Single Fiber Efficiency Research Articles

Numerical simulation of non-spherical microparticles' deposition on single fiber

As a classical gas-solid two-phase flow system, the processes of fiber filtering microparticles are prevalent in nature and engineering. However, the impact of microparticle shape on fiber filtration processes is still largely unexplored. Herein, using the self-developed spheropolyhedral-based discrete element lattice Boltzmann method, the filtration process of non-spherical microparticles through a single fiber is investigated. Results show that the single fiber efficiency (SFE) for non-spherical particles exhibits a trend of initially increasing and subsequently decreasing trend with the increase in Stokes number (St), which is similar to the case of spherical particles. However, it is interesting to note that the peak values of SFE increase significantly with decreasing particle sphericity (ψ) and the corresponding St values become larger. As ψ decreases from 1.0 (sphere) to 0.671 (tetrahedron), the SFE increase from 0.205 to 0.49 and the corresponding St rises from 1.0 to 1.75. The enhanced SFE can be explained by elevated collision probability and adhesion probability, based on detailed particle kinematics and dynamics behavior analysis as well as microscopic depositional structure evaluation. The depositional structures of the non-spherical particles have larger capture areas, leading to higher initial collision probabilities. Meanwhile, the anisotropic collisions between non-spherical particles and fibers greatly contribute to higher secondary collision probabilities. In addition, compared to spherical particles of the same volume, the non-spherical particles experience greater fluid resistance, resulting in lower initial collision velocities and larger initial adhesion probabilities. The face-to-face contacts between non-spherical particles also lead to stronger interparticle adhesion and enhanced adhesion probabilities.

Theoretical and experimental study for enhancement of filtration performance of nonwoven fibrous media by nonuniform compression

In this study, the effect of uniform and nonuniform compression on the quality factor of filtration of nonwoven fibrous media has been investigated theoretically and experimentally. Given that the effect of nonuniform compression on filtration has not been reported, it is a new way for enhancement the performance of filters. The “quality factor”, which includes filtration efficiency and pressure drop, has been used to compare different types of filter media compression. Theoretical calculations have been performed using the classical single fiber efficiency theory. The results showed that with compression, the filtration efficiency increases only for a range of particle sizes. Experiments were carried out on 10 polypropylene fibrous filters with different compression ratios and types. For each sample, the efficiency and pressure drop were measured. It was found that the trend of changes in filtration efficiency due to compression is the same for the theoretical and experimental results. The experimental of uniform and nonuniform compression results showed that the filtration efficiency increases only for particles smaller than 3 μm. For particles smaller than 3 μm, the effect of compression on increasing efficiency increases. The increase is lower in nonuniform compression because some particles can escape from uncompressed spaces. Due to the lower pressure drop in nonuniform compression an increase in the quality factor is achieved in comparison to the uniform compression. The increase in the quality factor of the PP1 filter is up to 25.83%, and for the PP2 filter is up to 25.93%. It was also observed that with the increase of the compression ratio, the quality factor of the filter decreases.

Prediction of air filtration efficiency and airflow resistance of air filter media using convolutional neural networks and synthetic data derived from simulated media

In the absence of experimental air filtration test data, it is difficult to predict the air filtration efficiency and airflow resistance of filter media. In this novel work, an effort is undertaken to predict the air filtration efficiency and airflow resistance of filter media based solely on scanning electron microscopy (SEM) images of the filter fabric. The analytical single fiber efficiency (SFE) model is incorporated into a machine-learning model designed to analyze digital imagery. A convolutional neural network (CNN) is developed and optimized using synthetic training data produced from digital replication of air filter media. Simulated air filter media were created to replicate the physical characteristics of actual nonwoven nanofibrous air filter media. Digital grayscale images of the simulated media provided the input data for the CNN. Analytical calculations of efficiency and resistance based on the SFE model provided target data for the CNN. The regression model included seven convolution layers and two hidden fully connected layers along with width reduction and depth expansion methods. Twelve model parameters were optimized using training and validation data of 2100 iterations of simulated media. The model was then employed to predict the air filtration behavior of actual air filter media based on an SEM image of the media. The resulting predictions of air filtration efficiency and airflow resistance based solely on an SEM image suggests the potential viability of using synthetic data derived from simulated media to train machine learning models for real-world applications. To the knowledge of the authors, this is the first such application of a CNN to predict air filtration efficiency and airflow resistance using an SEM image, synthetic data created from simulated media, and the SFE analytical model.

