Microgravity Sci. Technol. (2012) 24:307–325
DOI 10.1007/s12217-012-9315-8
ORIGINAL ARTICLE
Nucleate Pool Boiling Experiments (NPBX)
on the International Space Station
Vijay Kumar Dhir · Gopinath R. Warrier ·
Eduardo Aktinol · David Chao · Jeffery Eggers ·
William Sheredy · Wendell Booth
Received: 5 December 2011 / Accepted: 13 June 2012 / Published online: 19 July 2012
© Springer Science+Business Media B.V. 2012
Abstract During the period of March–May 2011, a series of boiling experiments was carried out in the Boiling Experimental Facility (BXF) located in the Microgravity Science Glovebox (MSG) of the International
Space Station (ISS). The BXF Facility was carried to
ISS on Space Shuttle Mission STS–133 on February 24,
2011. Nucleate Pool Boiling Experiment (NPBX) was
one of the two experiments housed in the BXF. Results
of experiments on single bubble dynamics (e.g., inception and growth), multiple bubble dynamics (lateral
merger and departure, if any), nucleate pool boiling
heat transfer, and critical heat flux are described. In
the experiments Perfluoro-n-hexane was used as the
test liquid. The system pressure was varied from 51 to
243 kPa, pool temperature was varied from 30◦ to 59◦ C,
and test surface temperature was varied from 40◦ to
80◦ C. The test surface was a polished aluminum disc
(1 mm thick, 89.5 mm in diameter) heated from below
with strain gage heaters. Five cylindrical cavities were
formed on the surface with four cavities located at the
corners of a square and one in the middle. During experiments the magnitude of mean gravity level normal
This work was initially supported under the NASA
Microgravity Fluid Physics Program.
V. K. Dhir (B) · G. R. Warrier · E. Aktinol
Henry Samueli School of Engineering and Applied Science,
UCLA, Los Angeles, CA, USA
e-mail: vdhir@seas.ucla.edu
D. Chao · W. Sheredy
NASA Glenn Research Center, Cleveland, OH, USA
J. Eggers · W. Booth
Zin Technologies, Cleveland, OH, USA
to the heater surface varied from 1.2 × 10−7 ge to 6 ×
10−7 ge . The results of the experiments show that a
single bubble continues to grow to occupy the size of
the chamber without departing from the heater surface.
During lateral merger of bubbles, at high superheats
a large bubble may lift off from the surface but continues to hover near the surface. Neighboring bubbles
are continuously pulled into the large bubble. At low
superheats bubbles at neighboring sites simply merge to
yield a larger bubble. The larger bubble mostly locates
in the middle of the heated surface and serves as a
vapor sink. The latter mode continues to persist when
boiling is occurring all over the heater surface. Heat
fluxes for steady state nucleate boiling and critical heat
fluxes are found to be much lower than those obtained
under earth normal gravity conditions. The data are
useful for calibration of results of numerical simulations. Any correlations that are developed for nucleate
boiling heat transfer under microgravity condition must
account for the existence of vapor escape path (sink)
from the heater, size of the heater, and the size and
geometry of the chamber.
Keywords Bubble dynamics · Nucleate boiling ·
Critical heat flux · Microgravity
Introduction
At earth normal gravity boiling is known to be a very
efficient mode of heat transfer, and as such, it is employed in component cooling and in various energy
conversion systems. For space applications, boiling can
also be a preferred mode of heat transfer since for
a given power rating the size of a component can
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be significantly reduced. Applications of boiling heat
transfer in space can be found in the areas of thermal
management, fluid handling and control, power systems, on-orbit storage and supply systems for cryogenic
propellants and life support fluids, and for cooling of
electronic packages associated with various instrumentation and control systems. Recent interest in exploration of Mars and other planets, and the concept of insitu resource utilization on Mars highlights the need to
understand the effect of gravity on boiling heat transfer
at gravity levels of 1 ≥ g/ge ≥ 10−7 .
Studies of boiling at low gravity can be grouped
into two periods- the studies that were conducted in
the 1960s mostly at NASA Glenn Research Center
and the studies that have been conducted during the
last three decades. In the earlier studies, single bubble
dynamics (bubbles growth and departure) and nucleate boiling heat transfer on ribbons and wires were
studied. Although these studies provided valuable insights to the phenomena, the duration of experiments
at low gravity was only a few seconds and did not
represent quasi-static conditions. In the later studies
boiling experiments at g/ge ∼
= 10−2 and g/ge ∼
= 10−4
have been conducted for much longer durations of low
gravity. However, these experiments have often yielded
contradictory data and have not been able to provide
understanding of the phenomena up to a level that is
necessary for development of models or correlations.
As such at present we neither have a basis for scaling of
the effect of fluid properties and gravity nor have correlations for nucleate and maximum heat fluxes which
can be used for design purposes.
Amongst the studies conducted in the 1960s, Siegel
and Keshock (1964) studied the dynamic behavior of
bubbles on an isolated site formed on a very smooth
horizontal nickel surface. The experiments were conducted for g/ge varying from 1 to 0.014, and saturated
water at one atmosphere pressure was used as the test
liquid. From the measurement of growth rate and bubble diameter at departure it was concluded that none of
the correlations reported in the literature at that time
yielded predictions that were in agreement with data as
g/ge was reduced. Also, it was found that at reduced
gravity, after a large bubble departed several smaller
bubbles growing at the same site were sucked into the
larger bubble before the cycle repeated itself. Furthermore, it was noted that bubble diameter at departure
and growth period increased with decrease in gravity
and the growth rate of the bubble at departure had
some influence on the bubble diameter at departure.
However, the magnitude of gravity had little effect on
the contact angle which was found to remain nearly
constant during the growth period.
Microgravity Sci. Technol. (2012) 24:307–325
Using the bubble growth rate data, Keshock and
Siegel (1964) evaluated the magnitude of the forces that
lead to the bubble departure. They noted that bubble
departure was governed by the balance of buoyancy,
surface tension, and inertial force. For slow growing
bubbles, buoyancy was balanced by surface tension
forces whereas for the fast growing bubbles it was
the liquid inertia and surface tension that determined
the bubble diameter at departure. Thus it was found
that for fast growing bubbles, there was no effect of
gravity on bubble diameter at departure, whereas for
slow growing bubbles the bubble diameter at departure
increased as g−1/2 .
Siegel and Usiskin (1959) studied nucleate boiling
on electrically heated vertical and horizontal ribbons
under free fall conditions. During the free fall the platform carrying the test section traveled about 8 ft. From
photographic observations it was found that during the
free fall vapor remained adjacent to the heated surface
and did not appear to push away from the heater surface. Subsequently, Usiskin and Siegel (1961) measured
critical heat flux on a 1 mm diameter platinum wire
under the low gravity conditions that lasted about 1 second. For gravity levels of 1 ≤ g/ge ≤ 0.04, it was found
that observed critical heat flux was generally consistent
with the g1/4 dependence given by the hydrodynamic
theory while nucleate boiling data were comparable to
those obtained at earth normal gravity. Siegel (1967)
reviewed the reduced gravity boiling studies and concluded that the effect of magnitude of gravity on nucleate boiling heat transfer is small. Referring to the work
of Cochran et al. (1966), he concluded that the magnitude of gravitational acceleration becomes even less
important with liquid subcooling. It should be stressed
that although in studies prior to 1967, gravity levels up
to 10−5 ge were obtained, the duration of experiments
in reduced gravity was less than 7 sec. Transient effects
must have played an important role in the nucleate and
critical heat flux data obtained in these short duration
tests.
