Thermocapillary Convection in Floating Zone with Axial Magnetic Fields

Numerical simulations on the effects of axial magnetic fields on the thermocapillary convection in a liquid bridge of silicone-oil-based ferrofluid under zero gravity have been conducted. The Navier-Stokes equations coupled with the energy conservation equation are solved on a staggered grid, and the mass conserving level set approach is used to capture the free surface deformation of the liquid bridge. The obvious effects of the magnetic fields on the flow pattern as well as the velocity and temperature distributions in the liquid bridge can be detected. The axial magnetic fields suppress the thermocapillary convection and a stagnant flow zone is formed between the circulating flow and the symmetric axis as the magnetic fields increase. The axial magnetic fields affect not only the velocity level inside the liquid bridge but also the velocity level on the free surface. The temperature contours near the free surface illustrates conduction-type temperature profiles at moderate strength fields.     

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.     

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.     

Heat Transfer Through the Interface and Flow Regimes in Liquid Bridge Subjected to Co-Axial Gas Flow

We present results of an extensive numerical study on the thermocapillary (Marangoni) convection and a heat transfer through the interface in a liquid bridge of Pr?=?68. The geometry of the physical problem is a cylindrical and non-deformable liquid bridge concentrically surrounded by an annular gas channel under conditions of zero gravity. The gas flow is co- or counter-directed with respect to the Marangoni flow. The forced gas flow along the interface provides two actions: via shear stresses and heat exchange. Usually the cooling of the interface enhances the flow while the heating slows down. This general trend may not hold when shear and thermocapillary stresses are comparable. The results show that when gas enters from the cold side the heat transfer through the interface is considerably larger than that when gas enters from the hot side.     

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.     

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.     

Numerical Investigation of Thermocapillary Convection in a Liquid Layer with Free Surface

Effects of Marangoni number, aspect ratio and gravity level on thermocapillary convection in a liquid layer is investigated numerically, in which the level set method is employed to capture free surface deformation. The computational results show that, with the increase of Marangoni number the free surface deformation is increased and it can lead to free surface rupture if the Marangoni number is large enough. The end walls has a damping effect on the free surface deformation, and as the aspect ratio (A =L/(0.5H)) decreases the deformability of free surface is reduced. The gravity can damp the free surface deformation, particularly as gravity level varies from 0.0001g ₀ to g ₀ the free surface deformability decreases steeply.     

Effect of motion of a local heat source on thermocapillary deformation of a thin liquid film flowing down under the action of gravity

The influence of a moving local heat source on the structure of flow in a thin liquid film flowing down on an inclined substrate under the action of gravity has been theoretically studied. Two-dimensional steady-state and conjugated hydrodynamic heat transfer problem has been solved in a long-wave approximation. The characteristics of flow are compared for various regimes: from the liquid film flowing down on a vertical surface with an immobile heat source to the behavior of a horizontal liquid layer under the action of a moving heat source. It is shown that changes in the flow velocity profile related to an increase in the velocity of the heat source motion and a decrease in the substrate slope under other equal conditions (constant flow rate, film thickness, and heat release) lead to a sharply increased thermocapillary deformation of the liquid film.     

Hydrothermal wave in a shallow liquid layer

The oscillatory thermocapillary convection and hydrothermal wave in a shallow liquid layer, where a temperature difference is applied between two parallel sidewalls, have been numerically investigated in a two-dimensional model. The oscillatory thermocapillary convection and hydrothermal wave appear if the Marangoni number is larger than a critical value. The critical phase speed and critical wave number of the hydrothermal wave agree with the ones given analytically by Smith and Davis in the microgravity environment, and it travels in the direction opposed to the surface flow. Another wave traveled downstream in addition to the hydrothermal wave traveled upstream was observed in the case of earth gravity condition.     

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.     

Report on Microgravity Experiments of Dynamic Surface Deformation Effects on Marangoni Instability in High-Prandtl-Number Liquid Bridges

This paper reports an overview and some important results of microgravity experiments called Dynamic Surf, which have been conducted on board the International Space Station from 2013 to 2016. The present project mainly focuses on the relations between the Marangoni instability in a high-Prandtl-number (Pr=?67 and 112) liquid bridge and the dynamic free surface deformation (DSD) as well as the interfacial heat transfer. The dynamic free surface deformations of large-scale liquid bridges (say, for diameters greater than 10 mm) are measured with good accuracy by an optical imaging technique. It is found that there are two causes of the dynamic free surface deformation in the present study: the first is the time-dependent flow behavior inside the liquid bridge due to the Marangoni instability, and the second is the external disturbance due to the residual acceleration of gravity, i.e., g-jitter. The axial distributions of DSD along the free surface are measured for several conditions. The critical parameters for the onset of oscillatory Marangoni convection are also measured for various aspect ratios (i.e., relative height to the diameter) of the liquid bridge and various thermal boundary conditions. The characteristics of DSD and the onset conditions of instability are discussed in this paper.     