Particle Collection of Electret Media under Different Filtration Pressures

Electret media have the advantage of enhancing the particle collection efficiency while keeping/reducing the media pressure drop by the electrostatic mechanisms in addition to the mechanical ones. This study investigated the effect of operational pressure on the filtration performance of electret filter media. Experiments was conducted at the pressures in the range of 0.33–3 atm and using DMA-classified particles ranging in electrical mobility sizes of 10–600 nm. The penetration of particles in the neutral, singly charged and Fuchs’ bipolar charge states of two electret media (both charged and discharged) were measured. The single fiber efficiency due to the induced and Coulombic forces, i.e., ηIn and ηC, were obtained. It is found that both efficiencies are a function of the filtration pressure (i.e., fiber Kn#) in addition to those parameters already included in existing models. A simple modification was also proposed for the existing model (based on the single fiber theory), enabling it to predict the particle collection efficiency of electret media operated under various filtration pressures.

Efficiency Analysis of Covid 19 Filtration with The Theoretical Approach Developed for Single Fiber Efficiency

It is necessary to understand the efficiency of the filters, which are the most important part of the ventilation systems, in order to hinder the spreading of the Covid-19 virus. In this article, a model was created using theoretical solutions developed for single fiber efficiency and with this created model, the previous theoretical data were confirmed. Then, the theoretical data in the model created with the experimental data were compared. Also, factors such as fiber diameter, particle diameter, or flow velocity affecting single fiber efficiency were changed and the effects of these parameters were investigated. As a result of this investigation, it was seen that, as the Peclet number increased, the single fiber efficiency decreased, and the efficiency of the single fiber increased when the particle diameter increased. Finally, when the solidity, which can be considered as the fiber density in the filter, increased, the single fiber efficiency increased along with it. When solidity was increased from 0.011 to 0.03, single fiber efficiency increases from 0.023 to 0.0269. That is, there was an increase of approximately 16.9 %.

Microstructure and particle filtration behavior of multimodal fibrous filter media made of fibres of different sizes for engine intake air filtration

This paper reports a set of theoretical expressions for describing the microstructure of multimodal fibrous filter media comprising fibres of different cross-sectional sizes along with the application of such expressions to predict the filtration performance of clean and clogging media for engine intake air filtration. It is found that the overall filtration efficiency corresponds very well with the classical theory of air filtration after suitably modifying the single fibre efficiency and mean fibre diameter. The square of mean fibre diameter is found to be a weighted harmonic mean of the square of fibre diameter of individual component. The single fibre efficiency of multimodal filter media, when expressed as a summation of single fibre efficiencies of individual component of fibres weighted by the corresponding surface area fraction, correlates very well with the experimental results. The evolution of pressure drop is explained satisfactorily based on Berman’s approach after suitably modifying the Davies equation.

Modeling Study on Oil Particle Filtration Performance of a Composite Coalescing Filter

The traditional theoretical model is not suitable for the simulation of the oil particle filtration process of a composite filter due to its huge difference in fiber diameter. In this paper, the concept of fiber dispersion σ was introduced into a mathematical model for describing the dynamic filtration process of a composite filter. The results show that the model is in good agreement with the experimental data. As the packing density is constant, the higher the fiber dispersion, the better performance is. In addition, the effect of different factors on the efficiency of different mechanisms was discussed. For fine particles (<0.1 μm), diffusion is the dominant mechanism. For coarse particles (>1 μm), the inertia impaction mechanism dominates the filtration efficiency. The fiber diameter has a significant effect on the inertia impaction mechanism. The single-fiber efficiency by inertia impaction mechanism is obviously affected by filtration velocity. The packing density has an effect on all mechanisms except for the diffusion mechanism. Moreover, such a model would contribute to an accurate prediction of the dynamic filtration performance of composite filters with polydisperse fiber diameter and improve performance by adjusting parameters reasonably.

