à contact direct. Ainsi, généralement utilisé pour des gaz très solubles (acide fluorhydrique, ammoniac), le principe d’absorption de gaz dans
les goutelettes du rideau d’eau a montré des facteurs de dilution élevés.
En comparaison avec la dispersion mécanique, l’absorption consiste à une
réduction de concentration par transfert de masse. L’effet n’est donc pas
simplement local. L’abattement obtenue au niveau du rideau se concrétise
par une disparition de la masse du polluant dans le nuage aval.
Conclusion
La dispersion naturelle d’un nuage de gaz lourd est généralement lente mais
peut être ammeliorée par la presence d’un rideau d’eau. La mise en place
de ces dispositifs est donc primordiale pour diminuer les distances de risque
en cas d’accident. Généralement, les études réalisées concernent une approche unique (soit modélisation soit essais experimentaux). Une approche
integrée permettant de confronter et valider les résultats issus de ces approches manque. C’est pourquoi cette thèse se propose de produire, confronter et valider les résultats issus d’essais expérimentaux en grandeur réelle,
d’essais sur maquette en soufflerie et de simulations numériques.
Ce travail concerne uniquement l’effet de la dispersion mécanique.
Essais “Terrain”
Les essais “terrain” ont été conduits sous la responsabilité de l’équipe Risques
Industriels et Naturels du Centre Génie de l’Environnement et des Risques
de l’Ecole des Mines d’Alès.
Trois campagnes d’essais ont été effectuées.
6
Résumé
L’objectif étant d’investiguer la dispersion mécanique des rideaux d’eau sur
des nuages de gaz lourds, ces campagnes d’essais ont mis en jeu des lâchers de
chlore et de dioxyde de carbone, gaz présentant la particularité d’être lourds,
non inflammables et très peu solubles dans l’eau.
Cette dernière caractéristique permet d’étudier la dispersion forcée, c’està-dire la dispersion mécanique liée à la présence de rideau d’eau (dispersion
additionnelle à celle due aux turbulences de l’atmosphère).
Des bouteilles de chlore liquéfié (B 20 - Air Liquide), pressurisées à l’azote, et
équipées de tubes plongeurs afin d’obtenir un débit de rejet gazeux constant,
ont été utilisées comme source de gaz lors des scénarios de fuite.
Le débit de gaz était contrôlé au moyen d’un manodétendeur placé en sortie
de chaque bouteille. Celui-ci était réglé dans la gamme 3-5·105 Pa, de façon
à obtenir un débit total de rejet de 4 à 8 kg/min. Le débit de gaz moyen
sur la durée du rejet, a été déterminé par pesée des bouteilles de gaz avant
et après chaque scénario et par mesure du temps d’émission du gaz (variable
selon les expériences mais proche de 8 minutes).
Pour le dioxyde de carbone, le système source était constitué d’un réservoir
de gaz (camion citerne), d’une piscine remplie d’eau et constituant un bainmarie, ainsi que d’un bloc de détente. Le dioxyde de carbone liquide était
acheminé vers le bain-marie où il subissait une vaporisation avant d’être
détendu et libéré à l’atmosphère. Le rejet s’effectuait à 50 cm du sol, horizontalement et était contrôlé par l’intermédiaire d’un débitmètre volumique.
Un schéma est présenté dans la figure 2.
Vanne
111111
000000
0
0000001
111111
0
1
0
1
11
00
00
11
00
11
11
00
00
11
00
11
00
0011
11
00
11
Camion citerne
11111
00000
0
1
0
1
0000
11111
0
1
0
1
0
1
0
1
0
1
00
11
0
1
00
11
Bain-marie
Detendeur Rejet
Figure 2: Schéma du montage de la source de carbone de dioxide
7
Résumé
La difficulté principale de ce système a consisté à générer des rejets en phase
gazeuse ayant un débit constant au cours du temps. Il s’est avéré nécessaire
de contrôler et réajuster l’ouverture de la vanne régulièrement afin de maintenir le débit rejeté à une valeur constante comprise de 20 kg/min. En
revanche, la température, variait durant le rejet de gaz en raison du refroidissement du bain chaud.
Les techniques de mesures ont fait appel au piégeage du chlore dans une
solution de soude (analyse ultérieure au laboratoire des échantillons). Ce
principe de mesures permet d’accéder à des valeurs moyennes de la concentration. Les mesures de concentration (instantanées) du dioxyde de carbone
ont été réalisées au moyen de sondes infrarouges. Les mesures de concentrations se situent au niveau du sol à différentes distances en aval du rideau
d’eau.
Le rideau d’eau mis en œuvre lors des essais terrain était constitué d’une
tuyauterie (montée sur un ensemble métallique) équipée d’une distribution
uniforme de buses espacées de 20 cm.
Les buses, de type industriel à cône plein avec un angle d’injection de 90◦
ont été caractérisées au sein du Water Spray Facility de l’IVK en terme de
distribution de gouttes dans le spray (figure 3).
√
Le diamètre d’orifice est 8.1 mm et le nombre d’écoulement 0.9 ·10−4 kg/s/ Pa.
La pression d’alimentation en eau du rideau de sprays était contrôlée à l’aide
de deux manomètres fixés sur la tuyauterie.
Les conditions météorologiques jouent un rôle important (notamment la
vitesse du vent) sur les performances du rideau d’eau. Ainsi la direction
et l’intensité du vent, ainsi que l’humidité et la température ont été mesurées
durant chaque scénario, à l’aide d’une station météorologique positionné à
10 m de haut. Un anémomètre ultrasonique mesure également les trois composantes de la vitesse du vent à 2 m de haut.
Un essai consistait en une séquence de deux expériences. La première phase
correspondait à la dispersion libre du nuage de gaz durant laquelle le rideau
d’eau n’était pas actionné (figure 4(a)). Cette expérience était directement
suivie par une phase de dispersion forcée durant laquelle le rideau d’eau était
8
Résumé
0.08
PDA
Rosin Rammler
0.07
0.06
[cc/cc]
0.05
0.04
0.03
0.02
0.01
0
0
(a) Buse tangentielle
200
400
600
d [µm]
800
1000
1200
(b) Distribution de diamètre
Figure 3: La buse tangentielle et la distribution de diamètre correspondante
pour D0 =5.1 mm
alimenté (figure 4(b)).
Le positionnement des capteurs instantanés pour le dioxide de carbone et
une mesure typique sont présenté dans la figure 5.
Cette chronologie a permis de reproduire celle utilisée lors des essais réalisés
dans la galerie à vent.
Les essais “terrain” ont permis d’étudier l’influence de différents paramètres
tels que le débit de fuite (de 1 à 20 kg/min), les conditions météorologiques
(vent faible, vent modéré, ...), le type de buses et la pression du rideau d’
eau, sur l’efficacité de dispersion du rideau d’eau.
Résultats typiques
En dispersion libre, les concentrations suivent une distribution gaussienne
dans la direction latérale jusqu’à une distance de 20 m de la source. Le
nuage se disperse rapidement dans la direction lateral avec la distance de
9
Résumé
(a) Dispersion libre
(b) Dispersion forcée
Figure 4: Vue d’essais de dispersion libre et forcée par rideau d’eau
la source. La figure 6 présente la distribution latérale de concentration de
chlore mesurée au sol en aval du rideau lors d’essais “terrain” effectués pour
différentes configuration de rideaux d’eau et de conditions météorologiques
ce qui se traduit par différentes valeurs de RM. En dispersion forcée, les
résultats sont dependants du facteur de quantité de mouvement RM. Pour
des RM faibles (typiquement RM=2), les distributions de concentrations
sont similaires à celles de dispersion libre: elles sont gaussiennes. Dès que
RM excède une valeur seuil d’environ 5, le rideau uniformise la distribution de concentration (figure 6(b)). La distributions est très affectée par la
présence du rideau d’eau et la largeur du nuage augmente de façon importante bien que cela soit très peu abordé dans la littérature. On constate
10
30
3
25
2.5
20
2
15
10
C [%]
Distance de la source [m]
Résumé
Points de mesure
Rideau d’eau
Disp. libre
Disp. forcee
Essai A3
Essai A2
Essai A1
1.5
1
0.5
5
0
−15
−10
Source
−5
0
5
Distance laterale [m]
10
0
0
15
5
(a) Capteurs instantanés
10
Temps [min]
15
(b) Mesure instantanée
Figure 5: La buse tangentielle et la distribution de diamètre correspondante
pour D0 =5.1 mm
à la figure 6 que l’efficacité de dispersion croı̂t avec le ratio de quantité de
mouvement et atteint des valeurs de l’ordre de 90% quand RM approche de
10.
Essai 7_3
4
3
x 10
Disp. libre
Disp. forcée
2.5
Disp. libre
Disp. forcée
2
C [ppm]
C [ppm]
x 10
2.5
2
1.5
1.5
1
1
0.5
0.5
0
−15
Essai 8_2
4
3
−10
−5
0
5
Position latérale [m]
(a) RM =1.4
10
15
0
−15
−10
−5
0
5
Position latérale [m]
10
15
(b) RM =7
Figure 6: Distribution latérale de concentration au sol derrière le rideau d’eau
11
Résumé
La visualisation du nuage de gaz montre une bulle de recirculation en amont
du rideau d’eau. Cet effet augmente également avec le rapport de quantité de
mouvement RM. Le nuage de gaz est donc affecté dans la direction vertical
(visualisé figure 4(b)) et latérale (mesuré figure 6(b)).
Par les mesures intantanées, le temps de réponse du rideau d’eau a pu être estimé comme le présente la figure 7 en evaluant le facteur de dispersion normé
en fonction du temps. Le temps de réponse du rideau d’eau est estimé par
le temps jusqu’à ce que la courbe atteint un plateau, à l’ordre d’une minute.
Dans ce cas, le RM est inférieur à 1.
1
0.9
essai A1
essai A3
0.8
max
0.5
FD
/FD
0.6
dose
0.7
0.4
0.3
0.2
Temps de
reponse
0.1
0
0
0.5
1
*
t [min]
1.5
2
Figure 7: Facteur de dilution en fonction du temps
Ces essais “terrain” ont permis d’évaluer le facteur de dispersion pour différentes conditions d’utilisation des rideaux d’eau.
12
Résumé
Essais “Galerie à Vent”
Parallèlement aux essais “terrain”, des simulations expérimentales à petite
échelle se poursuivaient dans la galerie à vent de l’Institut von Karman.
Cette démarche s’affranchit de la variabilité des conditions atmosphériques
en travaillant notamment à vitesse de vent constante. Ces expériences de
laboratoire ont permis de couvrir une plus grande gamme de RM et plus
particulièrement de placer l’accent sur l’effet de la hauteur du rideau d’eau
par rapport à l’épaisseur du nuage de polluant.
Critère de similitude
Les essais laboratoire reposent sur des critères de similitude établis lors
d’études précédentes [72].
La similitude géométrique est basée sur un facteur d’échelle fixé à 1/10. La
similitude de Reynolds est impossible et le nombre de Reynolds est environ 10
fois plus petit dans la simulation, mais l’écoulement autour des gouttes reste
turbulent. Les transferts de chaleur et de matière, dans ces conditions, sont
accélérés par un facteur 2, mais ceci est compensé par un entraı̂nement de gaz
par unité de liquide pulvérisé qui est aussi augmenté d’un facteur 2. Les variations de concentration en phase liquide et l’élévation de température du gaz
seront donc globalement du même ordre de grandeur qu’en “vraie grandeur”.
La structure de l’écoulement gazeux autour du rideau d’eau et les trajectoires
des gouttes sont reproduits à l’échelle pour des vents réduits d’un facteur 5
et une pression d’alimentation du rideau d’eau réduite également d’un facteur 5. Ces échelles correspondent exactement au rapport des quantités de
mouvement du rideau et du vent, et ont été confirmées par des simulations
numériques. La paroi supérieure de la veine, nécessaire pour les essais Galerie
à Vent, modifie la structure de l’écoulement gazeux pour les vitesses de vent
inférieures à 1 m/s en vraie grandeur. Sa présence cause la formation d’une
zone de recirculation en amont du rideau d’eau à basse vitesse.
13
Résumé
La Galerie à vent
La soufflerie de l’IVK réalisée pour étudier le comportement des rideaux de
liquide pulvérisé en présence de vent respecte les conditions de similitudes
présentées ci avant. Elle a été baptisée Galerie à vent dans la mesure où
elle ne simule pas le profil de vitesse turbulent de la couche atmosphérique
et que la perturbation locale induite par la présence du rideau de fluide est
nettement plus importante que la turbulence atmosphérique.
La Galerie à vent de l’IVK permet l’expérimentation de fuite gazeuse face
à un rideau d’eau sous des conditions de vent réel allant jusque 50 km/h.
Le schéma de principe de la Galerie à vent est présenté à la figure 8. Elle
comprend une section d’entrée, la veine d’essai proprement dite, une sortie
et un échappement atmosphérique. Les dimensions principales de la Galerie
sont de 11,5 m de long, 4 m de large et 5 m de haut. La section d’essai est
de 7 m de long, 1 m de haut et 1,3 m de large. Elle est constituée de parois
transparentes pour permettre tout type d’accès optique.
Le convergent d’entrée est conçu pour amener un écoulement d’air uniforme
dans la veine d’essai. Il s’agit d’une contraction simple, constituée d’une
entrée rectangulaire se contractant simultanément dans les deux directions.
Une mousse et un nid d’abeilles sont ajoutés à l’entrée pour créer une perte
de charge et uniformiser l’écoulement et le niveau de turbulence.
Quatre gros éjecteurs positionnés en sortie induisent une faible dépression
dans le tunnel, qui provoque l’écoulement d’air. Ces éjecteurs se caractérisent
par un flux d’air très stable et ce même pour les débits les plus faibles. Un
anémomètre à boule chaude sert à la détermination du profil de vitesse dans
la section d’essais. La plage de vitesse testée est 0,2 à 1,5 m/s, ce qui correspond à des vents réels d’environ 4 à 25 km/h.
Le gaz utilisé dans la présente étude est le dioxyde de carbone. C’est un gaz
lourd de masse volumique 1,98 kg/m3 , peu soluble dans l’eau, donc approprié
pour étudier la dispersion mécanique. Il se présente dans des bouteilles de 37
kg de gaz pressurisé à 5 MPa. Le nuage de polluant est généré par injection
au travers de toute la largeur du plancher. A la source, le gaz est pur (100
%). Une vanne et un débitmètre de type rotamètre permettent de fixer et
14
Résumé
contrôler la valeur du flux de polluant durant l’expérience.
Figure 8: Schéma de principe de la Galerie à Vent
Les mesures de concentrations sont effectuées à l’aide d’un anémomètre à
fil chaud. Cette technique a été développée à l’Institut von Karman [24].
Elle permet une mesure locale et instantanée [52].
Un échantillonnage isocinétique du gaz est effectué au moyen de peignes
verticaux constitués de tubes de prélèvement connectés à un collecteur mis
en dépression par une pompe à vide. Ces tubes sont séquentiellement mis
en communication avec un second collecteur via une batterie d’électrovannes
dont l’ouverture et la fermeture sont commandées par un programme Testpoint qui contrôle aussi l’acquisition de mesures.
Ce second collecteur alimente la sonde à fil chaud qui placée derrière un
trou sonique fixe la vitesse de l’écoulement. Le fil chaud est placé dans un
pont d’impédances dont le signal ne dépend que des propriétés du gaz et
donc de sa concentration en polluant. La relation voltage-concentration est
déduite d’un étalonnage effectué sur un banc spécialement conçu. La sensibilité de l’instrument 0,01 % CO2 /mV.
15
Résumé
Le rideau d’eau consiste en une tuyère longue de 1,3 m équipé par une rampe
de buses distribuées uniformément. Ces buses sont du type tangentielles cône
plein - Lechler 422.406. Ce modèle de buse est à l’échelle 1/4 de celui utilisé
lors des essais “terrain”. Il a un orifice avec un diamètre de 1,45 mm, √
un angle
◦
−5
de sortie de 90 et un nombre d’écoulement FN = 3, 96 · 10 kg/s/ Pa. Le
rideau occupe toute la largeur de la soufflerie. Les pulvérisations impactent
sur un sol poreux, pour que l’eau puisse être récupérée dans un système de
recyclage.
Résultats
La figure 9 est une visualisation du comportement du nuage de polluant au
contact du rideau d’eau. Le comportement est similaire à celui obsrevé lors
des essais “terrain” comme présenté à la figure 4. On constate la même
bulle de recirculation amont signe d’un effet dispersif du nuage. Par consequent, les tests en laboratoire reproduisent fidèlement l’effet du rideau d’eau
à grande échelle.
(a) RM =2
(b) RM =10
Figure 9: Visualisation de la bulle de recirculation pour différentes valeurs
de RM
La figure 10 montre un profil vertical de la concentration du polluant mesuré
lors d’essais en galerie à vent. L’action dispersive du rideau d’eau se fait es16
Résumé
sentiellement sentir au niveau du sol où les niveaux de concentration restent
très importants en dispersion libre. Elle se concrétise aussi par une bonne
uniformisation de la distribution verticale. C’est la raison qui nous conduit
à caractériser la performance du rideau d’eau uniquement sur la base de la
concentration échantillonnée au niveau du sol.
3
2.5
z/Hc
1.5
1
1
0.5
0.5
5
10
15
Concentration en masse [%]
(a) RM =2
wc
2
1.5
0
0
Disp. libre
Hwc=30cm
Hwc=40cm
H =50cm
2.5
wc
2
z/Hc
3
Disp. libre
Hwc=30 cm
Hwc=40 cm
H =50 cm
20
0
0
5
10
15
Concentration en masse [%]
20
(b) RM =7
Figure 10: Profil vertical de concentration et l’effet du rapport des hauteurs
Hwc /Hc
La figure 10 met également l’accent sur l’effet de la hauteur du rideau,
Hwc , par rapport à celle du nuage, Hc . Tous paramètres étant identiques,
l’élévation du rideau d’eau entraı̂ne une meilleure efficacité et ce d’autant
que la valeur de RM est grande.
17
Résumé
Simulations numériques
La simulation numérique comprend deux volets. La première partie est dédiée
à reproduire le comportement aéro-hydraulique d’une pulvérisation qui se
décharge dans une atmosphère au repos et plus précisément à quantifier le
phénomène d’entraı̂nement gazeux. La deuxième partie, se consacre à la
modélisation de l’effet de dispersion mécanique produit par un rideau d’eau.
Méthodes numériques
Deux approches totalement différentes sont généralement suivies pour simuler
numériquement les écoulements dispersés [15]. On peut traiter les phases
gazeuse et liquide comme deux milieux continus qui interagissent en moyennant les propriétés de la phase dispersée dans un volume de contrôle. Cette
démarche débouche sur la méthode à deux fluides ou l’approche eulérienne/
eulérienne. Les équations phasiques ont la même structure et peuvent être
donc résolues suivant la même technique numérique.
La seconde façon de décrire une pulvérisation est l’approche eulérienne/lagrangienne. Elle consiste à considérer toujours la phase porteuse comme un
milieu continu mais à décrire la phase dispersée par un réseau de trajectoires de gouttelettes. L’écoulement gazeux est modélisé par les équations
de Navier-Stokes avec un modèle de fermeture pour la turbulence tandis
qu’une description lagrangienne de suivi de gouttelettes permet de calculer
leur vitesse ainsi que les échanges de masse et de chaleur. Le couplage entre
les deux phases s’effectue itérativement selon une boucle à deux échelons:
tout d’abord on calcule les propriétés de la phase dispersée dans le milieu
gazeux. Puis on détermine les termes “sources” produit par les gouttelettes
dans les équations de la phase gazeuse ce qui va introduire une modification
des champs de vitesse, de température et de concentration du gaz. A partir
de ces nouveaux champs on corrige les caractéristiques des gouttelettes et ce
jusqu’à convergence des calculs.
Dans la présente étude les simulations CFD sont réalisées suivant l’approche
r La turbulence est décrite
eulérienne/ lagrangienne avec le code FLUENT.
18
Résumé
par le modèle RNG k − ε avec un traitement de paroi.
La pulvérisation est décrite par une série de points d’injection au niveau
de la buse. Chaque point d’injection est l’origine d’une trajectoire qui se
caractérise par l’angle par rapport à l’axe du spray, la granulométrie et la
vitesse des gouttelettes ainsi que le débit liquide qu’elle transporte.
Typiquement, les simulations se basent sur environ 20 trajectoires, chacune
étant porteuse d’une distribution de taille de type Rosin-Rammler définie
par 20 diamètres de gouttelettes. Le modèle eulérien/ lagrangien d’une
pulvérisation demande aussi une finesse de grille de calcul qui soit en accord avec la taille des gouttelettes.
Une nouvelle méthode pour modéliser l’effet rideau d’eau est prśentée. Puisque
le couplage entre les 2 phases dans l’approche Eulerienne-Lagrangienne se fait
par un terme source de quantité de mouvement dans la phase gaseuze l’idée
que l’utilisateur puisse la définir directement est intéressante.
Le terme source global SG reprénte la quantité de mouvement total de la
phase dicrète dans le domaine. Dans FLUENT, il peut être definit dans la
direction horizontal et vertical par SG = ṁl · U0 /V où ṁl est le débit, U0 la
vitesse initial et V le volume du rideau d’eau. En deux dimensions il peut
être traduit par le débit unitaire et la superficie latérale:
SG = ṁl,u · U0 /A.
(5)
SG est définit en [N/m3 ]. Cette méthode facilitera la simulation de l’effet
d’un rideau d’eau, reduira le temps de calcul. Cependant, il peut être utilisé
uniquement pour l’effet mécanique du rideau d’eau (les transferts de masse
et de chaleur sont negligés).
Les simulations illustrant le comportement d’un simple spray ont été discutées et validées. Elles montrent que le phénomène d’entraı̂nement gazeux,
mécanisme clef qui contrôle la dilution mécanique par pulvérisations d’eau,
peut être reproduit numériquement.
Dans le cas d’un rideau d’eau placé dans un vent, l’écoulement est traité
dans un domaine physique bidimensionnel. Quand elles atteignent le sol, les
gouttelettes disparaissent du domaine. L’intensité et l’échelle de longueur de
19
Résumé
la turbulence sont respectivement fixées à 2 % et 10 cm. Le profil de vitesse
du vent suit une loi en puissance 1/7.
u(y) = u0 [
y 1/7
] ,
δ0
(6)
oú δ0 est la hauteur de couche limite. Le nuage de polluant est soit simulé
entièrement en plaçant la fuite de polluant au sol soit prescrit à l’entrée du
domaine de calcul par son épaisseur et sa concentration.
Résultats
La figure 11 présente l’entraı̂nement gazeux induit par un spray dans une
atmosphère au repos. Une comparaison entre les deux approches numŕique
testées est proposée. Un accord général satisfaisant est observé entre la
3
3
X Velocity
-0.923061
-1.85313
-2.78319
-3.71326
-4.64332
-5.57339
-6.50345
-7.43352
-8.36359
-9.29365
z [m]
2
1.5
1
0.5
0
0
1
x [m]
2
3
(a) Approche Eulerienne-Lagrangienne
2.5
X Velocity
-0.686728
-2.05238
-3.41803
-4.78368
-6.14933
-7.51498
-8.88063
-10.2463
-11.6119
-12.9776
2
z [m]
2.5
1.5
1
0.5
0
0
1
2
3
x [m]
(b) Approche Euler-Source
Figure 11: Champs de vitesse autour du spray. Comparaison entre les 2
techniques CFD
méthode Eulerienne-Lagragienne et la méthode Euler-Source plus simple qui,
par ailleurs semble mieux simuler l’entrainement d’air près de la buse.
20
Résumé
Air velocity [m/s]
15
PDA 20 cm
PDA 50 cm
PDA 140 cm
CDF (E−L) 20 cm
CFD (E−L) 50 cm
CFD (E−L) 140 cm
CDF (E) 20 cm
CFD (E) 50 cm
CFD (E) 140 cm
10
5
0
0
0.1
0.2
0.3
Radial direction [m]
0.4
0.5
Figure 12: Profils radiaux de la composante vertical de vitesse du gaz
La figure 12 compare les profils de vitesse du gaz mesurés par l’anémomètre à
phase Doppler et calculés numériquement et ce pour deux distances de la buse
(z = 0.85m et 1.05m). Compte tenu des difficultés associées à la simulation
numérique de tout écoulement polydispersé en particulier la pertinence des
conditions aux limites, la modélisation de la turbulence dans un système avec
couplage de phases, on peut admettre que l’accord expérience-numérique est
très satisfaisant.
Les deux méthodes Eulerienne-Lagrangienne et Euler-Source sont également
confrontées dans le cas de la simulation de l’effet du vent. Dans la figure
13 la bulle de recirculation est présentée pour un cas à RM=7 où le débit
unitaire d’eau est ṁl,u =3.2 kg/s/m. La vitesse du vent est V =2 m/s.
Par l’approche Euler-Source, la bulle de recirculation est nettement plus importante que par l’approche Eulerienne-Lagrangienne.
21
Résumé
(a) Approche Eulerienne-Lagrangienne, RM =7
(b) Approche Euler-Source, RM =7
Figure 13: Streamlines in forced dispersion cases for various RM values
Par les concentrations au niveau du sol (maximales), l’évolution du facteur
de dispersion forcée est evalué par rapport à la distance à la source. Elle est
présentée à la figure 14 pour deux valeurs de RM.
Le rideau d’eau est placé à x=15 m. L’effet du rideau (en terme de réduction
de concentration) se fait déjà en amont du rideau. Cependant, la région aval
est plus importante puisque c’est généralement la région a protéger.
Le facteur de dispersion F D décroit avec la distance en accord avec la
littérature. Ensuite, pour petite valeur de RM, la zone où F D > 1 est restreint à l’alentour du rideau. Pour RM=7, non seuleument F D augmente,
22
Mass concentration [%]
Résumé
100
Free disp.
RM=2
RM=7
80
60
40
20
0
0
5
10
15
x [m]
20
25
30
5
RM=2
RM=7
FD
4
3
2
1
0
5
10
15
x [m]
20
25
30
Figure 14: Evolution du facteur de dispersion en fonction de la distance de
la source
mais la zone où F D ¿ 1 est largement agrandit.
Les simulations numériques ont proposé la nouvelle méthode Euler-Source,
plus simple pour modéliser les effets d’un rideau d’eau. Elle montre une très
bonne concordance avec des valeurs experimentales sur l’entrainement gazeux
dans un spray et simule également en concordance avec les visualisations des
essais expérimentaux les effets du vent.
23
Résumé
Synthèse des differentes approches
La synthèse focalise sur la comparaison des différentes approches utilisées
dans la thèse pour evaluer l’effet du vent sur la dispersion forcée par rideau
d’eau. Ensuite, la formulation de Bosanquet [14] est utilisée pour estimer
l’effet du facteur de dispersion en fonction de la distance du rideau d’eau. Au
final, un nouveau modèle de l’effet du vent, qui est introduit dans CASIMIRE
est expliqué.
Par visualisation, la bulle de recirculation créée par le rideau d’eau sous
l’effet du vent, montre des caractères similaires entre les approches différente
pour des valeurs de RM équivalentes. Les faibles RM augmentent faiblement
la hauteur du nuage. Lorsque le RM dépasse 5, une bulle de recirculation
qui se crée en amont du rideau est crée devient importante.
Les mesures de concentration ont été effectuées à differentes distance de la
source par rapport à la hauteur du rideau d’eau (évaluées par le facteur
Xc /Hwc ). Pour comparer les differentes approches expérimentales, il a fallut
estimer un facteur de correction pour tenir en compte de ce fait:
Essais “terrain′′ , chlore :
Essai en soufflerie, dioxide de carbone :
Xc
= 1.75
Hwc
Xc
=4
Hwc
(7)
(8)
(9)
La correction de l’effet de Xc /Hwc étant faite, la comparaison entre les differentes approches peut être menée. Elle est présentée à la figure 15 où on
porte l’efficasité de dispersion forcée η en fonction de RM.
Les trois outils concordent. L’éparpillement des données autour d’une courbe
moyenne résulte de différences entre les conditions opératoires comme par exemple formation du nuage (confiné ou développé), le niveau de température,
la valeur du rapport Hwc /Hc . Le point marquant de cette figure est qu’une
efficacité de dispersion forcée par rideau d’eau de 90% peut être attendue si
un ratio de quantité de mouvement de 10 ou plus est garanti.
Ce rsultat encourageant laisse présager l’existence d’une corrélation entre
24
Résumé
100
90
80
70
η [%]
60
50
40
30
Essais "terrain"
Galerie a vent
Galerie a vent (ancien)
CFD
Moyenne
20
10
0 −1
10
0
1
10
10
2
10
RM
Figure 15: Facteur de dispersion par rapport au rapport de quantité de
mouvement
la dispersion mécanique et l’effet du vent. En utilisant le fait que le débit de
gaz entraı̂né et donc Ug est proportionnel au débit d’eau injecté on en conclut
que le rapport Ug /V varie comme la racine carrée du rapport des quantités
de mouvement rideau/vent RM. Dès lors, en se basant sur le formalisme de
Bosanquet on démontre que:
√
F D = [1 + C · RM ]2 .
(10)
C’est la corrélation qui est comparée aux données expérimentales à la figure
15. Le paramètre C inclut les effets de nombreux paramètres comme Ds , voire
Hc en essais “terrain” et d’autres qui n’ont pas pu être systématiquement
déterminés expérimentalement. La valeur du coefficient C n’est pas facilement accessible par calcul. La valeur de 0,65 montre un accord satisfaisant
avec l’expérience. On constate que la corrélation reprend bien la tendance
expérimentale et qu’elle tend bien vers une efficacité nulle quand le rideau ne
fonctionne pas.
Pour modéliser l’effet du vent dans CASIMIRE, un modèle basée sur l’equation
10 est proposé. Il doit prendre en compte la hauteur du nuage et doit se recaler sur le calcul de CASIMIRE sans vent. Pour ce faire en développant 10
25
Résumé
on definit la norme suivante:
√
2C h · rm + C 2 h · rm
FD − 1
√
f=
=
F D0 − 1
2C RMM AX + C 2 RMM AX
(11)
où h = Hc /Hwc , rm le ratio de quantité de mouvement basé sur Hc et la
vitesse moyenne des gouttes sur la hauteur du rideau et RMM AX la valeur
de RM qui donne F D0 , facteur de dispersion calculé par CASIMIRE sans
vent. f représente alors le facteur de correction qu’il faut appliquer pour
l’influence du vent. Pour des conditions de grand vent F D → 1 alors f → 0;
aux faibles vents f → 1. D’où simplement
F D = 1 + f · (F D0 − 1).
(12)
La comparaison du modèle avec les essais terrains présentée à la figure 16
montre qu’un très bon accord est obtenue pour C=1.7.
7
6
FD
5
4
3
2
1
0
Field test
C=1.5
C=1.7
C=2
1
2
3
4
Wind velocity [m/s]
5
Figure 16: Modèle de l’effet du vent
26
6
Résumé
Conclusions générales & perspectives
Conclusions
L’objective de cette étude qui était de modéliser la performance d’un rideau
d’eau sur un nuage de gaz lourd en réduisant les concentrations par un effet
mécanique est atteint. Cette technique peut être utilisée industriellement
pour diminuer le risque et l’environnement de fuite de gaz dangeureux.
Une méthodologie impliquant trois approches fondamentales de la recherche
appliquée - la modélisation numérique, la simulation en laboratoire et l’essai
terrain - a été suivie.
Les différentes approches visent à la détérmination des concentrations de polluant dans un nuage de gaz lourd, sans et avec fonctionnement d’un rideau
d’eau. Les essais “terrain” prennent en compte les effets tri-dimensionnels
à grande echelle. Les expériences en soufflerie se limitent aux effets bidimensionnels en petite echelle. Tandis que les simulations numériques modélisent les écoulements observés dans les experiences.