Oka et al. (1995) have studied pool boiling of nPentane, R-113, and water on transparent heaters under parabolic flight conditions. During the flight, significant variation of the gravity level occurred and only
for about 5 seconds, reduced gravity, g/ge , of about 0.02
persisted normal to the heater surface. It was noted that
during stable nucleate boiling of n-Pentane and R-113,
bubble merger at the heater surface occurred by sliding
of the bubbles along the surface. However in water,
coalescence of bubbles occurred in the direction normal
to the heaters by suction of smaller, newer bubbles into
larger bubbles. The difference in bubble merger behavior for water and the two other liquids was attributed to
Microgravity Sci. Technol. (2012) 24:307–325
differences in surface tension and wettability characteristics. It was postulated that vapor/liquid/solid contact
behavior attains significant importance at low gravities.
However, the authors reported no quantitative value
of physical parameters (e.g., contact angle) which could
be used to relate to the observed behavior. During the
period of low gravity no bubbles were seen to detach
from the heater surface. Nucleate boiling heat fluxes
under low gravity condition for R-113 and n-Pentane
were found to be comparable to those obtained under
earth normal gravity conditions. However, with water,
a substantial reduction in nucleate boiling heat fluxes
at a given wall superheat was found at the low gravity
levels. All of the reported data were obtained for subcooled liquid with a liquid subcooling as high as 20 K.
No critical heat flux condition (CHF) was achieved in
water, but CHF with n-Pentane and R-113 was found
to be about 40% of that under earth normal gravity
conditions.
Abe et al. (1994) have studied pool boiling of a
mixture of ethanol and water under free fall conditions
of a drop tower. In the experiments, reduced gravity of
the order of 10−5 ge existed for about 10 seconds. It was
found that during boiling with this non-azeotropic mixture, the nucleate boiling heat transfer coefficients were
about 20% higher than those under normal gravity conditions. Also, with 11.3% weight mixture of ethanol in
water, the critical heat flux observed at 10−5 ge , was only
about 20–40% lower than that obtained at the earth
normal condition. This finding again suggests that for
these short durations of microgravity, the dependence
of critical heat flux on gravity is very weak. From visual
observations it has been suggested by Abe et al. that
the Marangoni effect along the bubble causes the liquid
to flow into micro/macro layer underneath the bubble.
The inflow of liquid is also responsible for lifting of
the bubbles from the surface. The bubbles, however,
continued to position themselves near the surface. At
high heat fluxes a double layer of bubbles was formed
on the heater surface with secondary bubbles sucking
the primary bubbles and enlarging themselves.
Straub (1994) has reviewed the microgravity boiling
heat transfer work conducted in his laboratory since
1980. He and his co-workers have conducted saturated
and subcooled boiling experiments in a drop tower
facility a ballistic rocket and in parabolic flights. In
the drop tower the duration of microgravity was about
10 seconds, in the aircraft 20 seconds, and in the ballistic rocket about 6 minutes. Both electrically heated
wire heaters and flat plate heaters were used in the
experiments. During subcooled boiling of R-113 on a
horizontal wire in the ballistic rocket flight (g/ge <
10−4 ), a vapor film appeared to surround the wire once
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power was supplied to the wire. The vapor film was
observed to pulsate and during receding period of the
vapor film front, a liquid film was deposited on the wire.
Rewetting of the wire led to activation of nucleation
sites on both sides of the oscillating film. Condensation at the vapor-liquid interface occurred and by
Marangoni effect hotter liquid from near the wall was
pushed into the colder bulk liquid. For pure vapor,
existence of Marangoni convection cannot be justified.
Thus the authors postulated that there were some noncondensibles in the liquid which, upon evaporation of
liquid, tended to accumulate at the outer edge of the
film. The accumulation of the non-condensibles caused
local saturation pressure of the vapor to decrease and
reduce the interfacial temperature. This mode of boiling was termed as nucleate boiling and magnitude of
nucleate boiling heat fluxes at a given wall superheat
was found to be comparable to that at g/ge = 1, under
similar subcooling conditions. On a flat plate heater a
large vapor bubble occupying the whole heater surface
formed upon nucleation. During the rapid growth of
the bubble, a foam of smaller bubbles was created in the
thin liquid film held between the heater and the large
bubble. Also, it has been noted that a thermocapillary
flow existed from the base of the bubble to the top and
it lifted up the back of the bubble. Smaller bubbles were
observed to be present on the heater only when the
liquid was subcooled.
In the parabolic flights, when the gravity level
changed from low to high values, little change in the
heat transfer coefficient during nucleate boiling on
a platinum wire was noted, although the size of the
bubbles was observed to shrink. A similar observation
was made for the data obtained on flat plate heaters.
To explain the lack of dependence of nucleate boiling
on the level of gravity, Straub has identified primary
and secondary mechanisms for nucleate boiling. The
primary mechanism for heat transfer during nucleate
boiling is the evaporation of the thin film between the
vapor and the heater surface. The flow in the thin
film is supported by the capillary pressure gradient.
The evaporation ceases and a dry region in the central
portion of the base of the bubble is formed when the
wall superheat is sufficiently high to dislodge the molecules attached to the heater surface. This qualitative
description of the evaporation process is similar to the
quantitative analysis performed by Lay and Dhir (1995)
for fully developed nucleate boiling heat transfer. It
was noted that the evaporation of the microlayer is
mainly determined by capillary forces and as such is
not influenced by gravity. The secondary mechanisms
were responsible for transfer of heat and mass from
the wall to the bulk. These included mass and energy
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carried by departing bubbles, and convection induced
by bubble motion and condensation at the top of the
bubbles. Surface tension was claimed to be the dominant force that led to merger of bubbles horizontally
and vertically, migration of secondary bubbles to larger
bubbles, and lifting of larger bubbles by nucleation
of secondary bubbles underneath. In subcooled boiling, Marangoni convection tended to hold the larger
bubbles against the heater surface. No quantitative
analyses to support these qualitative observations were
provided. However, it was noted that to develop a physical understanding of boiling under microgravity conditions, basic studies dealing with boiling heat transfer
and physical processes associated with single bubbles
should be performed. The single bubble studies should
include bubble inception, bubble growth, bubble dynamics, evaporation and condensation around bubbles
attached to the heater, bubble coalescence, and stability
of dry spots underneath bubbles.
