Boiling in microgravity

Transcription

1 University of Ljubljana Faculty of Mathematics and Physics Department of Physics Seminar I 1 st year, Second cycle degree Boiling in microgravity Author: Anja Dobravec Advisor: doc. dr. Miha Ravnik November 29, 2017 Abstract Experience with fluids (liquids and gases) seems common in Earth environment. However, this is different in microgravity. Boiling is a very complicated phenomenon and analytical solutions are only rough approximations. In this seminar I will present some basic effects of microgravity on boiling of liquids and their consequences. Specifically I will explain basic principles of pool boiling and heat transfer, bubble growth model and experimental observations of bubble growth in microgravity. A basic comparison between experimental observations and related theory will be given. More generally, the aim of this seminar is to present some fascination, which emerges in fluids when subjected to microgravity conditions.

2 Physics of fluids in microgravity 1 INTRODUCTION Contents 1 Introduction 1 2 Microgravity 2 3 Boiling regimes 2 4 Bubble growth model Influence of gravity on heat transfer correlation Experimental observations of bubble dynamics 6 6 Conclusion 9 References 9 1 Introduction Explorations in microgravity environment are interesting in many different areas, such as space and Earth applications. Many things behave differently in microgravity than on Earth. Crystals grow better and have more perfect shapes, flames of fire are more round, fluids behave differently. In this seminar we will concentrate on boiling phenomenon in microgravity. Boiling is one of the most efficient heat transfer mechanisms [1]. Even though it has been studied for more than 50 years, we still do not understand the whole process. Boiling is a very complex phenomenon, which combines heat and mass transfers, hydrodynamics, and interfacial phenomena [2]. In order to use it in any application, we have to understand the fundamentals of the process itself, bubble dynamics and heat transfer during the first phase of boiling, called nucleate boiling. Then we can optimize the design and safe operation of heat exchange equipment that uses nucleate boiling as a heat transfer method in extreme environments such as in deep ocean or in microgravity. Research of boiling phenomena in low gravity started in the 1950s [3]. First experiments were performed in drop towers and later in parabolic flights, sounding rockets and aboard the space shuttle and they have shown that the stable boiling regimes exist in microgravity. Experiments in microgravity conditions aim to observe the effect of (micro)gravity on the boiling process because the dominance of Earth s gravity is excluded. Experiments show that the behaviour of boiling phenomena in microgravity is different than on Earth because of the absence of buoyancy, which means no separation between phases or components (i.e. no ordering of fluids due to density differences). There is no natural convection in microgravity which causes mixing of the boiling fluid on Earth, no hydrostatic pressure and no weight [4]. For example in low gravity bubbles grow bigger without departing from the heater surface as we will see later in the seminar [1]. In space technology boiling is used for cooling down engineering components through heat transfer as bubbles (vapour) are formed and detached from the surface. If we efficiently cool down the components, we can have smaller and lighter space exploration systems which means cheaper flights [5]. Nowadays there is an interest in long duration manned space flights such as a mission to Mars. Among all the challenges are also an increase of energy efficiency and reducing the weight and volume of the entire system [6]. Boiling heat transfer is also preferable for space applications because for a given power rating the size of a component can be significantly reduced. Other space applications using boiling heat transfer are fluid handling and control, power systems, on-orbit storage and supply systems for cryogenic propellants and life support fluids and electronics cooling. Because of the interest in Mars explorations we have to understand the effect of gravity on boiling heat transfer at gravity levels varying from 10 6 g µg /g 0 1, where g µg is a microgravity acceleration and g 0 10 m/s 2 is a gravity acceleration at the sea level. The seminar is structured as follows: in Paragraph 2 microgravity is described, in Paragraph 3 we explain the boiling phenomenon in microgravity and under normal gravity. In the last two paragraphs 1

