新型分离结晶法的三维数值模拟研究

本文借助三维数值模拟的方法,对微重力条件下新型分离结晶Bridgman生长过程中熔体内部的热毛细对流进行模拟计算。结果表明:(1)Ma数较小时,流动为稳态流动;当Ma数超过一定数值后,流动发展为非稳态热毛细对流。(2)狭缝宽度越大,流动越容易失稳。(3)随着高径比的增大,狭缝处流胞的流动范围增大。     

轴向磁场对分离结晶熔体中热毛细对流的影响

为了了解磁场对分离结晶过程中熔体内热毛细对流的影响,利用有限差分法进行了数值模拟.假定熔体为不可压缩流体,熔体深径比为1,自由表面无因次宽度为0.1,研究了哈特曼数分别为0、25,50和75时的碲锌镉晶体生长过程.结果表明:轴向磁场能够抑制熔体内部的流动,并随磁场强度的增加,抑制作用增强.     

无因次参数对新型分离结晶法制备CdZnTe晶体的影响

本文借助有限差分法对新型分离结晶法生长CdZnTe晶体过程中熔体内部的热毛细对流进行了三维数值模拟，获得了Marangoni数、高径比及狭缝宽度等无因次参数对流动的影响。结果表明：当Marangoni数较小时，流动为稳态流动，传热以导热为主，当Marangoni数超过一定数值后，流动发展为非稳态热毛细对流，对流的影响加...     

旋转环形液池内双组分溶液热毛细对流研究

通过毛细对流可视化实验系统和三维数值模拟研究了旋转环形液池内受Soret效应影响的双组分溶液热毛细对流,探究了泰勒数和液池深宽比对双组分溶液热毛细对流的稳定性及失稳后产生的振荡流型的影响。结果表明,旋转液池内受Soret效应影响的热毛细对流的基础流为逆时针流胞形成的轴对称稳态流动,液池旋转对基础流径向流动的影响较小,但会明显增大流体的相对周向流动。临界热毛细雷诺数随泰勒数的增大而增大,随深宽比的增大而减小。泰勒数和深宽比的变化会明显改变双组分溶液热毛细对流失稳后产生的振荡流型。随着泰勒数的增大,液池中会出现HTWs和内部波动共存的振荡流型,内部振荡由包含在基态流动中的旋转流胞产生。     

表面散热对双组分溶液热-溶质毛细对流的影响

为了了解表面散热对耦合热-溶质毛细对流的影响,对环形浅液池内考虑Soret效应的双组分溶液耦合热-溶质毛细对流进行了三维数值模拟。环形液池深宽比和半径比分别为0.1和0.5,工质为质量分数等于26.27%的甲苯/正己烷混合溶液。结果表明,当流动为二维稳态时,在Soret效应作用下,溶质会向内侧壁转移,浓度梯度和温度梯度方向相反。随着Biot数增加,流动失稳的临界热毛细雷诺数和临界振荡频率均减小,但波数增加。流动失稳后,在自由表面会出现与温度波动类似的浓度波动;随着热毛细雷诺数增加,不论自由表面是否绝热,流动均会过渡到混沌状态。     

Soret效应对具有自由表面的圆柱形浅池内双组分溶液热对流影响的实验研究

为了探究Soret效应对具有自由表面的圆柱形浅液池内双组分溶液热对流过程的影响, 通过实验观察了质量分数为50%的正癸烷/正己烷混合溶液在不同深宽比的液池内流动失稳后的自由表面耗散结构及液池内的温度波动. 结果表明, 双组分溶液流动失稳的临界热毛细Reynolds数小于纯工质的值, 且其随液层深宽比的变化规律与纯工质相同. 当深宽比小于0.0848时, 流动失稳后在自由表面观察到热流体波, 监测点处温度波动主频随热毛细Reynolds数增大而增加; 当深宽比大于0.0848时, 随热毛细Reynolds数的增大, 流动失稳后自由表面依次呈现轮辐状、花苞状、分离-合并-分离交替变化的条纹状结构.     

环形池内反常热毛细对流及稳定性线性稳定性分析

线性稳定性分析研究了环形池内反常热毛细对流及其稳定性,分析了液层深度对它们的影响。结果表明;当Ma较小时,反常热毛细对流为稳定的轴对称流动;当Ma超过临界值后,反常热毛细对流失稳,形成三维稳态波纹,波纹的形态与液层厚度有关。当液层深度H(H=d/ΔR,d为环形池深度,△R为内外径宽)小于0.0833时,失稳波纹为波数较多、位于环形池外环边沿、短小细密的"边沿波";当H大于0.0833时,为波数较少、波纹较宽、几乎占据整个液层、呈轮辐状的"轮形波";H等于0.0833时,同时存在这两种波纹的可能性。随着液层深度的增加,对流失稳的临界Ma数逐渐减小。能量收支分析讨论了它们的失稳机理。     

