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.     

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.     

Attempt to study Marangoni flow of low-Pr-number fluids using a liquid bridge of silver

In order to experimentally investigate the Marangoni flow of low-Prandtl-number fluids in a liquid bridge geometry under the condition of small Marangoni numbers close to the critical Marangoni numbers Ma_c1 and Ma_c2, the formation of a liquid bridge of silver was attempted. The available temperature difference between the upper and lower rods to obtain a small Marangoni number, such as Ma = 50, was calculated for a 5 mm high liquid bridge for several molten metals. For molten silver, the possible temperature difference was estimated to be 16 K, whereas, for molten silicon, this was 0.38 K, which is unrealistic for the purposes of experiments. For silver, a free surface can be obtained in the wide range of oxygen partial pressures, whereas, for molten silicon, the available oxygen partial pressure range is very small; equilibrium oxygen partial pressure for SiO₂ formation is as low as 1.1 × 10⁻¹⁴ Pa. A liquid bridge of molten silver was successfully prepared and temperature oscillation was observed; the estimated Marangoni number was 160 and oscillation frequency was 0.26 Hz.     

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.      

Upwards migration phenomenon on molten lunar regolith: New challenges and prospects for ISRU

As part of our research on the feasibility of producing commodities from lunar regolith by thermal-driven processes with minimal terrestrial precursors we need to characterize, reproduce, and understand thermophysical properties of the molten regolith still unforeseen under the lunar vacuum conditions at a scalable sample size. Two unanticipated phenomena, apparently caused by lunar melt’s surface tension under vacuum, have been revealed in our research work, vacuum void formation and upwards migration. In this paper we present our findings and thinkable explanation on the upwards migration phenomenon experimentally observed and consistently replicated as JSC-1A lunar regolith simulant melted at high vacuum. Upwards migration of molten lunar regolith will make future lunar ISRU’s melting processes both challenging as molten bulk material would migrate upwards along the container’s walls, and also promising on new opportunities for alternative ISRU’s sustainable processes as regolith’s upwards migration takes place in uniformed thin-film pattern. Among the potential ISRU’s processes that might use controlled thermal thin-film-based migration without the necessity of terrestrial precursors are production of feedstock for 3D printing, fractional separation of regolith’s component’s (O₂, metals, and alloys) via pyrolysis, film coating, purification of valuables solid crystals including silicon, and fabrication of key elements for microfluidic, and MEMS devices. Thermal upwards migration phenomenon on JSC-1A’s melt is formulated and explained by the authors as due to thermal Marangoni effect (also known as thermo-capillarity) in which temperature gradients within the melt’s bulk and along the crucible’s wall yield the surface tension large enough to supersede the gravitational force and yield the experimentally observed upwards thin-film migration. As far as the authors know, upwards thermal migration of molten JSC-1A (or other lunar simulant regolith) under vacuum has not been reported in the literature. A thermal mathematical model accounting for thermal Marangoni effect on molten JSC-1A agrees with what experimentally was observed, the formation of the meniscus on the melt-wall surface interface along with an incipient upwards migration in thin-film pattern along the crucible wall that, according to the model, experiences large temperature gradient, an important factor to trigger the thermal Marangoni effect along with the fact that surface tension of the molten lunar regolith material is temperature dependent.     

The effect of temperature gradient on the motion of a bubble in reduced gravity

The bubble motion is described as a function of thermal variation of surface tension of a bubble, temperature gradient, gravity, bubble diameter, viscosity and density. The relations among those values are shown in graphs, from which the requirements for the measurements to ascertain the velocity due to thermal variation of surface tension are made clear. According to these requirements, the experiments on the velocity of the bubble in a temperature gradient were conducted and the Marangoni effect on a bubble motion was ascertained.     

Two-dimensional Marangoni and buoyancy convection related to crystal growth techniques in space

The natural convection in the horizontal liquid layer driven both by surface tension gradient and buoyancy was investigated experimentally and theoretically.The fluid motion was visualized by mixing fine flakes of aluminum into the liquid. At the same time, the temperature field was visualized by the Mach-Zehnder interferometry and the local temperature gradient distributions were visualized by making use of the refraction of parallel incident light.The numerical analysis using the Galerkin method was also carried out which agreed well with the experimental results.It was found that the velocity and temperature fields of Marangoni convection were varied depending on the aspect ratio of the liquid layer and on the coupled buoyancy convection.     

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.     

Conditions for the onset of temperature oscillations in a simulated floating-zone configuration

The floating-zone method for crystal growth is studied in simulated low-gravity conditions by suspending a drop of fluid vertically between two circular rods and heating the upper rod with a heater. Flow and temperature oscillations are observed under certain conditions. The temperature oscillation level and the oscillation frequency are measured against the Marangoni number for several aspect ratios using hexadecane and Fluorinert as the test fluids. It is observed that, contrary to what has been believed, the Marangoni number is not the proper parameter to describe the onset of oscillation.     

