Nucleate Pool Boiling eXperiment (NPBX) in microgravity: International Space Station

International Journal of Heat and Mass Transfer 83 (2015) 781–798

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Nucleate Pool Boiling eXperiment (NPBX) in microgravity: International Space Station Gopinath R. Warrier ⇑, Vijay K. Dhir, David F. Chao 1 Henry Samueli School of Engineering and Applied Science, Mechanical and Aerospace Engineering Department, University of California, Los Angeles, Los Angeles, CA 90095, USA

a r t i c l e

i n f o

Article history: Received 5 August 2014 Received in revised form 16 December 2014 Accepted 19 December 2014 Available online 20 January 2015 Keywords: Natural convection Nucleate boiling Maximum heat flux Microgravity

a b s t r a c t A series of nucleate pool boiling experiments were conducted in the Boiling Experimental Facility (BXF) located in the Microgravity Science Glovebox (MSG) on board the International Space Station (ISS) during the period March–May, 2011. Nucleate Pool Boiling eXperiment (NPBX) was one of the two experiments housed in the BXF. Results of experiments on natural convection, nucleate pool boiling heat transfer and critical heat flux are described. Perfluoro-n-hexane was used as the test liquid. The test liquid contained dissolved gas. The test surface was a polished aluminum disc (89.5 mm dia.) 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, to study bubble dynamics and initiate nucleate boiling. During experiments the magnitude of mean gravity level normal to the heater surface varied from 1.7  107ge to 6  107ge. The results of the experiments show that at low superheats, bubbles generated on the heater surface slide and merge to yield a large bubble located in the middle of the heater. At high superheats, the large bubble may lift off from the heater but continue to hover near the surface. In both these scenarios, the large bubble serves as a vapor sink. Natural convection heat transfer in microgravity was found to be consistent with that predicted by available correlations. Steady state nucleate boiling and maximum heat fluxes are found to be lower than those obtained under earth normal gravity conditions. The heat transfer coefficients for nucleate pool boiling are found to be weakly dependent on the level of gravity (h/hge / (g/ ge)1/8). Maximum heat flux also shows a weaker dependence on gravity than that given by the hydrodynamic theory of boiling. 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 (large vapor bubble acting as a sink) from the heater, relative size of the large bubble and heater, and the size and geometry of the chamber used. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Boiling is among the most efficient modes of heat transfer at earth normal gravity and as such is routinely used in energy conversion systems, component cooling and numerous other applications. Boiling can also be the preferred mode of heat transfer for space applications, since for a given power rating, the size of the component can be reduced significantly. Applications of boiling heat transfer in microgravity environments can be found in such areas as thermal management, on-orbit storage and supply systems for cryogenic propellants and life-support fluids, power conversion systems, fluid handling and control and electronics cooling. ⇑ Corresponding author. 1

E-mail address: [email protected] (V.K. Dhir). NASA Glenn Research Center.

http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.12.054 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved.

Recent interest in exploration of Mars and other planets highlight the need to understand the effect of gravity on boiling heat transfer over a wide range of gravity levels. The earliest studies of boiling in low gravity were by Siegel and Usiskin [22] who studied nucleate boiling on electrically heated vertical and horizontal ribbons under free fall conditions. Results showed the vapor mass remained adjacent to the heater surface under low gravity conditions. Usiskin and Siegel [29] did follow up experiments and measured the Critical Heat Flux (CHF) on small wires. For gravity levels of 1 6 g/ge 6 0.04, it was found that the CHF values was generally consistent with the g1/4 dependence given by the hydrodynamic theory [33], while nucleate boiling data were comparable to those obtained at earth normal gravity. Siegel and Keshock [21] studied the dynamics of bubbles on an isolated site formed on a very smooth horizontal surface, for 1 6 g/ge 6 0.014. None of the correlations available in the literature

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Nomenclature Cp D g h hfg k L Lc Lh P q Ra

specific heat (J/kg K) diameter (m) gravitational acceleration (m/s2) heat transfer coefficient (W/m2 K) latent heat of vaporization (J/kg) thermal conductivity (W/m K) characteristic length scale (m) capillary length scale (m) heater size (m) pressure (Pa) heat flux (W/m2 or W/cm2)   gbðTwTlÞL3c Rayleigh number ¼ ma

t T DT x y z

time (s) temperature (°C) temperature difference (°C) X-coordinate Y-coordinate Z-coordinate

at that time were found to predict the measured bubble growth rates and bubble departure diameters at low gravity. Both bubble departure diameter and bubble growth time increased as gravity level decreased. Keshock and Siegel [8] used the bubble growth data to evaluate the magnitude of the forces that lead to bubble departure. Buoyancy, surface tension, and inertial forces were found to govern bubble departure. Buoyancy balanced surface tension forces for slow growing bubbles, while liquid inertia and surface tension governed bubble departure diameter for fast growing bubbles. Thus they concluded that for fast growing bubbles, there was no effect of gravity on bubble diameter at departure. On the other hand, for slow growing bubbles, the bubble diameter at departure increased as g1/2. A review of the studies done in the 1960s can be found in Siegel [23]. Further investigations of boiling in low- and micro-gravity conditions were done in the 1990’s using the drop tower facility at NASA Glenn Research Center [3,4]. These were followed up by pool boiling experiments in the space station [14,15]. 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 fast growing bubbles. Evidence of both quasi-homogeneous and heterogeneous nucleation were reported. They also noted that long term steady-state nucleate boiling could be achieved 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. 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 experiments [16] on the space shuttle. Additionally, it has been noted that liquid subcooling enhances nucleate boiling heat transfer in microgravity. Pool boiling of n-Pentane, R-113, and water on transparent heaters under parabolic flight conditions was investigated by Oka et al. [17]. Visual observation showed 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. On the other hand, during nucleate boiling of water, bubble coalescence

Greek b d kd

thermal diffusivity (m2/s) volume expansivity (1/K) thermal conduction layer thickness (m) most dangerous wavelength (m), pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r=g ðql  qm Þ

q r t

density (kg/m3) surface tension (N/m) kinematic viscosity (m2/s)

a

 pffiffiffi = 2p 3

Subscripts e earth normal gravity l liquid max maximum s saturation v vapor w wall

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 differences in surface tension and wettability characteristics. It was postulated that vapor/liquid/solid contact behavior becomes important at low gravities. However, the authors reported no quantitative values for physical parameters (e.g., contact angle) that could explain the observed behavior. No bubble detachment from the heater surface was observed in these experiments. Nucleate boiling heat fluxes for R-113 and n-Pentane under low gravity conditions were comparable to those obtained under earth normal gravity conditions. However, for 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 (up to 20 °C subcooling). CHF was not achieved with water, but CHF with n-Pentane and R-113 was found to be about 40% of that for earth normal gravity conditions. A review of the microgravity boiling heat transfer work done by Straub and his co-workers can be found in Straub [26,28]. Electrically heated wires and flat plate heaters were used in these experiments. During subcooled boiling of R-113 on a horizontal wire in the ballistic rocket flight (g/ge < 104), a vapor film appeared to surround the wire once power was supplied to the wire. The vapor film was observed to pulsate and during the receding period of the vapor film front, liquid made direct contact with 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, since existence of Marangoni convection cannot be justified, the authors postulated that there were some non-condensables in the liquid which, upon evaporation of liquid, tended to accumulate at the outer edge of the film. The accumulation of the non-condensables caused a decrease in the local saturation pressure of the vapor and resulted in a reduction of 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 the 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. It