Straub and Micko (1996) have reported results of
subcooled and saturated boiling of R-134a on 0.05 and
0.2 mm diameter platinum wires uin the microgravity
environment of the space shuttle. Nucleate boiling heat
flux at a given wall superheat was found to be higher
in microgravity conditions than that obtained under
earth normal gravity conditions. The enhancement in
the rate of heat transfer was higher for the thicker wire.
For saturated liquid, the critical heat flux under microgravity condition was lower than that at earth normal
gravity; however it was much higher than that which
would be predicted from the hydrodynamic theory. The
liquid momentum created during bubble formation and
coalescence was attributed to lead to bubble departure
from the heater.
In another paper, Straub et al. (1996) have reported
results of bubble dynamics and pool boiling heat transfer on a 0.26 mm diameter hemispherical surface placed
in the BDPU (Bubble, Drop, and Particle Unit) facility.
This facility was carried in the space shuttle. Again,
little difference in the nucleate boiling data obtained
under 1 g and μg condition was found. The critical
heat flux for saturated liquid under microgravity was
found to be only 15% lower than that at 1 g. With R11 nucleate boiling heat fluxes as high as 90 W/cm2
were observed under microgravity conditions. Bubble
dynamics was observed to change significantly with
change in liquid subcooling, system pressure and wall
superheat. Surface tension, wetting behavior of the liquid, bubble coalescence and liquid momentum during
bubble formation was found to influence the boiling
process. Thermocapillary flow was found to play an
important role under subcooled boiling conditions.
Microgravity Sci. Technol. (2012) 24:307–325
Ervin et al. (1992) and Ervin and Merte (1993) have
studied transient nucleate boiling on a gold film sputtered on a quartz plate by using a 5 second drop tower
(g/ge ∼
= 10−5 ) at NASA Glenn Research Center. In the
experiments R-113 was used as the test liquid. From
the experiments, it was found that time or temperature
for initiating nucleate boiling was greater for a pool
at saturation temperature than that for a subcooled
pool. They also noted the occurrence of energetic boiling at relatively low heat fluxes. The energetic boiling
in which the vapor mass rapidly covered the heater
was postulated to be associated with an instability at
the wrinkled vapor-liquid interface. Merte (1994) and
Merte et al. (1995) have also reported results of pool
boiling experiments conducted in the space shuttle on
the same surface that was used in the drop tower
tests. Subcooled boiling under microgravity conditions
was found to be unstable. Because of a large step in
power input to the heater, the heater surface temperature rose rapidly. Nucleation generally occurred at
higher superheats and resulted in bubbles that grew
energetically. From analysis of the data the investigators found evidence of both quasi-homogeneous and
heterogeneous nucleation. It was noted that long term
steady state nucleate boiling could be maintained on a
flat plate heater under microgravity conditions when a
large bubble parked itself a short distance away from
the heater and acted as a vapor sink. Also, from runs
lasting a few seconds to up to about two minutes it
was concluded that nucleate pool boiling heat transfer
coefficients in microgravity are higher than those at
earth normal gravity. No mechanistic explanation was
given for this observation. Furthermore, because of the
onset of dryout, the maximum heat flux in microgravity
was reduced substantially.
These observations have been reinforced through
the results of two sets of recent experiments (Merte
et al. 1998) on the space shuttle. Additionally, it has
been noted that liquid subcooling enhances nucleate
boiling heat transfer in microgravity. A detailed review
of various studies has been reported by Dhir (2002).
In the present study of nucleate boiling heat transfer under microgravity conditions an approach is used
such that while providing basic knowledge of the phenomena, it also leads to development of simulation
models and correlations that can be used as design
tools for a wide range of gravity levels. In this study
a building block type of approach is used and only
pool boiling is investigated. Starting with experiments
with a single bubble, the complexity of the experiments
was increased to multiple bubbles placed on a twodimensional grid. A polished aluminum wafer was used
Microgravity Sci. Technol. (2012) 24:307–325
as the test surface because on such a surface cavities
of desired size and shape can be easily fabricated. In
the experiments, liquid subcooling and wall superheat
were varied parametrically. The system pressure in the
experiments was varied over a narrow range around
one atmosphere. In the experiments, the heater surface
temperature is to be maintained nearly constant by controlling power input to different regions on the heater.
Data were taken for heater temperatures, power input
to heaters and liquid temperature in the pool. Visual
observations provided quantitative data on bubble inception, bubble growth, bubble merger and bubble departure processes. The data were obtained under nearly
steady state microgravity conditions.
Modeling/complete numerical simulation of the boiling process is an integral part of the experimental
effort. Scaling of the effect of gravity in the range 1 ≥
g/ge ≥ 10−7 has been a prerequisite for the model. A
quantitative comparison of data from experiments for
bubble dynamics including bubble growth, merger and
departure process has been made.
Overall Objective
Develop a mechanistic model for nucleate boiling under microgravity conditions. The model is to be supported by experiments on the ISS. Use a building block
approach to validate different components of the model
and increase complexity in steps.
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minum wafer (6061-T6 aluminum alloy, surface roughness between 16–19 nm) with five artificial cavities as
the boiling surface. The aluminum wafer (shown in
Fig. 1) was bent at the edge to join with the housing
holding the wafer. The overall diameter is 89.5 mm
and its thickness is 1.0 mm. Five artificial cavities
were etched on the heater surface using the Electrical
Discharge Machining (EDM) technique. Four of the
cavities are located in the corners of a square (38.18 mm
per side), while the fifth cavity is located at the center.
Hence the diagonal distance between the central cavity
(denoted as cavity 1) and the other cavities (denoted as
cavities 2, 3, 4, and 5) is 27.0 mm (see Fig. 1). Single
bubble departure diameter predicted at 10−4 ge , using
numerical simulations, was used in deciding on the
spacing between the prefabricated cavities. The spacing
chosen was such that lateral bubble merger would occur
prior to departure when multiple nucleation sites are
activated on the heater surface. Each of the cavities was
designed to have the following nominal dimensions:
diameter ∼ 10 μm and depth ∼ 100 μm. However as it is
extremely difficult to precisely control the dimensions
of the cavities during the EDM machining process,
some variation in cavity dimensions are to be expected.
Based on inspection of the cavities it was found that the
cavity diameters varied from 16.3 to 17.6 μm. Figure 2
shows a photograph of one of the etched artificial
cavities (D ∼ 16.3 μm).
Specific Objectives
•
•
•
•
Single bubble dynamics (nucleation, growth and
departure) under microgravity conditions under
constant wall temperature.
Effect of liquid subcooling, wall superheat and system pressure on single bubble dynamics.
Heat flux variation on the heated surface during
single bubble evolution.
Lateral merger of bubbles formed at neighboring
sites and mechanism of vapor removal from the
surface.
Experiments
NPBX Experimental Apparatus
The Nucleate Pool Boiling eXperiment (NPBX) is one
of two experiments housed in the Boiling eXperiment
Facility (BXF). NPBX uses a diamond turned alu-
Fig. 1 Schematic of the aluminum heater
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Fig. 2 Photograph of an etched artificial cavity (D ∼ 16.3 μm)
The heating of the aluminum wafer was accomplished using strain gage heaters bonded to the backside of the wafer. In addition, thermistors were also
bonded to the backside of the wafer to monitor the
wafer temperature at several locations. The strain gage
heaters and thermistors are grouped such that each of
the five cavities could be activated independently.
