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。     

环形浅液池内浮力-热毛细对流的渐近解

为了了解水平温度梯度作用下环形浅液池内浮力-热毛细对流的基本特性,采用渐近线方法获得了环形浅液池内浮力-热毛细对流的近似解析解,得到了主流区速度场和温度场的表达式,通过与数值模拟结果的比较表明,所得结果是合理的。     

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。     

浮力对环形液池内热毛细对流的影响

为了了解浮力的影响,对水平温度梯度作用时环形液池内的热毛细对流进行了非稳态三维数值模拟,环形液池外壁被加热,半径为40 mm,内壁被冷却,半径为20 mm,液池深度为(1-17)mm,流体为0．65cSt的硅油,其Pτ数为6．7．模拟结果表明,当水平温度梯度较小时,流动为轴对称稳态流动,随着温度梯度的增加,流动将会失去其稳定性,在浅液池内,转化为热流体波,浮力对失稳后的流型无影响,但会使热流体波的振幅下降;在深液池内,在常重力条件下,转化成三维稳定流动,在微重力和小重力条件下,转化为三维振荡流动．     

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。     

深径比对双层热毛细-浮力对流的影响

三维数值研究了环形液池内双层流体热毛细-浮力对流的流动特性,考察了液池深径比对流动的影响,其中外壁面维持高温,内壁面维持低温。结果显示,上、下层流体热毛细-浮力对流的流动结构依赖于液池深径比的大小,其内在机制是由浮力和热毛细对流相对强度所决定;随着深径比的减小,热毛细-浮力对流不稳定性的周向温度波数先增大后减小,存在发生流动不稳定性的最佳深径比;热毛细对流的振荡周期随着深径比的减小逐渐减小,而周向速度平均扰动幅度逐渐增大.     

环形浅液池内热毛细对流的渐近解

为了了解微重力下水平温度梯度作用时环形浅液池内的热毛细对流特征,采用渐近线方法求得了环形浅液池内热毛细对流的近似解析解,得到了主流区速度场和温度场的表达式,并与数值模拟结果进行了比较,两者基本吻合.     

Soret效应对环形浅池内溶液热毛细对流的影响

采用三维数值模拟方法研究了Soret效应对深宽比为0.1的环形浅液池内双组分溶液热毛细对流的影响。结果表明,当热毛细Reynolds数较小时,溶液流动为二维轴对称稳态流动,相同热毛细Reynolds数下双组分溶液热毛细对流强于纯工质。由于双组分溶液中Soret效应的作用,冷壁面附近溶质浓度大于热壁面附近溶质浓度,且溶质毛细力方向与热毛细力方向一致,因此,溶液流动加强,相比纯工质更易失稳。流动失稳后,双组分溶液中呈现出热流体波与三维振荡流动并存的流动结构,且Soret效应对热流体波波数和主频有明显影响。     

数值分析环形池硅熔体旋转-热毛细对流及其稳定性

数值模拟了内半径20 mm、外半径40 mm、深5 mm环形池内硅熔体在旋转和热毛细力共同驱动下的热对流,通过线性稳定性分析确定了旋转-热毛细对流失稳的临界Marangoni数等临界条件。研究结果表明,液池低速旋转会降低轴对称热毛细对流的稳定性,而较高速度的旋转能增强热毛细对流的稳定性。临界条件下旋转-热毛细对流耗散结构波纹的传播方向与液池的旋转方向相同,临界周向波数随旋转速度的增加而增加。在较大的旋转速度下,液池底部出现涡胞,底部涡胞对热毛细对流的稳定性具有削弱作用。     

重力水平对热毛细-浮力对流耗散结构的影响

为了了解重力水平对环形液池内热毛细-浮力对流耗散结构的影响,利用有限容积法进行了非稳态三维数值模拟,环形液池外壁被加热,内壁被冷却,流体为0．65cSt硅油,其Pr数为6．7。结果表明,在微重力条件下,流动为三维振荡流动;当重力水平增加到0．1go时,流动结构转化为沿周向运动的一组滚胞,其轴线与温度梯度方向一致;当重力...     

