Marangoni convection and mass transfer from the liquid to the gasphase

Marangoni convection can be introduced by instabilities in the evaporation of a component of a solution, under isotermal conditions. Three experiments were performed under micro-gravity conditions, to prevent density induced Rayleigh convections, during the D1-Spacelab flight. With a flat gas-liquid interface the system was stable, while a two-dimensional linear stability analysis predicts the system to be unstable. Deformation of the interface introduced concentration and thus surface tension gradients. The resulting Marangoni convection will enhance the mass transfer of the evaporation process.     

Study of the Marangoni convection around a surface tension minimum under microgravity conditions

At equilibrium, aqueous fatty alcohol solutions presents a surface tension minimum versus temperature. The influence of such an extremum on the Marangoni convection is studied. Two experiments have been performed under microgravity conditions (Texus 8 (1983) and Texus 9 (1984) flights). The velocity fields are determined by following the paths of tracer particles and furthermore, in the Texus 9 experiment, differential interferograms have been recorded.     

EURECA and SGF performances

The EURECA platform offers unique characteristics for microgravity research: a very low level micro-gravity spectrum and a long operation time for recoverable experiments.

Five core facilities on board will perform an impressive number of experiments. The main purpose of the Solution Growth Facility (SGF) is growing crystals at low temperatures by using the double diffusion technique in three compartment reactors. The micro-gravity eliminates the convection in the liquid, while Marangoni convection is avoided by the absence of free surfaces. The thermal gradient in the buffer zone is better than 0.01°C/cm. Turbulence is eliminated by control of the valve rate and by a pressure compensation system. All surfaces are coated with Halar. The diffusion rate can be controlled by the use of filters. The SGF contains three independently controlled reactors.

A forth reactor contains an experiment aiming at measuring the Soret coefficient of twenty binary organic mixtures and aqueous electrolyte solutions.     

Marangoni convection when the surface tension increases with the temperature in normal and low gravity conditions

Thermocapillary convection has been studied in n-heptanol aqueous solutions whose surface tension is increasing with the temperature.

The fluid was confined in a parallelipipedic enclosure and a thermal gradient was imposed parallel to the free liquid/gas interface. The motions induced by the thermocapillary forces have been studied under low gravity conditions during Texus sounding rocket experiments and during the Spacelab D1 mission.

The combined thermocapillary and buoyancy convection have been extensively studied during ground based experiments.

The influence of the aspect ratio and of the alcohol concentration were investigated. Detailed velocity field was determined at the steady state under normal gravity conditions by Laser Doppler Anemometry (LDA).

The aspect ratio was of 0.3 and the temperatures imposed to the lateral sides of the cell were respectively 45°C and 60°C. With these experimental conditions, two superposed contrarotative cells were observed with rapid motions in the surface from the cold to the hot side. This convective pattern was also observed during the normal and high gravity periods of parabolic flights but during the low gravity period of the parabola the motions stopped everywhere in the fluid even in the surface and reappeared immediatly at the pull out of the parabola.     

Microgravity experiments on the transition from laminar to oscillatory thermocapillary convection in floating zones

The onset of oscillatory thermocapillary convection in floating zones has been measured under microgravity in two sounding rocket experiments TEXUS and on board of the satellite SPAS-01. We report on the critical Marangoni numbers measured under μ-g and in the 1-g reference experiments     

Hydrothermal instability of thermocapillary convection in large-Prandtl-number liquid bridges under microgravity

Linear stability analysis was performed to study the mechanism of transition of thermocapillary convection in liquid bridges with liquid volume ratios ranging from 0.4 to 1.2, aspect ratio of 0.75 and Prandtl number of 100. 2-D governing equations were solved to obtain the steady axi-symmetric basic flow and temperature distributions. 3-D perturbation equations were discretized at the collocation grid points using the Chebyshev-collocation method. Eigenvalues and eigenfunctions were obtained by using the Q–R method.The predicted critical Marangoni numbers and critical frequencies were compared with data from space experiments. The disturbance of the temperature distribution on the free surface causes the onset of oscillatory convection. It is shown that the origin of instability is related to the hydrothermal origin for convections in large-Prandtl-number liquid bridges.     

