Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 16.7.1 - Influence of Subcooling on Transition from Nucleate to Film Boiling in Microgravity
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 Material Information
Title: 16.7.1 - Influence of Subcooling on Transition from Nucleate to Film Boiling in Microgravity Boiling
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Zhao, J.-F.
Li, J.
Yan, N.
Wang, S.-F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: critical heat flux
pool boiling
subcooling
microgravity
 Notes
Abstract: An experimental investigation on bubble dynamics and heat transfer in pool boiling on plate in microgravity had been carried out aboard the Chinese recoverable satellite SJ-8. The transition process from nucleate to film pool boiling was studied in the present paper. The plate heater with an effective heating area of 15*15 mm2 was used in the space experiment. Degassed FC-72 was used as the working fluid. A quasi-steady heating method was adopted, in which the heating voltage was controlled to increase exponentially with time. The heating rate was set low enough to obtain quasi-steady, continuous pool boiling curves. It was observed that small, primary bubbles formed and slid on the surface, which coalesced with each other to form a large coalesced bubble. The coalesced bubble had a smooth surface at high subcooling. It was difficult to cover the whole heater surface, resulting in a special region of gradual transitional boiling in which nucleate boiling and local dry area can co-exist. On the contrary, strong oscillation of the coalesced bubble surface at low subcooling may cause re-wetting of local dry-outs and activating more nucleate sites, resulting in an abrupt transition to film boiling.
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: VID00409
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 1671-Zhao-ICMF2010.pdf

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



Influence of Subcooling on Transition from Nucleate to Film Boiling in Microgravity


Jian-Fu ZHAO, Jing LI, Na YAN, Shuang-Feng WANG


Key Laboratory of Microgravity (National Microgravity Laboratory), Institute of Mechanics, Chinese Academy of Sciences.
Beijing 100190, China





Keywords: critical heat flux, pool boiling, subcooling, microgravity






Abstract


An experimental investigation on bubble dynamics and heat transfer in pool boiling on plate in microgravity had been carried
out aboard the Chinese recoverable satellite SJ-8. The transition process from nucleate to film pool boiling was studied in the
present paper. The plate heater with an effective heating area of 15*15 mm2 was used in the space experiment. Degassed
FC-72 was used as the working fluid. A quasi-steady heating method was adopted, in which the heating voltage was
controlled to increase exponentially with time. The heating rate was set low enough to obtain quasi-steady, continuous pool
boiling curves. It was observed that small, primary bubbles formed and slid on the surface, which coalesced with each other
to form a large coalesced bubble. The coalesced bubble had a smooth surface at high subcooling. It was difficult to cover the
whole heater surface, resulting in a special region of gradual transitional boiling in which nucleate boiling and local dry area
can co-exist. On the contrary, strong oscillation of the coalesced bubble surface at low subcooling may cause re-wetting of
local dry-outs and activating more nucleate sites, resulting in an abrupt transition to film boiling.


Introduction


Pool boiling is an increasing significant subject for
investigation, since many potential applications exist in
space and in planetary neighbours due to its very high heat
fluxes. There exists interrelation of numerous factors and
effects as the nucleate process, the growth of the bubbles,
the interaction between the heater's surface with liquid and
vapor, the evaporation process at the liquid-vapor interface,
and the transport process of vapor and hot liquid away from
the heater's surface, and so on. Many of these sub-processes
mentioned above are influenced by the buoyant force in
normal gravity due to great difference between the liquid
and its vapour. Therefore, boiling is an extremely
complicated and illusive process, and then fewer studies
have focused on the physics of the boiling process than
have been tailored to fit the needs of engineering
applications. As a result, the literature has been flooded


with the correlations involving several adjustable, empirical
parameters. These correlations can provide quick input to
design, performance, and safety issues and hence are
attractive on a short-term basis. However, the usefulness of
the correlations diminishes very quickly as parameters of
interest start to fall outside the range of physical parameters
for which the correlations were developed. Thus, the
physics of the boiling process itself is not properly
understood yet, and is poorly represented in the most
correlations, despite of almost seven decades of boiling
research.
In microgravity environment, the influence of the
buoyant force can be weakened greatly, or even eliminated.
The local phenomena of flow and heat transfer near the
heating surface in boiling may loom large, which is very
helpful to reveal the elementary mechanisms underlying the
boiling process. The earliest experiments of boiling in
microgravity were carried out more than half of a century