THE NATURE OF NATURAL CONVECTION IN BINARY GASES DUE TO HORIZONTAL TEMPERATURE AND VERTICAL CONCENTRATION GRADIENTS

Abstract

Results are presented that illustrate the effect of augmenting or opposing thermal and solutal body forces on the flow, temperature and concentration distributions. For augmenting buoyancy forces, the flow field is very similar to that for a single-component fluid. Multicellular flow patterns are observed for opposing buoyancy forces that depend on the buoyancy parameter. The solutal buoyancy force does not dominate the flow field for all values of opposing body forces, because the solutal gradient is vertical. The concentration gradient affects the natural convection through both the additional buoyancy force and the thermophysical properties of the gas mixture.     

THERMOSOLUTAL CONVECTION WITH COUPLED HEAT AND MASS TRANSFER PROCESS IN A SQUARE ENCLOSURE

Thermosolutal convection flow and its effect on the heat and the mass transfer in a square enclosure is studied experimentally. Both thermal and solute diffusion are induced from the sides, and natural convection is initiated by the combined thermal and solutal buoyancies, which either augment or oppose to each other. The solute diffusion is initiated in an electrochemical system that uses copper sulfate-sulfuric acid solution as an electrolyte. Depending on the magnitude of buoyancy ratio, three different kinds of flow regimes and structures can occur, which lead to different distributions of concentration in the enclosure. The formation and growth of layered flow structure is attributed to the solutal boundary-layer flow that can intrude and accumulate along the horizontal wall. The nearly stagnant layer that occurs can reduce the heat transfer rate. The Nusselt numbers at different flow regimes are measured and correlated in terms of relevant nondimensional parameters. This suggests the correlation of Sherwood number in different ranges of buoyancy ratio. The visualization of flow structures and measurements of both heat and mass transfer allow better understanding of the complicated system.     

定向凝固宏微观热质传递及双扩散流作用

针对氯化铵水溶液定向凝固实验,从枝晶微尺度到宏观尺度耦合模拟了相变热质传递过程,并考察了双扩散流的影响.研究结果表明,NH4CI-H2O溶液的枝晶微界面浓度变化与非平衡态的谢尔方程趋于一致.两相区内液相率较高,易于流动的产生;热浮升力形成逆时针回流,溶质浮升力形成顺时针回流;热浮升力作用大于溶质浮升力.但双扩散流几乎不影响凝固沿着结晶室高度方向进行的一维定向特性.数值模拟中低估了两相区的渗透率,是造成两相区厚度模拟结果小于实验值的原因.     

强制对流和自然对流作用下枝晶生长的数值模拟

建立了一个基于格子玻尔兹曼方法 （LBM） 的二维模型,对强制对流和自然对流作用下合金凝固过程中的枝晶生长行为进行了模拟研究. 与传统的基于求解Navier-Stokes方程计算流场的方法不同,本模型采用基于分子动理论的LBM对凝固过程中的传输现象进行数值计算. 用三组粒子分布函数分别建立了计算流场、由对流和扩散所控制的浓度场和温度场的LBM演化方程. 通过求解LBM演化方程获得固/液界面前沿的浓度和温度分布. 然后,基于溶质平衡方法计算了枝晶生长的驱动力. 为了对模型进行验证,将模拟在强制和自然对流作用下枝晶上游尖端的稳态生长特征分别与Oseen-Ivantsov 解析解和修正的Lipton-Glicksman-Kurz 模型预测结果进行了比较, 模拟结果和理论预测结果符合良好. 模拟结果还表明,对流使热量和溶质从上游传输到下游,从而加速了枝晶在上游方向的生长,而抑制了下游方向的生长,形成了非对称的枝晶形貌. 关键词： 微观组织模拟 枝晶生长 对流 格子玻尔兹曼方法     

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.     

