Single Fiber Collection Efficiency Research Articles

Modelling Particle Capture Efficiency with Lattice Boltzmann Method

The transport and deposition of particles over a fixed obstacle set in a fluid flow is investigated numerically. A two-dimensional model, based on lattice Boltzmann (LB) method and discrete element (DE) method, is used to simulate particle deposition. The corresponding method is two-way coupling in the sense that particle motion affects the fluid flow and reciprocally. The particle capture efficiency, as a function of particle size and Stokes number, is investigated using one-way (effect of the particle on the fluid is not considered) and two-way coupling respectively. The numerical simulations presented in this work are useful to understand the transport and deposition of particles and to predict the single fiber collection efficiency. The effect of obstruction shape on single fiber collection efficiency is investigated with LB-DE methods. Results show that the influence of particle on the flow field cannot be neglected for particles with large size. Numerical results for circular fiber collection efficiency are in good agreement with theoretical prediction and existing correlations. Enhanced collection efficiency is achieved by changing the fiber shape.

Numerical study of particle capture efficiency in fibrous filter

Numerical simulations are performed for transport and deposition of particles over a fixed obstacle in a fluid flow. The effect of particle size and Stokes number on the particle capture efficiency is investigated using two methods. The first one is one-way coupling combining Lattice Boltzmann (LB) method with Lagrangian point-like approach. The second one is two-way coupling based on the coupling between Lattice Boltzmann method and discrete element (DE) method, which consider the particle influence on the fluid. Then the single fiber collection efficiency characterized by Stokes number (St ) are simulated by LB-DE methods. Results show that two-way coupling method is more appropriate in our case for particles larger than 8 μm . A good agreement has also been observed between our simulation results and existing correlations for single fiber collection efficiency. The numerical simulations presented in this work are useful to understand the particle transport and deposition and to predict the capture efficiency.

Modeling particle-loaded single fiber efficiency and fiber drag using ANSYS–Fluent CFD code

In this paper, we present a methodology for simulating pressure drop and collection efficiency of a filter medium during instantaneous particle loading using the ANSYS–Fluent CFD code, enhanced with in-house subroutines. The simulations are comprised of a numerical solution of the Stokes equations for obtaining the air flow field, and Lagrangian particle tracking, for determining the particle collection efficiency and particle deposition sites on the fibers. The modeling strategy presented in this work allows one to track particles of different sizes and simulate the formation of 3-D dendrite particle deposits in the presence of aerodynamic slip on the fiber surface. In particular, the deposition of particles on a fiber, and on previously deposited particles, is made possible by developing in-house subroutines, which mark the cells located at the deposition sites and modify their properties so that they resemble solid particles. Fiber drag and single fiber collection efficiencies are obtained from simulations for fibers and particles of different diameters for demonstration purposes. Effects of particle capture mechanisms on a filter’s pressure drop and collection efficiency are presented and discussed with respect to the studies reported in the literature. More specifically, two fiber diameters of 1 and 20μm are used to demonstrate that the normalized single fiber collection efficiency increases with increasing mass of the loaded particles on the fibers (i.e., time) if the particle capture mechanism is interception or diffusion, but stays almost invariant if the capture mechanism is inertial impaction. Fiber drag (resembling the filter’s pressure drop) seems to increase because of particle deposition, but at different rates for different particle capture mechanisms.

Influence of Filter Inhomogeneity on Air Filtration of Nanoparticles

Thermal rebound of nanoparticles on the surface of fibers is of big concern in collecting nanoparticles. Another factor influencing the collection of nanoparticles is the inhomogeneity of fiber packing. The present work studied the influence of filter inhomogeneity on the collection efficiency of nanoparticles by using wire screens as uniform structure in comparison to real filters. As a result, the single fiber collection efficiencies of nanoparticles through wire screens are in good agreement with those predicted by Kirsch and Fuchs (ηD=2.7Pe(superscript -2/3)), but the dependence of real filters on Pe is somewhat smaller than –2/3 and in agreement with (ηD=0.84Pe^(-0.43), Wang et al., 2007). The dependence of single fiber collection efficiency on Pe for the real filters is well explained by considering non-uniformly packed filter consisting of two regions of densely packed and loosely packed fibers.

