Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 17.7.4 - Condensation/Collapse Processes of Vapor Bubbles Injected in Subcooled Pool
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 Material Information
Title: 17.7.4 - Condensation/Collapse Processes of Vapor Bubbles Injected in Subcooled Pool Boiling
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Ueno, I.
Hosoya, R.
Hattori, Y.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: vapor bubble
condensation
collapse
micro-bubble formation
instability
 Notes
Abstract: We focus on condensation and collapse processes of vapor bubble(s) in a subcooled pool. We generate the vapor in the vapor generator and inject it/them to form vapor bubble(s) at a designated temperature into the liquid at a designated degree of subcooling. In order to evaluate the effect of induced flow around the condensing/collapsing vapor bubble, two different boundary conditions are employed; that is, the vapor is injected through the orifice and the tube. Through this system we try to simulate an interaction between the vapor bubble and the subcooled bulk in a complex boiling phenomenon, especially that known as MEB (microbubble emission boiling) in which a higher heat flux than critical heat flux (CHF) accompanying with emission of micrometer-scale bubbles from the heated surface against the gravity is realized in a narrow range of subcooled condition.
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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Bibliographic ID: UF00102023
Volume ID: VID00435
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 1774-Ueno-ICMF2010.pdf

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


Condensation/Collapse Processes of Vapor Bubbles Injected in Subcooled Pool


Ichiro Ueno1, Ryota Hosoya2 & Yasusuke Hattori3

'Tokyo University of Science, Faculty of Science & Technology, Department of Mechanical Engineering
2Undergraduate, Tokyo University of Science, Faculty of Science & Technology, Department of Mechanical Engineering
3Tokyo University of Science, School of Science & Technology, Division of Mechanical Engineering
presentt affiliation: Hitachi High-Technologies Corp., Japan)
2641 Yamazaki, Noda, Chiba 278-8510, Japan
1 k i ,- n, ,d,, l,, '1



Keywords: vapor bubble, condensation, collapse, micro-bubble formation, instability




Abstract

We focus on condensation and collapse processes of vapor bubble(s) in a subcooled pool. We generate the vapor in the vapor
generator and inject it/them to form vapor bubble(s) at a designated temperature into the liquid at a designated degree of
subcooling. In order to evaluate the effect of induced flow around the condensing/collapsing vapor bubble, two different
boundary conditions are employed; that is, the vapor is injected through the orifice and the tube. Through this system we try to
simulate an interaction between the vapor bubble and the subcooled bulk in a complex boiling phenomenon, especially that
known as MEB (microbubble emission boiling) in which a higher heat flux than critical heat flux (CHF) accompanying with
emission of micrometer-scale bubbles from the heated surface against the gravity is realized in a narrow range of subcooled
condition.


Introduction

Development of heat exchange system with higher
efficiency under microgravity is indispensable for
long-duration exploration in space. In the recent decades,
'microbubble emission boiling (MEB)' has been focused as
a potential phenomenon to be applied to such systems. The
MEB realizes a higher heat flux than the critical heat flux
(CHF) accompanying with emission of micrometer-scale
vapor bubbles around the heated surface against the gravity
under a certain degree of the subcooling. This unique
phenomenon was reported by Inada et al. (1986), and a large
number of researches have followed to grasp an occurring
condition and heat transfer characteristics of this
phenomenon in the boiling on the flat surface (Suzuki et al.
2002, 2009), on the thin wire (Tange et al. 2004), and in the
microchannel (Wang & Cheng 2009). These fabulous
researches have indicated that this boiling process emerges
in a narrow range of subcooled condition with a higher heat
flux than the CHF accompanying with continuous formation
and emission of a number of micrometer-scale bubbles from
the heated surface. A series of experimental work were
recently carried out by a group of The University of Tokyo
(Tange et al. 2004) to indicate growing and collapsing
process of the vapor bubble on a thin wire. The dynamics of
the MEB itself, however, is not fully understood at all. One
of the reasons for this might be complex interactions among
liquid, gas, and solid phases. Interaction between bubbles
themselves also prevents a fine observation of the
phenomenon.
We have focused on the collapsing processes of vapor


bubble injected into a subcooled pool to extract the
vapor-liquid interaction from such a complex system of the
boiling (Ueno & Arima 2007). We employ a vapor
generator to supply vapor at designated flow rate to the
subcooled pool instead of using an immersed heated surface
for the boiling experiment. We try to detect a spatio-
temporal behavior of a single bubble of superheated vapor
exposed to a subcooled liquid through this system. In the
present study, we especially focus on effect of induced flow
around the condensing/collapsing vapor bubble by changing
the injection direction and the boundary conditions. We also
focus on interaction between/among the condensing/
collapsing vapor bubbles laterally injected to the pool.


