Evaporation-induced natural convection of a liquid slug of binary mixture inside a microchannel: effect of confinement

Both experimental and simulation studies have been carried out on internal convection of an evaporating liquid slug of aqueous NaCl solution inside a microcapillary. Effect of confinement due to the extended channel length beyond the interface of the liquid slug has been investigated by placing the liquid slug inside the microcapillary of different lengths. Micro-PIV technique has been used for measurement of velocity field inside the liquid slug. Simulation studies have been carried out using COMSOL Multiphysics software for reporting the evaporative flux distribution on the meniscus and the concentration field distribution inside the liquid slug. The combined experimental and simulation studies successfully explain the underlying flow physics. Evaporation from the liquid–air interface of the slug induces buoyancy-driven Rayleigh convection. Evaporative flux of the interface depends on the extended length of the microcapillary beyond the liquid slug. The presence of extended channel region beyond the meniscus suppresses the evaporation from the meniscus due to the absence of evaporation flux normal to the channel wall. Evaporation occurs primarily from only one meniscus when the slug is located at one end of a long channel. Evaporation occurs from both the menisci when both the menisci are directly exposed to the atmosphere. Evaporation from only one meniscus of a slug leads to one recirculation bubble inside the liquid slug, whereas evaporation from both the menisci leads to two recirculation bubbles inside the liquid slug. Liquid slug with asymmetric extended channel length beyond the liquid slug interface leads to asymmetric evaporative flux, concentration field distribution and recirculation bubble size. The extended channel length beyond an evaporating liquid slug can influence/control the performance of a digital microfluidic system/device.     

Experimental investigation of enhanced spreading and cooling from a microgrooved surface

A heat transfer cell is specifically designed to analyze the heat spreading capacity of a microgrooved surface. V-shaped microgrooves are etched on silicon wafers using standard lithographic process. The shapes of the liquid menisci in the microgrooves are accurately measured using image analyzing interferometry as functions of heat input and opposing body force (angle of inclination). The relevant parameters that govern the spreading and cooling process of an evaporating curved microfilm, e.g., the adsorbed film thickness, contact angle, and curvature at the thicker end of the meniscus are accurately measured. The trends in these values are found to be consistent with the physics of the process. The temperature profiles are measured for the microgrooved and non-grooved silicon substrates under identical conditions of heat input and inclination and the enhanced spreading of the film in presence of microgrooves toward the hot region is quantified. The axially average values of a dimensionless temperature, defined for this study, are used to quantify the enhanced cooling and temperature homogenization potentials of microgrooved surfaces along with the effect of opposing body forces. The study clearly indicates the beneficial effects of change-of-phase heat transfer from an evaporating microfilm on microgrooved surfaces and its potential use in a miniature passive cooling device.     

降膜蒸发过程的数值模拟和传热传质分析

利用CFD软件对逆流降膜蒸发过程进行了实验模拟研究,研究了速度边界层、热边界层和浓度边界层的变化对降膜蒸发传热传质特性的影响规律;通过建立一维逆流降膜蒸发的数学方程编程求解出了对流传热传质的Nu数和Sh数,利用Fluent软件模拟出的实验结果采用回归分析得出了气液流量比Raw、流道的长宽比αL、空气进口无量纲温度θai以及空气进口Re数与Nu数、Sh数之间的无量纲关系式,可为降膜蒸发换热器的设计提供参考。     

微型多槽道平板热管传热特性分析及最大传热量预测

对三角形槽道的微型平板热管,进行流动、传热性能的理论分析和模拟.建立了微型平板热管的一维稳态模型,以分析工质在热管内的轴向分布,液体和蒸汽压力、流速的轴向变化,并计算其最大传热量.该模型更精确地考虑了具有互相连通蒸汽空间的平板热管,汽液界面剪切摩擦力对其传热性能影响,模型预测结果与实验数据取得较好的符合,能够更准确地预测微热管的最大传热量.该模型可用来帮助微型多槽道平板热管的结构设计.     

Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions

Motivated by quantification of micro-hydrodynamics of a thin liquid film which is present in industrial processes, such as spray cooling, heating (e.g., boiling with the macrolayer and the microlayer), coating, cleaning, and lubrication, we use micro-conductive probes and confocal optical sensors to measure the thickness and dynamic characteristics of a liquid film on a silicon wafer surface with or without heating. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz which is more suitable for rapid process. The optical sensors are therefore used to measure the instantaneous film thickness in an isothermal flow over a silicon wafer, obtaining the film thickness profile and the interfacial wave. The dynamic thickness of an evaporating film on a horizontal silicon wafer surface is also recorded by the optical sensor for the first time. The results indicate that the critical thickness initiating film instability on the silicon wafer is around 84 μm at heat flux of ~56 kW/m². In general, the tests performed show that the confocal optical sensor is capable of measuring liquid film dynamics at various conditions, while the micro-conductive probe can be used to calibrate the optical sensor by simultaneous measurement of a film under quasi-steady state. The micro-experimental methods provide the solid platform for further investigation of the liquid film dynamics affected by physicochemical properties of the liquid and surfaces as well as thermal-hydraulic conditions.     

