Columbia International Publishing

Transcription

Columbia International Publishing
American Journal of Heat and Mass Transfer
(2015) Vol. 2 No. 2 pp. 59-75
doi:10.7726/ajhmt.2015.1005
Research Article
Numerical Simulation of Heat and Mass Transfer
in the Evaporation of Glycols Liquid Film
along an Insulated Vertical Channel
Abderrahman Nait Alla1, M’barek Feddaoui1*, and Hicham Meftah1
Received 4 December 2014; Published online 28 March 2015
© The author(s) 2015. Published with open access at www.uscip.us
Abstract
A numerical study is reported to investigate the evaporation in mixed convection of a pure glycol liquid film
such as ethylene and propylene glycol. The model solves the coupled governing equations in both phases
together with the boundary and interfacial conditions. The systems of equations obtained by using an implicit
finite difference method are solved by TDMA method. The influence of inlet temperature of liquid film and
inlet pressure in the gas flow are examined. The results indicate that the propylene glycol evaporates in more
intense way in comparison to ethylene glycol, due to the volatility difference. Additionally, In the operating
system, the heat transfer by sensible mode is more important than latent mode.
Keywords: Mixed convection; Heat and mass transfer; Evaporation, Ethylene and propylene glycol; Vertical
channel
1. Introduction
Heat and mass transfers are important in many processes such as evaporative cooling for waste
heat disposal, cooling of a high temperature surface by coating it with a phase-change material,
liquid film evaporators, turbine blade cooling, combustion premixing, and distillation of a volatile
component from a mixture with in volatiles. The evaporation of liquid film in air is an essential
physical phenomenon that may be encountered in the most diverse fields. Because of its practical
importance, a large number of theoretical analyses have been made concerning the evaporation of
liquid film.
A vast amount of works relating to water have been conducted by many authors. Chang et al.
(1986) solved the Navier-Stokes equations using the thin shear flow assumption. The wall of the
______________________________________________________________________________________________________________________________
*Corresponding e-mail: [email protected]
1 Laboratoire Génie Energie, Matériaux et Systèmes (LGEMS), Ibn Zohr University ENSA B.P. 1136, Agadir,
Morocco.
59
Abderrahman Nait Alla, M’barek Feddaoui, and Hicham Meftah / American Journal of Heat and Mass Transfer
(2015) Vol. 2 No. 2 pp. 59-75
tube was considered to be covered by a stationary water film at a specified uniform temperature.
This study was extended later by Chiang and Kleinstreuer (1991) by allowing the liquid film to fall
under the action of gravity and removing the thin shear flow assumptions. Both studies involve
Boussinesq approximation and the flow was assumed to be laminar. The evaporative cooling of
liquid film in mixed convection channel flows was explored by Yan et al. (1991); Yan and Lin
(1991). Their results show that the liquid film cooling is mainly caused by latent heat transfer
associated with its evaporation. The evaporation of the pure liquid film by mixed convection of
humid air in a vertical channel has been studied by Yan (1992). Their study shows that the
assumption of extremely thin film thickness is valid only for low mass flow rate. He et al. (1998)
was interested in cooling of the wall in a vertical tube uniformly heated by a water film. Turbulent
flow is considered in the gas flow and laminar in the liquid film. Two different modes of heat
transfer are identified. When water is supplied at relatively high temperature, the system operates
in evaporating mode. When it is low the system operates in the direct film cooling mode. Debbissi et
al. (2001) investigated numerically the coupled heat and mass transfers by natural convection
during water evaporation into air in a vertical heated channel by including radiative heat transfer
between plates. They observed that the evaporative cooling disturbs considerably the velocity and
temperature profiles in particular near the exit section of the channel. Feddaoui et al. (2001, 2003b)
studied the thermal transfer taking place in the event of film evaporation in the presence of a gas
flow inside a vertical heated tube. They reported that the evaporation is improved for a system with
higher heat flux and smaller inlet liquid film. They also treated the case of insulated wall for both
cylindrical configuration and vertical channel (Feddaoui et al. (2003a, 2006)) for examining the
effectiveness of evaporative cooling process.
As far as the evaporation of other pure liquids films was concerned, Tsay et al. (1990) undertook a
numerical and experimental study of cooling wall by using an ethanol film on a vertical plate in the
