微重力环境下横向旋转磁场对热表面张力流的影响

微重力环境下外加磁场可以有效地控制浮区法晶体生长中的热表面张力流,从而提高半导体晶体生长的质量。在比较相同强度(7mT)的横向静态磁场与横向旋转磁场对热表面张力流影响的基础上,研究了外加横向旋转磁场(旋转频率50Hz)对三维半浮区熔体热表面张力流的控制。结果表明:横向旋转磁场对熔体产生周向搅拌作用和轴向抑制作用,其有助于三维热表面张力流转变为二维轴对称流动。浮区法晶体生长中,横向旋转磁场是一种比较理想的熔体对流控制方法。

二维方腔内热表面张力流的格子Boltzmann方法模拟

热表面张力驱动的对流是微重力下浮区法晶体生长中熔体最重要的物质与热输运方式。采用单松弛双分布函数格子Boltzmann模型,自主开发了相应的格子Boltzmann方法的串行和MPI并行程序包,并应用该程序包对开口方腔内流体的二维热表面张力对流进行了数值模拟研究。其中串行程序合并碰撞迁移过程和引入临时数组以连续读入分布函数,相比分开碰撞迁移过程,计算性能提高了二倍;在此基础上,采用单向计算区域分区和非阻塞通信模式,实现了MPI版格子Boltzmann并行程序包开发。对比基于传统有限体积法CFD程序计算结果表明,串行和MPI并行版格子Boltzmann程序包计算结果精确可靠;并行程序具有较好的性能。

非均匀旋转磁场对液桥表面张力对流控制的研究

半导体晶体作为各类电子器件的基础材料,它的发展对现代电子行业的发展起着重要的推动作用.为了探索生长高品质晶体的优化方案,建立了半浮区液桥的三维磁流体动力学模型,采用有限体积法数值研究了微重力环境下外加二极对非均匀旋转磁场对液桥表面张力对流的控制作用.研究结果表明,由二极对非均匀旋转磁场(7mT,50 Hz)所产生的周向强迫搅拌作用,有效地提高了液桥的最大周向速度,同时抑制了液桥的最大轴向速度,该作用使无磁场作用下的三维液桥表面张力对流得到有效控制转变为二维轴对称定常流动.

组合线圈磁场下的液桥热表面张力流

为了优化外加磁场对对流控制作用,该文主要研究了轴向载流线圈磁场,横向四载流线圈磁场及其组合磁场对液桥热表面张力对流的控制。研究结果表明:轴向载流线圈磁场可有效抑制熔体的径向流动,并改善熔体对流的轴对称性;而横向载流线圈磁场可有效地抑制熔体轴向的对流,但是会破坏熔体对流的轴对称性。合理布置的轴向载流和横向四载流线圈的组合磁场同时保留了轴向载流线圈磁场的轴对称影响和横向四载流的轴向抑制作用,可以达到更好的控制熔体对流的效果,有利于从浮区法晶体生长中获得高质量晶体。

微重力下成一定夹角平板间的表面张力驱动流动的研究

空间微重力环境中,由于重力基本消失,表面张力等次级力发挥主要作用,流体行为与地面迥异,因此有必要深入探究微重力环境中的流体行为规律和特征.板式贮箱利用板式组件在微重力环境中对流体进行管理,从而为推力器提供不夹气的推进剂,这对航天器精确进行姿态控制、轨道调整具有重要意义.板式组件中常包含成一定夹角的平板结构,比如蓄液叶片之间.本文研究了微重力环境中成一定角度平板间的表面张力驱动流动问题,考虑了液体与壁面的动态接触角、对流引起的压力损失、黏滞阻力、液池内弯曲的液面等因素的影响,推导出了表面张力驱动流动中液体爬升高度的二阶微分方程.该方程可用四阶Runge-Kutta方法求解.通过同时考虑两个主导力,可将流动过程分为三个阶段,并得到了不同阶段内的爬升高度的近似方程.本研究建立了6个不同尺寸的计算模型、选用3种不同型号的硅油,利用有限体积法开展仿真工作,仿真结果与理论结果吻合良好,验证了理论解的正确性.本文的研究结果可为板式贮箱的研制和空间流体管理提供理论依据和数据支撑.

A New Model for Prediction of Surface Tension of Pure Fluids

A new model based on the theoretical work of Boudh-Hir and Mansoori was developed for prediction of surface tension of pure fluids. The new model has the advantage of not requiring densities in the calculation, and the input parameters are critical temperature and connectivity indices. A total of 209 compounds covering a wide variety of substances were used to develop the model, and the overall correlative AAD is 4.21%. To test its predictive ability, the model is further used to predict the surface tension of 25 more compounds that were not included in the model development. The overall predictive AAD is 4.07%, which illustrates that the model is reliable. The model proposed is simple and easy to apply, with good predictive accuracy.