ago. Limited by unique and stringent constraints in terms of
size, power and weight of experimental apparatuses, and of
number and duration of the experiments, the study on
boiling in microgravity is still insufficient. On the progress
in this field, several reviews are available. Among many
others, Straub (2001) issued a comprehensive review of his
own activity on this field from the early 1980s to date,
while Di Marco (2003), Kim (2003), Ohta (2003), and Zhao
(2009) issued reviews of researches of boiling and other
related topics in microgravity in Europe, in US, in Japan,
and in China, respectively.
The steady heating method was usually adopted, in
which the heat flux or surface temperature was adjusted
step-by-step. For each step, the heating time lasted for a
period long enough to obtain a steady state of boiling. It
may, however, cause some difficulties in determining the
trend of boiling curves due to the large scattering of
measured data points. Furthermore, the mechanism of the
transition from nucleate to film boiling is still not in a
substantial agreement with each other due to no observation
of the real process of the transition using the steady heating
method.
This article presents the results of a space experiment
on pool boiling performed aboard the Chinese recoverable
satellite SJ-8. Quasi-steady pool boiling of degassed FC-72
was established on a plane plate with an effective heating
area of 15*15 mm2. The bubble dynamic and transition
from nucleate boiling to film boiling were recorded using a
video camera. The influence of subcooling on the transition
from nucleate to film boiling was analyzed based on the
recorded images and data obtained in the space
experiments.


Nomenclature


pressure (kPa)
voltage (V)
temperature (K)
heat flux(W. c in-)


Greek letters
r heating time (s)


Subsripts
sub subcooling


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

w heater wall
sat saturation
CHF critical heat flux


Experimental Facility


A device using a transient heating method was
developed to conduct experiments on pool boiling in
microgravity aboard the Chinese recoverable satellite SJ-8
(Zhao 2009). The boiling chamber, as shown in Fig. 1, was
filled with degassed FC-72 and fixed inside an air-proof
container in which the pressure was initially about 100 kPa.
A bellows connected with the chamber will allow the
pressure in the chamber to be approximately constant
during the boiling processes. An auxiliary heater was used
for adjusting the temperature of the bulk liquid from the
ambient temperature to about the middle between the
ambient and saturation temperature at the corresponding
pressure.
fl-gil


(CD OkmW-9
,- o a.d-b

Mtf m
Uli~nt JMI-B
r*' .


Tsuduc


Thfmw-cwpf


"Mdsk


A4m.
Figure 1: The boiling chamber and its accessories.


The plane plate heater, as shown in Fig. 2, has an
A1203 ceramic substrate with a size of 28x20x1 mm3
embedded in the PTFE (Polytetrafluoroethylene) base with
a thickness of 25 mm. An epoxy-bonded composite layer of
mica sheets and asbestos was set between the ceramic
substrate and PTFE base to reduce the heat leakage. The
effective heating area with an area of 15x15 mm2 was
covered by a serpentine strip of multi-layer alloy film which
was of 300 ugm in width and about 10 ugm in thickness. The
roughness of the heater surface is shown in Fig. 3. The
multi-layer alloy film comprised several layers of metals
(Cr, Cu, Ni, Au), and had nominally a resistance of 6 2.
A transient heating method is adopted, in which the
heating voltage was controlled as an exponential function
with time, namely U = Uo exp(r/To), here r denotes the











heating time, and the period to determines the heating rate.
The period was set as to = 80 s in the space experiments in
order to make the process as a quasi-steady state of boiling,
which was verified in the preliminary experiments on the
ground. Furthermore, the period used in the present study is
about 3-4 order of magnitude larger than those in Johnson
(1971), which guaranteed the fulfillment of quasi-steady
condition, although different structure of the heater and
working fluid employed here.


