陶瓷过滤管管壁内气体流动的LBM并行模拟

采用格子Boltzmann方法,编制并行程序,计算了陶瓷过滤管管壁内的流体流动,从微观角度对滤管多孔介质结构内的流动进行分析。以陶瓷过滤管管壁扫描电镜图片为基础,根据实际滤管的厚度,确定计算中的多孔介质结构。分析了无膜滤管微细通道内的速度及压力随入口参数的变化情况以及多孔介质结构对速度的影响,给出了压力沿滤管厚度方向的变化曲线;分析了有膜滤管内的流动情况,给出了压力变化曲线,由计算结果可知,滤膜压降占滤管压降的比例较大。对过滤管微孔结构内流动的研究,可为陶瓷过滤管的性能优化提供理论指导。     

The Permeability Distribution (PD) Method for Filter Media Characterization

Cake filtration is frequently used for the removal of particulate solids from fluids in industrial processes. The build up of a filter cake is usually accompanied by a decrease in overall permeability of the filter leading to an increased pressure drop over the filter medium. For an incompressible filter cake that builds up on a homogeneous filter cloth (surface filtration mode), a linear pressure drop profile is expected over time. However, occasionally experiments show curved pressure drop profiles. Whereas pressure drop profiles with increasing slope are generally ascribed to cake compression and/or depth filtration, pressure drop profiles with decreasing slopes are only ascribed to inhomogeneities in the filter. Such inhomogeneities can arise due to filter cake patches and/or an inhomogeneous filter cloth itself. In this work a method is proposed that transforms the pressure drop profile of a filter into a permeability distribution (PD) of the filter medium, thus accounting for possible inhomogeneities of the medium. The determination of the PD is looked at as an inverse problem of an integral transformation. The method is applied to experimental filter pressure drop data of laboratory scale jet-pulse cleaned bag filter plants. It is found that even clean filter media can exhibit a significant permeability profile.     

Direct numerical simulation and experimental validation of flow resistivity of nonwoven fabric filter

We numerically study and then experimentally validate the flow resistivity of commercial nonwoven fabric filters used for a bag filter system. To represent a realistic flow field inside the filters during simulation, a numerical method that coordinates the filter structure obtained by X-ray computed tomography imaging with computational fluid dynamics using the immersed boundary method is developed. The effects of superficial velocity, porosity of the filter domain, and type of filter on pressure drop are investigated and analyzed based on Darcy's law. The predictions from our numerical method are quantitatively in good agreement with the experimental measurements. We demonstrate that the Kozeny constants of the filters can be estimated by utilizing the solid volume fraction. These results demonstrate that our simulation method can be used to clarify the effects of porosity, fiber arrangement, and fiber shape on the pressure drop. Finally, its application to water droplet permeation is demonstrated.     

Optimization of Pleated Filter Designs Using a Finite-Element Numerical Model

A numerical model has been developed to optimize the design of pleated filter panels. In this model, the fluid flow is modeled by a steady laminar flow and the filter media resistance is governed by the Darcy-Lapwood-Brinkman equation. A finite element method with a nine-node Lagrangian element is used to solve the governing equations. For the rectangularly pleated filter panel, the numerical results agree well with the analytical model of Yu and Goulding (1992) and with his experimental data. The pressure drop increases at small pleat count due to increased media face velocity, and at large pleat count due to increased viscous drag in the pleat spacings. Therefore, an optimal pleat count for minimum pressure drop exists at a certain pleat height for each filter media type. The optimization of rectangular pleated filters, e.g., mini-pleated filter panels, has been performed for six commercial filter media. The optimal pleat count is shown to increase with decreasing media permeability of the filter media. A generalized correlation curve has been found for the six filter media by using a nondimensional parameter analysis. The results can be used to design pleated filter panels with minimum pressure drop.     

