低温回热制冷机内交变流动与换热的格子-Boltzmann方法模拟

采用格子-Boltzmann方法(LBM)分别通过对二维通道及多孔介质内交变流动与换热进行了数值模拟,以研究低温回热制冷机中脉冲管内及回热器内交变流动与换热规律.结果表明:二维通道交变流动结果与解析解吻合得很好.当交变流动数较大时,出现速度环形效应,温度表现为周期波动变化.多孔介质内各点速度总体均呈现交替波动变化,但振...     

基于格子波尔兹曼方法的回热器数值模拟

利用格子玻尔兹曼方法,直接对蚀刻薄片和层叠丝网回热器的微观结构流场进行了模拟.得到了两种回热器填料的微观流场和两端的压差.模拟结果显示,当回热器的直径、水力直径和填充率相近情况下,不同流速下蚀刻薄片卷裹式回热器的稳态阻力系数均比层叠丝网回热器小.稳态阻力系数的模拟变化趋势与实验一致.     

基于格子Boltzmann方法的疏水表面润湿性数值模拟

润湿性对固体表面上液体的各种动力学行为具有重要影响,疏水表面的特殊润湿性是其在减阻、降噪、防污等领域有着广泛应用前景的根本原因。基于Shan-Chen模型的格子Boltzmann方法对疏水表面润湿性进行数值模拟,获得了材料属性和微形貌对疏水表面润湿性的影响规律。研究表明,要使疏水表面处于Cassie-Baxter润湿状态,微形貌高度必须大于某一临界值,而当疏水表面一旦处于Cassie-Baxter润湿状态后,继续增加微形貌高度也不会提高其疏水性能;疏水表面的表观接触角随气液界面分数先增大后减小,且存在一个最佳的气液界面分数使表观接触角达到最大。     

多孔介质模型镁合金流变铸轧LBM模拟

根据REV尺度多孔介质格子-Boltzmann方法基本理论,建立求解高固相率半固态浆料渗流过程的热流耦合数值模型.应用模型对变孔隙率Poiseuille流动及纯扩散凝固进行模拟.LBM模拟结果与有限差分模拟吻合.分析了压差作用下浆料在平板间渗流冷却与热流耦合过程.模拟结果表明浆料渗流速度随温度降低而减慢,相同时刻下凝固潜热较大的浆料渗流Re数高.利用提出的模型初步探索了AZ91D镁合金半固态流变铸轧过程的热流场分布.     

改善瓶用聚酯结晶性能的方法

本提出了几种通过共聚改性提高聚酯冷结晶温度的方法，同时探索了缩聚工艺条件对聚酯结晶性能的影响，实验结果表明，通过降低缩聚初期的温度，可显减慢聚酯的结晶速度。     

微通道气体流动的格子波尔兹曼模拟

通过隐式格子波尔兹曼方程,并采用壁面平衡边界条件以及二阶关系,模拟了微通道气体流动中的非线性压力和壁面滑移速度,模拟结果与Arkilic的解析结果十分吻合,验证了格子波尔兹曼方法在滑移流区的有效性.     

基于格子玻尔兹曼方法的低We数液滴铺展凝固模拟研究

为了研究3D喷墨打印中液滴在壁面的铺展、凝固现象与机理,基于格子玻尔兹曼方法(Lattice Boltzmann method,LBM),建立了三维多组分相变模型,模拟计算了单液滴在低We数条件下与低温基板碰撞后的演变过程。在模拟过程中考虑了壁温、壁面润湿性等因素对于液滴的铺展、凝固的影响。模拟结果表明,在非润湿性壁面,液滴铺展产生震荡阻尼现象,通过改变壁温控制液滴凝固速度可以达到阻碍或者促进液滴铺展;而在润湿性壁面,凝固会阻碍液滴铺展,液滴最终铺展因子随接触角减小而降低,并且与壁温成正比关系。此外,壁温在低于一定范围后才会对液滴形貌(铺展因子、接触角)造成明显影响。     

霜层生长过程中的导热模型

在逾渗理论的基础上,对霜层的导热系数进行分析.现有研究表明由于冰晶体的不断生长,会形成不同的集团,当在霜层中形成大的连通集团的时候,霜层导热系数的上升速度会突然加快.在此基础上提出了基于逾渗理论的导热模型,并采用数值模拟的方法验证了上述过程.以临界点为分界线,分别导出了霜层生长过程中,不同孔隙率下的导热系数的通用数学表达式.与其他导热模型的结果比较表明,本模型更加符合实际,并且有较大的使用范围.     

