Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 15.7.1 - Rewetting of Heated Surfaces by Intermittently Bursting Liquid-Metal Contacts – An Experimental Study
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Title: 15.7.1 - Rewetting of Heated Surfaces by Intermittently Bursting Liquid-Metal Contacts – An Experimental Study Boiling
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Ilyas, M.
Hale, C.P.
Walker, S.P.
Hewitt, G.F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: rewetting
sputtering
cyclic bursting
explosive boiling
 Notes
Abstract: This paper describes an experimental study on the rewetting of a hot vertical surface by a descending liquid film. The transition between the upper wetted zone and the lower hot zone (the “rewetting front”) moves relatively slowly down the surface. As the liquid progresses from the wetted region to the dry region over a hot surface, it is expelled off the surface at the rewetting front as a result of violent boiling (“sputtering”). The use of a fast response infra-red thermal imaging system, for temperature measurement at a location near the rewetting front through an infrared transparent substrate coated with platinum, and a high speed video camera both demonstrated the existence of a cyclic bursting phenomenon. The bursting frequency depends on the inlet degree of subcooling of the feed fluid where as liquid flow rate has little effect on it. Multiple events of this type gradually remove heat from the metal allowing the rewetting front to progress slowly down the surface. It was possible to measure the temperature gradient over an axial length of several millimetres near the quench front. The temperature measurements also indicated that the metal surface temperature at the rewetting front was close to the homogeneous nucleation temperature, consistent with earlier measurements (Pereira 1997) using a different technique.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Resource Identifier: 1571-Ilyas-ICMF2010.pdf

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PaperNo 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Rewetting of Heated Surfaces by Intermittently Bursting Liquid-Metal Contacts An Experimental Study


Muhammad Ilyas*, Colin P. Halet, Simon P. Walker* and Geoffrey F. Hewittt

Department of Mechanical Engineering, Imperial College London, London, SW7 2AZ, UK
tDepartment of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK
m.ilvas06(Aimperial.ac.uk, g.hewittA(imperial.ac.uk



Keywords: Rewetting, sputtering, cyclic bursting, explosive boiling




Abstract

This paper describes an experimental study on the rewetting of a hot vertical surface by a descending liquid film. The
transition between the upper wetted zone and the lower hot zone (the ic ceilingg front") moves relatively slowly down the
surface. As the liquid progresses from the wetted region to the dry region over a hot surface, it is expelled off the surface at the
rewetting front as a result of violent boiling ("sputtering"). The use of a fast response infra-red thermal imaging system, for
temperature measurement at a location near the rewetting front through an infrared transparent substrate coated with platinum,
and a high speed video camera both demonstrated the existence of a cyclic bursting phenomenon. The bursting frequency
depends on the inlet degree of subcooling of the feed fluid where as liquid flow rate has little effect on it. Multiple events of
this type gradually remove heat from the metal allowing the rewetting front to progress slowly down the surface. It was
possible to measure the temperature gradient over an axial length of several millimetres near the quench front. The temperature
measurements also indicated that the metal surface temperature at the rewetting front was close to the homogeneous nucleation
temperature, consistent with earlier measurements (Pereira 1997) using a different technique.


Introduction

Rewetting of heated surfaces has many important
technological applications in cryogenic processes, filling of
liquefied natural gas pipelines, high temperature
metallurgy and the nuclear industry. It is one of the most
crucial phenomena to be considered for the safety analysis
of the design basis loss of coolant accident (LOCA) in
light water reactors (Pressurized Water and Boiling Water
Reactors, PWR's and BWR's). In such an accident, the
coolant is rapidly expelled from the reactor vessel through
a large break in a feed pipe and the nuclear fuel rods
undergo a drastic increase in their temperature. To mitigate
the consequences of LOCA, water is fed into the reactor
core via an emergency core cooling system; in the PWR
this water is fed to the core via the lower plenum ("bottom
re-flooding") and in the BWR this water is sprayed onto
the top of the core ("top re-flooding"). In the work
described here, the emphasis is on the top re-flooding case
but the detailed local mechanisms are believed to be very
similar for both cases. The rewetting process is a very
violent one with the rewetting front moving rather slowly
over the hot surface. If the water is at a temperature close
to saturation, the rate of progression of the rewetting front
is independent of the flow rate of the water approaching
the rewetting front; this is an indication of the fact that the
rewetting process is governed by events local to the
rewetting front (Bennet et al, 1966).
In top re-flooding, excess water is thrown off the
surface by a process known as sputtering. In bottom re-
flooding, there is a similar process, though in this case the