沸腾情形下开放式毛细微槽中的干涸特性实验研究

利用高速摄像仪、CCD以及宽视场体视显微技术对液体在微槽内的润湿、蒸发和沸腾现象进行了可视化观察和描述,对发生沸腾换热现象时的竖直矩形毛细微槽群热沉中的液体沿微槽槽道方向的干涸点高度(润湿高度)进行了测量,并对微槽几何尺寸、工质等因素对润湿高度的影响进行了实验研究.实验结果表明:微槽群的高强度的强化换热特性是由薄液膜蒸发和核态沸腾换热的共同作用造成的;发生沸腾换热现象时,微槽内的液体润湿高度随着输入加热功率的增加能得到有限度地升高;一定热负荷下,微槽较深、较宽以及微槽群密度较大时液体的润湿高度较高;在微槽内发生核态沸腾的情形下,甲醇的润湿高度要远大于乙醇和蒸馏水.     

Effects of a surface-tension gradient on the performance of a micro-grooved heat pipe: an analytical study

We investigate effects of surface-tension gradients on the performance of a micro-grooved heat pipe in this work. The surface-tension gradient force is accounted for in the present model, and expressions for radius of curvature, liquid pressure, liquid velocity, and maximum heat throughput are found analytically using a regular perturbation technique. With a favorable surface-tension gradient, the liquid pressure drop across the heat pipe can be decreased by ∼90%, and the maximum heat throughput can be increased by ∼20%. In contrast, using an unfavorable surface-tension gradient, the liquid pressure drop increases by ∼150%, and the maximum heat throughput decreases by ∼15%. For the same values of the favorable and unfavorable surface-tension gradients, the unfavorable effect is more pronounced than the favorable one. The effects of the surface-tension gradients are found to be increasing with the corner angle of a polygonal heat pipe. Adverse effects of the surface-tension gradient could be due to the variations in the liquid temperature and/or surfactant concentration. Nevertheless, a favorable situation where the surface-tension gradient can facilitate the liquid flow in a heat pipe can also be obtained using a suitable surfactant, surface charge, etc., and then the performance of a micro heat pipe can be improved.     

基于有限元模拟的薄膜热流计结构设计及优化

精确的热流测量对航空航天领域发动机设计及使用过程至关重要.薄膜热流计以其体积小、热容量小、干扰小、不破坏部件表面气流等显著优势,成为发动机热端部件表面热流测量的新方法.针对传统工程经验设计薄膜热流计精确度不高且迭代耗时长的缺点,基于有限元仿真模拟方法,建立了一种薄膜热流计有限元分析模型,综合分析了热流密度、热阻层厚度、热电堆厚度等因素对热流计冷热结点温度梯度的影响,提出薄膜热流计优化思路.分析结果表明,优化后的薄膜热流计具有更出色的热学性能与电学性能.     

Thin film evaporation in microchannels with interfacial slip

We investigate the role of interfacial slip on evaporation of a thin liquid film in a microfluidic channel. The effective slip mechanism is attributed to the formation of a depleted layer adhering to the substrate–fluid interface, either in a continuum or in a rarefied gas regime, as a consequence of intricate hydrophobic interactions in the narrow confinement. We appeal to the fundamental principles of conservation in relating the evaporation mechanisms with fluid flow and heat transfer over interfacial scales. We obtain semi-analytical solutions of the pertinent governing equations, with coupled heat and mass transfer boundary conditions at the liquid–vapor interface. We observe that a general consequence of interfacial slip is to elongate the liquid film, thereby leading to a film thickening effect. Thicker liquid films, in turn, result in lower heat transfer rates from the wall to liquid film, and consequently lower mass transfer rates from the liquid film to the vapor phase. Nevertheless, the total mass of evaporation (or equivalently, the net heat transfer) turns out to be higher in case of interfacial slip due to the longer film length. We also develop significant physical insights on the implications of the relative thickness of the depleted layer with reference to characteristic length scales of the microfluidic channel on the evaporation process, under combined influences of the capillary pressure, disjoining pressure, and the driving temperature differential for the interfacial transport.     