Lors des essais terrains, les distributions de concentration au sol ont montré
une augmentation de largeur de nuage en presence du rideau d’eau. Cet
effet n’est souvent pas souligné dans la littérature. Le temps de réponse du
rideau d’eau a été évalué par des mesures instantanées. Il est de l’ordre d’une
minute, ce qui n’est pas négligable.
La hauteur du rideau est un paramètre de dimensionnement à prendre en
considération. Elle doit être supérieure à celle du nuage mais pas trop élevée
pour éviter la perte de la tenue au vent et la réduction du taux d’entraı̂nement
dans la partie basse du rideau. Un rapport de 2 à 3 semble être un bon compromis.
En ce qui concerne les simulations numériques, une nouvelle approche EulerSource est proposés. Dans un modèle Eulerien-Lagrangien, chaque gouttes
apporte une source quantité de mouvement dans la phase gazeuse. Dans un
modéle Euler-Source bi-dimensionnel, le terme source est défini sur la latérale
27
Résumé
du rideau d’eau. Cette méthode simule donc l’effet perturbateur du rideau
plus simple manière.
Les résultats numériques reproduisent correctement l’entrainement d’air induit par un spray. L’approche Euler-Source apparait plus présice que le
modèle Eulerien-Lagrangien. Pour la simulation de l’effet du vent elle crée
une bulle de recirculation plus importante que l’approche Eulerien-Lagrangien,
mais semble plus en concordance avec les visualisations expérimentales.
Enfin, un synthèse des résultats montre une bonne concordance entre les
différentes approches.
L’action dispersive mécanique d’un rideau d’eau se concrétise par une uniformisation des distributions de la concentration du polluant accompagnée
d’un abaissement de sa valeur dans le champ proche en aval des pulvérisations
d’eau. Dans les essais “terrains” cette uniformisation a été mesurée dans la
direction latérale et en soufflerie dans la direction verticale. Les effets tridimensionnel sont donc à considérer.
L’effet du rapport de quantité de mouvement RM sur l’efficacité du rideau
d’eau est évident. Il est considéré faible en dessous de RM=2. Entre
2 < RM < 5, une bulle de recirculation est créée et F D augmente. Ensuite, c’est effet ne fait que croı̂tre. Ce résultat est retrouvé dans les trois
approches.
Un dimensionnement pertinent d’un rideau d’eau doit garantir un ratio de
quantité de mouvement élevé de l’ordre de 10 et une hauteur deux fois
supérieure à celle du nuage. Si ces conditions sont satisfaites, une efficacité
de dispersion forcée de 90% peut être obtenue.
Enfin, un modèle de l’effet du vent est présenté par l’equation de Bosanquet. Il permettera la modélisation de l’effet du vent dans CASIMIRE.
28
Résumé
Perspectives
La méthode numérique Euler-Source méritterait une extension au cas tridimensionnel. Ils pourraient être comparés aux effets tri-dimensionnels mesurés
dans les essais “terrain”.
Ce projet a démontré tout interet de suivre la methodologie proposée dans
cette recherche. Une perspective générale pourrait donc être de répéter cette
methodologie pour l’étude des effets d’absorption d’un nuage de gaz soluble
et de réchauffement d’un nuage froid par rideau d’eau.
En cas d’absorption, une campagne d’essais “terrain” serait plus compliquée
à mettre en place, vu la necessité de prélever toute l’eau du rideau pour
analyse et eventuellent rincage.
En cas d’effet thermique, les essais devraient être comparés à des essais de dispersion mécanique pour en évaluer la contribution de ces deux mécanismes.
Enfin, une enquête sur les accidents industriels qui ont fait l’objet de rideau
d’eau et le retour d’experience vécue permetterait de créer une base de
données judicieuse pour completer cette recherche.
29
Résumé
30
Part I
Introduction
31
Technological advances have resulted in an important progress, especially in
the chemical industry. In the same time, the concern of the hazard on these
sites has resulted in a raise in safety systems. However, the risk of accidents
is unavoidable, and taking the dimensions of the storage tanks and the nature of the gases (under pressure in a liquid phase) into consideration, the
hazard is rather fatal [23].
Special attention is given to heavy gas clouds, as their accidental release
is more frequent than for other gases [26]. In addition, their consequences
(from toxicity and / or flammability) are particularly severe due to their
behaviour in the atmosphere. Their density, controls the dispersion by gravitational effects, and the cloud remains in a dense layer on the ground for
large distances [60].
In 1984, an accidental gas release in Bhopal (India) resulted in the worst
industrial disaster in history [22]. It consisted of more than 40 tons of
methylisocyanate (MIC) used in the production of sevin. The washing of
pipelines may have lead water into a storage tank, initiating an exothermic
reaction with the gas. The pressure and temperature inside the tank increased until safety valves opened and the gas escaped through the vent gas
scrubber. Studying this particular case demonstrate the crucial importance
of design, operating procedure, safety systems, maintenance and many other
steps in the process industry.
• Operating procedures: MIC has to be stored in a liquid form. The
refrigeration unit designed to cool the liquid was known shut down
as an economy measure. The gas was therefore stored under nearly
atmospheric pressure for more than a month before the accidents.
• Safety systems: Several safety systems were installed at the sevin plant
in Bhopal:
– A standby tank was available for transfer of MIC in occasions of
emergency;
– a Vent Gas Scrubber (VGS) had been provided in the design for
neutralizing 13 tons of MIC per hour;
– a flare system could neutralize the gas by incineration, and
33
– water curtains were installed to mitigate toxic vapours.
• Design: It concerns the plant as well as the safety systems. For the
initial problem, “the water washing theory” is determined through personnel observations in the plant just before the leak. This theory leads
to an interrogation and investigation on the plant design. In addition,
the VGS that could neutralize 13 tons of MIC per hour was installed
for the use on storage tanks that had capacities of 90 tons. The proportions were inadequate. And the upward water curtain could not reach
the height of the release location.
• Maintenance: Water reacts with MIC but the reaction time is long.
However, the water carried iron rust filings from corroded pipe walls
and other contaminants into the storage tank. These elements acted as
catalysts for the exothermic reaction that lead to the burst. In addition,
a corroded piece of pipeline to the flare tower for gas incineration was
removed for replacement. However, it was not replaced and the flare
tower could not be used.
This example demonstrates under which conditions gases have been stored
and the dramatical consequences the accident had and still has 20 years later
on its environment and victims. Some industries have taken this tragedy as
an example to increase the safety measures on their plants [7].
Therefore, tools to mitigate the consequences of such a gas dispersion are
still of interest. The water curtain is commonly used for its dispersion and
absorption abilities on a gas cloud. Fixed water curtains are installed around
storage tanks in the process industry. And mobile water curtains are commonly used by fire-fighters in emergency cases [36], [5], [27].
This study concerns more particularly the influence of a water curtain on
a heavy gas cloud. Emphasis is given on the mechanical dispersion induced
by the sprays in terms of air entrainment. The project concerns fixed installation used on industrial site around storage tanks. The water curtains
efficiency is investigated with respect to the design and the operating conditions by different means.
The objectives of the following project are twofold. The first aim is to mea34
sure the water curtain efficiency with respect to the operating conditions.
Secondly, the more influencing parameters are defined in order to validate
the efficiency. These investigations are undertaken by various means, both
experimental and numerical.
An engineering code CASIMIRE has been developed for the evaluation of
the water curtain performances. The lack of validation with large-scale tests
has given the motivation for the following work. The present project is a part
of a contract collaboration between the von Karman Institute and Ecole des
Mines d’Alès, TOTAL and Gaz de France.
The methodology of the manuscript is described hereunder:
The first part presents a description of heavy gas cloud formation and the
following dispersion in the atmosphere. In fact, the gas type, the release
circumstances and the ambient conditions are necessary information to describe its natural dispersion. Various means of mitigation that have been
investigated are presented and lead to the water curtain, which is commonly
used in the process industry. Its mechanisms on a gas cloud are explained.
Then, the water curtain design is discussed with respect to its efficiency in
dispersing and diluting a heavy gas cloud.
Field tests ensure the motivation of the project by performing an experimental approach in large scale. In the second part, they are presented in
detail. The experimental procedure that includes the set-up, the operating
conditions and the performed measurement are described. Next, the results
mostly concern the gas cloud identification by concentration measurements.
Comparisons between tests without and with water curtain, operating under
different conditions, leads to the efficiency of this mitigation tool.
Since some variables are uncontrolled in the field tests (typically meteorological conditions), a more parametrical investigation is undertaken in a wind
gallery. These experiments are described in the third part. It consists of additional investigations to complement conclusions drawn from the field tests.
Controllable wind speeds and regulation of the curtain-to-cloud height ratio
are the parameters of highest interest in this part.
In the fourth part, numerical simulations are presented. A new technique
35
to simulate the water curtain effect on a gas cloud by an Eulerian model
with a user defined momentum source is compared to the standard EulerianLagrangian model. Comparisons with measured spray characteristics and
visualisations from field tests and Wind Gallery are given.
Next, in the fifth part, a synthesis from the different approaches is given
presenting a comparison in between the approaches. A model of the wind
effect on the forced dispersion is presented.
This leads to the general conclusions of the work. They are presented in
the last part with the resulting perspectives.
36
Part II
Literature survey: Atmospheric
dispersion and mitigation of
heavy gas clouds
37
Introduction
The most important features of gas dispersion and its mitigation are presented. The use of water curtain is explained and the influence of its operating parameters on heavy gas cloud dispersion is described.
The motivation is to present the investigations that have led to the description of heavy gas dispersion and the mitigation by water curtains. Models
and experimental approaches are recalled.
In this part, the dispersion of a heavy gas cloud is described in chapter
1. From the type of release, the formation of the gas cloud and its natural
dispersion are detailed. Special emphasis is given on heavy gas clouds, as
this is the concern in this project. Various mitigation means that exist or
have been presented in the literature are also described. Chapters 2 and 3
present the water curtain mitigation by mechanical dispersion, absorption
and heating of a heavy gas cloud. Each of these mechanisms is explained
and previous investigations are presented by experimental work and modelling respectively.
39
40
Chapter 1
Dispersion and mitigation of
heavy gas clouds
This chapter describes briefly in three sections the release, the formation and
the natural dispersion of a heavy gas cloud. The first section 1.1 presents
typical storage conditions of gases on industrial sites and examples of the
resulting accidental releases. The second section 1.2 defines the “heavy gas
cloud” formation with respect to density, temperature and phase characteristics. Section 1.3 finally describes the natural dispersion of a gas cloud. Some
definitions of its characteristics are given, and its main features are presented.
At last, different methods to mitigate the consequences of a heavy gas release
are presented in section 1.4.
1.1
The release
The nature of the release plays an important role and dominates the cloud
dispersion essentially close to the source. Emphasis is given on gases that
are stored liquefied under pressure. In these storage tanks, the liquid phase
is overlaid with a gaseous phase in the upper part of the tank.
In this case, the nature of the release depends on the location of the rup41
Chapter 1. Dispersion and mitigation of heavy gas clouds
ture on the tank. In figure 1.1 some possibilities are sketched. A distinction
between three various releases are given in the following sections.
Gaseous release
11
00
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
00
11
11
00
00
11
00
11
11
00
Liquid release
1
0
0
1
0
1
0
1
0
1
0
1
1
0
0
1
0
1
0
1
0
1
0
1
Two-phase
release
Figure 1.1: Gas storage tank with various accidental release options
1.1.1
Liquid phase
Also called run-out; it is occurring when a rupture of important dimensions
is located in the bottom of the tank where the liquid gas phase is located. It
usually creates a pool under the release, and when the gas is evaporating a gas
cloud is formed. The liquid release results in the maximum mass release [13]
in comparison with a two-phase or gaseous release under identical conditions
(storage pressure, diameter of release, . . . ).
1.1.2
Two-phase
A two-phase release is a mixture of liquid and gas. It may take place in the
canalisation or in a rupture close to the interface (between liquid and gas)
inside the tank. Under pressure, this release behaves as a jet. Depending of
the density of liquid phase, there might be a deposit of liquid phase under
the release location and to a certain distance (due to the jet). This is called
rain-out.
An investigation of the quantity of rain-out have been measured experimentally for a flashing water jet [9]. It was found that the reservoir tempera42
1.1.3. Gaseous phase
ture had more significant influence on the total rain-out fraction than the
reservoir pressure. Rain-out occurred up to a distances between 5 and 9 m
downstream the source for reservoir pressures ranging from 5 < P < 10 bar
and temperatures ranging from 110 to 170◦ C.
1.1.3
Gaseous phase
Due to a rupture for example on the top of the tank (in the location of the
gaseous phase) the release consists only of gas. In this way, the gas cloud is
directly generated.
1.2
Heavy gas cloud formation
In fact, a heavy gas cloud is not only formed by gases that are heavier than
air due to their molecular weight; some release circumstances may form a
cloud that behaves like one. Heavy gas clouds may be due to ([13], [60]):
• A heavy gas with higher molecular weight than air (chlorine, carbon
dioxide, propane, butane)
• A gas initially lighter than air whose vapour is heavier than air at low
temperatures (for example methane evolved from refrigerated Liquid
Natural Gas (LNG) at its boiling point),
• A gas initially lighter than air containing a large amount of small liquid
droplets (typically ammonia). An estimation of the quantity of ammonia aerosols needed in order to form a cloud denser than air is between
16 and 20 % in a two-phase release [38]. Its density will however depend on the ambient humidity. The lower the humidity, the higher the
density.
• A gas initially lighter than air which may form dense mixtures due to
molecular associations (hydrofluoric acid).
43
The term “heavy gas cloud” must therefore be used for a gas cloud that
behaves like one due to its release or meteorological conditions.
After the gas is released, there is a formation of a cloud or a plume. A
distinction between these two terms is given with respect to the release conditions [56]. A steady continuous release is typically formed in pressurized
canalisations or by a small rupture. At the release location, a jet is formed.
It is characterized by the pressure in the storage tank and the size of the rupture. It may consist of high pressure and velocity release, before a plume is
a formed. Steady conditions are achieved inside a plume. An instantaneous
release forms into a cloud. It may be the consequences of a large rupture in
a storage tank.
1.3
Natural dispersion
The dispersion of heavy gas clouds has been investigated by experimental
approaches and described by modelling. It has lead to a general description
of its behaviour.
1.3.1
Experimental investigation
The atmospheric or natural dispersion of heavy gas clouds have been investigated experimentally for a long time. From the seventies, large-scale tests
have been undertaken and compared to laboratory experiments and led to
the development of models.
The most famous large scale dispersion tests performed may be The Thorney
Island trials in 1984. It consisted in the first part of instantaneous releases
of three to four tons of a mixture of freon and nitrogen. In some few trials,
a continuous release was tested [55]. The second part concerned the interaction of a heavy gas cloud with obstacles. The Tortoise Desert tests in 1983
investigated the dispersion of an instantaneous ammonia release of 15 to 60
m3 under pressure.
44
1.3.2. Description of the phenomena
Usually, the investigations concern concentration measurements and visualization of the cloud dispersion. The gas cloud dimensions have been evaluated by this means and correlated to the meteorological conditions. The
tests have lead to a general understanding of the gas cloud behaviour even
with the large variability of the test conditions.
Due to the cost and complexity of performing field tests, investigations are
more easily performed in wind tunnels. For this purpose, scaling rules have
been defined in order to relate the wind tunnel tests to full scale ([59], [65],
[10], [4], [43]). However, in wind tunnel modelling, constraints and limitations are encountered due to scaling problematic or wind tunnel capabilities of modelling the field. The Froude number scaling requires reduced
wind speeds and enhanced atmospheric density gradients at reduced scales
to generate equivalent levels of atmospheric stability [43]. This is often the
main scaling constraint in wind tunnel modelling. Next, creating a steady
wind profile, an accurate atmospheric turbulence for the lowest speeds and
Reynolds number are typical difficulties [58].
1.3.2
Description of the phenomena
The buoyancy of the cloud is associated to the clouds behaviour in the atmosphere with respect to its characteristics. A heavier-than-air gas cloud is
negatively buoyant. The dispersion of a heavy gas cloud may be described
in two steps.
The first step (after the jet release) is the “heavy gas phase” in which the
cloud breaks down due to its density and creeps at ground level. The dilution of the cloud is weak; it is bound to the atmospherical turbulence and
turbulence due to the displacement of the cloud. This permits an air entrainment inside the cloud (lateral and through the top). The gravitational
effects dominate this phase, and without wind and atmospherical turbulence,
the cloud height would be minor.
As the dispersion progress, the cloud loses its density and the gravitational
effects become negligible. This is the transition phase between the one dominated by gravitation and the one dominated by atmospherical turbulence.
45
The cloud tends to a dilution phase bound only to atmospherical turbulences:
the passive dispersion. It results in a vertical turbulent mixing and the height
increases [19].
It is worth noting that many parameters influence on the atmospherical dispersion of a gas cloud. First, the conditions under which is has been released
(velocity, temperature, nature of gas . . . ) will results in the nature of the gas
cloud (passive or non-passive). The height and release direction are also important. In addition, the meteorological conditions, specially the wind by the
means of advection, the atmospherical turbulence (generally characterised by
atmospheric stability classes), the temperature and relative humidity of the
ambient air play a dominating role on the dispersion of the gas cloud since
they interact on the transport, the air entrainment and the heat exchanges.
In addition, the environment of the gas cloud has to be taken into consideration, because the ground roughness and the presence of obstacles on the gas
cloud trajectory can also modify the gas cloud behaviour by increasing the
dilution and / or slowing it down its evolution (with obstacles).
1.3.3
Modelling
Nowadays, over hundred models are available (publicly or for sale) to calculate the dispersion of accidental releases of hazardous gases [44]. In general
a distinction is given between three types of models:
• The Gaussian model. It is based on the conservation of momentum and
presents the simplest dispersion modelling. It is generally defined for
the case of continuous releases of passive gases. Therefore, it is suitable
for the second dispersion phase of a heavy gas cloud.
• Integral models, also called box models or top hat models. These
models use the physical properties in order to describe the heavy gas
cloud characteristics (temperature, density, concentration, height and
radius). The dispersion is splitted in two phases as described earlier; the
first is dominated by the gravitational effect. The cloud will increase in
radius and decrease in width until the transition to the second phase.
46
1.4. Different means of mitigation
Then the cloud will disperse naturally as a passive cloud and the Gaussian model may be used.
The limitations of this type of model are for example the case of a
release with strong momentum and the variations of the entrained air
in the different models. Their efficiency in terms of simulation time
and adaptability to change the topography, the release rate with time
and low wind conditions make them the more used models in heavy
gas dispersion investigations [26].
• Special models for gaseous jets exist, they are either empirical or integral models (MICAR [25]). Horizontal or vertical sprays can be modelled. Their applicability is gas releases with velocities higher than
wind. If the jet reaches the ground, the model has to be transferred to
a more appropriate one (heavy or passive gas model).
• At last, three dimensional models relay on numerical solutions of differential equations. Their complexity allows the modelling of topography
and special release conditions. However their efficacy is difficult to
evaluate [26].
Dense gas dispersion models are mainly used to define hazard ranges for toxic
irritant gases. However, it is much easier to predict the dispersion distances
for flammable gases (within a few percents) than the longer dispersion distance of toxic gases (between 10 and 100 ppm). Therefore, less investigations
are made on the latter matter [37]. In addition, the toxicity of some gases
on human beings is not identified and there exists a wide range of opinions
for the exposure - response relationships [37].
Ignoring density effects of a plume has been investigated [37] and the authors
highlight the consequences in under-predicting gases toxicity with respect to
an inconvenient modelling.
1.4
Different means of mitigation
Now that the dispersion of heavy gas clouds has been described, ways to
decrease the hazard in terms of toxicity or flammability are presented. It is
47
called mitigation. The gas concentrations in the cloud have to be reduced
under toxicity or flammability levels in order to decrease the hazardous area.
Different manners to mitigate the consequences of a heavy gas release, either
by forced dispersion or by gas removal exist. They are described in the
following sections.
1.4.1
Foam
To reduce the evaporation from a liquid pool of toxic or flammable gas, the
use of foams has been tested ([19], [53], [30], [73]). Foam, also called neutralization agent, acts as a barrier for the gas evaporation and as a neutralization
slowing down the evaporation. An ideal foam should be stable over a long
period, without reaction [19].
For example, the use of foam on a liquid chlorine pool can reduce the evaporation from 0 to 38 % owing to the authors [73], [30]. The foam helps forming a
hydrate on the chlorine surface. A three centimetres layer has been observed
[30].
A comparison of the evaporation rate of non-boiling chorine has been given
for an uncovered surface, a foam covered surface and a plastic covered surface
[30]. The evaporation rate was reduced about 10 % with plastic film and 20
% with detergent foam compared to uncovered surface.
For LNG, high expansion foams are used to decrease the distance to Lower
Flammable Limit [53].
However, reactions may occur when the foam is placed on the liquid pool.
It can be chemical reactions, or reactions due to the physical disturbance.
Such reactions may increase the vaporization rate during neutralization and
therefore increase the downwind concentrations for some time even if the
length and total quantity of the chlorine released is reduced ([73], [30]).
48
1.4.2. Greenbelts
1.4.2
Greenbelts
The use of vegetation in the vicinity of storage installations may improve the
dispersion of gas releases. This is due to a forced dispersion of the gas cloud
encountering a porous media.
The greenbelt was defined by [47] as bands of vegetation made of trees alternated with shrubs. A close planting was recommended to achieve a greater
leaf surface by having more trees per surface area.
Small scale field tests measured the efficiency of greenbelts by evaluating
the free-to-forced concentration ratio i.e. the concentration in a free dispersion by the concentration using a greenbelt downstream [32]. Figure 1.2
presents the experimentation field. Five meters downstream the gas source,
or 3.5 m downstream the greenbelt, the concentrations were reduced by a
factor 4.
Figure 1.2: Greenbelts [31]
Greenbelts offer landscaping of industrial areas and in addition a local mitigation of a gas release. It may delay the spreading of a toxic or flammable
cloud, however, the mitigation factor is important only in the greenbelts
vicinity.
49
1.4.3
Air fans
Enhancing the dilution of a heavy gas cloud increases the cloud volume and
reduces its concentrations. A common way to dilute a gas cloud consists in
exposing it to airflow, such as ventilation. In this manner, a forced dispersion
of the heavy gas cloud takes place.
A mechanical ventilation induced by air fans has then been considered as
a possible mean of mitigation. The advantages consist in the simple installation and maintenance. Moreover, there is no need of additional equipment
that should be stored. Precautions to prevent sparks from the fan have to
be taken into account to avoid an explosion of the gas cloud if it is explosive
or flammable [39].
However, due to the large air flows required, the cost of the fans and ducting
[57] it has not been recognized as an optimal mean.
1.4.4
Fire curtain
Fire produces vertical air entrainment. Therefore, a fire curtain could be
placed downwind a gas release in order to dilute a non-flammable gas cloud.
Dilution by fire curtains have been investigated experimentally on the field
[30]. However, results were difficult to assess. The suggestions are to create
high flames to elevate the gas cloud and to position the fire curtain close to
the source, but avoiding radiation to increase the evaporation speed in case
of a liquid pool.
The choice of fire curtains must be evaluated with respect to the type of
gas, such that chemical reaction due to the heat does not lead to other hazardous materials.
50
1.4.5. Water & Steam curtains
1.4.5
Water & Steam curtains
The air fan and fire curtain techniques use an induced airflow to enhance the
dilution of the cloud. The same idea is used with water or steam curtains.
A water or steam curtain consists of a rack equipped with a uniform distribution of nozzles. As the spray induces an air entrainment, the gas cloud
is diluted.
The action of a water curtain is threefold, namely the mechanical dispersion by air entrainment, the mass transfer by chemical absorption and the
heat transfer due to temperature differences. These actions are explained
with more details in the next chapter.
In fact, this mitigation is commonly used in the process industry because
of the readily available water, the simplicity of the technique, the applicability to other types of hazard. . . Enhanced investigation have been performed
on this mitigation tool due to its applicability to different physical mechanisms. However, a validation of its efficiency with respect to its design and
operating conditions is still missing.
1.5
Water curtain definition
Two types of water curtain exist: mobile water curtains commonly used by
fire-fighters and fixed installations used on industrial sites. A water curtain
is composed of ramps equipped with nozzles. They may be switched on automatically in case of an accidental release. Usually, these water curtains
generate a vertical spray, in downward or upward operating mode.
Water curtains are characterized by the nozzle spacing, the width and height
as sketched in figure 1.3. Several ramps may also be placed in the downwind
direction [14].
The nozzle spacing is defined as Ns and is given in meter. The water curtain
should develop into a fully two-dimensional screen such that the gas does not
51
Figure 1.3: Sketch of water curtains
go through. Varying the distance between the nozzles changes the zone of a
fully developed water curtain. If the distance between the nozzles is large,
the cloud may bypass the water curtain through empty spaces. If the nozzle
spacing is very small, the induced air entrainment is reduced.
A nozzle flow rate is given by its flownumber FN that relates the flow rate Qw
to the operating
pressure ∆P for the specific nozzle. It is usually presented
√
in kg/s/ Pa by the following equation
Q̇w
FN = √ .
Pa
(1.1)
Experimental investigations have demonstrated that the flownumber varies
with the square of the orifice diameter (FN ∼ D02 ) as presented in figure 1.4
[15].
From the choice of nozzle, the spacing and the operational pressure, the
water flow rate per meter of water curtain may be defined with respect to
the nozzle flownumber FN , spacing Ns and operational pressure ∆P as
ṁl,u =
52
FN
· ∆P
Ns
(1.2)
1.5. Water curtain definition
100
4
10 FN [l/s/Pa
−1/2
]
150
50
0
0
5
10
15
D0 [mm]
20
25
30
Figure 1.4: Flownumuber versus orifice diameter FN ∝ D02 [15]
The nozzle characteristics are of prime interest for the mitigation efficiency.
With the flownumber, the spacing and the operational pressure, the flow rate
per unit length of water curtain may be obtained. This latter influence is
rarely discussed in literature but it is a factor more related to both the type
and spacing of the nozzles.
There exist mainly three types of nozzles, which produce different flow patterns:
• The full cone nozzle develops in a circular cone. The spray angle may
vary typically from 30 to 130◦ . A large nozzle orifice results in a large
flownumber and a coarse droplet spray. A schematic of the full cone
nozzle is given in figure 1.5(a).
Some nozzle are equipped with a swirl chamber such that the liquid is
put into rotation.
• A hollow cone nozzle has a similar envelope of that of the full cone
nozzle, however, the inner part of the spray is free of droplets. Typical
use of this nozzle is air-humidification in air conditioning systems or
gas cleaning in chemical and environmental engineering installations.
53
(a) Full cone
(b) Hollow cone
(c) Flat fan
Figure 1.5: Different types of nozzles
A schematic is presented in figure 1.5(b).
• The flat fan nozzle produces a flat elliptic cone (also rectangular or
trapezoidal distribution of liquid). The flow in this spray is more complex than that of cone nozzle. Particularly high-energy jets are generated with spray angles up to 60. Nozzles with small flow rates are
especially suited for humidifying and spraying in general. Flat fan nozzles are commonly used for cleaning operations, in steel making and in
many other fields of surface treatment. A schematic of this nozzle is
given in figure 1.5(c).
For each of these types of nozzles, fog or steam variants exist. Their aim is
to produce very small droplets, typically needed for absorption effects (see
section 2.3).
Other nozzles are designed with an obstacle in front of the orifice. For example the PROTEX nozzle presented in figure 1.6(a) is commonly used in the
process industry for mitigation purposes. It develops into a full cone spray.
Another type is the hydroshield nozzle presented in figure 1.6(b). Close to
the orifice a circular flat plate is placed such that when the liquid jet impacts,
a flat water curtain is generated. Placed on the ground it produces a wide
angle (up to 180◦ ) upward spray as shown in figure 1.6(c). It is the tool used
by fire fighters in emergency situations as a mobile curtain.
54
1.5. Water curtain definition
The droplet size distribution is commonly presented by the number of droplets
corresponding to different diameters as in figure 1.7. For this, the droplets
are grouped in classes. Each class, of range ∆d, is referred to with the index
i and characterised by its average diameter di. On the figure, the y-axis
presents the number of droplets, ni , belonging to the class i, normalized by
the total number of droplets ntot .
A mathematical representation of experimental data is given by the Rosin-
(a) Full cone protex nozzle
(b) Hydroshield nozzle
(c) Hydroshield mobile water curtain
Figure 1.6: Different nozzle types
55
0.08
PDA
Rosin Rammler
0.07
0.06
[cc/cc]
0.05
0.04
0.03
0.02
0.01
0
0
200
400
600
d [µm]
800
1000
1200
Figure 1.7: Droplet size distribution
Rammler distribution. The analytical expression of the volumetric fraction
f (di) of the drops of diameter di is expressed in terms of the average size
d¯ and a dispersion factor both determined from best fitting of experimental
data.
di
dσ−1
(1.3)
f (di ) = σ i¯σ exp [− ¯ ]σ
d
d
where σ is the spreading diameter generally ranging between 2 and 4. For
full cone nozzle it is usually closer to 2. A droplet size distribution for a full
cone spray fitted with a Rosin Rammler distribution is presented in figure 1.7.
For a poly-dispersed spray, many diameter definitions exist in order to model
it as a mono-dispersed spray. Then the spray is featured by one mean droplet
diameter. The definitions can be cast under the following equation
(dmn )
m−n
P
with m 6= n. Typical mean diameters are
• The arithmetic mean diameter d10 ;
56
ni m
i ntot di
ni n
i ntot di
= P
(1.4)
1.6. Conclusion
• The surface mean diameter d20 that were the diameter of the droplets
within a mono-dispersed spray equivalent to the actual spray in interfacial area;
• The volume mean diameter d30 that were the diameter of the droplets
within a mono-dispersed spray equivalent to the actual spray in interfacial volume;
• The Sauter diameter d32 is defined as the mean ratio of the total volume
to the total surface of the droplets. In this way, it represents the ratio
of inertia to drag. This diameter models properly the hydrodynamic
behaviour of the spray [15], [70].
The spray is regarded as direct-contact-exchanger / reactor composed by a
poly-dispersed particulate flow. Therefore, modelling of transport phenomena in the spray has to take this aspect into account.
1.6
Conclusion
The dispersion of heavy gas clouds has been described in this chapter. Heavy
gas cloud dispersion is characterized by a slow gravitational dispersion for
some considerable distance. It has an enhanced lateral spreading compared
to a neutrally buoyant plume. Slowly, a transition to a passive state leads to
standard turbulent spreading.
The different classes of heavy gas cloud dispersion models have been described, from the Gaussian model, integral models to three dimensional models.
Different means of mitigation have been presented for the purpose of reducing heavy gas cloud hazard in term of concentrations. Some act as barriers,
such as greenbelts in front of a gas release or foam on an evaporating liquid
pool. Others induce airflow in order to dilute the gas cloud by enlarging
its volume and thus decreasing its concentration. Air fans, fire and water
curtain have been succinctly presented.
57
The water curtain have been recognized as a suitable mitigation technique
in the process industry. However, a validation of its efficiency is still needed
because the mitigation from the water curtain is complex. It consists of three
different physical mechanisms that are described in the next chapter. And
for each one, different requirements (flow rate, droplet size distribution, . . . )
are proposed to enhance the efficiency. Therefore, a water curtain design
and operating conditions should be validated with respect to the concerned
physical mechanism.
58
Chapter 2
Experimental approach
Three physical mechanisms may occur during mitigation of a heavy gas cloud
with a water curtain, namely the mechanical dispersion relying on the air entrainment, the heating of cold cloud by the spray action and the absorption
of pollutant in the liquid phase.
The dispersion effect has for long time been used by fire-fighters to disperse
smoke and by miners in coal mines for local ventilation and dust removal [20],
[21]. It is caused by a mechanical displacement inducing air entrainment in
a spray. This phenomenon will be further investigated in this thesis.
