高温氧化物熔体中表面张力对流效应研究实验

利用高温实时光学观察方法,实时地观察了高温氧化物熔体中的表面张力对流效应．稳态的热毛细对流流线呈轴对称形式,非稳态的热毛细涡流运动伴随熔液温度呈周期性变化,同时还观察到另一种由熔体自由表面的弯月面曲率的变化引起的非稳态的热毛细对流现象．最后测定了相对熔液温度下非稳态热毛细对流的振荡频率．     

Thermal patterns in evaporating liquid

In this paper, some of the preparatory experiments of the ESA sponsored space program CIMEX-1 are presented. A liquid layer of variable thickness is subject to a flow of inert gas. The non-uniform evaporation induced by the gas flow creates a temperature gradient parallel to the interface triggering in that way thermocapillary convection. The combined action of evaporation, thermocapillarity and gravity has been not completely clarified both theoretically and experimentally. The experiment presented in this work concerns a liquid layer of ethanol of 2.2 mm thickness in presence of a mass flow of Nitrogen whose intensity varies in the range of hundreds of milliliter per minute. The experiments were performed at an initial liquid temperature of 21°C. The patterns observed are strongly dependent on the flow rate of inert gas. A change in the instability patterns has been observed for a gas flow of about 1.7 l/min.     

Experimental study on the transition process to the oscillatory thermocapillary convections in a floating half zone

The transition process of the thermocapillary convection from a steady and axisymmetric mode to the oscillatory mode in a liquid bridge with a fixed aspect ratio and varied volume ratio was studied experimentally. To ensure the surface tension to play an important role in the ground-based experiment, the geometrical configuration of the liquid bridge was so designed that the associated dynamic Bond number B_d≈1. The velocity fields were measured by Particle Image Velocimetry (PIV) technique to effectively distinguish the different flow modes during the transition period in the experiments. Our experiments showed that as the temperature difference increased the slender and fat bridges presented quite different features on the evolution in their flow feature: for the former the thermocapillary convection transformed from a steady and axisymmetric pattern directly into an oscillatory one; but for the latter a transition flow status, characterized by an axial asymmetric steady convection, appeared before reaching the oscillatory mode. Experimental observations agree with the results of numerical simulations and it is obvious that the volume of liquid bridge is a sensitive geometric parameter. In addition, at the initial stage of the oscillation, for the former a rotating oscillatory convection with azimuthal wave number m=1 was observed while for the latter a pulsating oscillatory pattern with azimuthal wave number m=2 emerged, and then with further increase of the temperature difference, the pulsating oscillatory convection with azimuthal wave number m=2 evolved into a rotating oscillatory pattern with azimuthal wave number m=2.     

轴向磁场下分离结晶熔体内热毛细对流的数值模拟

为研究轴向磁场对分离结晶Bridgman法生长CdZnTe晶体熔体热毛细对流的影响,采用有限差分法进行了三维数值模拟。结果表明轴向磁场能有效抑制熔体内的热毛细对流;轴向磁场对熔体内部温度分布也有较大的影响,能使等温线分布变得平缓;当磁场强度不变时,随着狭缝宽度的增大熔体内部的流动减弱。     

环形池内双层流体浮力-热毛细对流的实验研究

液封提拉法生长晶体过程中,液封流体与熔体形成了不相溶的双层流体。除了自由表面的热毛细力作用外,液-液界面的热毛细力以及浮力共同驱动了双层系统内浮力-热毛细对流的产生。为了了解环形池内双层流体的复杂流动和传热特征,本工作对环形池内0.65cSt硅油与水组成的双层流体浮力-热毛细对流进行了实验观察。实验记录了不同的液层厚度比和深宽比条件下流动失稳的临界温差与流动形态。结果表明,液层总厚度的增大会削弱系统的流动稳定性。流动失稳的流动形态有三种,即第一种热流体波、第二种热流体波和三维稳态流动。实验还观察到了流动分岔现象。     

Gravitational effects on evaporative convection at microscale

Thermocapillary convection induced by phase change (evaporation) has been investigated in confined environment. This paper introduces some insight into the physics of evaporatively-driven thermocapillary convection and emphasizes on the interaction between the observed convection and gravity. Non-equilibrium interfacial conditions lead to temperature/surface tension gradients which drive convective patterns. The latent heat of evaporation leads to an important cooling effect near the triple contact line. Evaporation of volatile liquids in capillary tubes is experimentally investigated to demonstrate the above effects. The size of the capillaries is found to be an important factor in the effect that gravity could have on thermocapillary convection. The oscillatory behaviour observed when buoyancy affects thermocapillary convection could be explained through the coupling between interfacial temperature and the flow within the liquid. The three dimensional nature of the flow structure is found to extend the effect of gravity to the horizontal section of the flow.     

