采用DSMC方法对微槽流动统一滑移模型的讨论

基于扰动分析理论,在等温假设条件下,由二维Navier-Stokes方程和统一滑移模型推导得到压力分布、速度分布和质量流率理论表达式.以氩气为介质,采用直接模拟Monte Carlo(DSMC)方法计算了出口克努森数(Kno数)处于0.052～0.254的多个二维微槽流动,并将不同滑移模型的扰动分析解与DSMC结果进行了对比分析.结果表明,在模拟计算范围内,它们的压力分布基本一致;速度分布和质量流率均有一定差异,但在微槽道中心线附近流动高速部分仍十分接近.对于流动高速部分,当Kno数为0.052时,一阶滑移模型更接近于DSMC;当Kno数为0.073,0.101,0.15时,二阶模型更接近于DSMC;当Kno数为0.197,0.254时,所有滑移模型的速度和质量流率均小于DSMC.相比较,二阶滑移边界模型Cercignani与DSMC结果有更好的一致性.     

近壁面静止圆球的尾迹特性

圆球在大空间均匀来流下的绕流特性已得到了广泛的研究,但是壁面对圆球绕流特性的影响还未清楚。通过实验方法研究了与壁面接触的静止圆球在明渠流中的绕流特性,重点关注了圆球后尾迹的特性。研究发现,圆球尾迹存在三种状态：稳定状态、非稳定对称状态和非稳定非对称状态。当Re<150时,尾迹呈稳定状态,尾迹关于垂直于壁面的平面对称;当150<Re<400时,尾迹呈对称状态,Reynolds数较高时,圆球后存在规则的、周期性的涡脱落,Strouhal 数为0.26~0.33,脱落涡关于垂直于壁面的平面对称,此时尾迹功率谱的频带分布较宽;而当Re>400时,圆球后的涡脱落不再对称,尾迹随着Reynolds数的增大呈现混沌特征。     

微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

三角形螺旋夹套内流体的湍流流动及换热模拟

对三角形螺旋夹套内流体的湍流流动及换热性能进行了模拟,得到了充分发展条件下恒定热流加热时釜内湍流流体的速度场,分析了雷诺数(Re)和无量纲曲率(k) 对流体阻力和换热性能的影响,并由模拟数据拟合出平均阻力系数及平均努赛尔数的关联式. 结果表明,湍流流动中,夹套内流体的二次流动为稳定的二涡结构,随雷诺数增大,二次流强度和湍动能均增强. 由于离心力的作用,外壁面的阻力系数远大于内壁面. 换热面上局部努塞尔数的峰值出现在靠近二次涡中心位置的换热壁面处,换热面中心处的局部努塞尔数约为峰值的85%. 随Re和k增大,峰值处的局部努塞尔数值增大最明显,流体的平均努塞尔数及阻力系数均增大. 在所模拟的范围内,三角形螺旋夹套的效率因子E>3.7,且随Re和k增大,E逐渐增大.     

螺旋半圆管夹套内层流流动及换热特性研究

将螺旋半圆管夹套的物理模型简化为半圆形截面螺旋管,对4种不同结构夹套内的三维层流流动及换热进行了模拟求解,所得的结果与文献中的实验数据进行了对比。给出了发展段以及充分发展段的流场和温度场的分布,通过坐标变换得到了二次流的矢量图;分析了雷诺数Re、量纲一曲率δ和量纲一螺距λ对夹套内流体流动及换热特性的影响。结果表明:二次流对螺旋半圆管夹套的换热起强化作用,λ的影响很小,仅增大,δ流动阻力及壁面平均努塞尔数Nu均增大。     

