Air flows and pressure drop modelling for different pleated industrial filters

A dimensionless model was developed to determine the pressure drop across clean pleated filters, according to filter medium type, geometric characteristics of the pleating (distance between two pleats, pleat height, etc) and air flow parameters (filtration velocity, air density, etc). The model was derived from both experimental and numerical results obtained from nuclear and automotive filters — high efficiency particulate air (HEPA) and low efficiency particulate (LE), respectively. The major findings were that a more homogeneous air flow distribution occured over the surface of the pleated HEPA filter, while geometric characteristics had a greater influence on the initial pressure drop across the LE filter. The numerical model highlighted the fundamental importance of the filter medium's air flow resistance on air flow distribution.     

The influence of a 2-component model on the computed regeneration behaviour of an uncoated diesel particulate filter

The present work deals with the modelling of diesel particulate filters. Model extensions as well as further development regarding soot monitoring during regeneration are investigated. A slip flow model, leading to an improvement of computed pressure drop profiles, a shape correction term, considering an increasing perimeter which changes the wall flow through the particle layer, and a 2-component model, which distinguishes between soot as cake upon and soot within the filter wall, have been implemented and the effects on previous results discussed. However, the main focus of the work is laid upon a sensitivity analysis regarding soot combustion within the pore combined with a discussion upon judging the accuracy of computed results. In order to compare experimental and computed results an uncoated SiC filter with 200 CPSI and 15 mil wall thickness was used. All experiments were conducted under real-world conditions on an engine test bench, which includes filter loading and regeneration. It can be shown, that the 2-component model effects positively the simulation of filter loading, as the filtration can be divided into deep bed and cake filtration. Due to the sensitivity analysis on kinetic parameters different effects on pressure drop and soot loading profiles during regeneration have been investigated. A faster soot combustion within the filter wall shows only a slight effect on the total soot loading curve. However, there is a strong influence on the pressure drop profile, which lowers the deviation between experiment and simulation.     

Clogging of fibrous filters by liquid aerosol particles: Experimental and phenomenological modelling study

Fibrous filters are the most common means used to separate liquid aerosol particles from an industrial gas stream. The pressure drop and penetration (=1-efficiency) are the most important performance criteria of the filter. In this study, experimental and modelling results describing the pressure drop and penetration evolution of a glass microfibre HEPA filter are presented. For the experimental part, the pressure drop and penetration evolutions of a HEPA filter are described as well as the influence of the filtration velocity on those evolutions. For the modelling part, the physical collection mechanisms taken into account and their mathematical expressions which are the basis of the phenomenological model are described in a first step. After that the experimental values are compared to their modelled counterparts. Different efficiency models from the literature have been tested in order to determine the one closest to the experimental values. The influence of the filtration velocity on the model is studied in the last part. The model presented here is capable of describing the pressure drop and penetration evolution of a HEPA filter over the whole filtration period.     

A Simple Numerical Model of Pressure Drop Dynamics During the Filtration of Liquid Aerosols on Fibrous Filters

We present a new numerical model of filtration of liquid aerosols on fibrous filters. The main goal of the model is to describe the dependence of pressure drop on time during the nonstationary filtration process. The main difference between the current model and others present in the literature is that it contains very few parameters—in its minimal form only two—and still describes the results of experimental measurements with a very good accuracy. We also estimate the dependence of the parameters of the model on the process conditions (i.e., gas flow velocity and the geometrical parameters of the filter) which enable the use of the presented model to predict the filtration process evolution in any fibrous filters.     