Effect of Surface Evaporation on Steady Thermocapillary Convection in an Annular Pool

In order to understand the effect of surface evaporation on thermocapillary convection in an annular pool, a series of numerical simulation on thermocapillary convection of the fluids with Prandtl number from 0.01 to 50 in the pure vapor environment were carried out. The results show that thermocapillary convection is always coupled with the evaporation process on the free surface. With the increase of evaporation Biot number, the surface temperature decreases, and the evaporation mass flux near the hot wall increases obviously. However, near the cold wall, the evaporation mass flux increases first, and then decreases. When Marangoni number is small, the total evaporation mass rate at free surface increases with the increase of evaporation Biot number; when Marangoni number is larger, it increases first and then approaches a constant value. The aspect ratio of the annular pool has a positive influence on the thermocapillary convection strength and the total evaporation mass rate. With the increase of Prandtl number, the surface temperature rises gradually and the evaporative mass flux increases, and the thermocapillary convection cell moves gradually toward the outer wall and the free surface. This effect decreases with the increase of evaporation Biot number When evaporation Biot number is smaller, the total evaporation mass rate increases with the Prandtl number; when Biot number is larger, Prandtl number has little impact on the total evaporation mass rate.     

Thermocapillary Convection in a Binary Mixture with Moderate Prandtl Number in a Shallow Annular Pool

This paper presented a series of numerical simulations on thermocapillary convection for mixed toluene/hexane solution with mass fraction of \(c_{0}=?26.27\)% in a shallow annular pool. The Prandtl number of the binary solution is 5.54. Results indicate that when the annular pool subjects to a radial temperature gradient, the solute shifts toward the inner wall under the Soret effect, which leads to a concentration gradient with the opposite direction to the temperature gradient. With the increase of surface heat dissipation, thermocapillary convection is enhanced and the flow is more prone to destabilization. Therefore, the critical thermocapillary Reynolds number of the flow destabilization and the corresponding critical oscillation frequency all decrease, but the critical wave number increases. After flow destabilization, the concentration fluctuation similar to the temperature fluctuation on the free surface appears. No matter the free surface is adiabatic or not, the flow always undergoes a transition from two-dimensional steady axisymmetric flow to the hydrothermal waves, and then to chaos with the increase of thermocapillary Reynolds number.     

Stability of thermocapillary convection and regimes of a fluid flow acted upon by a standing surface wave

It has been established that, in the case where a standing surface wave acts on a thermocapillary-convection flow in a cylindrical volume, there arises an oscillating-convection zone between the laminar and turbulent regimes of flow. It is shown that the boundary between these regimes is determined by the amplitude δ and the number of periods n of the standing wave and is practically independent of the Marangoni number and the oscillation frequency of this wave. At n = 2, in the range 0.004 < δ < 0.006, the parameters of the fluid cease to oscillate. The mechanisms by which the thermocapillary convection in closed volumes loses its stability are discussed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 5, pp. 108–115, September–October, 2007.     

Asymptotic Solution of Thermocapillary Convection in a Differentially Heated Thin Annular Two-Layer Pool

The steady laminar two-dimensional thermocapillary convection of two immiscible liquid layers in a thin annular pool with one free surface, one liquid/liquid interface subjected to a radial temperature gradient was investigated using asymptotical analysis. The pool is heated from the inner cylindrical wall and cooled at the outer wall. Bottom and top surfaces are adiabatic. The asymptotic solution is obtained in the core region in the limit as the aspect ratio, which is defined as the ratio of the lower layer thickness to the gap width, trends to zero. The numerical experiments are also carried out to compare with the asymptotic solution of the steady two-dimensional thermocapillary convection. The results indicate that the expressions of velocity and temperature fields in the core region are valid in the limit of the small aspect ratio.     

Experimental study of surface deformation and flow pattern on buoyant-thermocapillary convection

Free surface deformation is one of the most important physical phenomena in fluids with free surface. In the present paper, convection and surface deformation caused by thermocapillary effect in a rectangular cavity were investigated. In ground experiments, the convection was also affected by gravity. The cavity has a horizontal cross section of 52mm×42mm and the thikkness of the liquid layer is 4mm. Temperature difference between two sides of the liquid layer was increased gradually, and the flow in liquid layer will develop from steady to unstable convection. An optical diagnostic system consisting of a revised Michelson interferometer with image processor was developed to study fluid surface deformation in convection, and the displacements of free surface oscillation were determined. PIV technique was adopted to observe the evolution of flow pattern, and the velocity fields were obtained quantitatively. The present experiments demonstrate that surface deformation is quite distinct in buoyant-thermocapillary convection. in order to understand the mechanism of buoyant-thermocapillary convection, not only the hydrothermal wave instability but also the surface wave instability should be discussed.     