3 Physics of fluids in microgravity 3 BOILING REGIMES are a review of bubble growth model and experimental observations of boiling process in microgravity, followed by the conclusion. 2 Microgravity Gravity is an attractive force between any two objects. Under classical physics the following Newton s equation [1] is used to calculate the gravitational force between two massive objects (m 1 and m 2 ): #» F grav = G m 1 m 2 r 2 #» r, (1) where G is the gravitational constant, m 1 and m 2 are the masses of corresponding objects and r is the distance between the centres of these two masses. If we relate this equation with the Newton s second law, which says #» i F i = m a, #» we can calculate the acceleration a of the object. We know that the acceleration due to Earth s gravity is g = 9.81 m/s 2 on the Earth s surface. If we go further away from the surface, the acceleration is getting smaller, but always different than zero. If we take a look on ISS (International Space Station), it orbits the Earth at about 330 km to 435 km height. The acceleration due to Earth s gravity at this height is around 90% of the acceleration on the Earth s surface, so the value is around 9 m/s 2. Thus, the centripetal acceleration due to the Earth gravity accelerates the ISS towards the center of the Earth and it is falling down all the time. But the ISS orbits the Earth at a precise speed ( km/h 7.7 km/s). At this speed gravity curves the trajectory of the ISS just as much as the Earth below it which keeps the ISS in a circular orbit. The astronauts are inside the ISS at the same distance from the center of the Earth, so gravity accelerates all equally. They are falling freely. Due to small residual accelerations of the spacecraft, including aerodynamic drag, vehicle rotation and venting of gases, the environment inside an orbiting space station is called microgravity [7]. There is no sharp cut-off where micrograity begins. Microgravity environment is defined as the acceleration conditions that exist on a microgravity platform, where the Earth s gravitational force is almost entirely balanced by the inertia force, which is proportional to the change in time of the velocity vector. A gravity acceleration inside the ISS, which orbits the Earth in Low Earth Orbit (LEO) is g µg 10 6 g 0 [4]. 3 Boiling regimes Boiling is a very efficient mode of heat transfer because it removes large amounts of heat by generating vapour from liquid. Therefore it is interesting in many technical applications. In this section boiling regimes are described for better understanding of the boiling phenomenon, which occurs in microgravity environment and under terrestrial conditions (by that expression is meant Earth gravity as it is on the Earth surface). Boiling occurs when we heat up the liquid until we reach a critical temperature, called boiling point or saturation temperature. At this temperature the ambient pressure is equal to vapour pressure of the liquid and the liquid vaporizes into gas phase. The higher the ambient pressure, the higher the boiling point. There are two types of boiling, depending on saturation temperature. If the liquid is at saturation temperature, we call it saturated boiling. If the liquid is at lower temperature than saturation, we call it subcooled boiling. When we provide enough heat to the liquid that it reaches the boiling point, we see bubbles. Molecules gain enough energy for phase transition and they transit from the liquid phase to the gas phase. When we boil the liquid, we see vapour of the liquid as bubbles. Figure 1 shows a comparison between nucleate pool boiling in microgravity and in Earth s gravity [5]. The experiment was done in a chamber. At its bottom is a heater surface with 5 artificial nucleate sites, cavities. The test liquid in this case was water, but experiments have been done with different liquids. We can see a difference in the bubble formation. In low gravity we see one big bubble on the heater surface. Small bubbles formed at the heater surface coalesce with the bigger one and therefore it is growing. It is attached to the surface and prevent liquid to reach the heater surface under the bubble. Under terrestrial conditions we see many small bubbles, ascending upward due to buoyancy [1]. 2

