热学微流控系统中瞬态相变传热研究

微尺度相变传热广泛存在于微反应器、微型燃料电池、微蒸发器、微冷凝器、微热管、微汽泡执行器等微流控器件中，研究微流控系统中的相变问题对于微流控器件的设计和运行具有重要的科学意义。本文针对三类典型的微尺度相变问题，即微尺度流动沸腾、微尺度流动凝结以及微加热器上的汽泡动力学进行了深入细致的研究，实验研究中所采用的实验件均为标准MEMS微加工工艺制作，克服了常规机械加工所造成的表面粗糙度的影响。 考虑到微流控系统中大量应用交叉型、弯曲型等复杂结构的微通道，在微尺度流动沸腾研究中，设计了一种具有交错微通道结构的微流控芯片，并以丙酮为工质，对该芯片内的流动沸腾进行了研究。发现了周期为毫秒量级微时间尺度的流型结构，整个周期包括单相液体充液、两相分层流以及部分蒸干的液膜流三个阶段；在单个微通道区域，由于蒸发动量力的作用，液膜沿流动方向呈非均匀分布，蒸干首先发生在上游；由于液相弗劳德数较小，导致微通道中依然存在分层流流型。由于毕渥数较小，芯片背面温度几乎与芯片内壁面温度保持同步变化。虽然红外热像仪的响应频率较低，但仍然可以鉴别出由于流型周期性转换导致的壁面温度脉动。 在微尺度流动凝结换热研究中，为便于获取凝结过程的动态流动特性，设计了一种低高宽比的单微通道，并以水为工质，对该微通道中的流动凝结换热进行了研究。实验中采取了空气自然对流冷却和 水强制对流冷却两种冷却强度。研究发现，该微通道中的凝结换热呈周期性，其周期在毫秒量级。在通道上游入口处，存在一个呈准静止状态的长汽弹，汽弹前端周期性脱离汽泡。增加冷却强度会使汽泡的脱离频率增大，脱离直径减小；长汽弹前端周期性脱离汽泡是由于汽液界面具有较大的韦伯数。汽泡在该微通道内的运动过程中直径基本不变是由于汽泡在通道内的滞留时间远小于汽泡完全冷凝所需的总时间。 为澄清并联通道的多通道效应对微尺度凝结换热的影响，作者设计了由三个矩形通道组成的并联微通道冷凝器。研究发现，通道中的流型结构与单通道凝结过程类似，均为上游呈准静止状态的长汽弹和下游周期性的汽泡脱离。在中间通道和侧通道中，总共发现了三种不同的汽泡脱离模式，即单汽丝断裂模式、双汽丝同步断裂模式以及双汽丝非同步断裂模式。多通道效应主要表现在由于硅基固体导热的影响，三个通道中具有不同的温度分布，中间通道的温度关于其中心线成对称分布，而两侧通道中的高温区域均靠向中间通道。虽然硅具有良好的导热性，整个硅基上的温差很小，但在微尺度下，小温差依然可以导致较大的温度梯度，造成中间通道的双汽丝关于其中心线成对称分布，并且总是发生同步断裂；侧通道中的双汽丝偏向中间通道，并且在靠近中间通道的一侧汽丝总是首先发生断裂。由于温度梯度引起的Maragnoni对流效应，侧通道中的汽泡脱离后便靠向高温侧。 在微汽泡动力学研究中，设计了一种尺寸为 的Pt薄膜微加热器，研究了脉冲控制参数对微加热器上汽泡动力学特性的影响。研究发现在该微加热器上发生汽泡核化时，核化温度均达到液体的过热极限，因此为均质核化过程。在不同的脉冲控制参数下，存在三类不同的汽泡动力学特性，即（1）汽泡爆炸性生长和冷凝以及汽泡二次生长；（2）汽泡爆炸性生长继而分裂、吸引并聚合；（3）汽泡振荡生长而后持续生长并最终达到稳定状态。在第（1）类中，汽泡二次生长是由于脉冲加热过程中在玻璃基片上储存了热量；在第（2）类中，汽泡冷凝过程中的Marangoni效应导致分裂后的汽泡互相吸引并最终聚合。在第（3）类中，汽泡尺寸最终达到稳定是由于汽泡内蒸汽的发生量与汽液界面上蒸汽的凝结量相等。 本文的研究将为微反应器、微型燃料电池、微换热器、微汽泡执行器等相变微流控系统的设计和运行提供科学指导。