A schematic of the strain gages and thermistors on
the backside of the wafer is shown in Fig. 3. The
strain gages were arranged such that each cavity had
two groups of strain gage heaters associated with it;
one strain gage heater directly underneath the cavity
Fig. 3 Schematic of the strain
gage and thermistor
arrangement on the backside
of the wafer
Microgravity Sci. Technol. (2012) 24:307–325
(called the cavity heater and labeled as ‘C’ in Fig. 3)
and another surrounding it (called the surround heater
and labeled as ‘S’ in Fig. 3). In addition, there are
two groups of background heaters (labeled background
heaters ‘B1’ and ‘B2’ in Fig. 3). The background heaters
are not directly associated with the cavities. These
heaters are used to heat parts of the wafer that are not
heated by the cavity or surround heater groups. As such
there were a total of 12 heater groups on the backside
of the wafer. In Fig. 3, each of the heater groups associated with a particular cavity is identified by the corresponding cavity number; for example, heater groups
C1 and S1 are the cavity and surround heaters groups
associated with cavity 1 (center cavity), respectively.
The strain gages used in the experiment were manufactured by Vishay Precision Group. The strain gages used
for the surround and background heaters are Model:
EA-06-250AF-120, which has dimensions of length =
11 mm and width = 7 mm. The cavity heaters are EA06-062TT-120, which are dual gages each with a nominal resistance of 120 ohms (total length = 8 mm, total
width = 7 mm). All strain gages have are approximately
0.05 mm thick.
The arrangement of the thermistors on the backside
was similar to the arrangement of the heater groups.
For each cavity, one thermistor was used to measure
Microgravity Sci. Technol. (2012) 24:307–325
313
Fig. 4 Schematic of heater
assembly
the temperature almost directly below the cavity while
another was used to measure the temperature of the
surrounding area. With this arrangement, the temperature of each heater group was controlled by adjustment of the power to the heater. For example, power
to heater group C1 was controlled using output from
thermistor T1, while power to heater group S1 was
controlled using output from thermistor T2. A similar
arrangement was used for the other cavities. Note that
power to the background heater groups B1 and B2 is
controlled using output from thermistors 12 and 11,
respectfully. The 12 thermistors bonded to the backside
of the aluminum wafer were manufactured by Omega
(Model: TH-44007-36-T). The maximum bead diameter
of these epoxy encapsulated thermistors is 2.4 mm and
they have a nominal resistance of 5000 ohm at 25 ◦ C.
Figure 4 shows the cross section of the complete
heater assembly. The aluminum wafer (with strain gage
heaters and thermistors bonded to the backside) was
bonded to a G-11 base using 3M Scotchweld 2216
epoxy. Four additional thermistors were provided in
the G-11 base. These thermistors were placed at distances of 5.3, 8.6, 14.7, and 24.5 mm from the bottom of the aluminum wafer. Lead wires soldered to
the strain gage heaters and thermistors were used to
connect them to the power supply and data acquisition
system, respectively. Note that the lead wires are not
shown in Fig. 4. The backside of G-11 base was filled
with an insulating epoxy (3M Scotchcast 251 epoxy)
to a depth of approximately 19 mm. Hence three of
the thermistors in the G-11 base were embedded in
the insulating epoxy, while the fourth was located in
the fluid (just below the insulating epoxy). The temperatures measured by the three thermistors in the
insulation and the thermistor located in the fluid were
used to estimate the heat loss through the backside of
the heater assembly. The four thermistors embedded in
the insulation were made by YSI (Model: 014-55034NA-IT-ST). These thermistors are glass encapsulated
and have a maximum bead diameter of 2.4 mm. Their
nominal resistance is 5000 ohms at 25 ◦ C. Figures 5a
and b show photographs of the heater assembly. The
(a)
Fig. 5 Photographs of heater assembly a boiling surface and b backside
(b)
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top side of the wafer (boiling surface) is shown is Fig. 5a
while the backside is shown in Fig. 5b before the backside was filled with Scotchcast 251 epoxy. The strain
gage heaters (with soldered lead wires) and thermistor
arrangement can be clearly seen on the backside of the
wafer.
The schematic of the experimental apparatus is
shown in Fig. 6. It consists of the test chamber, heater
assembly, bellows, bulk fluid heater and a pump. The
heater assembly was located at the bottom of the test
chamber. The pressure (measured using three pressure transducers) in the test chamber was controlled
by changing the position of the bellows. The bellows
are controlled by external means to minimize any
oscillations. The temperature of the fluid in the test
chamber was maintained by the fluid conditioning loop
which consists of the pump, three inline heaters (total
power = 180 W) and associated plumbing. The test
chamber is also provided with six thermistors (labeled
#1 through #6 in Fig. 6) for measurement of fluid
Fig. 6 Schematic of test
chamber
Microgravity Sci. Technol. (2012) 24:307–325
temperatures. Four sapphire windows are provided on
the test section for visual observation. Two cameras
(29.97 fps) are used to record two orthogonal views of
the boiling process occurring on the aluminum wafer.
The test fluid is filtered, degassed Perfluoro-n-hexane.
The test chamber is made of aluminum and has
the following internal dimensions: height = 228.6 mm,
width = 114.3 mm. Additionally, the test chamber has a
square cross section (114.3 × 114.3 mm). The thermistors (#1 through #6) used to measure the bulk fluid temperature are located at distances of 168.7, 114.8, 112.0,
66.5, 40.6, 19.0 mm, respectively, from the top of the
aluminum wafer as indicated in Fig. 6. The bellows have
an effective diameter of 16.5 cm and an approximate
displacement volume of 690 cm3 . As mentioned earlier,
four windows were provided on the test chamber. Each
window has dimensions of 80.0 × 80.0 mm. Two orthogonal windows are used for visual observation (using
cameras 1 and 2), while the other two windows are
used for lighting. For safety considerations, the entire
Microgravity Sci. Technol. (2012) 24:307–325
experimental apparatus is mounted inside a secondary
containment vessel. Figure 7 shows a photograph of
BXF located inside MSG onboard the ISS. The data
recorded during the NPBX experiments consists of the
following:
(i)
(ii)
(iii)
(iv)
(v)
Pressure – at three locations
Bulk liquid temperature – at six locations
Wafer temperature – at 12 locations
Insulation temperature – at four locations
Wafer power – for each heater group (12 heater
groups)
(vi) Acceleration levels – in three orthogonal directions
(vii) Video – two orthogonal views
All data, except the acceleration levels, are recorded at
a sampling rate of 20 Hz, while the video were recorded
at 29.97 fps. The acceleration levels were recorded
aboard the ISS by the Microgravity Acceleration Measurement System (MAMS) (low frequency data, low
pass filtered at <1 Hz) and the Space Acceleration
Measurement System (SAMS) (high frequency data,
sampled at 500 Hz and low-pass filtered at 250 Hz)
modules for the entire duration of the experiments. It
must be noted that the SAMS module located inside
MSG was not operational. The MAMS module is located in the U.S. Laboratory Module, Destiny, Express
Rack No. 1. The MAMS module and the Microgravity
Science Glovebox (MSG) are located in the same bay,
with MAMS mounted on the ceiling and MSG located
on the starboard side. The distance between MAMS
and MSG is approximately 2.3 m. The wafer was oriented such that the magnitude of the acceleration was
highest pointing towards and normal to the heater (zdirection as shown in Fig. 6). No vibration isolation
platform was used in the experiments.