Lead
Connection area


Epoxy-bonded Lead
Heating area Composite layer Connection area


Cerainic,.
Substrate





Figure 2: The heater structure.


1.010-

1.005-

1.000-


-6 -3 0
X, mm


INot lt scale


3 6


Figure 3: The roughness of the heater surface.


The absolute pressure within the boiling chamber is
measured using a pressure transducer with a range of 0 -
0.2 MPa and an uncertainty of 0.25% FS (full scale). Two
thermocouples with a range of 0-100 C and an uncertainty
of 0.5 C are used to measure the bulk temperature of the
fluid in the boiling chamber. The outputs for thermocouples
and pressure transducer are sampled at rate of 1 Hz. A CCD
video camera, at the direction of 450 with respect to the
heater surface, is used to obtain images of the motion of
vapor bubble or film around the heater, which is digitized
and recorded at a speed of 25 fps in MPEG. Four LEDs
(light-emitting diode) are used to light the boiling chamber
through two optical windows.
There are two stages with different number of runs in
the space experiments. Every run consists of pre-heating,


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

stabilizing and boiling phases except the first run without
pre-heating phase. The first stage is conducted in an
ambient pressure condition with the initial pressure of about
100 kPa. After its completion, a solenoid valve is opened to
vent air from the container to the module of the satellite,
and then the pressure inside the container was reduced to
the same of that in the module of the satellite, namely
40-60 kPa. Thus, in the second stage of the space
experiment, the pressure in the boiling chamber is different
from those in the first stage. One more stage is added in
actual flight, which is also performed in the reduced
pressure condition as the second stage.


Results and Discussion


There were 8 runs performed in the space experiment.
Unfortunately, video images were obtained only in the first
five runs of the first stage. Table 1 gives the corresponding
experimental conditions, which also listed estimated values
of the critical heat flux (CHF) and the corresponding
superheats. Figs. 4 and 5 show some typical processes of
bubble growth, heating history, and the corresponding
boiling curves in the space experiments. The curve of the
heater temperature vs time in the run I-5 in Fig. 5 was
shifted-up 30 C to distinguish it from that in the run I-1.


Table 1: Space experimental conditions and the estimated
CHF values.


Run" p (kPa) A Tsb (K) qcHF (W/cm2) Tsa (K)
I-1 90.8 36.9 8.3 10.0 28 66
I-2 97.3 25.8 6.6 9.1 34 76
I-3 102.3 21.8 7.0 7.6 40 56
I-4 105.7 19.5 7.7 8.2 20 29
I-5 111.7 18.4 8.6 8.9 11 17
II-1 57.2 24.5 5.7 6.9 24 42
II-2 91.1 18.8 7.4-9.5 26-55
III-1 65.5 27.5 6.3 6.6 30 35


In the five runs of the first stage, the first appearance
of bubbles is observed at 21.89 s, 8.68 s, 8.12 s, 4.54 s, and
4.84 s, respectively. Comparing with the first runs, the
nucleate boiling occurred significantly earlier in the
following runs. Considering the experimental procedure, it
may indicate that there could be residual micro-bubbles in








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


cavities after the preceding runs. These micro-bubbles
would make the cavities easier to be activated, and the
boiling would thus be initiated at a lower wall superheat.
Furthermore, bubbles attached on the surface seemed to be
able to suppress the activation of the cavities in its
neighborhood according to the detailed analyses of the
video images.


b)










Figure 4: Typical processes of bubble growth (a: I-1; b:
1-5).


In the first run I-1, the explosive boiling was observed,
in which a great amount of vapor appeared abruptly and
explosively at the incipience of boiling. Then, several
segregate bubbles with the smooth interface were formed
under action of surface tension. An obvious drop of the
heater temperature was observed in the curve of the heating
history, correspondingly. This drop caused an additional
heat flux from the ceramic substrate to the liquid, and may
result in a local maximum of the heat flux to the liquid in
the transition region from the incipience to quasi-steady
nucleate boiling despite of the monotonous increasing of the
heating rate. The following runs were different, in which a
gradual growth of the first bubble was observed. The
process of bubble growth even appeared an obvious
standstill after its first appearance. Correspondingly, no
obvious over-shooting or drop of the heater temperature can
be observed in the curves of the heating history in the
following runs.