A filtration model for the flow of dilute,stable emulsions in porous media—I. Theory

Flow of dilute, stable emulsions in porous media is important in several oil recovery processes. Because underground media have relatively low permeabilities, the emulsion drop sizes may overlap the pore sizes. Hence, strong interaction occurs between the emulsion droplets and pore constrictions, and local flow redistribution occurs within the porous medium. To predict quantitatively how emulsions are transported in underground media, a theoretical model is required which correctly accounts for the interactions between the flowing droplets and the pore walls.In Part I of this work, we present a simplified filtration model describing the flow of stable, dilute emulsions in unconsolidated porous media. In the model, emulsion drops are captured in pores by straining and interception and, thus, reduce the overall permeability. Transient flow behaviour is characterized by there parameters: a filter coefficient, a flow-redistribution parameter and a flow-restriction parameter. The filter coefficient controls the sharpness of the emulsion front, the flow-redistribution parameter dictates the steady-state retention, as well as the flow redistribution phenomenon, and the flow-restriction parameters describes the effectiveness of retained drops in reducing permeability.Critical comparison is made between the new filtration theory and the current continuum—viscous and retardation models for emulsion flow in porous media. Only the filtration picture is able to explain all the experimental observations. Quantitative comparison between the filtration flow theory and experiment is presented in Part II.     

空气滤清器过滤阻力多元关联式的研究

为减低空气滤清器过滤阻力,对空气滤清器流场特性进行的实验测试和模拟计算,旨在于为空气滤清器的优化设计提供相关的理论指导和依据。采用CFD模拟技术,结合实验测试的方法,建立了空气滤清器的CFD模拟的两维和三维模拟模型,计算机模拟计算结果与实验测试结果吻合,计算得出空气滤清器的过滤阻力与其各自影响因素的变化规律,通过数值模拟计算结果的回归分析,给出了空气滤清器总成过滤阻力与其显著影响因素的单因素关联式。利用大量的过滤压降与不同滤清器结构参数、过滤速度和过滤介质特性参数的模拟数据,回归得出总成过滤阻力与入口速度、滤芯褶数、进气管长、进气管径、出气管长、出气管径和滤纸厚度的无量纲的多元关联式。     

Development of high efficiency particulate absorbing filter materials

We have developed new high efficiency particulate absorbing filter materials by bonding the fiber web with the help of high pressure water jets emerging from micron sized nozzles and subsequently coating the filters with a chemical binder. Two different types of nonwoven filters are produced by varying the water jet pressure during the bonding process. The performance characteristics of the filter materials are evaluated in terms of filtration parameters, such as filtration efficiency, dust holding capacity, and pressure drop. Filtration efficiency depends on the pore characteristics, namely pore size and their distribution in the filters. The developed filter materials have shown promising performance characteristics by capturing higher amount of dust particles with a relatively low pressure drop during use. These filter materials can be used for a wide range of industrial applications, where high filtration efficiency is required at low energy consumption. A fluid flow simulation is carried out by computational fluid dynamics (CFD) to understand flow pattern during the bonding process. The CFD is also used to predict the pressure drop in the nonwoven filter materials during filtration process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

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...     

基于气固两相流的双循环流化床提升管压降模型的预测和实验研究

为研究提升管颗粒循环流率对提升管压降的影响，搭建双循环流化床冷态实验系统，采用差压变送器进行提升管轴向区域压降的实验研究。基于提升管不同的颗粒速度计算方法，充分考虑加速区和充分发展区的不同压降机理，建立加速区、充分发展区和整个提升管压降模型，与实验结果比较发现：加速区颗粒速度采用滑移系数方法所得压降与实验值较吻合，在充分发展区进行压降计算时颗粒速度采用滑移速度等于终端速度计算所得结果较精确；在提升管压降计算时可综合考虑加速区和充分发展区适用的压降模型进行计算，可为实际生产运行中采用压差法进行提升管轴向颗粒浓度的分布提供一定参考，为提升管压降的在线监测提供指导。     

Clogging modeling in pleated filters for gas filtration

A model of clogging of a pleated filter in gas filtration is presented. The model is obtained by combining a semi-analytical model of the flow in a pleated filter and an empirical model of clogging of the planar filter medium applied locally along the pleated filter channels. The model takes into account the formation of a filtration cake of variable thickness at the porous wall of the pleated filter entrance channels and the resulting evolution of the entrance channels aperture distribution during the filtration/clogging process. Based on the numerical predictions, two main clogging scenarios are identified and analyzed. The optimum pleat density, defined as the pleat density maximizing the filter capacity, is determined and shown to be greater than the pleat density minimizing the pressure drop for a clean filter. Predictions of the evolution of overall pressure drop across the pleated filter due to clogging are compared with experimental data.     