聚氧化乙烯等温结晶过程的计算机模拟

用Monte Carlo方法模拟了聚氧化乙烯(PEO)在预先成核条件下的等温结晶过程,并用Avrami方程进行了结晶动力学处理.结果显示,Avrami方程能够成功地描述被模拟的PEO等温结晶的初期过程.随着结晶温度的升高,结晶速率和球晶的线生长速率减小,而Avrami指数值基本不变,都接近于3.采用热台偏光显微镜(HSPOM)实验证实了模拟实验模型和结果的正确性.     

Numerical study of natural convection in a cavity of high aspect ratio by using the lattice Boltzmann method

A comprehensive numerical study has been conducted to investigate two‐dimensional, steady heat transfer of natural convection in a divided enclosure of high aspect ratio. The vertical walls of the enclosure are maintained at different temperatures, while the horizontal walls are adiabatics. A numerical hybrid scheme with lattice Boltzmann for fluid velocity and finite difference for the temperature is adopted. Parametric studies of the effects of aspect ratio, number and length of partitions attached to the cold wall of the enclosure on heat transfer and fluid flow have been performed. Copyright © 2007 John Wiley & Sons, Ltd.     

Coupling lattice Boltzmann and material point method for fluid-solid interaction problems involving massive deformation

This paper presents a coupling lattice Boltzmann and material point method (LBMPM) for fluid-solid interaction problems involving massive deformation. The convected particle domain interpolation-based material point method is adopted to solve the structure responses due to the particularly advantage on dynamic massive deformation simulations and the lattice Boltzmann method is utilized for its reliability and simplicity to simulate the complex fluid flow. The coupling strategy for these two methods is based on the consistent conditions with respect to displacement, velocity, and force, respectively, on the interface between fluid and solid parts, including the unified interpolation bounce-back scheme for curved boundaries, the Galilean invariant momentum exchange method for hydrodynamic forces, the force imposing strategy particularly for massive deformation, and the refilling algorithm for moving boundaries. There is no remeshing operation needed in the proposed LBMPM for both solid and fluid parts even when solid massive deformation and fluid complex flow are considered. Three representative numerical examples are carried out and the simulation results demonstrate that the proposed LBMPM is capable of simulate complex bidirectional fluid-solid interaction processes with the superiority for the problems involving solid dynamic large deformation behaviors and complex fluid flow.     

Instability and treatments of the coupled discrete element and lattice Boltzmann method by the immersed moving boundary scheme

The immersed moving boundary (IMB) scheme has been extensively used to couple the discrete element method (DEM) with the lattice Boltzmann method (LBM). In the literature, only the formulation of IMB for lattice nodal cells covered by a single-solid particle was given. The treatment of situations where a nodal cell is covered by two or more solid particles is seldom discussed. It is found that some numerical instability can occur for such situations due to an inappropriate computation of the weighting function in the IMB formulation. This work presents an enhanced treatment that can resolve the issue and validates it using some benchmark tests. Furthermore, to avoid the extra costs associated with the treatment and simplify the complicated procedure introduced, a simplified IMB scheme is proposed. The accuracy of both enhanced and simplified IMB schemes are validated by test cases including single-particle sedimentation, two-particle drafting-kissing-tumbling phenomenon, and multiple-particle sedimentation. Then, the robustness of both schemes is examined and discussed using a specially designed flow past cylinders test. The simplified IMB scheme is proved to be robust and sufficiently accurate and simpler and more effective than the enhanced scheme.     