Fig. 7
ISS
Boiling Experimental Facility (BXF) in MSG onboard
315
On ground, prior to filling the chamber with the test
liquid (Perfluoro-n-hexane), the fluid was distilled using a custom built distillation facility. After distillation,
the liquid was degassed using a custom built degassing
setup. The degassing was continued until the measured
dissolved gas concentration was less than 20 ppm. The
test chamber was then evacuated and filled with the
distilled, degassed test liquid. The total dissolved gas
concentration in the test liquid was below 100 ppm at
the time of initial filling the chamber.
During preliminary tests performed on ground, the
contact angle was measured from the bubble shape.
The measured contact angle varied between 33◦ –38◦ .
Experimental Anomalies
1) Though the experiments were designed with negligible content of dissolved gas, as the experimental period progressed, the dissolved gas content
increased with time and subsequently stabilized.
The maximum measured dissolved gas content was
about 740 ppm. The presence of dissolved gas
changes the saturation temperature by 1.5 to 6.2 ◦ C
(depending on the system pressure). Additionally,
it also affected the nucleation characteristics significantly and also the bubble dynamics when the
liquid was subcooled.
2) Although precautions were taken to minimize/
eliminate bubble formation at the edge, where
the wafer was embedded into the base, numerous
bubbles were observed at the edge. As a result of
variations in gravity, the bubbles were observed to
move along the edge.
3) Contaminant particles were observed in the test
liquid in spite of a filter being present in the fluid
circulation loop.
4) When the vapor production rate was high, the
bellows were unable to maintain constant system
pressure. This was due to the slow movement of
the bellows.
5) After about three weeks of testing, a major anomaly occurred. The anomaly was accompanied by
a sudden drop in the voltage of one of the 24 V
buses. The faulty bus provided power to the liquid heaters, cavity and surround heaters as well
as to heaters of the second experiment housed in
BXF (MicroArray Boiling Experiment, MABE).
The voltage drop coincided with erratic readings of
pressure sensors and some temperature sensors.
6) Fortunately the background heaters of NPBX were
powered by a second 24 V bus. As a result, subsequent to the anomaly, we were able to conduct
316
Microgravity Sci. Technol. (2012) 24:307–325
several single bubble, bubble merger and integral
nucleate boiling experiments. However, there were
significant constraints on the range of parameters
over which experiments could be conducted.
Experimental Procedure
Two different sets of experiments were conducted in
the NPBX setup. These experiments can be divided
into (i) single and multiple bubble experiments, and
(ii) integral nucleate boiling heat transfer experiments.
In the single and multiple bubble experiments, single
and/or multiple nucleation sites were activated, with the
goal of studying the bubble inception, growth, merger,
lift off processes and the associated heat transfer. On
the other hand, in the integral boiling experiments,
the goal was to obtain the boiling curve on the whole
surface by parametrically varying the test conditions
such as wall temperature, pressure and bulk liquid
temperature. As such, no attempt was made in the
integral experiments to systematically study nucleation
at specified sites. The experiments were conducted in
the temperature controlled mode rather than the heat
flux controlled mode.
One of the key aspects of the single and multiple
bubble experiments was the detection of bubble inception in the absence of visual information. The following procedure was adopted to detect bubble inception.
The entire wafer is initially maintained at a prescribed
temperature. The temperature of the site to be nucleated is increased linearly at a prescribed rate, which
can be varied. The power to the cavity and surround
heaters automatically adjusts to attain the specified
temperature. The temperature of the site is increased
until bubble inception occurs. When nucleation occurs,
the temperature at the site decreases. This decrease in
temperature is detected by the temperature sensors on
the backside of the wafer (underneath the cavity). The
time lag between bubble inception and the measured
temperature drop on the backside is approximately
one second. This was determined by syncing the video
recordings and the temperature measurements. At this
point, the power to the cavity and surround heaters
associated with the particular nucleation site is automatically cut off or reduced to maintain a prespecified
temperature. Thereafter the entire wafer is maintained
at the preset temperature. Note that, if desired, it is
possible to maintain different portions of the wafer at
different temperatures.
At the beginning of each experimental run the bulk
liquid was heated to the desired temperature. The maximum temperature to which the bulk fluid is heated by
the bulk liquid heaters is 59 ◦ C. In order to prevent any
boiling from occurring on the bulk liquid heaters during
this process, the test chamber pressure is increased to
202.65 kPa (2.0 atm., Tsat = 79.3 ◦ C). As a result, the
liquid is always subcooled (minimum liquid subcooling
is approx. 20 ◦ C) as it is being heated by the bulk
liquid heaters. In the worst case scenario, if boiling does
occur on the bulk fluid heaters, the vapor generated
on the heaters will likely condense as the bulk liquid
is highly subcooled. The liquid is then pumped from
the bottom of the test chamber through the inline bulk
fluid heaters and back into the test chamber. The hot
liquid is discharged close to the top of the test chamber.
Once the liquid has attained the required temperature,
the pump and bulk fluid heaters are turned off. The
liquid is then allowed to settle for about 5 mins. before
continuing with the rest of the experiment.
During the liquid settling time, the various parameters for the particular experiment are uploaded to
the BXF controller. For single and multiple bubble
experiments the input parameters include the following: (i) test chamber pressure, (ii) temperatures of the
different regions of the wafer, and (iii) nucleation sites
to be activated, and (iv) temperature ramp rate at the
nucleation site. For integral boiling curve experiments,
the input parameters include (i) test chamber pressure
(ii) initial wafer temperature and (iii) magnitude of
temperature increments and (iv) duration for which a
specified temperature was to be maintained.
Once these parameters are uploaded, the experiment is ready to proceed. The experiment is conducted
remotely with downlink of video and data. The test
chamber pressure is first set to the prescribed value.
The wafer temperature is then set to the prescribed
value. For single and multiple bubble experiments, the
temperatures of the cavity and surround heaters associated with the site(s) to be activated were increased
until bubble nucleation occurs. During this process the
temperature of the rest of the wafer was held constant at the prescribed temperature. For the integral
experiments, the temperature of the entire wafer was
increased by increments of 1–3 ◦ C every 2 minutes. The
total time for each experiment was approximately 15–
20 minutes. At the end of each experiment, power to
all heaters was cut off and the pressure of the system
was increased to 253.31 kPa so as to condense the vapor
present in the test chamber. The typical time taken for
the vapor to condense was about 10–15 mins.