Next, heat exchange may also occur between the ambient air and a gas cloud
more efficiently with the use of a water curtain due to its mechanical effect.
At last, depending on the gas solubility, water droplets may provoke a chemical absorption or adsorption. Some gases are then captured by the spray
droplets. This phenomenon is an additional effect to the mechanical dispersion.
59
Chapter 2. Experimental approach
2.1
Air entrainment
A visualization of the air entrainment in a single spray initially at rest, i.e.
without wind effects, is given in figure 2.1. Smoke is introduced in the vicinity
of the spray, and is rapidly entrained into it in the vertical direction. The air
entrainment takes place in the developing region of spray, that is equivalent
to the region where the spray diameter Ds increases with the distance to the
nozzle.
Figure 2.1: Air entrainment visualization in a single spray [15]
2.1.1
Single spray at rest
Single spray behaviour has been investigated in the VKI-Water-Spray facility
to determine the main hydrodynamic characteristics of a spray. It is further
described in section 7.1.
Its functioning has been investigated and described [69]. Later, it has been
used for spray characterisation in various configurations (water sprays [66],
60
2.1.2. Wind effect
[46], [3] and flashing [74]).
2.1.2
Wind effect
The air entrainment for a single spray has been introduced. However, a water curtain will be positioned in a wind such that the resulted air entrained
introduced in the previous part may be affected.
High wind speed is an effective tool of mitigation to disperse a dense cloud
by itself. The use of water curtains is appropriate for low wind speed cases:
a movement of the gas will be induced by the air entrainment in the sprays.
This air entrainment is strongly affected in high wind speeds [57].
The difference between the functioning of upward and downward water curtains is described in a simple and comprehensive manner [61]. Imagine a
heavy gas cloud near the ground encountering a water curtain under no or
low wind conditions as sketched in figure 2.2:
• Downward water curtain: As the air entrainment of a spray is highest close to the nozzle, this water curtain will draw air into the gas
cloud and an outflow both up- and down-stream will occur close to the
ground. Now, if the outflow has sufficient velocity upstream, a mixing
zone, or a recirculation is created.
• Upward water curtain: For this configuration, the entrainment takes
place close to the ground. It is therefore the gas that is entrained
upward, mixed with air and diluted in the wake of the barrier. An
important matter for upward water curtain is that the droplets have
longer trajectory, therefore it is beneficial for absorption, as the contact
time is longer.
For higher wind velocities yields [61]:
• Downward water curtain: When the wind speed increases, the recirculation bubble is reduced and replaced by a high cloud height that
61
Figure 2.2: Air entrainment in upward and downward water spray curtain
for no or low wind speeds
travels through the barrier. Also, the barrier effectiveness decreases
with high wind speeds more than for upward curtains.
• Upward water curtain: For low wind and high flow rates, a recirculation
zone is present in the immediate wake of the barrier. For high wind
speeds, the recirculation bubble is pushed towards the ground.
The most famous large-scale tests that have been performed to investigate the
mechanical effect induced by the air entrainment in sprays in real conditions
(i.e. with wind effects) are:
• Buxton test series [60]: Continuous releases of carbon dioxide with release rates of about 1.5 kg/s for 3 minutes were performed. In this
time, concentrations in a free dispersion case (without water curtain)
and a forced dispersion case (with water curtain) were measured at different distances from the source. The tests evaluated the performances
of different nozzles and water curtain orientations.
• Field tests described by Moore & Rees [63]: Eight experiments with
steam and water curtain were performed in 1981. Liquid commercial
propane was released at rates from 0.15 to 3 kg/s. Some water curtain
design parameters were investigated. However, the water curtain was
positioned extremely close to the release point (1 to 2 m).
• EMA chlorine field tests[31]: Small scale continuous releases of chlorine were investigated for the hazard in small storage compartments.
62
2.1.2. Wind effect
Nozzles of flat fan and full cone types were compared for the mitigation
purpose.
During these tests, only gases with low solubility in water such that the absorption in the water from the curtain is negligible, have been used.
The dilution factor which is defined as the concentration ratio without and
with operating water curtain:
FD =
Concentrationwithout water curtain
Concentrationwith water curtain
(2.1)
It has been evaluated for various test configurations (gas type and flow rate,
water flow rate, positioning of the water curtain with respect to the gas
source, meteorological conditions...). The results of the various tests show a
high degree of mitigation in term of dilution factor, especially in low wind
speed and high water flow rate scenarios. Upward water curtains were proved
more efficient than downward ones. The dilution factor was greater by a factor 2 to 5 with upward curtains and reached more than 6 [63].
In smaller scales, laboratory experiments have been performed in wind tunnels [14], [50], [?]. The VKI-Wind-Gallery will be presented in part IV, where
numerous investigations on water curtains effect on gas clouds have been performed, also in the present thesis [3], [45].
For wind speeds higher than 5 m/s, the water curtain is changing shape,
bends in the wind direction and the resulting impact on the gas cloud is
reduced [12].
If a water curtain has a higher momentum than wind, then forced dispersion
can take place [41]. A water-to-wind momentum ratio can be defined as
RM =
ṁl,u · U0
ρc V 2 Hwc
(2.2)
where ṁl,u is the water flow rate per unit length defined in equation 1.2, U0
is the initial velocity of the spray at the orifice, ρc the cloud density, V the
wind velocity and Hwc the height of the water curtain. The dispersion factor
F D increases with RM. However, as the wind is squared in RM there is
63
a limit in increasing the water momentum when the wind is high (typically
V > 3 or 4 m/s).
2.2
Thermal effect
As gases are often stored under pressure, they are very cold (carbon dioxide
is at -20◦C at 20 bar [2]). In the conditions of an accidental release the temperature gradient may therefore be large.
If a very cold gas is heated by mixing with an induced air entrainment,
a buoyancy effect will take place and the gas cloud will increase in volume,
and reduce in concentration. This air entrainment may come from a water
curtain as described in section 2.1.
Water sprays and curtains are currently used by fire-fighters for their ability
to absorb heat. On industrial sites, water curtains or liquid films of water are
used to protect storage tanks by thermal shielding from a heat flux generated by neighbouring fire [40], [66], [17]. Small droplets with large interfacial
surface absorb more heat by radiative scattering and evaporating processes.
This effect has been measured in the VKI-Wind-Gallery. In a mixing box,
liquid nitrogen droplets were injected in an air flow. The evaporation of
the nitrogen in the air stream results in a cold gas cloud. The temperature
was tuned varying the flow rates of nitrogen and air. The water curtain was
designed with hydroshield nozzles, specially conceived for laboratory use [49].
Temperature measurements were performed initially at the gas source and
downwind of the water curtain at a distance equivalent to 4.5·Hwc, where
Hwc is the water curtain height, with a rack of thermocouples.
Generally, the downward water curtain has been proved to be more efficient to heat up the gas cloud than the upward curtain. This is due to the
fact that the gas cloud goes through the empty spaces between the upward
nozzles at ground level. The addition of a small wall that blocks the passage
of the cloud improves the heating, but the downward water curtain remains
64
2.3. Absorption
the most efficient. In addition, downward mode nozzles bring more fresh air
in the cloud than upward mode.
A typical example of a non-dimensional downstream temperature profile
(T − Tmin )/(Tmax − Tmin ) measured in the Wind Gallery is presented in figure 2.3(a) in the case with and without water curtain. The initial gas cloud
temperature is −30◦ , the wind speed is V =0.25 m/s and the position of the
measurements is 2.25 m downstream the water curtain or 5.25 m downstream
the gas source.
The efficiency is defined by the heating factor given by
FR =
−
−
Tmin
− Tamb
+
+
Tmin
− Tamb
(2.3)
where the indices amb is related to the ambient temperature, − is related to
the configuration without water curtain et + is related to the configuration
with operating water curtain.
The heating factor is evaluated with respect to the water-to-wind momentum
ratio RM defined in equation 2.2. It increases and reaches a plateau value
of about 4, then it seems to stabilize for a range of RM > 10 as shown in
figure 2.3(b).
2.3
Absorption
The next action of the water curtain is the absorption of the pollutant by
water droplets. It depends on the gas solubility in water and may be negligible under certain conditions. For example, for low soluble gases as chlorine
(solubility in water of 7.3 g/l or 0.7 % at 20 ◦ C) absorption is very poor.
However, it can be an efficient mean to enhance the water curtain mitigation
for more soluble gases. Additives in the water of the curtain may accelerate
or create a chemical reaction for low soluble gases.
In opposition to mechanical dispersion, absorption consists in a pollutant
removal from the cloud. In this way, the mitigation is not only local around
65
1.4
5
Full cone
Flat fan
Spray ON
Spray OFF
1.2
4
1
FR
Z/H
wc
3
0.8
0.6
2
0.4
1
0.2
0
0
0.2
0.4
(T−T )/(T
min
0.6
−T
max
0.8
)
min
(a) Vertical temperature profiles
1
0
0
10
1
10
RM
2
10
(b) Heating factor with respect to RM
Figure 2.3: Typical results of the thermal behaviour in Wind Gallery [51]
the water curtain but remaining to be felt far away downstream [33].
The water curtains absorption effect is commonly used for highly soluble
gases (hydrofluoric acid, ammonia).
• Goldfish HF spill tests [11]: This campaign is one of the older large
scale investigations on the absorption effect of water curtains. It consists of a few numbers of tests under difficult meteorological conditions.
Therefore, few conclusions are drawn out of these tests.
• Hawk test series [68]: These tests concern detailed investigation of the
water curtain efficiency on hydrogen fluoride. A base case was defined
for a water curtain design (nozzle type, spacing, operating pressure)
changing the water-to-gas ratio. Then the influence of a large number
of operating parameters was evaluated with respect to the base case.
• EMA ammonia field tests [26], [5]: Medium scale, continuous releases
of ammonia were performed. The efficiency of the mobile water curtain
“hydroshield” was evaluated.
Gases that are highly soluble in water (typically hydrofluoric acid [11], [68]
and ammonia [36], [35]) have demonstrated removals up to 36 % with one
66
2.3. Absorption
water curtain and 47 % using both up and downward mode water curtains
[11]. In the field tests with a hydroshield nozzle, the efficiency reached 20
%. This water curtain is not the best for the absorption effects as it creates
large droplet distributions [26]. However, it may consist of an exothermal
reaction: the heat generated by the dilution of 529 g in one litre of water at
20 ◦ C provokes evaporation of 32 % of the initial water [12]. Therefore the
heat of dilution for ammonia may limit the absorption efficiency of the water
curtain.
For low soluble gases few solubility measurements are made in the field.
In small-scale chlorine gas releases (0.8 kg/min), a large variation of results
was obtained for various configurations. A maximum of 5 % of the released
chlorine was dissolved in the water from the curtain [31]. This case corresponds to confined release conditions and very small droplet diameter (that
enhances absorption) while the other disposed of unconfined release conditions and larger droplet distributions. However, using additives in the water
curtain to enhance a chemical reaction increases the absorption. In field tests,
chlorine was let in a liquid pool to evaporate. The absorption by a single
screen using soda ash solution absorbed 20 % of the amount of chlorine [19].
A list of different additives that have been tested is given in table 2.1. For
chlorine, which has low solubility, more tests concern mass transfer in single
drops [67]. Sodium thiosulfate and potassium ioxide absorbed more and enhancement factors reached 56 %. For practical reasons, sodium thiosulfate
is suggested as the most suitable substance. For hydrofluoric acid with high
solubility, small improvement is shown with sodium hydroxide [68].
The droplet diameter is the critical parameter for absorption. A smaller
droplet absorbs faster because the interfacial area is more important and the
contact time is longer [70]. This fact has been confirmed with chlorine by
an enhancement of 40 % for drop diameter of 2.65 mm versus 4.82 mm [67].
For hydrofluoric acid the mitigation efficiency increased from 10 to 20 % decreasing the droplet size to the half (320 to 120 µm) [68].
Due to the favourable action of small droplets on the absorption part, the use
of steam curtain (generating small droplets) has been evaluated. However,
in comparison with water curtain, the cost is the most important difference
as water curtains consumes about 100 times less energy ([57], [33]). Never67
Gas
Chlorine
Solubility
0.7 % by weight
Hydrofluoric acid [68]
AUA
100 % by weight
Additive
soda ash [19]
sodium thiosulfate [67]
sodium hydroxide [67]
potassium ioxide [67]
anhydrous caustic [73]
dry powder soda ash [73]
NaHCO3 [68]
NaHCO3 [68]
AFFF [68]
sodium hydroxide [68]
Table 2.1: Additives in water curtains to enhance absorption
theless, a steam curtain may be favourable in industrial sites where steam is
readily available. It might be more affected under wind conditions.
2.4
Conclusions
From a single spray at rest, to a water curtain in a cross-wind, the different
physical mechanisms of sprays have been determined by experimental approaches.
The physical mechanisms (air entrainment, heat transfer and absorption)
are sensitive to different parameters in the water curtain.
For the mechanical effect, upward water curtains have generally been recognized most efficient compared to downward curtains. In addition, large
water flow rates are found necessary to enhance dispersion.
Next, for a cloud heating, the downward water curtain is found more efficient than the upward curtain. More fresh air is entrained in the cloud and
enhances the heating and dilution.
68
2.4. Conclusions
At last, the absorption effect of the cloud is of course highly dependent on
the gas solubility in the water. In case of low solubility, additives may be
added to the water to enhance chemical reactions that leads to absorption.
In this case, the droplet diameter is the critical parameter. Smaller droplet
has larger interfacial area and longer contact area than larger droplets.
In the next chapter, these features will be described and further explained
by models.
69
70
Chapter 3
Modelling of transport
phenomena in liquid sprays
Several models exist to evaluate the water curtain behaviour on a gas cloud.
The more complex are multi-dimensional models, and the most simple are
the semi-empirical models.
3.1
Multi-dimensional approach
Complex models are based on the solution of differential equations of mass
and momentum conservation for the droplet and the gaseous phase. The system of equations has to be solved by numerical methods over the entire flow
field, on imposition of appropriate boundary conditions. In these models, the
discrete phase is generally modelled by a Lagrangian approach, and coupled
to the gas phase modelled in a standard Eulerian model.
71
Chapter 3. Modelling of transport phenomena in liquid sprays
3.1.1
The gaseous phase
The gaseous phase is considered as an incompressible fluid, comprising a mixture of inert gas (ambient air) and several vapour species (originating from
the mass transfer with the dispersed liquid phase and from pollutant).
The classical Reynolds decomposition is employed to separate mean flow
and turbulent fluctuations. The turbulent character of the flow is modelled
by the total momentum and thermal eddy diffusities, νe and ωe , respectively.
They take into account the contributions of all the significant strain rates in
the generation of turbulence (production and dissipation) and of the presence of particles in the flow field through source terms SΦ expressed for each
droplet class i [15]. The full description of the gas flow can be expressed by
the following general formulation:
∂
∂Φ
∂
(ρg Φ) +
(ρg Uj Φ − ΓΦ
) = SΦ
∂t
∂xj
∂xj
(3.1)
where Φ, ΓΦ and SΦ may be defined for mass, momentum, energy and turbulence modelling. Table 3.1 gives these definitions. The formulation of the
Φ
Mass
Xv
Momentum
U
Energy
h
Turbulence diffusivity
νt
Turbulent kinetic energy
k
Turbulent dissipation rate ǫ
ΓΦ
ρg De
ρg νe
ρg Cp αe
νt
νt
νt /Cǫ
SΦ
P m
Si
P
−∇P + Siu pt
P
Sa dv + Sih pt
P
Prod-Diss + Siturb pt
Prod-Diss
Prod-Diss
Table 3.1: Terms of the gas-phase equation [18]
source terms is:
Mass source
dmdi
dt
(3.2)
dt
t
where ∆ti is the time taken for the particles of class i to cross the control
volume and ṅi is their rate of injection.
m
Sdroplet,i
72
= ṅi
Z
t+∆ti
3.1.2. The droplet phase
Momentum source
u
Sdroplet,i
= ṅi
Z
t+∆ti
t
[mdi (
dudi
dmdi
− g) + (u − udi )
]dt
dt
dt
(3.3)
dmdi
dt
takes account for possible liquid phase change.
Enthalpy source
h
Sdroplet,i
= ṅi
3.1.2
Z
t+∆ti
t
[mdi
dmdi
dhdi
− (hν − hdi )
]dt
dt
dt
(3.4)
The droplet phase
The droplets are regarded as rigid spheres with uniform internal temperature
(small Bi number approximation) and none of the droplet-to-droplet interactions like collision or particle break-up occurs: the spray is assumed to be
a loose suspension. The droplet phase is modelled by a discrete distribution
of droplets of varying diameter, di (section 1.5). In addition, the liquid injection is described by a number of droplet trajectories with different initial
angles, velocities and mass fluxes.
Each droplet class is followed along its trajectory from the nozzle until it
hits a solid obstacle (the ground in the present case). Full droplet absorption
or rebounding on the obstacle can be simulated.
Assigning appropriately a portion of the total quantity of liquid injected per
unit of time to each of the trajectories and determining the drag force along
each trajectory allow representation of the droplet-gas momentum exchange.
The same procedure is followed for the thermal behaviour of the droplet so
that the equations of the liquid phase can be generalised as follows
dΘi
∆Θi
=
+ Si
dt
τi
(3.5)
where Θi is defined for momentum, heat and mass transfer in table 3.2.
τ is the response time of the droplet of diameter di. The momentum response time is related to the drag coefficient CD,i and the droplet Reynolds
number Rei based on the diameter and the relative velocity of the class i.
73
Momentum
Θi
Udi
Heat transfer
Tdi
Mass transfer
mdi
τi
Si
ρl d2i
4
·
3 µg Rei CD,i
ρl Cp,l d2i
6kg N ui
ρl d2i
6ρg Dnu ∆X∞,S Shi
∆ρl,g
g
ρl
L
i)
· dLn(m
Cp,l
dt
+ Sri
0
Table 3.2: Terms of the droplet-phase equation [18]
The thermal behaviour of the droplet depends on the temperature difference with respect to the gas phase ∆Ti = Tg − Ti . The thermal response
time of the droplets is governed by the convective heat transfer coefficient
expressed in terms of the Nusselt number Nui .
Mass transfer related to phase change (evaporation) leads to include effects
of droplet size variation in the source term. The subsequent rate of change of
the droplet diameter is modelled via a response time, which is now function
of the Sherwood number Sh and the difference of the vapour mass concentration ∆ρi∞ = ρvi − ρv∞ .
Mass transfer could be also due to physico-chemical absorption of gaseous
pollutant species within water droplets. In such a process where the variation
of the droplet size can be ignored, the mass flux is modelled by a two-film
approach that leads of equalling the pollutant mass transfer from the gas
phase to the mass transfer into the liquid phase
ϕg,i =
Dν Shi
(ρp∞ − ρpi ) = Ea κl,i (Cpi − Cb ).
di
(3.6)
In the gaseous phase, it is given by the concentration difference between the
gas cloud, ρp∞ , and the surface of the droplet ρpi multiplied by the mass
transfer coefficient by convection given with respect to the molecular diffusion coefficient Dν , the Sherwood number Shi and the droplet diameter di.
In the liquid phase, the flux is equivalent and represented in the same manner between the concentration inside the droplet surface Cpi and the bulk
concentration in the centre of the droplet Cb multiplied by the mass transfer
coefficient in the liquid phase κl,i . If an additive is used in the water to enhance the absorption, its effect may be modelled by an enhancement factor
74
3.1.3. Numerical models
Ea .
The phenomenological particulate transport coefficient such as the drag coefficient CDi , the Nusselt number Nui and the Sherwood number Shi are
determined from classical correlations involving the Reynolds number Rei
([15], [66], [70]). Given appropriate initial conditions, this system of ordinary
differential equations is solved until the flow exits from the computational
domain.
3.1.3
Numerical models
Different numerical methods have been used in order to solve the set of
equations for the multi-dimensional approach.
• The finite difference solver was used typically in HGSPRAY [34] and
NEWSPRAY [18].
• Finite element solver was developed by DeMulder [29]. Simulations of
the air flow entrained by water sprays are given as an example.
• Finite volume solver has also been tested with Fluent applications [51].
The solving method is always a two-step iterative procedure as commonly
reported for gas-droplet flows in the literature. In the first step, the droplet
trajectories and the momentum source terms are calculated on the basis of
the most updated gas flow field. This latter is then adjusted in the second
step using the newly obtained source terms. The process is repeated until
convergence of the flow pattern is reached. The numerical techniques adopted
reflect the different nature of each set of equations.
3.2
One-dimensional approach
As the multi-dimensional are complex and require long calculation time, simpler model are attractive. In this case, a one-dimensional model is defined
75
on the basis of the multi-dimensional model above.
An engineering model has been developed. It models the spray in a onedimensional manner (in the vertical direction). The detailed derivation of the
conservation and constitutive equations that compose the model is presented
in [15], [66] or [70]. The physical modelling relies on the mass, momentum
and energy balance of the two-phase flow along the axial distance of the spray
considering average quantities in the cross section area of the two-phase flow.
According to flow visualisation illustrated in figure 2.1, it is assumed that
the gas flux enters perpendicularly to the spray sheath. A velocity normal
to the spray envelope, which verifies the mass continuity equation, models
this entering gas flow. The mass balance expresses that the gas flow rate ṁ
inside the spray varies due to the external gas entrainment provoked by the
momentum exchange between the two phases and due to the liquid droplet
evaporation.
∇ṁg = ∇ṁg,ent −
nc
X
i
∇ṁdi
(3.7)
The momentum change of the two-phase flow, uṁ, is due to the presence
of the body force modelled by the apparent weight of the particulate phase
F~vol :
∇(ug ṁg ) +
nc
X
i
∇(udi ṁdi ) =
X
F~vol
(3.8)
The enthalpy change of the two-phase flow, he, results from the external-gas
entrainment and the possible thermal radiation absorbed by the gas phase
(water vapour content) as well as by the droplets.
∇(ṁg heg ) +
nc
X
i
∇(ṁdi hedi ) =
nc
X
i
Q̇r,di + Q̇r,g + ∇(ṁg,ent heg,ent )
(3.9)
The behaviour of the droplet phase is modelled by the generic equation 3.5
and the spray edge is described by a droplet trajectory equation.
The one dimensional approach is the basis of the modelling included in the
code MARRS. This model has later been incorporated as the hydrodynamical
part of the engineering code CASIMIRE.
76
3.3. Typical results
(a) Flow pattern
(b) Gas velocity profiles
Figure 3.1: Single spray entrainment
3.3
Typical results
3.3.1
Air entrainement
Figure 3.1(a) pictures the gas flow pattern induced by a downward liquid
spray developing in still air and hitting the ground [29]; the continuous lines
represent the streamlines and the dash lines correspond to the droplet trajectories. Close to the floor, the spray impingement provokes a gas wall jet
and subsequently a recirculation zone.
Figure 3.1(b) shows typical gas velocity profiles in a free downward spray.
The plotted trajectories of some droplet classes indicated that as the twophase flow develops the core of the spray is more and more occupied by the
smallest droplets whilst the coarser droplets tend to remain on the envelope.
77
13
20
12
18
distance from the nozzle z
Exp z = 0,25 m
x
Exp z = 1,05 m
Num z = 0,25 m
Num z = 1,05 m
xx x
xx
11
axial velocity magnitude (m/s)
droplet velocity (m/s)
16
14
12
10
x
8
x
x
x
x
droplet diameter d
6
Exp d=0,132 mm
Exp d=0,697 mm
Exp d=0,965 mm
Num d=0,126 mm
Num d=0,660 mm
Num d=0,966 mm
x
4
2
0
0
0.2
0.4
0.6
10
9
8
7
6
5
x
4
3
2
x
1
0.8
1
distance from nozzle (m)
(a) Droplet velocities
1.2
1.4
0
xx
0
0.1
0.2
0.3
radial position (m)
(b) Gas phase velocity
Figure 3.2: CFD simulations of droplet and air velocity in a single spray [48]
The velocity of droplets and the gaseous phase are presented in figure 3.2
for a single spray simulation.
Good agreement was found for the discrete phase. The mean droplet velocity
at different distances from the nozzle and for various diameters is equivalent
to the measured ones (figure 3.2(a)).
The conclusions are however different for the gaseous phase presented in
figure 3.2(b). The radial velocity of the gas phase does not show a good
agreement in the spray axis. Close to the nozzle, at 0.25 m, the difference is
almost 100 %, and as the distance to the nozzle increases, the comparisons
are improved. Two explanations are given for this discrepancy; the first is
the concentration of discrete phase close to the nozzle. The droplet volume
fraction is too high for a proper coupling between the phases. The second
reason is that the model under predicts the turbulent dissipation close to the
nozzle [48].
Figure 3.3 presents a comparison of the induced gas flow rates for various
operating pressures predicted by the one-dimensional model MARRS and
measured with a PDA.
78
3.3.1. Air entrainement
1.2
Gas flow rate [m3/s]
1
P=124 kPa, Model
P=264 kPa, Model
P=640 kPa, Model
P=124 kPa, Exp.
P=264 kPa, Exp.
P=640 kPa, Exp.
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
Distance from nozzle [m]
0.8
1
Figure 3.3: Comparisons between MARRS and experimental results in the
Water-Spray-Facility for the gas flow rate in a spray [71]
The air entrainment in axis-symmetric sprays has been modelled theoretically and compared to experimental data [54]. It can been shown by dimensionless analysis that the air-to-water flow ratio in a spray is a function of the
water density ρw , the nozzle flownumber FN , the spray diameter Ds (defined
as the diameter of the fully developed spray) [54]:
√
ρw FN
Qa
= f(
).
(3.10)
Qw
Ds2
A synthesis of various experimental approaches comparing experimental results from
from 7·10−7 to 9·10−4
√ different authors, with flownumbers ranging
√
[kg/s/ kPa] in a plot of Qa /Qw with respect to ρw FN /Ds2 shows a reasonable well concordance. It is shown in figure 3.4 where some one-dimensional
and two-dimensional simulations are also plotted.
The figure 3.4 shows two different regions corresponding to large and small
√
values of ρw FN /Ds2 where the air entrainment is respectively wake and jet
controlled:
√
• High values of ρw FN /Ds2 : The entrainment of a single drop is modelled by the turbulent wake of an isolated body moving in an infinite
79
2
10
McQuaid 1975 mean
McQuaid
Eggleston et al.
Watts
Beresford
Heskestad et al.
Benatt et al.
Browning
Davies et al.
Wilcox et al.
VKI
1
Qa/Qw
10
0
10
−1
10
−4
10
−3
10
−2
−1
10
10
0
10
1
10
1/2
ρw FN/D2
Figure 3.4: Synthesis of experimental data and McQuaids air entrainment
correlation
medium. Then, the total entrained airflow rate Qa in a spray is given
by the sum of air entrained by all the droplets in a spray.
√
ρw FN −5/6
Qa
∝(
)
.
(3.11)
Qw
Ds2
A line with slope −5/6 fits the trend of experimental points in figure
√
3.4 for high values of ρw FN /Ds2 .
√
• Low values of ρw FN /Ds2 : A model for an unbounded jet suggests that
the total quantity entrained by the jet up to a given axial distance from
the nozzle is proportional to that distance, such that
√
ρw FN −1/2
Qa
∝(
)
.
(3.12)
Qw
Ds2
80
3.3.1. Air entrainement
A line with slope -1/2 fits well with low values of
3.4.
√
ρw FN /Ds2 in figure
High FN values correspond to coarse-droplet sprays because the nozzle orifice is larger, and FN values characterise fine-droplet sprays. In addition,
coarse droplets develop with a larger spray diameter than fine droplets as
√
sketched in figure 3.5. Therefore, high values of ρw FN /Ds2 correspond to
fine-droplet sprays and low values to coarse-droplet sprays. A coarse droplet
spray will finally entrain more gas than fine droplet sprays are able to do [14].
Fine droplet spray
Coarse droplet spray
α
α
D
D
Figure 3.5: The spray angle α and diameter D
The mechanical dispersion from a water curtain on a heavy gas cloud increases with the droplet size. The higher the droplet mass, the higher the
kinetic energy and the higher the drag force that induces the air entrainment
[12].
The air entrainment increases with the water flow rate. However, the flow
rate may be changed in two different manners. Increasing the operating pressure increases the flowrate, but the droplet distribution decreases accordingly.
The resulting air entrainment may therefore not have improved. To be sure
to augment the air entrainment in a spray, it is better to increase the mass
flow rate by enlarging the nozzle orifice. Then, both the flow rate and the
droplet size increase and the air entrainment rises up.
81
It is worth noting that the droplet velocity has little influence on the quantity
of air entrained by its wake: a drop moving slowly disposes of more time to
entrain air at low rate while it is the opposite for a fast moving drop [54].
3.3.2
Thermal effect
The thermal behaviour of a gas cloud in contact with a water curtain has
been investigated with the one-dimensional model (MARRS) in the vertical
direction [16].
Different scenarios of a chlorine gas cloud moving slowly on the ground towards a water curtain are considered. The gas cloud is 1 m high, its average
temperature is -30◦ C and the molar fraction is 20%. The mass fraction of
water vapour in the cloud is 0.001. The surrounding air is at 10◦ C and
contains a mass vapour fraction of 0.004. The characteristics of the water
curtain are given in table 3.3. It is worth noting that the water is warmed
at a temperature of 30◦ C.
Nozzle
◦
Initial angle
√[]
FN [kg/s/ Pa]
Water temperature [◦ C]
∆P [kPa]
Full cone
60
0.717 · 10−3
30
700
Table 3.3: Water curtain characteristics
Figure 3.7 presents two scenarios with water curtain in downward and upward
operating mode. The vertical distribution of the gas and droplet temperature
averaged over the cross section of the spray is plotted. The nozzle position
is chosen as the vertical axis origin.
In the downward operating mode (figure 3.6(a)), the water curtain height
is 2 m. Therefore, in the upper part (from 0 to 1 m from the nozzle) clean
air is entrained and is mixed to the cold pollutant entrained in the lower
82
3.3.2. Thermal effect
0
2
T
g
0.5
1
T
g
1.5
2
0
d=75 µ m
d=375 µ m
d=675 µ m
10
20
°
T
30
40
(a) Downward water curtain, F R=3.8
1.5
d=75 µ m
d=375 µ m
d=675 µ m
1
0.5
0
−40
−20
0
20
40
°
T
(b) Upward water curtain, F R=2.6
Figure 3.6: One-dimensional simulation of vertical thermal behaviour
part (from 1 to 2 m). Very close to the nozzle, the important liquid enthalpy
induces a rather fast jump of the gas temperature (Tg ). Then, as the gas
entrainment proceeds, the droplet temperature decreases. As observed, the
tiny droplets (75 µm) reach temperature below that of the gas due to the
cooling provoked by evaporation. In this region, there is a mass transfer from
the droplets to the gas (evaporation) and heat transfer from the gas to the
droplets. In hitting the cold cloud, the spray brings an important amount
of fresh air that induces a significant increase of the cloud temperature by
mixing. Since in the cloud the droplets are in contact with a gas mixture
above the freezing point and because evaporation rate is not strong enough,
no ice formation occurs.
Positioning the nozzles at the ground to work in upward mode leads to quite
different behaviour as shown in figure 3.6(b). In such a design the droplets
exhaust first in the cold cloud. The liquid phase cools down rapidly. The
small droplets experience freezing already after 0.4 m and are fully frozen at
1.5 m from the injection point. The cold pollutant is heated rapidly near the
nozzle where the droplet concentration is high; then, the temperature is stabilised because the convective heat transfer balances the incoming enthalpy
of the entrained gas at low temperature. At the top of the cloud (1 m), the
mixing with the warmer air enhances the cloud heating, which is, however,
restrained by the presence of the frozen small droplets.
83
The thermal performance is defined by the heating factor given in equation 2.3. It is 3.8 and 2.6 for the downward and upward mode, respectively.