presence of a co-current gas flow. It was observed that the interfacial heat flux is predominantly
determined by latent heat transfer connected with film evaporation, and significant results are
obtained for the system with a high inlet liquid temperature or a low inlet liquid film. Senhaji et al.
(2009) conducted a numerical study of evaporating liquid film of pure alcohol by mixed convection.
They considered turbulent liquid film falling on the inner face of a vertical tube with a laminar flow
of dry air entering the tube with a constant temperature. Recently, Oubella et al. (2014) presents a
numerical simulation of the evaporation in laminar mixed convection along a vertical channel
formed by two parallel plates; one is isothermal and wetted by a thin liquid film of water or
acetone, the second plate is insulated. They showed that the dimensionless mass evaporating rate
increases noticeably with the decrease of inlet temperature and the use of more volatile liquid film.
Computational studies of liquid film evaporation for glycols have been carried out by a number of
investigators. Palen et al. (1994) conducted an experimental study in the case of mixtures of water ethylene glycol and water - propylene glycol in a vertical tube. They observed that for some
experimental conditions, the local heat transfer coefficient between the partition and the liquid
mixture can fall by 80% in relation to the relative value of the pure water, a value that is as low as
the one got with the pure ethylene glycol in the same conditions. Agunaoun et al. (1998) presented
a numerical analysis of the heat and mass transfer in a binary liquid film flowing on an inclined
plate. The most interesting results are obtained in mixed convection, particularly in the case of
ethylene glycol water mixture. Cherif and Daif (1999) considered the evaporation of a thin binary
liquid film by mixed convection in a vertical channel. They showed the importance of the film
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(2015) Vol. 2 No. 2 pp. 59-75
thickness and composition in the mass and heat transfers. Armouzi et al. (2005) investigated
numerically the evaporation by mixed convection of a binary liquid film flowing down of two
coaxial cylinders. They showed that the volatilities of the mixture influence the heat transferred
through the latent mode, which is more pronounced for a mixture composed of volatile
components. Nasr et al. (2011) performed a numerical analysis of the evaporation of binary liquid
film. The film is falling down on one plate of a vertical channel under mixed convection. The first
plate of a vertical channel is externally submitted to a uniform heated flux while the second one is
dry and isothermal. The liquid mixture consists of water and ethylene glycol while the gas mixture
has three components: dry air, water vapour and ethylene-glycol vapour. Recently, Debbissi et al.
(2013) investigated numerically the evaporation of a binary liquid film flowing on one of two
parallel vertical plates under mixed convection channel. This study shows that in some operating
conditions, the water evaporates better when mixed with ethylene glycol than water alone.
It is important to note that the evaporation of the pure substances such as ethylene or propylene
glycols have not received much attention, despite their importance in industry as the most common
antifreeze fluids. In addition most of the studies conducted are not treating the effect of pressure of
the surrounding gas on the evaporation phenomena. However, when the gas enters with a high
pressure, this will decrease the difference between the gas pressure and its saturation pressure and
consequently a disadvantage for the evaporation phenomena. All these considerations motivate the
present work by analysing the coupled heat and mass transfers during the evaporation of ethylene
glycol and propylene glycol. In the following, the physical and mathematical model will first be
presented, and then the numerical approach will be described.
2. Analysis
2.1 Physical model and assumptions
We consider a glycol liquid film falling along the internal face of a vertical channel (Fig 1). The liquid
flows down over the left wall with an inlet liquid temperature TL 0 and inlet liquid mass flow rate 0 .
The flow of dry air enters the channel at temperature T0 and constant velocity u 0 .
For mathematical formulation of the problem, the following simplifying assumptions are taken into
consideration:
• Vapour mixture is ideal gas.
• The gas flow is laminar.
• The vapour and liquid phases are in thermodynamic equilibrium at the interface.
• Radiation heat transfer, viscous dissipation and other secondary effects are negligible.
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(2015) Vol. 2 No. 2 pp. 59-75
Fig. 1. Physical model
2.2 Governing equations
2.2.1 Liquid phase
-
-
-
Continuity equation
  L u L    L v L 