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was also postulated that 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. During the parabolic flights, the heat transfer coefficient during nucleate boiling on a platinum wire and a flat plate heater was found to change little when the gravity level changed from low to high values. However, the size of the bubbles was observed to shrink. Straub identified primary and secondary mechanisms for nucleate boiling as a means to explain the lack of dependence of nucleate boiling heat transfer on gravity level. The primary mechanism for heat transfer during nucleate boiling is the evaporation of the thin liquid 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 [11], 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 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 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 et al. [27] reported results of bubble dynamics and pool boiling heat transfer using 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. The nucleate boiling data again showed little difference under 1ge and lg conditions. The critical heat flux for saturated liquid under microgravity was found to be only 15% lower than that at 1ge. With R-11 nucleate boiling heat fluxes as high as 90 W/cm2 were observed under microgravity conditions. Changes in liquid subcooling, system pressure and wall superheat were observed to significantly influence the size of the primary bubble. Additionally, surface tension, wetting behavior of the liquid, bubble coalescence and liquid momentum during bubble formation were found to influence the boiling process. Under subcooled boiling conditions, thermocapillary flow was found to play an important role. A detailed review of various studies conducted in low gravity has been given by Dhir [1]. Pool nucleate boiling in microgravity was investigated by Zhao et al. [31] aboard the Chinese recoverable satellite SJ-8. An Al2O3 ceramic substrate with a multi-metal alloy heater on top (15  15 mm heated area) was used for the experiments. FC72 was the test liquid; the concentration on noncondensable gas was not measured. The gravity level was estimated to be between 103ge and 105ge. The overall duration of the experiments was approximately 100 s. During nucleate boiling, small bubbles merged to form a large vapor bubble on the heater surface. Boiling

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heat transfer rates in lg were found to be lower than that obtained at earth normal gravity; Critical heat flux values were lower by about a factor of three (3) compared to that obtained at 1ge. Recently, Kannengieser et al. [7] conducted boiling experiments on board the Sounding Rocket Maser 11. Novec™ HFE 7000 was used as the test fluid and boiling surface was flat copper surface (1  1 cm2) heated from below by a resistive heater element. The experiments conducted were classified into two groups: (i) those for Tw < Tsat and (ii) those when Tw > Tsat. Since boiling cannot be justified when Tw < Tsat, they noted that non-condensables gas (N2) was dissolved in the liquid; N2 gas was used to pressurize the test chamber prior to liquid filling. The concentration of N2 gas dissolved in the liquid was estimated to be 0.01 mol/mol, just prior to the start of the boiling experiments. In the experiments it was observed that initial bubble nucleation triggered additional bubbles to nucleate in its vicinity. In addition to boiling spreading all over the heater surface, bubble coalescence resulted in the formation a large primary bubble located in the middle of the heater surface. Small bubbles generated on the heater were observed to slide along the heater surface and merge with the large primary bubble. Marangoni effect was postulated to cause the bubble sliding observed in the experiments. The size of the primary bubble was found to be almost constant when Tw < Tsat but increased rapidly when Tw > Tsat. The large primary bubble stayed attached to the heater surface during the entire duration of the experiment. Based on experimental results, they concluded that at low wall temperatures (Tw < Tsat) Marangoni convection was the dominant heat transfer mechanism, while at higher wall temperatures (Tw > Tsat), evaporation at the bubble base was the dominant heat transfer mechanism. Very little difference was found in the boiling heat transfer rates obtained in lg and 1 g. Further details of the boiling heat transfer studies performed in low- and micro-gravity conditions can be found in Dhir et al. [2]. Recently, Raj et al. [20] conducted pool nucleate boiling experiments using the Microheater Array Boiling Experiment (MABE) located in the BXF facility aboard ISS. Perfluoro-n-hexane was the test liquid; noncondensable gas concentration was measured periodically and is given in Table 2. A smooth Quartz substrate with a microheater array (7  7 mm2, 5.6  5.6 mm2 and 4.2  4.2 mm2 heated area) were used in these experiments. The gravity level was reported to be less than 106ge. The effect of heater size, liquid subcooling, wall superheat and pressure were investigated. Increase in liquid subcooling and pressure resulted in increased boiling heat transfer rates. The boiling curves obtained during experiments were categorized into two regimes based on heater size and gravity level, namely the ratio pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi of heater size (Lh) to capil lary length scale Lc ¼ r=gðql  qm Þ : (a) Surface tension Dominated Boiling (SDB) (Lh/Lc < 2.1) – where boiling heat transfer was dependent on heater size and (b) Buoyancy Dominated Boiling (BDB) (Lh/Lc > 2.1) – where boiling heat transfer rate was independent of heater size. For ISS microgravity experiments, all boiling experiments were found to be in the SDB regime. In the ISS experiments, the authors reported a large vapor bubble covering the entire heater surface. This bubble did not lift-off from the heater. Based on experimental data, the authors hypothesized that in the SDB regime the boiling heat transfer rates were independent of gravity level, since a vapor bubble covered the entire heater surface. No photographs of the boiling phenomena were provided in this study to support the fact that nucleate boiling exists on the heater surface. Results of studies performed to date show that we still do not have a basis for quantifying the effect of fluid properties and gravity on nucleate pool boiling. We also do not have any correlations for nucleate and maximum heat fluxes that can used for design purposes in low gravity conditions. In this work, we focus on the experimental data for nucleate pool boiling obtained under

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microgravity conditions. Specifically, experimental results for (i) natural convection, (ii) nucleate boiling, and (iv) maximum heat flux are presented.

2. Experiments 2.1. NPBX experimental apparatus The Nucleate Pool Boiling eXperiment (NPBX) was housed in the Boiling eXperiment Facility (BXF) on ISS. A diamond turned aluminum wafer (6061-T6 aluminum, surface roughness between 16 and 19 nm) with five artificial cavities was the boiling surface used in NPBX. The edge of the aluminum wafer (Fig. 1) was bent to join with the housing holding the wafer. The wafer was 1 mm thick and 89.5 mm in diameter. The five artificial cavities were fabricated using the Electrical Discharge Machining (EDM) technique. Four of the cavities were located at the corners of a square (38.18 mm per side), while the fifth cavity was located at the center. The center cavity is denoted as cavity 1, while the other cavities are denoted as cavities 2, 3, 4 and 5, respectively (Fig. 1). The diagonal distance between the center cavity and the other cavities is 27.0 mm. Single bubble departure diameters predicted at 104ge were used to determine the distance between the cavities, since we were not sure what level of gravity would be available on ISS. The spacing was such that lateral merger of bubbles nucleating at different sites would occur prior to departure, when multiple nucleation sites are activated on the heater surface. The nominal dimensions of each of the cavities were: diameter – 10 lm and depth – 100 lm. However some variations in the cavity dimensions were observed due to the variability inherent in the EDM process; cavity diameters varied from 16.3 to 17.6 lm. Fig. 2 shows a photograph of one of the cavities (D  16.3 lm).

The aluminum wafer was heated using strain gage heaters bonded to the backside of the wafer. The wafer temperature was measured using thermistors bonded to the backside of the wafer at several locations. The strain gage heaters and thermistors were grouped such that each of the five cavities could be activated independently. Fig. 3 shows the layout of the strain gage heaters and thermistors on the backside of the wafer. The strain gages were arranged such that each cavity had two groups of heaters associated with it; one strain gage heater directly underneath the cavity (called the cavity heater) and another heater group surrounding it (called the surround heater). In Fig. 3, the cavity heaters are labeled as ‘C’ while the surround heaters are labeled as ‘S’. In addition, there are two groups of background heaters (labeled ‘B1’ and ‘B2’

Fig. 2. Photograph of an etched artificial cavity (D  16.3 lm).