硅熔体CZ结构浅池内热毛细对流转变滞后特性

为了了解水平温度梯度作用时Czochralski(CZ)结构浅池内硅熔体热毛细对流的转变滞后特性,利用有限差分法进行了非稳态三维数值模拟,坩埚外壁被加热,半径为50mm,晶体半径为15mm,液池深度为3mm,坩埚外壁与晶体生长界面温差变化范围为6-27K。模拟结果表明,当逐渐增加温差时,在△T=9K处,二维轴对称流动转变为三维稳态流动,在△T=20.6K处,三维稳态流动转变为三维振荡流动;当逐渐减小温差时,在△T=19.5K处,三维振荡流动才转变为三维稳态流动,因此,二次流动转变存在滞后,滞后温差约为1.1K。     

毛细力比对环形液池内耦合热-溶质对流的影响

为了了解毛细力比R_σ对耦合热溶质毛细对流的影响,对深宽比为0.15的环形浅液池内双组分溶液耦合热-溶质毛细对流进行了三维数值模拟。液池内、外壁分别维持恒定温度和浓度,工质为甲苯/正己烷混合溶液,Prandtl数为5.54,Lewis数为25.8。结果表明,当热毛细Reynolds(ReT)数较小时,耦合热-溶质毛细对流为三维稳态流动,当ReT数超过临界值后,流动将转变为三维周期性振荡流动;在计算范围内(-1≤R_σ≤0),随着毛细力比的增加,流动失稳的临界Re_T数会逐渐减小。     

液桥耦合热-溶质毛细对流自由表面变形的数值研究

采用二维轴对称模型数值研究了液桥耦合热-溶质毛细对流流动特性,自由表面变形使用Level set方法进行捕获,分析了热Marangoni数与溶质Marangoni数比值(R_σ)对流场及自由表面变形的影响。结果显示,当R_σ-1,流场由一个顺时针对流组成,自由表面在底部凸起,而在上部凹陷,且随着R_σ减小自由表面变形幅度增大。当R_σ=-1时,流场由两个反向旋转的对流涡组成,自由表面的变形幅度较小。当R_σ-1时,流场由一个逆时针旋转的对流涡组成,自由表面在上部凸起而在下部凹陷,随着R_σ增大自由表面变形幅度增大。     

Coupling of Evaporation and Thermocapillary Convection in a Liquid Layer with Mass and Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interracial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.     

Experimental Investigation of Thermocapillary Convection in a Liquid Layer with Evaporating Interface

Thermocapillary convection coupling with the evaporation effect of evaporating liquids is studied experimentally. This study focused on an evaporation liquid layer in a rectangular cavity subjected to a horizontal temperature gradient when the top evaporating surface is open to air, while most previous works only studied pure thermocapillary convection without evaporation. Two liquids with different evaporating rates are used to study the coupling of evaporation and thermocapillary convection, and the interfacial temperature profiles for different temperature gradients are measured. The experimental results indicate evidently the influence of evaporation effect on the thermocapillary convection and interfacial temperature profiles. The steady multicellular flow and the oscillatory multicellular flow in the evaporation liquid layer are observed by using the particle-image-velocimetry method.     

Experimental Study on Liquid Free Surface in Buoyant-Thermocapillary Convection

We investigate the surface deformations of buoyant-thermocapillary convection in a rectangular cavity due to gravity and temperature gradient between the two sidewalls. The cavity is 52mm×42 mm in horizontal cross section, the thickness of liquid layer h is changed from 2.5 mm to 6.5 mm. Surface deformations of h = 3.5 mm and 6.0mm are discussed and compared. Temperature difference is increased gradually, and the flow in the liquid layer will change from stable convection to unstable convection. Two kinds of optical diagnostic system with image processor are developed for study of the kinetics of buoyant-thermocapillary convection, they give out the information of liquid free surface. The quantitative results are calculated by Fourier transform and correlation analysis, respectively. With the increasing temperature gradient, surface deformations calculated are more declining. It is interesting phenomenon that the inclining directions of the convections in thin and thick liquid layers are different. For a thin layer, the convection is mainly controlled by thermocapillary effect. However, for a thick layer, the convection is mainly controlled by buoyancy effect. The surface deformation theoretically analysed is consistent with our experimental results. The present experiment proves that surface deformation is related to temperature gradient and thickness of the liquid layer. In other words, surface deformation lies on capillary convection and buoyancy convection.     

Modeling and simulation of thermocapillary flows using lattice Boltzmann method

To understand how thermocapillary forces manipulate droplet motion in microfluidic channels, we develop a lattice Boltzmann (LB) multiphase model to simulate thermocapillary flows. The complex hydrodynamic interactions are described by an improved color-fluid LB model, in which the interfacial tension forces and the Marangoni stresses are modeled in a consistent manner using the concept of a continuum surface force. An additional convection–diffusion equation is solved in the LB framework to obtain the temperature field, which is coupled to the interfacial tension through an equation of state. A stress-free boundary condition is also introduced to treat outflow boundary, which can conserve the total mass of an incompressible system, thus improving the numerical stability for creeping flows.The model is firstly validated against the analytical solutions for the thermocapillary driven convection in two superimposed fluids at negligibly small Reynolds and Marangoni numbers. It is then used to simulate thermocapillary migration of three-dimensional deformable droplet at various Marangoni numbers, and its accuracy is once again verified against the theoretical prediction in the limit of zero Marangoni number. Finally, we numerically investigate how the localized heating from a laser can block the microfluidic droplet motion through the induced thermocapillary forces. The droplet motion can be completely blocked provided that the intensity of laser exceeds a threshold value, below which the droplet motion successively undergoes four stages: constant velocity, deceleration, acceleration, and constant velocity. When the droplet motion is completely blocked, four steady vortices are clearly visible, and the droplet is fully filled by two internal vortices. The external vortices diminish when the intensity of laser increases.     

