Pore network simulation of fluid imbibition into paper during coating: II. Characterization of paper's morphology and computation of its effective permeability tensor

We describe a method for characterization of the microstructure of a paper, in terms of the statistical distribution of the effective radii and lengths of the channels between the paper's fibers, and the average coordination numbers that characterize the connectivity of such channels, both in the main planes of the paper and in the transverse direction perpendicular to these planes. The microstructure of the paper is then represented by a three-dimensional network of interconnected flow channels between the fibers. The geometrical characteristics of the channels, such as their effective radii and lengths, are selected from the statistical distributions obtained by the characterization method, as are the two average coordination numbers. Extensive computer simulations are carried out to compute the effective permeability tensor of the paper, and to investigate the effect of the various microstructural parameters on the tensor. These simulations indicate that the two average coordination numbers, as well as the spatial distribution of the fibers and their length, strongly influence the flow properties of a paper.     

Pore network model of deactivation of immobilized glucose isomerase in packed-bed reactors. Part III: Multiscale modelling

A widely used method for converting glucose to fructose is by enzymatic isomerization. This process, which uses immobilized glucose isomerase, takes place in a packed-bed reactor that consists of microporous particles with a range of pore sizes, characterized by a pore size distribution. The micropores are also interconnected, giving rise to a three-dimensional (3D) network of pores with distributed sizes and connectivities. The particles themselves generate a 3D pore network at the reactor level with distributed pore sizes, but with a fixed connectivity. In this paper, Part III of a series, we develop a multiscale modelling approach to this problem, beginning with the relevant phenomena at the scale of the micropores, and integrating them into the particle and reactor length scales. As the efficiency of the process is significantly affected by deactivation of the microporous particles, we take this phenomenon into account at all the relevant length scales. We use a real random packing of particles, originally constructed by Finney (Proc. R. Soc. Lond. A, 319 (1970) 479), and map its pore space onto an equivalent 3D Voronoi network in which the pores are represented by the edges of the Voronoi polyhedra. The flow field in the Voronoi network is determined, and the convection-diffusion-reaction equation is then solved in the Voronoi network, taking into account the gradual deactivation of the microporous particles. Several plausible mechanisms of deactivation of the microporous particles are considered, and their effect on the performance of the reactor is investigated. Good agreement is found between the results of the computer simulations and the relevant experimental data.     

油水两相变径管流动模拟研究

油水两相混合流动是输油管路中经常遇到的流动现象,其流动规律研究具有重要的科学价值和广泛的工程应用价值。利用FLUENT软件对油水两相流在变径管中的流动进行模拟发现,管路内压力、速度在管径突然变化的区域有明显变化,且随含油量的不同其变化程度也有所不同。通过模拟分析突变管内油水两相流动规律,为实际油品输送提供一定的参考依据。     

聚合物流动的多尺度模拟

基于聚合物分子运动论,提出了一种新的计算聚合物流体的多尺度方法。该方法在宏观尺度上应用无网格方法求解速度场,在微观尺度上应用随机模拟技术计算聚合物对应力的贡献,从而避免需要本构方程来封闭连续性方程和动量守恒方程。对Hooke哑铃模型、FENE哑铃模型、FENE-P哑铃模型,模拟了突然起动平面Couette流动;对Hooke哑铃模型,模拟了方腔驱动流动。从而验证了该方法的有效性和计算结果的可靠性。     

Numerical simulation of transient roll-waves in two-phase pipe flow

This paper presents a transient roll-wave simulator based on a one-dimensional incompressible two-fluid model. Using an efficient numerical method and a grid cell length of one pipe diameter, the simulator is able to accurately predict experimental data in the roll-wave regime. Essential to obtaining the stable roll-wave solutions is the use of a modified version of the Biberg friction model. Based on an algebraic eddy viscosity model, the Biberg model yields mechanistic expressions for the wall and interfacial shear stresses in stratified two-phase flow. Input to the model is closure relations for the turbulence levels at either side of the interface. In order to model the increased dissipation of energy in breaking wave fronts, we propose to modify the original closures by adding a term proportional to the negative gradient of the liquid height. As the waves grow and come close to breaking, this term redistributes the shear stresses such that the waves are stabilized. The numerical method used in the simulator is an extension of a previously presented pseudospectral Fourier method. In particular, spectral vanishing viscosity terms are introduced to stabilize short wavelengths inconsistent with the long wavelength approximation of the two-fluid model. The result is a simulator that provides reliable and convergent numerical solutions for all stratified flow conditions. In the last part of the paper, the simulator is tested against roll-wave experiments with horizontal and upward inclined flows of water and a dense gas in a 10 cm pipe. Wave heights and speeds as well as mean pressure drops and holdups are predicted within 20% accuracy when compared with 474 experiments. Furthermore, the transient simulations enable us to study the dynamics and interactions between waves. In agreement with experimental observations, two different wave growth mechanisms are identified, and their influence on the evolution of the roll-wave flow regime is analyzed.     