Fig. 1. Details of NPBX boiling surface (diameter = 89.50 mm).

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Fig. 3. Strain gages and thermistor arrangement on the backside of the wafer.

in Fig. 3) that are not directly associated with the cavities. These heaters were used to heat parts of the wafer that were not directly heated by the cavity or surround heater groups. As such there are 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, respectively. All strain gages used in the experiment were manufactured by Vishay Precision Group. The strain gages used for the surround and background heaters were Model: EA-06-250AF-120 (length = 11 mm, width = 7 mm, nominal resistance = 120 ohms). The cavity heaters were Model: EA-06-062TT-120, which are dual gages each with a nominal resistance of 120 ohms each (total length = 8 mm, total width = 7 mm). All strain gages were approximately 0.05 mm thick. Epoxy encapsulated thermistors (diameter = 2.4 mm, nominal resistance = 5000 ohms at 25 °C) were used to measure the temperature of the wafer. 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 the temperature almost directly below the cavity while another was used to

measure the temperature of the surrounding area. With such an arrangement, for a specified temperature of each heater group, the power to that particular heater group adjusts to yield the specified temperature. 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 was controlled using output from thermistors T12 and T11, respectively. The 12 thermistors bonded to the backside of the aluminum wafer were manufactured by Omega (Model: TH-44007-36-T). The cross section of the complete heater assembly is shown in Fig. 4. 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 (YSI, Model: 014-55034-NA-IT-ST) were provided in the G-11 base at distances of 5.3, 8.6, 14.7, and 24.5 mm from the bottom of the aluminum wafer. Copper wires soldered to the strain gage heaters and thermistors were used to connect them to the power supply and data acquisition system, respectively. Fig. 4 does not show the lead wires. The backside of G-11 base was filled with an insulating

Fig. 4. Details of the heater assembly.

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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 were used to estimate the heat loss through the backside of the heater assembly. These thermistors are glass encapsulated and have a maximum bead diameter of 2.4 mm and a nominal resistance of 5000 ohms at 25 °C. Photographs of the top (boiling surface) and bottom of the heater assembly, before the backside was filled with epoxy, is shown in Fig. 5. Most of the shiny surface of the wafer is covered by a shadow and as such appears dark in the photograph. The strain gage heaters (with soldered lead wires) and thermistor arrangement can be seen on the backside of the wafer. Fig. 6 shows the schematic of the experimental apparatus. The main components are the test chamber, heater assembly, bellows, bulk fluid heater and pump. The heater assembly was located at

Strain gage

Thermistor (a)

(b)

Fig. 5. Photographs of heater assembly (a) boiling surface and (b) backside.

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 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 and depth = 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. Due to safety considerations, the entire experimental apparatus is mounted inside a secondary containment vessel. Fig. 7 shows a detailed 3D CAD drawing of BXF, a facility in which NPBX was contained, while Fig. 8 shows a photograph of BXF located inside MSG onboard the ISS. The data recorded during the NPBX experiments consists of the following:

Fig. 6. Schematic of test chamber.

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MABE Side View Camera Manifold

MABE Focus Motor Pressure Transducers (3)

Mirrors (8)

MABE Side View Backlight MABE Backlight Bulk-Fluid Heaters (3)

NPBX Backlight

Bulk Fluid Thermistors (6)

NPBX Camera (2)

NPBX Wafer Fig. 7. Boiling eXperiment Facility (BXF).

Fig. 8. Boiling eXperimental Facility (BXF) in MSG onboard ISS.

(i) (ii) (iii) (iv) (v) (vi) (vii)

Pressure: 3 locations. Bulk liquid temperature: 6 locations. Wafer temperature: 12 locations. Insulation temperature: 4 locations. Power to each heater group: 12 heater groups. Acceleration levels in three orthogonal directions. Video: 2 orthogonal views.

All data, except the acceleration levels, were 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 (z-direction as shown in Fig. 6). No vibration isolation platform was used in the experiments. 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 liquid was degassed 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° and 38°.

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2.2. 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 1.5–6.2 °C (depending on the system pressure). The presence of dissolved gas resulted in bubble nucleation occurring at very low wall superheats. This behavior was observed during ground testing as well. (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, occasionally 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 24V 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 24V bus. As a result, subsequent to the anomaly, we were able to conduct 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. 2.3. 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. The procedure for only the integral nucleate boiling experiments is described here. In the integral boiling experiments (the intergal nucleate boiling experiments are defined as those experiments in which the entire heater surface was heated), 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 key aspect 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, while the rest of the wafer surface is maintained at the prescribed temperature. 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 with 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, which is the same as the rest of the wafer. 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 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 approximately 20 °C). 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 over 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 min., before continuing with the rest of the experiment. During the liquid settling time, 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. The temperature of the entire wafer was increased by increments of 1–3 °C every 2 min. The total time for each experiment was approximately 15–20 min. 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 min. 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 were recorded: (i) temperature 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 evaluated at regular intervals (typically once a week) during the duration of the experiments. The dissolved gas concentration was determined by the

G.R. Warrier et al. / International Journal of Heat and Mass Transfer 83 (2015) 781–798 Table 1 Test parameters for NPBX experiments. 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

51–203 kPa 30–59 °C 40–80 °C 1.7  107 to 6.0  107 ge 1.2  107 to 3.5  107 ge 46–737 ppm

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

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  105) [6] used is the one measured for air in FC72 in the temperature range 31–60 °C. FC-72 is the commercial grade version of Perfluoro-n-hexane made by 3 M. 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. 2.4. Data reduction and uncertainty in measured quantities 2.4.1. Data reduction From the measurements, the power (Q) supplied to each heater group is calculated as,

Q ¼ I2 R

ð1Þ

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 prior to launch have shown that resistance of the heater groups change very little for the range of temperatures encountered in these experiments (30–85 °C). The heat flux (qw) on the wafer surface is calculated as,

P qw ¼

Q  Q loss Aw

ð2Þ

P 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 overall 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

789

assuming that the unheated edge of the wafer acts as a fin. Based on the energy balance, 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 used to connect heater groups and thermistors to their respective control circuits are embedded in it. The energy balance was performed assuming the temperature of the wafer surface to be uniform and equal to the 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 variation in wafer temperatures was between ±0.3 °C and ±0.5 °C as the total power was increased from 18 W to 70 W, respectively. For the microgravity experiments, the temperature differences varied from ±0.2 °C and ±0.4 °C as the total power was varied from 2.4 W to 7.4 W, respectively. The effect of these small temperature differences on calculation of heat loss trough the insulation is expected to be negligible. Based on the determined effective thermal conductivity (=2 W/mK), heat loss from the backside of the wafer in microgravity was calculated. The variation between the liquid temperatures measured at the six different locations was between 1.5 °C and 2.0 °C, depending on the power level (maximum heater temperature = 80 °C). The liquid temperatures and liquid subcoolings reported in this study are arithmetic averages of the six readings. In this paper, the gravitational acceleration values reported are the arithmetic average values of the gravity levels recorded by MAMS. They are recorded at a frequency of 0.06 Hz. For example, for the nucleate boiling experiment at 125 kPa discussed in this paper, the arithmetic and Root Mean Square (RMS) values of g/ge are: X-axis (plane of the wafer) – mean = 1.3  107, RMS = 2.1  107; Y-axis (plane of the wafer) – mean = 2.2  107, RMS = 2.3  107; Z-axis (normal to the wafer) – mean = 2.5  107, RMS = 2.7  107. 2.4.2. Uncertainty analysis The uncertainty in the thermistors (min. and max. values depending on the particular thermistor) is as follows: (i) bulk liquid thermistors: ±0.2–0.5 °C, (ii) wafer thermistors: ±0.2– 0.3 °C, and (iii) insulation thermistors: ±0.3–0.5 °C. The temperatures measured at 12 locations on the wafer were area-averaged to determine the average temperature of the wafer. The maximum temperature deviation from the area-averaged temperature was also noted. The uncertainty in the pressure measurement is ±1 kPa. The uncertainty of the current measurement varied between 6 and 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, respectively. As the power supplied to the strain gage heaters increases from 4 W to 120 W, the fraction of heat lost through the back side of the wafer varied from 22% to 13% of the total power supplied to the wafer. The uncertainty in the calculated heat flux from the top surface of the heater surface decreased from 72% to 6% as the heat flux increased from 0.01 W/cm2 to 1.3 W/cm2. 3. Results and discussion 3.1. Natural convection The first set of experiments conducted on ISS was designed to investigate natural convection heat transfer in microgravity conditions. All natural convection experiments on ISS were conducted at a system pressure of 182 kPa with subcooled liquid (Tl = 46 and

