Local Distribution of the Heat Transfer in Water Spray Quenching

Local Distribution of the Heat Transfer in Water Spray
Quenching
F. Puschmann, E. Specht, J. Schmidt
University of Magdeburg, Institute of Fluid Dynamics and Thermodynamics,
Magdeburg
Introduction
In most cases efforts are made to use continuous casting water spray quenching to
achieve uniform cooling with a locally independent heat transfer. However, the heat
transfer coefficient obtained in this cooling task with spray water depends on various
conditions. Among them, the surface temperature of the material subjected to cooling
and the characteristics of the spray generated by nozzles exert an influence on the heat
transfer coefficients achieved /1/-/3/. The spray characteristics for full-cone nozzles, e.g.
impingement density, drop diameter and drop velocity, in a defined plane in front of the
nozzle are a function of the radial distance, even in the ideal case of a symmetrical spray
jet. In the continuous casting process materials are frequently cooled by means of panels
of flat-spray nozzles, often causing areas without water impingement and overlap areas
of individual nozzles. The results of the locally non-uniform spray characteristics and
interactions in nozzle panels are that water is not applied in a uniform way to the entire
material subjected to cooling and, hence, cooling intensities vary locally. Investigations
focussed on determining the influence exerted by surface temperature, water
impingement density, drop diameter and drop velocity on local heat transfer in water
spray quenching in the film boiling range.
Experimental Set-up
Drop Diameter and Velocity
The measuring set-up sketched in Fig. 1 was constructed to investigate drop size and
velocity. It served to measure the distribution of drop sizes and drop velocities of water
sprays by means of a 2D-Phase-Doppler-Anemometer (PDA). The PDA is an optical
measuring system which is suited to performing non-contact and simultaneous
measurements of the velocity and the diameter of spherical particles. The sensing
volume of the PDA is fairly small, ensuring high resolution in terms of time and area. It
is able to identify individual drops passing through the sensing volume. Due to its high
laser performance of 4W it is an excellent device for performing measurements in misty
and steamy environments.
Figure 2 depicts the distribution of drop size and drop velocity for a flat-spray
nozzle as used in continuous casting. The mean volumetric diameter is presented as a
function of the measuring position. The measuring plane was located at a distance of
200mm in front of the nozzle. The measuring position was the distance measured across
Local Distribution of the Heat Transfer in Water Spray Quenching
2
the length of the flat jet from its centreline. A pressure of 5bar was applied to the nozzle
resulting in a water flow rate of 318kg/h. The mean volumetric diameter was about
110µm in the centre of the spray jet. The drop size increased towards the border.
Reducing towards the border, the mean drop velocity was about 6.4m/s. It should be
noted that the aperture angle of the flat-jet nozzle was 105° and, hence, the overall
length of the spray jet in the measuring plane was about 520mm. The figure shows a
section of the centre exhibiting a width of 200mm.
Air
Water
Nozzle
Probe volume
Laser (max. 4 W)
30 ° (off-axis angle)
Transmitter
Receiver
Analysis unit
150
7
140
6,5
130
v
6
120
D30
5,5
110
100
90
5
Nozzle: D25381-13-105/20-1
Measuring planes: 200 mm
Nozzle pressure: 5bar
Flow rate: 318kg/h
80
-100
-50
Mean drop velocity [m/s]
Mean volumetric drop diameter [µm]
Figure 1. PDA System
4,5
0
50
4
100
Measuring position [mm]
Figure 2. Drop Size and Velocity
Impingement Density
A patternator as depicted in Figure 3 was used to measure the water impingement
density. The water drops of a spray were collected by means of collecting tubes, which
3
Puschmann, Specht, Schmidt
were arranged in the spray jet and exhibited a diameter of DR=10mm, over a period ∆t.
The amount of water mW collected can be used in the equation
4 ⋅ mw
(1)
m& S =
∆t ⋅ π ⋅ D R2
to compute the water impingement density. The water impingement density can be also
measured by means of the PDA, but due to the high error rate is necessary to employ
another measuring system, i.e. a patternator as in this case.