组合线圈磁场下的液桥热表面张力流

为了优化外加磁场对对流控制作用,该文主要研究了轴向载流线圈磁场,横向四载流线圈磁场及其组合磁场对液桥热表面张力对流的控制.研究结果表明:轴向载流线圈磁场可有效抑制熔体的径向流动,并改善熔体对流的轴对称性;而横向载流线圈磁场可有效地抑制熔体轴向的对流,但是会破坏熔体对流的轴对称性.合理布置的轴向载流和横向四载流线圈的组合磁场同时保留了轴向载流线圈磁场的轴对称影响和横向四载流的轴向抑制作用,可以达到更好的控制熔体对流的效果,有利于从浮区法晶体生长中获得高质量晶体.     

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．     

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。     

Numerical Simulation of Thermocapillary Convection in Detached Solidification under Microgravity

In order to investigate the fundamental characteristics of thermocapillary convection in the detached solidification under microgravity, the finite-difference method was adopted to perform the numerical simulations. The results show that the flow of molten liquid is steady with the low Marangoni number and it only exists in the vicinity of the free surface. Moreover, with the Marangoni number increasing, the flow is expanded toward the inner part of molten liquid gradually, and at the same time the flow velocity on the free surface increases. However, the flow is unstable when the Marangoni number exceeds the critical Marangoni number.     

微重力条件下分离结晶过程中熔体热毛细对流的数值模拟

用有限差分法对微重力条件下分离结晶生长中的熔体热毛细对流进行了数值模拟,熔体的深径比A取1和2,自由界面无因次宽度B取0.05,0.075和0.1;得到了分离结晶Bridgman生长过程中熔体热毛细对流的流函数分布和温度分布图,研究了流型的演变过程和流动的失稳机制.结果表明:当Marangoni(Ma)数比较小时,流动为稳态流动并只存在于自由界面附近,随着Ma数的增加,流动增强并逐步向熔体内部扩展,熔体内部温度分布的非线性性增加,自由界面速度增大;Ma数超过某一临界值后,流动转化为非稳态流动;流动失稳的物理机制是流速的变化和阻力的变化之间存在滞后.     

Heat Transfer from a Local Heat Source to Subcooled Liquid Film

An experimental investigation is performed of heat transfer from a local heat source to films of water and low-boiling dielectric liquid that flow down a vertical plate by gravity. The liquids are substantially subcooled. In the case of perfluorotriethylamine flow, regular structures are formed in the film at the threshold value of the heat flux density. After the heated layer of liquid comes to the film surface, three characteristic modes of heat transfer are observed, which are associated with the variation of the modes of liquid flow caused by thermocapillary convection. At low values of the Reynolds number of the film, a specific form of critical heat transfer is observed, which is characterized by disintegration of the jet into droplets and their separation from the heater.     

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.     

Oscillatory thermo capillary convection in a spinning cylindrical liquid bridge under micro-gravity condition

In a viscous liquid column thermocapillary convection arises under the action of an axial temperature gradient. If this temperature gradient is oscillatory it induces surface oscillations and may lead to large amplitudes, if the frequency of the temperature fluctuation is in resonance with that of the liquid column. The response of the liquid column (non-rotating and rotating) has been determined and the effect of changing aspect ratio, Prandtl number, thermal expansion coefficient and rotational speed of the column in the case of steady temperature gradient may deteriorate for fluctuating temperatures, especially, if the fluctuating frequency is in reasonance with one of the lower natural frequencies of the liquid. The streamlines for the rotating and non-rotating liquid bridge are also presented and show the complex morphology of the convectional flow field, which is due to rotation and temperature oscillations of the liquid system.     

Numerical Simulation of Thermocapillary-Buoyancy Convection in Detached Solidification

In order to understand the characteristics of thermocapillary-buoyancy convection in detached solidification, we conducted a series of numerical simulations of thermocapillary flow of CdZnTe melt with different aspect ratio and gap width under gravity using the finite-difference method. The results indicate that, at a small Marangoni number, the buoyancy has a great effect on the temperature distribution; however, its influence on the maximum stream function value is so little that it can be neglected. When Marangoni number exceeds a threshold value, the steady flow converts into the unstable convection. In addition, the buoyancy makes the critical Marangoni numbers decrease by one order of magnitude comparing the results obtained in gravity and those in microgravity condition.