Nucleate pool boiling in microgravity: Recent progress and future prospects

Pool boiling on flat plates in microgravity has been studied for more than 50 years. The results of recent experiments performed in sounding rocket are presented and compared to previous results. At low heat flux, the vertical oscillatory motion of the primary bubble is responsible for the increase in the heat transfer coefficient in microgravity compared to ground experiments. The effect of a non-condensable gas on the stabilisation of the large primary bubble on the heater is pointed out. Experiments on isolated bubbles are also performed on ground and in parabolic flight. The effect of a shear flow on the bubble detachment is highlighted. A force balance model allows determining an expression of the capillary force and of the drag force acting on the bubble.     

Design and performance of an experiment for the investigation of open capillary channel flows

In this paper we report on the set-up and the performance of an experiment for the investigation of flow-rate limitations in open capillary channels under low-gravity conditions (microgravity). The channels consist of two parallel plates bounded by free liquid surfaces along the open sides. In the case of steady flow the capillary pressure of the free surface balances the differential pressure between the liquid and the surrounding constant-pressure gas phase. A maximum flow rate is achieved when the adjusted volumetric flow rate exceeds a certain limit leading to a collapse of the free surfaces. The flow is convective (inertia) dominated, since the viscous forces are negligibly small compared to the convective forces. In order to investigate this type of flow an experiment aboard the sounding rocket TEXUS-41 was performed. The aim of the investigation was to achieve the profiles of the free liquid surfaces and to determine the maximum flow rate of the steady flow. For this purpose a new approach to the critical flow condition by enlarging the channel length was applied. The paper is focussed on the technical details of the experiment and gives a review of the set-up, the preparation of the flight procedures and the performance. Additionally the typical appearance of the flow indicated by the surface profiles is presented as a basis for a separate continuative discussion of the experimental results.     

Laminar bubbly flow in an open capillary channel in microgravity

The study of a bubbly laminar two-phase flow in an open capillary channel under microgravity conditions was conducted aboard the sounding rocket, Texus-45. The channel consists of two parallel plates of width b = 25 mm and distance a = 10 mm. The flow along the length l = 80 mm is confined by a free surface on one side and a plate on the opposite side. The bubbles are injected at the nozzle of the capillary channel via six capillary tubes of 100 μm in inner diameter. Different liquid and gas flow rates were tested leading to different liquid free surface shape and bubble size.     

热毛细对流速度场测试研究

为了探索微重力状态下表面张力驱动流速度场的测试技术，本文在地面进行了模拟实验，对位于热壁下流体中气泡周围的表面张力驱动流进行了研究，并用ＰＩＶ技术测量了流场的速度分布．     

Experimental techniques in natural convection

Experimental techniques in natural convection heat transfer employed in the author's laboratory are introduced. The techniques are mostly related to visualization of flow, temperature field, and heat flux distribution in fluids. Three topics are presented, the first being natural convection in a horizontal rectangular liquid layer driven by surface tension and buoyancy. The patterns of flow were visualized by suspending fine aluminum flakes in the liquid. At the same time, the distribution of the temperature gradient in the liquid was visualized by an optical method making use of the refraction of light. The second topic is the onset of oscillatory convection in the Czochralski growth melt. In this case a forced flow due to rotation of the crystal and the vessel is superimposed on the buoyancy convection, resulting in an oscillatory flow under certain circumstances. The flow pattern and the temperature distribution in the liquid were visualized simultaneously by suspending in the liquid a microencapsulated temperature-sensitive liquid crystal. Periodical oscillation of the flow and the temperature was clearly recognized. The third topic is the rollover of double liquid layers that were stratified stably due to a density difference. A small-scale experiment was carried out to clarify the basic mechanism of rollover. The tracer method was used to visualize boundary layer flow along the vertical side wall and the shadowgraph technique to visualize the density distribution in the liquid layers. The article emphasizes the importance of visual observation in the investigation of natural convection phenomena.     

Buoyancy effects on flow transition in low-density inertial gas jets

Effects of buoyancy on transition from laminar to turbulent flow are presented for a momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2 s drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior with change of buoyancy.     

Analysis of efficiency of using hybrid propulsion for accelerating small-size rockets starting from the earth surface

The flight trajectories of a hypothetical small-size rocket starting from the Earth surface and driven by hybrid propulsion, which involves a solid-propellant rocket motor (SRM) and a ramjet-type air-breathing engine (ramjet), are calculated and analyzed. The basic varied parameter is the ratio of the fuel masses used for operation of various engine types with the total fuel mass being unchanged. The objective function is the flight range of the rocket under different initial conditions. Based on a model problem considered as an example, it is demonstrated that the flight range can be substantially increased by using hybrid propulsion (SRM and ramjet), as compared with the vehicle having identical mass and size characteristics and using the SRM only.     

