Chemical etching of a GaSb crystal incorporated with Mn grown by the Bridgman method under microgravity conditions

A GaSb crystal incorporated with Mn has been grown by the Bridgman method on the Polizon facility onboard the FOTON-M3 spacecraft. Structural defects and growth striations have been successfully revealed by the chemical etching method. By calculating various parameters of the convection, the striation patterns can be explained, and the critical value of the Taylor number, which characterizes the convective condition of the rotating magnetic field induced azimuthal flow, was shown. The stresses generated during crystal growth can be reflected by the observations of etch pit distribution and other structural defects. Suggestions for improving the space experiment to improve the quality of the crystal are given.

Small-scale turbulent fluctuations beyond Taylor's frozen-flow hypothesis

The space-time cross-correlation function C-T(r, tau) of local temperature fluctuations in turbulent Rayleigh-Benard convection is obtained from simultaneous two-point time series measurements. The obtained C-T(r, tau) is found to have the scaling form C-T(r(E), 0) with r(E)=[(r-U tau)(2)+ V-2 tau(2)](1/2), where U and V are two characteristic velocities associated with the mean and rms velocities of the flow. The experiment verifies the theory and demonstrates its applications to a class of turbulent flows in which the requirement of Taylor's frozen flow hypothesis is not met.

Kinetic Studies of Gas Flows

Our recent studies on kinetic behaviors of gas flows are reviewed in this paper. These flows have a wide range of background, but share a common feature that the flow Knudsen number is larger than 0.01. Thus kinetic approaches such as the direct simulation Monte Carlo method are required for their description. In the past few years, we studied several micro/nano-scale flows by developing novel particle simulation approach, and investigated the flows in low-pressure chambers and at high altitude. In addition, the microscopic behaviors of a couple of classical flow problems were analyzed, which shows the potential for kinetic approaches to reveal the microscopic mechanism of gas flows.

Extinction of lean near-limit methane/air flames at elevated pressures under normal- and reduced-gravity

perimentally at evaluated pressures and under normal- and micro-gravity conditions utilizing the 3.5 s drop tower of the National Microgravity Laboratory of China. The results showed that under micro-gravity conditions the natural convection is minimized and the flames become more planar and symmetric compared to normal gravity. In both normal- and micro-gravity experiments and for a given strain rate and fuel concentration, the flame luminosity was found to enhance as the pressure increases. On the other hand, at a given pressure, the flame luminosity was determined to weaken as the strain rate decreases. At a given strain rate, the fuel concentration at extinction was found to vary non-monotonically with pressure, namely it first increases and subsequently decreases with pressure. The limit fuel concentration peaks around 3 and 4 atm under normal- and micro-gravity, respectively. The extinction limits measured at micro-gravity were in good agreement with predictions obtained through detailed numerical simulations but they are notably lower compared to the data obtained under normal gravity. The simulations confirmed the non-monotonic variation of flammability limits with pressure, in agreement with previous studies. Sensitivity analysis showed that for pressures between one and 5 atm, the near-limit flame response is dominated by the competition between the main branching, H + O2 ? OH + O, and the pressure sensitive termination, H+O2+M? HO2 + M, reaction. However, for pressures greater than 5 atm it was determined that the HO2 kinetics result in further chain branching in a way that is analogous to the third explosion limit of H2/O2 mixtures. 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Evaporation of Ethanol Drops on a Heated Substrate Under Microgravity Conditions

We present in this paper results obtained from a parabolic flight campaign regarding ethanol sessile drop evaporation under reduced gravity conditions. Drops are created using a syringe pump by means of injection through a PTFE (polytetrafluoroethylene) substrate. The drops are recorded using a video camera and an infrared camera to observe the thermal motion inside the drop and on the heating substrate. The experimental set-up presented in this paper enables the simultaneous visualization and access to the heat flux density that is transferred to the drop using a heat flux meter placed between the heating block and the PTFE substrate. We evidence original thermal spreading phenomena during the ethanol drop creation on a heated PTFE substrate. The drop exhibits specific behaviour which is discussed here. This work is performed in the frame of a French-Chinese collaboration (project IMPACHT) for future experiments in a Chinese scientific satellite.

Three-Dimensional Linear Instability Analysis of Thermocapillary Return Flow on a Porous Plane

A three-dimensional linear instability analysis of thermocapillary convection in a fluid-porous double layer system, imposed by a horizontal temperature gradient, is performed. The basic motion of fluid is the surface-tension-driven return flow, and the movement of fluid in the porous layer is governed by Darcy's law. The slippery effect of velocity at the fluid-porous interface has been taken into account, and the influence of this velocity slippage on the instability characteristic of the system is emphasized. The new behavior of the thermocapillary convection instability has been found and discussed through the figures of the spectrum.

