Inertial Efficiency of Cylindrical Collectors at an Angle to the Mean Direction of Flow

A model is given for the inertial deposition of an electrified filter whose fibers are oriented at an angle to the mean direction of flow. When the particle is polarized by the field resulting from a charged or polarized fiber, a fast numerical method for computing the efficiencies is presented for small Stokes numbers. A comparison of the accuracy of this method is made to the usual shooting method. The effective deposition rates due to inertia over a range of fiber orientations are computed. An average value of the effective deposition rate is computed, assuming a uniform distribution of orientations to obtain an approximation to the mean of the deposition rate of a filter having randomly oriented fibers. This average deposition rate is compared to the deposition rate for fibers which are strictly transverse to the mean direction of flow.     

Simulation of particle deposition on filter fiber in an external electric field

Particle deposition onto a filter fiber was numerically simulated when a uniform external electric field was applied. The effects of electric field strength, particle inertia, and electrical conductivity of particles on particle deposition characteristics such as particle loading patterns and collection efficiency were qualitatively investigated. As a result, the electrostatic forces between a newly introduced particle and the already captured particles on the fiber were found to have a great influence on the particle deposition patterns compared with the results where the electrostatic forces were neglected. Conductive particles and filter fibers lead to higher collection efficiency and more linear structure of particle deposits than those of dielectrics, and the particle inertia could also be more important to the collection efficiency of a fibrous filter when electric fields are present. The simulated particle deposits obtained from this work agreed well with the existing experimental results, in which the photographs of particle loaded fibers, within an external electric field, were reported.     

Effects of an external electric field on the collection efficiency of air filters: Filtration mechanisms with an external e-field

An external electric field was applied on the filter to improve its collection efficiency, and the collection efficiencies of the different filters under various conditions were evaluated. Dominant electrical filtration mechanisms for each condition were investigated using experimental and theoretical approaches. Four types of air filters were used as test filters: a charged fiber filter, a low-grade filter with 50% collection efficiency in the most penetration particle size (MPPS) zone, and two high-grade filters with more than 95% collection efficiency in the MPPS zone. Three different particle charge states—neutralized, single-charged and uncharged—were considered. For neutralized particles, the external electric field led to a 14.5%p. and 2.5%p. increase in the collection efficiencies of the low-grade filter and charged fiber filter, respectively. With the electric field, the collection efficiency of the low-grade filter increased by 30%p. for single-charged particles. The electric field also affected the collection efficiencies of the charged filter and high-grade filters, but the effect was not significant. For uncharged particles, the electric field did not lead to a remarkable increase in the collection efficiencies of any of the filters. Through experimental and theoretical analysis, it was found that the polarization force imposed on the charged fiber was the dominant factor for the charged fiber filter regardless of application of the external electric field. The Coulombic force imposed on the electric field was the dominant factor for the low-grade filter, while both the Coulombic and the polarization forces affected the collection efficiency of the high-grade filter.

Copyright © 2017 American Association for Aerosol Research     

Estimation of collection efficiency enhancement factor for an electret fiber with dust load

In a previous study to investigate how the morphology of particles accumulating on an electret fiber evolves and affects the collection efficiency of the filter at dust-loaded condition, a three-dimensional stochastic model is utilized to simulate the deposition and agglomeration of particles on a cylindrical electret fiber via two different electrical effects, namely, induced force (for uncharged particles) and Coulombic force (for charged particles). In the present study, the additional effect of Brownian diffusion is incorporated in the model and the morphology of particle agglomerates obtained in the simulated results is found to agree well with experimental observations obtained by Hiragi and Kanaoka et al. for both uncharged and charged particles. In addition, the ratio of dust-loaded collection efficiency, η, to the clean-fiber collection efficiency, η₀, can still be approximated as linear function in the case of weak electrical effects. However, when the electrical parameters are large, the normalized collection efficiency has to be represented by two linear relations, i.e., at low dust load and high dust load. Estimates of the initial collection efficiency and efficiency enhancement factor are given graphically and tabulated as function of Peclet number, the interception parameter and the two electrical parameters.     