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1000

McAdams

Nu

100

Present study -7 g/ge = 10

10

g/ge = 1 Zell et al. -4 g/ge = 10 g/ge = 1

1

Kobus & Wadekind

Merte et al. g/ge = 1 Parker & El-Genk g/ge = 1

0.1 0 10

10

1

10

2

10

3

10

4

10

5

10

6

10

7

10

8

10

9

10

10

Ra Fig. 9. Natural convection data for various gravity levels.

54 °C) and the entire wafer surface maintained at uniform temperature. Similar calibration experiments were also performed at earth normal gravity prior to launch. The data obtained under microgravity conditions are shown in Fig. 9. The characteristic length (L) used in defining Nusselt (Nu = hL/k) and Rayleigh num  3 bers Ra ¼ gbðTwTlÞL was taken to be the heater surface area ma divided by the perimeter (for the wafer used in NPBX, L = D/4). Note that solid stars are the data that were obtained during NBPX runs at earth normal gravity (Ra  108  109) while solid squares are the microgravity data (Ra  102) 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. The uncertainty in the measured heat transfer rate is larger (±35–74%) because the power levels for the microgravity experiments are very low (2.4–7.4 W). The microgravity experiments were not video recorded, however visual observations showed discretely spaced streamers leaving the surface. The predictions from natural convection correlations of Kobus and Wedekind [10] and McAdams [13] for natural convection are shown in Fig. 9 as solid and dashed lines, respectively. These correlations are given below:

McAdams :

Nu ¼ 0:54RaL1=4

Kobus and Wedekind :

104 6 RaL 6 107

Nu ¼ 1:759Ra0:13 4L

ð3Þ

300 6 Ra4L < 104 ð4Þ

Nu ¼ 0:9724Ra0:194 4L

104 6 Ra4L 6 3  107

Note that Kobus and Wadekind’s correlation was developed using the 4L as the characteristic length (for a disk, 4L = diameter of disk), while McAdams’s correlation is based on L. The Ra range and the corresponding Nu predicted by Kobus and Wadekind’s correlation have been scaled by a factor of 64 and 4, respectively, so that they can be plotted in Fig. 9. The data for microgravity conditions, with Ra about seven orders of magnitude smaller than that for earth

normal gravity are well predicted by Kobus and Wedekind’s correlation. McAdams correlation predicts the data obtained at earth normal gravity well. Natural convection data found in the open literature at different gravity levels are also plotted in Fig. 9. The experimental data from the following studies are shown: (i) Zell et al. [32] (g/ge = 1 and 104, 20  40 mm plate, L = 26.66 mm), (ii) Merte et al. [15] (g/ ge = 1, 19.05  38.1 mm plate, L = 25.4 mm), and (iii) Parker and El-Genk [18] (g/ge = 1, 10  10 mm plate, L = 10 mm). The characteristic length defined earlier is used to calculate Ra and Nu. The data of Merte et al. and Parker and El-Genk (both at g/ge = 1) is correlated well by McAdams’s correlation. Zell et al.’s experimental data at g/ge = 1 and 104 is always higher than the values predicted by the correlations. The critical Ra (Rac) for natural convection to begin on upward facing heated plate is given to be approximately 1800 pffiffiffiffiffi [24]; Rac is based on the conduction layer thickness (d ¼ 2:77 at). Based on Rac = 1800, for the present study, the time required for natural convection to be fully established is estimated to be about 24,000 s. Since the duration of the microgravity experiments never exceeded 1600 s, it is possible that natural convection has not been fully established. However, as noted earlier, discretely spaced streamers leaving the heater were clearly observed. Also little change in the power supplied to the wafers was observed. As such present data obtained at longer periods of time, is considered to be nearly steady state data. For Zell et al.’s data set, the time required for natural convection to be fully established is estimated to be about 200 s; since bubble nucleation occurred about 1–1.5 s after power is applied to the heater, it is very likely that transient conduction was the heat transfer mode prior to bubble nucleation. This could explain why their data are much higher than those predicted by correlations. Kobus and Wedekind’s correlation was developed based on data obtained for air at earth normal gravity. For these experiments based on the lower limit of the applicability of their correlation, the critical Rayleigh number was estimated to be 248, which is lower than the Rac = 1800 value suggested by Sparrow et al. [24].

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3.2. Nucleate boiling

Fig. 11 shows the boiling curves obtained under microgravity conditions, before the anomaly occurred. The curves shown are for mean pressures of P  125, 145 and 164 kPa. The liquid was subcooled in all cases. The dissolved gas concentration in these experiments was approximately 275 ppm. It should be noted that in conducting these experiments, the temperature of the entire wafer surface was initially at a preset temperature. The temperature of the central heater group (cavity and surround) was then ramped up until nucleation occurred at the central cavity. Once nucleation was detected visually, the temperature of the central heater group was reduced to the preset temperature. Once quasi steady-state conditions were achieved the data were recorded and then the entire wafer temperature was increased in steps to obtain the boiling curve. Due of the fact that boiling was already initiated on the wafer, boiling could be sustained at low wall superheats. The temperature at which nucleation occurred is also shown in Fig. 11. The wall superheat at which nucleation occurred is seen to decrease with increase in system pressure. In Fig. 11, the pressure and the variation in the wall superheats for each data point are shown. With all heaters operational the temperature variation across the wafer varied from ±0.3 °C to ±0.7 °C. Because both the pressure and liquid subcooling changes simultaneously, it is difficult to discern the effects of each of these variables independently on the boiling curve. The boiling curves shift to the left (i.e., lower wall superheats) with increase in pressure and/or liquid subcooling. This is consistent with data available in the literature (e.g., [5,20]. As shown in Fig. 11, for the microgravity data, the dependence of the heat flux on wall superheat decreases from 1:1 q / DT 4:1 w to q / DT w as pressure and liquid subcooling vary from 125 kPa (DTsub = 7.6–13.5 °C) to 164 kPa (DTsub = 14.4–17.6 °C). The nucleate boiling data obtained at earth normal gravity for varying P and DTsub show a similar trend. For example, Parker and El-Genk’s data for nucleate boiling at earth normal gravity show the dependence of heat flux on wall superheat decreases 4:2 from q / DT w to q / DT 2:8 w as DTsub increases from 0 to 30 °C, at P = 102 kPa. Similarly, Guan et al’s data for saturated nucleate boiling at earth normal gravity shows dependence of heat flux on wall 4:1 superheat from q / DT 5:2 w to q / DT w as P increases from 150 to