天舟一号货运飞船空间实验装置蒸发相变地面实验

以天舟一号货运飞船为依托,开展空间蒸发相变传热规律的科学实验研究,探索重力对蒸发传热传质过程的影响规律.设计了一套地面蒸发实验平台,以蒸发相变液体FC-72为研究对象,通过红外热像仪测温、热流量计、差分热电偶等手段,观测FC-72液层在不同台面温度、注液量等情况下的相变界面变化、蒸发表面特性、流体物性及Marangoni对流涡胞的变化等,获取其蒸发两相流体的液层温度差、表面温度场、热流量值、蒸发速率和涡胞结构等.实验结果表明:在其他条件不变的情况下,FC-72液层与蒸发台面的温差越高,其蒸发速率越快;注液量越大,蒸发速率也越大;在蒸发过程中出现了浮力对流涡胞和Marangoni对流涡胞.此外,通过地面蒸发实验可以确定空间科学实验选用的实验介质和材料,进而优化确定空间科学实验的工况、参数及流程等,部分地面实验结果也将直接成为天地对比实验的科学成果.      

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.     

Thermocapillary migration of bubbles and droplets

The migration of a droplet in a large liquid body possessing a uniform temperature gradient is analyzed in the creeping flow limit for small values of the Marangoni number (ε). The method of matched asymptotic expansions is used for the solution of the field equations. The migration velocity is calculated to 0(ε²) and the result reduces to that of Young, Goldstein and Block [10] in the limit of zero Marangoni number.     

微重力池沸腾中的气泡和传热行为数值模拟

基于标准动量传输方程、连续性方程以及能量方程,建立了单个气泡的二维数值模型,考察了微重力过冷核态池内沸腾中的气泡动力学和传热行为.在动量传输方程中耦合了表面张力和Marangoni力,连续性方程和能量方程中耦合了相变效应,考虑了加热面上过热液体层的影响,并引入相场函数捕捉气液界面的动态变化.结果表明:气泡在生长过程中,形状由最初的半球形变为椭球形,最后变为脱附时的梨形;气泡在加热面上自由迁移,呈非轴对称结构,并且气泡上部的温度场呈现为蘑菇云状;气泡的脱附直径正比于g-0.488,脱附周期正比于g-1.113,加热面上的平均热通量正比于g0.229.      

Correlation of oscillations of flow and temperature in floating zone under microgravity

The correlation between oscillations of flow and temperature of the Marangoni convection in a cylindric floating zone are studied. Photographs of the oscillating flow patterns are taken by triggering with a thermocouple signal of the temperature oscillation to reveal the frequency and phase correlations. Whilst both frequencies coincide, phase shift between temperature and flow oscillations exists.     

Direct observation of processes in a solidifying dispersion

The evolution of a dispersion under the action of temperature gradients and solidification was followed optically in a transparent molten salt (CsCl) with inclusions of Pb-droplets and gas bubbles. This system is believed to model a solidifying metallic alloy. Rejection of Pb-particles by the solidification front was observed, while large gas bubbles were incorporated. Thermocapillary convection at the gas bubbles considerably distorted the temperature field and even caused local remelting. Marangoni migration of bubbles was not observed, contrary to expectations.     

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.      

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.      

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。     

Convective stability of immiscible fluids in parallel shearing motion —role of the diffusion

Coupling between the Laplace and the Marangoni effects in a parallelly moving diffusion system is studied. Convective instability arises from either pure Marangoni effect or Laplace effect which is always influenced by the Marangoni contribution. Near to , a common stable region is found under disturbances of concentrations and micro interface deformations. Gravity plays a role through the Laplace condition.     

纵向非均匀多孔介质Marangoni对流不稳定性

理论研究了纵向非均匀多孔介质中流体表面张力驱动的对流不稳定性.充满液体的多孔介质层从下方加热,上方自由表面冷却,形成可引起多孔介质液层Marangoni-Benaxd对流流动的纵向温度梯度.采用线性化的Brinkman-Forchheimier方程作为控制方程组,对孔隙率分别为线性函数、正弦三角函数分布的非均匀多孔介质液层的Marangoni-Benaxd问题进行了线性稳定性分析.通过采用Chebyshev-Tau谱方法求解广义特征值问题,得到了系统临界Maxangoni数随无量纲波数变化的中性稳定性曲线,分析和比较了孔隙率的变化对液层对流稳定性和流场结构的影响,获得了纵向非均匀多孔介质液层不稳定性现象的新特征.