Dendritic deposition of uncharged aerosol particles on an uncharged fiber in the presence of an electrical field

A Monte Carlo simulation method based on particle trajectory calculations is used to study the effect of a uniform electrical field on the dendritic deposition of uncharged dielectric aerosol particles on uncharged dielectric fibres. The system parameters are chosen so that the main mechanisms of deposition are interception and electrical attraction. The main electrical force on an oncoming particle is that exerted by the polarized fiber and dendrites. It is found that simple superposition of the electrical fields of the polarized fibre and of the individual deposited particles gives an approximation to the actual field that is adequate for practical purposes. Based on such superposition, particle trajectories are calculated and the dendritic deposition phenomenon is studied. The effects of the electrical field are found to be very important and to increase with increasing field strength. In general, the electrical field increases the number of dendrites per unit length of fibre, and produces dendrites which are long, slender and tend to follow the force lines of the electrical field. The enhancement of the overall rate of deposition is also drastic and increases with increasing electrical field strength.     

Numerical Study of an Electret Filter Composed of an Array of Staggered Parallel Rectangular Split-Type Fibers

The flow field through a staggered array of parallel, rectangular split-type electret fibers was numerically modeled. The particle trajectory and the collection efficiency were simulated by solving the equation of particle motion, taking into account the effects of diffusion, interception, inertial impaction, and electrostatic forces. The model was validated against results calculated from semiempirical expressions. The model was applied to investigate the role of the inertial impaction and the interception mechanisms in the particle collection by an electret fiber, the particle trajectories under various filtration conditions, the effect of the aspect ratio of the rectangular fiber on the filter penetration, and the distribution of the deposited particles on the surface of the fiber. The simulated results indicate that the inertial impaction and interception mechanisms account for a major portion of neutral particles collected by an electret fiber when the Stokes number is higher than 0.5. For neutral particles, fibers with an aspect ratio of 38/10 have almost the same penetration as fibers with an aspect ratio of 10/38; while for singly charged particles, fibers with an aspect ratio of 38/10 achieve a much lower penetration when the electrophoretic collection mechanism dominates. In addition, it is predicted that a filter composed of fibers with an aspect ratio of 38/10 will result in a lower flow resistance and thus a slower clogging process when the dielectrophoretic collection mechanism dominates.     

A Simulation Study on the Collection of Submicron Particles in a Unipolar Charged Fiber

In this study, we developed a simulation method to predict the initial collection efficiency of a unipolar charged fiber and the particle deposition morphology in the electret filter composed of unipolar charged fibers. The particle sizes considered in this study were in the submicron range, and in the simulation method, Brownian motion of particles was also taken into consideration along with electrostatic forces acting on the particles. The simulation results were compared with other investigator's initial collection efficiency data, and it was found that simulation results are in good agreement with the experimental data. Based on this, we analyzed the effect of operating variables on the particle deposition morphology, which in turn affects the collection efficiency and pressure drop of the filter. In view of the simulation results on particle deposition morphology, it is clear that in the case of electret filters, particle deposition tends to take place onto the entire perimeter of fibers relatively uniformly, which may reduce the increase of pressure drop with time or extent of particle deposition compared to the conventional fibrous filter.     

COLLECTION OF INERTIALESS PARTICLES ON SPHEROIDS AND SPHERES WITH ELECTRICAL FORCES AND GRAVITATIO

Collection efficiencies are predicted for the capture of fine, incrtialess, charged particles on a single spheroidal collector in a gaseous flow field by the action of coulombic and external electric field forces and gravity. With the flow and external fields parallel to the axis of symmetry, collection efficiencies for spheroidal collectors are found by determining particle trajectories. For three-dimensional nonsymmetric systems resulting from the flow being at an arbitrary angle to both the axis of symmetry and the external fields, collection efficiencies are found by determining particle fluxes to the collector. For single force cases, particle deposition is independent of collector geometry and, for point panicles, is the same for all stationary incompressible flows. Using a cellular model of flow around a sphere, droplet density in wet scrubbers is shown to have little hydrodynamic effect on single droplet collection efficiencies.     