4 Physics of fluids in microgravity 3 BOILING REGIMES Figure 1: Comparison between boiling in Earth s gravity and in microgravity environment. In Earth s gravity (left image) the action of buoyancy allows the bubbles to overcome surface tension forces. The bubbles rise upward away from the heater surface. In microgravity (right image) the buoyancy is weak and does not play role in the bubble dynamic. Therefore the bubbles remain attached to the surface of the heater because of large surface tension which is a consequence of produced vapor due to the continuous input of energy from the heater. Reproduced from [5]. Boiling curve on Figure 2 (left picture) represents various boiling regimes, described below. Figure 2: Sketch of a boiling curve in microgravity and and terrestrial 1g environment is on the left side. It is a plot of the heat flux density q = Q/A vs. the temperature difference T w or wall superheat, reffered to the driving temperature difference between the heater surface and the saturation temperature. A quantitative curve depends on the fluid, liquid state and heater configuration. Reproduced from [4]. On the right side is a boiling curve obtained from experiments for three different levels of gravity: microgravity (red), Earth gravity (blue) and high gravity (black) for various subcoolings. Normal and high gravity are similar but we see that boiling curve in microgravity is different. It has lower CHF. CHF increases with gravity and subcooling. Reproduced from [9]. 1. Nucleate boiling Nucleate boiling begins at lower wall superheat in microgravity than in 1-g, point C 1g, and µ g, point C µg but the first one at the higher heat flux. If the temperature of the surface is precisely controlled, the heat flux suddenly increase to point D. Vapour bubbles are formed at preferred 3

5 Physics of fluids in microgravity 3 BOILING REGIMES positions; these are microscopic cavities or cracks on the heater surface called nucleate sites. Any further increase in heat flux or saturation temperature activates more nucleate sites. We see this as growing of the boiling curve to the point E and F. Nucleate boiling is important for technical applications because it effectively transfers high heat flux. In this region a small rise of the surface temperature (saturation temperature) results on Earth in a large non-linear increase of the heat flux as q ( T sat ) m, 3 m 5. Up to the point E on the curve the system is in the isolated bubble regime where the active sites are few and widely separated. With increasing surface superheat, more sites become active and more bubbles are generated. The active sites are spaced so closely, that bubbles from adjacent sites start to interact with each other and merge together [8]. Vapor is being produced so rapidly that bubbles form columns of vapor slugs that rise up to the free surface under terrestrial conditions. This is a segment EF [4]. From experiments we know that the onset of nucleate boiling in microgravity occurs at lower wall superheat than in 1g environment. This is a consequence of gravity. If we have thermal convection like on Earth, it cools down the heated plate and delays the onset of boiling. In microgravity only liquid close to the heater surface reaches the saturation temperature. Liquid away from the heater surface has much lower temperature [2]. Experiments also showed that the heat transfer efficiency for saturated flow nucleate boiling is higher in microgravity than on Earth. This means that at the same saturation temperature we have higher heat flux in microgravity and lower in terrestrial conditions. This is because in microgravity the main bubble levitates above the surface for a long period and the liquid rewets the heater surface which leads to new bubble nucleation and high heat transfer that is observed as lower surface temperature. For subcooled flow boiling the heat transfer efficiency for low heat fluxes is larger in microgravity than on Earth and for high heat fluxes is lower than on Earth. Small bubbles coalesce into bigger one on the heater surface. On the other side, in nucleate pool boiling the heat transfer efficiency is lower in microgravity than on Earth. [2]. 2. Critical heat flux (CHF) It is reached at point F on the Figure 2 and marks the upper limit of the nucleate boiling regime or maximum of the heat flux. At that point the liquid can not move down toward the heater surface because of upward moving bubbles (vapour). The liquid flow rate is not big enough to keep the surface wet. On the heater surface occur dry spots. If the cooling is insufficient, the surface temperature can not be reduced and dry spots spread over the entire surface. Thus the slope of the curve on the Figure 2 starts to decrease [8]. We see the line F G on the Figure 2. It occurs at a constant heat flux and the temperature of the heater surface increases very fast. If the temperature reaches the melting point of the heater material, it may be disturbed. Gravity has a strong influence on CHF values because in microgravity the heater sufrace gets dry faster than on Earth. CHF decreases as gravity level decreases, so CHF is (much) lower in microgravity environment [1]. 