The BXF controlled video cameras are programmed
to automatically record two orthogonal views of the
heater surface during each NPBX experiment. These
videos are recorded on video tape at a frame rate
of 29.97 fps. In addition, during the duration of each
experiment, the following data was recorded: (i) tem-
Microgravity Sci. Technol. (2012) 24:307–325
317
Table 1 Test parameters
System pressure
Test liquid temperature
Test surface temperature
Mean magnitude of level of
gravity normal to the wafer
Mean magnitude of level of
gravity in the plane of the wafer
Dissolved gas content
=
=
=
=
51 kPa to 243 kPa
30 ◦ C to 59 ◦ C
40 ◦ C to 80 ◦ C
1.7×10−7 to 6.0×10−7 ge
= 1.2×10−7 to 3.5×10−7 ge
= 0 to 737 ppm
Data Reduction and Uncertainty
in Measured Quantities
Data Reduction
The power (Q) supplied to each heater group is calculated as,
Q = I2 R
perature at various locations on the backside of the
wafer (12 thermistors), (ii) temperatures in the insulation (4 thermistors), (iii) bulk fluid temperatures (6
thermistors), (iv) current supplied to each heater group
(12 heater groups), and (v) pressure at various locations in the test chamber (3 transducers). All the data
were recorded at a sample rate of 20 Hz. The gravitational acceleration data from sensors in the Microgravity Science Glovebox (MSG) on ISS was continuously
recorded onboard the ISS. These data are cataloged
based on date and time and were available for download. Table 1 gives the range of the test parameters that
were varied during the experiments.
The dissolved gas concentration was calculated at
regular intervals (typically once a week) during the
duration of the experiments. The dissolved gas concentration was determined by the following procedure: (i)
at a given fluid temperature, extend bellows slightly so
as to set the pressure in the chamber to be between
34 and 46 kPa (ii) calculate the saturation pressure
(for the given temperature) for the test fluid using the
Antoine equation, assuming it does not contain any
dissolved gas (iii) calculate the difference between the
set pressure and the calculated saturation pressure (iv)
if there is a difference, use Henry’s law to calculate
the gas concentration of the dissolved gas. Note that
Henry’s law constant (= 5.4 × 10−5 ) used is the one
measured for air in FC-72 in the temperature range
31 ◦ C to 60 ◦ C. FC-72 is the FC-72 is the commercial
grade version of Perfluoro-n-Hexane made by 3M. The
variation in the measured dissolved gas concentration
over time is shown in Table 2. Note that the NPBX
experiments were conducted on days 89–91, 95–96, and
129–133.
Table 2 Measured dissolved gas concentration
Day
P (kPa)
T (◦ C)
PPM
81
91
94
101
117
129
132
35.46
46.61
31.41
42.56
41.54
37.49
46.61
30.3
33.1
23.1
24.8
23.6
23.1
30.1
46
299
261
712
737
589
543
(1)
where I is the measured current and R is the resistance
of the heater group. Note that the current supplied
to each heater group was recorded digitally in counts;
the counts were then converted to engineering units
(Amperes) using conversion factors determined during
calibration. Tests conducted earlier (at earth gravity)
have shown that the change in resistance of the heater
groups is negligible for the range of temperatures encountered in these experiments (30–85◦ C). The heat
flux (qw ) on the wafer surface is calculated as,
Q − Qloss
qw =
(2)
Aw
where
Q is the sum of the power supplied to each
heater group, Qloss accounts for all the losses, and Aw is
the surface area of the wafer.
In order to develop a procedure to determine Qloss ,
several steady-state natural convection experiments
were performed, at earth normal gravity. In these tests
an energy balance was performed. While making this
energy balance, the heat transfer coefficients on the
top of the wafer and side of the heater assembly were
determined from standard textbook correlations. The
heat loss at the edges of the wafer was accounted for
by assuming that the unheated edge of the wafer acts
as a fin. Based on the energy balance performed, the
effective thermal conductivity of the insulation (3M
Scotchcast 251 epoxy) was determined. The thermal
conductivity of the insulating epoxy differs from the
value given by the manufacturer because copper lead
wires are embedded in it. These wires were used to connect the heater groups and thermistors to their respective control circuits. In performing the energy balance,
the temperature of the wafer surface was assumed to be
uniform (= area averaged value of the 12 thermistors
bonded to the backside of the wafer). The variations in
the wafer surface temperature were small. For example,
for the earth normal gravity experiments, the temperature differences varied from ±0.3 ◦ C to ±0.8 ◦ C as
the power supplied increases from 18 W (T = 7.8 ◦ C)
to 70 W (T = 20.9 ◦ C), where T = Tw –Tl . For the
microgravity experiments, the temperature differences
vary from ±0.2 ◦ C to ±0.4 ◦ C as the power supplied
varies from 2.4 W (T = 8.6 ◦ C) to 7.4 W (T = 24.6 ◦ C).
318
The effect of these small temperature differences is
expected to be negligible. Once the effective thermal
conductivity was determined, a similar energy balance
was performed for the microgravity natural convection
experiments to determine Qloss . It must be noted, that
in order to determine the heat transfer coefficients at
the side of the wafer assembly in microgravity, the standard textbook correlation for natural convection on a
vertical plate was used with the appropriate magnitude
of the gravitational acceleration.
The gravitational acceleration values reported in this
paper are the arithmetic average values of the gravity
levels recorded by MAMS at a frequency of 0.06 Hz.
For example, for the single bubble case discussed in this
paper, the arithmetic and Root Mean Square (RMS)
values of g/ge are: X-axis (plane of the wafer) – mean =
1.1 × 10−7 , RMS = 1.9 × 10−7 ; Y-axis (plane of the
wafer) – mean = 2.1 × 10−7 , RMS = 2.3 × 10−7 ; Z-axis
(normal to the wafer) – mean = 2.5 × 10−7 , RMS =
2.6 × 10−7 .
Uncertainty Analysis
For integral experiments, the temperatures measured
at 12 locations on the wafer were area-averaged to
determine the average temperature of the wafer. The
maximum and minimum temperature deviation from
the area-averaged temperature was also noted. The
uncertainty in the temperature measurements (wafer
thermistors, insulation thermistors and the bulk fluid
thermistors) is ±0.2 ◦ C and the uncertainty in the
pressure measurement is ±1 KPa. The uncertainty of
the current measurement varied between 6–13 mA,
depending on the heater group. As mentioned earlier,
the change in the resistance of the heater groups in the
temperature range 30–85 ◦ C was found to be negligible.
Based on the measurement uncertainties given above,
the uncertainty of the calculated total power supplied
decreased from 17% to 0.8% as the power increased
from 1.4 W to 200 W. Similarly, the uncertainty in the
calculated heat flux decreased from 26% to 1.6% as the
heat flux increased from 0.01 W/cm2 to 1.3 W/cm2 .
Microgravity Sci. Technol. (2012) 24:307–325
was held below the saturation temperature of liquid.