Model Development of the Transient Solid Hold‐up and Pressure Drop in Fabric Filters

There are different non‐idealities related to cloth filters, which are cleaned by jet pulses. The well‐known phenomenon of cake compaction causes progressive curves of the pressure drop versus time. Some experiments show degressive shapes of the pressure drop, though. It will be shown, that this behavior can be explained by dividing the filter in segments. Each model segment can have a different cake thickness, whereas the pressure drop of every single segment is the same at any time during filtration. The capacity of this model is pointed out to determine the cake load on existing filters and to simulate unknown operating points.     

Characterization of An Open-Pored Nickel Foam with Respect to Aerosol Filtration Efficiency by Means of Measurement and Simulation

The deposition of micron and submicron particles in metallic, ceramic, or synthetic open-pored foams is a special field of aerosol filtration in porous media. This is due to the more complicated pore structure than, for example, fibrous filter media. Therefore, the measurement as well as the simulation of aerosol filtration in open-pored foams involves certain custom-built techniques.

The filter efficiency for micron and submicron particles can be measured by differential electrical mobility analyser systems (DEMAS) or optical particle counters (OPC). Empirical formulas are available in literature for open-pored polyurethane foams to determine their aerosol filtration efficiency and pressure drop. An additional method for characterization is direct numerical simulation (DNS) by means of a three-dimensional (3D) model of the pore structure. These models can be obtained either by tomography or by mathematical generation.

In this work, the filter efficiency of an open-pored nickel foam with a cell diameter of 450 μm is determined by the methods previously mentioned. The experimental results are in good agreement with the results of the 3D simulation and a semi-empirical approach for polyurethane foams is adapted for a nickel foam.

Copyright 2015 American Association for Aerosol Research     

Thermally bonded nonwoven filters composed of bicomponent polypropylene/polyester fiber. I. Statistical approach for minimizing the pore size

A statistical approach involving the uniform design of experiments and regression analysis is used to investigate the effects of thermal bonding process parameters, dwell time, thermal bonding temperature, and hot air velocity, on the pore size of three‐dimensional (3D) nonwoven filters. These filters are produced from polypropylene (PP)/polyester (PET) (sheath/core) bicomponent staple fibers. The pore structures of the filter samples were examined using the capillary flow porometer. Results reveal that the statistical approach is effective in identifying the effects of the investigated process parameters on both the bubble point pore diameter and the mean flow pore diameter for the thermally bonded nonwoven filter samples. Under the optimized processing parameters for achieving the minimized pore size, the predicted minimum bubble point pore diameter is 111.12 μm and the predicted minimum mean flow pore diameter is 63.4 μm for the filter sample with the area density of 60 g/m². They are in good agreement with the experimental values of 111.71 and 60.91 μm, respectively. Microstructure features observed using scanning electron microscope indicate that the effects of the investigated process parameters on the pore size are closely related to the thermal energy delivered to the fibers and the pressure drop acting on the fabrics. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2689–2699, 2006     

A case study of simulating submicron aerosol filtration via lightweight spun-bonded filter media

The most common method of filtration is via fibrous nonwoven media. Fibrous filters are generally characterized by their collection efficiency and pressure drop. Traditional computational studies in this area are typically based on unrealistic 2-D geometries with the fibers simply placed in a lattice (regular array) perpendicular to the flow. The traditional approaches however, do not permit studying the relation between the 3-D structure of a filter media and its performance. In this study, for the first time, a virtual 3-D web is generated based on the fiber orientation information obtained from analyzing microscopic images of lightweight spun-bonded filter media. Pressure drop and collection efficiency of our virtual filter are simulated and compared with the previous 2-D analytical and numerical models as well as experiment. Our pressure drop calculation, unlike the previous models, showed a perfect agreement with the predictions of the Davies’ empirical equation. The collection efficiencies obtained from simulating a thin spun-bonded filter media challenged with submicron particles having diameters ranging from 50 to 500 nm showed a similar trend as that of the previous 2-D models. For the solid volume fraction (SVF), filter thickness, and the fiber and particle diameters considered in this study, we found collection efficiencies higher than that of the above mentioned 2-D models with a relatively good agreement with experimental data obtained from a TSI 8130 filter tester.     