Both operating modes of the water curtain may yield sufficient heating so
that the cloud can become buoyant enough to be more easily dispersed by
atmospheric currents. However, it should be stated that the heating process
results mainly from the mixing of warm air and cold pollutant. For that
reason the downward mode appears more efficient.
3.3.3
Absorption
Two efficiency definitions are possible when dealing with pollutant absorption
effect. The first is the net absorption efficacy ηabs defined as the ratio of the
pollutant really absorbed by the droplets to the pollutant entrained in the
spray.
ṁp,abs
(3.13)
ηabs =
ṁp,ent
The global efficacy η includes also the effect of the dilution and it is expressed
in terms of relative concentration reduction as
η=
ρp,cloud − ρp,exit
.
ρp,cloud
(3.14)
Figure 3.7(a) presents the evolution of absorption efficiencies in the vertical
direction with a spray as predicted by the one-dimensional model MARRS.
The efficiencies are plooted with respect to the gas cloud to water curtain
height ratio Hc /Hwc. The scenarios concern a gas cloud with initial molar
concentration of 20%,√a water curtain equipped with nozzle with flownumber
FN = 9.7 · 10−4 ks/s/ Pa and a height of 1.65 m.
For small cloud height, the amount of pollutant entrained within the spray is
small and the absorption rate is high. Two causes are invoked: the pollutant
concentration in the liquid phase is yet rather low and the gas has a longer
contact time with the droplet phase. Moreover, a large amount of fresh air
is mixed to the pollutant so that the global efficiency is big. As the cloud
height increases more and more pollutant becomes present in the spray replacing fresh air, the contact time lessens and at the same time the pollutant
84
3.3.4. Wind effect
1
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6
c
η
H /H
wc
abs
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
η
ηabs
0.2
0.4
0.6
0.8
0
200
1
(a) Soluble gas absorption efficiency
within an upward liquid spray [16]
250
300
350
400
d [µ m]
450
500
550
600
(b) Effect of droplet size on the absolute
absorption efficiency of a liquid spray [16]
Figure 3.7: One-dimensional simulation of vertical thermal behaviour
concentration in the droplet increases. As a net result, the absorption and
the global efficiency drop accordingly.
The effect of the spray distribution on the absorption efficacy is analysed
in figure 3.7(b) by varying the mean droplet diameter of the Rosin-Rammler
size distribution from 200 µm to 600 µm.
The figure shows that to enhance gas trapping, sprays formed of small
droplets should be used. This behaviour is closely related to the increase
of interfacial area. However, sprays of small droplets are less resistant to
wind effect.
3.3.4
Wind effect
In CFD simulations, the modelling of the wind effect may be done in a twodimensional domain, defining the spray as presented in the single spray case
and defining a wind profile. A comparison between experimental results from
the wind gallery and NEWSPRAY model are given in figure 3.9 for water
curtains in downward and upward operating mode.
85
(a) Downward mode
(b) Upward mode
Figure 3.8: CFD simulations from NEWSPRAY with wind effect [18]
1
0.8
1
Exp.
NEWSPRAY
Exp.
NEWSPRAY
0.8
0.6
Z/H
Z/H
0.6
0.4 2 m
Upstream
2m
Downstream
0.2
0
0
Exp.
NEWSPRAY
Exp.
NEWSPRAY
0.2
0.4
(T−T )/(T
min
2m
Downstream
0.2
0.6
−T
max
2m
0.4 Upstream
0.8
)
min
(a) Downward water curtain
1
0
0
0.2
0.4
(T−T )/(T
min
0.6
−T
max
0.8
1
)
min
(b) Upward water curtain
Figure 3.9: NEWSPRAY simulations compared with experimental results
[18]
Due to the entrainment of fresh air mixed with the cold gas cloud, the downward operating mode of the curtain leads to better heating than the upward
operating mode and would facilitate therefore, the subsequent natural dispersion process further downstream [18].
Numerical simulations of cold gas mitigation have also been investigated
with simulations performed with FLUENT code [49]. The spray is defined
by a Lagrangian method in a Eulerian continuum. The sprays are flat nozzles types. Upward and downward spray orientations have been investigated.
86
3.4. Semi-empirical approach
The results are defined with respect to the heating factor F R (equation 2.3).
The numerical simulations mimic the flat fan experiments carried out in the
Wind Gallery. The heating factor increases as the water-to-wind momentum
ratio RM increases and then it stabilizes at 4 in agreement with the experimental observations (see figure 2.3(b)).
Fluent simulations have been followed and concern the physical model of
droplet phase change and the heating effects of a water curtain on a heavy
cold gas cloud [8].
(a) Downward mode
(b) Upward mode
Figure 3.10: CFD simulations [14]
3.4
Semi-empirical approach
The modelling of the heavy gas cloud mitigation by water curtains has also
been modelled by simple semi-empirical approaches.
The CURTAIN modelling [14] is similar to the one given by Moore & Rees
[63]. It consists of splitting the physical domain in three regions. The first
and the last regions are located up and downwind the water curtain and
involve atmospheric dispersion. The second region is the water curtain and
concerns the interaction between the water curtain and the gas cloud. The
evolution of the cloud concentration is modelled by Bosanquet equation in
each region:
2+α
Ck (X) = Qg [αVw (Zik + ǫ
Xk )2 ]−1
(3.15)
2α
87
where k is the region number (I, II or III), Xk is the distance from the
beginning of the region, Zi is the corresponding initial cloud height, Qg is
the effective flow rate of gas and α is the aspect ratio of the cloud section
(W/Hwc). The entrainment coefficient ǫ is the basic parameter. It accounts
for the atmospheric stability class and the liquid spray action under the form
ǫ = ǫatm + ǫs
Ve
Vw
(3.16)
where the spray efficacy coefficient ǫs depends on the nozzle type and ranges
typically from 0.2 and 1. The value is obtained through dedicated experiments carried out in the Wind Gallery.
The CURTAIN simulations have been compared successfully with experimental laboratory data for upward and downward operating water curtains.
The upward mode is demonstrated to be more efficient than the downward
mode as for the mechanical effect of the spray is concerned. The concentration reduction is typically improved by a factor 2 in the upward mode when
a small wall is complemented.
The variation of the dilution factor with respect to the downwind distance
has been simulated and is presented up to 500 m downwind the water curtain in figure 3.11 for different atmospheric stability classes. The fact that
the dilution factor is most important close to the curtain is stated. Further
downwind, the dilution factor decreases and tends to unity.
It can be concluded that the mechanical effect of the water curtain is effective in decreasing significantly the hazardous area. However, it is more
appropriate to reduce the gas concentration below the lower flammability
level (for ammonia or LNG) than below toxicity level over proper downwind
distances (chlorine or HCl).
In the CURTAIN model [14], a heat transfer module has been incorporated
to evaluate the vaporization rate of aerosols present in the cloud. A macroscopic energy balance is applied in each region (I, II or III). The amount of
heat absorbed by the cloud is the sum of the convective exchange with the
ground, and the heat brought by the entering fresh air and the water phase.
Phase changes process such as evaporation and freezing are taken into account.
88
3.4. Semi-empirical approach
15
Stab. class F
Stab. class D
Stab. class A
FD
10
5
0
50
100
150
200
250
300
x [m]
350
400
450
500
550
Figure 3.11: Dilution factor with respect to the distance to the water curtain
In the wake of the curtain, the heating process destroys the plumes negative buoyancy and the cloud loses its stratification. It is then assumed that
the atmospheric stability in the wake jumps locally to class A such that the
natural entrainment factor increases. The length of the wake region is evaluated by regarding the water curtain as an obstacle. Typical wake lengths
of 6 to 10 Hwc are found. As for the mechanical dispersion, the mitigation
effect is mostly located in the vicinity of the water curtain.
In the CURTAIN model [14], a new global mitigation factor of the water
curtain is defined incorporating the mechanical dilution factor F D and the
inhibition coefficient F I which is the percentage of the toxic gas flow absorbed:
F D − F Ip1
(3.17)
FC =
1 − FI
where p1 is the molar fraction of the toxic gas at the curtain inlet. The conditions F D > 1 and 0 < F I < 1 yields such that the global mitigation factor
F C increases monotonically with the inhibition factor F I. Now, combining
the Bosanquet equation from the mechanical dispersion (equation 3.15) and
89
the global mitigation factor F C the following expression was derived:
F Dt (X) =
q
F C(1 − F I)Z1,I + ǫ̃(XIII − X2,II ) 2
1
[
]
1 − FI
Z1,I + ǫ̃(XIII − X2,II )
(3.18)
where ǫ̃ is the modified entrainment factor incorporating the aspect ratio
function ǫ(2 + α)/2α. The limit limx→∞ F Dt = 1/(1 − F I) points out that
the contribution of the absorption in the concentration reduction remains as
an offset in comparison to the mechanical dispersion that was located in the
vicinity of the water curtain.
A Discontinuous Virtual Source (DVS) model has been presented [28] to
predict the influence of the hydroshield water curtain on the dispersion of a
heavy gas cloud by providing concentrations of toxic gas downwind the water curtain. It requires a minimum of input data (meteorology, water curtain
dimensions and position) in order to be used in crisis situations.
The dimensions of the cloud vary with the impact on the water curtain
which is then defined as a porous wall allowing a part of the cloud to pass
through it. In addition, the wake of the water curtain is compared to the
one of an obstacle.
The DVS model have been compared to a database of field tests results
and demonstrated good estimations of centreline concentrations in the wake
of the water curtain.
3.5
Conclusions
The modelling of transport phenomena in liquid sprays is complex. In this literature survey, different methods, from multi-dimensional approach, through
one-dimensional model, to semi-empirical approaches are presented. Results
from different applications are often compared to experimental data for validation. Generally, reasonable concordance is presented.
The different mechanisms of the water curtain on a gas cloud are simulated.
90
3.5. Conclusions
For instance, air entrainment induced by a single spray in a still atmosphere,
the wind effect, the thermal effects on a cold cloud and absorption effects.
91
92
Chapter 4
Conclusions
In this literature survey, the main concluding aspects on gas dispersion, different means of mitigation and especially water curtains and their design
have been presented. It is shown that water curtains are recognized as an efficient tool of mitigation to disperse, absorb and heat a gas cloud. Its design
is essential for its efficiency and differs for dispersion, absorption and heat
transfer mechanisms. It is therefore important to design a water curtain for
the gas cloud it will mitigate.
This thesis is related to the mechanical dispersion induced by a spray, typically for the use on heavy gases that have low solubility in water. Important
concentration reductions are observed in the vicinity of the water curtain,
but decrease with the distance to the source approaching unity far from the
source ([33], [42] and [65]). However, most of these projects use low water
flow rates in the curtain (ṁl,u < 5 kg/s per meter of water curtain).
When the gas is absorbed in the water curtain, either for its solubility in water
or by chemical reaction with additives, this effect is additional to the mechanical dispersion. The absorption decreases the concentrations also downwind
as the gas is trapped, and not only displaced. The water may need to be
collected and cleaned in order not to pollute the environment.
Little investigation is made on heat transfer induced by a water curtain on a
93
Chapter 4. Conclusions
cold heavy gas cloud. This is probably due to a more complex experimental
set-up to measure and quantify the temperature. However, some laboratory
investigations have been performed and present the heat transfer efficiency.
The advantages of the water curtains are the readily available water, the
operating cost and the simple functioning. In case of a gas dispersion on
an industrial site, an early detection [57] and the wind direction (only the
part downwind needs to be activated, [57]) are essential information. Water
canalisation should be constantly under pressure; however this is not always
the case in practise.
The disadvantages of downward water curtains are the limited access for
maintenance and for upward curtains the possible damage being placed at
ground level [57]. If there is a contact between the water and the tank where
the release is occurring, the water enhances the eventual corrosivity of the
gas. If the water curtain is positioned to close to a liquid pool, it will enhance
vaporisation and increase the downwind concentrations. If it positioned to
close to a jet release, the gas might pass through the water curtain (due to
high inertia) and decrease the efficiency. The maintenance cost of the water
curtain is evaluated and has been estimated to rapidly be high [12]. The ambient temperature may be a concern under freezing conditions. The droplet
size distribution has to be conceived for a scenario (mechanical dispersion,
heating and / or absorption) as they may be dragged by the wind, or easily
saturated (absorption).
Nevertheless, it is important that a water curtain is designed for a scenario
with respect to its location and its needed physical mechanism with respect
to the gas characteristics. For low soluble gases, the air entrainment effect
is enhanced by big droplets, thus by nozzles with high flownumbers, while
for highly soluble gases, the mitigation is enhanced by using small droplet
diameter distribution (steam).
The performed studies on the water curtain efficiency usually present results
from a single approach (field tests, wind tunnel experiments or modelling).
Some comparative work has been performed specially by modelling. However, a project could be performed with the intention to validate the results
from different approaches.
94
In the present work, this method is used for the investigation of the mechanical dispersion of a spray by field tests, wind gallery experiments and
numerical simulations in the following methodology:
• Field tests are performed with the same nozzles for various configurations. Chlorine and carbon dioxide are used, as their solubility in water
is low. The water curtain efficiency is evaluated for the mechanical dispersion.
• Next, to compare a wind gallery’s possibility in simulating large scale
tests, experiments have been carried on with a smaller type of industrial
nozzles. The wind gallery also gives the possibility of a more parametrical study than in the field and presents various types of results.
• For each of these steps, numerical simulations are made to compare and
investigate their ability in modelling air entrainment and gas dispersion.
• At last, a synthesis of these different approaches is presented with respect to the air entrainment induced in a spray, and the wind effect on
a water curtain.
In this manner, this thesis should present a detailed investigation on the
mechanical dispersion by water curtains.
95
Chapter 4. Conclusions
96
Part III
Field tests
97
Introduction
The objective of the field tests is to measure the changing behaviour of a
heavy gas cloud in the presence of a water curtain in real conditions that
may be encountered during an accident. The gas cloud will be influenced by
the water curtain with respect to its operating conditions (nozzle type and
spacing, pressure, height, width) and the meteorological conditions.
The motivations to perform field tests are threefold; first, the validation
of using industrial nozzles is essential. In the past, many investigations have
been performed on water curtain efficiency in wind galleries. In these tests,
the nozzles are scaled down and have small flownumbers. Next, performing
tests approaching real case scenarios are of interest due to variations of meteorological conditions. The wind is fluctuating in velocity and direction, and
induces three dimensional effects on the cloud dispersion. In addition, the
gas cloud is not canalised; it may therefore disperse in the lateral as much
as in the vertical direction. Such effects are not reproducible in laboratories.
At last, a database is created for comparison, validation and improvement of
the engineering model CASIMIRE described in section 3.2.
The worst case scenario, where no water curtain is used to disperse the gas
cloud will be referred as free dispersion. Next, operating the water curtain
to introduce a mixing effect between the gas and the ambient will be referred
to as forced dispersion. The methodology of this experimental approach is
to perform free and forced dispersion tests and compare concentrations for
various operating conditions in order to create a database for the modelling.
In this part, a detailed description of the field tests preparation is given
99
in chapter 5. Chapter 6 presents all the results that have been extracted
from these campaigns. The most investigated parameter is the ground concentration.
100
Chapter 5
Description of the set-up
This chapter describes in details the preparation of the different arrangements on the field tests. The field sites, the gas sources and water curtains
disposition and functioning are defined for the various campaigns presented
in chapter III. Then the measurement techniques for the concentration, temperature and meteorological conditions are given. At last, the experimental
procedure is defined before concluding on the methodology of the field tests.
5.1
Objectives of the different campaigns
Three different series of trials have been performed with different scales procedures.
• First campaign: The downwind cloud behaviour was identified in a
large field (500 m2 ) for free and forced dispersion tests. The tests were
performed on a large flat terrain in ”Camp des Garrigues”, France (figure 5.1(a)).
This campaign resulted in concentration measurements in a large field
downstream the source for different wind speeds and various water
curtain designs and operating conditions. However, the changes in
the meteorological conditions made an evaluation of the water curtain
101
Chapter 5. Description of the set-up
performance difficult. Therefore, the experimental procedure was improved in the next campaign.
• Second campaign: During these trials performed at Lavera (site of
TOTAL, France, figure 5.1(b)), the free dispersion case was directly
followed by a forced dispersion case to minimize changes in the meteorological conditions between the tests. Larger nozzles were used to
increase the water curtain momentum that was shown to be low in the
first campaign.
• Third campaign: Tests were performed in a larger scale in the same
conditions as in the second campaign. Increasing the gas release and
water curtain dimensions created a more realistic hazard scenario.
In all cases, the experiments were controlled by fire-fighters and a security
area was defined during the trials.
(a) Camp des Garrigues
Figure 5.1: The field sites
102
(b) Lavera
5.2. Gas source
5.2
Gas source
This thesis concerns the investigation of heavy gas dispersion and particularly
mechanical dispersion induced by a water curtain. Therefore, gases have been
chosen for their density and solubility characteristics: density to ensure the
behaviour of a heavy gas cloud (ρg > ρair ), and low solubility to make the
absorption of the gas in the water curtain negligible. The thermal aspect
has been measured and found negligible for the considered release rates and
distances. In this way, only the mechanical dilution will be investigated.
Chlorine and carbon dioxide were chosen, and the details of the source term
are described hereunder.
5.2.1
Chlorine gas
In the two first campaigns, chlorine gas was chosen for the dispersion investigation. It is a heavy gas; therefore it disperses slowly at ground level. Its
solubility in water is also low, such that the effect of the water curtain is
mainly a mechanical dispersion. The main chlorine characteristics are given
in the table 5.1.
CAS nb.
Characteristics
Molar mass
Vapour Density
Boiling point at atm. pressure
Specific heat at constant pressure
Solubility in water at 20 ◦ C
7782-50-5
Non-flammable gas in air
Heavy oxidant
70.906 g/mol
2.49 kg/m3
-34.06 ◦ C
477 J/kg · K
7.3 g/l or 0.7% by weight
Table 5.1: Chlorine gas characteristics
The gas source consisted of one or two cylinders of liquefied chlorine (B20Air Liquide), pressurized at 10 bar and fitted out with a dip pipe to achieve
steady gaseous continuous releases. The release is moderate for safety rea103
sons, but also such that the thermal aspects are negligible compared to the
forced dispersion of the spray. When two bottles are used, they are placed
together (as shown in figure 5.2), and in the following the source will be
considered as a single release.
Figure 5.2: Disposition of chlorine bottles
A manometer is used to control the release while weighting the bottle before
and after the release and timing the duration determines the total discharge
rate. In addition, a manometer reduces the pressure and velocity of the release. Typical releases last 4 minutes and release rates range from 1 to 5
kg/min. The chorine gas is emitted horizontally, at ground level. At the
source, it behaves as a jet, but due to the ground friction its velocity is
evaluated as the wind velocity at the level of the water curtain.
5.2.2
Carbon dioxide gas
In the last campaign, carbon dioxide which is a more user-friendly gas was
chosen in order to investigate, in a safer manner, a larger scale effect. Its
main characteristics are given in the table 5.2.
The gas source involves a tanker (20 tons at 50 bar), a heating pool, and
104
5.2.2. Carbon dioxide gas
CAS nb.
Molar mass
Relative Vapour Density
Boiling point at atm. pressure
Specific heat at constant pressure
Solubility in water
Concentration in air
124-38-9
44.01 g/mol
1.521 kg/m3
-75.5◦ C
0.85 J/kg · K
2 g/l
0.03%
Table 5.2: Carbon dioxide characteristics
a depressurizer as shown in picture 5.3. This complex system is set up in
order to achieve a constant gaseous release.
The liquid carbon dioxide is heated in the pool until vaporization, and transferred to a depressurizer, such that the release pressure and velocity are
reduced. The gaseous source is located 50 cm above ground level, horizontally directed and controlled by a volumetric flow meter.
(a) Warm bath
(b) Release system
Figure 5.3: Carbone dioxide gas source system
In order to generate a constant gaseous flow rate with time, it was necessary
to control and readjust the opening valve situated between the heating pool
and the depressuriser. Figure 5.4 presents the evolution of the volume flow
rate with time (at the top) and the exit temperature of carbon dioxide (middle). The temperature is decreasing during the release due to the cooling
105
of the pool. The mass flow rate may then be calculated with respect to the
density changes for the respective temperatures (bottom).
800
3
Volume flow rate [m /h]
600
400
200
0
0
30
5
10
15
20
10
15
20
10
Time [min]
15
20
Temperature [°C]
20
10
0
0
20
5
Mass flow rate [kg/min]
10
0
0
5
Figure 5.4: Evolution of the carbon dioxide release
However, the carbon dioxide density variation in this temperature range is
small (1.951 kg/m3 at 0◦ to 1.754 at 30◦ [1]), and the mass flow rate is stable
at about 20 kg/min.
Table 5.3 summarizes the gas and respective flow rate for the various campaigns.
Campaign
First
Second
Third
Gas
Chlorine
Chlorine
Carbon dioxide
Gaseous rate
[kg/min]
1 < mg < 4
4 < mg < 8
20
Table 5.3: Gas flow rate for the various campaigns
The gas source was in all cases placed in the origin of the defined coor106
5.3. Water-curtain & Nozzles
dinates; the x-axis represents the downwind direction, and the y-axis the
lateral direction. The release direction was oriented in the wind direction.
5.3
Water-curtain & Nozzles
The water-curtain consists of a pipeline equipped with a uniform distribution
of pressure nozzles as sketched in figure 5.5.
5m
20 cm
2m
water-pipe
Figure 5.5: Sketch of the water-curtain
5.3.1
Nozzle characteristics
For the mechanical dispersion, nozzles that induce a large amount of air
entrainment (large orifice diameter and flownumber) are chosen. The used
nozzles are full cone nozzles with a spray angle of 90◦ and are manufactured
by Lechler. A picture is presented in figure 5.3.1.
Three different nozzles sizes are tested in the field tests; their characteristics are presented in table 5.4. The nozzle diameter D0 varies from 3.6 to
8 mm√and the resulting flownumbers range is 2.35 · 10−4 < FN < 9.32 · 10−4
kg/s/ Pa. For two nozzles, the flownumber was measured in the water spray
facility (see section 7.1) by weighting and timing the water flow for various
107
Figure 5.6: Full cone tangential nozzle
pressures. The errors with respect to the flownumber given by the manufacturer is inferior to 3.5%. In addition detailed spray characteristics for these
nozzles are given in section 8.1.
Ref. nr.
422.966
422.846
422.726
Nozzle diameter
[mm]
8
5.1
3.6
Manufacturer
FN
√
[kg/s/ Pa]
9.32 · 10−4
4.66 · 10−4
2.35 · 10−4
Measured
√ FN
[kg/s/ Pa]
9.00 · 10−4
4.76 · 10−4
-
Error
%
3.4
2.1
Table 5.4: Field tests nozzle characteristics
The generated droplet distribution is large. Measurements and modelling
state sauter mean diameters are ranging from 300 < D32 < 400 µm. Maximum droplet sizes are of the order of one millimetre.
5.3.2
The water curtain
The first water curtain is 5 m long and 2 m high (figure 5.5). The second
is 10 m long with a system to adjust the height between 2 and 3 m. Their
maximum number of nozzles is respectively 25 and 50. The water supply is
in both cases made simultaneously in both extremities and two manometers
evaluate the pressure on the water curtain at both sides. The water curtains
maximum flow-rate is 270 l/min per meter of water curtain at 1000 kPa. The
108
5.4. Measurement points & technique
operating pressure is 300, 500 or 700 kPa.
The water curtain orientation is downward. The rack is then rotated and
put on the ground; the nozzles are 40 cm above the ground.
Table 5.5 summarizes the choice of nozzles with respect to their orifice diameter D0 , the nozzle spacing Ns and the resulting water flow rate per meter
of water curtain for operating pressure in the range 300 < ∆P < 700 kPa in
the various campaigns.
Campaign
First
Second
Third
D0
Ns
[mm]
[cm]
3.6 & 5.1 40
8
20
8
20
Water curtain rate
[kg/min/m]
20 < ml,u < 60
150 < ml,u < 225
190 < ml,u < 225
Table 5.5: Water curtain characteristics for the various campaigns
The choice of nozzles and spacing changes drastically the flow-rate of the
water curtain. In the field test a range from 20 to 225 kg/min per meter of
water curtain has been tested. The water curtain is adjusted perpendicular
to the wind direction and the release axis of the gas.
5.4
Measurement points & technique
Instantaneous and mean concentrations are recorded at different positions
around and downstream the water curtain. Temperatures have also been
measured in order to evaluate the gradient with the ambient air. The measurement techniques and positions are described in this section.
109
5.4.1
Instantaneous concentration measurements
In order to evaluate the gas cloud behaviour in terms of concentrations with
respect to time, instantaneous measurements provide detailed information.
Chlorine
Photo Ionisation Detectors (PID Mini RAE 2000 from RAE System, see figure 5.7(a)) are used to measure instantaneous chlorine concentrations during
the first campaign.
The mixture of air and gas is pumped through a filter (for humidity) to a
cell. The gas is ionised by means of an UV light and the concentration is
deducted from the electric discharge from the ionised gas.
The captors are equipped with discharge lamps of 11.7 eV. In the concentration range 1 to 10000 ppm, the resolution is 1 ppm, the response time 2 s
and precision ±20%. A data logger in the captor ensures the storage of the
measured concentrations.
The lamp used to ionise the chlorine is extremely fragile and very expensive, therefore the use of these instantaneous captors was limited to one test
day. During this day, 15 captors were used. Their positioning downwind the
water curtain is sketched in figure 5.8(a). Due to their sensitivity to water,
they could not be placed close to the water curtain as fine droplets are entrained by the wind. They were therefore placed about 10 m downwind the
water curtain.
Carbon dioxide
In the last campaign, the carbon dioxide concentrations were measured instantaneously by means of Infra red sensors (MultiWarn II, Draeger, see
figure 5.7(b)). The instrument is operated with a hose probe. The mixture
of air and gas is then pumped through a water and dust filter with a flow
rate between 0.2 and 0.6 l/min.
The instrument can be used to measure carbon dioxide gas in mixtures con110
5.4.2. Mean chlorine concentrations
taining air in volumetric concentrations up to 25%. Its precision is higher
than 0.01%.
During the tests, five captors were used and positioned 4 m downwind the
water curtain or 14 m downwind the gas source as presented in figure 6.13(a).
The dilution factor can then be evaluated in the vicinity of the water curtain.
(a) PID captors for chlorine
(b) Multiwarn captors for carbon dioxide
Figure 5.7: Instantaneous concentration measurement positions
5.4.2
Mean chlorine concentrations
The mixture of air and chlorine is absorbed and bubbled in a sodium hydroxide solution (0.1 M) in order to trap the chlorine. The bubbling starts
30 sec after the beginning of the gas release (in order to skip the transient
start and the gas cloud covers the measurement field) and lasts till the end of
the measurement. Previous tests in laboratory demonstrated that more than
95% of the chlorine was trapped by the bubbling. Under these conditions,
concentrations are average concentrations over 3.5 min. The chlorine concentration is then deduced directly by UV spectrophotometer of hypochlorite
ions formed during the reaction (between chlorine and soda) with respect to
111
15
15
Measurement
points
10
Lateral distance [m]
10
5
Source
0
Water
curtain
−5
−10
−15
−5
5
Measurement
points
Source
0
−5
Water
curtain
−10
0
5
10
15
20
Downwind distance [m]
(a) PID captors for chlorine
25
30
−15
−5
0
5
10
15
20
25
(b) Multiwarn captors for carbon dioxide
Figure 5.8: Instantaneous concentration measurement positions
the volume of the solution and the volume of air bubbled through the solution. The lowest detectable value is 11 ppm. The estimated error in the
measurements is 10%, the uncertainty of the concentration is close to 15%.
A remote antenna controls the mean concentration measurements.
Figure 5.9: Mean concentration measurement set-up
112
30
5.4.2. Mean chlorine concentrations
In the first campaign, about forty measurement points were distributed, at
ground level, within in a circular mesh (with the source as the origin) downwind of the source in different lengths from the source at 7.5, 10, 15, 20 m
as sketched in figure 5.10(a).
In the second campaign, the test procedure was modified such that a forced
dispersion test directly follows a free dispersion test. For this purpose, the
measurement points are activated by the means of two different frequencies
for the remote control. This new procedure requires therefore two captors
in each measurement point (in figure 5.10(b) one point represents two captors), one switched on during the free dispersion test and one during the
forced dispersion test. The number of measurement points is then reduced
to the half compared to the first campaign. Therefore, the positioning of the
points was restricted in the area close to the curtain, up- and downstream,
in order to measure the concentration reduction in this zone. Some points
were placed between the source and the water curtain and on the sides of the
water curtain in order to evaluate whether the gas cloud was surrounding
the water curtain or not.
15
15
Measurement
points
10
10
5
Source
0
−5
Water
curtain
−10
−15
−5
5
Measurement
points
Source
0
Water
curtain
−5
−10
0
5
10
15
(a) First campaign
20
25
−15
−5
0
5
10
15
20
25
(b) Second campaign
Figure 5.10: Mean chlorine measurement positions
113
5.4.3
Temperature measurements in the gas cloud
In order to evaluate the temperature difference between the gas cloud and
the ambient, the temperature was measured at different locations from the
gas source at ground level.
Up to five thermocouples type K connected to data loggers were used during
experiments. The error was estimated to less than 1◦ C by calibration. The
sampling rate was set at 1 or 3 seconds. They were positioned at ground
level, either close to the source, or at 10 m from the source at the same
location as the concentration measurement points.
The knowledge of the temperature gradient between the gas cloud and the
ambient air will help providing information on the heat transfer importance
during the cloud dispersion.
5.5
Meteorological conditions
The meteorological conditions (wind speed and direction, temperature, relative humidity) were measured by two different means, a vane propeller
anemometer and an ultrasonic anemometer.
5.5.1
The vane propeller anemometer
This wind monitor measures the horizontal wind speed and direction respectively in function of the propeller rotation and the vane position.
The wind speed range is zero to 60 m/s and resistant to gust of 100 m/s. Its
resolution and accuracy are respectively 0.1 m/s and ± 0.3 m/s.
The vane position is transmitted by a potentiometer in which a constant
applied voltage deliver the output signal as an analogue voltage directly proportional to the azimuth angle. The range of the vane position is 360◦ , its
114
5.5.2. The ultrasonic anemometer
resolution and accuracy are 1◦ and ± 3◦ respectively.
In addition, a relative humidity and temperature probes are used. The sensor
for the relative humidity is capacitive polymer with an accuracy of 3% from
0 to 100% relative humidity. A platinum sensor is used for the temperature
probe. Its standard accuracy is ±0.3◦ at 0◦ C.
The vane propeller anemometer is fixed at 10 m above the ground to measure
the wind speed and direction.
5.5.2
The ultrasonic anemometer
An ultrasonic anemometer performs tri-dimensional measurements. It measures the wind velocity through the transit time of the ultrasonic signal sent
between the transducers.
The resolution and accuracy of the velocity are 0.01 m/s and ± 1% RMS
between 0 and 30 m/s respectively. For the wind direction, the resolution is
0.1◦ , and accuracy 2◦ RMS.
This anemometer is fixed 2 m above ground level in order to evaluate the
wind behaviour at the height of the water curtain.
Meteorological conditions (especially wind speed and direction) are very important. Therefore, much attention is given to these measurements. These
captors were located in the secured area upwind the source defined in the
test.
5.6
Experimental procedure
The experimental procedure was changed in the different campaigns for the
reasons presented before. However some essential features have been kept
regular.
115
The water curtain was oriented perpendicular to the wind direction in the
release axis of the gas. The measurement points were adjusted such that
they covered the expected width of the gas cloud with respect to the release
axis.
The measurements started when the cloud had reached a homogeneous behavior (typically after 30 s after opening the source).
The instantaneous concentration and temperature measurements had to be
started before the release, such that they contain the arrival of the cloud.
In the first campaign, 30 seconds after the release, the mean concentration
measurements started and lasted approximately 3.5 min. The meteorological
recording starts with the release. Figure 5.11 presents the procedure.