环形浅液池内低Prandtl数流体超临界热毛细对流演化

为研究低Prandtl数（Pr）流体热毛细对流演化过程，对环形浅液池内Pr=0.011的流体热毛细对流进行三维数值模拟.研究发现:当Marangoni数较小时，流动为轴对称稳态流动；当Marangoni数超过某一临界值后，流动失稳并转变为热流体波，其波数随Marangoni数增加而减小，而波动主频增大；随着Marangoni数增加，流动加强，沿周向运动的热流体波演变为沿径向运动的径向波，其波数大大减小；当Marangoni数继续增加时，波动频谱曲线噪声增加，呈广谱特性.因此，在计算范围内热毛细对流的演化过程为:轴对称稳态流动-热流体波-单周期径向波-多周期三维振荡流动.      

Oscillatory thermocapillary convection in floating zones under normal- and micro-gravity

The thermocapillary convection in a floating zone becomes oscillatory, exceeding a Marangoni number of approximately 10⁴. The oscillatory state of thermocapillary convection has been verified under 1 g and 10^?4 g and the features seem to be fairly independent of the gravity level. The transition point from the laminar to the oscillatory state (critical Marangoni number), however, depends on the magnitude and direction of the buoyant force.     

Marangoni convection in a liquid layer under the simultaneous action of a transverse magnetic field and rotation

Asymptotic stability characteristics with respect to the onset of Marangoni convection in a liquid layer under the simultaneous action of a large transverse magnetic field and a large rate of rotation are investigated. In certain parameter ranges there is a decrease in critical Marangoni number for increasing magnetic field and rotation depending on the coupling between interfacial perturbations and rotation.     

Recent development of Marangoni flows theory and experimental results

Microgravity fluid dynamics deals with the thermodynamics and dynamics of fluid systems in low-gravity environments and, thus, covers a rather large variety of subjects /1/. Among them, free convection in systems with fluid-fluid interfaces has received renewed attention because these systems are found in many materials sciences processes and exhibit a number of new, interesting phenomena. This paper surveys some recent studies by the author and his coworkers on the free convection occurring in a system of two interfacing, immiscible fluids subject to an imposed temperature gradient in a gravity environment. In such a system, three classes of free convection may be distinguished: natural, Marangoni, and combined free convection, according to whether it is caused by thermally induced buoyant forces, Marangoni stresses, or both. For each class one may further distinguish different regimes according to the relative importance of convective and diffusive transport of momentum and internal energy.First, the use of order of magnitude analysis is illustrated to identify and determine the types of flow regimes that can be established for each class of free convection. Subsequently, the attention is focused on the study of the main features of the two classes of flow regimes that can be established when the leading driving action is the Marangoni stress: Marangoni boundary layers and Marangoni-Stokes flow. Finally, a brief review of the experimental experience of the author's team is reported, looking in particular at the space experiments performed on board the missions SL1 and D1 of the Spacelab.     

Two-dimensional convection in liquid layer related to crystal growth techniques in space

In this paper, the Marangoni convection including buoyancy convection in two-dimensional fluid model was investigated experimentally and theoretically. The transparent liquid in the test cell was heated and cooled under various conditions. The fluid flow driven by surface tension and by buoyancy force was visualized and measured. The flow patterns, velocity distributions and temperature field were obtained and they were compared with the results of the numerical solutions. The effect of the configuration and the convection in microgravity on crystal growth was discussed. The temperature field of the crystallizing surface was predicted.     