Nomenclature


t
T
AT,ub
q


time (ms)
temperature (K)
degree of subcooling (K)
heat flux (W/m2)


Greek letters
X wavelength (ipm)


Experiment

Experimental apparatus is illustrated in Fig. 1. This system
consists of three major components; the subcooled pool, the








vapor generator and supplier, and the measuring system.
The experimental system is almost the same as introduced
by the present authors' group (Ueno & Arima 2007) except
the vapor generation system. The vapor of the distilled
water is generated in the generator, and supplied to the
subcooled pool filled with the same distilled water through a
heated stainless-steel tube of 1.5 mm in inner diameter. The
tank for the subcooled pool is made of Pyrex@ glass, and its
inner width, depth and height are of 280 mm, 280 mm and
200 mm, respectively. Temperature of the injected vapor is
confirmed to keep almost constant at 101 OC through the
series of the experiments. The injection rate of the vapor is
controlled by the valve on the vapor generator. In the
present study, the authors vary the injection direction to the
subcooled pool; downward injection as the same as Ueno &
Arima (2007), and upward injection. Furthermore, the
authors prepare two different condition of the tip of the
injection tube in order to grasp the effect of the induced
convection around the growing/condensing vapor bubble;
one condition is a bare tip, and the other a tip immersed in
the plate to realize a kind of 'orifice.' The temperature of
the pooled fluid, or the degree of subcooling ATsub, is
controlled with cooling channel made of copper immersed
in the pool. The authors are convinced with pre-experiments
that the spatio-distribution of the degree of subcooling in the
pool is almost uniform. The injected vapor bubble is
captured by high-speed video camera at a frame rate up to
0.1 million fps with a back-lighting system.


Cooling channel I1/

T/C PC for DA
Light High-speed
camera



Const. PC for
Temp. E HS-camera
Bath Vapor generator

Figure 1: Experimental apparatus.


Results and Discussion

Typical example of growing and condensing processes of
the vapor bubble injected upward through the orifice at a
constant rate to the subcooled pool at ATsub = 32 K is
presented in Fig. 2. Bottom part in each frame corresponds
to the surface of the substrate with the orifice. Scale bar in
the first frame of the above image corresponds to 2.0 mm.
The instance indicated in the first frame is referred as t = to.
This successive images are captured from a single movie
taken at 30000 fps (frame per second) with a shutter speed
of 1/30 ms. The injected vapor condenses with keeping
rather smooth free surface even under such a large degree of
subcooling of the pool. Noted that the vapor is continuously
supplied to the bubble until the bubble departs from the
orifice. After a while of growing stage, a significant necking
process takes place on the bubble right above the orifice (at


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

about t = to + 2.0 ms in this case) accompanying with a
condensation near its neck part; this is driven by the fluid
flow induced by the growth and departure of the bubble
from the orifice. The bubble detaches from the orifice to
drastically decrease its volume because the bubble is
isolated to be exposed to the subcooled pool without
supplying the vapor to the bubble. Tiny bubbles of O(pm) in
diameter are formed in the collapsing process, which is seen
as the black clouds at the top region in the last frame (at t =
to + 3.6 ms in this case). The abrupt condensation and
collapse processes of the vapor bubble are described more
in detail later in this section.


t= t ma] t +0.4


3.8 to+1.2 t,+1.6



I..


to +2.0 to+2.4 to+2.8 to + .2 to + .6
Figure 2: Typical example of growing and condensing
processes of the upward-injected vapor bubble into the pool
of 32 K in the degree of subcooling through the orifice.
Scale bar in the first frame corresponds to 2.0 mm. Shutter
speed is of 1/30000 s.