excess water is entrained as droplets in the flow of steam
generated at the rewetting front. This steam/droplet flow
passes over the hot surface above the rewetting front and
can cool it significantly (here, we call this process long
range precursory cooling in order to distinguish it from the
dramatic cooling events which occur close to the rewetting
front by the intermittent contact sequence which we will
describe below, and which we call short range precursory
cooling).
The great importance of the rewetting processes in the
reactor safety is reflected from the wide range of studies on
this subject. Reviews of this work are given in
(Butterworth and Owen 1975), (Collier 1982) and (Pereira
1997).
(Duffey and Porthouse 1973) performed the rewetting
experiments with tubes and rods with wide range of
material conditions. The physical processes involved in the
rewetting of high temperature surfaces have been shown to
be identical for the both falling water films and bottom re-
flooding. The variation of rewetting velocity with mass has
been shown to be independent of hydraulic diameter over a
range of practical interest. During the bottom flooding, a
well defined and clearly visible quench front followed by a
region of nucleate boiling was observed to move upwards
at nearly constant velocity.
(Pereira 1997) measured the temperature at the liquid-
metal interface by using plated gold-platinum micro-
thermocouple during the passage of the rewetting front. He
observed that intermittent liquid-metal contact occurs over
a very short axial range with a frequency of the order of 1
KHz for the initial wall temperature of 600 C. The






Paper No


rewetting front itself is at a temperature close to the
homogeneous nucleation temperature but the temperature
gradient away from the wall is too great to allow bubbles
to grow from ordinary nucleation centers in the immediate
region at and upstream of the front. The ordinary boiling
does not play a significant role in the rewetting process.
(Saxena et al. 2001) carried out experiments to study the
rewetting behavior of a vertical annular channel, with hot
inner tube, for bottom flooding and top flow rewetting
conditions. The experimental study showed that for a given
initial surface temperature of the tube, the rewetting
velocity increased with an increase in flow rate of water
and it decreased with an increase in the initial surface
temperature for a given water flow rate. The cold flooding
velocities were much higher than the rewetting velocities
for initial surface temperature of 2900C and more.
However, the rewetting velocities were close to cold
flooding velocities at initial surface temperature of 2000C.
The effect of flow rate was predominant at this
temperature. The rate of increase of rewetting velocity with
respect to flow rate was large at initial surface temperature
of 2000C, whereas it was much less at higher initial surface
temperatures. At a given flow rate of water, the rewetting
velocity decreased with an increase in initial surface
temperature.
(Mitsutake et al. 2003) measured transient boiling heat
transfer on a vertical surface of a rectangular
parallelepiped made of copper, brass and carbon steel at
300 C during bottom flooding. The heat transfer area is
divided into three regions, namely, film boiling, nucleate
boiling and natural convection. The wetting front,
separating non-wetted and wetted area, moved up behind
the rising water level. The distance between the water level
and the wetting front showed a tendency to be longer with
decrease in liquid subcooling or increase in flooding
velocity and thermal inertia pck of the block.
To understand the mechanism of heat transfer at the
quench front, it is necessary to carry out detailed
temperature measurements to quantify and exhibit the
intermittent contacts. With the thermal imaging system, it
was possible to perform these measurements. In what
follows, we first give a brief description of the
experimental set up; this is followed by results and
discussion. Conclusions are given in the last section.


Nomenclature


temperature (C)
thermal conductivity (W K 'm 1)


Greek letters
a thermal diffusivity (m2s-')

Subsripts
int interface
S solid
1 liquid


Experimental Study

In this work, the phenomenon of rewetting of a hot plate


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

by a falling film flow has been studied. The experimental
set up is shown in Fig. 1. The water tank (10) contains de-
mineralized water. An electrical heating system is fitted in
the tank to pre-heat the water at the desired temperature.
The pump (8) runs the water in a closed loop through a
control valve (9) to ensure steady flow rate and uniform
temperature of the inlet water. The water is directed to the
top of the test section (1) through the injection nozzle (5)
at different flow rates. The nozzle is filled with beads of
equal size to ensure uniform distribution of the water as it
feeds over the plate width. The flow rate meter (6)
measures the flow rate. Once the water was passed over
the plate, the drain water is collected in the tray (12).