3. Transition boiling The average heater surface temperature is controlled and we have a maximum heat flux in point F. With increasing temperature, the heat flux starts decreasing and more bubbles are generated than detached from the heater surface. Bubbles merge and form vapour films over portions of the surface and this leads to further decrease of the contact area between the heating surface and the saturated liquid. Layer of vapour conducts energy poorely than liquid. The surface temperature is not uniform, it varies with location and time. Any further rise of the surface temperature lowers the heat flux and dry spots increase. The upper temperature limit of the transition boiling, where the minimum heat flux is reached (point H on Figure 2) is called Leidenfrost temperature. The part of the boiling curve where unstable film is combined with partial nucleate boiling, F H on the graph, is called transition boling [8]. 4. Film boiling 4

6 Physics of fluids in microgravity 4 BUBBLE GROWTH MODEL If we raise the temperature above the Leidenfrost point, a stable vapour film on the heater surface is formed. It separates the bulk liquid and the heating surface. The energy is transferred to the liquid above vapour by radiation and conduction. We are now in film boiling regime shown on Figure 2 as a segment HG and beyond G. The heat flux is monotonically increasing function of the excess temperature. Pool boiling continues in this regime until the surface temperature reaches the maximum allowable temperature of the heating surface. In this case the phase change occurs at a liquid-vapour interface [4]. 4 Bubble growth model Bubbles are formed in the process of boiling. When liquid gets enough heat, a liquid-vapour phase transition occurs. Vapour bubbles are formed in the nucleation sites and they start to grow. Under terrestrial conditions buoyancy lifts the bubbles up to the surface and because of that, they are also expanding. There is no buoyancy in microgravity, so bubbles remain at the heater surface for a long time. Small bubbles coalesce with the bigger one, so there is only one big bubble, not many smaller like on Earth. There exists a correlation which connects the diameter of the bubble at the time of departure from the surface, called departure diameter D B, the gravity and surface tension. If we consider a static force balance between buoyancy and surface tension (σ), we can write a characteristic length scale - the Laplace coefficient L: ( ) σ 1/2 L = D B = C L, (2) g(ρ l ρ g ) where C is a constant dependent on wetting condition and fluid properties and g is an actual acceleration value [4]. This equation is based on the assumption of the bubble departing if the buoyancy overcomes the holding surface tension force. The gravity dependence of the departure diameter is expressed as: ( ) D B,µg g 1/2 =. (3) D B,1g g 0 The departing bubble is 10 times larger in the case of g/g 0 = 10 2 than under terrestrial conditions. 4.1 Influence of gravity on heat transfer correlation Many heat transfer correlations have been developed and all of them are empirically or semiempirically based. These correlations were developed because of the demand of industrial application to develop technical equipment and they are restricted to special fluids and limited in the liquid range. Here we are interested in correlation which describes gravity as a correct physical parameter. Rohsenow correlation (Eq. 4) is often used because it is valid for different fluids and under different gravity acceleration g. Rohsenow assumed that the heat transfer depends on successive bubble detachments and convection [10]. [ ] g(ρl ρ g ) 1/2 [ ] cpl (T w T sat ) 3.0 q = µ l h P r 5.15 σ C sf h l (4) where C sf is an empirical constant related to the fluid-heater surface combination and is between and , µ l is dynamic viscosity of the liquid, h is phase change enthalpy, g is gravity acceleration, ρ l and ρ g are densities of the liquid and of the vapour, σ is surface tension, c pl is isobaric specific heat capacity of the liquid, T w and T sat are the temperatures of the wall and of the saturation and P r is the Prandtl number [4]. The exponents were obtained by fitting experimental results. In this correlation we can see the gravity dependence and if we assume identical liquid, heater and superheat conditions, we get: ( ) q µg g 1/2 = (5) q 1g g 0 5