Effects of convective fluid motion upon oxide crystal growth in high temperature solution

For understanding of the influence of convective flow on crystal growth, space high temperature in situ observation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystals in high temperature up to 1,000°C. Model experiments using transparent liquids such as KNbO₃ and a mixture of Li₂B₄O₇+KNbO₃ were chosen to investigate effects on ground and in space. On the earth, an investigation of growth kinetics of KNbO₃ crystal related to two different states of convection: diffusive-advective flow and diffusive-convective flow, has been performed. The per unit length of a step e is calculated from the experimental data for two different states of convection. Analyses of these data show the effect of buoyancy convection is to enhance the sharpness of the interface. The growth of KNbO₃ crystals from solution of KNbO₃+Li₂B₄O₇ was investigated in space. The streamlines of the steady thermocapillary convection in Li₂B₄O₇ solvent was observed. Due to thermocapillary convection, KNbO₃ crystal grains grew and filled the whole solution homogeneously. Earth-based quenching experiments are designed in order to study polyhedral instability of KNbO₃ crystal, which is controlled by diffusion mechanism limitation. In all cases, when the crystal was nucleated near air/solution surface, it lost its polyhedral stability and varied from polyhedrons to dendrites. The thickness of diffusion mechanism limitation layer is about 60 μm.     

Effects of convective fluid motion upon oxide crystal growth in high temperature solution

For understanding of the influence of convective flow on crystal growth, space high temperature in situ observation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystals in high temperature up to 1,000°C. Model experiments using transparent liquids such as KNbO₃ and a mixture of Li₂B₄O₇+KNbO₃ were chosen to investigate effects on ground and in space. On the earth, an investigation of growth kinetics of KNbO₃ crystal related to two different states of convection: diffusive-advective flow and diffusive-convective flow, has been performed. The per unit length of a step e is calculated from the experimental data for two different states of convection. Analyses of these data show the effect of buoyancy convection is to enhance the sharpness of the interface. The growth of KNbO₃ crystals from solution of KNbO₃+Li₂B₄O₇ was investigated in space. The streamlines of the steady thermocapillary convection in Li₂B₄O₇ solvent was observed. Due to thermocapillary convection, KNbO₃ crystal grains grew and filled the whole solution homogeneously. Earth-based quenching experiments are designed in order to study polyhedral instability of KNbO₃ crystal, which is controlled by diffusion mechanism limitation. In all cases, when the crystal was nucleated near air/solution surface, it lost its polyhedral stability and varied from polyhedrons to dendrites. The thickness of diffusion mechanism limitation layer is about 60 μm.     

Effect of G-jitter on the thermocapillary convection experiment in ISS

Effects of g-jitter on the measurement of dynamic surface deformations (DSD) caused by the oscillatory thermocapillary convection in a liquid bridge have been studied experimentally. A vibration table driven by a piezoelectric actuator is constructed to generate g-jitters of realistic amplitude and frequency spectra. A microimaging displacement meter is used to measure DSD in the presence of g-jitter. It is shown that the natural DSD in the oscillatory state of thermocapillary convection is well separated in the frequency domain from that caused by harmonic and non-harmonic g-jitters. It is concluded that the DSD measurement in the future ISS experiment is feasible even in the presence of g-jitter there.     

Oscillation Transition Routes of Buoyant-Thermocapillary Convection in Annular Liquid Layers

There are various oscillation transition routes of buoyant-thermocapillary convection in an annular liquid layer. Three types of transition routes including quasi-periodic bifurcation, period-doubling bifurcation and tangent bifurcation have been observed. In our ground experiments, the depth of liquid layer is in a range of 1.6–2.4 mm. The silicone oil with Prandtl number of 28.6 is selected as the liquid medium. The temperature oscillation is detected by a single-point temperature measuring system and the surface oscillation is measured by a laser displacement-sensor with high resolution. The step-heating mode is adopted in the experiments. Transition routes of temperature oscillation and surface oscillation are studied systematically, and the relationship between them is discussed, too.     