Improved Correlation for the Volume of Bubble Formed in Air-Water System

In order to address the bubble formation and movement in air-water two-phase flow,single bubble rising in stagnant water is experimentally studied by digital image processing.Bubbles are released individually from the submerged orifices with different diameters(1.81 mm,2.07 mm,2.98 mm,3.92 mm)at different detachment frequency.Images are recorded by a high-speed video camera and processed by digital image processing technique. The factors impacting the formed volume of bubble are discussed.The experimental results showed that a threshold of gas flow rate(400 mm 3 ·s- 1)divides the bubble formation into two regimes:the constant volume regime and the growing volume regime.Especially for the growing volume regime,the surface tension is taken into account.The bubble volume is consisted of two parts:the surface tension impacting part and the gas volume flow rate impacting part.An improved correlation for bubble volume prediction is developed for the two regimes and better coincidence with the experiment data than the previous models is obtained.

Modeling of Surface Tension and Viscosity for Non-electrolyte Systems by Means of the Equation of State for Square-well Chain Fluids with Variable Interaction Range

The equation of state(EOS)for square-well chain fluid with variable range(SWCF-VR) developed in our previous work based on statistical mechanical theory for chemical association is employed for the correlations of surface tension and viscosity of common fluids and ionic liquids(ILs).A model of surface tension for multi-component mixtures is presented by combining the SWCF-VR EOS and the scaled particle theory and used to produce the surface tension of binary and ternary mixtures.The predicted surface tensions are in excellent agreement with the experimental data with an overall average absolute relative deviation(AAD)of 0.36%.A method for the calculation of dynamic viscosity of common fluids and ILs at high pressure is presented by combining Eyring’s rate theory of viscosity and the SWCF-VR EOS.The calculated viscosities are in good agreement with the experimental data with the overall AAD of 1.44% for 14 fluids in 84 cases.The salient feature is that the molecular parameters used in these models are self-consistent and can be applied to calculate different thermodynamic properties such as pVT,vapor-liquid equilibrium,caloric properties,surface tension,and viscosity.

Heat Transfer in a Liquid-Solid Circulating Fluidized Bed Reactor with Low Surface Tension Media

Heat transfer characteristics between the immersed heater and the bed content were studied in the riser of a liquid-solid circulating fluidized bed, whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of liquid velocity, particle size, surface tension of liquid phase and solid circulation rate on the overall heat transfer coefficient were examined. The heat transfer coefficient increased with increasing particle size or solid circulation rate due to the higher potential of particles to contact with the heater surface and promote turbulence near the heater surface. The value of heat transfer coefficient increased gradually with increase in the surface tension of liquid phase, due to the slight increase of solid holdup. The heat transfer coefficient increased with the liquid velocity even in the higher range, due to the solid circulation prevented the decrease in solid holdup, in contrast to that in the conventional liquid-solid fluidized beds. The values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.

Numerical simulation of dynamic process for liquid film spreading by lattice Boltzmann method and its experimental verification

Combined with the kinetic model of liquid film spreading, a new numerical method of solid-liquid-gas three-phase flow was developed for the moving of contact line, which was a hybrid method of computational fluid dynamics and lattice Boltzmalm method （LBM）. By taking the effect of molecule force in droplet and the wall surface on liquid film into account, the changing law of contact angle with different surface tensions was analyzed on glass and aluminum foil surfaces. Compared with experimental results, the standard deviation by using LBM is less than 0.5°, which validates the feasibility of LBM simulation on the dynamic process of liquid film spreading. In addition, oscillations are discovered both at the initial and end phases. The phenomenon of retraction is also found and the maximum retraction angle is 7.58°. The obtained result shows that the retraction is proved to be correlative with precursor film by tracking the volume change of liquid film contour. Furthermore, non-dimensional coefficient 2 is introduced to measure the liquid film retraction capacity.

Study on Surface Properties for Non-polar Fluids with Density Functional Theory

The density functional theory, simplified by the local density approximation and mean-field approxi-mation, is applied to study the surface properties of pure non-polar fluids. A reasonable long rang correction is adopted to avoid the truncation of the potential. The perturbation theory is applied to establish the equation for the phase equilibrium, in which the hard-core chain fluid is as the reference fluid and the Yukawa potential is used as the perturbation term. Three parameters, ε/κ, d and ms, are regressed frorn the vapor-liquid equilibria, and the surface properties, including density profile, surface tension and local surface tension profile are predicted with these parameters.