COLLECTION OF INERTIALESS PARTICLES ON SPHEROIDS AND SPHERES WITH ELECTRICAL FORCES AND GRAVITATIO

Collection efficiencies are predicted for the capture of fine, incrtialess, charged particles on a single spheroidal collector in a gaseous flow field by the action of coulombic and external electric field forces and gravity. With the flow and external fields parallel to the axis of symmetry, collection efficiencies for spheroidal collectors are found by determining particle trajectories. For three-dimensional nonsymmetric systems resulting from the flow being at an arbitrary angle to both the axis of symmetry and the external fields, collection efficiencies are found by determining particle fluxes to the collector. For single force cases, particle deposition is independent of collector geometry and, for point panicles, is the same for all stationary incompressible flows. Using a cellular model of flow around a sphere, droplet density in wet scrubbers is shown to have little hydrodynamic effect on single droplet collection efficiencies.     

Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity

Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete?, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5–2.5 m/s) representative of some high volume sampling applications was characterized. Experimental data and analytical theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Additionally, electrostatic fiber charge density is difficult to quantify, and measurements of aerosol collection efficiency are often used to calculate this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge density were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equilibrium Boltzmann charge distribution, and then using an electrostatic precipitator to separate out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70–85% at 30 nm. Filter performance was reduced significantly (40–50% collection efficiency) when the electrostatic filtration component was eliminated. Experiments performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, experimental data at 0.5 m/s were compared to theoretical predictions and good agreement was found for both electrostatic and nonelectrostatic effects.

© 2013 American Association for Aerosol Research     

Continuum Model Evaluation of the Effect of Saturation on Coalescence Filtration

Abstract

Coalescing filters are widely used throughout industry for removal of liquid aerosols from gases or the separation of liquid droplets from emulsions. Typical filters are constructed of non-woven fibers. Fibrous filters are capable of efficient removal of micron and submicron sized droplets and particles. The filtration process is highly complex due to variability in fiber sizes, particle sizes, mixtures of particles and droplets, mixture of types of droplets (oil, water, etc.), and effects of viscosity, surface tension, and chemical reactions between components or with the filter fibers. Prediction of filter performance under such complex conditions is difficult.

Performance of a filter depends on many factors like particle and fiber sizes, flow rate, surface properties of the fibers etc. One of those parameters is the saturation of the filter medium. Saturation is a measure of the amount of liquid present in the void space. Prior models assume that the saturation is uniform along the depth of the medium. In real media, the liquid holdup at steady state need not be uniform with position. Local velocity increases when the saturation is high.

In this paper, a steady state model for a coalescing filter is used to evaluate the effects of saturation on void fraction and its subsequent effect on filter performance. Single fiber mechanisms of direct interception and diffusion deposition are used to model droplet capture efficiencies and drag forces. These mechanisms are applied to volume averaged continuum equations in which the saturation is varied linearly with position in the filter. The results show the minimum pressure drop and largest quality factor occurs with a uniform saturation profile and that variation in average saturation has a greater effect on filter performance than does the slope of the linear saturation profile. The model predicts that uniform saturation profile performs better than the other profiles.     

High-Volume Aerosol Filtration and Mitigation of Inertial Particle Rebound

The performance of electrostatically charged blown microfiber filter media was characterized for high-volume sampling applications. Pressure drop and aerosol collection efficiency were measured at air pressures of 55.2 and 88.7 kilopascals (kPa) and filter face velocities ranging from 2.5 to 11.25 meters per second (m/s). Particle penetration was significant for particles above 0.5 micrometers (μm) in aerodynamic diameter where the onset of particle rebound was observed as low as 200 nanometers (nm). Particle retention was enhanced by treating filters in an aqueous solution of glycerol. Adding this retention agent eliminated electrostatic capture mechanisms but mitigated inertial rebound. Untreated filters had higher nanoparticle collection efficiencies at lower filter face velocities where electrostatic capture was still significant. At higher filter face velocities, nanoparticle collection efficiencies were higher for treated filters where inertial capture was dominant and particle rebound was mitigated. Significant improvements to microparticle collection efficiency were observed for treated filters at all air flow conditions. At high air pressure, filter efficiency was greater than 95% for particles less than 5 μm. At low air pressure, performance enhancements were not as significant since air velocities were significantly higher through the fiber mat. Measured single fiber efficiencies were normalized by the theoretical single fiber efficiency to calculate adhesion probability. The small fiber diameter (1.77 μm) of this particular filter gave large Stokes numbers and interception parameters forcing the single fiber efficiency to its maximum theoretical value. The adhesion probability was plotted as a function of the ratio of Stokes and interception parameter similar to the works of others. Single fiber efficiencies for inertial nanoparticle collection were compared to existing theories and correlations.