球形气泡界面变化对尾涡性质和尺寸的影响

利用计算流体力学法研究了中等Reynolds数下(25≤Re≤500)气泡界面污染程度对其尾流的影响。借鉴圆球绕流和停滞帽模型,提出了一种模拟中等Reynolds数下受污染球形气泡尾流的三维模型,气泡界面污染程度取决于帽角(θ)的大小,帽角越大表示气泡表面污染程度越小。研究发现:Re=25~200时,污染程度的减小会减小尾涡长度(s)、分离角(φ)以及涡中心位置(l)和(h)的数值,但不会改变其与Reynolds数表征的关系;污染程度的减小会使Re=250~500时尾涡的三维特性减弱,使Re=350时有序脱落的尾涡的强度减小并最终使其不发生脱落,使Re=500时无规律脱落的尾涡的无序性减弱并最终使其不发生脱落。     

球形气泡界面变化对尾涡性质和尺寸的影响

利用计算流体力学法研究了中等Reynolds数下（25 ≤ Re ≤ 500）气泡界面污染程度对其尾流的影响。借鉴圆球绕流和停滞帽模型，提出了一种模拟中等Reynolds数下受污染球形气泡尾流的三维模型，气泡界面污染程度取决于帽角（θ）的大小，帽角越大表示气泡表面污染程度越小。研究发现：Re=25~200时，污染程度的减小会减小尾涡长度（s）、分离角（φ）以及涡中心位置（l）和（h）的数值，但不会改变其与Reynolds数表征的关系；污染程度的减小会使Re=250~500时尾涡的三维特性减弱，使Re=350时有序脱落的尾涡的强度减小并最终使其不发生脱落，使Re=500时无规律脱落的尾涡的无序性减弱并最终使其不发生脱落。     

过渡流下钝体绕流特性的可视化实验对比

为解明钝体绕流机理,设计了开式低速循环水槽,建立了钝体绕流实验平台。采用粒子图像测速法(PIV)和染色法,对钝体绕流特性进行实验研究。采用INSIGHT 4G进行全面调控。通过检测水槽不同高度激光面上的流动状态验证实验台稳定性。将PIV及染色法所得数据与已有模拟及实验结果进行对比,验证了实验方法正确性。选取过渡流下特定钝体(圆柱)进行周期研究,探寻钝体绕流周期性。并分析圆柱、方柱绕流在不同Reynolds数下的流动特性异同。实验表明:Re=200时,在一个周期内,圆柱周围正涡、负涡周期性脱落,正涡逐渐形成、增强、消散,负涡也在对应的位置循环发展。通过圆、方柱流动实验对比可知,圆柱绕流分离点不固定而方柱绕流分离点固定,位于其前后锐边上。     

折流板换热器壳程湍流流动与换热的三维数值模拟

利用FLUENT软件对折流板换热器壳程湍流流动与换热进行了三维数值模拟。得到了折流板换热器的温度场、速度场、质点迹线图、压降分布图等。根据模拟得到的结果,从多个方面对折流板换热器壳程湍流流动与强化传热进行了有益的探讨。     

A new expression for spherical aerosol drag in slip flow regime

A 3D simulation study for an incompressible slip flow around a spherical aerosol particle was performed. The full Navier–Stokes equations were solved and the velocity jump at the gas–particle interface was treated numerically by imposition of the slip boundary condition. Analytical solution to the Stokesian slip flow past a spherical particle was used as a benchmark for code verification, and excellent agreement was achieved. The simulation results showed that in addition to the Knudsen number, the Reynolds number affects the slip correction factor. Thus, the Cunningham-based slip corrections must be augmented by the inclusion of the effect of Reynolds number for application to Lagrangian tracking of fine particles. A new expression for the slip correction factor as a function of both Knudsen number and Reynolds number was developed. The particle total drag coefficient was also correlated against Re and Kn over the range of gas–particle relative speeds yielding the incompressible slip flow from the Stokesian regime up to the threshold of compressibility. Inclusion of gas slip on the particle surface enhances the accuracy of particle drag force prediction up to 40.9% in the range of 0.01<Kn<0.1 and 0.125<Re<20 compared to the no-slip continuum drag values.     