移动副速度对空气轴承影响的CFD仿真分析

研究空气轴承优化控制问题,针对高速移动副空气轴承对空气轴承气膜特性会影响定位精度特点,空气轴承采用非结构化网格实现了多节流器气膜的耦合,设计的密度可控的非均匀网格划分方案克服了最大最小尺寸比偏大给实际带来的困难。根据FLUENT用三维双精度耦合隐式标准k-ε粘性湍流两方程模型,仿真得到了不同移动副速度时,阵列多节流器耦合后的质量流量、气膜压强分布和速度分布,为辨识高速移动副时多节流器耦合的气膜数学模型提供了详实数据。当增大移动副速度时,耗气量减小但气膜承载力增加,为高速移动副空气轴承的优化设计提供了依据。     

Theoretical analysis of capillary performance of triangular microgrooves

The purpose of this study is to analyze the capillary performance of triangular microgrooves. The influences of several major of parameters on the capillary performance (wetted axial length) of triangular microgrooves are discussed theoretically. One dimensional, non-linear and contact angle possessed differential and algebraic equations are both used for the theoretical analyses of triangular microgrooves. In this study, the curvature radius, cross section area, and distribution of pressure and velocity of working fluid in microgrooves are considered. Besides, the mutual effects among inertial force, body force, capillary force, and friction force are also discussed. The significance of contact angle and hydraulic diameter to the prediction of capillary performance for microgrooves are both demonstrated by the proposed algebraic solutions.     

Thin film evaporation effect on heat transport capability in a grooved heat pipe

A theoretical model predicting the heat transfer performance occurring in a grooved heat pipe is developed. The model includes the effects of groove geometry, thin film evaporation, contact angle, and film condensation. The numerical results show that the groove geometry significantly affects the thin film evaporation and condensation. The thin film evaporation plays a key role in the total effective thermal conductivity and determines a limit for the maximum amount of heat transport through the micro regions for a given evaporator geometry. While the contact angle can influence the capillary limitation, it significantly affects the thin film evaporation and the total effective thermal conductivity of a groove heat pipe. In order to verify the theoretical analysis, an experimental investigation on a grooved heat pipe was conducted. The current investigation will result in a better understanding of thin film evaporation and its effect on the maximum heat transport in a grooved heat pipe.     

Symmetry-to-asymmetry transition of Marangoni flow at a convex volatizing meniscus

Plenty of studies have been made to have a comprehensive understanding of heat and mass transport at an evaporating meniscus. Recently in this journal Gazzola et al. (2009) reported an asymmetrical flow pattern generated at a convex meniscus while the boundary conditions were symmetrical. In their experiment a vertical micro well was filled up with liquid and the meniscus was held convex above the well outlet. The flow pattern at the meniscus was found having only one single vortex, which was distinct from those symmetrical paired vortexes in literatures (Buffone and Sefiane 2004). In the present work a simulation is conducted to clarify the mechanisms behind the abnormality. Factors including liquid evaporation, vapor transport, and Marangoni effect are considered. It is found that for a convex meniscus as that in the experiment, symmetrical flow pattern is not stable. The transition of symmetry to asymmetry occurs and this is greatly affected by random perturbations. After the asymmetrical flow pattern is established, the perturbations have relatively minor effects on the flow pattern.     

Heat and mass transfer computations for laminar flow in an axisymmetric sudden expansion

Detailed heat or mass transfer rate predictions are made using a finite difference computer model for laminar flow in an axisymmetric sudden expansion. The structure of the recirculation zone and the distribution of mass transfer rate downstream of the expansion are calculated as functions of Reynolds number, inlet velocity profile, geometry and Schmidt number. It is shown that a Couette flow analysis of the appropriate scaled equations gives the essential details of the mass transfer behaviour.     

超声速旋转火箭弹气动特性仿真和分析

采用CFD方法,基于剪切应力输运（Shear Stress Transport,SST）湍流模型,求解大长细比卷弧翼火箭弹在超声速情况下的气动力和气动热问题．对火箭弹流场进行数值计算,与实验数据进行对比．采用薄壁模型模拟结构耦合传热,计算在一定海拔和旋转情况下火箭弹的气动加热,并与不旋转的情况进行对比．计算结果表明该数值方法能较好地计算气动力因数和气动热分布．在特定的低转速和海拔情况下的火箭弹温度分布比不旋转的稍微大一点,在旋转情况下的火箭弹尾部截面压力分布不对称,尾部流线更加紊乱;弹头和尾翼前缘温度较高,应当在火箭弹设计中予以考虑．     

Chemical reaction and mixing inside a coalesced droplet after a head-on collision