Marangoni Convection Instabilities Induced by Evaporation of Liquid Layer in an Open Rectangular Pool

In order to investigate the Marangoni convection instability of 0.65cSt silicone oil induced by evaporation in liquid layer, a series of experiments are carried out in an open rectangular pool. The effects of side wall temperature as well as ambient temperature on competitions between BM convection and thermocapillary convection are analyzed thoroughly. Increasing of the side wall temperature would inevitably enhance thermocapillary convection and suppress the formation of BM cells by transferring hot fluid from border to surface. As long as the side wall temperature is high enough, BM cells would disappear completely and multicellular rolls as well as hydrothermal waves would occur in the whole layer. Increasing ambient temperature would enhance both BM convection and thermocapillary convection, but the later one benefits more from it because hydrothermal waves can occur at a lower Ma number. Critical Marangoni numbers for the incipience of hydrothermal waves and that disappearance of BM convection cells are obtained under different ambient temperatures.     

Effect of ambient fluid flow upon onset of oscillatory thermocapillary convection in half-zone liquid bridge

Onset of the oscillatory thermocapillary convection in a half-zone liquid bridge is known to be sensitive to heat transfer at free surface of the liquid bridge and the ambient air motion. The effect of the heat transfer through the free surface upon the onset of the transition mechanism and three-dimensional nature of the oscillatory flow, however, is not yet fully understood. In the present study, the thermal fluid behavior of the ambient gas and its effect upon the criticality were investigated experimentally and numerically. In the experiment, flows in the liquid bridge and the ambient air were visualized by suspending tracer particles in both fluids. Volume of the ambient air region was adjusted by placing two partition disks perpendicular to the liquid bridge. The onset of oscillation depended on the distance between the partition disks; the critical Marangoni number increased with the decreasing distance. Three-dimensional simulation of the liquid bridge and the ambient air has been performed by the finite difference method in order to determine the onset of the oscillation. The present calculations demonstrate significant influence of the heat transfer at the free surface on the onset of oscillation. The results of calculations yield a good agreement with the experimental critical values.     

Thermo-capillary and residual natural convection in an orbiting spherical liquid system

The influence of a given temperature distribution applied at the surface of a spherical liquid system in a circular orbit has been determined analytically. Three basic convectional flows inside the liquid have been investigated: Marangoni convection, residual gravity natural convection and the natural convection due to self-attraction. For small drops thermocapillary convection is dominant, while for large liquid spheres the convection due to self-attraction is predominant for orbits around the sun, while for orbits of smaller diameter, as for instance around the earth, the natural convection due to the residual gravity is dominant for large spherical drops.     

Experimental Study on the Combined Evaporation Effect and Thermocapillary Convection in a Thin Liquid Layer

The coupling mechanisms and flow characteristics of thermocapillary convection in a thin liquid layer with evaporating interface were studied. The planar liquid layer, with the upper surface open to air, was imposed externally horizontal temperature differences. The measured average evaporating rates and interfacial temperature profiles indicated the relative importance of evaporation effect and thermocapillary convection under different temperature gradients. A temperature jump was found at the interface, which was thought to be related to the influence of evaporation effect. All above mentioned results were repeated in a rarely evaporating liquid to compare the influence of evaporation effect.     

Influence of Acoustic Perturbation and Acoustically Induced Thermal Convection on Premixed Flame Propagation

Interference between an acoustic field and premixed flame is investigated. Two kinds of transport processes are considered to cause combustion promotion in the acoustic field. One is diffusive transport. It has long been considered that the acoustic oscillation promotes diffusion, like turbulence does. The other is convective transport. Acoustically induced thermal convection that is driven by a kind of acoustic radiation force in a standing acoustic field has been previously found (Tanabe et al., Proc Combust Inst 28:1007–1013, 2000). The burning rate of an isolated single droplet is promoted by this thermal convection. In this report, the influence of the acoustic oscillation on the premixed flame propagation was examined through experiment and numerical simulation. The gravitational force was not taken into account since it complicates the combustion phenomena. The experiment and numerical simulation were done in microgravity condition. Each influence of turbulent diffusion and thermal convection were evaluated from the burning velocity, flame shape and flame speed, since the diffusive effect has been considered to have an influence on the burning velocity, while the convective effect has an influence on the flame speed and flame shape. As a result, it was clarified that the diffusive effect has a minor influence on the burning enhancement of the premixed flame in the standing acoustic field. On the contrary, the acoustically induced thermal convection has a significant influence on it. Premixed flame behavior in the standing acoustic field can be explained through the characteristics of the acoustically induced thermal convection.