Effect of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel

The objective of this paper is to investigate the effects of channel surface wettability and temperature gradients on the boiling flow pattern in a single microchannel. The test section consists of a bottom silicon substrate bonded with a top glass cover. Three consecutive parts of an inlet fluid plenum, a central microchannel and an outlet fluid plenum were etched in the silicon substrate. The central microchannel had a width of 800 mu m and a depth of 30 mu m. Acetone liquid was used as the working fluid. High outlet vapor qualities were dealt with here. The flow pattern consists of a fluid triangle (shrinkage of the liquid films) and a connected long liquid rivulet, which is generated in the central microchannel in the timescale of milliseconds. The peculiar flow pattern is formed due to the following reasons: (1) the liquid rivulet tends to have a large contact area with the top hydrophilic channel surface of the glass cover, but a smaller contact area with the bottom silicon hydrophobic surface. (2) The temperature gradient in the chip width direction at the top channel surface of the glass cover not only causes the shrinkage of the liquid films in the central microchannel upstream, but also attracts the liquid rivulet populated near the microchannel centerline. (3) The zigzag pattern is formed due to the competition between the evaporation momentum forces at the vapor-liquid interfaces and the force due to the Marangoni effect. The former causes the rivulet to deviate from the channel centerline and the latter draws the rivulet toward the channel centerline. (4) The temperature gradient along the flow direction in the central microchannel downstream causes the breakup of the rivulet to form isolated droplets there. (5) Liquid stripes inside the upstream fluid triangle were caused by the small capillary number of the liquid film, at which the large surface tension force relative to the viscous force tends to populate the liquid film locally on the top glass cover surface.

Multi-channel effect of condensation flow in a micro triple-channel condenser

Multi-channel effect is important to understand transport phenomenon in phase change systems with parallel channels. In this paper, visualization studies were performed to study the multi-channel effect in a silicon triple-channel condenser with an aspect ratio of 0.04. Saturated water vapor was pumped into the microcondenser, which was horizontally positioned. The condenser was cooled by the air natural convention heat transfer in the air environment. Flow patterns are either the annular flow at high inlet vapor pressures, or a quasi-stable elongated bubble at the microchannel upstream followed by a detaching or detached miniature bubble at smaller inlet vapor pressures. The downstream miniature bubble was detached from the elongated bubble tip induced by the maximum Weber number there. It is observed that either a single vapor thread or dual vapor threads are at the front of the elongated bubble. A miniature bubble is fully formed by breaking up the vapor thread or threads. The transient vapor thread formation and breakup process is exactly symmetry against the centerline of the center channel. In side channels, the Marangoni effect induced by the small temperature variation over the channel width direction causes the vapor thread formation and breakup process deviating from the side channel centerline and approaching the center channel. The Marangoni effect further forces the detached bubble to rotate and approach the center channel, because the center channel always has higher temperatures, indicating the multi-channel effect. 

Determination of the interdiffusion coefficients of liquid Zn and Sn using Ta/Zn-Sn/Si trilayers

In this paper we present a new method for measuring diffusion coefficients in liquid metals under convection-less conditions with solid/liquid-liquid/solid trilayer. The advantage of this kind of trilayer is that effects from gravity-induced convection and Marangoni-convection can be omitted, so that the diffusion coefficient is determined more accurately. The Ta/Zn-Sn/Si trilayer were prepared with a multi-target ion-beam sputtering deposition technique and annealed in an electric furnace under an argon atmosphere. The interdiffusion of liquid zinc and tin at 500 degrees degree C was investigated. The diffusion concentration profiles were determined by energy dispersive spectroscopy. The interdiffusion coefficients range from 1.0x10(-6)cm(2)/s to 2.8x10(-6)cm(2)/s, which is less than previous values measured by capillary reservoir technique under 1g-environment where various convection exist. The precise interdiffusion coefficients of liquid zinc and tin result from the removing of disturbances of various kinds of convection.