3-D simulation of particle filtration in electrospun nanofibrous filters

Virtual 3-D geometries resembling the internal microstructure of electrospun fibrous materials are generated in this work to simulate the pressure drop and collection efficiency of nanofibrous media when challenged with aerosol particles in the size range of 25 to 1000 nm. In particular, we solved the air flow field in the void space between the fibers in a series of 3-D fibrous geometries with a fiber diameter in the range of 100 to 1000 nm and a Solid Volume Fraction (SVF) in the range of 2.5 to 7.5%, using the Fluent CFD code, and simulated the flow of large and fine particles through these media using Lagrangian and Eulerian methods, respectively. Particle collection due to interception and Brownian diffusion, as well as the slip effect at the surface of nanofibers, has been incorporated in the CFD calculations by developing customized C++ subroutines that run in the Fluent environment. Particle collection efficiency and pressure drop of the above fibrous media are calculated and compared with analytical/empirical results from the literature. The numerical simulations presented here are believed to be the most complete and realistic filter modeling published to date. Our simulation technique, unlike previous studies based on oversimplified 2-D geometries, does not need any empirical correction factors, and can be used to directly simulate pressure drop and efficiency of any fibrous media.     

A hybrid 3D/2D finite element technique for polymer processing operations

A hybrid 3D/2D finite element method is proposed to simulate polymeric fluid motions for various polymer processing operations that may involve complex flow geometries. The flow regime is divided into 2D and 3D regions, and the fluid flow is assumed to follow the Hele Shaw model in the 2D region. Mathematically the 2D and the 3D regions are linked by the conservation of mass and the continuation of pressure variations. Two example calculations have been used to illustrate the accuracy and efficiency of the new numerical technique, and it was found that a substantial amount of computing time can be saved to obtain numerical solutions that are as accurate as the complete 3D calculations.     

螺旋折流板换热器数值模拟及入口结构改进研究

利用FLUENT软件,采用雷诺应力湍流模型模拟了不同流量下常规螺旋折流板换热器壳程的流动与传热性能,并用实验验证了模拟的可靠性。为了减少换热器壳侧入口处压降,对螺旋折流板换热器壳侧入口结构进行改进,将流体垂直流入壳体改为入口与壳体形成一定角度。对结构改进后的螺旋折流板换热器进行的数值模拟表明,采用倾斜入口结构时进口处压力降比采用垂直入口结构时低52%以上,且流量越大,这种降低幅度越明显。     

Theoretical Modeling of Filtration by Nonuniform Fibrous Filters

Modeling commercial filter media using classical theories results in incorrect filter pressure drop and efficiencies and this is attributed to media inhomogeneity. The use of an arbitrary inhomogeneity factor is seen to be inadequate in accounting for the effect of operating conditions on the performance of inhomogeneous filter media. A simplistic theoretical modeling approach is described here to account for the variations in filter packing densities and to estimate their effect on the media particle capture characteristics. The theoretical modeling results are used in obtaining an equivalent filter packing density distribution from media local efficiency measurements. Considering this variation in the theoretical models is seen to result in better prediction of media performance over a wide range of operating conditions. The observed deviation of experimental efficiencies from the theory at smaller Peclet numbers can be explained from the results of the theoretical analysis.     

Extruded monoliths for gas separation processes: Height equivalent to a theoretical plate and pressure drop correlations

Extruded monoliths are generally adopted in the chemical industry for the advantage of reduced pressure drops at high throughput, but a systematic procedure to evaluate their performance is not readily available. Key performance indicators for a monolith are the height equivalent to a theoretical plate (HETP) and the pressure drop of its channels. This contribution presents for the first time a systematic approach to the analysis of several extruded monoliths of industrial relevance. A procedure to derive the HETP correlation for an arbitrary extruded monolith is presented, and pressure drop correlations from literature are reviewed. The HETP correlations have been validated against three dimensional (3D) numerical simulations. A reduced-order model is derived from the HETP and is shown to capture the overall dynamics of the 3D simulations with high accuracy without adjustable parameters. Finally, a comparison between the extruded monoliths and packed beds is reported and includes pressure drops.     