% 2 8 100
1 20 40 60
Injection rate [m/s] 0 ATb [K]
(b)
Figure 3: Condensation rate of injected vapor bubble to
subcooled pool; (a) the cases of upward- and
downward-injections of vapor through the tube and (b) the
cases of injection from the orifices of different material
(only for the case of upward injection).






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


t, + l0. \(0/


t" + O.Z


t + ;.zoo t, + O.Z.to/


t" + ;.;0


to + 3.333 t + 3.367 to+ 3.4 to + 3.433 to + 3.467


Figure 4: Successive images of the condensing/collapsing vapor bubble through the orifice. Images are captured from the
same movie but with a different frame interval shown in Fig. 2. The instance t = to in this figure corresponds to that in Fig. 2;
this figure indicates the vapor bubble behavior during the last three frames in Fig. 2.


2 mm
"


t = t [ms] tl+ 0.13
r -


tl+ 0.39 tl+ 0.52

-2 mm


t = t1 [ms]


t,+ 0.26





t,+ 0.65


tl+ 0.13 tl+ 0.26


Figure 5: Zoomed view of the top region of the vapor bubble exhibiting such a fine disturbance under (a) 31
in the degree of subcooling ATsub. Each frame has a common scale bar (2 mm). Shutter speed is of 1/30000 s.


We have examined the condensation and collapse processes
as functions of the degree of subcooling of the pool, the
injection rate of the vapor, the direction of vapor injection,
and the boundary condition of the vapor injection (tube or
orifice). Figure 3 indicates the maximum condensing rate of
the vapor bubble as functions of the degree of subcooling of
the pool liquid and the vapor injection rate at different
boundary conditions; (a) the cases of upward- and
downward-injections of vapor through the tube and (b) the
cases of injection from the orifice of PDMS and copper. The
bubble volume is evaluated under an assumption that the


K and (b) 73 K


bubble is axisymmetric by detecting the periphery of the
bubble and integrating the dish of a pixel in height along the
axis. The volume is normalized with the maximum volume
in the growing and condensing processes in the concerned
experimental run as discussed in Ueno & Arima (2007). The
condensation rate has a jump at ATsub of 30 K for both cases.
Tiny bubble formation in the collapsing process of the vapor
bubble as indicated in Fig. 2 accompanies with such a
vigorous condensation above the threshold of ATsub. This
trend corresponds well with the occurrence condition of the
MEB at around ATsub of 30 K at atmospheric pressure as









indicated by Suzuki et al. 2" iI2) and by others. One can
clearly see almost the same trends of the jump of the
maximum condensation rate at the threshold ATsub
nevertheless the differences of the vapor injection direction
and of the boundary condition. There exist quantitative
differences of the condensation rate among the cases; the
differences of the injection direction and the boundary
condition might lead a difference of induced flow by the
abrupt condensation of the vapor bubble, which brings a
difference of the thermal boundary condition around the
condensing/growing bubble.
Now we focus on the collapsing behavior of the vapor;
Figure 4 indicates successive images of the condensing/
collapsing vapor bubble through the orifice. These images
are captured from the same movie but with a different frame
interval shown in Fig. 2. Note that the instance t = to in Fig.
4 corresponds to that in Fig. 2, so that this figure indicates
the bubble behavior during the last three frames in Fig. 2. It
is clearly seen that the bubble keeps a smooth interface at
the early stage of the condensation. After a while of the
necking and condensation processes, at t = to + 3.267 ms, a
tiny but significant disturbance of 0(100 ipm) in wavelength
emerges at the top region of the bubble. The bubble is now
departed from the vapor flow through the orifice and
isolated from the vapor supply through the orifice, thus the
bubble is fully exposed the subcooled pool. Sudden
condensation near the interface of the bubble takes place,
and results in an abrupt decrease of the pressure inside the
bubble. Liquid around the vapor bubble does not follow
such a sudden decrease of the pressure because of the inertia.
Such a sudden drop of the pressure inside the vapor bubble
and an inertia effect lead the fine disturbances over the free
surface. In addition, those disturbances over the bubble
surface do result in the increase of the surface area to further
enhance the heat transfer through the bubble surface, which
bring an acceleration of the condensation in the bubble.