半浮区液桥热毛细振荡流

采用非定常、三维直接数值模拟方法研究大Pr数半浮区液桥热毛细对流从定常流向振荡流的过渡过程．文中详细描述了热毛细振荡流的起振和振荡特征,给出了液桥横截面上振荡流的流场和温度分布．在地面引力场条件下计算的结果与地面实验的结果进行比较,得出液桥水平截面上的流场和温度分布图样以一定的速度旋转,自由表面固定点处流体的环向流速正、负交替变化的一致结论．     

Dynamic detachment of a gas bubble from a solid flat substrate performing normal harmonic vibrations

This paper presents an investigation of the detachment of a bubble from a solid flat substrate according to a dynamic scenario, i. e., due to strong vibrations of the surface shape of the bubble caused by normal nonacoustic harmonic vibrations of the substrate. The Layzer’s model based on an analysis of single-mode solutions near the bubble top was used to study its detachment in microgravity, where the dynamics of the bubble surface is due to competition between liquid inertia forces and surface tension forces. Detachment of the bubble from the substrate was determined from the condition of its elongation during vibrations by a magnitude comparable to the radius of the bubble in equilibrium. The dependence of the vibration intensity required for the detachment of the bubble on the problem parameters was determined using a number of empirical assumptions. The volume of the detached bubbles was estimated.     

Effect of force fields on pool boiling flow patterns in normal and reduced gravity

This paper reports the observations of boiling flow patterns in FC-72, performed during a microgravity experiment, recently flown aboard of Foton-M2 satellite, in some instances with the additional aid of an electrostatic field to replace the buoyancy force. The heater consisted of a flat plate, 20 × 20 mm², directly heated by direct current. Several levels of liquid subcooling (from 20 to 6 K) and heat fluxes up to 200 kW/m² were tested. A complete counterpart test, carried out on ground before the mission, allowed direct comparison with terrestrial data. The void fraction in microgravity revealed much larger than in normal gravity condition: this may be attributed to increased bubble coalescence that hinders vapor condensation in the bulk of the subcooled fluid. In several cases, an oscillatory boiling behavior was detected, leading to periodical variation of average wall overheating of some degrees. The electric field confirmed to be very effective, even at low values of applied voltage, in reducing bubble size, thus improving their condensation rate in the bulk fluid, and in enhancing heat transfer performance, suppressing the boiling oscillations and preventing surface dryout.     

Solidification of a binary mixture in a square cavity with a free surface

Solidification of an NH₄Cl-H₂O solution in a square cavity with a free surface has been studied numerically and experimentally. Solidification is characterized by the coexistence of solid, mushy (solid plus liquid), and liquid regions, and flow within the liquid and mushy regions is driven by surface tension gradients, as well as by opposing thermal/solutal buoyancy forces. Theoretical and experimental results indicate that (i) surface tension forces spawn a cellular flow at the top of the cavity and enhance its development with increasing time, (ii) water enrichment of the top cell induces remelting of adjoining solid, and (iii) final steady-state conditions are characterized by a liquidus front whose thickness increases with increasing depth. However, predicted and observed results differ with respect to the chronological pattern of multicellular flow development and the time required to achieve steady-state conditions. Differences are attributed to selected model assumptions and to uncertainties concerning mushy region structure.     

The effect of residual contamination on Marangoni convection in a spherical liquid system

The thermocapillary convection which results from both a temperature field as well as residual contamination applied at the surface of a spherical liquid system in a microgravity environment has been studied both analytically and numerically. Such an investigation is relevant to containerless materials processing in a microgravity environment. The analysis is linear. Temperature and concentration fields are steady, but have a non-axisymmetric spatial variation. The compound fluid drop system as well as the simple drop system are studied. Results are compared to those of Marangoni flow, driven only by a thermal field.     