Thermophoresis in colloidal suspensions driven by Marangoni forces

In a hydrodynamic approach to thermophoretic transport in colloidal suspensions, the solute velocity u and the solvent flow v(r) are derived from Stokes' equation, with slip boundary conditions imposed by thermal Marangoni forces. The resulting fluid velocity field v(r) significantly differs from that induced by an externally driven particle. We find, in particular, that thermophoresis due to surface forces is insensitive to hydrodynamic interactions. As a consequence, the thermal diffusion coefficient D(T) of polymer solutions is independent of molecular weight and concentration.     

A HOLOGRAPHIC INTERFEROMETRY STUDY OF THE DOUBLE-DIFFUSIVE LAYERED SYSTEM

This study investigates double-diffusive convection in a two-layer, salt-stratified solution destablized by lateral Keating and cooling. The two-wavelength holographic interferometry method was used to measure the transient temperature, concentration, and density distributions. The evolution of the two-layered system can be divided into three stages. In the first stage, the thermally driven convective layers form rapidly, and the existing diffusion layer adjusts itself into a thin interface by convection motion. In the second stage, a quasi-steady state is attained. The temperature distribution is S-type and the temperature difference across the diffusion region does not change much. The concentration distribution is uniform in the two fluid layers, but the concentration and density differences decrease linearly with time. When the interface becomes very thin, unstable finger-type convection appears. Finally, the interface is destroyed by the boundary layers at the side walls in the third stage. The interfacial Nusselt number and Sherwood number art found to increase with the thermal Rayleigh number, and the effect of the solutal Rayleigh number seems to be less significant. The dimemumless mixing time is found to correlate well with thermal and solutal Rayleigh numbers. Results from numerical simulation are demonstrated and compared with the experiments.     

双颗粒在溶解条件下沉降的多相流动特性

对牛顿流体内溶解双颗粒在垂直管道中的沉降运动进行了直接数值模拟. 流体运动由守恒方程计算, 密度和黏性的变化考虑流场温度变化的影响, 通过积分粘性应力和压力获得颗粒的受力跟踪颗粒运动, 溶解引起的相变及其形状的变化由溶解潜热、溶解质量与分散相边界处的温度梯度的关系建立的方程决定. 通过颗粒和流体间相互的作用力和力矩及边界条件的施加实现相间耦合. 对双颗粒在等温流体无溶解条件和非等温流体溶解条件下的沉降过程进行了计算. 结果表明, 在一定雷诺数内, 热对流产生的颗粒尾迹处涡的脱落以及溶解引起的颗粒质量、颗粒表面形态的变化引起了颗粒的横向摆动, 并使颗粒沉降速度发生了变化.     

Heat transfer analysis of MHD rotating flow of Fe_3O_4 nanoparticles through a stretchable surface

The flow of a magnetite-H_2O nanofluid has been considered among two rotating surfaces,assuming porosity in the upper plate. Furthermore, the lower surface is considered to move with variable speed to induce the forced convection. Centripetal as well as Coriolis forces impacting on the rotating fluid are likewise taken into account. Adequate conversions are employed for the transformation of the governing partial-differential equations into a group of non-dimensional ordinary-differential formulas. Numerical solution of the converted expressions is gained by means of the shooting technique. It is theoretically found that the nanofluid has less skin friction and advanced heat transport rate when compared with the base fluid. The effect of rotation causes the drag force to elevate and reduces the heat transport rate. Streamlines are portrayed to reveal the impact of injection/suction.     

Magnetohydrodynamic Natural Convection in a Rotating Enclosure

Magnetohydrodynamic natural convection heat transfer in a rotating, differentially heated enclosure is studied numerically in this article. The governing equations are in velocity, pressure and temperature formulation and solved using the staggered grid arrangement together with MAC method. The governing parameters considered are the Hartmann number, 0≤$Ha$≤70, the inclination angle of the magnetic field, 0$^◦$≤$θ$≤90$^◦$, the Taylor number, 8.9×10$^4$≤$Ta$≤1.1×10$^6$ and the centrifugal force is smaller than the Coriolis force and the both forces were kept below the buoyancy force. It is found that a sufficiently large Lorentz force neutralizes the effect of buoyancy, inertial and Coriolis forces. Horizontal or vertical direction of the magnetic field is the most effective in reducing the global heat transfer.