In the present study, nucleate boiling experiments were performed at two gravity levels, namely earth normal gravity (g/ ge = 1) and microgravity (g/ge  107). The experiments conducted in terrestrial gravity were performed using an identical heating surface installed in an experimental apparatus in our laboratory. In terrestrial gravity experiments, the heater surface temperature was controlled using an algorithm similar to that used in NPBX. The entire surface was heated and the cavity temperature ramp up procedure discussed earlier was not utilized. FC72 was the test liquid. A comparison of the temperature controlled nucleate boiling curve obtained in this study at earth normal gravity (P = 107 kPa, DTsub = 10.1 °C) with those of Parker and El-Genk [18] for similar conditions (P = 101 kPa, DTsub = 10 °C) is shown in Fig. 10. In Parker and El-Genk’s experiments, heat flux was controlled. As such a temperature drop (hysteresis) was observed when nucleation occurred. Also shown in Fig. 10 are the predicted natural convection curve based on McAdams’s [13] correlation and nucleate boiling curve from Stephan and Abdelsalam [25] correlation for saturated conditions. In addition, the CHF values predicted from hydrodynamic theory [33] is also plotted in Fig. 10, for DTsub = 0 °C and 10 °C. Due to power constraints, the data from the present study only covers the natural convection and partial nucleate boiling regimes. From Fig. 10 it can be seen that the agreement between the experimental data obtained in this study and those obtained by Parker and El-Genk [18] is good. Also, McAdam’s correlation predicts the natural convection data well. The heat flux predicted from Stephan and Abdelsalam’s correlation is however lower than the data. Several experiments were conducted on ISS with the entire surface heated. A few of these were performed before the significant anomaly occurred when all the heaters were operational. A number of experiments were performed after the anomaly occurred, with only the background heaters operational. As a result, the temperature variations across the heater surface were larger in these experiments.

40 o

g/ge = 1 FC72

10

Maximum heat flux ΔTsub = 10 C o P = 101 kPa ΔTsub = 0 C (Zuber et al.)

2

qw (W/cm )

Parker & El-Genk o P = 101 kPa, ΔTsub = 10 C Present study o P = 107 kPa, ΔTsub = 10.1 C

1

Saturated fully developed nucleate boiling (Stephan & Abdelsalam)

0.1 Natural convection (McAdams)

0.01 -20

0

20 o

ΔTw ( C) Fig. 10. Comparison of nucleate boiling curves at earth normal gravity (g/ge = 1).

30

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2 175.1

158

169.9

1

165.5 1.1

147.4

q ∝ ΔΤ

126.4 121.5

135.9 3.1

q ∝ ΔΤ

14 5

0.1 ⎯p

⎯p

≈

118.2

Nucleation

≈1

kP a

4 16

a kP

25 kP a

2

≈

131.3

141.9

152.5

⎯p

139.9 4.1

159.7

q ∝ ΔΤ

qw (W/cm )

152.2

117 o

ΔTsub = 14.4-17.6 C

-7

g/ge ≈ 10

o

ΔTsub = 10.3-12.9 C o

ΔTsub = 7.6-13.5 C

0.01 1

2

3 o ΔTw ( C)

4

5

6

7

8

9 10

Fig. 11. Comparison of nucleate pool boiling curves at different system pressures (dissolved gas concentration  275 ppm).

375 kPa. Thus limited data under microgravity conditions show that weakening of the dependence of heat flux on wall superheat, in comparison to that at earth normal gravity, is enhanced when either pressure or liquid subcooling is increased. It is important to note that the data plotted in Fig. 11 were obtained in the presence of noncondensables dissolved in the liquid (275 ppm). The presence of noncondensables reduces the saturation temperature which in turn increases the wall superheat and decreases the liquid subcooling. Note that Tsat used in Fig. 11 accounts for the presence of noncondensables. For a given dissolved gas content, the change in Tsat is dependent on the system pressure; larger change in Tsat occurs at lower system pressures. The curves plotted in Fig. 11 are expected to shift to the left (lower wall superheats) as the dissolved gas content increases [30]. Fig. 12 shows a sequence of photographs showing the boiling process during nucleate boiling. These photographs correspond to the boiling curve obtained for P  125 kPa. The sequence of photographs is from left to right and from top to bottom. The bubbles that are generated on the heater surface stay on the surface and merge together to form a large bubble at the center of the heater. Further increase in wall superheat results in more bubbles nucleating on the heater, move radially inwards and merge with the large central bubble. The last two photographs (t = 655 and 818 s) show that the heater surface is almost completely occupied by the large central bubble. In this run, the large bubble did not depart the surface. In some experiments conducted at higher wall superheats, the large bubble, formed as a result of merger of smaller bubbles, lifted off and continued to hover just above the heater with smaller bubbles continuously merging with it. In either case, the vapor sink is located on or close to the heater surface. In contrast, at high gravity levels (g/ge > 102), the bubbles lift off from discrete locations (nucleation sites) and move away from the heater. At gravity levels (g/ge > 102), buoyancy forces dominate over other forces such as surface tension force and gas/liquid inertia forces. On the other hand, at microgravity conditions, where buoyancy forces are significantly lower, bubbles grow to a very large size before buoyancy force become dominant. In microgravity conditions, we observe a large vapor bubble sitting in the middle of the heater surface while smaller bubbles move radially inwards and merge with it. This

large bubble would conceivably lift off the heater if the heater (and chamber) size was large enough to accommodate the bubble. Qualitatively behavior of the vapor mass shown in these photographs are very similar to those observed by Merte et al. [15], Straub [28], and Kannengieser et al. [7]. Fig. 13 shows a comparison of the nucleate boiling data obtained at earth normal gravity and microgravity conditions with that available in the literature, for similar experimental conditions. The data plotted in Fig. 13 include the following: (i) Merte et al. [15]: Fluid – R113 (a) g/ge = 1, DTsub = 11.0 °C, P = 150 kPa. (b) g/ge = 104, DTsub = 11.0 °C, P = 150 kPa. (ii) Straub [28]: Fluid – R113 (a) g/ge = 1, DTsub = 17.0 °C, P = 102 kPa. (b) g/ge = 104, DTsub = 17.0 °C, P = 102 kPa. (iii) Kannengieser et al. [7]: Fluid – HFE7000 (a) g/ge = 1 (without noncondensables), DTsub = NA, P = NA. (b) g/ge = 105 (with noncondensables), DTsub = 10.5 and 11.9 °C, P = 140 and 150 kPa. (iv) Parker and El-Genk (2007): Fluid – FC-72 (a) g/ge = 1, DTsub = 10.0 °C, P = 101 kPa. (v) Raj et al. [20]: (a) Fluid – Perfluoro-n-hexane, g/ge < 106, DTsub = 11 °C, P = 101.0 kPa, dissolved gas content < 250 ppm. (b) Fluid – FC72, g/ge = 1, DTsub = 11 °C, P = 101.0 kPa. (vi) Present study: (a) Fluid – Perfluoro-n-hexane, g/ge  107, DTsub = 7.6– 13.5 °C, P = 125 kPa, dissolved gas content = 261 ppm. (b) Fluid – FC72, g/ge = 1, DTsub = 10.5 °C, P = 128 kPa. Also shown in Fig. 13 is the post-flight nucleate boiling data obtained in experiments conducted in our laboratory at earth normal gravity using a heater that was identical to the one used in NPBX. This data was plotted earlier in Fig. 10. Focusing on the data obtained in the present study (g/ge = 1 and 107), it is clear that

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8 cm

8 cm

s

t = 156 s

t = 291 s

t= 410

t = 536 s

t = 655 s

t = 818 s

Fig. 12. Visual observation of nucleate boiling (P = 125 kPa, Day 95, dissolved gas concentration  261 PPM).