-600
500
400
300
200
100
n1


S- -20 40 60 80
ATsub [K]


Figure 5: Wavelength of disturbances arisen on
condensing vapor bubble as a function of ATsub. Averaged
value of wavelength is plotted as a mark, and its existence
range is indicated as a bar.


Figure 5 indicates a zoomed view of the top region of the
vapor bubble exhibiting such a fine disturbance under
different degrees of subcooling of ATsub = (a) 31 K and (b)
73 K. Constant frame interval is employed to describe the
bubble collapse for both cases, thus one can clearly see a
difference of the characteristic time for the collapse process
in changing the degree of subcooling. Abrupt collapse of the
isolated vapor bubble exposed to the subcooled pool never


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

takes place without fine disturbance over the free surface.
This disturbance is the resultant of the sudden drop of the
pressure inside the vapor bubble due to the condensation. As
the degree of subcooling increases, the disturbance over the
bubble becomes finer, and the characteristic time for bubble
collapse becomes shorter. The averaged wavelengths raised
over the free surface is plotted in Fig. 6. We must
emphasize that we seldom have any such a fine disturbance
on the bubble surface if ATsub < 20 K with moderate and
low injection rate of the vapor. This trend shows a good
agreement with the trend of the microbubble formation in
the general boiling experiment as aforementioned, that is,
we have a threshold to realize the MEB at ATsub of 30 K.
We carry out another series of experiments of subcooled
pool boiling on a thin platinum wire in order to compare
such growing/condensing processes of the vapor bubble
injected to the subcooled pool with the microbubble
emission boiling (MEB). The apparatus for this
experimental series is basically the same as introduced in
Tange et al. (2004). Typical example of the MEB realized in
this system is presented in Fig. 6. The test wire is made of
platinum of 300 pim in diameter. The degree of subcooling
is of 65 K, and the heat flux through the test wire is of 2.6
M/mm2. One can clearly see a bubble at right hand side in
the frame, which exhibits an abrupt growth and collapse on
the heated wire. The authors evaluate the condensation rate
in this process as conducted in the case of vapor injection
experiments. Figure 7 represents a comparison of the vapor
bubble condensation rate between the cases of the vapor
injection experiments and the thin-wire boiling experiment.
The condensation rates as a function of the degree of
subcooling for both cases show almost the same tendency;
the condensation rates exhibit a significant change at around
ATub = 40 K. The behavior of the vapor bubble
condensation through the tube or orifice can be regarded to
simulate that of the microbubble emission boiling through
the high-speed observation as well as the evaluated
condensation rates.


Figure 6: Vapor bubble growth and condensation process
in subcooled pool boiling on platinum wire of 300 pim in
diameter; degree of subcooling ATub = 65 K and heat flux q
= 2.6 M/mm2. Scale bar: 1.0 mm.


In the series of the experiments of the boiling on the thin
wire, one can clearly see a formation of a number of tiny
bubbles of O (pm) in diameter in the collapse of the vapor
bubble towards the heated-wire surface. Those bubbles flow
away from the heated wire on the stream of high
temperature plume as observed in the right-half of the
frames in Fig. 5. Figure 7 represents a measured velocity of
ejected microbubbles in the abrupt collapse of the vapor
bubbles. This figure makes a comparison of the velocities
among the cases of the boiling on thin wire (example is


1 1 atm
orifice



l I I









shown in Fig. 6), the vapor injection through the orifice, and
that through the original tube.



S1500
S :- vapor injection
-r n --El boiling on
1000- thin wire


500-


o 0
O 9,
4 0 100
2 0 40 860
Ejection rate[m/s] Degree of subcooling [K]

Figure 7: Condensation rate of vapor bubbles injected
downwardly through the tube to subcooled pool (circle) and
those in boiling on thin wire (square).


of 6__o t Boiling
E on thin wire
5 d Orifice
a o v Tube
5 o 8

> 3 3


)1 O 0

0o0 20 40 60 80 100
S Degree of subcooling [K]

Figure 8: Comparison of emitted microbubbles after
abrupt collapsing of the vapor bubble among the cases of
boiling on thin wire, vapor injection through the orifice
(tube immersed in solid plate) and that through the tube.