管型混合澄清槽内的液-液两相流的数值模拟

管型混合澄清槽在工业生产中具有广阔的应用前景。通过计算流体力学分别对管型混合澄清槽内的混合室和澄清室进行数值模拟,系统地探究了分散相液滴尺寸（d₃₂₌ 100~500 μm）、进料油水比（O∶A = 1∶1~1∶5）、入口挡板及入口位置对混合澄清效果的影响,并将模拟结果与传统方型混合澄清槽进行对比。结果表明,管型混合室内的流场分布更合理,不易形成流动死区。管型混合室内搅拌桨上方和下方形成压力更小的低压区,流体的湍动能更大,搅拌性能更好。在混合室中,降低分散相d₃₂和进料油水比能够提高混合性能。在澄清室中,提高分散相d₃₂和降低进料油水比能够提高澄清性能,入口挡板能够有效提高澄清性能。     

流化床内颗粒流体两相流的CFD模拟

采用先进的CFD模拟技术分析流化床内两相复杂体系的非线性流体动力学特征已得到普遍认同,但是由于不同研究者对颗粒与颗粒以及颗粒与流体之间相互作用力认识的差异,导致欧拉-欧拉框架下动量守恒方程的不同表达形式。本文在总结文献中有关颗粒黏性力、固相压力和两相间作用力的基础上,从双流体理论出发,提出了一个考虑拟平衡态下固体颗粒对流体相和固相动量守恒方程均有影响的简捷流体动力学模型。该模型的主要特点是表征颗粒离散属性的特征长度视为颗粒直径的同一数量级。随后在CFX4.4商业化软件平台上通过增加用户自定义子程序,对网格尺度、时间步长和最大颗粒堆积率的无关性进行检验,介绍了作者近年来采用该模型模拟二维/三维流化床内液固体系的散式流态化、气固Geldart A类物料的散式/聚式流态化和床层塌落特性以及Geldart B/D类物料的鼓泡/射流流态化和床层塌落的研究进展。模拟的主要结果与经典理论、本研究实验和文献报道数据相一致,说明该模型可以用来预测流化床内密相颗粒流体体系的动力学特性。     

Analysis of Fluid Energy Mill by gas-solid two-phase flow simulation

The particulate motions and collisions inside the Fluid Energy Mill were simulated by coupling the Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD). The influences of the operating conditions on the particulate motions and collisions were investigated to further explain size reduction process. The high-speed grinding air streams introduced through narrow inlets selectively accelerate the particles located near the inlets. Those particles are more likely to hit the wall at a high speed, or collide with other particles due to the velocity difference. The simulation results also reveal that abrasion is the dominant breakage mechanism during the particle-particle collisions. On the other hand, with the increase of number of particles in the chamber, the particle-particle collision becomes more important for milling, compared to the particle-wall collision. The side-swipe particle-particle collisions also facilitate transferring of coating materials among particles, which explains the simultaneous milling and coating process recently developed in our lab.     

Interaction of droplets with porous structures: Pore network simulation of wetting and drying

In fluidized bed spray agglomeration, the time evolution of a liquid droplet deposited on a porous particle is of paramount importance for the success of the process. The combination of droplet penetration into the pores and evaporation, either directly from the droplet surface or from the surrounding wet pores, determines how long free liquid remains on the particle surface so that other particles can bind via liquid bridges. In this work, a two-dimensional pore network model that combines the algorithms of liquid migration and drying is developed to track the full droplet evolution, from its deposition on network surface to complete evaporation of the liquid. The influence of the pore structure for mono-modal and bi-modal networks with different spatial correlation of the pore size on the evolution of the liquid droplet is investigated. The effect of the liquid viscosity on the evolution of the droplet in the pore network is studied. Moreover, pore network simulations with multiple depositions of liquid droplets on the same network are presented as a rough approximation of spray agglomeration process.     

Supercritical fluid flow in porous media: modeling and simulation

The present work aims the modeling and simulation of supercritical fluid flow through porous media. This type of flow appears in several situations of interest in applied science and engineering, as the supercritical flow in porous materials employed in chromatography, supercritical extraction and petroleum reservoirs. The fluid is constituted of one pure substance, the flow is monophasic, highly compressible and isothermal. The porous media is isotropic, possibly heterogeneous, with rectangular format and the flow is two-dimensional. The heterogeneities of porous media are modeled by a simple power law, which describes the relationship between permeability and porosity. The modeling of the hydrodynamic phenomena incorporates the Darcy's law and the equation of mass conservation. Appropriated correlations are used to model, in a realistic form, the density and the viscosity of the fluid. A conservative finite-difference scheme is used in the discretization of the differential equations. The nonlinearity is treated by Newton method, together with the conjugate gradient method. The results of the simulation for pressure and mobility of supercritical and liquid propane flowing through porous media are presented, analyzed and graphically depicted.     