We investigated the phenomena of a chemical reaction inside a coalesced droplet after a direct (head-on) collision. A droplet containing an alkaline solution collided with a droplet containing a pH indicator on a surface with a wettability gradient. We used a high-speed camera to observe the color-changing reaction inside the coalesced droplet. Compared with a traditional dye-mixing test, the chemical reaction inside the coalesced droplet facilitated the mixing of two counter-reactive fluids and was more than 100 times as efficient as for unreactive fluids mixing inside the coalesced droplet. Instead of mere mixing, a chemical reaction inside a coalesced droplet is valuable for applications in a digital microfluidic open system. In droplet coalescence, the characteristics of the fluids and the ratio of volumes of two droplets caused a varied profile of the droplet coalescence, especially the neck curvature that affects the shape of the material interface between the two droplets at an initial phase. We observed the evolution of the chemical reaction with a varying radius of neck curvature inside the coalesced droplet. For the case of a small radius of neck curvature, the small interfacial area between two reactive fluids accumulated an intense heat of reaction and induced a rapid growth of the fingers. For the case of a large radius of neck curvature, the growth of fingers was slight and the interface was uniform across the large interfacial area. Our work illustrates a correlation between the rate of chemical reaction and the profile of a coalesced droplet, which is a significant reference in droplet-based microfluidic systems for biochemical applications.     

Thin water films driven by air shear stress through roughness

A model of thin water film transport over small scale surface roughness is developed in the context of a high Reynolds number boundary layer theory. The surface water is found to be described by a lubrication equation. It is shown that small scale surface roughness can first effect the water flow at roughness heights which are much less than those of first nonlinear response in the air. A number of well known phenomenon are encountered when using this model, such as pooling of water between roughness elements and rivulet formation. A linearized subsonic heat transfer analysis is also presented, and water protuberances and roughness are found to enhance the ambient heat flux. Solitons are calculated for two-dimensional films, and a linear stability analysis shows that two-dimensional film fronts can become unstable and develop into rivulets.     

A rollable,organic electrophoretic QVGA display with field‐shielded pixel architecture

Abstract— A 100‐μm thin QVGA display was made by combining a 25‐μm thin organic transistor active‐matrix backplane with an electrophoretic display film. High contrast and low crosstalk was achieved by the addition of a field shield to the backplane. The display can be bent repeatedly to a radius of 2 mm without any performance loss. Extended mechanical tests at a radius of curvature of 7.5 mm show that the display can be rolled at least 30,000 times without noticeable degradation.     

Performance of thin film silicon MEMS on flexible plastic substrates

The fabrication and characterization of thin film silicon MEMS microbridges on flexible polyethylene terephthalate substrates are described. Surface micromachining using an aluminum sacrificial layer and a maximum processing temperature of 110 °C was used for device fabrication. These microbridges are electrostatically actuated and their deflection at resonance and at low frequencies is measured optically. Quasi-DC deflection with a quadratic dependence of the actuation voltage is observed, and resonance frequencies up to 2 MHz and quality factors of around 500 are measured in vacuum. Bending measurements are performed by subjecting these devices to tensile and compressive strain. The low frequency response (bridge deflection as a function of the applied voltage) was measured in air before bending and after every bending step. Under tensile strain, 16.6% of the devices survive the maximum bending with a radius of curvature of 1 cm, equivalent to a tensile strain 1.25%. In contrast, for compressive strain, 50% of the devices survive the bending corresponding to a radius of curvature of −0.5 cm, equivalent to a compressive strain of −2.5%. Thin film silicon microresonators on flexible plastic substrates can withstand more compressive strain than tensile.     

Film drainage mechanism between two immiscible droplets

The interaction between two deformable droplets consists of unique dynamic characteristics that are not present during the interaction of solid bodies. A thin film of surrounding fluid is entrapped between the droplets and then drains out under the influence of an external force before the droplets can adhere or coalesce. The drainage process during the coalescence of two similar droplets has received significant research interest due to the presence of dynamical interactions between the droplets. Surprisingly, the film drainage process between two partial engulfing immiscible droplets has not been studied yet. Using a numerical study, we investigate the film drainage between two partial engulfing immiscible droplets. We vary the interfacial tensions between the droplets and surrounding fluid in wide ranges to observe the film drainage time between the droplets. Based on our simulations, we identified three regimes of fast, intermediate and delayed drainage. We found that the film drainage of two immiscible droplets exhibits additional flow into or out of the film, which does not exist in the film drainage of identical droplets. This additional flow can either increase or decrease the rate of film drainage between the droplets, depending on the interfacial tension of droplets with the surrounding fluid and the interfacial tension of the two immiscible droplets.