0
x
y
(1)
Momentum equation
  L u L u L    L u L v L 
dp   u L 



L
 L g
x
y
dx y 
y 
Energy equation
  L C pLu L TL 
x

  L C pL v L TL 
y

  TL 
L
y 
y 
(2)
(2)
2.2.2 Gas stream
-
-
-
Continuity equation
 G uG   G vG 

0
x
y
(4)
Momentum equation
  G u G u G    G u G v G 
dp   u G 



G
 G g
x
y
dx y 
y 
(5)
Energy equation
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 G C pGuGTG   G C pGvGTG    TG  
w

 G

 G D(C Pv  C pa )TG

x
y
y 
y  y
y

-

(6)
Concentration equation of vapour
 G uG w  G vG w  
w 

 G D 
x
y
y 
y 
(7)
In the above equations  , u , v , T , w ,  ,  and D , respectively, stand for the density, axial
velocity, transverse velocity, temperature, mass fraction of vapour, dynamic viscosity, conductivity
and mass diffusivity; C pG , C pv and C pa are the specific heat of gas, glycol vapour and air,
respectively, and g is the gravitational acceleration. The subscripts L and G denote the liquid
and gas phases, and 0 the condition at inlet. The pressure gradients in equations (2) and (5) can be
written as:
p p d

 0 g
x
x
Thus the term:
p
p

 g   d  (    0 ) g
x
x
where the term    0 g represents the buoyancy forces due to the variations in temperature and
concentration.
2.2.3 Boundary and interfacial conditions
The boundary conditions are:
- x  0:
;
uG  u 0
- y  0:
u L  vL  0
- yd:
uG  vG  0
;
TG  T0
TL
;
0
y
TG
;
0
y
w  w0
;
;
TL  TL 0
w
0
y
The interfacial matching conditions specified at y   x are described as follows:
- Continuities of velocity and temperature:
;
u I  x   uG , I  u L , I
TI x   TG , I  TL, I
(8)
- Continuity of shear stress
 u 
 u 
  

 y  L , I  y  G , I
 I  
(9)
Transverse velocity of the air-vapor mixture is deduced by assuming that the interface is
semipermeable (Eckert and Drake (1972)) that is, the solubility of air in the liquid film is negligibly
small and the y-directed velocity of air is zero at the interface:
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(2015) Vol. 2 No. 2 pp. 59-75
D w
1  wI  y
vI  
(10)
By assuming the interface is in thermodynamic equilibrium and the air-vapor mixture is an ideal
gas mixture, the mass fraction of water or glycols vapor at the interface can be evaluated by the
relation:
wI 
M v pv ,i
M a  p  pv ,i   M v pv ,i
(11)
where pv ,i , is the partial pressure of vapour at the interface, and M v , and M a , are, respectively,
the molar masses of vapour and air;
- Heat balance at the interface implying


T 
T 
 I h fg
  y     y   m

 L,I 
 G ,I
 I , the vapour generation rate ( G vI )
where h fg is the enthalpy of evaporation and m
(12)
The local heat exchange between the air stream and liquid film depends on two related factors: the
interfacial temperature gradient on the air side results in sensible convective heat transfer, and the
evaporative mass transfer rate on the liquid film side results in latent heat transfer. The total
convective heat transfer from the liquid film to the air stream can be expressed as follows:

T 
 I h fg
q I  qSI  q LI   
m
y  G , I

(13)
The local Nusselt number at the interface defined as:
hT 2d 
Nu x 
Can be written as:
G

q I 2d 
G TI  Tb 
Nu x  Nu S  Nul
(14)
(15)
where Nu S , and Nul , are, respectively, the local Nusselt numbers for sensible and latent heat
transfer, and are defined as:
Nu S 
qSI 2d 
G TI  Tb 
(16)
Nul 
q LI 2d 
G TI  Tb 
(17)
And
The local Sherwood number is defined as:
Shx 
 1  wI 2d 
hM 2d  m
 I
D
 G DwI  wb 
(18)
where the subscript b denote the bulk quantities, the local bulk temperature Tb and the bulk mass
fraction wb in the channel are respectively defined as follows:
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(2015) Vol. 2 No. 2 pp. 59-75
d
Tb 
 