20 10

Present: -7 PFnH, 10 ge

2

qw (W/cm )

1

0.1

Straub: R113 -4 10 ge

0.01

convection

-10

1ge

Kannengieser et al.: HFE7000 -5 10 ge 1ge

Natural

1E-3 -20

FC72, 1ge

Merte et al.: R113 -4 10 ge 1ge

Raj et al.: -6 PFnH, < 10 ge

0

10

20

30

FC72, 1ge

40

50

o

ΔTw ( C) Fig. 13. Comparison of nucleate boiling data at various gravity levels.

heat fluxes obtained during natural convection and partial nucleate boiling at g/ge = 1 are larger that the heat fluxes observed during fully developed nucleate boiling under microgravity conditions. For example at g/ge  107, the highest heat flux obtained during fully developed nucleate boiling is 1 W/cm2 at a wall superheat 5.8 °C; on the other hand at g/ge = 1, a heat flux of 1.1 W/cm2 is obtained during partial nucleate boiling at a wall superheat of 5.8 °C. Similarly, for g/ge  107, qw = 0.09 W/cm2 at DTw = 3.3 °C, while for g/ge = 1, qw  0.65 W/cm2 (based on interpolation) at DTw = 3.3 °C. The experimental data shows that the magnitude of the heat fluxes obtained for a given wall superheat is reduced as

the gravity level is decreased. A similar trend is seen in Straub’s data, where the heat fluxes during nucleate boiling at earth normal gravity are not only larger than those obtained under microgravity conditions but they also occur at lower wall superheats (e.g., for g/ ge = 104, qw = 3.9 W/cm2 at DTw = 14.4 °C, while for g/ge = 1, qw = 7.5 W/cm2 at DTw = 14.8 °C). The data of Raj et al. [20] shows that at lower wall superheats (<21.5 °C), microgravity boiling heat fluxes are larger than those obtained at 1ge (e.g., for DTw = 16.5 °C, qw = 0.76 W/cm2 at g/ge = 1 while qw = 1.2 W/cm2 at g/ge  106). This trend is reversed at higher wall superheats (>21.5 °C). In spite of the high wall superheats obtained, the authors did not report

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occurrence of dry out. The boiling curve obtained at g/ge = 104 by Merte et al. [15] is at lower wall superheats and only has three data points in the nucleate boiling regime before partial dry out (qw = 3.5 W/cm2 at DTw = 19 °C) occurs. On the other hand, the heat fluxes for the boiling curve at g/ge = 1 increase from 3.6 to 8.1 W/cm2 for wall superheats varying from 24.7 to 28.3 °C. The two data points of Kannengieser et al. show that there is little difference between nucleate boiling heat fluxes at g/ge = 1 and 105. The limited experimental data available show a large variation in the range of wall superheats and that different heat transfer regimes can occur at the same wall superheat when the level of gravity is reduced. For example, at earth normal gravity, natural convection or partial nucleate boiling occur at a particular wall superheat (about 6 °C), while microgravity experiments show that fully developed nucleate boiling occurs at the same wall superheat. The presence of noncondensables dissolved in the liquid is another factor to consider, as noncondensables cause bubble nucleation to occur at lower wall temperatures and as a result shift the entire boiling curve to the left as Tsat decreases with gas content in the liquid. For a more quantitative comparison of pool nucleate boiling heat transfer at various gravity levels we compare the heat transfer coefficients obtained during boiling. Fig. 14 shows a comparison of the heat transfer coefficients obtained at microgravity and earth normal gravity for the test cases plotted in Fig. 13. Note that only data for boiling is shown in Fig. 14. The experimental data from the present study and that from Straub clearly show that the heat transfer coefficients decrease with decrease in gravity level. For example, for the present study, for g/ge  107, h = 1101 W/m2 K at DTw = 5.8 °C (fully developed nucleate boiling), while for g/ge = 1, h = 1915 W/m2 K at DTw = 6.9 °C (partial nucleate boiling). Similarly, for Straub’s data, at g/ge = 104, h = 2435 W/m2 K at DTw = 11.3 °C (fully developed boiling), while for g/ge = 1, h = 3446 W/m2 K at DTw = 10.7 °C (partial nucleate boiling). Raj et al.’s data show that the heat transfer coefficients decrease with decrease in gravity level for DTw values > 21.5 °C. At DTw > 21.5 °C heat transfer coefficients for 106ge are smaller than those for 1ge (e.g., for DTw = 24 °C,

h = 2955 W/m2 K at 1ge while h = 1485 W/m2 K at 106ge). Note that for DTw = 21.5 °C, heat transfer coefficients for 1ge and 106ge are almost identical. Raj et al.’s microgravity data shows that the heat transfer coefficients are almost independent of DTw (i.e., h  1400 W/m2 K for DTw P 21.5 °C). This dependence of h on DTw is similar to the dependence found during film boiling rather than nucleate boiling. Due to the small heater sizes and the high wall superheats used in these experiments, it is possible that film boiling is occurring on the heater surface. Based on the ratio of the heater size (Lh) to capillary length scale (Lc) (Lh/Lc = 0.0053, 0.0071, 0.0089 for Lh = 4.2 mm, 5.4 mm and 7.0 mm, respectively), Raj et al. reported that their boiling data was in the Surface tension Dominated Boiling (SDB) regime, where boiling heat transfer is independent of gravity level. In our ISS experiments, the ratio Lh/Lc = 0.036 at 107ge. Based on the criterion given by Raj et al., our boiling data are also in the SDB regime. However our data plotted in Figs. 11, 13 and 14 and the photographs of the boiling process shown in Fig. 12 show nucleate boiling similar to that for high gravity levels except that the vapor removal mechanism is different. Now a large vapor bubble located in the middle of the heater acts as a vapor sink while smaller bubbles move radially inward and merge with it. Merte et al.’s data shows that in most cases, heat transfer coefficients at g/ge = 1 are higher than those obtained at g/ge = 104, except, the heat transfer coefficient for g/ge = 1 at DTw = 24.7 °C (h = 1347 W/ m2 K) is slightly lower than that obtained for g/ge = 104 at DTw = 19 °C (h = 1655 W/m2 K). Note however that the wall superheats are quite different in these two cases. Higher heat transfer at 104 g/ge may be a result of the flashing that occurred in the superheated liquid. These are not considered to be steadystate data. Kannengieser et al.’s data for g/ge = 105 gives h = 1974 and 1544 W/m2 K for DTw = 6.4 and 13.6 °C, respectively. For g/ge = 1, h = 1861 and 1863 W/m2 K for DTw = 7.6 and 13.3 °C, respectively. Comparison of data at the two gravity levels shows that the heat transfer coefficient for one of the data points for microgravity experiments is almost the same while the other is lower than the heat transfer coefficients obtained at g/ge = 1, for similar wall superheats.