Onset of thermal ripples at the interface of an evaporating liquid under a flow of inert gas

The dynamics of thermal ripples at the interface of a volatile pure liquid (C₂H₅OH) is studied experimentally and numerically. Liquid evaporates under a flow of inert gas (N₂) circulating along the interface. The evaporation rate is varied by regulating both the gas flow rate and the gas pressure. Experiments in microgravity environment allowed to identify a transition to “interfacial turbulence,” along which some particular events such as nearly periodic and possible intermittent behaviors. Direct numerical simulations have been performed, and compare qualitatively well with experimental results, offering new insights into the physical mechanisms involved. Small-scale ripples appear to arise from a secondary instability of large-scale convection cells and their motion seems to follow the corresponding large-scale surface flow. The relative role of surface tension and buoyancy in triggering these flows is assessed by comparing experiments and simulations in both microgravity and ground conditions. Qualitative features compare satisfactorily well such as typical speed and orientation of the thermal ripples, as well as spiral flow in the bulk.     

Numerical and experimental study of the heat transfer and fluid flow by thermocapillary convection around gas bubbles

 At liquid–gas or liquid–liquid interfaces thermocapillary or Marangoni convection develops in the presence of a temperature or concentration gradient along the interface. This convection was not paid much attention up to now, because under terrestrial conditions it is superimposed by the strong buoyancy convection. In a microgravity environment, however, it is the remaining mode of natural convection. During boiling in microgravity it was observed at subcooled conditions. Therefore the question arises about its contribution to the heat transfer. Thus the thermocapillary convection was intensively studied at single gas bubbles in various liquids both experimentally and numerically. Inside a temperature gradient chamber, the overall heat transfer around single bubbles of different volume was measured with calorimetry and the liquid flow with PIV and LDV. In parallel to the experiment, a 2-dimensional mathematical model was worked out and the coupled heat transfer and fluid flow was simulated with a CV-FEM method both under earth gravity level and under microgravity. The results are described in terms of the dimensionless Nusselt-, Peclet-, Marangoni-, Bond- and Prandtl-number. Received on 23 August 1999     

Two-phase flow and pool boiling heat transfer in microgravity

Researches on two-phase flow and pool boiling heat transfer in microgravity, which included ground-based tests, flight experiments, and theoretical analyses, were conducted in the National Microgravity Laboratory/CAS. A semi-theoretical Weber number model was proposed to predict the slug-to-annular flow transition of two-phase gas–liquid flows in microgravity, while the influence of the initial bubble size on the bubble-to-slug flow transition was investigated numerically using the Monte Carlo method. Two-phase flow pattern maps in microgravity were obtained in the experiments both aboard the Russian space station Mir and aboard IL-76 reduced gravity airplane. Mini-scale modeling was also used to simulate the behavior of microgravity two-phase flow on the ground. Pressure drops of two-phase flow in microgravity were also measured experimentally and correlated successfully based on its characteristics. Two space experiments on pool boiling phenomena in microgravity were performed aboard the Chinese recoverable satellites. Steady pool boiling of R113 on a thin wire with a temperature-controlled heating method was studied aboard RS-22, while quasi-steady pool boiling of FC-72 on a plate was studied aboard SJ-8. Ground-based experiments were also performed both in normal gravity and in short-term microgravity in the drop tower Beijing. Only slight enhancement of heat transfer was observed in the wire case, while enhancement in low heat flux and deterioration in high heat flux were observed in the plate case. Lateral motions of vapor bubbles were observed before their departure in microgravity. The relationship between bubble behavior and heat transfer on plate was analyzed. A semi-theoretical model was also proposed for predicting the bubble departure diameter during pool boiling on wires. The results obtained here are intended to become a powerful aid for further investigation in the present discipline and development of two-phase systems for space applications.     

The effect of an insoluble surfactant on the skin friction of a bubble

In this paper, we develop a novel moving mesh method suitable for solving axisymmetric free-boundary problems, including the Marangoni effect induced by surfactant or temperature variation. This method employs a body-fitted grid system where the gas–liquid interface is one line of the grid system. We model the surfactant equation of state with a non-linear Langmuir law, and, for simplicity, we limit ourselves to the situation of an insoluble surfactant. We solve complicated dynamic boundary conditions accurately on the gas–liquid interface in the framework of finite-volume methods. Our method is used to study the effect of a surfactant on the skin friction of a bubble in a uniaxial flow. For the limiting case where the surface diffusivity is zero, the effect of a tangential stress generated by the surface tension gradient, allows us to explain a new phenomenon in high concentration regimes: larger surface tension, but also larger deformation. Furthermore, this condition leads to the formation of boundary layers and flow separation at high Reynolds numbers. The influence of these complex flow patterns is examined.     