Solute Transportin Sand Columns as Affected by Effluent Surface Tension

Transport of nonreactive solutes in soils is principally controlled by soil properties, such as particle-size distribution and pore geometry. Surface tension of soil water yields capillary forces that bind the water in the soil pores. Changes in soil water surface tension by contaminants may affect flow of soil water due to decreased capillary forces, caused by lowered soil water surface tension. This study aimed at assessing solute transport in sand columns as affected by effluent surface tension. Miscible displacement （MD） tests were conducted on sand columns repacked with sands sieved from 2.0, 1.0, 0.5 and 0.25 mm screens. The MD tests were conducted with 0.05 M bromide solutions prepared using water with surface tension adjusted to 72.8, 64, 53.5 and 42 dyne/cm2. Obtained breakthrough curves were modeled with the convection-dispersion equation （CDE） model. Coefficient of hydrodynamic dispersion and pore-water velocity responded inconsistently across decreased particle-sizes and water surface tensions and this was attributed to non-uniform effect of lowered effluent surface tension on solute transport in different pore-size distribution.

Modeling and Parametric Study of the Heat Transfer Inside a Lithium Bromide Flow Confined by Wire Screen

The present study deals with the numerical analysis of heat transfer inside a lithium bromide-water solution flowing down between finely meshed plastic wire screens. These screens confine the flow through capillary action while allowing the water vapour transfer inside an innovative absorber technology. The complex menisci shape formed on the confinement grid level, where the surface tension forces are of first importance, are reconstructed by a volume-of-fluid model. A continuum surface force model is used to account for the surface tension force. A static contact angle is used to define the wall adhesion. A new algorithm, consisting to set an unique constant temperature at the liquid/vapour interface and to determine the evolution of heat transfer characteristics over the simulation domain, has been implemented and validated by analytical solution. A parametric study has been conducted to determine the effect of the geometry, the contact angle and the shape of the wire on the heat transfer.

Computational fluid dynamic simulations on liquid film behaviors at flooding in an inclined pipe

The complex liquid film behaviors at flooding in an inclined pipe were investigated with computational fluid dynamic(CFD) approaches. The liquid film behaviors included the dynamic wave characteristics before flooding and the transition of flow pattern when flooding happened. The influences of the surface tension and liquid viscosity were specially analyzed. Comparisons of the calculated velocity at the onset of flooding with the available experimental results showed a good agreement. The calculations verify that the fluctuation frequency and the liquid film thickness are almost unaffected by the superficial gas velocity until the flooding is triggered due to the Kelvin–Helmholtz instability. When flooding triggered at the superficial liquid velocity larger than0.15 m·s-1, the interfacial wave developed to slug flow, while it developed to entrainment flow when it was smaller than 0.08 m·s-1. The interfacial waves were more easily torn into tiny droplets with smaller surface tension, eventually evolving into the mist flow. When the liquid viscosity increases, the liquid film has a thicker holdup with more intensive fluctuations, and more likely developed to the slug flow.

Study on asymmetric interior corner flow in microgravity condition

The capillary flow in asymmetric interior corner consisting of straight vane and curved wall is studied with analytical solution.The concept of equivalent interior corner angle is proposed to convert the asymmetric interior corner model into symmetric interior corner model.Then the governing equations of interior corner flow are established,and based on which the interior corner flow is calculated.This method is used to analyze the capillary flow in cylindrical vane-type surface tension tank with outer vanes.The research can provide beneficial reference to the design of vane-type surface tension tank.

The impact of interphase forces on the modulation of turbulence in multiphase flows

The modulation of turbulence by particles has been rigorously investigated in the literature yielding either a reduction or an enhancement of the turbulent kinetic energy at different spatial length scales.However,a general description of the turbulence modulation in multiphase flows due to the presence of an interphase force has attracted less attention.In this paper,we investigate the turbulent modulation for interfacial and fluid-particle flows analytically and numerically,where surface tension and drag define the interphase coupling,respectively.It is shown that surface tension and drag appear as additional production/dissipation terms in the transport equations for the turbulent kinetic energies(TKE),which is of particular importance for the turbulence modelling of multiphase flows.Furthermore,we study the modulation of turbulence in decaying homogenous isotropic turbulence(HIT)for both types of multiphase flow.The results clearly unveil that in both cases the energy is reduced at large scales,while the small-scale energy is enhanced compared to single-phase flows.Particularly,at large scales surface tension works against the turbulent eddies and hinders the ejection of droplet from the corrugated interface.In contrast,at the small scales,the surface tension force and the velocity fluctuations are aligned leading to an enhancement of the energy.In the case of fluid-particle flows,particles retain their energy longer than the surrounding fluid increasing the energy at the small scales,while at the large scales the particles do not follow exactly the surrounding fluid reducing its energy.For the latter effect,a considerable dependence on the particle Stokes number is found.