Thermophoresis in Rarefied Gas Flows

Numerical calculations are presented for the thermophoretic force acting on a free-molecular, motionless, spherical particle suspended in a rarefied gas flow between parallel plates of unequal temperature. The rarefied gas flow is calculated with the direct simulation Monte Carlo (DSMC) method, which provides a time-averaged approximation to the local molecular velocity distribution at discrete locations between the plates. A force Green's function is used to calculate the thermophoretic force directly from the DSMC simulations for the molecular velocity distribution, with the under-lying assumption that the particle does not influence the molecular velocity distribution. Perfect accommodation of energy and momentum is assumed at all solid/gas boundaries. Earlier work for monatomic gases (helium and argon) is reviewed, and new calculations for a diatomic gas (nitrogen) are presented. Gas heat flux and particle thermophoretic forces for argon, helium, and nitrogen are given for a 0.01 m spacing between plates held at 263 and 283 K over a pressure range from 0.1 to 1000 mTorr (0.01333- 133.3 Pa). A simple, approximate expression is introduced that can be used to correlate the thermophoretic force calculations accurately over a wide range of pressures, corresponding to a wide range of Knudsen numbers (ratio of the gas mean free path to the interplate separation).     

Concentration slip and its impact on heterogeneous combustion in a micro scale chemical reactor

The rarefied gas effect on concentration slip and on heterogeneous combustion in microscale chemical reactors was investigated. First, a concentration slip model to describe the rarefied gas effect on the species transport in microscale chemical reactors was derived from the approximate solution of the Boltzmann equation. Second, the model was verified using the direct Monte-Carlo method for the pure diffusion problems at different Knudsen numbers. The comparison showed that the present analytical model for the concentration slip boundary condition reasonably predicted the rarefied gas effect in the slip regime. Finally, the impact of the concentration slip on the coupling between the surface catalytic reactions and the homogeneous gas phase reactions in a microscale chemical reactor was examined using the one-step overall surface reaction model with a wide range of Knudsen and Damköhler numbers. It was shown that the rarefied gas effect significantly reduced the reaction rate of the surface catalytic oxidization for large Knudsen numbers. Furthermore, it was shown that the impact of slip effects on catalytic reactions strongly depends on the competition between the reaction rate and diffusion transport. It was found that the concentration slip causes a nonlinear reaction rate distribution at large Damköhler numbers. The results also showed that an accurate prediction of the rarefied gas effect on catalytic reactions in microscale reactors has to consider both the temperature slip and the concentration slip.     

滑移效应下纤维绕流场及过滤阻力的数值计算与分析

考虑实际纳米/微米纤维表面的流体滑移效应,采用数值方法求解滑移流动机理下纳米/微米纤维绕流场及过滤阻力,分析讨论了Knudsen数Knf和填充率C对纤维近壁面速度分布及纤维过滤阻力的影响规律.结果表明,对纳米/亚微米纤维过滤情形,纤维表面流体的滑移效应导致纤维绕流场与非滑移条件下情形有显著差异,尤其在高填充率下,纤维表...     

An analytical model for real gas flow in shale nanopores with non‐circular cross‐section

An analytical model for gas transport in shale media is proposed on the basis of the linear superposition of convective flow and Knudsen diffusion, which is free of tangential momentum accommodation coefficient. The present model takes into the effect of pore shape and real gas, and is successfully validated against experimental data and Lattice–Boltzmann simulation results. Gas flow in noncircular nanopores can be accounted by a dimensionless geometry correction factor. In continuum‐flow regime, pore shape has a relatively minor impact on gas transport capacity; the effect of pore shape on gas transport capacity enhances significantly with increasing rarefaction. Additionally, gas transport capacity is strongly dependent of average pore size and streamline tortuosity. We also show that the present model without using weighted factor can describe the variable contribution of convective flow and Knudsen diffusion to the total flow. As pressure and pore radius decrease, the number of molecule‐wall collisions gradually predominates over the number of intermolecule collisions, and thus Knudsen diffusion contributes more to the total flow. The parameters in the present model can be determined from independent laboratory experiments. We have the confidence that the present model can provide some theoretical support in numerical simulation of shale gas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2893–2901, 2016     