Filtration of liquid aerosols with a horizontal fibrous filter

The aim of this study was to determine the filtration efficiency and pressure drop in liquid aerosol capture mechanisms using a fibrous coalescent filter. The experimental set-up was designed to measure filtration efficiency and pressure drop through the filter. To avoid the gravimetric effect, the filter was placed horizontally. A series of experiments highlighted the importance of operating conditions such as filtration velocity and aerosol concentration. The results demonstrated the advantage of working at high velocities to minimise the resistance of the medium and to enhance the mass efficiency of the coalescent filter. In parallel, no effect of aerosol concentrations in the pressure drop increase was noted.     

Numerical simulation of the flow and the collection mechanism inside a scale hybrid particulate collector

Numerical technology has been widely used for the study of the electrostatic precipitators (ESP) and the bag filters. This paper presents a numerical model for a scale hybrid particulate collector (HPC), which combines the ESP technology and the filtration technology together. The collection process of the HPC is unsteady as the pressure drop across the bag filter increases with the deposition of the particles. The physical processes of the model include the corona discharge, the fluid flow, the particle charging and the filtration. The corona discharge field is solved by using a finite volume method. For the fluid field, the unsteady and incompressible Navier–Stokes equations with the RNG κ–ε turbulence equations are solved. The effect of the electric field on the fluid field named electro-hydrodynamic is also considered. For the particle charging, the filed-diffusing combined model of Lawless (1996) [37] is adopted. For the filtration, an unsteady cake formation model is proposed. The pressure drop across the cake is calculated according to the mass density of the cake. The coefficient between the pressure drop and the mass density of the cake comes from the experimental data. Applying the numerical model to the HPC, the influence of the hole diameter of the perforated-plate on the collection efficiency of the electrostatic zone is analyzed. Numerical results show that the collection efficiency of the electrostatic zone of the HPC has no certain relation with the hole diameter of the perforated plate. The effect of the hole diameter of the perforated-plate on the collection efficiency of the electrostatic zone becomes weaker with increasing the applied voltage.     

Numerical simulation of soot filtration and combustion within diesel particulate filters

The numerous benefits offered by diesel engines, compared to gasoline ones, are balanced by a drawback of increasing concern, namely soot emissions. Nowadays, soot emissions can be reduced by physically trapping the particles within on-board diesel particulate filters (DPF). The filter gets progressively loaded by filtering the soot laden flue gases, thus causing an increasing pressure drop, until regeneration takes place. The aim of this work is to develop a fully predictive three-dimensional mathematical model able to accurately describe the soot deposition process into the filter, the consequent gradual modification of the properties of the filter itself (i.e. permeability and porosity), the formation of a soot filtration cake, and the final regeneration step. The commercial computational fluid dynamics (CFD) code Fluent 6.2.16, based on a finite-volume numerical scheme, is used to simulate the gas and particulate flow fields in the DPF, whereas particle filtration sub-models and regeneration kinetics are implemented through user-defined-subroutines (UDS).Model predictions highlight uneven soot deposition profiles in the first steps of the filtration process; however, the very high resistance to the gas flow of the readily formed cake layer determines the evolution into an almost constant layer of soot particles. The ignition of the loaded soot was simulated under different operating conditions, and two regeneration strategies were investigated: a “mild regeneration” at low temperature and oxygen concentration, that operated a spatially homogeneous ignition of the deposited soot, and a “fast regeneration”, with an uneven soot combustion along the axial coordinate of the filter, due to strong temperature gradients inside the filter itself. These findings are supported by comparison and validation with experimental data.     