Figure 4 shows the distribution of water impingement density obtained with the flatspray nozzle as a function of the measuring position. As can be seen, the mean
impingement density amounting to 4.5kg/m2/s is fairly constant under the operating
conditions described above, forming individual skeins at the nozzle. The results of two
measurements are shown to document the reproducibility of the measuring results
obtained with this measuring procedure.
Water spray
Patternator
Collective
containment
Figure 3. Patternator System
Water impingement density [kg/m²/s]
5,5
5,3
5,1
4,9
4,7
4,5
4,3
4,1
Nozzle: D25381-13-105/20-1
Measuring planes: 200 mm
Nozzle pressure: 5bar
Flow rate: 318kg/h
3,9
3,7
3,5
-50
-30
-10
10
Measuring position [mm]
Figure 4. Impingement Density
30
50
Local Distribution of the Heat Transfer in Water Spray Quenching
4
Heat Transfer
The measuring procedure presented in Figure 5 was developed to determine the heat
transfer within a fairly short period of time and with locally high resolution /4/. It is
based on determining the surface temperature by means of infrared thermal imaging. To
determine heat transfer, a thin metal sheet was arranged in front of the spray-generating
nozzles and supplied with a constant electric current. The water leads away the heat
from the hot metal sheet surface. In a stationary measuring process the metal sheet
temperature assumed a value as a function of the local heat transfer coefficient. The
higher this coefficient, the lower was the local metal sheet temperature. In a nonstationary measuring process the metal sheet was heated to an initial temperature
without being cooled by water spray. Subsequently, the spray jet was released cooling
down the sheet. Due to the low thickness of the metal sheet of 0.1mm to 0.3mm both
measuring procedures yielded an almost identical temperature distribution on both the
side sprayed on and the side not sprayed on. The local distribution, and time distribution
in the non-stationary case, of the surface temperature of the non-approached side were
recorded by means of an infrared camera. On that relevant side the sheet exhibited a
specific coating with an emission capability that had been determined before as a
function of temperature. By using a telephoto lens with a supplementary lens a local
temperature resolution of up to 0.2mm/pixel can be achieved. The local distribution of
the heat transfer coefficient can be calculated from the temperature distribution. The
difference between the surface temperature distribution measured on the rear side and
the required surface temperature distribution can be determined through a numerical
solution of the problem of thermal conduction. Thereby the multidimensional
conduction of heat within the sheet was taken into account.
PDA-Analysis unit
PDAReceiver
PDAData
Metal sheet
0.1 - 0.3 mm
IR Analysis
unit
IRpicture
Registration
of operation
conditions
Nozzle
150 l/h
5bar;22°C
IR Radiation
Transmitter
Pump
IR Camera
0.2 mm/Pixel
300 A
Direct current
source
Water
Laser (4 Watt)
Figure 5. IR System
Stationary measuring procedure
In the stationary measuring case surface temperature distribution assumed a constant
value under cooling conditions which was recorded by means of an infrared camera.
This surface temperature distribution can be used to compute the distribution of the heat
5
Puschmann, Specht, Schmidt
transfer coefficient. The heat flux resulting from the current flow through the metal
sheet and, hence, from the source of heat applied, is calculated by means of the electric
power Pel supplied and the area A of the metal sheet as follows
P
I 2 ⋅ R  ρ el 
I2
q& H (ϑ H ) = el =
=
⋅
(
ϑ
)
,
(2)
H

A
b⋅l
b2
 s 
where ϑH is the corrected metal sheet temperature, I the electric current passed through
the metal sheet, R the electrical resistance of the metal sheet, (ρel/s) the temperaturedependent specific resistance of the metal with reference to the sheet thickness s, and b
the width of the sheet. The specific resistance of the metal sheet was established in
special measurements as a function of temperature.