7










13
--------4)






5 14
I






Figure 1: The experimental set up: 1. Stainless steel plate
2. Sapphire disk 3, 4. Copper clamps 5. Spray nozzle 6.
Flow rate meter 7, 9. Control valves 8. Pump 10. Water
tank 11. Power Supply 12. Sink 13. Infra red camera 14.
Computer



In order to heat the test section, there is a provision to
connect it to the power supply (11) through the copper
clamps (3) and (4). The top clamp (4) is held in place
firmly by two tufnol pieces while the bottom one is left
hanging loosely to allow thermal expansion for the test
section. Due to unavailability of the power supply (11) at
the time of experiment, an auxiliary heating system was
used to heat the test section. The uniformity of the test
section temperature was ensured by taking temperature
measurements at different axial locations using fast
response K-type thermocouples.
Figure 2 shows the details of the test section. It consists of
a 482 mm by 50 mm by 1.2 mm vertical flat stainless steel
(AISI 304) plate. Midway down the steel plate (1), a hole





Paper No


was drilled to accommodate the sapphire disk (2) with
dimensions of 10 mm (diameter) by 1 mm (thickness). To
measure the temperature at the liquid-solid interface using
infra red camera, the face of the sapphire disk opposite to
the camera was coated with the platinum (-0.2 ntm) by the
ion deposition method. The disk was mounted to be flush
with the surface of the plate so as to provide minimal
disturbance to the advancing rewetting front.


482








,, J


4I













2












3
-12-


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

1) are shown in Fig. 3. At an angle of 900 from the infra
red camera, a high speed video camera (at a rate of 2000
frames/sec) was used to film the phenomenon occurring
during the process of rewetting. The video images for the
above mentioned case are shown in the Figs. 7 and 8. An
important piece of information obtained from the visual
observation is that the liquid film is lifted-off as a result of
explosive vaporization at the quench front. The lifted-off
film behaves like harmonic wave and the front itself serves
as a simple harmonic oscillator. Images shown in the Figs.
7 and 8 indicate the wave like behavior of the lifted-off
liquid sheet.


h. .. .Al


Figure 3: Representative thermal images at a series of time
steps demonstrating the wetting of the sapphire disk


Results and Discussion

The effect of two flow parameters the flow rate and the
inlet degree of subcooling of the feed fluid has been
studies in this experiment. For this purpose, several runs
with different flow conditions were performed; the
experimental matrix is given in Tab. 1.


Table 1: Flow condition for the experiment


w
4- 50 -*


Figure 2: Bottom, side and top view of the test section with
sapphire disc embedded at the centre. All dimensions are in
millimetres

The test section was heated near to 400 OC prior to the start
of water flow over it. The sapphire disk was also heated up
to a temperature close to that of the surrounding stainless
steel; since sapphire has a thermal conductivity close to
stainless steel it would be expected that the behavior of the
descending wetting front passing over the disks would be
similar to its behavior in passing over the adjacent stainless
steel surface.
As the liquid flows over the plate on the front side, the
temperature was measured through the sapphire disc from
beneath the surface using Cedip Titanium 560 M infra red
thermal imaging system at a rate of 1000 frame per second.
The field of view of the infra red camera was 8.5 square
millimeters and the resolution was set to 30x30 pixels. The
thermal images for a typical case (serial number 3 in Tab.


Inlet degree of Flow Initial wall
S. No subcooling rate temperature
(C) (L/min) (C)

1 70 1.2 400

2 70 1.9 420

3 70 2.5 405

4 60 1.2 410

5 60 1.9 408

6 60 2.5 427

7 40 1.9 375

8 40 2.5 385

9 40 3.2 400






Paper No


400-


o 300-


- 200-

100


100 200 300 400 500
Time (ms)


Figure 4: Temperature
number 3 in Tab. 1)


390-


385-
0
380-

E
375-


600 700


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

temperature arising due to the impact of the wavy film
passing over the surface or droplets that are being
generated at the quench front far upstream. Figure 8 shows
the visual images when the quench front was far upstream.
From these images, the waves can be seen very clearly, the
troughs and crests are encircled. Although the surface
temperature is recovered after each impact, however, there
is an overall cooling of the surface due to these impacts.
This is called as long range precursory cooling.


tTime axis


trace for a typical case (serial


50 100 150 200
Time (ms)