Thermocapillary migrations of drops and bubbles

The transport phenomenon of drops or bubbles is a very important topic in fundamental hydrodynamics research and practical applications such as material processing and the chemical engineering. In microgravity environment, if drops or bubbles stay in a continuous phase with non-uniform temperature ¯eld, they will start to move as a result of the variance of the interface tension. This kind of movement is called the Marangoni migration. This review tries to sum up the main results in this ¯eld on theoretical analysis, numerical simulations and experiments. So far the theoretical analysis is still limited to the linear or weak nonlinear steady questions, while the current numerical simulations can already obtain the time- dependent process of the bubble/drop migration when the e®ect of heat convection is small. For strong heat convection problem, or when the Marangoni number is bigger than 100, no numerical result is in consistence with those of experiments so far. Some of the lastest numerical results are shown when heat convection is strong, and the main di®erence between strong and weak heat convection is analyzed. Finally, we also discuss the main unresolved problems in this ¯eld and some possible directions in the future.

Effect of volume ratio on thermocapillary flow in liquid bridges of high-Prandtl-number fluids

In present study, the transition of thermocapillary convection from the axisymmetric stationary flow to oscillatory flow in liquid bridges of 5cst silicon oil (aspect ratio 1.0 and 1.6) is investigated in microgravity conditions by the linear instability analysis. The corresponding marginal instability boundary is closely related to the gas/liquid configuration of the liquid bridge noted as volume ratio. With the increasing volume ratio, the marginal instability boundary consists of the increasing branch and the decreasing branch. A gap region exists between the branches where the critical Marangoni number of the corresponding axisymmetric stationary flow increases drastically. Particularly, a unique axisymmetric oscillatory flow (the critical azimuthal wave number is m=0) in the gap region is reported for the liquid bridge of aspect ratio 1.6. Moreover, the energy transfer between the basic state and the disturbance fields of the thermocapillary convection is analyzed at the corresponding critical Marangoni number, which reveals different major sources of the energy transfer for the development of the disturbances in regimes of the increasing branch, the gap region and the decreasing branch, respectively.

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。

Pool Boiling on Wire in Microgravity - What Have We Learned

In the past years, steady pool boiling of degassed R113 on thin platinum wires has been studied systematically in our lab, including experiments in long-term microgravity aboard RS-22, in short-term microgravity in the Drop Tower Beijing / NMLC, and in normal gravity on the ground. Slight enhancement of nucleate boiling heat transfer is observed in microgravity, while dramatic changes of bubble behaviors are much evident. The value of CHF in microgravity is lower than that in normal gravity, but it can be predicted well by the Lienhard-Dhir correlation, although the dimensionless radius in the present case is far beyond its initial application range. The scaling of CHF with gravity is thus much different from the traditional viewpoint. Considering the influence of the Marangoni effects, the different characteristics of bubble behaviors in microgravity have been explained. A new bubble departure model has also been proposed, which can predict the whole observation both in microgravity and in normal gravity.

Transformation from ordered islands to holes in phase-separating P2VP/PS blend films by adding triton X-100

Our previous investigation showed that the ordered hexagonal island pattern in the phase-separating polymeric blend films of polystyrene and poly(2-vinylpyridine) (PS/P2VP) formed due to the convection effect by proper control of PS molecular weight, solvent evaporation rate, and the weight ratio of PS to P2VP. In this paper, we further illustrate that, by adding a proper amount of the surfactant Triton X-100 to the PS/P2VP toluene solution, the ordered hexagonal island pattern can be transformed to the ordered honeycomb pattern. The effects of the amount of Triton X-100 on the surface morphology evolution and the pattern transformation are discussed in terms of the collapse of Triton X-100, phase separation between Triton X-100/P2VP and PS, the interfacial interaction between Triton X-100/P2VP and the mica substrate, and the Benard-Marangoni convection.