7 Physics of fluids in microgravity 5 EXPERIMENTAL OBSERVATIONS OF BUBBLE DYNAMICS In the space shuttle with g/g 0 = 10 4 the heat flux is reduced to 1% compared with Earth gravity. But the exponent in Eq. 5 varies with different theoretical models from 0.3 to 1.5 for most of the usual corelations. But these correlations anticipate lower heat flux than observed in experiments. It was suggested that g is a constant and does not affect the heat transfer in microgravity, which is domiated by the surface tension and depends on the heater size [2]. In the presence of gravity a large part of the entire heat transfer at boiling is transported by convection while in microgravity convection is an almost neglecting part of the heat transport. The bubbles in microgravity environment are not affected by external forces such as buoyancy and shear force. This tells us that there are other important mechanisms for the heat transfer. In microgravity heat transfer is mainly due to evaporation at the foot of the primary bubble. It was also found out that gravity has a weak influence on heat transfer but it strongly affects the process when heater plate becomes dry and reduce CHF in microgravity [2]. Another observation is that with increasing heat flux, heat transfer decreases. We can understand it because with increasing heat flux, more bubbles are formed and there are more dry spots on the heater surface which decrease the heat transfer. In 1969 scientists observed in microgravity experiments that the frequencies of the fluctuations of the heater surface temperature under the bubbles and their departure are the same. This was an experimental discovery for a thin liquid microlayer which is formed underneath the bubble at the solid surface. The heat transfer occurs across this layer and the evaporation process takes place here. The thickness of the microlayer is wedge-shaped and at the lifetime of the bubble (t 0 ) with radius R 0 is: δ 0 = 2 (3 νl t 0 ) 1/2 ( r R 0 ) 3/2, (6) where, ν l is kinematic viscosity [4]. Bubbles are formed in the process of nucleation and after that their growth is controlled by the evaporation process at the liquid-vapour interface. The heat flux density at the vapour-liquid interface (bubble surface) is: ( ) T q(r, φ) = λ l = f(r, φ), (7) r r=r,φ where, λ l is the thermal conductivity of the liquid. Bubbles grow rapidly and therefore push the surrounding liquid away. The generated vapour volume can be up to three orders of magnitude larger than the volume of the liquid which evaporates [4]. The extension of the vapour bubble presses the bubble at the heater surface, flattens the bubble base and because of the viscous effects between moving bubble s interface and the solid heater form a thin wedge-shaped liquid microlayer. In microgravity some assumptions can be made to simplify the equations: the main heat transport to the bubble takes place across the microlayer δ(r, t) from the heater wall to the liquid-vapour interface where, the liquid is evaporating and only conduction takes place in the microlayer. In this case we can write the local heat flux at the bubble base as: q(r, t) = λ l δ(r, t) [T w(r, t) T i (r, t)] (8) where, T w is the wall temperature and T i is the liquid-vapour interface temperature. T i is a little bit higher than the saturation temperature to overcome the kinetic resistance at the evaporation. The highest heat fluxes are expected beneath the centre of the bubble because the thickness of the microlayer is very thin in this region. The prediction of boiling heat transfer still remains a difficult task regardless more than 50 years of experiments because many mechanisms such as bubble nucleation, coalescence, bubble detachment, oscillations, heater size, evaporation and condensation are involved [2]. 5 Experimental observations of bubble dynamics We can study physics under microgravity conditions in different ways, each has different duration of microgravity conditions. We know drop towers (5 s - 10 s), parabolic flights (20 s), sounding rockets 6