Copyright 2014 American Association for Aerosol Research     

Electrostatic effects on inertial particle transport in bifurcated tubes

Most aerosols found naturally in the ambient environment or those dispersed from artificial devices such as dry powder inhalers, are electrically charged. It is known that a strong electrostatic charge on aerosols can result in transport behavior dramatically different from that of uncharged aerosols, even in the absence of an external electric field. In the present work, we study pneumatic transport of corona‐charged particles in bifurcated tubes. This is accomplished by tracking the motion of discrete particles numerically under the influence of drag, gravitational, and electrostatic forces. The model aerosol is fly ash powder, whose size and charge distributions have been determined experimentally. The electrical mobility of the charged particle cloud is modeled through coulombic interactions between discrete point charges. For the case of polydispersed particles electrically charged across a distribution, the deposition efficiency was found to be greater than what is indicated by the mean charge and size. In particular, use of negatively charged fly ash powder of mean size of 2 μm and mean charge of ?1.5 C/kg led to significant increase in deposition efficiency (～29%) compared with uncharged fly ash powder of the same size distribution (～8%). Analysis of particle residence times suggests significant interaction between electrical and drag forces. These findings could have implications for pneumatic powder conveying or pulmonary drug delivery applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Deposition of flexible and stiff fibrous particles

The object of this paper is an analysis of motion and deposition of fibrous particles. A particle moves with the air flowing around the fiber which makes the filter element. Two limiting cases are studied: for flexible particle with low value of elasticity and for stiff, neutral and electrically charged, particle. The efficiency of particle deposition is estimated.     

Optimizing the Design of Room Air Filters for the Removal of Submicrometer Particles

Room air filters, which usually closely resemble high-efficiency particulate air (HEPA) filters, should be designed to maximize the clean air delivery rate (CADR) rather than operate at the very high collection efficiencies and relatively high pressure drops associated with HEPA filters. That is, for fixed electrical energy consumption, filters should remove the most particles possible. This can be accomplished by designing room filters that operate at lower collection efficiencies and higher airflow rates than HEPA filters. Based on filtration theory, the CADR of a fibrous filter is maximized with respect to filter thickness and air velocity at fixed energy consumption and filtration area. For very small particles for which diffusional deposition is the predominant collection mechanism, it is shown that a filter thickness resulting in a collection efficiency of 82% is optimal. For somewhat larger particles having diameters close to the filter's most penetrating size, direct interception is included in the analysis. The importance of inertial impaction and electrostatic deposition is also considered. This article supports the belief that room air filters used for enhancing indoor air quality can be improved significantly and suggests a methodology to accomplish this improvement.     

Inertial deposition of “heavy” aerosol particles in fibrous filters

The inertial deposition of aerosol particles of a high-density material on filter fibers is studied. By the boundary trajectory method, the capture coefficients are calculated as functions of the size and density of particles under various gas filtration conditions at different filter parameters with simultaneous allowance for the particle inertia, interception, gravity, van der Waals forces, and gas slip on fine fibers. The case is considered where an undisturbed flow upstream of the filter is directed at an angle to the sedimentation velocity vector. The conditions under which particles are not deposited on fibers are found.Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 1, 2005, pp. 50–55.Original Russian Text Copyright © 2005 by Kirsh.     