G
C pGuGTdy
(19)
x
d

G
C pGuG dy
x
And
d
wb 

G
uG wdy
x
(20)
d

G
uG dy
x
At every axial location, the overall mass balance in the gas flow and liquid film should be satisfied:
d
x
d   0  G u0    G uG dy   G vI dx
x
x
x
0
0
0    L u L dy    G v I dx
(21)
0
(22)
To improve the understanding of heat and mass transfer process, a cumulative evaporation rate is
introduced:
x
x
0
0
 I dx    G vI dx
Mr   m
(23)
Note that in the above formulation, the variations of the thermophysical proprieties with
temperature and air-vapour composition are included. They are calculated from the pure
component data by means of mixing rules applicable to any multicomponent mixture. For further
details, the thermophysical proprieties are available in (Reid and Sherwood (1984)).
3. Numerical Method
The conjugated problem is defined by the parabolic systems equations, (Eqs. 1–7) with the
appropriate boundary conditions are solved by a finite difference numerical scheme. Since the
impossibility of obtaining an analytic solution for the non-linear coupling differential equations, the
axial convection terms are approximated by the backward difference and the transversal
convection and diffusion terms are approximated by the central difference. Each system of the
finite-difference equations forms a tridiagonal matrix equation which can be solved by the Thomas
algorithm ( Patankar (1980)).
3.1 Marching procedure
After specifying the flow and thermal conditions, the numerical solution is advanced forward and
step by step as follows:
1. For any axial location x , guesses
dp d
and  x .
dx
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(2015) Vol. 2 No. 2 pp. 59-75
2. Solve the finite-difference forms of equations (1)-(7) simultaneously for u , T and w .
Numerically integrate (1) and (4) to find v .
3. Check the mass conservation of both liquid film and gas flow by examining the satisfaction of the
equations (21) and (22). If not, adjust
dp d
and  x and repeat procedures 2 to 4.
dx
4. Check the satisfaction of the convergence of velocity, temperature and mass fraction. If the relative
error between two consecutive iterations is small enough, i.e.:
max  in, j  in, j 1
max 
n
i, j
 10 4
(24)
where  represents the variables u , T and w . If not, repeat procedures 1 to 5.
3.2 Stability mesh
To obtain enhanced accuracy in the numerical computations, grids are chosen to be non-uniform in
both axial and transverse directions. Accordingly the grids are compressed towards the interface
gas-liquid and towards the entrance of channel. Grid independence tests were carried out by
comparing the total heat transfer of ethylene and propylene glycol for different values of I, J and K
(Table 1). It is noted that the differences in the cumulative evaporation rate from computations
using grids ranging from 51 51  21 to 201 201  61 were always less than three percent. In
light of those results all further calculations were performed with the 101 101  41 grid.
In order to fix the position of the liquid-gas interface, the cartesian y coordinate transformed into 
coordinates:
-

-

y x
x
for
0  y  x
y x
for  x  y  d
d x


Table1 Comparisons of cumulative evaporation rate Mr  10 5 kgm1s 1 for various grid
arrangements and T0  300 K , TL 0  400K , 0  0.02kgm s , Re  2000, p0  1atm,
d  0.02m .
51 51  21
51 101  21
101 101  41 201 101  61 201 201  61
E. gly
P. gly
E. gly
P. gly
E. gly
P. gly
E. gly
P. gly
E. gly
P. gly
1.78
2.82
1.92
4.12
1.88
3.00
1.80
2.86
1.92
3.04
5.96
8.94
6.82
11.04
6.14
9.86
5.98
9.58
6.28
10.00
12.70 20.76 13.86 22.50 13.36 21.50 12.74 20.48
13.16
21.10
20.86 33.82 22.20 35.78 21.78 34.92 21.62 34.60
22.12
35.32
26.90 43.96 28.10 44.90 27.92 44.38 27.80 44.32
28.16
44.86
31.82 51.50 33.16 52.50 33.16 52.28 33.04 52.26
33.22
52.46
1 1
x
d
1.09
5.07
16.87
41.03
66.90
96.08
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I: total grid points in the axial direction; J: total grid points in the transverse direction at the gas
side; K: total grid points in the transverse direction at the liquid side
3.3 Comparison with previous studies
To further check the adequacy of the numerical scheme for the present study, the results of the
present model were compared with study of Yan (1992). The solutions agree well between the
present predictions and those of Yan (1992) as shown in figure 2 which presents the axial evolution
of local Nusselt number. In view of these validations, the present numerical algorithm and
employed grid layout are adequacy to obtain accurate results for practical purposes.
200
Present prediction
Simulation of Yan [Yan, 1992]
160
-2
qW=500W.m
qW=1000W.m
Nux
120
-2
qW=3000W.m
-2
80
40
0
0.00
0.04
0.08
0.12
0.16
0.20
2.x/(b.Re)
Fig. 2. Comparison of our present work with those of Yan (1992). validation of calculated Local
Nusselt number along the channel for Re  2000, T0  20C
4. Results and Discussions
In this study, the results were obtained for impute parameters. The range of each parameter for the
analysis is listed in Table 2. The chosen common parameters are the length of the channel L  2m ,
the dry air at T0  300 K and the width of the channel d  0.02m .
Table 2 The ranges of the physical parameters.
Inlet liquid temperature, TL0 K 
Inlet pressure, p0 atm 