Free dispersion:
0 0.5
4
Measurement time [min]
Forced dispersion:
0 0.5
4
Measurement
time [min]
Water curtain on
Figure 5.11: Experimental procedure in the first campaign
In the forced dispersion case, the water curtain was turned on before the gas
source to ensure a stabile water curtain when the release started. The mean
concentration measurement still started after 30 seconds after the gas release.
Six test days resulted in 34 tests. From these tests, 20 tests were exploitable;
their characteristics are presented in table 5.6. Changing wind direction
or other practical problems are the reasons for which some tests were unexploitable.
116
5.6. Experimental procedure
One can see that the influence of the wind speed on RM is very important
as it is squared. This will be proved as the most important parameter for the
mechanical dispersion performance of the water curtain. In this campaign,
poor water flow rate and high wind conditions resulted in a large number of
test with RM < 1.
Exp
nr
12
14
16
21
22
25
26
31
32
41
42
43
44
45
61
62
63
64
65
66
Wind
[m/s]
2.3
1.0
2.4
3.5
2.3
1.1
0.9
6.3
5.6
2.9
3.6
1.8
2.3
3.7
4.0
3.0
3.4
3.4
2.8
3.0
Gas
[kg/min]
2.5
2.2
4.3
3.4
2.9
4.0
4.2
3.2
0.9
1.8
2.4
3.5
5.0
3.3
3.0
3.1
2.2
4.0
3.5
8.6
Water
RM
[kg/min/m]
39
<1
50
4.0
50
1.5
39
<1
50
5.6
50
8.3
39
<1
39
<1
39
1.3
39
<1
19
<1
19
<1
19
<1
∆P
[kPa]
300
500
500
0
300
500
500
0
300
300
0
300
0
300
300
0
0
300
0
300
Table 5.6: The first campaign test characteristics
In the second campaign, a trial consisted of a free dispersion case followed by
a forced dispersion case. The mean concentration sensors were switched on
30 seconds after the beginning of the gas release and lasted 3.5 min. Then
117
the water curtain was operated, and 30 seconds later, the second mean concentration captors were switched on. The schema sketched in figure 5.12
illustrates the experimental procedure.
0
0.5
4
4.5
Free dispersion measurement
Gas
8
Forced dispersion measurement
time [min]
Water curtain on
Figure 5.12: Experimental procedure for the second campaign
Table 5.7 presents the characteristics of ten exploitable tests from three days.
The water flow rates were kept high such that RM > 1 in all cases.
Exp
nr
71
72
73
82
83
84
91
92
93
93
Wind
[m/s]
4.2
4.0
5.3
2.1
1.9
1.4
6.2
5.6
3.4
3.8
Gas
[kg/min]
3.7
5.8
5.9
6.6
7.7
5.2
3.8
5.1
4.8
3.9
Water
[kg/min/m]
191
191
226
191
148
191
148
191
191
191
RM
1.7
1.8
1.4
7.0
5.4
16.6
0.6
1.3
2.6
2.4
∆P
[kPa]
500
500
700
500
300
500
300
500
500
500
Table 5.7: The second campaign test characteristics
In the third campaign, the water flow rate was not sufficient to reach RM > 1.
In the last campaign, only instantaneous captors were used, therefore, once
the gas source was opened, several trials of free and forced dispersion were
followed. Table 5.8 presents the characteristics of the exploitable tests.
118
5.7. Conclusions
Exp
nr
A1
A2
A3
Wind
[m/s]
3.7
3.7
3.7
Gas
[kg/min]
20
20
20
Water
RM
[kg/min/m]
120
<1
120
<1
120
<1
∆P
[kPa]
200
200
200
Table 5.8: The third campaign test characteristics
5.7
Conclusions
The set-up of the field tests has been presented for three different test campaigns. The table 5.9 summarizes the main used flow rates for gas and water
in the campaigns.
Campaign
First
Second
Third
Gas
Chlorine
Chlorine
Carbon dioxide
Gaseous rate
[kg/min]
1 < mg < 4
4 < mg < 8
20
Water curtain rate
[kg/min/m]
20 < ṁl,u < 50
150 < ṁl,u < 225
190 < ṁl,u < 225
Table 5.9: The different campaign characteristics
The water flow rate was highly increased after the first campaign due to
preliminarily results stating that for high wind conditions, the water curtain
needs to be stronger in terms of momentum.
The gas flow rate was increased to approach real case scenarios and in this
occasion a less hazardous gas, carbon dioxide was used.
Measurements of temperature and meteorological conditions ensure the knowledge of the “uncontrollable” parameters. Measurements allow quantification
of the gas cloud concentrations and temperatures at different downwind positions, with and without operating water curtain. In this way, the water
curtain efficiency in dispersing a heavy gas cloud can be deduced.
119
120
Chapter 6
Results
The temperature measurements are described to evaluate the importance of
heat transfer in these tests. The features of free and forced dispersion are
described in detail. Most of the results in these sections are taken from the
measurements of the first campaign. The results from the second campaign
focus on the dilution factor achieved by performing free and forced dispersion
tests under similar meteorological conditions. At last, the conclusions are
given.
6.1
Temperature measurements
Temperature measurements aim to evaluate the temperature difference with
the ambient and to determine its effect on the mitigation.
6.1.1
Temperature measurements in the near field of
the source
Three temperature measurements were performed at 1 m from the source in
an arc in front of the release. At this distance, the gas cloud is very narrow
121
Chapter 6. Results
(∼ 1 m), therefore, only the thermocouple placed in the release axis of the
gas has measured temperature differences with the ambient air.
Figure 6.1 presents a typical temperature measurement in the gas release
axis. Large variations (up to 15◦ C) are measured. This is due to the sensitivity of the gas cloud to wind fluctuations and ground roughness. No temperature differences due to the operation of the water curtain (positionned 3
m downwind) are observed.
40
30
7_2: Qg=5.8 kg/min, V=4.0 m/s
Ambient
temperature
Gas release
Temperature [°C]
20
10
0
−10
−20
−30
−6
−4
−2
0
2
4
Time [min]
6
8
10
12
Figure 6.1: Ground temperature in the centre axis of a chlorine gas cloud at
1 m from the release under free dispersion
Mean temperatures (ambient temperature, gas cloud temperature and temperature difference ∆T between ambient and cloud temperatures) of four
tests are given in table 6.1. The temperature before releasing the gas is defined as the ambient temperature (at ground level 1 m in front of the gas
source position). The differences in the various tests are due to the season
under which the tests were performed. The gas cloud temperature is highly
dependent on wind fluctuations and ground roughness; variations of 10◦ between the tests are observed. Typical temperature differences ∆T are of 20
122
6.1.2. Temperature measurements far from the source
to 30◦ C.
Test
Ambient temperature [◦ C]
Gas cloud temperature [◦ C]
∆T [◦ C]
62
63
72 73
11.4 10.0 26.2 28.2
-6.4 -17.3 -6.2 -7.2
17.8 27.3 32.4 35.4
Table 6.1: Mean temperatures 1 m from the gas source
From these measurements, one could think that the temperature difference
between the gas and the ambient could enhance heat transfer mechanisms in
the cloud dispersion. Therefore, further measurements have been performed
downwind of the water curtain.
6.1.2
Temperature measurements far from the source
The next temperature measurements were performed 10 m downwind the gas
source, or 6 m downwind the water curtain. There were five measurement
points, equally spaced, but only one of the thermocouples measured a temperature difference. Its position corresponds to the position of the maximum
measured concentration at 10 m from the source.
In figure 6.2, temperature measurements are presented for a free and a forced
dispersion cases together. At this distance from the source, the gas temperature is close to the ambient temperature and differences are in the order of
1◦ C under free dispersion. The temperature difference when the water curtain is operating (forced dispersion) is much more important (∼ 4◦ C). The
thermocouples are not directly in contact with the water curtain but probably in contact with fine droplets that are entrained by the wind downwind
of the water curtain.
Mean temperatures are presented in table 6.2 for free and forced dispersion cases. In the forced dispersion case, ∆T between the ambient and the
gas cloud is evaluated by the ambient temperature when the water curtain
is operating. In this manner, temperature difference variations between free
123
Chapter 6. Results
20
Free disp. 2_1: V=3.5 m/s; Q =3.4 kg/min
g
Forced disp. 2_2: V=2.3 m/s; Qg=2.9 kg/min
15
°
Temperature [ C]
Ambient
temperature
Gas release
10
Water curtain on
5
−8
−6
−4
−2
0
Time [min]
2
4
6
8
Figure 6.2: Ground temperature in the center axis of the gas cloud 10 m
downwind from the source for free and forced dispersion
and forced dispersion cases are measured ±0.5◦ C.
Ambient temperature [◦ C]
Forced dispersion temperature [◦ C]
Gas cloud temperature [◦ C]
∆T [◦ C]
Free dispersion
15.5
14.6
0.9
Forced dispersion
14.4
10.8
9.6
1.2
Table 6.2: Mean temperatures 10 m from the gas source
In the free dispersion case, the temperature differences between the ambient
and the gas source are negligible at 10 m from the source in these measurements (< 1◦ ). In the forced dispersion case, the temperatures are influenced
by the water temperature, however, the temperature difference due to the gas
cloud is of the same order than in the free dispersion case (∼ 1◦ ). Therefore,
cloud heating due to the water curtain is neglected in this thesis.
124
6.2. Free dispersion
6.2
Free dispersion
The behaviour of the gas cloud dispersion has been investigated downwind
the release. The results concern the lateral concentration distribution through
the cloud, the concentration reduction, and the width of the cloud.
The operating conditions as wind and gas flow rate are varying between
the tests: the wind conditions vary between 1.4 to 6.3 m/s and the gas flow
rate from 2.2 to 7.7 kg/min. No tests could be performed under similar conditions to check the repeatability of the results. However, tests with similar
wind or gas flow rate were achieved, such that the influence of one or the
other parameter has been evaluated.
6.2.1
Gaussian distribution
The concentration distribution has been investigated in the clouds lateral
direction for different distances to the gas source. A Gaussian distribution
follows as presented in figures 6.3 and 6.4.
In this case, the Gaussian distribution is given along an arc at a given distance
to the source. One can deduce from these results that the cloud distribution
is close to the one of a passive cloud, which may be due to the weakness
of the release rate. The distribution is not given for comparative motives,
but only as descriptive tool, which is fitting with the results. An error could
be evaluated; however this is not made, as the Gaussian distribution in the
literature is given for passive cloud dispersion.
The Gaussian distribution tends to deviate from the release direction x = 0
in most cases. This is due to the change in wind direction during the trials. The influence of the wind and the gas release rate has been investigated
keeping one of the parameter constant and varying the other one.
Figure 6.3 presents two tests performed with similar gas flow rates (∼ 3.5
kg/min). Higher concentrations are measured close to the source (at 7.5 m)
at high wind conditions (see figure 6.3(a)). However this difference is de125
Chapter 6. Results
creasing with the distance to the source and at 15 m the concentration is of
the same order.
Test 6_5
10000
5000
0
−15
10000
5000
0
−15
10000
5000
0
−15
Test 2_1
7.5 m
−10
−5
0
5
10
15
15
10000
5000
0
−15
15
10000
5000
0
−15
15
10000 C [ppm]
5000
0
−15
−10
10 m
−10
−5
0
5
10
15 m
−10
10000 C [ppm]
5000
0
−15
−10
−5
0
5
10
20 m
−5
0
5
Lateral position [m]
10
(a) V=3.5 m/s, Qg =3.4 kg/min
7.5 m
10000
5000
0
−15
−10
−5
0
5
10
15
10 m
−10
−5
0
5
10
15
15 m
−10
−5
0
5
10
15
20 m
−5
0
5
10
15
(b) V=2.8 m/s, Qg =3.5 kg/min
Figure 6.3: Gaussian distribution, influence of wind speed
Figure 6.4 presents two tests performed under similar wind speed (∼ 3.5
m/s). Close to the source, the two cases are rather similar exept for the
cloud width which is larger under less gas flow rate (see figure 6.4(b)). This
might be due to the release conditions: for a higher flow rate, the gaseous jet
will keep its momentum for a longer duration and therefore spread less in the
lateral direction. However, as the distance to the source increases the case of
higher flow rate keep high concentration up to 15 m from the source where
the case of lower flow rate is already dispersed to very low concentrations.
An increase of gas flow rate of 30% increases the concentration at 10 m by a
factor 3 and 15 m by a factor 5.
Assuming a Gaussian distribution, its evolution with respect to the distance
to the source is presented in figure 6.5.
126
6.2.1. Gaussian distribution
Test 2_1
Test 4_2
5000
5000
7.5 m
7.5 m
0
−15
5000
−10
0
−15
5000
−10
−5
0
5
10
15
0
−15
5000
−10
0
−15
5000
−10
−5
0
5
10
15
10 m
10 m
−5
0
5
10
15
−5
0
5
10
15
15 m
15 m
0
−15
5000
−10
C [ppm]
−5
0
5
10
15
0
−15
5000
−10
C [ppm]
−5
0
5
10
15
20 m
20 m
0
−15
−10
−5
0
5
10
0
−15
15
(a) V=3.5 m/s, Qg =3.4 kg/min
−10
−5
0
5
10
15
(b) V=3.6 m/s, Qg =2.4 kg/min
Figure 6.4: Gaussian distribution, influence of gas release rate
Test 6_5
15000
7.5 m
10 m
15 m
20 m
C [ppm]
10000
5000
0
−10
−5
0
5
10
Figure 6.5: Gaussian distribution with respect to the source
127
Chapter 6. Results
6.2.2
Concentration with the distance to the source
Figure 6.6 presents the maximum concentrations (whatever their lateral position) as a function of the distance to the source.
4
5
x 10
7_1: V=4.2 m/s, Q =3.7 kg/min
g
7_2: V=4.0 m/s, Qg=5.8 kg/min
7_3: V=5.3 m/s, Qg=5.9 kg/min
8_2: V=2.1 m/s, Qg=6.6 kg/min
9_1: V=6.2 m/s, Qg=3.8 kg/min
9_2: V=5.6 m/s, Qg=5.1 kg/min
2_1: V=3.5 m/s, Qg=3.4 kg/min
3_1: V=6.3 m/s, Qg=3.2 kg/min
4_2: V=3.6 m/s, Qg=2.4 kg/min
4_4: V=2.3 m/s, Qg=5.0 kg/min
6_2: V=3.0 m/s, Qg=3.1 kg/min
6_3: V=3.4 m/s, Qg=2.2 kg/min
6_5: V=2.8 m/s, Qg=3.5 kg/min
4.5
4
Max C [ppm]
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
Figure 6.6: Concentration reduction with the distance to the source
At 2.5 m from the release, the gas cloud is narrow. As there as only 3
measurement points at this distance, the results depend on the cloud orientations with respect to the points. However, the concentration decreases
quickly with the distance in the region between 2 and 7.5 m.
Between 7.5 and 20 m from the source, the evolution with respect to the
distance to the source varies less. The higher maximum values represent the
test cases with high flow rates and low wind speeds as test 4 4.
128
6.2.3. Cloud width as a function of the distance to the source
6.2.3
Cloud width as a function of the distance to the
source
The width of the cloud is estimated on the Gaussian distribution fitting the
experimental points as presented in section 6.2.1. It is defined as the width
of the concentrations exceeding the 10% of the maximum value (Gaussian
peak) [ref].
Results are given in table 6.3. The cloud width varies between 6.2 and
8.4 m. The gas cloud therefore increases more than one meter in width per
meter downwind. The influence of the wind conditions and the release rates
are hardly noticible in these cases.
Test
Width [m]
71 72
6.3 8.4
73 82 83
6.3 6.6 8.4
84
7.8
91 92 93
6.2 6.2 7.0
94
6.8
Table 6.3: Cloud width at 7.5 m from the source in the second campaign
6.3
Forced dispersion
When the water curtain is operating, the gas cloud may change shape, concentration distribution, and behaviour. The concentration distribution, the
concentration reduction with distance and the cloud width are investigated
for various forced dispersion cases.
6.3.1
Influence of the RM ratio
Recirculation bubble
Pictures and records of the field tests have illustrated the importance of the
water-to-wind momentum ratio RM with the visualisation of a recirculation
129
Chapter 6. Results
bubble upstream the water curtain. An example is presented in figure 6.7
for two different RM values.
(a) RM =5
(b) RM =16
Figure 6.7: The recirculation bubble visualisation for different RM values
This is due to the fact that the entrained air (in the upper part of the water
curtain) that is rejected upstream (at ground level) is increasing with RM.
In this manner, the cloud approaching the water curtain encounters an air
flow, and is redirected upward before it is influenced by the air drag in the
upper part of the water curtain.
Concentration distribution
The mean concentration distribution is evaluated in the lateral direction of
the cloud at different distances from the source for two different water-towind momentum ratios RM in figure 6.8 .
For test 6 1 presented in figure 6.8(a), the wind speed is moderate (4.0 m/s)
and therefore the resulting RM is very low. The concentration distribution
fits a Gaussian distribution for different distances to the source as for the free
dispersion cases. This is mainly due to the moderate wind speed (4 m/s).
However, for low wind conditions, a high RM value may be reached even for
130
6.3.1. Influence of the RM ratio
Test 6_1
Test 2_5
5000
7.5 m
0
−15
5000
−10
0
−15
5000
−10
0
−15
5000
0
−15
5000
−5
0
5
10
15
10 m
−5
0
5
10
15
15 m
−10
C [ppm]
−10
−5
0
5
10
15
20 m
−5
0
5
(a) RM =0.2
10
15
7.5 m
0
−15
5000
−10
0
−15
5000
−10
0
−15
5000
0
−15
−5
0
5
10
15
10 m
−5
0
5
10
15
15 m
−10
C [ppm]
−10
−5
0
5
10
15
20 m
−5
0
5
10
15
(b) RM =5.6
Figure 6.8: Concentration distribution, influence of RM
low water flow rate in the water curtain. For example, in test 2 5 (RM=5.6),
the concentrations do not follow a Gaussian distribution any longer.
The correlation between the ground level concentrations measurements at
different distances from the source is examined and sketched on figure 6.9.
Figure 6.9(b) consists of a forced dispersion test case, where the wind was
2.4 m/s at 10 m, the water curtain placed at 2.5 m from the source has an
operating pressure of 500 kPa, and the resulting RM was 1.5.
The shape of instantaneous concentration measurement may be described
by peaks of different height, width and intervals. However, similar shapes for
neighbouring captors in lateral and downwind direction are measured. This
observation leads to correlation investigations of the instantaneous results
whose results are the following:
• Downwind direction: For measurements spaced by 5 m (from 15 to 20
or 20 to 25 m) the correlation between the measurements are above
85% and up to 94%. These present the higher correlation values.
In 10 m distance (from 15 to 25 m) the correlations have decreased but
are still very satisfactory from 76 to 89%.
131
Chapter 6. Results
30
700
700
x=25 m
x=25 m
0
0
2
700
x=20 m
20
4
6
4
6
2
4
time [min]
6
0
0
2
700
x=20 m
4
6
4
6
4
6
15
Selected points
0
0
2
700
x=15 m
10
5
Water curtain
0
−15
−10
Source
−5
0
5
0
0
2
700
x=15 m
C [ppm]
25
10
(a) Chlorine captor position
15
0
0
0
0
2
(b) Resulting instantaneous measurements
Figure 6.9: Instantaneous chlorine captors position and measurements
• Lateral direction: In the lateral direction, the correlation increases with
the distance to the source from 74% at 15 m to 86% at 25 m.
• Diagonally: Correlation between measurements in diagonal positions
results in the worst correlation from 65 to 76%.
Thus, a cloud that travels through the field disperses the same concentration
distribution over a large field.
During the free dispersion case, the wind turned and the results could not
be exploited. Therefore, comparisons of free and forced dispersion case could
be made.
For the concentration distribution with time, the concentrations presents
the similar cloud behaviour with respect to peak values in a large field.
132
6.3.2. Concentration decrease with distance to the source
6.3.2
Concentration decrease with distance to the source
As the water-to-wind momentum ratio RM has been found to be a very important parameter in the forced dispersion cases, the concentration reduction
with the distance to the source is presented for the low RM cases in figure
6.10(a) and for the high RM values in figure 6.10(b). The first points at 2 or
2.5 m from the source are situated between the source and the water curtain.
For low RM values, the concentration reduction with the distance to the
source is similar to the free dispersion case.
Less variation between the tests is observed at 7.5 m from the source for
high RM values. Also, the maximal values are very low at 7.5 m from the
source and does practically not decrease until after 15 m. Between 15 and
20 m, some concentration drop is again observed. However, the few numbers
of measurements in this case can not confirm this conclusion.
4
x 10
4
7_2: RM=1.8
7_3: RM=1.4
9_2: RM=1.3
9_3: RM=2.6
9_4: RM=2.4
4_1: RM=0.6
4_3: RM=1.3
4_5: RM=0.3
6_1: RM=0.3
4
C [ppm]
5
3
2
1
0
0
x 10
8_2: RM=7.0
8_3: RM=5.4
8_4: RM=16.6
1_4: RM=4.0
2_5: RM=5.6
2_6: RM=8.3
4
C [ppm]
5
3
2
1
5
10
15
(a) Low RM values
20
0
0
5
10
15
20
(b) High RM values
Figure 6.10: Concentration decrease for different RM values
133
Chapter 6. Results
6.3.3
Influence of the water curtain on the width of
the cloud
The cloud width has been evaluated by the Gaussian distribution as in the
free dispersion cases for the test with low RM. No particular changes may
be concluded from this investigation.
For cases with high RM, the cloud width could not be evaluated as the
measurement points do not cover the cloud width (no longer Gaussian distribution to extrapolate).
In contrary with what was mentioned in the literature ([61], [73]), the gas
cloud width increases drastically for high RM values. This effect is three
dimensional and may not be reproduced in the Wind Gallery.
6.4
Dilution factor
The dilution factor DF is defined as the ratio of concentrations in the free
and forced dispersion case:
FD =
6.4.1
Cf ree dispersion
.
Cf orced dispersion
(6.1)
Different definitions
There are different ways to evaluate the dilution factor and here some emphasis is given on these methods. The dilution factor has been commonly
defined by the ratio of concentration in the free dispersion case to the concentration in the forced dispersion case. Under the assumption that a gas
cloud encountering the water curtain increases its height and keeps its width,
some even evaluate the dilution factor with respect to averages of a vertical
concentration profile in the cloud ([61], [73]).
134
6.4.1. Different definitions
In the present case, it is clear from figure 6.11 that a gas cloud encountering a strong water curtain (in terms of water-to-wind momentum ratio)
increases its width compared to the free dispersion case as discussed above.
Therefore, this assumption does not yield here.
Test 7_3
4
3
x 10
Free disp.
Forced disp.
2.5
x 10
Free disp.
Forced disp.
2.5
2
C [ppm]
2
C [ppm]
Test 8_2
4
3
1.5
1.5
1
1
0.5
0.5
0
−15
−10
−5
0
5
10
15
0
−15
−10
(a) RM =1.4
−5
0
5
10
15
(b) RM =7
Figure 6.11: Evolution of dilution factor with respect to the RM
The different manners to estimate the dilution factor are presented hereunder and figure 6.12 gives the following dispersion factors with respect to
the water-to-wind momentum.
• A local one: It may be calculated locally in every measurement point.
Then, in the positions close to the edge of the free dispersion cloud
width, the local dilution factor will be less than one, because the cloud
has increased in width and higher concentrations are measured in the
forced dispersion case. From these local dilution factors, the maximum
[63] or mean [60] values may be chosen.
F Dlocal =
Cf ree dispersion
Cf orced dispersion
(6.2)
Figure 11.2(e) presents the maximum local dilution factor downwind
the water curtain with respect to RM. The results do not follow the
135
Chapter 6. Results
trend line found from previous investigations on this matter (see section
11.2).
• A global one: It may be based on the average concentrations of several
measurement points for free and forced dispersion respectively. Then
the ratio represents a global dilution factor of the water curtain.
F Dglobal =
C̄x,f ree dispersion
C̄x,f orced dispersion
(6.3)
where x is the number of measurement points considered. In this particular case, three neighbouring points (with the highest concentrations)
were chosen. Figure 11.2(f) presents this method and a reasonable
agreement with the trend line is achieved.
• One based on the dose: From instantaneous measurements, the dose
represents the sum of the concentrations over a certain time t. It may
be calculated by the integral of the instantaneous concentration measurement in free and forced dispersion. The dose dilution factor F Ddose
may be defined as
Rt
Cf ree dispersion
F Ddose = R t0
0 Cf orced dispersion
(6.4)
where the time step t has to be chosen.
Some investigations on different dilution factor definitions are made. The assumption made in previous work that the gas cloud keeps a constant width
had to be excluded in this case. However, taking a concentration average over
a fixed number of measurement points downwind the water curtain seems to
give a reasonable agreement with the previous investigated trend line of the
dilution factor as a function of the water-to-wind momentum.
Another important fact, is that the dilution factor is dependent on the positions of the concentration measurement with respect to the water curtain
[14], [33], [31]. This aspect will be discussed in part VI.
136
6.4.2. Concentration distribution
2
2
10
global
field test
trend
1
FD
Max FDlocal
10
10
0
1
10
1
10
field test
trend
10
RM
(a) Maximum local dilution factor
0
1
10
10
RM
(b) Global dilution factor
Figure 6.12: Various F D definitions used on the field test results
6.4.2
Concentration distribution
An example of instantaneous measurements in free and forced dispersion is
presented in figure 6.13(b). Three tests A1, A2 and A3 are succeeding, each
consisting of a free and a forced dispersion case. The wind was 3.7 m/s at 10
m height, and the pressure of the water curtain was 200 kPa. The resulting
RM is smaller than one (very low), and a concentration reduction might not
be obvious.
However, in A1 and A3 a concentration reduction due to the water curtain
is acheived whereas it is not he case in test A2. Indeed the concentrations
are higher in the forced dispersion case than in the free dispersion.
By studying more carefully these figures, one can propose a description of
concentrations by counting in a measurement series, the number of times
different concentrations are measured. The measured concentration range is
divided in a number of classes and presented with histograms.
Concentration histograms are compared for free (on the left) and forced (on
the right) dispersion cases in figure 6.14. In a measurement series, the mea137
30
3
25
2.5
2
20
Measurement points
C [%]
Chapter 6. Results
15
10
Water curtain
Test A2
Test A1
Test A3
1.5
1
0.5
5
0
−15
Free disp.
Forced disp.
−10
Source
−5
0
5
(a) Carbon dioxide captors
10
15
0
0
5
10
Time [min]
15
(b) Resulting measurements
Figure 6.13: Instantaneous measurements of carbon dioxide concentrations
sured concentration range is spited in 10 classes. The figures present how
many times a concentration has been measured (y-axis) in each of these
classes (x-axis).
In the free dispersion case, the distributions presents a higher number of
large concentrations that in the forced dispersion. In addition, the concentration range is larger in the free dispersion case.
However, a large number of small concentrations may be as hazardous as
a small number of large concentrations. Therefore the dose has been calculated for the different cases.
A comparison of the different dilution factors defined by peak concentrations as done by [63] and [65], by mean concentrations, and by the ratio of
the dose is given in table 6.4.
It is interesting to notice that the dose dilution factor is 20 to 25% more
elevated than the other dilution factors. The dose actually represents the
most correct manner what a person would be exposed to if it was located
at the measurement point. Therefore, the dilution factor based on the dose
FDdose seems to be the more appropriate one. However, it require instanta138
6.5. Water curtain response time
Free dispersion
Forced dispersion
150
150
Test A1
Test A1
100
100
50
50
0
0
0.5
1
1.5
0
0
2
150
0.5
1
1.5
2
150
Test A3
Test A3
#
100
#
100
50
0
0
50
0.5
1
C [%]
1.5
2
0
0
0.5
1
C [%]
1.5
2
Figure 6.14: Histogram of carbon dioxide concentration in four positions
downwind the water curtain in free and forced dispersion cases
Test
A1
A3
FDlocal
2.6
1.7
FDmax
1.3
1.9
FDdose
3.3
2.5
Table 6.4: Various dilution factors from instantaneous measurements
neous measurements which was only available for test cases with RM <1.
6.5
Water curtain response time
The dose dilution factor F Ddose de fined above is investigated in a closer
manner with respect to time. In this way, the response time of the water
curtain i.e. the time it takes for the water curtain to decrease the concentrations may be defined.
139
Chapter 6. Results
The idea consists in evaluating the dilution factor with respect to time by
the following relation
R t∗
Cf ree disp.
F Ddose = R t0∗
(6.5)
0 Cf orced disp.
for various t∗ .
In figure 6.15, the dilution factor is presented with respect to time for two
tests, and two positions. In the beginning, the dilution factor continuously
increases, until it reaches a point where it stabilizes.
1
0.9
test A1
test A3
0.8
max
0.5
FD
/FD
0.6
dose
0.7
0.4
0.3
0.2
Time
response
0.1
0
0
0.5
1
*
t [min]
1.5
2
Figure 6.15: Dilution factor with function of time
The water curtain response time is represented by the time it takes for the
dilution factor to stabilize and is evaluated in the order of one minute.
The field tests have lead to a series of conclusions of a heavy gas cloud
behaviour in front of a water curtain. The definition of the water-to-wind
momentum RM, and the evaluation of the concentration behaviour with re140
6.6. Comparisons of various field tests
spect to this value seems appropriate in the present research. Therefore, two
papers are chosen, in which enough details are given to calculate the RM
values. In this way, a comparison with the literature is given in the next
section.
6.6
Comparisons of various field tests
In the past, investigations on heavy gas cloud dispersion have been performed
im more or less large scale, with toxic gases [63], [73] or tracers [60], [62],
[19]. The efficiency of a water curtain has generally been estimated with the
local dilution factor F D.
Some, but few studies present sufficient data to be exploited in a comparative manner. Furthermore, the concentration measurements are taken at
very different distances (with respect to the source or the water curtain) in
each case what makes the comparisons difficult. In this chapter the Buxton
test series [62] and the field tests of Moore & Rees [63] are chosen for further
comparisons as they concern the mitigation of low soluble gases and therefore
the mechanical dispersion.
To facilitate the comparisons, the data are traduced into the water-to-wind
momentum ratio RM and the dilution factor F D.
6.6.1
Operating conditions
Considering the performed field tests, a great variance is observed in the operating conditions. Table 6.5 presents the water curtain design with respect
to the nozzle flownumber FN , the nozzle spacing Ns , the operating pressure
∆P and the resulting water flow rate per meter of water curtain ṁl,u and
the gas flow rate mg . The presented values concern tests performed with a
downward operating water curtain.
The Buxton tests series present a larger variation in operating conditions than
the tests from Moore & Rees. From these test series, the nozzle flownumber
141
Chapter 6. Results
√
FN [kg/s/ Pa]
Ns [m]
∆P [kPa]
ṁl,u
mg [kg/s]
Buxton [62]
4.22 · 10−3
0.33 to 2
238 to 646
1.7 to 6.2
1.4
Moore & Rees [63] Present study
1.2 · 10−2
9.0 · 10−4
1.5
0.2 to 0.4
600
200 to 700
6.3
0.33 to 3.75
0.15
0.3
Table 6.5: Operating conditions of comparative field tests
is larger than in the present study, but the nozzle spacing is much larger.
The resulting water flow rate per meter of water curtain is still larger than
in the present study.
In the Buxton tests series, the release consists of carbon dioxide. The water
curtain is 34 m long and 3 m high and placed 15 m from the source. The
nozzles are hollow cone. The meteorological conditions are measured at 1.25
and 10 m height.
In the tests by Moore & Rees, the release of liquid commercial propane
was made in a liquid phase at a rate of 0.15 kg/s. This gas density is 1.5
kg/m3 . The water curtain is 5 m long and 2.1 m high. In these tests, the
water curtain was placed exceptionally close to the release at 1 or eventually
2 m. The nozzles are full cone 30◦ angle nozzle.