失重飞机上进行的电泳分离实验

介绍了用A3—l连续自由流电泳仪在俄罗斯失重飞机上进行的实验情况及结果．进行了4个架次的飞行试验，获得了飞行试验的数据和电泳分离图像．试验结果表明，电泳仪装置工作良好，从分离图像可以看出重力变化对电泳分离过程的影响并证实微重力环境对生物材料的电泳分离是有利的．     

Onset of Marangoni convection in an infinite layer of an electrically conducting liquid under magnetic field

The onset of Marangoni convection in an electrically conducting liquid is studied theoretically for the case that the liquid is subjected to a vertical temperature gradient and a magnetic field with arbitrary orientation.The effects of the intensity of magnetic field, the angle of inclination of magnetic field and the Biot number at the free surface on both the critical Marangoni number and the flow pattern are made clear.     

Numerical simulation of 3D convective motion disturbing the soret separation of the two components of a binary fluid mixture

Buoyancy driven convection arising in a fluid mixture confined in a cylindrical container designed for a measurement of the Soret (thermal diffusion) coefficient of a binary mixture, has been investigated in the case of a cylinder submitted to an axial temperature gradient, in low but steady gravity conditions. These 3D convective motions and their effect on the Soret separation have been numerically simulated, using a 3D finite differences technique, for a wide range of the characteristic parameters. The results will allow to interpret any Soret experiment performed in micro-gravity conditions.     

A sounding rocket experiment on the marangoni convection

During a 6-minutes free flight of a sounding rocket an experiment on the thermal Marangoni convection in a right circular cylindric floating zone was carried out to verify the thermal Marangoni convection by flow visualisation for the first time at 10^?4 g. From the motion picture analysis the velocity distributions are obtained. The interdependence between the two velocity maxima and the S-shaped temperature distribution on the free surface is discussed. The influence of the heat transfer caused by the flow of the surrounding air induced by the thermal Marangoni convection itself is considered briefly.     

过冷核态池沸腾中的Marangoni效应

分析了过冷核态池沸腾过程中气泡横向运动现象,指出其成因在于相邻气泡界面温度差引起的Mara-ngoni对流对周围液体的吸引.通过对该流动的尺度分析,得到了气泡横向运动特征速度及其可观测度的估算公式,其预测结果与实验观测相一致.特别是对极细小的初始核化气泡,该公式预测了强烈的横向Marangoni对流会导致气泡顶端微射流的形成.该效应在加热面水平向下或微重力沸腾等气泡脱落频率较低的情形中尤为重要.      

微重力环境下蒸发液层热毛细对流的数值模拟

提出了一种新模型来研究由单一物质构成的液层在其纯蒸气中的蒸发.液层置于微重力环境中并且受到水平方向温度梯度的作用,液层的热毛细对流和蒸发耦合在一起,使得气液界面的传热传质规律更加复杂.用理论分析的方法求解了不考虑热毛细效应的纯蒸发模型,得出温度场分布和界面质量流量的解析表达式.对于热毛细对流和蒸发耦合情况,采用有限差分的投影算法同时求解Navier-Stokes方程和能量方程,得到了不同蒸发Biot数和Marangoni数下流场和温度场的稳态数值解.论述了蒸发Biot数和Marangoni数对界面传热传质的影响,提出并解释了蒸发和热毛细对流耦合的三种模式.      

空间微重力条件下偏晶合金的研究

利用我国首次卫星搭载,进行了偏晶合金Zn—Pb、Al—Pb 的空间微重力下的重熔试验,研究了低于临界温度下Al—Pb 合金重熔过程中的物理规律;在临界温度之上重熔了Zn—Pb 合金,发现了Marangoni 对流对合金凝固过程的影响;在液态烧结条件下制备了均匀的粉末Zn—Pb 合金。在微重力下的研究表明:空间微重力条件对消除由于重力引起的宏观偏折和自然对流十分重要,但在重力消失后,非重力的其它因素的影响又显得格外重要。     

Some results from parabolic flights

On december 1984 and july 1985 parabolic flights with a KC 135 aircraft of NASA allow to perform preliminary tests on the Bénard instability problem which has been proposed to be studied during the D2 mission in the AFPM.On march 1986 the possibility to create a flat interface between two immiscible liquids has been investigated.