In the case of the boiling on the thin wire, the velocity of
emitted microbubbles increases as the degree of subcooling
of the pool. This can be explained by considering the
intensity of the collapse of the vapor bubble, which is
reflected to the intensity of the plume formation from the
heated surface. It is confirmed by the present observation
that the microbubbles flow on the plume formed by the
abrupt collapse of the vapor bubble near the solid surface.
This is the reason why the microbubbles are emitted radially
around the heated wire. In the cases of the vapor injection
through the orifice or the tube, on the other hand, the
microbubbles' velocity ranges in the same manner in spite
of the variation of the degree of subcooling. And the
velocity is larger than that in the boiling on the heated wire.
This is because the vapor bubble is supplied continuously to
the subcooled pool in the case of the vapor injection
experiments; the ambient fluid is vigorously driven by the
injected vapor. This must result in the difference of the


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

velocity of the microbubbles formed in the collapsing of the
vapor bubble.
Further researches are surely needed to indicate a physical
scenario to realize the fine disturbance over the bubble
surface after an abrupt drop of the vapor pressure, and a
scenario from the fine disturbances over the vapor bubble
when it is exposed to the subcooled pool to the micrometer-
scale vapor bubble formation.

Conclusions

In the present study a special attention is paid to the
growing and collapsing processes of vapor bubble injected
into a subcooled pool; we try to extract the vapor-liquid
interaction by employing a vapor generator that supplies
vapor at designated flow rate to the subcooled pool instead
of using a immersed heated surface to realize a vapor bubble
by boiling phenomenon. This system enables us to detect a
spatio-temporal behavior of a single bubble of superheated
vapor exposed to a subcooled liquid. We succeed to indicate
the condensation rates as functions of the injection velocity
of the vapor and the degree of subcooling of the pool. The
condensation rate exhibits a significant jump at the degree
of subcooling higher than 30 K. This tendency shows a
good agreement with the occurring condition of MEB in the
conventional systems. We also indicate that an abrupt
condensation of the injected vapor results in a fine
disturbance over the vapor bubble surface before the
collapse stage of the bubble. The wavelength is dependent
on the degree of subcooling of the pool. The threshold of
such a fine disturbance formation over the bubble
corresponds with that the occurring condition of the MEB.

Acknowledgements

The authors gratefully acknowledge Prof. Masahiro Shoji
and his students at Kanagawa University, Dr. Manabu
Tange at The University of Tokyo (present affiliation:
Shibaura Institute of Technology), and Prof. Koichi Suzuki
at Tokyo University of Science, Yamaguchi, for fruitful
discussion. A part of this research is financially supported
by the TEPCO Research Foundation, Japan.

References

Inada, S., Miyasaka, Y., Sakamoto, S., Chandratilleke, G.
R.: Liquid-solid contact state in subcooled pool transition
boiling system, Trans. ASME J. Heat Trans. 108, 219-221
(1986).

Suzuki, K., Saitoh, H., Matsumoto, K.: High heat flux
cooling by microbubble emission boiling, Ann. N. Y. Acad.
Sci. 974, 364-377 2." '2).

Suzuki, K., Inagaki, F., Ueno, I.: Heat transfer enhancement
in subcooled boiling with ultrasonic field, Trans. Japan
Society for Aeronautical & Space Sciences (JSASS) Space
Tech. Japan 7, Ph_67-Ph_70 (2009).

Tange, M., Yuasa, M., Takagi, S., Shoji, M.: Microbubbles
emission flow boiling in a microchannel and minichannel,
Proc. Microchannels and Minichannels 2004 (June 17-19,
2004, Rochester, New York, USA), CD-ROM (2'" 'i ,









Ueno, I. & Arima, M.: Behavior of vapor bubble in
subcooled pool, Microgravity Sci. Technol. XIX, 128-129
(2007).


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

Wang, G. & Cheng, P.: Subcooled flow boiling and
microbubble emission boiling phenomena in a partially
heated microchannel, Int. J. Heat Mass Trans. 52, 79-91
(2009).




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