蒸发相变与界面流动耦合机理研究

本文提出了一种新模型来研究液层在其纯蒸气中的蒸发热动力学特征,尤其是当蒸发界面张力驱动流占主导作用时(如微重力环境中)液层热毛细对流和界面蒸发始终耦合在一起. 气一液界面的传热传质规律有待深入研究. 本文数值模拟研究了蒸发相变界面热毛细对流与蒸发效应的耦合机质,得到了不同蒸发模式和不同强度热毛细对流蒸发液层的温度分布、蒸发速率以及对流流场分布的数值解. 论述了蒸发Biot数和Marangoni数对界面传热传质的影响,发现并解释了蒸发和热毛细耦合的三种模式

微重力环境下材料热物性研究的现状和发展

表面张力、润湿性、黏度和扩散等材料热物理性质是重要的物理化学参数之一,与之有关的诸多表面和界面现象一直吸引着科学研究者的浓厚兴趣,特别在微重力条件下,表面张力梯度引起的Marangoni对流现象等科学还需要人类不断的认识。本文介绍了近些年国内外在微重力环境下材料热物性研究的一些进展。

界面张力引起的流动及界面特征研究

通过对单层流体浮力－热毛细对流和两层流体 B$\acute{\rm e}$nard-Marangoni对流的实验研究，探讨界面张力梯度引起的自然对流的特征及机理问题。考虑到表面张力是温度的函数，对上面是空气(或其蒸汽)的薄液层，加载水平温度梯度将使得气液表面上表面张力分布不均匀，耦合于地面的重力作用，将会驱动薄层流体形成浮力－热毛细对流运动。液层厚度和温度梯度的改变(引起系统长高比、Bond数、Rayleigh数以及Marangoni数的变化)直接影响到薄层流体的对流模式的变化，还可能使得浮力－热毛细对流从稳定发展到不稳定。本研究中以硅油为实验介质，应用高分辨率PIV技术对薄层流体的对流速度场进行了测量，观察到了对流由单胞结构向多胞结构以及由稳定对流向振荡对流的转捩过程，分析给出了对流模式结构变化的规律以及状态转变的临界参数。在浮力－热毛细对流发展过程中，流体表面的变形(形貌)和表面振荡直接反映了热毛细作用与浮力作用的耦合规律以及热毛细对流表面波的基本特征。实验中应用激光干涉技术以及高精度位移传感器对薄液层体系(液层厚度1mm$\sim$5mm)作了系统的研究，获得了微米量级面形形貌变化规律及其亚微米尺度的表面振荡特性。用FFT以及小波分析方法研究了流体自由面振荡的分岔转捩过程及通往混沌的转捩途径。该研究对理解流体热毛细对流的机理具有重要的意义。在自然界里和工程技术中，多层流体体系对流现象更为普遍。近20年来，互不混溶的两层液体体系成为了很多理论和实验研究的重要对象，其主要原因有：(1)在两层流体体系中，由于上下层对流的耦合作用，在临界点上存在HOPF分叉，使得两层模型成为非线性理论研究的理想模型；(2)两层流体模型被应用于地壳运动的研究和空间晶体生长等领域。近年，很多学者通过理论分析和数值模拟对加载垂直温度梯度的上下两层流体B$\acute{\rm e}$nard-Marangoni对流问题进行了研究。上下液层对流的耦合与竞争可以导致上下液层出现多种对流耦合模式和振荡规律，外加温差、液层厚度以及液层厚度比的变化是形成不同对流模式的重要因素。本研究以FC70和KF90-10为实验介质，应用高分辨率PIV技术对两薄层流体B$\acute{\rm e}$nard-Marangoni对流进行了测量，从实验中清晰地观测到了3种临界对流模式：机械耦合、热耦合、临界振荡，分析给出了3种对流转换的临界参数，发现临界振荡可以在峰值液层厚度比附近一个较大的区域范围出现，并且峰值厚度比远离平衡厚度比，这些结果与目前的理论研究有明显的的差异。总之，两种不同外加温度梯度方式，会导致两种不同机制的对流--热毛细对流和Marangoni对流，他们是微重力流体物理研究的重要内容。

气泡/液滴运动的Level Set方法模拟研究

利用Level Set方法，结合投影法求解了描述气泡/液滴运动的Navier-Stokes方程。对地面常重力场中不同大小的空气泡在高黏度糖浆溶液中的自由上升运动现象，数值模拟结果与实验观测结果符合甚好，表明该方法能够计算大密度比和黏度比$(>1000:1)$情况下的气液两相流动。而对等密度液滴的热毛细迁移现象的数值模拟结果同样能够与实验结果相一致，表明该方法同样适于研究具有Marangoni效应的两相流动现象，特别是在空间微重力环境中的气液两相传热现象中的局部流动与传热问题。