6500 Present: -7 PFnH, 10 ge

6000

Raj et al. -6 PFnH, <10 ge

5000

Merte et al.: R113 -4 10 ge 1ge

FC72, 1ge

Straub: R113 -4 10 ge

FC72, 1ge

1ge

Kannengieser et al.: HFE7000 -5 10 ge 1ge

2

h (W/m K)

4000

3000

2000

1000

100 0

10

20

30

40

50

o

ΔTw ( C) Fig. 14. Comparison of heat transfer coefficients during nucleate boiling at various gravity levels.

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For a more quantitative comparison of the data, we compare the heat transfer coefficients for the data shown in Fig. 13, for various gravity levels for similar qw/qmax values. In fact, the qw/qmax values represent where we are on the nucleate boiling curve. Experimental values were used for qmax wherever available. For cases where experimental values for qmax are not available, the correlation of Zuber et al. [33] is used to predict it. In Fig. 15, the heat transfer coefficient for microgravity conditions is normalized by the corresponding value obtained at earth normal gravity while the gravity level is normalized with earth normal gravity. The experimental conditions for the data plotted in Fig. 15 are listed in Table 3. The qw/qmax values for the data points are also shown in Fig. 15. It must be noted that for the data obtained in this study for g/ ge = 1, the q/qmax value obtained from Parker and El-Genk was used (Fig. 10). Similarly, at g/ge  107, qmax values obtained at P = 84 kPa was used (see Fig. 16). Where ever possible, only data

points with similar q/qmax values for two gravity levels are shown in Fig. 15. Data sets where this was not possible are however included in Table 3. Additional data from Qiu et al. [19] for water and Kim et al. [9] for FC72 are included. From Fig. 15, focusing on the data for q/qmax  0.2, it can be seen that, h/hge decreases with decrease in gravity level. It is found that as the gravity level decreases seven orders of magnitude (g/ge from 1 to 107), the normalized heat transfer coefficient decreases from 1 to approximately 0.15 (i.e., 85% reduction). Based on the limited data available, h=hge / g=g 1=8 e . This dependence is shown in Fig. 15.The data obtained in the present study shows some dependence of h/hge on q/qmax. However, due to very limited data available currently, it is difficult to make definitive conclusions. Nucleate boiling data currently available is still very limited. Moreover due to differences in the size of the heater surfaces, experimental conditions (steady-state or transient), dissolved gas

1.1 1.0

Present, PFnH q/qmax = 0.05

Merte, R113 q/qmax = 0.23

q/q

q/qmax = 0.21

= 0.22

max

q/qmax = 0.13 Kim et al., FC72

q/qmax = 0.39

q/qmax = 0.22

q/qmax = 0.64 q/qmax = 1

Parker & El-Genk, FC72

h/hge

q/qmax = 0.22

0.5

Straub, R113 q/qmax = 0.19

1/8

h/hge ∝ g/ge

0.0 -8 10

-7

10

-6

10

-5

10

-4

-3

10

10

-2

10

-1

10

0

10

g/ge Fig. 15. Comparison of the nucleate boiling heat transfer coefficient at different gravity levels (data provided in Table 3).

Table 3 Experimental conditions for data shown in Fig. 15.

*

Study

g/ge

DTsub (°C)

P (kPa)

DTw (°C)

qw (W/cm2)

h (W/m2 K)

qmax (W/cm2)

qw/qmax

h/hge

Present Present Present Present Parker and El-Genk Present Parker and El-Genk Present Parker and El-Genk Present Parker and El-Genk Present Merte et al. Merte et al. Straub* Straub* Kim et al. Kim et al.

1 107 1 107 1 107 1 107 1 107 1 107 1 104 1 104 1 1.3  102

10.5 7.6 10.5 8.5 10 9.8 10 11.1 10 12.5 10 13.5 11.1 11.1 17 17 21.7 20

128.0 125.7 128.0 125.7 102.0 125.7 102.0 125.7 102.0 125.7 102.0 125.7 150 150 102 102 114 108

6.9 1.7 9.1 3.3 10.8 4.4 11.8 4.9 14.3 5.3 23 5.8 28.3 11.3 14.8 11.3 20.3 16.5

1.3 0.047 2.1 0.09 4.8 0.18 7.6 0.34 13.5 0.58 21.8 0.99 8.11 0.94 7.50 2.74 4.65 1.91

1,915 278 2,362 256 4,220 421 6,411 695 9,468 1,101 9,497 1,707 2,866 838 5,067 2,435 2,290 1,161

21.8 0.88 21.8 0.88 21.8 0.88 21.8 0.88 21.8 0.88 21.8 0.88 36.9 3.97 40.5 3.87 20 8.54

0.05 0.05 0.10 0.10 0.22 0.21 0.35 0.39 0.62 0.66 1.00 1.12 0.22 0.24 0.71 0.19 0.23 0.22

1.00 0.15 1.00 0.13 1.00 0.10 1.00 0.11 1.00 0.12 1.00 0.18 1.00 0.29 1.00 0.48 1.00 0.51

q/qmax not equal.

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2 CHF 1

90.9 69.2

88.1

65.8

86

63.7

83.6

60.5

o

ΔTsub = 4.3-6.7 C

⎯p ≈ 63

o

ΔTsub = 7.4-10.5 C

⎯p ≈

0.1

84 k P

a

kPa

77.5

2

qw (W/cm )

82.8

58.1

76.9

-7

g/ge ≈ 10 0.01 1

10

20

o

ΔTw ( C) Fig. 16. Nucleate boiling curves in microgravity where maximum heat flux was obtained.

content and the procedure adopted to conduct the experiments, large variations are observed in the boiling curves obtained. This makes analysis of the experimental data very challenging. Hence, any conclusions we draw from the available experimental data is tentative and has to be verified/validated with additional experiments and/or numerical simulations. 3.3. Maximum heat flux In the present study maximum heat flux (qmax) was achieved during nucleate boiling in a few low pressure experiments.

2x10

Maximum heat flux conditions were recognized when substantial increase in wafer temperature (3–4 °C) was accompanied by little increase in the supplied heat flux. Fig. 16 shows two nucleate boiling curves (P  63 and 84 kPa) in which maximum heat flux condition was obtained. Note that these experiments were performed after the anomaly occurred, hence only the background heaters were operational. As only the background heaters were operational, the variation in the wall superheats is larger (±1.3 to ±3.7 °C). Due to the large vapor generation rate, the chamber pressure could not be maintained constant during the duration of the run. The pressure corresponding to the each data point is given

0

Perfluoro-n-Hexane P = 72 kPa

0

10

o C 20 o C 10 o ub = C ΔT s ub =0 ΔT s b u ΔT s

=

qmax/qmax,Sat.,g/g

e

=1

R113 P = 120 kPa

-1

10

-7

Present study (g/ge ~ 10 ) (PnH) o

ΔTsub = 12.2 - 15.3 C -4

Merte et al. (1995) (g/ge = 10 ) (R113) o

ΔTsub = 0.3 - 11.1 C -4

Straub (2001) (g/ge = 10 ) (R113) o

ΔTsub = 0.0 C

Parker and El-Genk (g/ge = 1) (FC72) o

ΔTsub = 0, 10, 20 C -2

10

10

-8

10

-7

10

-6

10

-5

10

-4

10

-3

10

-2

g/ge Fig. 17. Maximum heat flux as a function of gravity level.