Simulation of dispersion and collection process of agglomerated particles in collision with fibers using discrete element method

The behavior of agglomerates in collision with fibers was simulated using the three-dimensional modified discrete element method and the influences of several factors on the fraction of collected particles were examined. Furthermore the single fiber collection efficiency for agglomerated particles was also investigated. In the case where gas velocity is quite low, agglomerates are only deformed but barely dispersed and thus collected as a single deformable particle. By contrast above some critical gas velocity, constituent particles are dispersed and at the same time partly collected on fibers. The fraction of collected particles first increases then decreases as the van der Waals attractive force between particle and fiber increases. The reason for the decrease in fraction of collected particles in strong adhesion region is that the smooth deformation of agglomerates along the fiber surface is inhibited by too strong adhesion. It was also suggested that there exists an optimum size ratio between the agglomerate size and fiber radius for the collection fraction. This is also closely related to the deformation of agglomerate along the fiber surface. In case of non-agglomerated particle collision, all the particles entering within the collision region are collected by fiber. By contrast in case of agglomerate collision, the dispersion of agglomerates as well as collection occurs in the same process and all the particles colliding with the fiber are not necessarily collected. Consequently the single fiber collection efficiency considerably decreases comparing to that for non-agglomerated particle collision.

Inertial Classification of Nanoparticles with Fibrous Filters

This paper proposes a new concept for the utilization of fibrous filters for the classification of nanoparticles. The present work confirmed that the filter employed in the present work can separate particles smaller than 100 nm. The main conclusions obtained in the present work are as follows: (1) Inertial filter utilizes inertial impaction of particles and the classification performance can be predicted by the log penetration law and the single fiber collection efficiency, (2) 50% cutoff size is reduced by increasing the filtration velocity and is predicted by Stk50 = 1, (3) Inertial filter developed in the present work has a low pressure drop compared to a low pressure impactor and therefore the volatilization of volatile organic compounds is suppressed during the atmospheric aerosol sampling.

Numerical analysis of particle behavior around a bipolar charged electret fiber

Charged and uncharged particle motions and collection characteristics around a bipolar charged rectangular shape electret fiber are studied numerically. Particle inertia, fluid drag, Coulomb force and polarization force are considered to predict the particle motion around the electret fiber. The effects of particle sizes, flow velocities, number of charges and polarities are also systematically investigated. For small size particles, the single fiber collection efficiency is greatly dependent on the charge polarity and the number of charges on a particle. However, particles larger than 5.mu.m do not show charging effect on collection efficiencies in the flow velocity ranges from 1.5 cm/s to 150 cm/s when the maximum charges are within +5 to -10. The results show that a strong electric field gradient at the corner of the bipolar charged fiber plays a very important role on collecting particles regardless of its charge polarity because of the polarization force. It also shows that the most penetrating particle size for a single electret fiber decreases as the flow velocity increases and the number of charges of a particle decreases.

ニードルフェルトフィルターの集塵特性

The present paper deals with the filtration performance of needle felt filters prepared from various kinds of materials. Single fiber collection efficiency was correlated experimentally with Reynolds number and the direct interception parameter, taking into account both volume fraction and needle punching density. Although the difference in the collection efficiency stemming from electrostatic charges on the fiber surface was observed, its dependence on the interception parameter showed the same tendency for all the materials.It was verified that the drag coefficient of felt filters varied inversely with Reynolds number and depended strongly on the volume fraction. Also an empirical expression of the drag coefficient applicable was obtained in a wide range of the volume fraction.