Inlet liquid mass flow rate, 0 kgm1s 1
Inlet Reynolds number, Re

360, 400
1, 2
0.01, 0.02
1000, 2000
For comparing the evaporation of some industrial fluids such as ethylene glycol and propylene
glycol, it is of interest to study the evolution of some physical properties which control the
evaporation phenomena. Figure 3a presents the effect of the liquid temperature on the evolution of
saturated pressure for two glycols liquids ethylene and propylene. It is clear that the saturated
pressure of propylene glycol is more important than ethylene glycol, consequently propylene glycol
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(2015) Vol. 2 No. 2 pp. 59-75
is more volatile than ethylene glycol. The effect of the liquid temperature on the specific heat
capacity of the two liquids is illustrated in figure 3b. It is apparent that the specific heat which
represents the amount of heat per unit mass required to raise the temperature by one degree
Celsius is higher for propylene glycol. This implies that the specific heat for propylene is greater
than ethylene. Figure 4 shows the development of the predicted velocity profiles along the channel
of glycols film. The velocity profiles develop gradually from the uniform distributions at the inlet
towards the parabolic ones when the flow goes downstream, with the maximum velocity shifting
towards the insulated wall. The results indicate that in the gas flow, velocity profiles of ethylene
and propylene glycol are similar except a small difference in the maximum velocity. Indeed,
  0.65 speed ethylene glycol is higher than a propylene glycol. This is due to the lower density of
ethylene glycol than propylene glycol. Figure 5 presents the distribution of axial temperature
profiles along the channel for various sections. In the gas flow the temperature increases as the flow
goes downstream which is due to the energy supplied by the liquid film to the gas stream. The later,
explains the liquid temperature decreases from the inlet to the outlet channel. It is interesting to
observe that distribution of axial temperature profiles along the channel of glycols film develop in
very similar fashion. This is apparently due to the small difference between the specific heats of the
two glycols studied. In the liquid phase, the temperature seems to be constant. This can be
explained by the fact that the liquid film is extremely thin, so the distribution of temperature inside
the liquid phase in the transverse direction is approximately constant, except at high inlet liquid
film (liquid flowing turbulently) when a slight decrease will be noticed from the temperature at the
wall to that at the gas-liquid interface. Figure 6 shows the evolution of the mass fraction of glycols
in the gas for different sections of the channel. Also it is noted that the mass fraction of propylene
glycol is higher than the mass fraction of the ethylene glycol close to the interface liquid-gas. This
increase is much more noticeable for more volatile component, i.e. propylene glycol than ethylene
glycol.
4000
4
(a)
3500
CPL(J/kg/K)
5
PSatX10 (Pa)
3
2
3000
2500
1
0
Ethylene glycol
Propylene glycol
(b)
Ethylene glycol
Propylene glycol
300
350
400
T(K)
450
500
2000
300
350
400
450
500
T(K)
Fig. 3. Evolution of thermophysicals propertises of propylene glycol and ethylene glycol with
thetemperature of: (a) Saturated pressure, (b) Specific heat
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(2015) Vol. 2 No. 2 pp. 59-75
0.0
0.2
Liquid
Gas
0.6
-1
u (ms )
0.4
0.8
Ethylene glycol
Propylene glycol
1.0
6.27
1.2
20.08
1.4
96.08
1.6
-1.0
-0.5
0.0
0.5