An improved CFD model of gas flow and particle interception in a fiber material

An improved CFD model of gas flow and particle interception in a fiber material which fiber size is Y-shape was developed in this work.The porous medium model was used to build the model of the whole size of fiber filter medium.Mixture model was adopted.The algorithm of particle interception in the whole size of fiber filter medium was derived and UDF (User Defined Function) that described kinds of particle filtering mechanisms in filter fibrous media was added to the Fluent default conservation equation as source term for simulation.The inertial resistance of the filter was taken into consideration,which provided a more precise measurement of the smoke flow and the particle interception in the filter under higher smoke speed conditions.The commercial software,Fluent 6.3,was used to simulate the smoke flow and particle interception in the filter in a single suction.The velocity and pressure profiles of smoke or nicotine particle in the filter,as well as nicotine particle volume fraction profile were well simulated.Finally.the comparisons of nicotine particle filtration efficiency between Fluent simulation results in this work and experimental results,as well as the model prediction in the literature were made to validate the simulation model.The comparisons showed that the particle entrapment model from simulation results was in good agreement with that from the experimental results.In addition,the Fluent simulation results are closer to reality both at the beginning and the end of the smoke process compating with the model predicted results in the literature.     

Particle Deposition in Parallel-Plate Reactors: Simultaneous Diffusion and External Forces

Particle deposition resulting from uniform external forces and Brownian motion is modeled in a parallel-plate reactor geometry characteristic of a wide range of semiconductor process tools: uniform, isothermal, downward flow exiting a perforated-plate showerhead separated by a small gap from a parallel, circular wafer. Particle transport is modeled using a Eulerian approach neglecting particle inertia and interception. Particles are assumed to originate in a planar trap located between the plates, such as would result for particles released from a plasma-induced particle trap after plasma extinction. Flow between infinite parallel plates is described by an analytic quasi-one-dimensional creeping flow approximation, where the showerhead is treated as a porous plate. An analytic, integral expression for particle collection efficiency (fraction of particles that end up on the wafer) is derived as a function of four dimensionless parameters: the flow Reynolds number, a dimensionless trap height, a dimensionless particle drift velocity, and the particle Peclet number. Numerical quadrature is used to calculate particle collection efficiency in terms of the controlling dimensionless parameters for external forces, which either enhance or inhibit particle deposition. Example calculations of collection efficiency are also presented in dimensional terms for a representative set of process conditions. Strategies to reduce particle deposition include the use of a protective external force and manipulation of the trap to keep it as far from the wafer as possible.     

The role of colloid chemistry in modeling deep bed liquid filtration

From mass balance on the suspension and filter and a charge balance on the filter, a set of hyperbolic partial differential equations is derived which describes how the suspension concentration, surface charges of the filter and particles, porosity of the filter, and the pressure drop vary with filter depth and time. These equations include a local deposition term which is evaluated by considering transport of the suspension particles to the filter particles by Brownian diffusion, interception, and sedimentation. The effect of the surface forces due to electrical double layer and van der Waals interactions was taken into account by treating the surface of the filter particles as possessing first order intermediate reaction kinetics, for which the rate constant is a function of the stability ratio of colloid chemistry.The governing equations were solved numerically, and the results compared with experimental data for unflocculated particles.The proposed filtration model is an advance over present models in that it contains no empirical factors which must be evaluated from filter runs and the effects of surfactants, pH, and ionic strength are accounted for.     

Pressure Drop and Interception Efficiency of Multifiber Filters

ABSTRACT

The viscous flow fields around multifiber filters have been investigated in a previous paper. The results of the previous work show that the flow becomes periodic immediately after the first fiber array downstream from the entrance if the fibers are arranged uniformly along the flow direction. The characteristics of such flow fields enable the pressure drop and the particle interception efficiency of a multifiber filter to be represented by single-fiber models. The total filtration efficiency, however, cannot be so represented since fibers interact during filtration processes. In this study, the pressure drop and the interception efficiency were investigated by making use of the viscous flow fields modeled in the previous research. The fiber separation ratio was found to have significant effects on pressure drop and efficiency. At a given volume fraction, changes in the fiber separation ratio will result in changes to the patterns of fluid flow and aerosol particle motion. Therefore, the fiber separation ratio significantly affects pressure drop and interception efficiency.