FRACTAL-BASED MODEL FOR EVALUATING THE FILTRATION EFFICIENCY OF THE NON-WOVEN FIBROUS COMPOSITES

The classic single-fiber efficiency model based on ordered packing of fiber composites has been widely used to predict filtration efficiency of fibrous filters for 40 years. However, the simplified single-fiber model often overestimates the filtration efficiency as most fibrous composite filters are composed of randomly distributed fibers. The numerical methods have been successfully employed to re-construct the realistic fiber composites, but simulation of the multi-mechanism filtration process in the complex fibrous architecture is computationally expensive and case-based and derivation of a compact versatile model for broad applications remain challenging. In this work, a fractal-based homogenization model is developed to predict the filtration efficiency of the fibrous composites, by considering the spatially randomly distributed fibers and the quasi-random distribution of pore size. A comparison with available experimental results collected from references shows that the proposed model is of high accuracy in predicting the filtration of submicron aerosol particles. The calculated results show that the Kuwabara hydrodynamic factor is sensitive to the pore fractal dimension. The filtration efficiency for different particle diameters can be divided into three stages, which increase as the proportion of fibers warped by the fluid decreases. In addition, the total efficiency decreases with the increasing pore fractal dimension in a nonlinear trend. For the given fractal dimension, the filtration efficiency increases with the increase of pore size ratio, indicating that the more uniform the pore size distribution, the higher the efficiency.

Prediction of filtration performance of compressed fibrous media

Fibrous filter media is usually subjected to compression in practice. However, the effect of compression on the pore structure, flow field and filtration performance of heterogeneous compressed filter media has not been studied well. In this work, the filtration performance of two fibrous filter media with different compressions was investigated by experiments, classical single fiber efficiency theory and simulations. Artificial 3-D structures were established via GeoDict software based on the parameters of real uncompressed fibrous filter media. There was good agreement between the experimental and simulated results. For compressed filter media, it was found that the equivalent fiber diameter increased with increasing compression ratio. After compression, a shift in the pore size distribution and an increase in the channel tortuosity can be observed. The filtration performance varies for different compressed filter media. In addition, the filtration efficiency decreases as the angle between the fibers and the X-Y plane increases. There is a decrease in the penetration for structures with a broader fiber size distribution. Not only the real fiber size distribution and orientation but also the variation in fiber diameters with compression should be considered when predicting or calculating the performance of compressed filter media.

Filtration of Submicron Soot Particles, Oil Droplets, and their Mixtures on Single- and Multi-layer Fibrous Filters

The dynamics of filtration efficiency and pressure drop during the simultaneous filtration of soot and oil aerosols on single- and multi-layer filters were investigated, and the determined filtration efficiency was compared with the theoretical efficiency obtained via the classical filtration theory. Additionally, the influence of liquid-phase aerosols on the morphology of the formed deposits was investigated. It was concluded that the addition of oil aerosols decreased the filtration efficiency and lowered the pressure drop increase rate for multi-layer filters. Additionally, for the filtration of aerosols containing soot and high oil concentrations, once maximum filtration efficiency was reached, an efficiency decrease occurred. The system imperfection factor was proposed as a mean to predict the efficiency of multi-layer filters. The modified version of the single fibre efficiency method was used to calculate filter mass change with reasonable accuracy.