微重力下液封对液桥内热毛细对流影响的研究

本文建立了具有液封的液桥（不相溶混的双层同轴液柱）内热毛细对流的物理模型和数学模型。采用涡量－流函数法对微重力条件下具有液封的液桥内热毛细对流进行了数值模拟，得到了双层液柱主流区的温度场和流场，证实了液封能够削弱液桥内热毛细对流，从而提高浮区晶体生长质量，并得到液封厚度对液桥内热毛细对流的影响规律。     

边界耦合的Marangoni对流边界层问题的近似解析解

研究了由温度梯度引起的Marangoni对流边界层问题.由于动量方程和能量方程的边界条件耦合,利用相似变换将偏微分方程组转化为常微分方程非线性边界值问题.通过巧妙引入摄动小参数对速度和温度边界层方程同时渐近展开求解,得到了问题的近似解析解,并对相应的动量、能量传递特性进行了讨论. 关键词： Marangoni对流 近似解析解 渐近展开     

Numerical simulation of natural convection in a sessile liquid droplet

Heat transfer in a sessile liquid droplet was studied with numerical methods. A computer code was developed for solving the problem of convection in an axisymmetric hemispherical droplet and in a spherical layer as well. The problem of establishing an equilibrium state in a droplet was solved using several variables: temperature, stream function, and vorticity. Simulation was performed for droplets of water, ethyl alcohol, and model liquids. Variable parameters: intensity of heat transfer from droplet surface, Rayleigh and Marangoni dimensionless criteria, and the characteristic temperature difference. It was revealed that the curve of convective flow intensity versus heat transfer intensity at droplet surface has a maximum. A dual-vortex structure was obtained in a stationary hemispherical profile of liquid droplet for the case of close values for thermocapillary and thermogravitational forces. Either thermocapillary or thermogravitational vortex might be dominating phenomena in the flow structure.     

Triple convective flow of micropolar nanofluids in double lid-driven enclosures partially filled with LTNE porous layer under effects of an inclined magnetic field

In this paper, free, forced and Marangoni convective flows within an open enclosure partially filled with a porous medium under impacts of an inclined magnetic field are investigated. The forced convection is due to the movement of the side walls, the free convection induces from the heated part in the bottom wall and the Marangoni convection is a responsible on the thermal interaction at the free surface (top wall). The flow domain is partially heated from below and partially filled by a porous medium. The local thermal non-equilibrium model (LTNEM) is used to represent the thermal field in the porous layer (bottom layer) while the two-phase model is used to simulated the micropolar nanofluid behavior. Two cases based on the direction of the movement of the side walls are considered, namely, assisting flow (downward lid motion) and opposing flow (upward lid motion). Numerical analysis based on the finite volume method is conducted and the obtained are presented in terms of the streamlines, isotherms, angular velocity, and the cup-mixing temperature θ_cup, the bulk-averaged temperature θ_ave and the average Nusselt numbers. The controlling parameters, in this situation, are the Darcy number Da, the Marangoni number Ma, the Nield number H, the vortex viscosity Δ, the Biot number Bi and the Hartmann number Ha. The results revealed that the increase in the Nield number enhances the cup-mixing temperature θ_cupand bulk-averaged temperature θ_ave regardless the direction of the side walls motion. Also, the average Nusselt number is boosted as the Marangoni number is grown.     

EXPERIMENTAL STUDY OF MARANGONI-BENARD CONVECTION IN A LIQUID LAYER INDUCED BY EVAPORATION

Marangoni-Bénard instability and convection in evaporating liquid layers have been studied experimentally through flow visualization and temperature profile measurement. Bénard cells have been observed in an evaporating thin liquid layer whether it is heated, adiabatic, or cooled from below. This experimental study has revealed a different mechanism from the traditional Rayleigh-Bénard and Marangoni-Bénard instabilities and convections, which require a negative temperature gradient in the thin liquid layer. Evaporation rate and enthalpy of evaporation have been found to be important parameters of instability and convection in an evaporating liquid layer. A modified form of Marangoni number, Ma^*, is proposed and its critical values, Ma^* _c, for alcohol and Freon-113 evaporating layers are determined experimentally. A quantitative comparison between Ma^* and the traditional Marangoni number, Ma, shows that Ma^* is an adequate indicator of the stability status in evaporating liquid layers.