150-



50-

A B
0 20 40 60 80 100
time (s)


12- b)-
10 0 1001
CHF..... ... CHF
8-









10 100
AT = T-T (K)


Figure 5: Heating histories (a) and boiling curves (b) in the
runs I-1 and 1-5. Symbols A, B, A' and B' in the left figure
are corresponding to the images in Fig. 4.


Compared with boiling in normal gravity, it was very
difficult for the bubbles to depart from the surface in
microgravity. Although the images were taken only from
the sole direction of 450 with respect to the heater surface, it
can be observed that primary bubbles generated continually,
slid on the surface, and coalesced with each other to form a
larger coalesced bubble. Some primary bubbles can also
generate under the coalesced bubble at the same time. The
coalesced bubble also engulfed small bubbles around it. It
can be inferred that a macro-layer may exist underneath the
coalesced bubble, where primary bubbles were forming.
For the case of high subcooling, e.g. I-1, it can be
observed that the coalesced bubble has a relative smooth
surface, and oscillated slowly near the center of the heater
surface. The behavior of the coalesced bubble is influenced
by interaction between the repulsion of the primary bubbles











and the pulls of liquid flow surrounding it. Its size increased
with the increase of the surface temperature, but it was very
difficult to cover the whole surface. Higher is the
subcooling, smoother the surface of the coalesced bubble
and slower its growth. This fact is caused by the strong
condensation near the top of the coalesced bubble, where
the vapor contacts directly with the subcooled liquid. Under
the action of the surface tension and the strong
condensation, the coalesced bubble shrank to an elliptical
sphere. Thus, the bottom of the coalesced bubble may dry
out partly at high heat flux, while on the other place,
particularly in the covers of the heater surface nucleate
boiling can be going on (Fig.6a). With the expanding of the
coalesced bubble and the local dry area, the boiling pattern
will gradually change to film boiling, as described by Oka
et al. (1995). This kind of transition boiling leads to a much
slow increase of the heater temperature and no maximum
on the boiling curve corresponding to the critical heat flux
(CHF) phenomenon (Fig. 5).

a)












b)













Figure 6: Schematic of the transition from nucleate to film
boiling in high (a) and low (b) subcoolings.


The bubble behavior and the characteristic of the
boiling curve at low subcooling, such as in the run 1-5, were
different from those at high subcooling. Due to relatively
lower subcooling, it is difficult for bubbles to condense. The


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

size of the coalesced bubble increased quickly, and a strong
oscillation caused by continuous coalescences appeared on
their surface. Furthermore, higher was the pressure, stronger
the surface oscillation. It may be caused by the fact that the
surface tension on the coalesced bubble decreased with the
increase of the pressure and the corresponding saturation
temperature. Thus, local dry spots underneath the coalesced
bubble cannot develop steadily. They may be re-wetted by
the surrounding liquid, and nucleate boiling may be kept on
the whole heater surface (Fig. 6b), even more nucleate sites
may be activated. Thus, unless the abrupt transition to film
boiling, the heat flux would keep increasing although the
surface temperature rose slowly or even fall down. An
abrupt increase of the heater temperature both in the curve
of heating history and in the boiling curve can be observed
obviously.
The gradual transition from nucleate to film boiling
brings some difficulties in determining the occurrence of
CHF. However, the trend of the increasing heater
temperature with the heating time can provide some
information of CHF. A rapid increase of the heater
temperature in the curve of the heating history may indicate
the beginning of the transition from nucleate to film boiling,
and a constant slope of the curve may suggest its
accomplishment. The corresponding ranges are marked in
Fig. 5, while the estimated values of CHF and the
corresponding superheats for all runs of the space
experiments are listed in Table 1. As shown in Fig. 5, the
estimated CHF values at low subcooling by the method
mentioned above are in good agreement with that
determined directly according to trend of the boiling curve,
and the corresponding transition range is evidently much
narrower than that at high one.
The estimated values of CHF in microgravity increase
with the increase of the subcooling at the same pressure, or
decrease with the decrease of the system pressure at the
same subcooling. These trends are similar with those
observed in normal gravity. Unfortunately, the pressure and
temperature of the liquid cannot be isolated completely
because of the capability of the passive control method of
the pressure inside the boiling chamber used in the present
study. Thus, some cross-influences of pressure and
subcooling on CHF exist.
Finally, a comparison of boiling curves in different
gravity conditions at the similar pressure and subcooling is