8 Physics of fluids in microgravity 5 EXPERIMENTAL OBSERVATIONS OF BUBBLE DYNAMICS (700 s) and orbiting platforms (space shuttle missions (Soyuz), space station ISS) (years). Bubble dynamics under boiling is a complicated phenomenon and is in standard approach under gravity solved numerically. The standard result for bubble radius growth as function of time is in the form: R B = C (at) 1/2 (9) where, a is the thermal diffusivity, t is time and C is a function of thermophysical properties and the wall superheat. Experiments in microgravity show that the bubbles in microgravity are growing slower than the numerical equations predict. Instead of the exponent 1/2 in Eq. 9 scientists found out that it decreases with the growing bubble. For small bubbles the exponent is about 0.5 and for bigger bubbles it decreases. At the top of bigger bubbles condensation takes place. These bubbles don t grow any more. If the condensation mass flow at the top of the bubble is greater than the evaporating one on the base, bubbles shrink [4]. Figure 3: The left plot shows the growth curve of the first bubble after onset of boiling. The data is from experiment TEXUS 10, liquid was Freon113. We see that R B t which is in a good agreement with Equation 9. The right plot shows the growth curves under different gravities. It represents a dependence of the bubble diameter to time. We can see that the bubble grows rapidly at the beginning and later it grows slower. Reproduced from [4, 11]. In experiments one big bubble is observed in the middle of the heater surface. At low superheat small bubbles generated on the heater surface merge together lateral to the surface and form a large bubble. This bubble grows to the size of the heater surface. In experiments with higher wall superheat a large bubble departs the surface, otherwise not. When two bubbles touch each other and coalesce, their mass centre is lifted from the heater surface. At high gravity levels, the bubbles detach from nucleation sites on the heater surface and move away fom the heater. Buoyancy forces dominates over surface tension force for gravity levels g/g 1g > In microgravity where buoyancy is much lower, bubbles grow to a large size on the surface before it departs. A large bubble is in the middle of the surface and smaller bubbles move radially inwards and merge with it. [1] The coalesced bubble oscillates perpendicular and tangential to the surface due to the released surface energy. Energy is released because of the difference of surface energy of the two bubbles and the coalesced bubble [4]. The new small bubbles are absorbed by bigger bubbles. The momentum induced by the liquid underneath the bubble pushes the bubbles away from the surface of the heater. This is a typical process of vapour transport in saturated boiling. The bubbles are getting larger and further away from the heater [12]. Another problem to solve is bubble detachment from the heater surface. In the presence of gravity the bubbles detach through buoyancy, and by the inertia force of the displaced liquid during growth which could effect in 1g and µg. It was found out from experiments that the relation (Eq. 2) for the departure diameter D D can be extended over a range of saturation pressures 0.1 < p/p c < 0.8. Only exponent m should be modified. This relation is based on the assumption that bubble departures if the buoyancy predominates the 7

9 Physics of fluids in microgravity 5 EXPERIMENTAL OBSERVATIONS OF BUBBLE DYNAMICS Figure 4: Visual observation of nucleate boiling in microgravity. We see how the main bubble is growing in time because small bubbles merge with it in lateral direction. Reproduced from [1]. holding surface tension force. This is true only for g/g 0 > The departure diameter can be empirically fitted as: ( ) 2σ m D D = β 0, (10) g(ρ l ρ g ) where β 0 is a contact angle, σ is surface tension, ρ l and ρ g are densities of the liquid and gas phase, respectively. The gravity dependence is expressed with the ratio: ( ) D D,µg g m =. (11) D D,0 g 0 m depends on the liquid and the geometry of the heater surface but usually 0.24 < m < 0.5. For a flat plate m 1/4 [4]. The observed bubble departure diameters in microgravity (g/g ) are 3-4 times larger than under terrestrial conditions. If we extrapolate this relation to g/g , the departure diameter should be timer larger. But this was not observed in the experiments. It follows that in high gravity buoyancy is not a driving force for the bubble detachment and there must be some other important mechanisms. It was also concluded that lower gravity results in higher bubble growth rate, see Fig.3. There is no effect of gravity on bubble departure radius for fast growing bubbles, so their departure diameter is very similar (slightly larger) to those in 1-g [11]. For slow growing bubbles m = 1/2 in Equation 11 [12]. In microgravity the bubble departure diameter is around 3-4 times larger than under terrestrial conditions. These bubbles carry times more energy as those on Earth. Time of detachment grows approximately with the square of the departure diameter, which means the bubbles in microgravity remain 10 times longer at the heater surface. The