气相生长硒化镉单晶体的生长速度研究

以晶体生长理论为基础，计算了气相提拉生长CdSe晶体时的生长速率。结果表明用提拉法气相生长CdSe晶体时，晶体的气相生长速率将会随着时间的延长以指数关系快速的趋近于提拉速度，以后不再变化。由此，在选定温场的前提下，对CdSe单晶体的气相生长速度进行了优化，确定了3mm／d的生长速度，得到了平界面生长的尺寸为Ф20mm×30mm，电阻率高达10^9Ω·cm，且未观察到深能级陷阱的优质CdSe大单晶体.     

An improved immersed moving boundary for hydrodynamic force calculation in lattice Boltzmann method

Particles suspension is considerably prevalent in petroleum industry and chemical engineering. The efficient and accurate simulation of such a process is always a challenge for both the traditional computational fluid dynamics and lattice Boltzmann method. Immersed moving boundary (IMB) method is promising to resolve this issue by introducing a particle-fluid interaction term in the standard lattice Boltzmann equation, which allows for the smooth hydrodynamic force calculation even for a large grid size relative to the solid particle. Although the IMB method was proved good for stationary particles, the deviation of hydrodynamic force on moving particles exists. In this work, we reveal the physical origin of this problem first and figure out that the internal fluid effect on the hydrodynamic force calculation is not counted in the previous IMB. An improved immersed moving boundary method is therefore proposed by considering the internal fluid correction, which is easy to implement with the little extra computation cost. A 2D single elliptical particle and a 3D sphere sedimentation in Newtonian fluid is simulated directly for the validation of the corrected model by excellent agreements with the standard data.     

Direct simulations of the linear elastic displacements field based on a lattice Boltzmann model

In this paper, a lattice Boltzmann model for simulating linear elastic Lame equation is proposed. Differently from the classic lattice Boltzmann models, this lattice Boltzmann model is based on displacement distribution function in lattice Boltzmann equation. By using the technique of the higher‐order moments of equilibrium distribution functions and a series of partial differential equations in different time scales, we obtain the Lame equation with fourth‐order truncation errors. Based on this model, some problems with small deflection are simulated. The comparisons between the numerical results and the analytical solutions are given in detail. The numerical examples show that the lattice Boltzmann model can be used to solve problems of the linear elastic displacement field with small deflection. Copyright © 2015 John Wiley & Sons, Ltd.     

The lattice Boltzmann method and the finite volume method applied to conduction–radiation problems with heat flux boundary conditions

This article deals with the implementation of the lattice Boltzmann method (LBM) in conjunction with the finite volume method (FVM) for the solution of conduction–radiation problems with heat flux and temperature boundary conditions. Problems in 1‐D planar and 2‐D rectangular geometries have been considered. The radiating–conducting participating medium is absorbing, emitting and scattering. In the 1‐D planar geometry, the south boundary is subjected to constant heat flux, while in the 2‐D geometry the south and/or the north boundary is at constant heat flux condition. The remaining boundaries are at prescribed temperatures. The energy equation is solved using the LBM and the radiative information for the same is computed using the FVM. In the direct method, by prescribing temperatures at the boundaries, the temperature profile and heat flux are calculated. The computed heat flux values are imposed at the boundaries to establish the correctness of the numerical code in the inverse method. Effects of various parameters such as the extinction coefficient, the scattering albedo, the conduction–radiation parameter, the boundary emissivity and the total heat flux and boundary temperatures are studied on the distributions of temperature, radiative and conductive heat fluxes. The results of the LBM in conjunction with the FVM have been found to compare very well with those available in the literature. Copyright © 2008 John Wiley & Sons, Ltd.     

Frost growth around a cylinder in a wet air stream

This paper describes a two-dimensional model which permits the evaluation of the local properties during the frost formation. To achieve this objective it is necessary to know the local coefficients of heat and mass transfer which were determined by solving the flow, temperature and humidity fields. To solve the flow field, the governing equations were developed in terms of the stream and vorticity functions. A numerical solution is obtained for low Reynolds number up to 400. With the flow field solved, the temperature and humidity fields are determined and hence the local heat and mass coefficients. These coefficients were then used in the solution of a two stage model for frost formation permitting the prediction of the local frost properties around the cylinder such as density, thickness and temperature.