微重力气液两相流动与传热

微重力条件下的气液两相流动与传热现象不仅在航天科技领域有重要的应用前景，而且由于抑制了重力和两相密度差所引起的浮力分层与相间滑移等因素的干扰，能够简化流动复杂性，突出流动中经由液气界面产生的相互作用，对揭示气液两相流动与传热现象内在控制机理极为有利，因此得到国际航天工程界和微重力流体力学界的高度重视，是目前相当活跃的研究前沿领域之一。中国科学院国家微重力实验室自20世纪90年代中期创建伊始，即将微重力气液两相流动与传热作为主要研究方向之一，先后完成了"和平号"空间站气液两相流实验、IL-76失重飞机气液两相流实验、第22颗返回式卫星和实践8号育种卫星搭载池内沸腾实验、NML落塔池内沸腾实验、NML落塔燃料电池内部气液两相流动及其电性能实验(与北京工业大学合作)等微重力实验研究项目，并通过地面对比实验及深入的数据分析和理论探索，得到如下结果：\newline (1)管内绝热气液两相流：首个长期、稳定微重力条件下圆管气液两相流型图和低重力条件下方管气液两相流型图，预测微重力气液两相弹状流－环状流转换的半理论Weber数模型，低重力条件下方管气液两相流摩擦压降数据及一个新的预测微重力气液两相泡状流压降的均相流模型等。\newline (2)池内沸腾：不同压力和过冷度条件下丝状表面和平板表面上的微重力池内沸腾传热曲线，临界热流(CHF)数据及其与重力相关的尺度关系，微重力池内沸腾现象中的气泡动力学行为及一个计入Marangoni效应的气泡脱落模型等。\newline (3)燃料电池微细通道气液两相流动：直接甲醇燃料电池(DMFC)内CO$_2$气泡生成与运动规律及其对燃料电池电性能的影响，H$_2$质子交换膜燃料电池(PEMFC)内水滴的生成与两相流动的发展及其对燃料电池电性能的影响等。\newline 本文首先对上述成果予以详细评述，然后结合该领域国际发展现状与我国航天(尤其是载人航天)事业的发展需求，对我国微重力气液两相流动与传热研究近期的发展趋势予以探讨。

液滴热毛细迁移流场和温度场拓扑结构的演变

热毛细迁移现象是流体颗粒(液滴/气泡)在非均匀温度场中由于界面温度梯度引起的非均匀界面张力驱动的运动。它不仅是流体力学中的经典问题之一,而且在诸如空间材料制备、空间流体和热管理系统等应用中也有着重要的应用。本文利用投影法求解了微重力条件下可变形液滴的轴对称热毛细迁移问题,控制方程组基于Level-set方法和连续界面张力模型。数值计算的终端迁移速度和空间实验结果相一致。计算结果表明,不同Marangoni数(Ma)情形具有相同的流场拓扑结构;但随着Ma数的增加,温度场的拓扑结构变化极大——在较小Ma数情形中,液滴内的最小温度发生在液滴尾部滞止点处。当Ma数超过某个临界值(10～20之间)后,最小温度点跳进液滴内部,并随着Ma数的增加而不断上移;液滴内部冷区最初呈球帽状,但其中部厚度随Ma数增加不断减小,同时向外扩展,形成外缘不断增厚的球壳状冷区;当Ma数超过另一临界值(约100)后,球壳状冷区在液滴轴线处破裂,冷区的拓扑结构转变为环状,最小温度点也随之远离轴线,不断向外、向下移动。温度场拓扑结构的变化反映了对流效应对液滴内部热量传输的影响不断增强,也强烈影响着液滴热毛细迁移终端速度随Ma数的变化特征。

An Electro-osmotic Fuel Pump for Dircet Methanol Fuel Cells

This work reports on the design and performance evaluation of a miniature direct methanol fuel cell(DMFC)integrated with an electro_osmotic(EO)pump for methanol delivery.Electro-osmotic pumps require minimal parasitic power while boasting no moving parts and simple fuel cell integration.Here ,aneletro-osmotic pump is realized from a commercially available porous glass frit.We characterize a custom-fabricated DMFC with a free convection cathode and coupled to an extennal electro-osmotic pump operated at applied potentials of 4.0,7.0,and 10V.Maximum gross power density of our free convection DMFC(operated at 50°)is 55 mW/cm2 using 4.0 mol/L concentration methanol solution supplied by the EO pump.Experimental results show that electro-osmotic pumps can deliver 2.0,4.0 and 8.0mol/L methanol/water mixtures to DMFCs while utilizing ~5.0% of the fuel cell power.Furthermore ,we discuss pertinent design considerations when using electro-osmotic pumps with DMFCs and areas of further study.

The electrochemical behavior of a system with a limited number of molecules

In this paper, based on Einstein relationship between diffusion and random walk, the electrochemical behavior of a system with a limited number of molecules was simulated and explored theoretically. The transition of the current vs time responses from discrete to continuous was clearly obtained as the number of redox molecules increased from 10 to 10(6).

Pattern formation in polymer films under the mask

This paper presents a straightforward method for patterning thin films of polymers, i.e. a prepatterned mask is used to induce self-assembly of polymers and the resulting pattern is the same as the lateral structures in the mask on a submicrometre length scale, The patterns can be formed at above T-g + 30 degreesC in a short time and the external electric field is not crucial. Electrostatic force is assumed to be the driving force for the pattern transfer. Viscous fingering and novel stress-relief lateral morphology induced under the featureless mask are also observed and the formation mechanisms are discussed.