In the experiments heat flux was quite low as a result
there is a larger uncertainty in the measured rate of
heat transfer. Figure 8 shows the data obtained in
microgravity conditions. The characteristic length used
in defining Nusselt and Rayleigh numbers was taken to
be the heater area divided by the perimeter. Solid and
dashed lines are the predictions from the correlation of
Kobus and Wedekind (2001) and McAdams (1954), respectively. Stars are the data that were obtained during
calibration runs at earth normal gravity (Rayleigh number ≈ 109 ) whereas solid triangles are the microgravity
data (Rayleigh number ≈ 102 ) which was obtained at
the beginning of the experimental activity when all of
the heaters at the back of the wafer were energized and
only small variation in temperature existed across the
wafer. These data with a Rayleigh number about seven
orders of magnitude smaller than that at earth normal
gravity appear to be correlated well by Kobus and
Wedekind’s correlation. However, the data obtained
after the anomaly when only background heaters were
operational show large scatter and is generally higher
than that predicted by the correlations. One reason
could be that the thermal layer was still developing
when the data were taken.
Single Bubble Dynamics
Several experiments were conducted to study single
bubble dynamics at the center cavity as well as the
surrounding cavities. During ramping up of the temperature of the test surface supporting the cavity and
the surrounding region, aside from the cavity a number
Results and Discussion
Natural Convection
Prior to boiling experiments, a number of test runs
were performed to study the rate of natural convection
heat transfer under microgravity conditions. In these
experiments liquid was subcooled and a uniform temperature existed on the wafer. The wafer temperature
Fig. 8 Comparison of natural convection data with correlations
Microgravity Sci. Technol. (2012) 24:307–325
of bubbles nucleated around the cavity. Upon visualization of the bubbles, the temperature of the heater
region supporting the cavity was brought down to the
temperature of the rest of the wafer. During this period
the bubbles continued to persist on the surface while
sometimes merging with the bubble at the nucleation
site. Eventually a single bubble at the cavity dominated
while supporting a necklace of smaller bubbles around
it. Figure 9 shows bubble size and shape at different
times during the growth. During the experiment system
pressure was not constant. It steadily increased during
the first 100 seconds of the growth period. At 100 seconds wall temperature was increased to compensate for
the increase in saturation temperature. As such during
the growth period of the bubble, pressure varied from
Fig. 9 Single bubble growth for Tw = 48.2–51.5 ◦ C, Tl = 34.6 ◦ C,
P = 60–78 kPa, gz /ge = 2.5 × 10−7
319
60.8 to 76 kPa. whereas the wall temperature varied
from 48.2 to 51.5 ◦ C. As a result, the liquid subcooling
varies from 5 to 1 ◦ C, while the wall superheat varies
from 4 to 7 ◦ C. At 178 seconds, the bubble has grown
to about 67 mm in diameter and shows no sign of
departure.
Quantitative data of bubble equivalent diameter,
defined as the diameter of a perfect sphere having the
same volume as the bubble, are plotted as a function
of time in Fig. 10a. The variation of wall superheat
with time is also shown. Figure 10b shows the variation
of system pressure and wall temperature during the
bubble growth. Using the liquid temperature and the
time dependent pressure, and wall temperature the numerical simulation tool developed by Son et al. (1999)
was exercised. In the numerical simulations, the areaaveraged heater surface temperature as a function of
time was given as input. No spatial variation of surface
temperature was considered for the single bubble cases.
The initial thickness of the thermal layer was obtained
from natural convection data. The solid line shows the
prediction from numerical simulations of bubble equivalent diameter as a function of time. The numerical
model accounted for condensation at the upper part of
the bubble and presence of dissolved gas in the liquid
(Wu and Dhir (2011)). The prediction from numerical
model is in remarkably good agreement with data.
Figure 11 compares at different times the bubble shape
and heat flux under the bubble and surround area
predicted from numerical simulations with those observed in the experiments. It is found that numerical
simulations do a good job in predicting bubble shape
and size. In the experiments, due to the fact that the
spatial resolution is not fine enough (a number of
heaters are grouped together), the heat flux under the
bubble cannot be determined when the bubble base
is small and occupies an area larger than the cavity
heater. However, if the bubble base occupies the area
covered by a bank of heaters around the cavity and
the heat input to the cavity and surround heaters is
zero, then the heat flux is zero. The results of numerical
simulations show that the heat flux at the bubble base is
very small (approx. 4 × 10−4 W/cm2 ). Note that the dry
area under the bubble base begins from the inner edge
of the microlayer. However, experimentally observed
heat flux on bubble unoccupied area of the heater is
about two times that obtained from numerical model
but at later periods the predicted heat fluxes are comparable to those observed in experiments. The numerical
model shows a peak in heat flux near the triple point.
This peak is not observed in the experiments because
of lack of availability of fine resolution heat transfer data.
320
Microgravity Sci. Technol. (2012) 24:307–325
Fig. 10 Comparison of single
bubble growth data with
results from numerical
simulations
Multiple Bubble Dynamics
Merger of bubbles nucleating at different cavities was
also investigated on ISS. Figure 12a shows two bubbles
formed at distinct nucleation sites prior to their merger.
After merger a single bubble, as shown in Fig. 12b,
forms and continues to grow. No premature bubble
departure, as a result of fluid inertia created during
merger, was observed at the low wall superheat that
existed during the experiment. Because of the occurrence of the anomaly after the third week of testing,
we did not get the opportunity to study interactions
of more than two bubbles nucleating simultaneously at
prescribed sites.
Figure 13 shows a snapshot from a video of bubble
dynamics when several bubbles merge. Visual observations showed that during initial merger of a number of
bubbles nucleating at the heater, a bubble may detach
Fig. 11 Comparison of experimental data with results from numerical simulations
Microgravity Sci. Technol. (2012) 24:307–325
321
Fig. 12 Two bubble merger,
gz /ge = 3.2 × 10−7
(a)
(b)
and depart away from the heater. However, subsequently a large bubble forms in the middle of the wafer,
while smaller bubbles are continuously pulling smaller
bubbles into it. Radial motion of smaller bubbles to
the middle of the wafer was seen before these bubbles
were pulled into the large bubble. The radial motion
of smaller bubbles is caused by the radial movement of
the downflowing liquid. At higher superheats the larger
bubble was found to lift off from the heater surface but
continue to hover near the surface while pulling smaller
bubbles into it. This vapor removal configuration continues to persist even when nucleate boiling occurs all
over the wafer. Thus during boiling under microgravity
conditions we see that the vapor sink (bubble layer)
exists on the surface or slightly away from the surface as
opposed to low gravity conditions where single bubbles
detach from the heater surface and move away from it.
Three-dimensional numerical simulation of merger
of five bubbles was carried out prior to the experiments
on the space station. Figure 14 shows a snapshot of the
Fig. 13 Multiple bubble merger – experiments, gz /ge = 4.5 × 10−7
Fig. 14 Multiple bubble merger – numerical simulations
322
process. After initial merger, a vapor bubble leaves the
heater surface and is seen moving through the pool.