1.0
Fig. 4. Distributions of axial velocity profile for TL 0  400K , Re  2000 and 0  0.02kgm1s 1
420
Liquid
Gas
400
380
b (n=1)
96.08
T(K)
360
20.08
340
6.27
Ethylene glycol
Propylene glycol
320
300
-1.0
Fig.
5.
Distributions
of
axial
-0.5
temperature
0.0
0.5

profile
1.0
for TL 0  400K ,
Re  2000 and
0  0.02kgm1s 1
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0.25
Gas
0.20
Ethylene glycol
Propylene glycol
w
0.15
0.10
6.27
0.05
20.08
96.08
0.00
0.0
0.2
0.4
0.6

0.8
1.0
Fig. 6. Distributions of axial mass fraction profiles for TL 0  400K ,
Re  2000 and
1 1
0  0.02kgm s
340
0
20
350
360
TI (K)
370
TI (K)
380
390
340
0
400
20
P0=1atm - TL0=400K
360
370
380
390
400
P0=1atm - TL0=400K
P0=2atm - TL0=400K
P0=2atm - TL0=400K
40
350
P0=1atm - TL0=360K
40
P0=1atm - TL0=360K
x/d
x/d
60
(a) Ethylene glycol
60
80
80
100
100
(b) Propylene glycol
Fig. 7. Distributions of temperature at the gas-liquid interface along the channel for (a)
Ethyleneglycol (b) Propylene glycol, for Re  2000 and 0  0.02kgm1s 1
70
(2015) Vol. 2 No. 2 pp. 59-75
0.00
0
0.05
WI
0.10
0.15
0.20
WI
0.25
0.00
0
20
0.05
0.10
100
0.25
P0=1atm - TL0=360K
60
P0=2atm - TL0=400K
P0=1atm - TL0=400K
P0=1atm - TL0=400K
P0=2atm - TL0=400K
40
x/d
P0=1atm - TL0=360K
40
x/d
80
0.20
20
(a) Ethylene glycol
60
0.15
80
100
Fig. 8. Distributions of mass fraction at the gas-liquid interface along the channel for (a) Ethylene
glycol (b) Propylene glycol, for Re  2000 and 0  0.02kgm1s 1
For more detailed analysis of heat and mass transfer characteristics, our attention is focused on the
effect of the inlet pressure and inlet liquid temperature. Figure 7 shows the effect of the inlet
pressure of the gas and inlet liquid temperature on the interfacial temperature along the channel. It
is noticed that the interfacial temperature decreases as the flow goes downstream. This decrease is
due to the heat transfer from the liquid film to the gas flow. Also the interfacial temperature profiles
for two glycols are develop in very similar fashion except some differences in the output of the
channel. It is clear that the interfacial temperature decreases from the inlet to the exit channel for
the lower pressure, which is evident for the perfect gas assumed in this study. Indeed when the gas
enters with a high pressure, this reduces the intermolecular distance and reduces the evaporation.
The distributions of the interfacial mass fraction of glycols vapour along the gas-liquid interface are
given in figure 8 for various inlet pressure and inlet liquid temperature. It is shown that the
interfacial mass fraction decreases with small inlet liquid temperature or a high inlet pressure.
Indeed, for a fixed value of inlet pressure, we observed that the interfacial mass fraction is high for
high inlet liquid temperature. The interfacial mass fraction increases with the decrease of inlet
pressure whereas the reverse trend is noticed for the effect of the inlet liquid temperature along the
channel. This is obviously due to the enhancement of the evaporation in the operating conditions.
71
(2015) Vol. 2 No. 2 pp. 59-75
-2
qS(Wm )
-2
0
qS(Wm )
200
400
600
800
0
0
0
20
20
P0=1atm - TL0=400K
40
400
60
800
P0=2atm - TL0=400K
x/d
P0=1atm - TL0=360K
600
P0=1atm - TL0=400K
40
P0=2atm - TL0=400K
x/d
200
P0=1atm - TL0=360K
60
(a) Ethylene glycol
80
80
100
100
Fig. 9. Axial evolution of sensible heat flux for (a) Ethylene glycol (b) Propylene glycol, for
Re  2000 and 0  0.02kgm1s 1
-2
-2
0
100
200
qL(Wm )
300
400
500
0
600
0
0
20
20
100
200
qL(Wm )
300
400
500
600
P0=1atm - TL0=400K
40
P0=2atm - TL0=400K
P0=1atm - TL0=360K
x/d
60
(a) Ethylene glycol
40
x/d
P0=1atm - TL0=400K
P0=2atm - TL0=400K
60
P0=1atm - TL0=360K
80
80
100
100
Fig. 10. Axial evolution of latent heat flux for (a) Ethylene glycol (b) Propylene glycol, for
Re  2000 and 0  0.02kgm1s 1