The disposition of the setup of the different tests (source, water curtain and
measurement points) is sketched in figure 6.16. It is worth noting that the
figures are in scale.
The Buxton test series is similar to the carbon dioxide test in the present
study, in a larger scale. The field tests from Moore & Rees are quite different
with respect to the very small distance between the source and the water curtain and the large distance between the water curtain and the concentration
measurement point.
142
6.6.2. Results
1
0
0
1
Concentration measurement point
Source
Present study:
Hwc=2 m
x=0
x=10
Buxton test series:
1
0
0
1
x [m]
x=14
Hwc=3 m
1
0
0
1
1
0
0
1
1
0
0
1
x=9
x=15 x=18 x=21
1
0
0
1
x=24
Moore & Rees:
Hwc=2.1 m
1
0
1
0
x=1
x=15
Figure 6.16: Experimental setup for the presented study, the Buxton test
series and the field tests of Moore & Rees
6.6.2
Results
Buxton
The results are based on concentration measurements downwind the water
curtain. They are smoothed values taken from the best fit curve through the
time averaged values.
Instantaneous measurements are taken for free and forced dispersion in a
continuous way. An example of a measurement series is given in figure 6.17.
When the water curtain is operated there is a clear concentration reduction
at 0.5 m height 9 m downstream the water curtain. Between the source and
the water curtain, at 9 m downstream the source, or 6 m upstream the water
curtain, no concentration reduction is observed, however some change in the
143
Chapter 6. Results
fluctuations seems obvious when the water curtain is operating. This may be
due to the increase of turbulence at the measurement point when the water
curtain is functioning.
upstream water curtain
downstream water curtain
Concentration [%]
8
6
4
2
0
0
0.5
1
1.5
2
Time [min]
2.5
3
3.5
4
Figure 6.17: An example of instantaneous concentration measurement from
Moodie [60]
Table 6.6 presents the characteristics of wind, RM and the resulting F D
at two positions downstream the source at 18 and 21 m; it represents 3 and
6 m downstream the water curtain position. The concerned dilution factor
F D is calculated locally in a measurement point.
Test
2
3
4
5
6
7a
7b
8
9
Nozzle spacing
[m]
0.33
1
1
1
1
1
1
2
2
V10
[m/s]
3.0
6.1
6.3
3.5
1.0
2.4
2.4
3.3
4.4
RM
4.1
0.3
0.5
1.5
18.6
3.2
4.0
2.1
1.2
F D at 18 m F D
1.2
3.6
2.8
1.9
1.1
at 21 m
1.9
1.3
1.1
4.3
4.1
6.6
3.8
-
Table 6.6: Characteristics of the Buxton tests series
144
6.6.2. Results
An increase of the dilution factor with the downwind distance of the water
curtain is observed. It takes place from 3 to 6 m downwind the water curtain.
In comparison with the present case, tests with RM values smaller than
1, result in poor dilution factors (∼ 1). An increase in RM generally results
in an increase of F D. In one case, RM=18.6, but the resulting dilution factor is poor for this particular case; it is of the same order than for test cases
with RM ∼ 4.
Field tests of Moore & Rees
These tests are performed under quite different experimental setup with respect to the distances between the source and the water curtain and, between
the water curtain and the measurement point (as illustrated in figure 6.16).
However, the results are translated into RM and F D values for comparative purposes. Table 6.7 presents the experimental conditions and results.
Only the dilution factor at 15 m from the source (or 14 from the water
curtain) is specified in the article. In this case, the dilution factor is calculated with respect to maximal local values from instantaneous measurement.
Test
11
12
13
14
15
16
17
V10
[m/s]
11.5
11.5
7.7
6.6
6.6
7.7
3.8
RM
0.6
0.6
1.3
1.8
1.8
1.3
5.4
F D at 15 m
2.1
2.1
1.8
2.4
3.3
1.1
2.7
Table 6.7: Characteristics of the field tests described by Moore & Rees [63]
In this case, the dilution factor is surprisingly high for RM values smaller
145
Chapter 6. Results
than 1. In fact, it does not seem highly dependent on the RM value. This
might be due to the distance to the water curtain. The largest RM results
in the highest dilution factor. However, the dilution factors are rather low in
this example.
From the free dispersion case (section 6.2) and the forced dispersion case
(section 6.3) in the present study, the concentrations at 15 to 20 m from the
source were found to be of the same order. However, this conclusion is based
on the chlorine tests with release rates inferior to the one of Moore & Rees.
This might be an explanation for the poor dilution factors in the referred
paper.
6.7
Conclusions
Heavy gas clouds behaviour in free and forced dispersion cases have been
investigated and compared by their concentration at different location downwind the source.
In the free dispersion case, the concentration distribution in the lateral direction at different positions downwind the gas source was Gaussian. The
cloud was shown to be more dependent on the gas flow rate than the wind.
At higher flow rates, it maintained high concentrations up to 20 m downwind. The concentrations rapidly decrease with the distance and at 20 m
little variation between the tests was observed. The cloud width increased
more than one meter per meter displaced downward.
In the forced dispersion cases, the results are highly dependent on the operating water-to-wind momentum ratio RM. For low values of RM, the
dispersed cloud has similar behaviour than a free dispersion case. For high
values of RM, the concentration distribution changes drastically as the cloud
width is increased. The concentration reductions are also larger for high RM
at high distances from the source.
When free and forced dispersion cases are performed under similar operating conditions in the second campaign, the comparative results were given.
146
6.7. Conclusions
It seems appropriate to define the dilution ratio with respect to an average
concentration of several measurement points downstream the water curtain.
The results showed reasonable agreement with the dilution factor trend line
with respect to RM.
From the instantaneous concentration measurements various dilution factor definitions were compared with respect to mean, maximum and sum and
variations of 25% were observed. The more appropriate dilution factor was
discussed to be the one based on the dose definition.
The water curtain response time was evaluated with instantaneous measurement to be in the order of 1 minute.
The results are finally compared to two field tests in the literature (Buxton tests series [62] and the field tests from Moore & Rees [63])
The temperature measurements confirmed that at the level of the concentration measurements, there are no longer temperature differences between the
gas and the ambient such that the mechanical dispersion of the cloud is the
dominant action of the water curtain.
147
Chapter 6. Results
148
Part IV
Laboratory experiments
149
Introduction
The laboratory part of the research project has been undertaken in collaboration with stagiaire or diploma course students who have been working
under my close supervision [46], [3], [45].
The objectives of performing Wind Gallery experiments are first to assess
the equivalence with the field tests to see if the Wind Gallery can reproduce
the main features of the field tests at small scale; and then, to undertake a
more parametrical investigation than could be done in the field tests. From
the latter, some variables were uncontrollable, like wind speed and gas cloud
height. In addition, gas concentration profiles in the vertical direction could
not be undertaken in the field tests for practical reasons. In the laboratory,
these measurements are more easily performed.
The motivations of performing Wind Gallery tests is therefore to complement the investigation that was made in the field tests.
The methodology consists of two main steps:
• First, the control of the wind speed is mandatory for a parametrical
study. In the field tests presented in chapter 6, the wind speeds are
often high. More precisely, the water-to-wind momentum ratio RM can
be controlled in Wind Gallery tests. In the previous part concerning
field tests, the results of low and high RM mitigation tests have been
discussed. In this part, the transition zones may be defined. Vertical
concentration profiles in the gas cloud estimate the gas cloud height at
different locations from the source. The change of behaviour between
151
the free and forced dispersion is analysed in the vertical direction.
• Next, the ratio of the water curtain to gas cloud height Hwc /Hc is
investigated. Varying the water curtain height for a fixed cloud height
provides a parametrical investigation of this parameter.
In this part the experimental set-up is presented in chapter 7. The results
are presented is chapter 8.
152
Chapter 7
Description & Preparation
In this chapter the Wind Gallery and the equipment required for the heavy
gas cloud dispersion by water curtain and its measurement are described.
The details of the Wind Gallery is given in section 7.2. Next, the installation
of the gas source is given in section 7.2.3. The water curtain is described
in section 7.2.4 with some nozzle characteristics. At last the measurement
technique and experimental procedure are given in section 7.2.5 and 7.2.6
respectively.
7.1
The Water Spray Facility
The main hydrodynamic characteristics of a spray are investigated in the
VKI-Water-Spray facility. A picture is presented in figure 7.1. The set-up is
composed of a hydraulic circuit supplying a single nozzle with a maximum
flow rate of 1 l/s at 800 kPa. The pulverized water is collected in a 12 m3
pool. The droplet size distribution in the spray and the entrained gas velocities are measured using Phase Doppler Anemometry (PDA). The influence
of pressure and position in the spray are studied.
The PDA instrument is a non-intrusive optical measurement technique. A
laser beam is splitted in two identical beams and their intersection defines
153
Chapter 7. Description & Preparation
Figure 7.1: PDA system in the VKI-Water-Spray facility
the measurement volume. A particle crossing this volume scatters light in
all directions by reflexion, refraction and diffraction. An optical receptor is
used to capture the scattered light. It measures scattered light by a particle
and deduces the particle size and velocity simultaneously.
In order to measure the air entrainment properties in a spray the smallest
droplets (typically below 20 µm) are assumed to behave as passive tracers.
In this manner, the gas phase velocity is measured.
As the PDA makes local measurements, profiles of the spray characteristics are acquired through a horizontal plane. In figure 7.2, the radial velocity
profile of the droplets and the gaseous phase are presented for different distances with respect to the nozzle axis [3].
7.2
The Wind Gallery
The VKI Wind Gallery has been specifically constructed for the investigation
of gas dispersion, absorption and heat transfer by water curtain mitigation.
The similarity criterions and a description of the Wind Gallery are presented
154
7.2.1. Similarity criterion
25
H=0.5 m
H=1.4 m
20
Axial velocity [m/s]
20
Axial velocity [m/s]
25
H=0.2 m
H=0.5 m
H=1.4 m
15
10
5
15
10
5
0
0
0.1
0.2
0.3
Radial position [m]
0.4
0.5
0
0
0.1
0.2
0.3
Radial position [m]
0.4
0.5
Figure 7.2: PDA measurements of droplet and air velocity in a single spray
[3]
hereunder.
7.2.1
Similarity criterion
Similarity criterions for laboratory tests have been established in previous
investigations [72].
The geometrical scaling is based on a scaling factor of 1/10. A scaling of
the Reynolds number is impossible and Re is about 10 times smaller in the
simulation. However, the flow around the water droplets remains turbulent.
Heat and mass transfer coefficients are higher by a factor two compared to
reality in these conditions, but it is compensated by a gas entrainment per
unit of injected liquid which is increased by a factor two. In case of pollutant
absorption, the concentration variations in the liquid phase and, in case of
cloud heating, the increase in temperature is therefore globally of the same
order that in the real conditions.
The structure of the gas flow around the water curtain and the droplet trajectories are reproduced in scale for winds reduced by a factor five and an
155
operating pressure in the water curtain reduced by a factor five. These scales
correspond exactly to the water-to-wind momentum ratio RM. That fact has
been confirmed by Wind Gallery simulations and field test situations [72].
7.2.2
Description of the Wind Gallery
A general view and a schematic of the Wind Gallery are presented in figure
7.3.
(a) Photography
(b) Schematic
Figure 7.3: VKI Wind Gallery
The test section is a rectangular channel of 1 m high, 1.3 m wide and 7
m long. The airflow is produced by a battery of four ejectors mounted at
the back end, thus producing a low pressure that keeps gas leaks towards
the inside of the test section. The gallery has demonstrated very uniform
velocity profiles. The wind speed in the Wind Gallery may vary from 0.2
to 1.5 m/s, which is equivalent to speeds ranging from 4 to ∼ 25 km/h at
full scale. Wind speed profiles are measured in the Wind Gallery with a hot
sphere and the velocity in the centre of the test section is defined as the free
stream velocity of the wind boundary layer.
The Wind Gallery does not need to simulate the turbulent atmospherical
156
7.2.3. Gas source
boundary layer, because the local perturbation induced by the water curtain
is obviously more important than the atmospheric turbulence.
The Wind Gallery was constructed in compliance with material resistance
requirements needed to use certain chemicals: For instance, typical pollutant
clouds have been simulated by releases of sulphur hexafluoride, low temperature nitrogen and carbon dioxide.
Upward and downward pointing curtains can be tested. The water is recuperated through a porous ground under the water curtain such that analysis
of chemical absorption can be made.
7.2.3
Gas source
The gas chosen for the investigation of the forced dispersion is carbon dioxide. A grid in the bottom of the tunnel that covers the width of the tunnel
assures a constant and uniform release flow in the lateral direction of the
tunnel as visualised in figure 7.4.
Figure 7.4: Photography of source injection (smoke for visualisation)
At the source, the gas is pure and the concentration at this point is 100%.
A gas flow meter measures constantly the mass-flow of the gas during an
157
experiment and typical release rate is of the order of 20 kg/h (or 0.006 kg/s).
The pollutant injection velocity in the tunnel was shown to be extremely
low such that no mixing was taking place near the source. Therefore, a
barrier has been placed in front of the source to perturb the flow and consequently increases the mixing. In this way the height of the gas cloud could
be adjusted without large variations of the ground concentration.
7.2.4
Water curtain
The water curtain in the Wind-Gallery is equipped with the same type of
nozzles as in the field tests at scale 1/4 (see figure 7.5(a)). The curtain
spreads over all the width of the test section. In downward operating mode
the nozzle ramp may be located at 0.3, 0.4 and 0.5 m height (see figure
7.5(b)) and the maximum number of nozzles which can be accommodated is
26 per meter. As already mentioned, the floor of the test section is porous
so that water is easily salvaged by a recirculation hydraulic system (closed
loop).
(a) Water curtain
(b) Fixation
heights
at
Figure 7.5: Water curtain in the Wind Gallery
158
different
7.2.5. Measurement points & technique
The nozzle is manufactured by Lechler under the reference number 422.406
−5
and has
√ an orifice diameter of 1.45 mm and a flownumber of 3.72·10
kg/s/ Pa. The Sauter mean diameter is 150 µm at 10 kPa. The spray
characteristics have been measured in details and further described [3].
7.2.5
Measurement points & technique
The concentrations measurements are performed by means of a special hot
wire technique developed at the von Karman Institute [24], [52].
In the Wind Gallery, vertical concentration profiles are easily obtained using
a comb of tubes sucking samples at different heights. This comb is presented
in figure 7.6.
Figure 7.6: Measurement point location in the Wind Gallery
It is usually placed 2 m downstream the water curtain in order not to absorb
water droplets. This corresponds to a larger distance than in the field tests,
taken with respect to the water curtain height:
Field test, chlorine :
Xc
= 1.75
Hwc
(7.1)
159
Field test, carbone dioxide :
Wind Gallery experiment, carbon dioxide :
Xc
=2
Hwc
Xc
=4
Hwc
(7.2)
(7.3)
(7.4)
where Xc is the distance between the water curtain and the concentration
measurement.
The gas sampling from the ten tubes on the comb demonstrated in a schematic
figure 7.7. All the ten samples are continuously aspired by vacuum pumps.
Each tubes are equipped with an electro-valve in a T-junction, such that the
flow can be directed to reservoir 1, or 2. In this manner, only one sample
is aquired in reservoir 2 (and a measurement takes place) while the rest are
aspired in reservoir 1 in order to have a continuous flow in all the tubes. This
reduces the time between the different samples.
Gas sampling
z [mm]
610
1
0
1
410 0
330
265
1
0
1
0
1
210 0
1
165 0
1
125 0
1
85 0
1
45 0
1
10 0
0
1
1
0
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
0
1
0
1
00
11
0
1
00
11
0
1
0
1
00
11
0
1
00
11
0
1
00
11
0
1
00
11
0
1
00
11
0
1
00
11
0
1
00
11
0
1
Electro-valve x 10
11
00
00
11
00
11
00
11
11
00
Pump
Pump
Reservoir1
0
1
1
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Hot wire
Reservoir2
Figure 7.7: Schematic of the measurement acquisition
A constant temperature hot wire anemometer is placed in a sonic hole in
order to be insensitive to the flow-velocity (see figure 7.8). In this way, the
signal of the hot wire depends only on the physical properties of the gas (density, heat conductivity, viscosity, . . . ). The underlying theory of this method
was presented by Houben [45].
160
7.2.5. Measurement points & technique
Figure 7.8: Schematic representation of the hot wire probe
Calibration of the hot wire as a function of the gas concentration in the
air is needed. For this purpose, two rotameters measure the amount of air
and gas supplied to a reservoir. At this point, hot wire measurements are
conducted for various flow rates of air and gas. Figure 7.9 presents a typical
calibration curve for carbon dioxide and forane. The carbon dioxide presents
a higher sensitivity than the forane gas and this is the reason why this gas
was chosen for the further investigations. The hot wire sensitivity for carbon
dioxide is 0.01% CO2 /mV.
0
carbon dioxyde
forane
−0.01
∆ V/V
0
−0.02
−0.03
−0.04
−0.05
−0.06
0
20
40
60
80
100
Figure 7.9: Experimental calibration curves for air-forane and air-carbon
dioxide mixtures
161
7.2.6
Experimental procedure
The experimental procedure is similar to the one of the field tests. The free
and forced dispersion cases are directly followed in order to ensure similar
operating conditions.
7.3
Conclusions
First, the Water Spray Facility is described for the quantification of the spray
characteristics. The droplet and gas behaviour in nozzles used in the field
tests and in the Wind Gallery experiments are described in the next chapter.
In the Wind Gallery experiments, emphasis is given on the parameters that
were not (or with difficulties) controlled in the field tests; typically water
curtain to gas cloud height ratio, low wind conditions (high RM values) and
vertical concentration measurements.
For this purpose, the water curtain height is easily changed between tests,
the gas source is optimised in order to reach a certain cloud height in the
test section, and vertical concentration ramps are used.
However, one has to keep in mind that the conditions in the wind tunnel
are two-dimensional, such that some three-dimensional effects observed in
the field tests could not be reproduced in the Wind Gallery. For instance,
the cloud width is fixed and constant in the free and forced dispersion case.
162
Chapter 8
Laboratory results
In the first section, some results from the Water Spray Facility concerning spray characteristics are given. Next, Wind Gallery experiments are
considered. Concentration profiles are presented in section 8.3 for various
water-to-wind momentum ratios RM. The influence of this parameter is explained and visualized. Next the influence of the curtain-to-cloud height ratio
Hwc /Hc is investigated in section 8.5 varying the water curtain height for a
fixed cloud height. At last, some instantaneous measurements are presented
and compared to the hot wire results in section 8.6.
8.1
Spray characteristics
The VKI Water Spray Facility where spray characteristics were measured
with Phase Doppler Anemometer (PDA) has been introduced in the literature survey in section 7.1. Now, some results obtained with sprays used in
this investigation are presented.
The different nozzles used in the field and Wind Gallery tests have been
described in section 5.3.1 and 7.2.4. Here, some hydrodynamic characteristics for these nozzles are given.
163
Chapter 8. Laboratory results
Figure 8.1 presents the Sauter mean diameter, d32 , for three different nozzles.
The droplet size distribution at 0.5 m from the orifice is highly dependent
on the orifice diameter, D0 ; the larger the orifice diameter, the bigger the d32 .
500
D0=5.1 mm
D0=3.6 mm
D =1.45 mm
450
0
400
300
250
d
32
[µ m]
350
200
150
100
50
0
−0.5
0
Radial position [m]
0.5
Figure 8.1: Sauter diameters in the radial direction for various nozzles,
∆P =10 kPa
The Sauter diameter increases with the orifice diameter D0 according to
the relation [3]
αl D02
ρl
¯
d = Cm [ 2
]1/3 · [ ]1/6 ,
(8.1)
θ0
ρg
CN sin( 2 )∆P
where αl is the liquid surface tension, θ0 the initial angle of the spray and
CN the discharge coefficient defined by:
q
FN = (CN πD02 /4) 2ρl .
(8.2)
The increase of the Sauter diameter in the centre of the spray indicates that
the small droplets have a tendency to migrate towards the centre of the spray
due to the air entrainment.
164
8.1. Spray characteristics
Table 8.1 presents the mean d32 for the different nozzles. These data are
important inputs of the CASIMIRE code.
D0
[mm]
5.1
3.6
1.45
d¯32
[µm]
350
280
150
Table 8.1: Mean Sauter diameter at different nozzle scales
The axial velocity of the droplets (figure 8.2(a)) and the entrained air (figure
8.2(b)) are presented for the different nozzles under the same operating and
measuring conditions than previously. The radial position corresponds to the
distance from the nozzle axis. A velocity distribution similar to that of single
phase free jet is found. The good symmetry of the velocity profiles observed
in figure 8.2(a) allows considering only one side of the spray.
25
Gas velocity [m/s]
20
Droplet velocity [m/s]
20
D0=5.1 mm
D0=3.6 mm
D =1.45 mm
0
15
10
15
10
5
5
0
−0.4
D0=5.1 mm
D0=3.6 mm
D =1.45 mm
0
−0.2
0
0.2
Radial position [m]
0.4
0
0
0.1
0.2
0.3
Radial position [m]
0.4
Figure 8.2: Droplet and gas phase velocities at 0.5 m from the nozzle (∆P =10
kPa)
165
The droplet velocities increase with the orifice diameter of the nozzle. This
is due to the droplet diameter being larger and keeping their inertia for a
longer duration.
The velocity of the gaseous phase is measured by the tiny droplets smaller
than 20 µm. It is generally recognized that these droplets behave like passive
tracers and are representative of the flow for the gaseous phase.
These data have been used to validate the predictions of the CASIMIRE
code. For instance in terms of the entrained gas flow rate after integrating
such velocity profiles (see figure 8.2(b)).
8.2
Wind Gallery visualisations
Visualisations in the Wind Gallery have been performed, and two examples
are presented in figure 8.3 for low (figure 8.3(a)) and high RM values (figure
11.2(d)).
(a) RM =2
(b) RM =7
Figure 8.3: Visualisation of various RM experiments
The gas is moving towards the water curtain which is placed on the right in
166
8.3. Concentration profiles
the picture. For low RM values, the cloud is not changing height encountering the water curtain. However, for high RM values, the cloud hits now a
barrier, which creates a recirculation zone due to the motion of the entrained
air and gas. Then, this recirculation zone enhances the mixing of the heavy
gas cloud. Such behaviour is in full agreement with field test observations.
8.3
8.3.1
Concentration profiles
Free dispersion
Figure 8.4 presents vertical concentration profiles in a free dispersion case,
measured respectively at 1.6 and 4 m downwind the source. The gas flow
rate is 0.006 kg/s and the wind speed is 0.4 m/s.
0.5
x=1.6 m
x=4 m
0.45
0.4
0.35
z [m]
0.3
0.25
0.2
0.15
0.1
0.05
0
0
10
20
30
40
50
Figure 8.4: Vertical concentration profiles in free dispersion
167
The heavy gas cloud is characterised with a large concentration variation and
a maximum value at ground level. As the distance to the source increase, the
gound concentration decrease, and the cloud height increase. These typical
concentration profiles state that the cloud height is between 0.2 and 0.3 m
in the Wind Gallery test section.
8.3.2
Forced dispersion
In the field tests, low and high RM values were differentiated through the
analysis of the lateral concentration measurements between low (< 2) and
high (> 4) values (see section 6.3 and 6.4).
Now, the influence of the RM is investigated in the Wind Gallery through
the analysis of the vertical concentration profiles. The test matrice is presented in table 8.2.
V
∆P
[m/s] [kP a]
0.6
510
0.6
360
0.6
250
0.6
150
0.6
100
# nozzles
7
7
7
7
7
ṁl,u
U0
[kg/s/m] [m/s]
0.143
16.1
0.120
13.5
0.100
11.3
0.0775
8.75
0.063
7.1
RM
10
7.1
4.9
3.0
2.0
Table 8.2: Heights of water curtain and gas cloud
Figure 8.5 presents typical vertical concentration distributions for free dispersion and various water-to-wind momentum ratios RM. The ordinate is
normalised by the cloud height. The measurements are taken at constant
wind speed (0.6 m/s) and constant gas flow rate (0.006 kg/s). In free dispersion, the gas cloud height Hc is 0.22 m at the location of the water curtain.
The water curtain height is set at Hwc = 0.5 m. The RM factor is varied by
changing only the nozzle pressure.
The highest concentrations are measured during free dispersion tests. The
168
8.3.2. Forced dispersion
3
Free disp.
RM=2
RM=5
RM=7
RM=10
2.5
z/H
c
2
1.5
1
0.5
0
0
5
10
15
20
Figure 8.5: Vertical concentration profiles for various RM values 2 m downwind the water curtain
maximum are located close to ground level which is typical behaviour of a
heavy gas cloud. At 7 m from the source, a concentration of about 20% is
measured.
Now, operating the water curtain, the forced dispersion profiles exhibit a
reduction of the maximum concentration close to ground level and an augmentation of the concentration at height larger than the initial Hc . This is
due to the mixing performed by the water curtain; the volume of the cloud
increases in the vertical direction but notice that it is bounded in the lateral
direction. In the field tests, the mixing was measured in the lateral direction
and observed in the vertical direction by visualisations.
Little difference is observed between RM=7 and RM=10. In fact, it is
worth noting that the concentration reduction of 1% is equivalent to the reproducibility of the measurement.
169
With these vertical concentration profiles, the dilution factor F D will be
defined by the ratio of ground concentrations for two reasons:
• The most hazardous concentrations are the one located at ground level
(populations, environment . . . )
• The effect of the water curtain is to uniform the vertical concentration
profile, such that for a high performance there is little difference in the
vertical direction.
8.4
Dilution factor
Figure 8.6 presents the dilution factor F D with respect to the momentum
ratio RM.
Now, a more parametrical investigation of the RM influence on the water
curtain efficiency may be given. Three ranges of RM values are defined:
• Low RM values, RM ≤ 3:
A low RM represents high wind conditions and/or low water flow rate
in the water curtain such that the water curtain has a weak momentum
compared to that of the wind. In this case, the heavy gas cloud is not
greatly affected by the water curtain. It may pass through it without
large dilution. The concentration profile downwind the water curtain
(RM=2 in figure 8.5) still presents a large gradient as the free dispersion case (with highest concentrations at ground level). This water
curtain has an unstable behaviour.
• Intermediate RM values, 3 < RM < 5
This range is a transition zone. The water curtain induces mixing
by air entrainment; however, the operating conditions are not at their
optimum. The dilution factor is increasing, but not drastically.
• High RM values, RM ≥ 5
High RM values usually represent low wind conditions and/or high
170
8.5. Influence of height ratio Hwc /Hc
2
FD
10
1
10
Transition
zone
0
10
1
3
RM
5
10
Figure 8.6: Dilution factor FD as a function of the water-to-wind momentum
ratio
water flow rates in the water curtain. In these conditions, the concentration profile is flat and consists of low concentrations. The higher the
RM, the lower the concentration.
8.5
Influence of height ratio Hwc/Hc
A height difference between the gas cloud and the water curtain is necessary
for an optimal dispersion function (see figure 8.7). For a downward water
curtain, the air entrainment in the upper part of the water curtain increases
the dilution in the cloud by mixing. Therefore, the curtain-to-cloud height
ratio Hwc /Hc is investigated in a parametrical manner.
The gas cloud height is evaluated at the position of the water curtain by
171
Hwc
Hwc
Hc
Hc
Figure 8.7: Sketch of cloud height variations
free dispersion concentration measurements. The height is defined as the
distance where the concentration exceeds 1% of the ground concentration.
This value is chosen in order to differentiate between free and forced dispersion cases. The cloud height was kept constant at 0.22 m. The water curtain
height Hwc was the changing variable in the height ratio.
Figure 8.8 presents concentration profiles for the different height ratios and
for different RM values. Low RM cases are given in figure 8.8(a) and high
RM values in figure 8.8(b).
3
2.5
z/Hc
1.5
1
1
0.5
0.5
5
10
15
(a) RM =2
wc
2
1.5
0
0
Free disp.
Hwc=30cm
Hwc=40cm
H =50cm
2.5
w
2
z/Hc
3
Free disp.
Hw=30cm
Hw=40cm
H =50cm
20
0
0
5
10
15
20
(b) RM =7
Figure 8.8: Concentration profiles for different water curtain to gas cloud
height ratio
For small RM values, the concentration at ground level remains high. How172
8.5. Influence of height ratio Hwc /Hc
ever, it decreases as the height of the water curtain increases. This is due to
the fact that the water curtain is then entraining more fresh air than pollutant and that enhances mixture compared to the case where Hwc ∼ Hc .
For large RM values, less variation is observed for the different height ratios
tested because the concentration reduction is now high. Nevertheless, a high
water curtain remains a bit more efficient.
The effect of the curtain-to-cloud height ratio Hwc /Hc , on the dispersion
factor is presented in figure 8.9.
14
Hwc/Hc=2.5
Hwc/Hc=1.4
12
FD
10
8
6
4
2
1
2
3
4
5
6
7
8
9
10
RM
Figure 8.9: Dilution factor with respect to RM
It is worth noting that the effect of the height ratio on the dispersion factor
becomes more significant as RM increases. As a practical rule, a watercurtain more than twice the height of the gas cloud is recommended.
173
8.6
Instantaneous measurements
In order to compose measurement techniques used in field and laboratory
tests a campaign of instantaneous measurements has been carried out. The
instantaneous infrared captor used for the field tests 5.4.1 was also used in
the wind gallery for this purpose.
Figure 8.10 presents instantaneous measurements at 2 and 3 m from the
gas source close to ground level (1 cm elevation). Time fluctuations of pollutant concentration in the Wind Gallery cloud are by far less important than
those measured during field tests: the RMS value does not exceed 0.5%.
Steady conditions are therefore measured in the Wind Gallery for a period
of 5 min. A concentration reduction of 3% when passing from 2 m to 3 m to
the source is observed.
18
2 m from source
3 m from source
16
Volume concentration [%]
14
12
10
8
6
4
2
0
0
1
2
3
4
5
Time [min]
6
7
8
9
Figure 8.10: Instantaneous pollutant concentrations with IR-captor
Comparison between the hot wire probe and the instantaneous measure174
8.7. Conclusions
ment in the Wind Gallery is plotted in figure 8.11 in terms of the vertical
concentration profiles for various RM values.
1.6
1.6
Inst. free
Mean free
Inst. Forced
Mean forced
1.4
1.2
1.2
1
z/Hc
z/Hc
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
Inst. free
Mean free
Inst. Forced
Mean forced
1.4
2
4
6
8
(a) RM =2
10
0
0
2
4
6
8
10
(b) RM =7
Figure 8.11: Comparsion between hot wire probe and IR-captor for different
RM values
The largest differences are about 10% and are often located at ground level.
8.7
Conclusions
The concentration profiles demonstrate that the highest concentrations are
situated at ground level.
At 7 m from the source, the cloud height increases by a factor two for low
RM values and by a factor three for high RM values.
The influence of the water-to-wind momentum ratio RM is investigated parametrically. A transition zone between low and high RM values is defined.
Visualisations support the concentration measurement showing the degree of
recirculation zone created upwind the water curtain. These observations are
in good agreement with the field tests.
175
The curtain-to-cloud height ratio is also investigated parametrically as this
parameter is difficult to control under field tests. The influence is dependent
of the RM value, and coordinating large RM with large height ratio provides
optimal mixing. A general recommendation is to have a water curtain twice
the gas cloud height.
Some instantaneous concentration measurement were taken and compared
to the hot wire concentration measurements. Form these tests, the gas cloud
is shown to be continuous. The concentration measurements are not as in the
field test with high variations. Comparisons of vertical concentration profiles
demonstrate in some cases differences up to 10%.
176
Part V
Numerical Simulations
177
Introduction
Two experimental approaches on the heavy gas dispersion by water curtains
have been presented in part III and IV. They have demonstrated good agreement in the heavy gas cloud behaviour observed during field and wind tunnel
test. Now, numerical investigations are conducted in order to demonstrate
its applicability to the problem.