10

-1

10

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G.R. Warrier et al. / International Journal of Heat and Mass Transfer 83 (2015) 781–798

in Fig. 16. The maximum (or critical) heat fluxes are also indicated in the figure. In microgravity conditions, the maximum heat flux value is very low. For the cases shown in Fig. 16, qmax varies from 0.73 to 0.79 W/cm2. Maximum heat flux normalized with that at earth normal gravity is plotted in Fig. 17 as a function of dimensionless gravity level. The predicted maximum heat flux values are obtained from the following correlation [33]: qmax ¼ 0:16qv hfg

 1=4 (  1=4  1=8 ) 1=2 ðkl cpl ql Þ ðT sat  T l Þ g rðql  qm Þ gðql  qm Þ q2m 1 þ 5:32 qm2 r qm hfg g rðql  qm Þ ð5Þ

The solid line is the prediction for Perfluoro-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 [28] and Merte et al. [15], respectively, obtained in the space shuttle with a gravity level g/ge = 104. The experimental data from Parker and El-Genk (2007) at g/ge = 1, at various liquid subcoolings) is also shown. 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. As discussed earlier vapor bubble (large bubble in the middle of the heater surface) removal configuration in microgravity conditions is very different from that at earth normal gravity (release of discrete bubbles or vapor columns). To access the effect of heater size used in this study on the maximum heat flux predicted from hydrodynamic theory, we calculate the ratio of the heater size (Lc = D = 0.0895 m) to the most dangerpffiffiffipffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ous Taylor wavelength (kd ¼ 2p 3 r=gðql  qm Þ, kd = 0.0084 m and 26.67 m for g/ge = 1 and 107, respectively). The ratio Lc/kd = 10.6 and 0.003 for g/ge = 1 and 107, respectively. Hence while the heater surface used in the present study can be considered an infinite flat plat for the experiments performed at g/ge = 1, it should be considered a very small heater for the microgravity experiments. For small heaters (i.e., Lc/kd < 3), the maximum heat flux predicted can be much larger than that for an infinite flat plat [12]. As such, the maximum heat flux obtained in microgravity experiments is much smaller than that would be predicted by hydrodynamic theory, when corrections for heater width (size) is taken into account. 4. Conclusions  Experiments conducted on the International Space Station were fairly successful. Due to the anomalies that developed in the BXF, only limited data could be acquired.  Rate of natural convection heat transfer in microgravity is found to be consistent with that predicted by existing correlation in the literature.  During nucleate boiling, at low wall superheats, multiple lateral bubble mergers result in the formation of a large bubble that locates itself in the middle of the heater surface. This large vapor bubble acts as a sink for vapor generated on the heated surface. At high superheats, after merger the large bubble may partially lift off from the surface. However this large bubble continues to hover over the surface while pulling smaller bubbles into it and growing in size. This vapor removal configuration is very different from that at earth normal gravity.  Observed nucleate boiling heat fluxes for nucleate boiling are lower compared to earlier data obtained on the space shuttle.

797

 Based on available data, normalized heat transfer coefficients for nucleate boiling were found to be very weakly dependent on gravity level. Normalized heat transfer coefficients decrease with gravity level as g=g 1=8 e .  Observed maximum heat flux is higher than that predicted by the hydrodynamic theory for an infinite flat plate extrapolated to microgravity. However when the width (size) of the heater is accounted for it is much smaller than that predicted from hydrodynamic theory.  Aside from experimental conditions, rate of nucleate boiling heat transfer under microgravity conditions can 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. Conflict of Interest None declared. Acknowledgement This was supported by NASA. References [1] V.K. Dhir, Boiling under microgravity conditions, in: Proceedings of the 12th Int. Heat Transfer Conference, Grenoble, France, 2002. [2] V.K. Dhir, G.R. Warrier, E. Aktinol, D. Chao, J. Eggers, W. Sheredy, W. Booth, Nucleate pool boiling experiments (NPBX) on the international space station, Microgravity Sci. Technol. (2011), http://dxdoi.org/10.1007/s12217-012-93158. [3] J.S. Ervin, H. Merte, R.B. Kellers, K. Kirk, Transient pool boiling in microgravity, Int. J. Heat Mass Transfer 35 (1992) 659–674. [4] J.S. Ervin, H. Merte, Boiling nucleation and propagation in microgravity, in: Proceedings of symposium Heat Transfer in Microgravity, New Orleans, LA, ASME HTD, vol. 269, 1993, pp. 131–138. [5] C. Guan, J.F. Klausner, R. Mei, Pool boiling CHF for pentane, hexane, methanol, FC-72, FC-87, and R113 on a smooth horizontal surface, Front. Heat Mass Transfer 2 (2011) 043002. [6] B. Horacek, K.T. Kiger, J. Kim, Single nozzle spray cooling heat transfer mechanisms, Int. J. Heat Mass Transfer 48 (2005) 1425–1438. [7] O. Kannengieser, C. Colin, W. Bergez, Pool boiling with non-condensable gas in microgravity: results of a sounding rocket experiment, Microgravity Sci. Technol. 22 (2010) 447–454. [8] E.G. Keshock, R. Siegel, Forces acting on bubbles in nucleate boiling under normal and reduced gravity conditions, NASA TND-2299, 1964. [9] J. Kim, J.F. Benton, D. Wisniewski, Pool boiling heat transfer on small heaters: effect of gravity and subcooling, Int. J. Heat Mass Transfer 45 (2002) 3919– 3932. [10] C.J. Kobus, G.L. Wedekind, An experimental investigation into natural convection heat transfer from horizontal isothermal circular disks, Int. J. Heat Mass Transfer 44 (2001) 3381–3383. [11] J.H. Lay, V.K. Dhir, Shape of a vapor stem during nucleate boiling of saturated liquids, J. Heat Transfer 117 (1995) 394–401. [12] J.H. Lienhard, V.K. Dhir, D.M. Riherd, Peak pool boiling heat-flux measurements on finite horizontal flat plates, J. Heat Transfer 95 (1973) 477–482. [13] W.H. McAdams, Heat Transmission, third ed., McGraw-Hill, New York, 1954. [14] H. Merte, Pool and flow boiling in variable and microgravity, in: 2nd Microgravity Fluid Physics Conference, Paper No. 33. Cleveland, OH. June 21–23, 1994. [15] H. Merte, H.S. Lee, R.B. Keller, Report on pool boiling experiment flow on STS47, STS-57, STS-60, Report No. UM-MEAM-95-01, 1995. [16] H. Merte, H.S. Lee, R.B. Keller, Dryout and rewetting in the pool boiling experiments flown on STS-72 (PBE-IIB) and STS-77 (PBE-IIA), Report No. UMMEAM-98-091, 1998. [17] T. Oka, Y. Abe, Y.H. Mori, A. Nagashima, Pool boiling of n-pentane, CFC-113, and water under reduced gravity: parabolic flight experiments with a transparent heater, J. Heat Transfer 117 (1995) 408–417. [18] J.L. Parker, M.S. El-Genk, Enhanced saturation and subcooled boiling of FC-72 di-electric liquid, Int. J. Heat Mass Transfer 48 (2005) 3738–3752. [19] D.M. Qiu, V.K. Dhir, M.M. Hasan, D. Chao, Single bubble dynamics during nucleate boiling under low gravity conditions, in: V.K. Dhir (Ed.), Microgravity Fluid Physics and Heat Transfer, Begell House, New York, 2000, pp. 62–71. [20] R. Raj, J. Kim, McQuillen, Pool boiling heat transfer on the international space station: experimental results and model verification, J. Heat Transfer 134 (2012) 101504-1. [21] R. Siegel, E.G. Keshock, Effects of reduced gravity on nucleate boiling bubble dynamics in saturated water, AIChE J. 10 (1964) 509–517.

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