shown in Fig. 7. There are three major features in the
comparison. First, the superheats at the incipience of boiling
are actually the same in different gravity, which means that
the gravity has little effect on the incipience of boiling.
Second, the value of CHF in microgravity is only about one
third of that in terrestrial condition. Third, the boiling curve
in microgravity has a slope obviously smaller than that in
normal gravity. Generally, boiling heat transfer in
microgravity is deteriorated comparing that in normal
gravity, particularly at high superheats or heat fluxes. Much
obvious enhancement, however, can be observed just
beyond the incipience, which is consistent with those in
steady state pool boiling experiments.


--1g, ATsub=33.3 K, p=107.7 kPa
- g, ATsub=36.9 K, p= 90.8 kPa
6 8 10 20 40 60 80100
AT =T -Ta (K)
sat w sat '


Figure 7: Comparison of boiling curves in different gravity.


Conclusions


In the present paper, quasi-steady pool boiling of
FC-72 on a plane plate have been studied experimentally in
microgravity aboard the Chinese recoverable satellite SJ-8.
Bubble dynamics, heat transfer, and particularly the
transition from nucleate to film boiling mode of boiling in
microgravity are analyzed.
In summary, bubble behaviors in pool boiling on plane
plate heater in microgravity have a direct effect on the
characteristic of the transition from nucleate to film boiling
at different subcooling. At high subcooling, it is difficult for
the coalesced bubble with a smooth surface and small size
to cover the whole heater surface, resulting in a special
region of gradual transition boiling in which nucleate
boiling and local dry area can co-exist. Correspondingly,
there is no maximum on boiling curves corresponding to
CHF. On the contrary, the strong surface oscillation of the


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

coalesced bubble at low subcooling may cause the
re-wetting of the dry spots and even more activated nucleate
sites, and then the surface temperature may keep constant or
even fall down with the increasing heat flux unless an
abrupt transition to film boiling occurs with a quick
increase of the heater temperature.


Acknowledgements


The present study is supported by the National Natural
Science Foundation of China under Grant No 10972225.


References


[1] Di Marco, P. Review of reduced gravity boiling heat
transfer: European research. J. Jpn. Microgravity Appl.,
20: 252-263 (2003).
[2] Johnson, H.A. Transient boiling heat transfer to water.
Int. J. Heat Mass Transfer, 14: 67-82 (1971).
[3] Kim, J. Review of reduced gravity boiling heat transfer:
US research. J. Jpn. Microgravity Appl., 20: 264-271
(2003).
[4] Lee, H.S., Merte, H., Chiaramonte F. Pool boiling
curve in microgravity. J. Thermophy. Heat Transfer, 11:
216 (1997).
[5] Ohta, H. Review of reduced gravity boiling heat
transfer: Japanese research. J. Jpn. Microgravity Appl.,
20: 272-285 (2003).
[6] Oka, T, Abe, Y., Mori, Y.H., Nagashima, A. Pool
boiling of n-pentane, CFC-113, and water under
reduced gravity: parabolic flight experiments with a
transparent heater. J. Heat Transfer, 117: 408-417
(1995).
[7] Straub, J. Boiling heat transfer and bubble dynamics in
microgravity. Adv. Heat Transfer, 35: 57-172 (2001).
[8] Zhao J.F., Li J., Yan N., Wang S.F. Bubble behavior
and heat transfer in quasi-steady pool boiling in
microgravity. Microgravity Sci. Tech., 21(S1):
S175-S183 (2009).




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