微重力条件下悬浮区中的Marangoni对流

本文研究微重力条件下悬浮区中的表面张力梯度驱动对流。采用有限元方法数值研究二维、轴对称悬浮区中的定常流动和温度分布,讨论P_r数及自由面上散热条件的影响。计算表明,P_r数(M_α数)增大时,流区大部分自由表面上温度梯度减小,流区流速较小,对流热输运较扩散效应增强。只要能维持自由面上一定的温度梯度,在较大的P_r数(M_α数)时,仍能得到收敛的定常解。

How to determine critical Marangoni number in half floating zone convection

In the present paper, the coordinated measurements of the temperature profile inside the liquid bridge and the boundary variation of Free surface, in addition to other quantities, were obtained in the same time for the half floating zone convection. The results show that the onset of free surface oscillation is earlier than the one of temperature oscillation during the increasing of applied temperature difference, and the critical Marangoni numbers, defined usually by temperature measurement, are larger than the one defined by free surface measurement, and the difference depends on the volume of liquid bridge. These results induce the question, ''How to determine experimentally the critical Marangoni number?'' Copyright (C) 1996 Elsevier Science Ltd.

Effects of high-frequency vibration on critical marangoni number

The influence of vibration on thermocapillary convection and critical Marangoni number in liquid bridge of half floating zone was discussed for the low frequency range 0.4-1.5 Hz and the intermediate frequency range 2.5-15 Hz in our previous papers. This paper extends the study to high frequency range 15-100Hz. This ground based experiment was completed on the deck of an electromagnetic vibration machine. The results of our experiment shows when the frequency of the applied acceleration is high enough, the amplitude of the time varying part of the temperature response is disappear and the shape of the free surface of the liquid bridge exhibits no fluctuations due to inertia. The critical Marangoni number which is defined to describe the transitions from a peroidical convection in response to vibration to an oscillatory convection due to internal instability is nearly the same as the critical Marangoni number for oscillatory flow in the absence of vibration.

Effects of g-jitter on the critical marangoni number

A half floating zone is fixed on a vibrational deck, which supports a periodical applied acceleration to simulate the effect of g-jitter. This paper deals with the effects of g-jitter on the fluid fields and the critical Marangoni number, which describes the transition from a forced oscillation of thermocapillary convection into an instability oscillatory convection in a liquid bridge of half floating zone with top rod heated. The responses of g-jitter field on the temperature profiles and flow pattern in the liquid bridge were obtained experimentally. The results indicated that the critical Marangoni number decreases with the increasing of g-jitter effect and is slightly smaller for higher frequency of g-jitter with fixed strength of applied gravity.

Rayleigh-Bénard热对流的动理论分析

利用基于分子动理论的直接模拟Monte Carlo(DSMC)方法,研究了 Rayleigh-Bènard问题.计算中,上下平板表面温度之比固定为0.1.Kn=0.01时,随着Ra数的增大,大约在1700附近,流动从热传导状态转变为热对流状态,DSMC计算得到的下平板热流与Ra数的关系与经典实验和理论结果相符.Kn=0.05时,流动保持稳定的热传导状态,Ra数的增大并不能引发热对流现象.

大Marangoni数下液滴热毛细迁移的数值研究

采用轴对称的非定常模型对大Marangoni数下的液滴热毛细迁移问题进行数值模拟研究，采用界面追踪方法，发现无量纲的液滴迁移速度会随着Marangoni数的增大而减小．数值模拟结果与神舟4号飞船实验中的结果进行了比较，得到一致的变化趋势．同时，这也是第一次在大Marangoni数范围下利用数值模拟的方法证实该实验现象．

大Reynolds数液滴Marangoni迁移的空间实验装置

讨论了液滴热毛细迁移的空间实验装置，该实验是中国神舟号飞船中的一个项目。与国际上同类装置比较，它具有不需要宇航员直接操作，领先程序自动控制及遥科学手段完成全部科学实验的能力。它不仅能完成大Reynolds数的液滴迁移实验，而且在这类空间装置上使用了等厚型光学干涉系统。

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.

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—Benard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—Benard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Marangoni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．

Oscillatory Instabilities of Two-Layer Rayleigh-Marangoni-Benard Convection

The Rayleigh-Marangoni-Benard convective instability (R-M-B instability) in the two-layer systems such as Silicone oil (10cSt)/Fluorinert (FC70) and Silicone oil (2cSt)/water liquids are studied. Both linear instability analysis and nonlinear instability analysis (2D numerical simulation) were performed to study the influence of thermocapillary force on the convective instability of the two-layer system. The results show the strong effects of thermocapillary force at the interface on the time-dependent oscillations at the onset of instability convection. The secondary instability phenomenon found in the real two-layer system of Silicone oil over water could explain the difference in the comparison of the Degen's experimental observation with the previous linear stability analysis results of Renardy et al.

Dynamic states in rotating Rayleigh-Bénard convection systems

A new geometry-independent state - a traveling-wave wall state - is proposed as the mechanism whereby which the experimentally observed wall-localized states in rotating Rayleigh-Bénard convection systems preempt the bulk state at large rotation rates. Its properties are calculated for the illustrative case of free-slip top and bottom boundary conditions. At small rotation rates, this new wall state is found to disappear. A detailed study of the dynamics of the wall state and the bulk state in the transition region where this disappearance occurs is conducted using a Swift-Hohenberg model system. The Swift-Hohenberg model, with appropriate reflection-symmetry- breaking boundary conditions, is also shown to exhibit traveling-wave wall states, further demonstrating that traveling-wave wall states are a generic feature of nonequilibrium pattern-forming systems. A numerical code for the Swift-Hohenberg model in an annular geometry was written and used to investigate the dynamics of rotating Rayleigh-Bénard convection systems.