Thermocapillary migration of liquid droplets in a temperature gradient in a density matched system

An experimental investigation of thermocapillary flow in droplets of a vegetable oil (partially hydrogenated soybean oil) immersed in silicone oil was conducted in a test cell with a heated top wall and a cooled bottom wall. The liquids are nearly immiscible and have equal densities at a temperature below the room temperature, thus providing a simulation of low-gravity conditions by reducing the buoyancy forces. The interfacial tension between the two oils was measured in the temperature range 20–50°C using a capillary tube and d/dT was determined to be negative. Droplets ranging in sizes from 3 mm to 1 cm diameter were injected into the silicone oil. The vertical temperature profile in the bulk liquid (silicone oil) produces temperature variations along the interface which induce variations in the interfacial tension. The flow inside the droplet driven by the resulting interfacial shear stresses was observed using a laser light-sheet flow visualization technique. The flow direction is consistent with the sign of d/dT. The observed maximum surface velocities are compared to the theoretical predictions of Young et al. (1959).For short times after injection, the droplets were driven by this flow towards the hot wall above the matched-density temperature until the droplets reached a point where the forces due to the flow and buoyancy were equal. After longer times, the droplets moved to the cold side due to suspected density changes caused by mass transfer from the droplets to the silicone oil. This was confirmed by tests under isothermal conditions, where it was observed that droplets of all sizes fell to the cold bottom eventually.Thus, even though the thermocapillary flow inside the droplets persisted for long times in spite of the mass transfer, the migration of droplets towards the hot side was eventually affected by uncontrolled buoyancy forces resulting from density changes due to mass transfer. While additional liquids are being tried, it is suggested from the present experience that reduced gravity experiments will probably be necessary to provide unambiguous data for the migration of droplets.     

Effect of magnetic field on the buoyancy and thermocapillary driven convection of an electrically conducting fluid in an annular enclosure

The main objective of this article is to study the effect of magnetic field on the combined buoyancy and surface tension driven convection in a cylindrical annular enclosure. In this study, the top surface of the annulus is assumed to be free, and the bottom wall is insulated, whereas the inner and the outer cylindrical walls are kept at hot and cold temperatures respectively. The governing equations of the flow system are numerically solved using an implicit finite difference technique. The numerical results for various governing parameters of the problem are discussed in terms of the streamlines, isotherms, Nusselt number and velocity profiles in the annuli. Our results reveal that, in tall cavities, the axial magnetic field suppresses the surface tension flow more effectively than the radial magnetic field, whereas, the radial magnetic field is found to be better for suppressing the buoyancy driven flow compared to axial magnetic field. However, the axial magnetic field is found to be effective in suppressing both the flows in shallow cavities. From the results, we also found that the surface tension effect is predominant in shallow cavities compared to the square and tall annulus. Further, the heat transfer rate increases with radii ratio, but decreases with the Hartmann number.     

On the motion of long bubbles in vertical tubes

Potential flow theory has been applied to study the shape and speed of an infinitely long bubble rising through flowing liquid in a vertical tube. In particular, the combined effects of surface tension and externally forced liquid motion are examined. An analytical formula for the bubble rise velocity in stagnant liquid is proposed, and shown to be in good agreement with experimental data for all values of surface tension. Numerical solutions for the bubble velocity in upward flowing liquid are obtained for laminar and turbulent velocity profiles. Approximate expressions for the bubble velocity, where the effects of liquid motion and surface tension are incorporated through the Reynolds and inverse Eötwos, are proposed and compared with experimental data. The predicted changes in bubble shape have, to a large extent, been confirmed through comparisons with photographic evidence for a wide range of parameters.     

Bubble generation and transport in a microfluidic device with high aspect ratio

Bubble generation and transport in a micro-device composed of a micro-T-junction and a following serpentine micro-channel was experimentally investigated. It has a rectangular cross-sectional with an aspect ratio of 7.425. Air and water were used as gas and liquid, respectively. Mixtures of water–glycerol and water–Tween-20 were used to study the effects of liquid viscosity and surface tension. Compared with previous T-junction bubble generation, the liquid and gas inlets orientation was switched in this work. The continuous flow was driven from the perpendicular channel and the dispersion flow was from the main channel. It shows that the break-up process has three periodic steps under certain operating conditions. The dimensionless bubble length L/w in the micro-channel with high aspect ratio is much larger than that in square microchannels. A correlation is proposed to correlate L/w with liquid flow rate J_L, gas flow rate J_G, and liquid viscosity μ_L. Surface tension σ can change the bubble shape but almost does not affect the bubble length in this fast break-up process. Additionally, a long bubble may be broken up at the corners at the same time because the locations of gas and liquid are exchanged relative to the concave and convex portions of an elbow after a turn which may result in the change of fluid velocities and gas–liquid pressure drop.