During the measurement the sheet was not only cooled by the spray jet. Also radiation
of energy and a convective heat transfer must be considered under the conditions of
high temperatures. This heat transfer is included as the heat loss. Hence, the heat flux
q& Sp (ϑH) leaded away by the spray yet is calculated by
q& Sp (ϑ H ) = q& H (ϑ H ) − q&V (ϑ H ),
(3)
where q&V (ϑH) is the temperature-dependent heat loss. Hence, the obtained heat transfer
coefficient αSp can be calculated using the corrected surface temperature ϑH and the
spray jet temperature ϑSp using the following equation
q& Sp (ϑ H (x, y ))
.
(4)
α Sp (x, y ) =
ϑ H (x, y ) − ϑ Sp
(
)
Non-stationary measuring procedure
Under the conditions of high heat transfer coefficients and high surface temperatures it
is difficult to obtain and keep a stationary operating point and in some cases it is even
impossible due to the limits of the electric power available. Hence, heat transfer
coefficients were determined by means of non-stationary techniques under the
conditions of high heat flux. To this end, the metal sheet was heated to an initial
temperature supplying a constant current and, subsequently, cooled down using a spray
jet. The time dependent distribution of temperature on the metal sheet surface was
measured.
For calculating the total heat transfer coefficients α with neglection of conduction
the differential equation
dϑ
ρ M ⋅ V ⋅ c M ⋅ H − Pel = α ⋅ A ⋅ ϑ Sp − ϑ H
(5)
dt
can be established through an energy balance at the metal sheet. Here, ρM is the density
of the metal sheet, V its volume, and cM its specific thermal capacity. Dividing by the
metal sheet area and converting the ratio Pel/A as described above, we obtain
dϑ
I2 ρ 
ρ M ⋅ s ⋅ c M ⋅ H − 2 ⋅  el  = α ⋅ ϑ Sp − ϑ H .
(6)
dt
b  s 
With the well known time dependent surface temperature it is possible to compute the
total heat transfer coefficient. This coefficient is corrected by heat losses. The IRCamera operates in line-scan-modus with a data rate of 2500Hz. In this case a high
resolution in time of surface temperature is obtained.
(
)
(
)
Local Distribution of the Heat Transfer in Water Spray Quenching
6
Results of Measurements
In preliminary investigations the nozzles were characterised. To this end, various
operating points of different nozzles were used to determine the drop size and drop
velocity distributions as well as the water impingement density. In the measurements
nozzles were examined which were different in only one of the measured variables
indicated. Thus, nozzles were examined which exhibited different drop velocities under
the conditions of an identical water impingement density and drop diameter distribution.
Establishing such operating points of nozzles, it is possible to examine the influence of
the drop diameter, drop velocity, water impingement density and surface temperature on
spray water quenching.
Influence of Drop Size
Figure 6 depicts the heat transfer coefficient as a function of the surface temperature,
the drop size serving as a parameter. Various nozzles were examined which were
different in drop diameter while producing identical water impingement densities and
identical drop velocities. The heat transfer coefficient was measured by means of a
stationary procedure. This procedure could be employed as the nozzles exhibited water
impingement densities which, amounting to about 0.25kg/m2/s, were fairly small and
suitable for the measuring procedure. As can be seen, in the investigated range the drop
diameter exerts no influence on the heat transfer obtained. The same dependence can be
established when analysing the influence of the drop diameter for other drop velocities.
Heat transfer coeffizient α [W/m²/K]
350
300
250
Nozzle 1: D30 = 107µm, w=3,7m/s
Nozzle 2: D30 = 46µm, w=3,7m/s
Nozzle 3: D30 = 71µm, w=3,7m/s
m& S = 0,25
kg
m2 ⋅ s
200
150
100
50
0
300
350
400
450
500
550
600
Surface temperature ϑ [°C]
Figure 6. Influence of Drop Size
Influence of Drop Velocity
Figure 7 shows the heat transfer coefficient as a function of surface temperature, the
drop velocity serving as a parameter. Two nozzles were examined which were different
in terms of velocity of the generated drops, while producing an identical water
impingement density of about 0.33kg/m2/s and an identical mean drop diameter of
66µm and 63µm with reference to the volume. As can be seen, the spray with the higher
7
Puschmann, Specht, Schmidt
mean drop velocity yielded a higher heat transfer coefficient. The heat transfer
coefficients were determined in a stationary measuring procedure.