Viscous flow past a porous sphere with an impermeable core : effect of stress jump condition

An arbitrary flow of a viscous, incompressible fluid past a porous sphere of radius `a' with an impermeable core of radius `b', using Brinkman's equation in the porous region is discussed. At the interface of the clear fluid and porous region, stress jump boundary condition for the tangential stresses along with the continuity of normal stresses and the velocity components are used. On the surface of the impermeable core no slip condition is used. The corresponding Faxen's laws are derived to compute the drag and torque acting on the surface r=a. It is found that the drag and torque not only change with the change of the permeability, but also a significant effect of the stress jump co-efficient is observed. The variation of drag and torque with permeability for different thickness (a-b) of the porous region as well as for different values of stress jump coefficient is discussed when the basic flow is due to uniform flow, two dimensional irrotational flow, doublet in a uniform flow, stokeslet, rotlet. In case of uniform flow the flow field has been plotted. In all the cases, a significant effect of the stress jump coefficient has been realized.     

Determination of the Scalar Friction Factor for Nonspherical Particles and Aggregates Across the Entire Knudsen Number Range by Direct Simulation Monte Carlo (DSMC)

The friction factor of an aerosol particle depends upon the Knudsen number (Kn), as gas molecule–particle momentum transfer occurs in the transition regime. For spheres, the friction factor can be calculated using the Stokes–Millikan equation (with the slip correction factor). However, a suitable friction factor relationship remains sought-after for nonspherical particles. We use direct simulation Monte Carlo (DSMC) to evaluate an algebraic expression for the transition regime friction factor that is intended for application to arbitrarily shaped particles. The tested friction factor expression is derived from dimensional analysis and is analogous to Dahneke's adjusted sphere expression. In applying this expression to nonspherical objects, we argue for the use of two previously developed drag approximations in the continuum (Kn → 0) and free molecular (Kn → ∞) regimes: the Hubbard–Douglas approximation and the projected area (PA) approximation, respectively. These approximations lead to two calculable geometric parameters for any particle: the Smoluchowski radius, R _S, and the projected area, PA. Dimensional analysis reveals that Kn should be calculated with PA/πR _S as the normalizing length scale, and with Kn defined in this manner, traditional relationships for the slip correction factor should apply for arbitrarily shaped particles. Furthermore, with this expression, Kn-dependent parameters, such as the dynamic shape factor, are readily calculable for nonspherical objects. DSMC calculations of the orientationally averaged drag on spheres and test aggregates (dimers, and open and dense 20-mers) in the range Kn = 0.05–10 provide strong support for the proposed method for friction factor calculation in the transition regime. Experimental measurements of the drag on aggregates composed of 2–5 primary particles further agree well with DSMC results, with differences of less than 10% typically between theoretical predictions, numerical calculations, and experimental measurements.

Copyright 2012 American Association for Aerosol Research     

Cluster-based drag coefficient model for simulating gas-solid flow in a fast-fluidized bed

Drag coefficient is of essential importance for simulation of heterogeneous gas-solid flows in fast-fluidized beds, which is greatly affected by their clustering nature. In this paper, a cluster-based drag coefficient model is developed using a hydrodynamic equivalent cluster diameter for calculating Reynolds number of the particle phase. Numerical simulation is carried out in a gas-solid fast-fluidized bed with an Eulerian-Lagrangian approach and the gaseous turbulent flow is simulated using large eddy simulation (LES). A Lagrange approach is used to predict the properties of particle phase from the equation of motion. The collisions between particles are taken into account by means of direct simulation Monte Carlo (DSMC) method. Compared with the drag coefficient model proposed by Wen and Yu, results predicted by the cluster-based drag coefficient model are in good agreement with experimental results, indicating that the cluster-based drag coefficient model is suitable to describe various statuses in fast-fluidized beds.