experimental departure diameters are below the numerically predicted. But the bubbles did not depart from the surface. According to numerical results, the boiling chamber would have to be much bigger to allow bubbles to grow enough to detach from the surface [13]. Bubble growth is a local and transient event of heat and mass transfer combined with the interaction and transient heat conduction in the wall. At the bubble base evaporation occurs and generates, together with dynamic growth, holding forces F h. These forces press the bubble at the wall like a spring, flatten and deform the bubble s base. The force balance is: Fh = 2σ R B πr 2 rest = 2πR B σsin 2 ψ (12) where, σ is the surface tension, R B is the bubble radius and R rest is a radius the bubble rests on the surface: R rest = R B sinψ [4]. After a short growth period of the bubble the wall temperature drops down due to the high heat flux at the base. The wall temperature oscillates with the frequency of bubble departure. The heat supply across the microlayer is slowed down and with it the recoil pressure. Therefore, the bubble growth and all dynamic forces which hold the bubble tends to zero. The surface tension transforms the 8

10 Physics of fluids in microgravity Figure 5: This schematic picture shows the detachment of the bubble and reformation to spherical shape due to surface tension forces. We can see the angle ψ too. Reproduced from [4]. bubble s shape into a sphere. Consequently, the bubble is elevated above the wall by z = R B cosψ. Liquid flow between the bubble and the wall can occur and detaches the bubble from the heater. In microgravity bubbles immediately detach the surface if the power of the heater is turned off. In the presence of gravity, bubbles are detached at a smaller diameter, before they ripen [4]. 6 Conclusion Boiling is a very efficient mode of heat transfer and there are different stages during the boiling process. The comparison of the results from microgravity boiling with results made under terrestrial conditions (in the presence of normal gravity) shows us that the heat transfer mechanisms in both environments are of similar nature. The main mechanism for boiling heat transfer in microgravity is evaporation process in the wedge-shaped microlayer which is formed by the rapid bubble growth beneath the bubble interface and the solid wall. Surface tension forces are the reason for bubble detachment. Further detachment and vapour transport are caused by coalescence mechanisms. Bubbles in microgravity are bigger, small bubbles merge with the bigger one and bubbles remain at the heater surface longer than on Earth. These research are important in many technical applications on Earth and in space. It can be used as a heat transfer mechanism in space vehicles. Improved efficiency in cooling systems can also lead to positive impacts on global economy and environment. References [1] G.R. Warrier, V.K. Dhir, D.F. Chao, "Nucleate Pool Boiling experiment (NPBX) in microgravity: International Space Station," International Journal of Heat and Mass Transfer 83, (2015). [2] C. Colin et al., "Nucleate pool boiling in microgravity: Recent progress and future prospects," Comptes Rendus Mecanique 345, (2017). [3] C. Konishi et al., "Flow boiling in microgravity: Part 1 Interfacial behavior and experimental heat transfer results," International Journal of Heat and Mass Transfer 81, (2015). [4] R. Monti, Physics of Fluids in Microgravity, pp (Taylor & Francis, London, 2001). [5] ( ) [6] C. Konishi et al., "Flow boiling in microgravity: Part 2 Critical heat flux interfacial behavior, experimental data, and model," International Journal of Heat and Mass Transfer 81, (2015). [7] ( ) [8] ( ) [9] J. Kim et al., "Pool boiling heat transfer on small heaters: effect of gravity and subcooling," International Journal of Heat and Mass Transfer 45, (2002). [10] J. M. Saiz Jabardo et al., "Evaluation of the Rohsenow Correlation Through Experimental Pool Boiling of Halocarbon Refrigerants on Cylindrical Surfaces," J. of the Braz. Soc. of Mech. Sci. & Eng. XXVI, (2004). [11] Y. Yang et al., "Effects of microgravity on Marangoni convection and growth characteristic of a single bubble," Acta Astronautica 100, (2014). [12] V.K. Dhir, G.R. Warrier, E. Aktinol et al., "Nucleate Pool Boiling Experiments (NPBX) on the International Space Station," Microgravity Sci. Technol. 24, (2012). [13] E. Aktinol, G.R. Warrier, V.K. Dhir, "Single bubble dynamics under microgravity conditions in the presence of dissolved gas in the liquid," International Journal of Heat and Mass Transfer 79, (2014). 9