螺旋片导流式分离器压力简化计算

应用计算流体力学CFD(ComputationalFluidDynamics)方法,对湍流状态下不同结构参数的螺旋片导流式气液分离器螺旋结构中的流体流动场进行了数值模拟。通过分析比较螺旋个数和螺距分别对压力降的影响,从而修正达西公式,拟合出螺旋结构压力降的简化计算公式,并通过了试验验证。结果表明,用此简化计算公式计算的结果与试验数据基本一致,可以用于螺旋片导流式气液分离器螺旋结构设计计算。     

Computational Fluid Dynamics Characterization of High-Capacity Structured Packing

The novel wire gauze structured packing, PACK-860, was characterized by means of numerical methods. The main features of PACK-860 such as height equivalent to a theoretical plate (HETP) and dry/wet pressure drop were evaluated. The flow structure in this packing was described by numerical simulation. To estimate the amount of HETP and dry/wet pressure drop, three-dimensional computational fluid dynamics (3D CFD) modeling with the Eulerian-Eulerian multiphase approach was applied. The average relative errors between the results obtained from CFD simulation and experimental findings for mass transfer efficiency and wet and dry pressure drop were assessed. Numerical observations were found to agree well with the empirical results, proving the reliability of CFD simulations for modeling separation processes.     

基于CFD-DEM方法的净化器流场模拟与结构优化

针对空气净化器能耗高的问题,使用离散元方法(DEM)在吸附滤网中建立随机堆积柱形活性炭模型,采用计算流体力学(CFD)方法对空气净化器内部流场进行数值模拟,在模拟与实验验证的基础上,考察了压降最小、流场最均匀的吸附滤网结构。结果表明,空气净化器压降主要发生在轴向,活性炭吸附滤网中回流、沟流现象严重,流体阻力是其他两种滤网的3倍。边数对多边形填充孔结构吸附滤网内压降与流场均匀性无影响,当孔结构改为圆形时,压降减小约52 Pa,节能18.4%(49 W);当孔直径由8 mm增至12 mm,压降减小约48 Pa,节能19.4%(45 W);滤网间距对空气净化器压降无影响,圆形、小孔径的吸附滤网内流场最均匀。     

Characterization of a New Structured Packing by Computational Fluid Dynamics

The novel wire gauze structured packing PACK‐1300Y with high specific surface area was characterized by computational fluid dynamics. The main features of PACK‐1300Y were investigated including the dry and wet pressure drop as well as the height equivalent to a theoretical plate (HETP). Moreover, the flow structure of this packing was described via numerical simulations. To evaluate the amount of HETP and dry and wet pressure drop, 3D computational fluid dynamic modeling with respect to the Eulerian‐Eulerian multiphase approach was applied. The average relative errors were determined between the findings achieved from computational fluid dynamic simulation and experimental findings for mass transfer efficiency and wet and dry pressure drop, respectively.     

管壳式换热器壳侧湍流流动与换热的三维数值模拟

综合应用体积多孔度、表面渗透度和分布阻力方法建立了适用于准连续介质的N-S修正控制方程.用改进的k-ε模型考虑管束对湍流的产生和耗散的影响,用壁面函数法处理壳壁和折流板的壁面效应, 对一管壳式换热器的壳侧湍流流动与换热进行了三维数值模拟.对计算结果进行了归纳,并与换热器冷态实验、前人的研究结果进行了对比分析,从而证明了该方法能更有效地模拟管壳式换热器壳侧的流动特性,压降实验数据和计算结果符合良好.