STime axis


Figure 5: Detailed
illustrated in Fig. 4


370-





350-


temperature trace of the region I as


------------- ------








300 400
Time (ms)


Figure 6: Detailed temperature trace of the region II as
illustrated in Fig. 4


The temperature history plot for a typical case (serial
number 3 in Tab. 1) is shown in Fig. 4. The temperature
plot can be divided into three distinctive parts. In the part I,
the temperature stays constant near to the initial
temperature. Detailed temperature trace of the region I is
shown in Fig. 5. There are some fluctuations in


Figure 7: Video image indicating intermittent contact


On the arrival of the quench front near the point of
observation, noticeable rapid temperature fluctuations
characteristic of the intermittent contacts occur. Figure 6
shows a closer view of the temperature trace for the region
II. The fall and rise of the temperature in the successive
intervals demonstrates the intermittent liquid-metal


I III


\~-~Xlcywy~,,






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


contacts and the subsequent explosive vaporization.
Analysis of the video images confirms this, Fig. 7 shows
successive images near the quench front indicating cyclic
contact and subsequent interface lift-off as a result of
explosive boiling, circles are drawn to mark the contact
and the interface lift-off. The surface keeps on cooling
during each brief contact. The frequency of these cyclic
lifting is a characteristic of the liquid-metal combination
under the given flow condition. The cooling caused by
these intermittent contacts in this region is called short
range precursory cooling.


Time axis


Figure 8: Video image indicating wavy pattern of the
lifted-off film far down stream of the quench front


The maximum limit on the degree of superheating of the
liquid is 308 C at the atmospheric pressure (Carey 1992).
At this temperature, homogeneous nucleation starts and
water is vaporized instantly. The interface temperature for
the situation of the contact of the liquid with the hot metal
considering both of these as semi infinite at uniform initial
temperature is given as (Carslaw and Jaeger 1959):


Tfnr- TM *rX
kg
.4q=-5---g-


Here 'k' is the thermal conductivity and a is the thermal
diffusivity of the material. If the interface temperature is at
308 C or above, the high heat flux will cause a small layer
of the liquid heated to a temperature where homogeneous
nucleation starts. The expulsion of the interface results
due to explosive vaporization. However, if the
instantaneous liquid-metal contact temperature is below
308 C, there will be no homogeneous nucleation,
resulting in the fluctuations to die out.
Equation (1) can be used to predict the solid temperature at
which the homogeneous nucleation in liquid (in contact
with solid) seizes. Considering the liquid temperature close
to saturation, the solid (stainless steel) temperature comes
out to be 355 C. When the solid temperature approaches
to this value, the liquid is no longer lifted-off indicating
that the permanent wetting has been established.
Examination of the Fig. 6 reveals that the steepest
temperature gradient starts around this temperature, as
shown by the dotted line in Fig. 4. This is followed by
rapid surface cooling by convection to near saturation
temperature as indicated by region III in Fig 4.
Information about the rewetting temperature, the quench
front velocity, the temperature gradient in the region
immediately ahead of the rewetting front and the bursting
frequency can be extracted from this experiment.

Bursting Frequency

At the quench front, the cyclic liquid-metal contacts and
subsequent interface lift-off is occurring as shown by the
temperature measurements and by the visual observation
(Figs. 4 to 8). The bursting frequencies have been
calculated for the different flow conditions, the results
being given in Tab. 3 and plotted in Fig. 9. The results
show that the bursting frequency increases with increasing
degree of sub-cooling whereas the flow rate has very little
effect on the bursting frequency.


40-

S20-

a0


10 15 20 25 30 35 4
Flow rate (L/min)


Figure 9: Dependence of bursting frequency on liquid flow
rate and inlet degree of sub-cooling


Paper No


* 40 C
* 60 C
70 C


I I I I I






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Rewetting Velocity

The temperature at different axial positions along the flow
direction separated by 0.85 mm has been obtained. The
time history of the temperature at these locations is plotted
in the Fig. 9. A sharp fall in temperature at any given point
indicates the arrival of the quench front to that point. The
time taken by the quench front to move from one location
to the other is obtained from Fig. 9. The knowledge of this
time and the fixed distance between the points give the
rewetting velocity. The rewetting velocities for the cases
studied are given in the Tab. 2. Derived data for the net
advancement per cyclic event (rewetting velocity divided
by the frequency) is given in Tab. 4. It is interesting to note
that this quantity is 3 x10-4 m for all the cases studied.