Subsequently a large bubble detaches from the surface
while continuing to hover near the surface. Smaller
bubbles move radially inward and are pulled into the
larger bubble. It is gratifying to note that these predictions made a priori are in good agreement with the
observations from the experiments as described above.
Integral Experiments
A number of experiments were conducted on ISS when
all of the surface was heated. Several of these experiments were conducted after the significant anomaly developed. In these experiments only background
heaters were operational and as a result variations in
temperature across the heater were enhanced. Results
for natural convection are given in Fig. 8 and were
discussed earlier.
Dependence of wall heat flux on wall superheat is
shown in Fig. 15, when boiling existed all over the wafer
surface. The reported data are for pressures varying
from 164 kPa to 63 kPa and for liquid subcoolings
from 14.6 to 5.2 ◦ C. As mentioned earlier, when the
vapor production rate is high the bellows were unable
to maintain constant pressure. As a result, for the data
reported in Fig. 15, the pressure (and consequently saturation temperature) increases as the wall temperature
increases. The pressure for each data point is shown in
Fig. 15 Nucleate boiling in
microgravity
Microgravity Sci. Technol. (2012) 24:307–325
Fig. 15. The dependence of wall heat flux on wall superheat is found to depend strongly on system pressure and
liquid subcooling. This behavior is somewhat similar
to that found at earth normal gravity. As mentioned
earlier, the integral experiments were performed both
before and after the anomaly occurred. In spite of the
fact that all the experiments were conducted in constant
temperature mode, there are temperature variations on
the heater surface as indicated in Fig. 15. The band in
the data represent the variation in temperature that
existed over the wafer surface. Note that the boiling
curves for P = 145 and 164 kPa were obtained when all
heaters were operational (i.e., before anomaly), while
the data for P = 63 and 84 kPa were obtained with only
the background heaters operational (i.e, after anomaly). As seen in Fig. 15, when all heater are operational,
the temperature variation across the entire heater surface is small (±0.3 to ±0.7 ◦ C). On the other hand,
when only the background heaters are operational, the
temperature variation across the heater is large (±1.3
to ±3.7 ◦ C). It is also important to note that in the
high pressure (P = 145 and 164 kPa) test cases boiling
was initiated by ramping up only the temperature of
the cavity and surround heaters to initate nucleation at
all five cavities. The temperature at which nucleation
occurred is also shown in Fig. 15. Subsequent to nucleation, the temperature of the entire wafer was decreased to the preset temperature. Once quasi steadystate conditions were achieved the data was recorded
and then the entire wafer temperature was increased
Microgravity Sci. Technol. (2012) 24:307–325
in steps to obtain the boiling curve. Due of the fact
that boiling was already initiated on the wafer using the
ramp up procedure, boiling could be sustained at low
wall superheats. In contrast the low pressure (P = 63
and 84 kPa) boiling curves were obtained by steadily
increasing the wall superheat over the entire wafer.
The nucleation temperature in these tests is higher but
generally consistent with that found when temperature
at a given site was ramped up. At pressures of 84 kPa
and 63 kPa, onset of critical heat flux condition was
noted when a sudden rise in temperature occurred
without any appreciable increase in the input power.
Thereafter, the experiment was terminated. Prior to
termination of the experiment no change in the vapor
structure on the surface was noted.
Present nucleate boiling data are compared in Fig. 16
with the previously reported data obtained on the space
shuttle by Lee et al. (1998) and Straub (2001), with R113 as the test liquid. In Fig. 16, the mean pressure
P during nucleate boiling is noted for the present
study. It is seen that for the same wall superheat the
present data correspond to lower heat flux. In microgravity conditions and low wall superheats, the bubbles
generated on the heater surface stay on the surface for
a long time and merge together to form a large bubble
at the center of the heater. At higher wall superheats,
the large bubble formed as a result of merger of smaller
bubbles lifts off and continues to hover just above the
heater with smaller bubbles continuously merging with
Fig. 16 Comparison of
nucleate boiling data with
that reported by Lee et al.
(1998) and Straub (2001)
323
it. In either case, the vapor sink is located on or close to
the heater surface. In contrast, at higher gravity levels
(g/ge ≥ 10−2 ), the bubbles lift off and move away from
the heater. The fractional area of the heater surface that
is dry (vapor) is much larger at lower gravity levels as
compared to that at higher gravity levels, which results
in a decrease in the overall heat transfer rate. Thus we
conclude that as level of gravity is reduced by about two
orders of magnitude from that in space shuttle, boiling
process becomes less efficient.
Maximum or critical heat flux normalized with that
at earth normal gravity is plotted in Fig. 17 as a function
of dimensionless gravity level. Solid line is the prediction for Perfluro-n-hexane from the hydrodynamic
theory corrected for liquid subcooling at a pressure of
72 kPa, whereas dashed lines are the predictions for
R-113 at 120 kPa. Filled circles and diamonds are the
data of Straub and Lee et al., respectively, obtained in
the space shuttle with a gravity level g/ge ∼
= 10−4 . The
observed critical heat fluxes are found to decrease as
the gravity level is decreased. The magnitude of the
critical heat fluxes observed in microgravity is generally higher than that predicted from the hydrodynamic
theory indicating a weaker functional dependence on
gravity. However, it should be noted that special attention needs to be paid to the relative size of the heater
and the chamber containing the heater aside from other
experimental conditions before we can generalize the
results.
324
Microgravity Sci. Technol. (2012) 24:307–325
Fig. 17 Comparison of
critical heat flux data with
predictions from
hydrodynamic theory
Conclusions
•
•
•
•
•
•
Intended experiments on ISS were fairly successful.
However, anomalies that developed resulted in acquisition of limited data.
Single bubbles were observed not to depart from
the heater surface except occasionally bubbles were
found to depart from the edge of the wafer. Lack
of bubble departure is consistent with predictions
from numerical simulations.
At low wall superheats lateral merger of bubbles
led to a single bubble that continued to grow on the
heater surface.
At high superheats after merger bubble may lift off
from the surface but was found to continue to hover
over the surface while pulling smaller bubbles into
it and growing in size. This is consistent with predictions from numerical simulations.
During nucleate boiling the above mode continued
with a large bubble sitting in the middle of the
surface and pulling smaller surrounding bubbles
into it. This large vapor bubble acted as a sink for
vapor generated on the heated surface.
Rate of natural convection heat transfer in microgravity is found to be consistent with that obtained
by extrapolation of existing correlations.
•
•
•
Observed heat fluxes for nucleate boiling are lower
compared to earlier data obtained on space shuttle. Functional dependence of heat flux on wall
superheat is found to show strong dependence on
pressure and liquid subcooling.
Observed critical heat flux is higher than that predicted by the hydrodynamic theory extrapolated to
microgravity.
Aside from experimental conditions, rate of nucleate boiling heat transfer will be dependent on
relative heater size and fluid confinement. As such
one should be extremely careful in extrapolation of
results from normal and low gravity experiments to
microgravity conditions.
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