Convective diffusional collection of polydisperse aerosols on a dust loaded fiber

A three-dimensional method for the stochastic simulation of dendritic growth of polydisperse aerosol particles on a fiber was proposed and its capability was demonstrated for the case of convective diffusion ( Pe = 200-25000, R = 0.1,0.2, σ g = 1.1, 1.2, 1.4). Based on the simulation result, time dependency of the morphology of particle dendrites and the collection efficiency were investigated. Particle dendrites on a fiber simulated by the proposed method grew up three-dimensionally and also randomly, and were similar to those in actual filters. The average distribution of the deposited particles on a fiber was largely dependent on the Peclet number, Pe, and interception parameter, R, but not so much on the deposited mass of particles nor on the polydispersity of aerosol particles. Furthermore, the maximum deposition of particles appeared at a certain angle from the front stagnation point and shifted to a larger angle as Pe increased but did not change so much even if the mass of deposited particles increased. A single fiber collection efficiency at zero dust load agreed well with the analytical solution, and that of a dust loaded fiber increases almost linearly with the loaded mass of deposited particles in a unit filter volume, m, regardless of filtration conditions. The coefficient in the linear function or the collection efficiency raising factor Δ increased as Pe increased and R decreased. However, values of Δ did not show a clear tendency by the change of geometric standard deviation of particle size σ g. Most of the conclusions obtained in this study were almost the same as those obtained for monodisperese aerosols although the measure of polydispersity of aerosol particles, the logarithmic standard deviation σ g, varied from 1 to 1, 4. Therefore, it could be concluded that in the studied range of σ g the polydispersity of aerosol particles does not affect the average performances of the dust loaded filter.

On the Extension of Mass Transferj-Factor Correlation to the Collection Efficiency of Fibrous Filters

Filtration of submicrometer aerosols in fibrous beds was considered as a mass transfer process. Earlier experimental data on the single-fiber collection efficiency of aerosols collected in fibrous filters were used to calculate j-factors for mass transfer. A correlation for j-factors as a function of the modified Reynold's number in the bed was developed. Comparison of the j-factors based on both experimental data and various correlations for mass transfer in packed beds indicated that aerosol filtration in the submicrometer regime possesses strong similarities to mass transfer in packed beds. Also, the experimental j-factors obtained from aerosol filtration were obtained over a larger range of void fractions of the fibrous filters (from 0.6 to 0.992) than mass transfer data and hence provide a more complete set of data to develop a comprehensive correlation.

Effect of Charging State of Particles on Electret Filtration

Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 μm in diameter and at different charging states. A single fiber collection efficiency ηED was found to be expressible as a function of dimensionless parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K In as, here, A, B, C, and D are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 μm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 μm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.

Deep-Bed Electret Filters: The Determination of Single Fiber Charge and Collection Efficiency

Deep-bed electret filters are used to separate small solid or liquid particles from gas flows. Compared with conventional deep-bed filters of similiar pressure loss, they operate with substantially higher collection efficiencies. During the course of filtration, however, the electrical charges of the deposited particles neutralize the electret fibers, temporarily decreasing the collection efficiency. In this paper, experimentally determined fractional separation functions of different types of electret filters are presented for the particle size range from 10 nm to 10 , um at a filter face velocity of 10 cm/s. The initial efficiencies are compared to that of a conventional (glass fiber) filter and also to those of the electret filters in discharged state. The results of long-term filtration experiments show that a complex time-dependent behavior exists for different filter materials. Furthermore, two experimental methods are presented and discussed which determine the charge characteristics of single electret fibers.

Simulation of the growing process of a particle dendrite and evaluation of a single fiber collection efficiency with dust load

The growing process of particle dendrites on a fiber and the time dependency of a single fiber collection efficiency under dust loaded conditions were studied. Monte Carlo simulation technique was used to express the growing processes of particle dendrites on a fiber in Kuwabara's cell. Fairly good agreement in shape of dendrites was obtained with experimental observations. Single fiber collection efficiency was estimated from the simulation results and it was further correlated with the mass of deposited particles in unit filter volume. Through these calculations for various combinations of Stokes number and interception parameter, the ratio of a single fiber collection efficiency to that of a clean fiber was approximated to a linear function of deposited mass in unit filter volume. The coefficient of the linear function, which was named collection efficiency raising factor, decreases as Stokes number and interception parameter increase.