With the numerical simulations, two main objectives are targeted. The first
consists in reproducing the hydrodynamical behaviour in a spray, more precisely the induced air entrainment in a single spray in a volume of gas at rest.
The next deals with the modelling of the mechanical dispersion of a heavy
gas cloud induced by a water curtain and influenced by wind effect.
The motivation is to define a simple manner to model the air entrainment
or the mechanical dispersion of a water curtain with numerical simulations.
In the part II, various models have been discussed. Here, the methodology
adopted includes and compares the Eulerian-Lagrangian approach and an
original method of simulating curtain called here the “Euler-Source” technique.
For a single spray, various characteristics are investigated, however, the induced air entrainment is the most important parameter. Next, for the mechanical dispersion, two special cases with different RM values are presented
and compared to a free dispersion case, as this parameter was recognized to
be the controlling key for forced dispersion.
179
180
Chapter 9
Model description
9.1
Governing equations
r
The behaviour of water curtain has previously been modelled with FLUENT
using a Eulerian-Lagrangian approach [48], [6], [8]. The discrete phase
(droplets) is then defined by a set of injections, in which the initial position, the material, the diameter distribution of size and velocity and the flow
rate are detailed. The gaseous phase is modelled by a standard Euler model.
9.1.1
The discrete phase
The droplet phase is described by a Lagrangian approach where single droplet
injections model the particulate flow at the nozzle exit. The droplet velocity
is calculated by solving the momentum equation, taking into account the drag
and gravity forces. No droplet-to-droplet interactions like collision or particle
break-up are considered. Injection of the droplets at the floor or at a given altitude, respectively, may reproduce upward or downward spray curtain. The
Rosin-Rammler droplet size distribution models the poly-dispersed nature of
the spray, which is also described by a sufficiently large number of droplet
trajectories initialised at the exit of the nozzles. The two-way coupling be181
Chapter 9. Model description
tween the gas and droplet flow is taken into account by the implementation
of momentum and energy source terms in the equations as further described
in section 9.1.3.
The definition of the injection velocities and mass flow rates can be obtained
in several ways [48]. The simplest way is to assume that the mass flow rate is
constant for each injection. Assuming a constant mass flow rate means that
is each injection has to have the same injection surface, which is the criteria
for determining the spray injection angles βi , which can be calculated from
the following equation:
tan βi =
q
i
I
+
1+
q
q
i−1
I
I−1
I
tan βI
(9.1)
where i = 1, 2, . . . , I − 1 represents the injection number and βI equals the
half of the opening angle of the spray.
The velocities u and v can now be calculated from the nozzle injection velocity u0 as follows:
u = |u0 | · sin βi
v = |u0 | · cos βi
(9.2)
(9.3)
The trajectory of the discrete phase (water droplets in this case) is computed by the momentum conservation law;
dup
g(ρ − ρp )
= Fd (u − up ) +
,
dt
ρp
(9.4)
where the index p stands for particle, and no index concerns the fluid phase.
The equation represents the balance of particle inertia with the forces acting
on the particle. The term Fd (u − up ),which is the drag force per unit particle
mass is written as
18µ CD Red
Fd =
,
(9.5)
ρp d2p 24
where dp is the particle diameter. The particle Reynolds number is defined
as
ρdp | u − up |
Red =
(9.6)
µ
182
9.1.2. The gaseous phase
The drag coefficient CD is correlated as folowing:
a3
a2
CD = a1 +
+
(9.7)
Re Re2
where a1 , a2 and a3 are constants that apply for smooth spherical particles
over several ranges of Re [64].
The set of parameters consists of the x and y position of twenty injected
droplet streams of different droplet diameters belonging to their injection,
the components of the initial droplet velocity, the distribution of the droplet
diameter, the mass flow rate of the injection, the type and material of the
particle.
9.1.2
The gaseous phase
The gaseous phase is described by the conservation equations of mass and
momentum. In FLUENT, the continuity equation is given as
∂ui
=0
∂xi
(9.8)
∂
∂p
∂
∂ui ∂uj
∂τij
(ρui uj ) = −
+
(µ(
+
)) +
+ Sd,i
∂xj
∂xi ∂xj
∂xj
∂xi
∂xj
(9.9)
and the momentum equation as
where Sd,i is a source term for the presence of the droplets in the flow. The
stress tensor τij is given by the Boussinesq formula
τij = µt (
∂ui ∂uj
2
+
) − ρkδij
∂xj
∂xi
3
(9.10)
where µt is the turbulent viscosity, k the kinetic energy and δij is the Kroneckers operator.
9.1.3
The coupling of the phases
As the trajectory of a particle is computed, FLUENT keeps track of the
heat, mass, and momentum gained or lost by the particle stream that follows
183
that trajectory and these quantities can be incorporated in the subsequent
continuous phase calculations. Thus, while the continuous phase always impacts the discrete phase, the effect of the discrete phase trajectories on the
continuum can also be incorporated. This two-way coupling is accomplished
by alternately solving the discrete and continuous phase equations until the
solutions in both phases reached prescribed convergence. The schema in figure 9.1 presents these steps.
Solve the continuous phase
Introduce discrete phase
Calculate the source term
Recalculate the continuous phase
Recalculate the discrete phase
Converged ?
No
Yes
Stop
Figure 9.1: Calculation steps in FLUENT
The coupling between the discrete and the continuous phase takes places
in the term Sd,i in equation 9.9. The momentum transfer from the continuous phase to the discrete phase is evaluated by examining the change
in momentum of a particle as it passes through each control volume. This
momentum change is computed as:
S=
184
X
(Fd (u − up ))ṁp ∆t
(9.11)
9.1.4. The new “Euler-Source” method
where Fd is given by equation 9.5, and ṁp is the particle flow rate. Heat and
mass exchange may also be computed in a simila manner [8].
The flowchart shown in figure 9.1 represents the steps of the coupled calculation. First, the continuous phase is solved through the equations 9.8
and 9.9. Then the discrete phase is introduced. The droplets positions and
velocities are calculated by equation 9.4 for all the streams and injections.
After, the source term 9.11 is computed and introduced in the continuous
phase which is recalculated. The discrete phase is recalculated with respect
to the gas changes. These two steps are repeated until convergence of the
solution in both phases.
9.1.4
The new “Euler-Source” method
As the coupling between the gas and the discrete phase takes only place
through the momentum source term Sd,i , the idea that the user could model
it diretly somehow is attractive.
The global source term SG represents the total momentum of the discrete
phase in the computational domain. In FLUENT it can be defined in the
horisontal and vertical direction with the flow rate ṁl in [kg/s], the velocity
U0 and the curtain volume V as
SG =
ṁl · U0
,
V
(9.12)
where ṁ is the total flow rate. SG is then given in [N/m3 ].
Now, to represent a water curtain in two dimensions, the flow rate per meter
of water curtain ṁl,u is used, with the initial droplet velocity at the nozzle
U0 and the lateral area of the water curtain A by the relation
SG =
ṁl,u · U0
A
(9.13)
In this manner, the source term represents the water curtain momentum
≡[N/m3 ] in two dimensions over the area A. It is worth noting that the
numerator of equation 9.13, namely ṁl,u · U0 , appears also in the definition
185
of RM.
This method facilitates the modelling of the water curtain effect on the environment, reduces the computation time and enhances convergence. However,
it can only be used for the mechanical effect of the water curtain (mass and
heat transfer are disregarded).
9.2
Domain
In numerical simulation, the computational domain has to be carefully defined. In this particular project, field test and wind gallery visualisations
have demonstrated an induced recirculation bubble by the water curtain for
low RM conditions. These effects should take place inside the computational
domain of the numerical simulations. It enhances convergence and therefore
diminishes the computational time.
9.2.1
Single spray at rest
For the simulation of a single spray in a continuum initially at rest, a domain
is designed as in figure 9.2.
Axis
symmetry
Pressure
inlet
Injections
Wall
Figure 9.2: The computational domain for a single spray simulation
186
9.2.2. Two dimensional approach with wind effect
As it consists of an axis-symmetrical case, only half spray angle is given.
The arc boundary has shown to enhance convergence in comparison with a
standard square shaped domain.
9.2.2
Two dimensional approach with wind effect
For field test simulation, a domain of 30 × 10 m is necessary. Figure 9.3
presents the area with the position of the water curtain and the boundary
conditions.
The boundary conditions are defined for the four extremities of the area.
The first is the velocity inlet, where a wind profile is defined (see section
9.5). The ground is defined as a wall (zero velocity). The outflow is a pressure outlet. The top should be defined far enough from the flow, that it may
be defined as symmetric.
Symmetry
Pressure
outlet
Velocity
inlet
Wall
Water
curtain
Figure 9.3: The computational domain with wind effect
For the grid, a boundary layer of 10 cm with quadric cells is chosen. The rest
of the domain is splitted in triangular cells because they improve the convergence especially for the water droplet injections. The domain contains two
areas, one for the water curtain and the other for the gas. In this manner,
the grid is refined in the water curtain and surrounding zone. This is due to
the complexity of the flow at that location. The number of cells is typically
200 000. A part of the grid is presented in figure 9.4 to demonstrate the fine
grid in the water curtain region.
187
3
z [m]
2
1
0
12
14
16
18
x [m]
Figure 9.4: The computational domain with wind effect
9.3
Gas source
The gas source is most conveniently defined as the boundary conditions of
the inflow. Several injection types have been tested from the ground [48], [6],
but, due to the zero velocity in this area (ground boundary condition), the
source is creeping upwind. Therefore, the source is inserted with the wind
at the velocity inlet.
It is defined by a constant pollutant concentration c0 in the vertical section
as
c(y) =
(
c0 for 0 ≤ y ≤ h0
0 otherwise
where the mass fraction c0 and its initial height h0 are chosen empirically,
typically 50 % for h0 =0.5 m.
The concentration level is chosen in order to investigate the effect in the
vicinity of the water curtain. Since the distance between the gas source and
the water curtain is large (15 m), only the flow pattern will be compared to
the field and Wind Gallery tests.
188
9.4. Water curtain
9.4
Water curtain
In the Eulerian-Lagrangian approach, the water curtain is defined in a discrete manner. The water curtain consists of a number of droplet injections in
the nozzle position. Each injection point is the origin of a trajectory which
is characterized by the angle taken with respect to the spray, the droplet size
and velocity distribution and their flow rate.
Typically, the simulations are based on 20 injections, each of them carrying
a Rosin-Rammler distribution defined by 20 droplet class diameters. This
approach requires a size of grid cells with respect to the droplet diameter [48].
In the Euler-Source approach, the water curtain is defined by the equation
9.13.
9.5
Wind profile
In the field-tests, the wind velocity is measured at 2 and 10 m height. These
values have been used to fit a velocity boundary layer in the simulations of
the 1/7 power law form
u(y) =
(
q
u∞ 7 y/y∞ for y ≥ y∞
u∞
otherwise
where u∞ is the wind measured at 10 m height in the field tests.
The chosen inputs for the turbulence modelling in FLUENT are the turbulence intensity I and the turbulence length scale, l. The turbulence intensity
is defined as the ratio of the root-mean-square of the velocity fluctuations,
u′ , to the mean flow velocity, u∞
I=
q
(u′)2
u∞
(9.14)
189
For wall-bounded flows the boundary-layer thickness d99 is used to compute
the turbulence length scale, l, from
l = 0.4 · d99 .
(9.15)
Typical turbulence intensity is chosen at 10%, with a length scale of 1 m.
The k-ε turbulent model was used in FLUENT.
9.6
Operating conditions
The variables are chosen in order to simulate the influence of the water
curtain, thus the free and forced dispersion case, and the influence of the
water-to-wind momentum rate RM.
For this purpose, the wind has been chosen as the changing variable in the
present cases. The water curtain is operating under ∆P = 5 kPa, which
was commonly used operating pressure in the field tests. The resulting RM
values are 2 and 7.
9.7
Conclusions
The governing equations of the Eulerian-Lagrangian approach are defined
as used in FLUENT. The coupling between the discrete and gas phase is
explained through the source term in the gas momentum equation. A new
technique, the “Euler-Source” method is then presented on the basis of a
user defined source term in the gas momentum equation. In this manner,
the mechanical effect of the discrete phase may be described in a simpler way.
Then, a brief description of the numerical simulations set-up is given. The
computational domain is described with the boundary conditions, the gas
source, the water curtain and the wind profile.
190
Chapter 10
Simulations
This chapter consist of two parts. Section 10.1 deals with single spray simulations compared with experimental data from section 8.1. Section 10.2
presents simulations of curtain in cross-wind like the field tests and the Wind
Gallery tests.
10.1
Air entrainment in a single spray
The induced gas entrainment by a spray is a major property of the forced
dispersion mechanism. Therefore, emphasis is given to this phenomenon.
The investigations is conducted with a nozzle used in the field test. It is
the full cone nozzle with orifice diameter D0 =5.1 mm presented in section
5.3. The experimental data obtained with this nozzle, have been presented
in section 8.1.
191
Chapter 10. Simulations
10.1.1
Spray envelope
In the developing part of the spray, the spray diameter increases with the
distance to the nozzle, and external air is entrained. When the gas entrainment vanishes, then it is worth quoting at what distance from the nozzle
the spray development is achieved: the spray envelope is a simple and good
indicator of such a state.
The spray envelope has been investigated by three different approaches (experimental, CASIMIRE modelling and numerical simulations). Results are
plotted in figure 10.1.
0
PDA
CASIMIRE
CFD Eul.−Lagr.
0.2
0.4
0.6
z [m]
0.8
1
1.2
1.4
1.6
1.8
2
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
Figure 10.1: Spray envelope at 1000 kPa
The experimental results are derived from Phase Doppler Anemometry measurements. As the measurement is local, profiles are taken in the radial
direction until the spray envelope. The spray diameter is then defined as the
192
10.1.2. Gas phase velocity
distance to the spray axis in which droplets are present. In this case the profiles are taken at 0.2, 0.5 and 1.4 m from the nozzle. CASIMIRE evaluates
the spray radius by the envelope droplet trajectory as described in section
3.2. In the numerical simulations performed with the Eulerian-Lagrangian
model, the spray envelope is represented by the droplet trajectory of the peripherical injections.
There is very good agreement between the approaches up to one metre from
the nozzle. After that distance, the discrepancies augment. The experimental data present the narrower spray diameter. The CASIMIRE predictions
disagree only by 15 %. The CFD simulations exhibit a different behaviour.
They do not show any bending of the envelope indicating that the spray continues to develop. At 1.4 m, the difference between the numerical simulation
and the experiment reaches 25 %.
The nozzle is simulated 2 m above a wall boundary condition. It is therefore
expected that air will not be entrained in the lower part of the spray as the
outflow from the entrained gas above is more important. In the following
section, the gas phase velocity and flow describe this matter.
10.1.2
Gas phase velocity
A comparison between numerical simulations and experimental data is given
in figure 10.2 where profiles of the vertical velocity component of gas are
plooted versus the radial position for different distances from the nozzle (0.2,
0.5 and 1.4 m).
The best agreement is found between the experimental data and the EulerSource approach. This is due to the fact that close to the nozzle, the discrete
phase in the Eulerian-Lagrangian approach is so dense that it exceeds a ratio
limit (in FLUENT) with respect to the gaseous phase.
Compared to the Eulerian-Lagrangian approach, the Euler-Source technique
model in a much more satisfactory manner the gas-phase velocities, both in
the radial and vertical direction:
193
Air velocity [m/s]
15
PDA 20 cm
PDA 50 cm
PDA 140 cm
CDF (E−L) 20 cm
CFD (E−L) 50 cm
CFD (E−L) 140 cm
CDF (E) 20 cm
CFD (E) 50 cm
CFD (E) 140 cm
10
5
0
0
0.1
0.2
0.3
0.4
0.5
Figure 10.2: Gas-phase velocity in the radial position
• The spreading of the velocities profiles in the radial direction slightly
over predicts the experimental data with the Euler-Source method.
Even larger spreading is observed with the Eulerian-Lagrangian manner.
• For the effect of the distance to the nozzle, the Euler-Source model
follows the experimental data with good agreement. The EulerianLagrangian model is not able to model this behaviour in a precise
manner.
The flow patterns predicted by the two CFD methods are presented in figure 10.3. The first simulation 10.3(a) represents the Eulerian-Lagrangian
approach, where the spray consists of water droplet injections. The next
10.3(b), represents the Euler-Source simulation, where the spray is simulated by a source term.
The induced flow in the spray region and near the ground level looks similar
in both approaches. However, in the Euler-Source approach (figure 10.3(b)),
the area representing the region under the nozzle results in larger zones of
large velocities.
194
10.2. Wind effect
3
3
X Velocity
-0.923061
-1.85313
-2.78319
-3.71326
-4.64332
-5.57339
-6.50345
-7.43352
-8.36359
-9.29365
z [m]
2
1.5
1
0.5
0
0
1
x [m]
2
3
(a) Euler-Lagrangian approach
2.5
X Velocity
-0.686728
-2.05238
-3.41803
-4.78368
-6.14933
-7.51498
-8.88063
-10.2463
-11.6119
-12.9776
2
z [m]
2.5
1.5
1
0.5
0
0
1
2
3
x [m]
(b) Eulerian approach
Figure 10.3: Radial velocity [m/s] comparisons for the two CFD approaches
The comparison of the two approaches show that the region with high droplet
concentration is not well simulated by the Eulerian-Lagrangian simulation.
This is due to the high concentration of droplets in this region, and that the
discrete phase is not coupled with the k − ǫ turbulent model [48]. The EulerSource simulation is more realistic in this region. However, the width of the
velocity profile in the radial direction is still larger than the experimental
ones.
10.2
Wind effect
The Euler-Source has shown promising results in the previous section for a
single spray. Now, two dimensional simulations of the field, with a gas release
and a water curtain are presented hereunder.
195
10.2.1
Free dispersion
The simulation of the free dispersion case is performed in the same grid as
for the forced dispersion case.
Figure 10.4 represents the chlorine mass fraction in the computational domain for V=2 m/s and low turbulence (intensity level of 10 % and length
scale of 1 m). The initial cloud released with a height of 0.5 m at the inlet
of the computational domain forms a cloud.
cl2
0.445
0.395
0.345
0.295
0.245
0.195
0.145
0.095
0.045
Figure 10.4: Chlorine mass fraction in the computational domain
One observes that the high concentration rapidly drops to the ground in
the first meter. In the same time, less concentrated gas disperse in the vertical direction. The vertical dispersion is rather poor since the cloud does not
exceed 2 m.
At the location of the water curtain, L=15 m (visible in the figure) the
cloud just reaches its height of 2 m with the lowest concentrations (10 % of
the inital).
10.2.2
Forced dispersion
The flow induced by a water curtain in a cross-wind is first investigated by
studying the velocity streamlines. In this manner, the recirculation bubbles
196
are well visualized for the various operating conditions tested.
Comparison of the methods
Figure 10.5 presents the velocity streamlines for a forced dispersion case and
compare the Eulerian-Lagrangian and the Euler-Source models. The water
flow rate is high. It is ṁl,u =3.2 kg/s/m. The wind velocity is V=2 m/s. The
resulting RM is 7.
(a) Euler-Lagrangian method, RM =7
(b) Euler-Source, RM =7
Figure 10.5: Streamlines in forced dispersion cases for various RM values
197
The Eulerian-Lagrangian simulation is presented in figure 10.5(a). The recirculation bubble is located very near the water curtain. Its shape is elliptic
and is longer in the horizontal direction. Its height is approximately the one
of the water curtain (2 m).
Figure 10.5(b) presents the Euler-Source simulation, which predicts a recirculation bubble larger than the Eulerian-Lagrangian method. Its height
is about twice the height of the water curtain. The consequences on the predictions of the gas cloud behaviour will be an enhanced dispersion compared
to the one of the Eulerian-Lagrangian simulation.
Visualisations from the field tests and Wind Gallery experiments, demonstrated large recirculation bubbles for RM values higher than five (larger
than Hwc ). As these visualisations are dependent on the visibility of the
cloud, the actual recirculation bubble is believed even larger. The EulerSource technique is therefore chosen as the most appropriate one and is used
in the following.
Influence of RM
As the water-to-wind momentum ratio RM was found in the field tests and
the Wind Gallery simulations to be a good indicative parameter of the flow,
its influence is investigated also by numerical simulations.
Figure 10.6 presents the chlorine mass fraction for different RM values. The
water flow rate is kept constant, ṁl,u =3.2 kg/s/m, and the wind velocity is
V =4 m/s for RM=2 and V =2 m/s for RM=7.
The influence of the RM value is evident also by the numerical approach.
For small RM (RM=2), the recirculation is small. It is located close to the
water curtain and its height is smaller than the one of the water curtain. For
higher RM (RM=7) the recirculation bubble increase in size.
The concentration distribution is highly affected by the recirculation bubble. Downstream the water curtain, high concentrations at ground level are
still observed for RM=2. In the case of RM=7, the recirculation bubble
198
(a) RM =2
(b) RM =7
Figure 10.6: Mass fraction of chlorine for different RM values
increases, and in the same time, it consists of higher concentrations than the
recirculation bubble for RM=2. Therefore, the dispersion is more important,
and the concentration downwind the water curtain is lower.
In the Wind Gallery, vertical concentration profiles were measured downwind
the water curtain. Here, such profiles will be extracted from the Euler-Source
simulations presented above.
Figure 10.7 presents vertical concentration profiles for RM value of 2 (low
regime) and 7 (high regime).
199
10
RM=2
RM=7
9
8
Height [m]
7
6
5
4
3
2
1
0
0
5
10
Concentration [%]
15
20
Figure 10.7: Vertical concentration profiles at 3.5 m downwind the water
curtain
The behaviour is similar to the one observed in the Wind Gallery (see figure
8.8). The concentration has its maximum value at the ground and decreases
rapidly with height (the vertical ordinate). At the high RM value, the ground
concentration is further reduced and the cloud height increases. This reflects
the mixing process, which enlarges the gas cloud in the vertical direction.
The difference between the RM values is not large, as optimised condition
leads to. This suggests that the curtain-to-cloud height ratio Hwc /Hc is not
large enough and not enough fresh air is mixed with the pollutant. This effect was described in section 8.5 and demonstrated in figure 8.8 with vertical
concentration profiles for different RM values and curtain-to-cloud height
ratios Hwc /Hc .
200
10.2.3. Dilution factor
10.2.3
Dilution factor
The downwind evolution of the ground concentration and the associated local
dilution factor F D have been evaluated from the source for two RM values.
Figure 10.8 presents the ground mass concentration distribution. The dilution factor is directly calculated from these distributions.
100
Free disp.
RM=2
RM=7
80
60
40
20
0
0
5
10
15
x [m]
20
25
30
5
RM=2
RM=7
FD
4
3
2
1
0
5
10
15
x [m]
20
25
30
Figure 10.8: Concentrations & dilution factor at ground level with respect
to the distance to the source
The water curtain is located at x = 15 m, and the concentration reduction
due to the water curtain occurs already upwind the water curtain. However, the region downwind the water curtain is more important, as the water
curtain usually will be placed as close to the release as possible since the
downwind area is usually the one to be protected.
The fact that the dilution factor decreases with the distance to the water
201
curtain has been presented in the literature [14]. In the present case, a new
observation is made: the dilution factor presents a highest value upstream
the water curtain, after it is continuously decreasing.
In case of RM=2, the area where F D > 1 is restricted in the vicinity of
the water curtain. In the case of RM=7, the F D values have increased drastically, and so has the area where F D > 1. In fact, 1 < F D < 2 at 15 m
from the water curtain.
10.3
Conclusions
In the case of a single spray in still atmosphere, the new approach “EulerSource” is very promising. It can simulate in a more precise manner the air
entrainment induced by a spray even in the vicinity of the nozzle.
In this way, simulations of a two dimensional field with a gas release and a water curtain, are more easily performed with this method. The Euler-Source
model predicts a larger recirculation bubble than the Eulerian-Lagrangian
model with is in better agreement with the experimental approaches.
Numerical vertical concentration profiles downwind the water curtain are
similar to the Wind Gallery experimental data.
The evolution of the ground dilution factor from the source shows that for
low RM values, F D > 1 only in the vicinity of the water curtain. As RM
increases, this region enlarges together with the F D value.
202
Part VI
Synthesis
203
Introduction
This part is elaborated in order to gather and compare in a synthetic manner the general findings obtained in along the three different approaches.
The objective is to propose a simple but consistent modelling of the wind effect on the mechanical efficiency of water curtain to disperse heavy gas cloud.
The field tests lead to dilutions factors with respect to various RM values performed under “real case scenarios”. Three dimensional effects were
observed and measured.
In Wind Gallery experiments, some of the field test observations could be
reproduced in a coherent manner. Therefore, a more parametrical investigation was performed in order to complete and validate the field test conclusion.
In addition, the effect of the water curtain to cloud height ratio was evaluated.
The numerical simulations demonstrated the applicability of simulating the
water curtain under a new technique; the Euler-Source method. In this manner, the water curtain is simply defined by a momentum source term in the
gas phase equation. The resulting flow fields agree with the experimental approaches in terms of the wind effect on the water curtain by a recirculation
bubble upstream the water curtain.
To summarize the different observations concerning the wind effect on forced
dispersion by water curtain, it is first worth comparing the cloud-curtain
interaction observed through the different visualisations performed and to
verify if the same effect of momentum ratio is observed in the different approaches. Then, emphasis is given to the evolution of the forced dispersion
205
factor to seek for a formulation of the wind effect to be implemented in the
engineering code CASIMIRE.
206
Chapter 11
The wind effect
The first section of this chapter is devoted to provide some elements to solve
the question about the equivalence between the three approaches. To support this comparison, the analysis of the upstream recirculation bubble is
conducted. The second section bears a general comparison of the variation of
the forced dispersion factor as the curtain-to-cloud momentum ratio evolves.
The third section deals with an analytical approach which, with the support
of the key outcomes of the three approaches, leads to a simple formulation
of the wind effect.
11.1
Recirculation bubble
From the different approaches, a recirculation bubble that is highly dependent of the water-to-wind momentum ratio RM has been visualized. In this
section, the size of this recirculation bubble is estimated in the various approaches.
Figure 11.2 show the cloud behaviour observed during field tests, laboratory
experiments, and numerical simulations for various water-to-wind momentum ratio RM. In all cases, the gas source is positioned at ground level,
and the cloud height at the position of the water curtain is evaluated in the
207
Chapter 11. The wind effect
free dispersion case to be smaller than half the height of the water curtain
(Hc < 0.5 · Hwc ).
Some similarities in the geometry of the recirculation bubble are observed
in the different approaches:
• RM=2:
At low RM values, the gas cloud is slightly affected by the water curtain. The wind effect is more important than the one of water curtain;
therefore, the cloud passes through the water curtain. No recirculation
bubbles are observed in the experimental tests, only an increase of the
gas cloud height at the level of the water curtain is demonstrated. In
the field, this growth approach 50%, while in the Wing Gallery it is
about 20%. In the numerical simulations a small recirculation bubble
is observed, however, one has to keep in mind that in the visualisations
in the experimental parts highly depend on the visibility of the cloud
(density and relative humidity).
• 3 < RM < 5:
In the Wind Gallery experiments, this range is stated as a transition
range between low and high RM values (see section 8.4). A recirculation bubble is evident for RM=5 in the experimental results. Its height
is of the order of the water curtain.
• RM>5:
For higher RM values, the visualisations show that the recirculation
bubble continue to increase in size. In the field tests (RM=16.6), the
bubble overtake the source location 4 m upstream the water curtain. At
ground level, it is also pushed upstream from the outflow of the water
curtain. Visualisations also demonstrate that when the gas cloud goes
over the water curtain, it is directly entrained in the water curtain on
the other side. In the numerical simulation, one can observe that the
gas concentrations are pushed upwind by the recirculation bubble.
One can conclude that the similar qualitative behaviour is observed under
equivalent RM values in the various approaches.
208
11.1. Recirculation bubble
(a) Free dispersion
(b) RM =2
(c) Free dispersion
(d) RM =2
(e) Free dispersion
(f) RM =2
Figure 11.1: The recirculation bubble in the different approaches
209
(a) RM =5
(b) RM =16.6
(c) RM =5
(d) RM =10
(e) RM =2
(f) RM =7
Figure 11.2: The recirculation bubble in the different approaches
210
11.2. Dilution factor F D and efficiency
11.2
Dilution factor F D and efficiency
The efficiency of the water curtain has been evaluated according to the concentration reduction downwind the curtain by the dilution factor. In section
6.4 the dilution factor F D was defined by equation 6.1 as the ratio of concentrations without ρp0 and with the water curtain ρpf . The dilution factor
has commonly been presented with respect to the water-to-wind momentum
ratio RM defined by equation 2.2.
Figure 11.3 presents the dilution factor with respect to the momentum ratio
RM for the different approaches.
2
10
FD
global
Field tests
Wind gallery exp.
1
10
0
10 0
10
1
10
RM
2
10
Figure 11.3: Dilution factor with respect to the water-to-wind momentum
ratio RM
The tendency of increasing F D with RM is obvious; however, net discrepancy between the two experimental tools exists. Several reasons are invoked:
• The effect of different concentration measurement positions downwind
the water curtain: The dilution factor is known to decrease with the
211
distance to the water curtain. Therefore, the position of the concentration measurements with respect to the water curtain is an essential
parameter. In the Wind Gallery experiments, the ratio Xc /Hwc=4 and
in the field tests Xc /Hwc =1.75 (see section 7.2.5). The ratio 2.3 between the approaches is large for comparative matter.
• The height ratio Hwc /Hc : Investigations of this parameter in the Wind
Gallery presented its influence. The effect on the height ratio was found
to be more significant as RM increases. Therefore, a large difference
between the approaches could lead to different dilution factor. However,
in the field tests, the cloud height was estimated to be half the water
curtain height by visual observations and thus with a similar height
ratio Hwc /Hc with the Wind Gallery experiments.
• Temperature differences: In the case of an additional thermal effect
by the water curtain, the measured dilution factor would be affected.
However, under the operating conditions both in the field and the Wind
Gallery tests, no important temperature differences between the gas
cloud and the ambient were measured.
From these facts, the effect of the measurement position with respect to the
height of the water curtain Xc /Hwc looks the more important.
A model based on a Bosanquet formulation [14] is given in order to evaluate the correction that has to be given the dilution factor to take account
for the different ratios Xc /Hwc in the field tests and the Wind Gallery experiments.
The downstream concentration evolution is given by the formulae:
ρp =
ṁp
αV [h0 + Ax]2
(11.1)
where ṁp is the pollutant flow rate, α is the aspect ratio of the cloud cross
section (width/height), V the wind velocity, h0 the initial cloud height, A an
entrainment coefficient and x the distance from the source.
Figure 11.4 presents the three regions counted for in the model.
212
Atmospheric
disperion
Water
curtain
Atmospheric
disperion
H
H
2
1
H0
x
x
0
1
x
2
x
3.5m
x
8m
Figure 11.4: Schematic of the modelling
The entrainment factor accounts for the atmospheric stability class (ǫatm )
and the liquid spray action through the velocity ratio Vent /V where Vent is
the entrained gas velocity in the spray.
ǫ = ǫs
Vent
V
(11.2)
and ǫs is the spray efficacy coefficient.
In the field test, the concentration measurements were taken at 3.5 m downwind the water curtain. Now, imagine a correction of the dilution factor from
the field test with respect to the Xc /Hwc value used in the Wind Gallery results. For Hwc = 2 m and Xc /Hwc = 4, Xc = 8 m.
A ratio of the dilution factors at these distances will provide the correction
factor κ that has to be applied:
F D3.5m
=κ
F D8m
(11.3)
The local dilution factors are given as
F D3.5m =
ρpf (3.5m)
ρp (8m)
and F D8m = f
ρp0 (3.5m)
ρp0 (8m)
(11.4)
They can be calculated from equation 11.1 what results in the following
relation:
Hp (3.5m) 2
Hp0 (8m) 2
F D3.5m
=( f
) ·(
)
(11.5)
F D8m
Hpf (8m)
Hp0 (3.5m)
213
where Hpf (3.5m) is the cloud height under forced dispersion and Hp0 (3.5m) is
the cloud height under free dispersion at the same location.