Filtration and regeneration behavior of polytetrafluoroethylene membrane for dusty gas treatment

With micron talcum particles and nano-CaCO₃ powder as test dust, a series of experiments have been carried out to systematically study the gas filtration and regeneration behavior of polytetrafluoroethylene membrane, and some comparisons were made with common filter media. The experimental results showed that the PTFE membrane had a filtration efficiency of above 99.99% for micron particles, and excellent regeneration behavior was obtained, though a much higher initial pressure drop existed. Based on the results, it was concluded that the PTFE membrane is an excellent surface-filtration media for micron particles. Effects of operation parameters, including airflow velocity, particle concentration and particle characteristics were also investigated. To better understand the evolution of pressure drop during the filtration process, a mathematical model with operation parameters and characteristics of particles was derived from the gas-solid two-phase flow theories. A novel method on the determination of regeneration period of the filter media was put forward based on the analysis of the pressure drop according to this model.     

Study on pressure drop characteristics of gas-liquid swirl annular flow

Considering the influence of swirl attenuation, the pressure drop characteristics of gas-liquid spiral annular flow are studied, and the pressure drop prediction model of spiral annular flow is deduced. The swirl-straight ratio of pressure drop is defined as the ratio of pressure drop of swirl flow to straight flow, used to characterize the effect of swirl decay on pressure drop. The expression of swirl-straight ratio of pressure drop is derived by the method of dimensional analysis, and it has a strongly dependence on Lockhart-Martinelli coefficient and gas phase Froude number. Finally, the prediction model of pressure drop for gas-liquid swirl annular flow is obtained. The pressure drop characteristics of the swirl annular flow are experimentally studied in a horizontal tube with an inner diameter of 50 mm. The range of the gas superficial velocity is 10—16 m/s and the range of the liquid volume fraction (LVF) is 0.6%—4.8%. Through comparison with experimental data, the relative error of the pressure drop prediction model is within ±15%, which provides a method reference for engineering applications.     

气液螺旋环状流压降特性研究

考虑旋流衰减的影响,对气液螺旋环状流的压降特性进行研究并推导出了螺旋环状流压降预测模型。定义压降旋-直比系数为气液两相螺旋环状流和气液两相直流的压降之比,以此来表征旋流衰减对压降的影响。基于量纲分析的方法对压降旋-直比系数进行分析,推导出其表达式,压降旋-直比系数依赖于Lockhart-Martinelli 参数和气相Froude数变化。最终,得出了气液两相螺旋环状流的压降预测模型。在50 mm内径的水平管内对螺旋环状流的压降特性进行了实验研究,其中气相表观流速变化范围为10~16 m/s,体积含液率（LVF）变化范围为0.6%~4.8%。通过与实验数据进行对比,压降预测模型的相对误差在±15%以内,为工程应用提供了参考。     

Measurement and prediction of filtration efficiency evolution of soot loaded diesel particulate filters

We here present laboratory based experimental and theoretical methods to characterize the filtration efficiency (FE) behavior of diesel particulate filters (DPFs) exposed to soot laden gas streams. Sensitivity of the FE behavior on filter microstructure and geometry properties have been studied, along with the impact of the hydrodynamic and aerosol flow conditions (flow rate, temperature, aerosol characteristics). Evolution of FE with soot load is reported from clean filter FE to maximum efficiency (100%), as the deposited soot in the filter wall itself acts as the filtering medium. The theoretical model considers different mechanisms (Brownian diffusion, particle interception and inertia) of soot capture and their impact on number based and mass based FE. The predictions from the theoretical model are in good agreement with experimental observations over a broad range of filter microstructure. Sensitivity of FE evolution on bare and coated filters has been reported, along with the impact of ash loading of the filters. Methods presented here are useful in determining the performance of DPFs under well-defined laboratory conditions and their extension to dynamic field conditions. These are also useful in determining filter properties for obtaining high FE and low pressure drop.     

环形狭缝通道内气液两相环状流流动特性

建立了环形狭缝通道内气液两相环状流的理论模型 ,该模型计及环状流气芯流动的可压缩性、气液两相间的相滑移、气相对液滴的夹带作用等因素 .考察了两相流质量流量和干度对压降、液滴速度相对变化和狭缝喉部气芯通流面积的影响 .用建立的理论模型对空气 -水两相环状流通过环形狭缝的两相压降进行预测 ,预测值和试验值吻合良好