Table 2: Rewetting velocity for different flow conditions
(m/s)

Flow Rate 1.2 1.9 2.5 3.2
(L/min)
Subcooling
(oC)
40 0.013 0.013 0.011

60 0.016 0.013

70 0.0181 0.015 0.021 -



Table 3. Rewetting Frequency (Hz)

Flow Rate 1.2 1.9 2.5 3.2
(L/min)
Subcooling
(oC)
40 34 36 36

60 48 50 -

70 67 67 70 -



Table 4. Advancement during each intermittent contact,
"rewetting velocity/ frequency ", (m)

Flow Rate
Fl R 1.2 1.9 2.5 3.2
(L/min)

Subcooling
(oC)

40 3.8E-4 3.7E-4 3.1E-4

60 3.4E-4 2.6E-4

70 2.7E-4 2.2E-4 3.0E-4


400

O
300

5 200
E
1-
100


200
Time (ms)


Figure 10: Temperature history at different axial positions


Temperature Gradient


Analysis of the data shows that a steep temperature
gradient exits near the quench front. For a typical case
(serial number 3 in Tab. 1), the temperature along the axial
direction at a time when quench front arrives at the top of
the sapphire disc is shown in the Fig. 11.


400


350-
0
300-

E
250-


200
0 1 2 3 4 5
Axial Position (mm)


Figure 11: Temperature gradient ahead of the quench front


It can be seen that the temperature varies from 215 C to
380 C in s-shaped profile over a spatial range of about
2.26 mm. This implies that there is a temperature gradient
of 72.8 C/mm. This also signifies the characteristic
length of the intermittent contact region.

Conclusions

From this study it is concluded that the main mechanism of
heat transfer at the quench front is the repeated bursting of
the liquid away from the surface. This bursting
phenomenon occurs at a frequency that is characteristics of
the liquid-metal combination for the given flow conditions.
It arises from an explosive boiling mechanism occurring
near the rewetting front; this mechanism is associated with
homogeneous nucleation of the liquid. During each burst,


Paper No





PaperNo 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

heat is taken by the liquid resulting in cooling of the
surface thereby allowing liquid to move along the surface
before the conditions for the next burst are established. The
characteristic length of the intermittent contact region is of
the order of few millimeters. The bursting frequency
increases with increasing degree on inlet sub-cooling
whereas the flow rate has very little effect on it. It is found
that the rewetting temperature is close to the homogeneous
temperature.

Acknowledgements

This work was carried out as part of the TSEC programme
KNOO, and we are grateful to the EPSRC for funding under
Grant EP/C549465/1. One of the authors (MI) would like to
acknowledge the Higher Education Commission (HEC)
Pakistan, Pakistan Institute of Engineering & Applied
Sciences (PIEAS) for funding his PhD studies at Imperial
College London.

References

Bennet, A.W., Hewitt, GF., Kearsey, H.A. and Keeys,
R.K.F.,1966,The wetting of hot surfaces by water in a
steam environment at high pressure. UKAEA Report No.
AERE R1546, May, 1966.
Butterworth D, Owen RG (1975) The quenching of hot
surfaces by top and bottom flooding-a review. In: AERE-
R: Harwell, UK
Carey VP (1992) Liquid-Vapor Phase-Change Phenomena
New York: Hemisphere Publishing Corporation
Carslaw HS, Jaeger JC (1959) Conduction of heat in solids
Oxford: The Clarendon Press
Collier JG (1982) Heat transfer in the post burnout region
and during quenching and reflooding. In: Handbook of
Multiphase Systems, Washington D.C.: Hemisphere
Duffey RB, Porthouse DTC (1973) The physics of
rewetting in water reactor emergency core cooling. Nuclear
Engineering and Design 25:379-394
Mitsutake Y, Masanori M, Kawabe R (2003) TED-AJ03-
296 Transient heat transfer during quenching of a vertical
hot surface with bottom flooding. Proceedings of the
ASME/JSME Thermal Engineering Joint Conference
2003:286
Pereira PM (1997) Heat transfer in rewetting of hot
surfaces. PhD Thesis, London: Chemical Engineering &
Chemical Technology, Imperial College London,
University of London
Saxena AK, Venkat Raj V, Govardhana Rao V (2001)
Experimental studies on rewetting of hot vertical annular
channel. Nuclear Engineering and Design 208:283-303




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