鼓泡塔中离子液体-空气两相流的CFD-PBM耦合模拟

针对咪唑类离子液体介质,采用Euler-Euler双流体模型与群平衡模型（PBM）耦合的方法,引入由实验结果拟合获得的适用于该介质的气液相间曳力系数模型,对内径0.203 m、高2 m的鼓泡塔中离子液体-空气两相流进行计算流体力学模拟,研究了不同表观气速下塔内气液两相速度场分布、气含率和气泡尺寸分布等流体动力学性质。与现有的相间曳力系数模型Schiller-Naumann模型模拟结果对比,采用本文模型得到的气含率与实验值吻合更好,气泡尺寸分布与实验结果一致。     

循环流化床换热器中液固二相流的数值模拟

在循环流化床换热器的流体中加入固体粒子,可以对边界层有扰动作用,加快换热器中的传质与传热.对流化床换热器中的液固二相进行数值模拟,分别讨论了固体粒子对液体动量方程、k-ε方程及能量方程的影响,在单液相方程的基础上进行修正,建立了相应的液固二相流控制方程,确定了物理模型及边界条件.利用FLUENT进行数值计算,分别讨论了...     

锌溴液流电池通道的计算流体力学模拟

针对不同类型的锌溴液流电池通道,利用流体力学计算软件 FLUENT对流体的流动状态进行了模拟。结果表明,在相同流速条件下,直角通道和弯角通道流体的局部流速最大值都出现边壁突变处;在液流电池反应区,流体的局部流速和湍流程度的分布存在不均匀现象。通过改变进出反应区的分流通道数量和结构,可以使电解液在反应区流动均匀,改进整个流动体系的传质性能,进而使锌溴液流电池电堆的充放电性能得到提高。     

SCX超细分级机进料管内气固两相流数值模拟

以两相流理论为基础,对SCX超细分级机进料管改进后内部的气固两相流动特性进行了数值模拟。分析了气相压力、速度、湍动能和固相颗粒浓度的分布情况,得到了进料管内气固两相的分布规律。结果表明,加入导流片之后,进料管内气相压力、速度、湍动能径向梯度减小,偏流现象减弱;导流片阻碍了固相颗粒的横向运动,改变了颗粒的运动轨迹,使得颗粒浓度分布更加均匀,为后续的分级提供了良好的基础。     

Finite-element modelling of combined free/porous flow regimes: simulation of flow through pleated cartridge filters

Mathematical modelling of creeping incompressible Stokes flow and low-permeability Darcy flow are well established and a number of reliable schemes for the simulation of these regimes are available in the literature. However, modelling of combined Stokes/Darcy regimes, such as those encountered in many types of industrial filters, still presents mathematical and practical challenges. In this paper, we present a finite-element model for the prediction and quantitative analyses of the hydrodynamic behaviour of deadend pleated cartridge filters. Elemental discretisation in this scheme is based on the use of unequal order approximation functions for velocity and pressure fields. We show that this discretisation generates unified stabilisation for both Stokes and Darcy equations and prevents ‘numerical locking’ whilst preserving the geometrical flexibility of the computational grid. Conducting a number of numerical tests, it is shown that the developed model is capable of yielding theoretically expected and accurate simulations for realistic industrially relevant problems. The model is tested for shear thickening non-Newtonian fluids, which represent fluids used in aeronautical applications and in some process industries. This study is part of a multi-disciplinary project undertaken by various investigators for the design and development of high-performance deadend pleated cartridge filters for aeronautical applications. It has been demonstrated that the developed model presents a cost effective, robust and reliable design tool to enable engineers to appraise the operation of such filters.     

并联管路气液两相流量均布特性数值模拟研究

为了确保大型液化天然气(LNG)板翅式换热器冷箱并联管路流量均布性,基于计算流体力学(CFD)方法,根据实际板翅式换热器冷箱并联管路的形式,简化并建立了并联管路物理模型,模拟研究了气液两相流的流量均布特性。研究结果表明:当气液两相流速固定时,增加两相的体积含气率会先降低后提高流量分配的均匀性;当两相的体积含气率固定时,流量分配均匀性随流速的增加而变好。不同管路布置方式对并联管路流量分配影响不同。为提高并联管路流量的均布性,当管道水平布置时,优先考虑水平上进下出式;当管道垂直布置时,优先考虑垂直向下流入式。上述研究成果将为大型LNG板翅式换热器冷箱并联管路运行参数和布置方法的设计提供重要的理论依据。