72
(2015) Vol. 2 No. 2 pp. 59-75
-1 -1
-1 -1
Mr (kgm s )
0.00
0
0.01
Mr (kgm s )
0.02
0.03
0.00
0
0.01
0.02
0.03
P0=1atm - TL0=400K
(a) Ethylene glycol
20
20
P0=2atm - TL0=400K
P0=1atm - TL0=360K
P0=1atm - TL0=400K
40
x/d
P0=2atm - TL0=400K
P0=1atm - TL0=360K
40
x/d
60
60
80
80
100
100
Fig. 11. Axial evolution of cumulative evaporation rate for (a) Ethylene glycol (b) Propylene glycol,
for Re  2000 and 0  0.02kgm1s 1
In figure 9 we present the axial distribution of the sensible heat flux at the interface. It is
remarkable that the heat transfer by sensible mode decreases monotonically for both glycols for
( x d  20) and tends towards an asymptotic value at the channel exit. The later, can be explained
by the decrease of the gradient of temperature between the hot liquid film and the cold gas as the
flow goes downstream. It is interesting to observe that the distribution of sensible heat flux at the
interface along the channel develop in very similar fashion for the two glycols. This is apparently
due to the small difference between the specific heat of the two studied glycols (Fig 3b). It is also
clear that the sensible heat q S increases whit the increase of the inlet liquid temperature. Whereas,
the effect of the inlet gas pressure on the sensible heat transfer is not very pronounced.
The effects of TL 0 and P0 on the evolution of latent heat transfer are illustrated in figure 10. The
first observations to be drawn from these curves indicate that the glycols are varied in similar
fashion, with a lower increase for propylene than ethylene. This is due to the higher saturated
pressure of propylene compared to ethylene (Fig 3a) . It is also noticed that latent heat transfer is
higher for a high inlet liquid temperature and lower inlet pressure, indicating the evaporation
enhancement in theses operating conditions. It remains to notice that the heat transfer by
evaporation is lower than the heat transfer by sensible mode due to temperature gradient. This
ascertainment is confirmed by figure 11 which represents the evolution of the cumulative
evaporation rate along the channel. It is clear that the evaporation rate increases when we increase
the inlet liquid temperature of the liquid film. Indeed, when the liquid film enters at 400K , it is
close to its boiling point and hence the evaporation is optimal. However, despite a higher inlet
liquid temperature, the effect of the pressure is remarkable. For TL 0  400 K and P0  1atm we can
reach more than 2.7% for propylene glycol and 1.7% for ethylene glycol, whereas for other inlet
liquid temperature and inlet pressure, the cumulated evaporation rate does not exceed 0.8% . In
73
(2015) Vol. 2 No. 2 pp. 59-75
addition, there is a significant difference between the evaporation of the propylene glycol and
ethylene glycol, which apparently due to the higher volatility of propylene glycol compared to
ethylene glycol as clearly seen from (Fig 3a).
5. Conclusion
The analysis presented in this paper has been developed by comparing the evaporation of two glycols
liquid film: ethylene glycol and propylene glycol along a vertical channel. The effect of inlet liquid film
temperature and inlet pressure on the momentum, heat and mass transfer in the two phases are
examined in detail. Based on the numerical results obtained, the following conclusion can be drawn:
(1). In the operating system, the heat transfer by sensible mode is more important than latent
mode.
(2). Heat and mass transfers by latent mode are more enhanced at lower pressure.
(3). Evaporation of glycols is improved for a system with higher inlet liquid film temperature.
(4). Propylene glycol evaporates in more intense way in comparison to ethylene glycol, due to the
volatility difference.
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