Now, the dilution factors measured during field tests are corrected and corresponds to the same Xc /Hwc as in the Wind Gallery experiments. The results
are presented in figure 11.5. The correction factor ranges between 1.4 and
1.8 for different wind and stability conditions.
14
Field tests
Field test corr.
Wind gallery exp.
12
FD
global
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
RM
Figure 11.5: Corrected dilution factor with respect to the water-to-wind
momentum ratio RM, all
It is observed that the influence of different Xc /Hwc is not negligible. It
can be modelled in a quite simple manner as presented above. The corrected
values of F D are used in the following of this chapter.
Now, a comparison of all the results that have been performed in this field
also from previous investigations at VKI is proposed in figure 11.6.
From the previous discussion, it can be understood where the data scatter is coming from and why the Wind Gallery data are in the lower range of
dilution factors.
214
20
18
16
Field tests
Wind Gallery
Wind Gallery (old)
CFD
Trendline
14
FD
12
10
8
6
4
2
0 −1
10
0
1
10
10
2
10
RM
Figure 11.6: Dilution factor with respect to the water-to-wind momentum
ratio RM, all
The trendline presented in the figure 11.6 is the outcome of a simple analysis
based on the Bosanquet formulation [14]. Evaluating equation 11.1 in x2 in
free and forced dispersion give
ρp0 =
M
M
and ρp2 =
2
[H0 + A0 x2 ]
[H0 + A0 x1 + As Ds ]2
(11.6)
where subscript 0 indicates free dispersion zone, s water curtain area and
M = ṁp /αV . Now assuming H0 + A0 x2 ≈ H0 + A0 x1 , which is quite acceptable for dense cloud behaviour in free dispersion if x2 − x1 is not to large,
leads to the expression
FD =
ρp0
As DS 2
≈ (1 +
)
ρp2
Hc
(11.7)
From [14] it is shown that As = Ug · Cs /V where Cs is an entrainment coefficient due to the water curtain itself and Ug the gas phase velocity in the spray.
In section 3.3.1 it was shown that Ug is proportional to the water flow rate.
215
Therefore, Ug /V varies as the root os the water to wind momentum ratios as
s
Qm,wc
Ug
= C0
V
Qm,wind
(11.8)
where C0 is a parameter dependent on the nozzle orifice D0 , the nozzle flow
number FN and the height of the water curtain. Substituting 11.8 in 11.7
give
√
(11.9)
F D = [1 + C · RM ]2
where C is a global parameter including all the effects not explicitly modelled
in this approach. It can not be easily calculated because some quantities such
as the cloud height in the field test are unknown. It is shown in figure 11.6
that a value of C=0.65 gives a satisfactory agreement with the experience.
The efficiency η in percent is related to the dilution factor by the relation
η=
ρp0 − ρpf
1
=1−
ρp0
FD
(11.10)
Figure 11.7 presents the evolution of water curtain efficiency η.
Efficiencies above 80% are quite easily reached, even for relatively low RM
values (∼ 2). To reach 90%, much higher RM values are required (∼ 10).
Such a finding is very promising as far as the industrial application is concerned.
11.3
Modelling the wind effect
The model MARRS was presented in the literature survey section 3.2. It
presents a one dimensional (vertical) model of a water curtains action on a
gas cloud in terms of mechanical dispersion by air entrainment. MARRS is
now part of a more general engineering model CASIMIRE, where the effect
of the wind has to be introduced.
The correlation in equation 11.9 can not be used straight forward as it is
216
11.3. Modelling the wind effect
100
90
80
70
η [%]
60
50
40
30
Field tests
Wind Gallery exp.
Wind Gallery exp. (old)
CFD
Trendline
20
10
0 −1
10
0
1
10
10
2
10
RM
Figure 11.7: Water curtain efficiency η with respect to the water-to-wind
momentum ratio RM, all
for several reasons. First, because it does not take explicitly into account the
curtain to cloud height ratio Hc /Hwc . This ratio was proved to be important
for low RM values in the Wind Gallery investigations (see figure 8.5). Secondly because, the dilution factor has to corresponds to the value predicted
by CASIMIRE for no wind conditions.
After developing the equation 11.9 a normalisation can be introduced as
following:
√
2C RM + C 2 RM
FD − 1
√
f=
=
(11.11)
F D0 − 1
2C RMM AX + C 2 RMM AX
f can be regarded as a correction factor to account for wind effect. F D0 is
the dilution factor without wind (predicted by CASIMIRE). For high wind
conditions, F D → 1 and f → 0. Low or no wind yields f → 1. Let RMM AX
be the RM values that provides F D0 . Then the wind effect can be modelled
by
F D = 1 + f · (F D0 − 1).
(11.12)
Figure 11.8 presents a comparison between predictions from equation 11.12
and field test results.
217
7
6
FD
5
4
3
2
1
0
Field test
C=1.5
C=1.7
C=2
1
2
3
4
Wind velocity [m/s]
5
6
Figure 11.8: Model of wind effect on dilution factor
Good agreement between the model and field tests is observed for C = 1.7.
It should be recalled that the parameter C is influenced by the
• the position of the measurement with respect to the water curtain
Xc /Hwc and the
• the height ratio Hwc /Hc as the measurement in the Wind Gallery suggests in figure 8.9.
The present wind model has been incorporated in the engineering model
CASIMIRE. The following illustrative exercise exemplifies the applicability
of this model to solve some design questions
218
11.4. Illustrative exercise
11.4
Illustrative exercise
To exemplify the applicability of the CASIMIRE code, an illustrative example of curtain pre-design is proposed.
The pollutant is chlorine and the leak is characterized by a continuous flow
rate of 3.5 kg/s. It is assumed that the cloud develops with a rectangular
cross section the aspect ratio, width/height, is equal to 10.
To mitigate the hazard consequence of such release a water curtain of 3 m
high is located at 15 m from the source. It is composed of 7 mm in diameter
full-cone nozzles, spaced by 0.5 m. The operating gauge pressure is 800 kPa.
Emphasis is given to the forced dispersion efficacy of the water curtain as
the wind speed rises from 1 m/s to 5 m/s. To calculate the characteristics
of the cloud just upstream the water curtain, the free dispersion module of
the VKI code CURTAIN, which relies upon the Bosanquet formulation (see
section 11.2), is used [14].
Table 11.1 lists the values of the height, Hc , and the concentration, ρp0 ,
of the cloud right to the curtain , the CASIMIRE predictions in terms of
the pollutant concentration ρpf just downstream the curtain and the forced
dispersion efficiency, η. The increase of the wind speed augments the cloud
height and reduces the initial pollutant concentration as a result of the free
dispersion enhancement. Up to about 2 m/s the water curtain maintain a
good efficacy. In the other hand above 3 m/s a net deterioration of the curtain performance occurs.
We have seen that the forced dispersion mechanism depends on the gas entrainment capacity of water sprays. Such a property, which results from the
momentum exchange between liquid and gas phases, is a direct function of
the droplet size. But at given pressure, the droplet diameter increases as the
nozzle orifice diameter increases D0 . It is then interesting checking the effect
of this parameter on the dispersion performance.
Table 11.2 presents the results when D0 varies from 5 mm to 12 mm. The
nozzle with a small diameter leads to small droplets which do not sustain the
219
Wind
[m/s]
1
2
3
4
5
Hc
[m]
0.69
0.72
1.02
1.29
2.48
ρp0
[g/m3]
736
342
113
53
11
ρpf
[g/m3 ]
90.5
85.5
39.1
22.6
8
η
[%]
87.7
75
65.4
57.4
27.5
Table 11.1: Example of efficiency η by wind; L=15 m, Hwc =3 m
wind strength. On the contrary the big orifice yields coarse droplets which
exhibit a good wind holding and consequently induce a significant gas entrainment.
V =1 m/s; Hc =0.69 m; ρp0 =736 g/m3
D0
ρpf
η
3
[mm] [g/m ]
[%]
5
156.8
78.7
7
90.5
87.7
10
61.8
91.6
12
72.1
90.2
V =3 m/s; Hc =1.02 m; ρp0 =113 g/m3
D0
ρpf
η
[mm] [g/m3 ]
[%]
5
54.5
51.8
7
39.1
65.4
10
26.2
76.8
12
24.5
78.3
Table 11.2: Effect of nozzle diameter D0 ; L=15 m and Hwc =3 m
Finally table 11.3 emphasizes the influence of the curtain height on the performance. Raising the curtain leads to increase the quantity of entrained
fresh air and therefore, due to mixing with the pollutant, to decrease the
downstream concentration. The result is an improvement of the curtain efficiency. However, high curtains are more sensitive to wind speed. That is
220
11.4. Illustrative exercise
the reason why table 11.3 points out the presence of an optimum value of
Hwc located at 3 m. Obviously, stronger is the wind, lower is the final efficiency of the forced dispersion: a relative D-diminution of 20% to 30% can
be experienced as the wind speed changes from 1 m/s to 3 m/s.
V =1 m/s; Hc =0.69 m; ρp0 =736 g/m3
Hwc
ρpf
η
[mm] [g/m3 ]
[%]
2
125.1
83
3
90.5
87.7
4
117
84.1
V =3 m/s; Hc =1.02 m; ρp0 =113 g/m3
Hwc
ρpf
η
3
[mm] [g/m ]
[%]
2
42.9
62
3
39.1
65.4
4
45.9
59.4
Table 11.3: Effect of water curtain height Hwc ; L=15 m
221
222
Part VII
General conclusions
223
Conclusions
The objective of this work is to model the performance of a water curtain
on a heavy gas cloud in reducing the pollutant concentration by mechanical
effect. This method is a mitigation mean, which can be used in the process
industry around storage tank of hazardous materials.
A methodology of complete applied research has been adopted. It consists
of field tests, laboratory experiments and numerical simulations.
For this purpose, these different approaches focuses on the level of pollutant concentration in a heavy gas cloud with and without the operation of
the water curtain; Field tests represent three-dimensional, large scale experiments with chlorine and carbon dioxide gas. Wind Gallery experiments rely
upon a two-dimensional, small scale investigation with carbon dioxide gas.
Numerical simulation in two-dimensions aims to mimic the induced flow patterns observed in the experimental parts.
In the field tests, the water curtain efficiency was evaluated on the basis
of downwind ground concentrations. The gas cloud behaviour was observed
changing between free and forced dispersion case, under high influence of the
water-to-wind momentum ratio RM. In free dispersion and forced dispersion
with low RM values, the concentration distribution in the lateral direction
remained Gaussian. Low RM values are typical of range 0 < RM < 2.
However, a concentration reduction could take place in this range, without
being significant (F D ∼ 2). As the RM value increases, the concentration
distribution becomes uniform and the dilution factor rises up to 10. Also,
high RM values enhanced both lateral and vertical spreading of the cloud.
The lateral cloud spreading due to the water curtain is generally neglected
in literature. Here, it is found important, but could not be evaluated due to
the lack of measurement points.
Most of the results are based on mean concentration measurements. Nevertheless, some examples with instantaneous captors are given. It is observed
225
that the concentration peaks are only advected with the cloud motion without significant diffusion. Correlations for captors spaced by 10 m in the
downwind direction up to 25 m from the source were of 75 to 90%.
The response time of the water curtain was obtained from the instantaneous
measurements, by evaluating the time for the dilution factor takes to reach a
plateau. It is found to be of the order of 1 minute. This time is not negligible
in case of an industrial hazard.
Some comparisons with literature is made, but the operating conditions,
the position of water curtain and the concentration probes with respect to
the source are generally quite different. Only, the Buxton test series [60],[62]
presents relatively concordance with the present results.
In the Wind Gallery experiments, different experiments are carried out with
the objective to reproduce the main features of the field tests, and next, to
undertake a more parametrical investigation that was not easy to perform
during field tests.
Visualisations in the Wind Gallery clearly pointed out the same main features as the field test, with respect to the influence of the RM value. It is
worth noting that the lateral spreading observed in the field tests can not be
studied in the lateral direction due to two-dimensional confinement.
In the Wind Gallery, vertical concentration measurements are performed.
They demonstrated that the maximum concentration is always at ground
level. But as the RM value increases, the profile becomes uniform. It means
that the concentration profile develops in height and that the maximum concentration is highly reduced. In fact, it is the same effect measured in the
lateral direction in the field tests. Also here, dilution factor F D=10 are
achievable.
The influence of the water curtain to gas cloud height ratio is also investigated. As a practical rule, water curtains more than twice the height of
the gas cloud are recommended. It is explained by the fact that a downward
water curtain entrains in this case more fresh air, such that the mixing with
226
the pollutant is enhanced.
Some instantaneous measurements in the Wind Gallery demonstrate that
the cloud behaviour reaches quickly a steady state. This is due to the low
level of turbulence and ground roughness in comparison with the field tests.
In addition, the release conditions are more stable in the Wind Gallery.
The two-dimensional numerical simulations compares a standard EulerianLagrangian model with a new approach called the Euler-Source technique.
In the Eulerian-Lagrangian model, each droplet is a momentum source while
in the Euler-Source technique, the momentum source is defined on the area
occupied by the water curtain.
In the case of a single spray operating in a continuum at rest, the EulerSource technique simulate in a more precise manner the gas entrainment
phenomenon.. Experimental data compared to the Euler-Source technique
shows promising results.
In the case of wind effect, the recirculation zone is larger with the EulerSource technique than with the Euler-Lagrange approach. However, it seems
more realistic in comparison with field and Wind Gallery tests.
In general, numerical methods reproduce in a satisfactory manner the experimental observations.
In a synthesis, comparison of the different approaches is achieved. In fact,
the recirculation bubble that occurs upstream the water curtain and that
grows with the RM values, is observed similarly in the field, in the Wind
Gallery experiments and in the numerical simulations as well.
Despite that the dilution factor F D data of the different approaches demonstrate some scatter, most probably due to the different position at which
the concentration is evaluated downstream the water curtain (Xc /Hwc), the
agreement is found to be very satisfactory.
227
A correction of the dilution factor is proposed to evaluate the influence of the
position of the concentration measurement with respect to the water curtain
Xc /Hwc . In this manner, the field test results was corrected to the Xc /Hwc
value used in the Wind Gallery and better agreement was achieved.
Furthermore, a wind model for the dilution factor is given. In the CASIMIRE
code, the dilution factor is estimated on the basis of the air entrainment.
Therefore, a correction factor due to the effect of the wind is searched for.
The simple model is fitted with experimental data from the field tests. This
wind model will be correlated to further experimental results before is will
be incorporated in the CASIMIRE model.
Now, for an industrial site where water curtain are mounted around storage tanks, some aspects have to be pointed out:
• A water curtain has to be designed for its purpose. In part II, the three
mechanisms of the water curtain were defined. If the gas is cold, heat
transfer may enhance dilution. If the gas is soluble, it may be absorbed.
However, the design of the water curtain differs for each mechanism.
• The release conditions may affect a water curtain efficiency: if the
release is a high pressure / velocity jet, it may go through the water
curtain without loosing inertia. The water curtain will then not be
able to disperse the gas jet. If the water curtain is placed to close to
a liquid pool, it will enhance evaporation and increase the downwind
concentrations.
• For the mechanical effect, the performance of the water curtain is highly
dependent on the wind speed. In case of high wind speed (> 5 m/s),
a high efficiency of the water curtain is hardly achievable. The water
curtain has to be designed with large flownumber nozzles to ensure a
high water flow rate and large orifice diameter such that the droplets
are large and stands more in wind. In addition it has to be used at
high operating pressures.
228
• The water curtain should be twice the height of the gas cloud in order
to entrain fresh air and enhance mixing.
• The water curtain response time is not negligible. Therefore, water
pipelines should be constantly under pressure.
A water curtain is generally conceived for one scenario. Therefore, it is essential that the operating conditions are adapted and functioning in the critical
moment.
Perspectives
Perspectives from the following thesis are presented first for each approach,
and then more general ideas are given.
In the field tests, a higher number of measurement points than used are
necessary to investigate the gas cloud behaviour in a three dimensional dispersion. In this case, ground concentrations were targeted; however, the
vertical dispersion is also important (recirculation bubble) and the increase
in gas cloud height should be evaluated. It was only measured in the Wind
Gallery but no lateral dispersion was possible.
It has been shown that instantaneous measurements were a key to understand the time dependent behaviour of the pollutant cloud in forced dispersion. It is then recommended to repeat such type of experiments with more
captors. With a more complete data base for various RM values, much more
information on the cloud dispersion could be drawn out.
As for the numerical simulations, many investigations remain to be done.
In particular, an extension to three-dimensional simulations is necessary. In
two dimensions, the Euler-Source technique gave promising results. In three
dimensions, the Euler-Source technique could quite easily be modelled compared to the Eulerian-Lagrangian approach which is to heavy. It would model
the cloud behaviour in front of the water curtain (lateral and vertical dispersion). Moreover, the influence of the atmospherical turbulence and of the
ground roughness is essential for proper comparisons with field tests.
229
General perspective of this work would be to suggest the present methodology to study the other experienced physical mechanisms within sprays. For
instance, the absorption effect through field tests, wind tunnel experiments
and numerical simulations. However, a project that integrates different approaches leads to more coherent presentation of the results. In the case of
absorption, tests are more complicated to set up, as the total amount of water should be recuperated for analysis and eventually treated before rejected.
In the case of the thermal effects, it should be compared to the case of mechanical dispersion in order to evaluate the difference as it finally leads to
dispersion.
230
Bibliography
[1] Air Liquide. Encyclopédie des gaz. 1976.
[2] Air liquide. www.airliquide.fr, 2005.
[3] Algieri A. Experimental study of water spray curtain. VKI Diploma
Course, Project Report 2003-01, 2003.
[4] Bächlin W., Theurer W. & Plate E. J. Wind field and dispersion in
a built-up area - a comparison between field measurements and wind
tunnel data. Atmospheric Environment, 25A(7):1135–1142, 1991.
[5] Bara A. & Dusserre G. The use of water curtains to protect firemen in
case of heavy gas dispersion. Journal of Loss Prevention in the Process
Industries, (10):179–183, 1997.
[6] Barillà D. Numerical simulation of single spray: modelling, validation
and application. VKI Stagiaire Report, 2003-09, 2003.
[7] Bathia K. K. Dupont’s inherently safe mic process after bhopal. Kanpur,
India, December 2004. International Conference on the 20th Anniversary
of the Bhopal Gas Tragedy.
[8] Beran J. Numerical simulation of heat transfer in liquid sprays. VKI
Diploma Course, Project Report 2004-05, 2004.
[9] Bigot J.-P., Touil A., Bonnet P. & Lacôme J.-M. Rain-out investigation: initial droplet size measurement. Kanpur, India, December 2004.
International Conference on the 20th Anniversary of the Bhopal Gas
Tragedy.
231
BIBLIOGRAPHY
[10] Blewitt D. N., Petersen R. L., Ratcliff M. A. & Heksestad G. Evaluation
of water spray mitigation system for an industrial facility. pages 483–
510. Modeling and mitigating the consequences of accidental releases of
hazardous materials, International Conference and Workshop, 1991.
[11] Blewitt D. N., Yohn J. F., Koopman R. P., Brown T. C., Hague W.
J. Effectiveness of water sprays on mitigating anhydrous hydrofluoric
acid releases. pages 155–180. International Conference on Vapour Cloud
Modelling, 1987.
[12] Bouchet S. Analyses des risques et prévention des accidents majeurs (dra-007). Ministère de l’amenagement du territoire et de
l’environnement, 2000.
[13] Britter R. E. & Griffiths R. F. The role of dense gases in the assesment
of industrial hazards. Journal of Hazardous Materials, (6):3–12, 1982.
[14] Buchlin J.-M. Mitigation of problem clouds. Journal of Loss Prevention
in the Process Industries, (7):167–174, 1994.
[15] Buchlin J.-M. Thermohydraulic modelling of liquid sprays. VKI Lecture
Series 1995-06 ”Two-Phase Flows with Phase Transition”, 1995.
[16] Buchlin J.-M. Water sprays as mitigation means. Cargèse, Corse,
France, October 2003. International Workshop on Multiphase and Complex Flow Simulation for Industry.
[17] Buchlin J.-M. Thermal shielding by water spray curtains. Kanpur, India,
December 2004. International Conference on the 20th Anniversary of the
Bhopal Gas Tragedy.
[18] Buchlin J.-M. & Alessandri E. Numerical simulation of the thermohydraulic behaviour of liquid sprays. Florence, Italy, July 1997. 13th
Annual Conference on Liquid Atomization and Spray Systems.
[19] Buschmann C. H. Experiments on the dispersion of heavy gases and
abatment of chlorine clouds. pages 475–488. 4th International symposium on transport of hazardous cargoes by sea and in land waterways,
1975.
232
BIBLIOGRAPHY
[20] Beresford T. C. The use of water spray monitors and fan sprays for
dispersing gas leakages. North Western Branch Papers, Institution of
Chemical Engineers, (5):6.1–6.3, 1981.
[21] Jones A. D. & James G. C. Air movement and cleaning by water sprays.
Annals of Occupational Hygiene, 31(2):161–179, 1987.
[22] Chouhan T. R. et al. Bhopal: The Inside Story. Second edition, 2004.
[23] Cole S. T. & Wicks P. J. European community research in major industrial hazards. Journal of Loss Prevention in the Process Industries,
7(2), 1994.
[24] Colin P. & Olivari D. Three applications of hot wire techniques for
fluid dynamic measurements. Intern. Congress on Instrumentation in
Aerospace Simulation Facilities, ICIASF, 1971.
[25] Crabol B. & Monfort M. Présentation du modèle MICAR1 de
dispersion des jets gazeux dans l’atmosphère.
Note technique
CEA/IPSN/DPEI/SEAC/93/123, 1993.
[26] Dandrieux A. Etude expérimentale de lefficacité des rideaux deau mobiles face à un rejet de gaz lourd (ammoniac-chlore) - Contribution à la
modélisation de linfluence des rideaux deau. PhD thesis, Université de
Provence, spécialité Biosciences de l’Environnement et Santé 2001.
[27] Dandrieux A., Dusserre G., Ollivier J. & Fournet H. Effectiveness of water curtains to protect firemen in case of an accidental release of ammonia: comparsion of the effectiveness for two different release rates of ammonia. Journal of Loss Prevention in the Process Industries, (14):349–
355, 2001.
[28] Dandrieux A., Dusserre G., Thomas O. The dvs model: a new concept
for heavy gas dispersion by water curtain. Environmental Modelling &
Software, (18):253–259, 2002.
[29] De Mulder T., Deconinck H. & Buchlin J.-M. Water spray simulation on
unstructured grids with a SUPG/PSPG stabilized P1/P1 finite element
solver. Venice, Italy, October 1995. 9th International Conference on
Finite Elements in Fluids New Trends and Applications.
233
BIBLIOGRAPHY
[30] Dickinson C., Ekedahl E., Frostling H., Harris C., Hertzberg O., Hultén
G., Lundstrom S. & Nordfors B. Experiment with active field measures
to reduce potential damage in chlorine accidents, 1977.
[31] Dimbour J. P. Contribution à l’étude expérimentale et à la modélisation
de l’influence de dispositifs de protection de type rideau d’eau sur la
dispersion atmosphérique d’un rejet de gaz lourd se produisant depuis un
local de stockage de chlore liquéfié sous pression. PhD thesis, Université
d’Aix - Marseille 1, 2003.
[32] Dimbour J. P., Dandrieux A. & Dusserre G. Reduction of chlorine
concentrations by using a greenbelt. Journal of Loss Prevention in the
Process Industries, (15):329–334, 2002.
[33] Fthenakis V. M. The feasibility of controlling unconfined releases of toxic
gases by liquid spraying. Chemical Engineering Commitee, 83:173–189,
1989.
[34] Fthenakis V. M. HGSPRAY: a complete model of spraying unconfined
gaseous releases. Journal of Loss Prevention in the Process Industries,
6:327–331, 1993.
[35] Greiner M. L. Water for application for ammonia vapor control. AIChE
Symposium: Control of Accidental Releases of Hazardous gases, 1989.
[36] Greiner M. L., Simplot J. R. Emergency response procedures for anydrous ammonia vapor release. Plant Operation Progress, 3(2), 1984.
[37] Griffiths R. F. & Fryer L. S. A comparison between dense-gas and passive tracer dispersion estimates for near-source toxic effects of chlorine
releases. Journal of Hazardous Materials, 19:169–181, 1988.
[38] Griffiths R. F. & Kaiser G. D. Production of dense gas mixture from
ammonia releases - a review. Journal of Hazardous Materials, (6):197–
212, 1982.
[39] Gupta J.P.
Dilution with air to minimise consequences of
toxic/flammable gas releases. Kanpur, India, December 2004. International Conference on the 20th Anniversary of the Bhopal Gas Tragedy.
234
BIBLIOGRAPHY
[40] Hald K. Study of thermal shielding by liquid sprays. Diploma Course
Project Report 2001-14, 2001.
[41] Hald K., Buchlin J.-M., Dandrieux A. & Dusserre G. Heavy gas dispersion by water spray curtains - a research methodology. Praha, Czech
Republic, June 2004. Loss prevention 2004, 11th International EFCE
Symposium.
[42] Hald K., Dandrieux A., Dusserre G. & Buchlin J.-M. A methodology
to investigate heavy gas dispersion by water-curtains. volume 1, pages
741–746, Maastricht, The Netherlands, June 2003. European Safety and
Reliability Conference.
[43] Hall D. J. & Walker S. Scaling rules for reduced-scale field releases of
hydrogen fluoride. Journal of Hazardous Materials, (54):89–111, 1997.
[44] Hanna S. R. Hazardous gas model evaluation. is an equitable comparison
possible? Journal of Loss Prevention in the Process Industries, 7(2),
1994.
[45] Houben D. Water spray curtains - scales model experiments and simulations. VKI, Stagiaire Report 2004-22, 2004.
[46] Imberdis O. & Zafer B. Investigation of spray droplets: 1-thermocouple
measurements in a flashing R134a spray, 2-PDA in a full cone water
spray. Stagiaire Report 2002-24, 2002.
[47] Khan F. I. & Abbasi S. A. Cushioning the impact of toxic release from
runaway industrial accidents with greenbelts. Journal of Loss Prevention
in the Process Industries, 13:109–124, 2000.
[48] Lewtak R. Numerical modelling of liquid sprays. VKI Diploma Course,
Project Report 2003-19, 2003.
[49] Malet J. Pulvérisation de type jets-plats. Rapport CEE, von Karman
Institute of fluid Dynamics, 1992.
[50] Malet J., Corieri P., Buchlin J-M. Mechanical and thermal actions of
liquid curtains using hydro-shield nozzle. Prag, Tcheck Republic, 1999.
European Aerosol Conference.
235
BIBLIOGRAPHY
[51] Malet J., Corieri P., Zimmer L. & Buchlin J-M. . An experimental
and numerical study of the mechanical and thermal actions of water
spray curtain for toxic cloud mitigation. volume 1, Toulouse, France,
July 1999. 15th Annual Conference on Liquid Atomization and Spray
Systems.
[52] Maroteaux D., Maroteaux F. & Murat M. Experimental study of a
hot wire, sonic nozzle probe for concentration measurements. Rev. Sci.
Instrum., 62(4), 1991.
[53] Martinsen W. E., Muhlenkamp S. P. & Olson L. J. Disperse LNG vapors
with water. Hydrocarbon Processing, International Edition, (33):261–
266, 1977.
[54] McQuaid J. Air entrainment into bounded axisymmetric sprays. Proc.
Instn. Mech. Engrs., (189), 1975.
[55] McQuaid J. Design of the Thorney Island continuous release trials.
Journal of Hazardous Materials, (16):1–8, 1987.
[56] McQuaid J. & Fitzpatrick R. D. Air entrainment by water sprays:
Strategies for application to the dispersion of gas plumes. Journal of
Occupational Accidents, (5):121–133, 1983.
[57] McQuaid J. & Moodie K. The scope for reduction of the hazard of
flammable or toxic gas plumes. Journal of Occupational Accidents,
(5):135–141, 1983.
[58] Meroney R. M. Wind-tunnel experiments on dense gas dispersion. Journal of Hazardous Materials, (6):85–106, 1982.
[59] Meroney R. M. Guidelines for fluid modelling of dense gas cloud dispersion. Journal of Hazardous Materials, (17):23–46, 1987.
[60] Moodie K. Experimental assessment of full-scale water-spray barriers
for dispersing dense gases. North Western Branch Papers, Institution
of Chemical Engineers, (5):5.1–5.13, 1981.
[61] Moodie K. The use of water spray barriers to disperse spills of heavy
gases. Plant Operation Progress, (4):234–241, 1985.
236
BIBLIOGRAPHY
[62] Moodie K., Taylor G. & Beckett H. Experimental assessment of full-scale
water-spray barriers for dispersing dense gases. Health and Safety Executive Research and Laboratory Services Divition, Section paper, 1981.
[63] Moore P. A. C. & Rees W. D. Forced dispersion of gases by water and
steam. North Western Branch Papers, Institution of chemical engineers,
(5):4.1–4.13, 1981.
[64] Morsi S. A. & Alexander A.J. An investigation of particle trajectories
in two phase flow systems. Journal of Fluid Mechanics, 55(2):193–208,
1972.
[65] Petersen R. L. & Diener R. Vapour barrier assessment programme for
delaying and diluting heavier-than-air HF vapour clouds. Journal of
Loss Prevention in the Process Industries, 3:187–196, 1990.
[66] Pretrel H. Etude du comportement thermo-hydraulique de pulvérisations
liquides sous l’effet d’un rayonnement infrarouge. Application à la protection incendie par rideau d’eau. PhD thesis, von Karman Institute &
Institut des Sciences Appliquées de Lyon, 1997.
[67] Puls E., Engelhardt F. & Hartwig, S. Investigation on the mitigation
during accidental release of heavy gas by technical devices. ?, (?).
[68] Schatz K. W. & Koopman R. P. Water spray mitigation on hydrofluoric acid releases. Journal of Loss Prevention in the Process Industries,
(3):222–233, 1990.
[69] St-Georges Buchlin J.-M., Riethmuller M. L., Lopez J. P., Lieto J. &
Griolet F. Fundamental multidisciplinary study of liquid spray for absorption of pollutant or toxic clouds. Loss Prev. Saf. Prom. Proc. Ind.,
2(65), 1992.
[70] St-Georges M. Etude Hydrodynamique des Pulvérisations Liquides pour
Application aux Rideaux d’Eau. PhD thesis, Université Claude Bernard
- Lyon 1, France, 1993.
[71] St-Georges M. & Buchlin J.-M. Detailed single spray - experimental
measurements and one-dimensional modelling. International Journal of
Multiphase Flow, 20(6), 1994.
237
BIBLIOGRAPHY
[72] St-Georges M., Buchlin J.-M. & Riethmuller M. L. Modélisation et validation en galerie à vent. Un contrôle des rejets accidentels par rideaux
de fluides. Projet CEE EV5V CT 92 0072, 1996.
[73] Thomerson J. R. & Billings D. E. Chlorine vapor suppression tests D.O.E. Nevada test site, 1990.
[74] Yildiz D., Rambaud P., van Beeck J. & Buchlin J.-M. Influence of orifice
diameter on flash evaporation of a two-phase R134a jet. Yokohama,
Japan, June 2004. 5th International Conference on Multiphase Flow,
ICMF’04.
238

Forced Dispersion of Heavy Gas Clouds by Water Curtains

Transcription

Similar documents

VISIONWALL Project Profile

O`hare South Air Traffic Control Tower

October 1, 2014

RP-ISO-hermetic 45N/60N RP-tec 55 / 55 plus (FERRO

ResIDeNces

iPPlus Sales Sheet - IP Plus Consulting, Inc.