Heat transfer coefficient α [W/m²/K]
350
Nozzle 4: D30=63µm, w = 6,7m/s
Nozzle 5: D30=66µm, w = 3,8m/s
300
250
200
150
m& S = 0,33
100
kg
m2 ⋅ s
50
0
300
350
400
450
500
550
600
Surface temperature ϑ [°C]
Figure 7. Influence of Drop Velocity
Influence of Water Impingement Density
Heat transfer coeffizient [W/m²/K]
450
Experiment
Fijimoto /6/
Müller / Jeschar /5/
400
350
300
250
200
150
ϑ = 550°C
w = 8m/s
D30 = 60µm
100
50
0
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
Water impingement density [kg/m²/s]
Figure 8. Influence of Impingement Density
Figure 8 presents the heat transfer coefficient as a function of the water impingement
density. Results of Müller/Jeschar /5/ and Fujimoto /6/ are also presented. The own
examinations were performed at a surface temperature of 550 °C, a drop velocity of
8m/s and a drop diameter of 60µm. It can be seen that the water impingement density
exerts a major influence on the heat transfer coefficients achieved. When the water
impingement density increases, the obtained heat transfer coefficient also increases. The
own examinations results higher heat transfer coefficients for constant impingement
density. In his investigations Fujimoto detected the same gradient of heat transfer
Local Distribution of the Heat Transfer in Water Spray Quenching
8
coefficient with increasing impingement density. The curve of Müller/Jeschar has a
lower gradient.
Influence of Surface Temperature
Figure 9 contains the heat transfer coefficient as a function of the water impingement
density. The surface temperature serves as a parameter. Operating points with constant
drop velocities and constant water impingement densities were examined at surface
temperatures of 350 °C, 450 °C and 550 °C. It can be seen that the heat transfer
coefficient achieved slightly decreases when the surface temperature increases and the
drop velocity and water impingement density remain constant.
Heat transfer coefficient [W/m²/K]
600
w = 8m/s
500
400
300
350°C
450°C
550°C
200
100
0
0
0,2
0,4
0,6
0,8
1
Water impingement density [kg/m²/s]
Figure 9. Influence of surface temperature
Conclusions
For performing heat transfer measurements with a locally high resolution an infrared
measuring device is available measuring the temperature distribution on an electrically
heated metal sheet. The metal sheet was cooled from the opposite side by means of
water spray. The water spray was analysed in terms of drop size, drop velocity and
water impingement density using a PDA. Examinations revealed that in the investigated
range the drop size has no influence on the heat transfer coefficient, whereas the surface
temperature exerts a low, drop velocity a bigger and water impingement density the
biggest influence. The investigated range of low impingement density has to be
enlarged to high impingement density used in continuous casing. With the known local
spray characteristics then the local heat transfer coefficients are computable. The
already investigated flat-jet nozzle used for cooling purposes in the continuous casting
process exhibited a fairly constant water impingement density. The drop velocity
reduces towards the border.
9
Puschmann, Specht, Schmidt
References
/1/
/2/
/3/
/4/
/5/
/6/
Jacobi, Kaestler, Wünnenberg; Heat transfer in cyclic secondary cooling during
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Köhler; Wärmeübertragung von heissen Oberflächen durch Wasserfilmkühlung im
Bereich der stabilen Filmverdampfung, Dissertation, TU-Clausthal, 1990
Mizikar; Sprya Cooling Investigation for Continous Casting of Billets and
Blooms; Iron and Steel Engineer (1970), S. 53-60
Boye, Schmidt; Einfluss von Oberflächentemperatur und Tropfenparameter auf
den Wärmeübergang bei der Sprühkühlung; Chem.-Ing.-Techn. 70, (1998), S
1177-1178
Müller, Jeschar; Untersuchung des Wärmeübergangs an einer simulierten
Sekundärkühlzone beim Stranggießverfahren, Archiv Eisenhüttenwesen, 44
(1973), S.589-594
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coefficient between spraying water and a hot surface above the Leidenfrost
temperature; ISIJ International 37 (5), 1997, S. 492-497