Liquid-mediated particle capture by nonwoven filter media for automotive engine intake air filtration

This article reports on development, characterization, and performance of liquid-treated nonwoven air filter media for automotive engine intake application. A polypropylene fiber-based needle-punched nonwoven fabric was prepared for treatment with four viscous liquids (glycerol, SAE 20W/50 engine oil, PEG 400, and deionized water) by liquid spraying technique. The filtration performance was evaluated in terms of initial and final gravimetric filtration efficiencies, fractional filtration efficiency, evolution of pressure drop, and dust holding capacity. The liquid-treated filter media registered higher gravimetric as well as fractional filtration efficiency and higher dust holding capacity as compared to the untreated ones. The initial and final gravimetric filtration efficiencies were found to be directly related to liquid add-on via a power law relationship. The liquid-treated filter media also exhibited higher fractional filtration efficiency than their untreated counterparts for all sizes of tested particles. Interestingly, the increase of fractional efficiency was more for smaller particles as compared to larger ones. This was explained quantitatively through single fiber efficiency due to adhesion. The viscosity of liquid was found to be a very crucial parameter as the dust deposition morphology was contingent to the flow of liquid onto the filter media. The stickiest liquid yielded highest filtration efficiencies, displayed slowest rise of pressure drop, and exhibited highest dust holding capacity.

Digital Twin Geometry for Fibrous Air Filtration Media

Computational modeling of air filtration is possible by replicating nonwoven nanofibrous meltblown or electrospun filter media with digital representative geometry. This article presents a methodology to create and modify randomly generated fiber geometry intended as a digital twin replica of fibrous filtration media. Digital twin replicas of meltblown and electrospun filter media are created using Python scripting and Ansys SpaceClaim. The effect of fiber stiffness, represented by a fiber relaxation slope, is analyzed in relation to resulting filter solid volume fraction and thickness. Contemporary air filtration media may also be effectively modeled analytically and tested experimentally in order to yield valuable information on critical characteristics, such as overall resistance to airflow and particle capture efficiency. An application of the Single Fiber Efficiency model is incorporated in this work to illustrate the estimation of performance for the generated media with an analytical model. The resulting digital twin fibrous geometry compares well with SEM imagery of fibrous filter materials. This article concludes by suggesting adaptation of the methodology to replicate digital twins of other nonwoven fiber mesh applications for computational modeling, such as fiber reinforced additive manufacturing and composite materials.

Effect of filtration pressure on the particle penetration efficiency of fibrous filter media

• Performance of fibrous filter media tested in the filtration pressure of 0.33–3.0 atm. • Single-fiber efficiency obtained by experiments compared to that by existing models. • A semi-empirical single-fiber efficiency formula for diffusive particles was proposed. The effect of filtration pressure on the particle collection performance of fibrous filter media was systematically investigated. The experiments were carried out in the filtration pressure range of 0.33–3 atm and using test particles in the electrical mobility size range of 20–400 nm. Two fibrous filter media: one made of glass fibers (GF) and the other made of metal fibers (SF), were selected for these tests. Our study shows that, at a face velocity of 0.1 m/s, by increasing the filtration pressure from 0.33 to 3 atm, the most penetrating particle size (MPPS) was slightly decreased, and the particle penetration was increased from 0.8% to 14% for media GF, and from 6.7% to 32.1% for media SF. In comparison to data calculated for existing single-fiber models, it was found that, for diffusive particles, the best fitting of available experimental data (i.e., those collected in this study and reported in others) was found under the assumption that the −1/3 power for the Péclet number is included in the single-fiber efficiency formula. Lastly, a semi-empirical single-fiber efficiency for diffusive particles is proposed to describe the reported experimental data.

Macroscale simulation of the filtration process of porous media based on statistical capturing models

In this work, a new model for filtration is developed using statistical capturing mechanisms. The model can be used in the macroscale simulation of filtration processes and can replace the current semi-empirical models based on the Single Fibre Efficiency. To validate the new model, firstly, simulation of the filtration process itself is implemented based on the methodology developed by Fotovati et al.(Separation and Purification Technology, 2012). Secondly, the novel statistical model for particle capturing was added in ANSYS Fluent by subroutines that calculate particle capturing and its influence on the membrane parameters. This includes recalculation of local properties of the membrane according to the collected mass and particle diameter to calculate the pressure drop with respect to the filter loading. The results from the numerical simulations were compared with experimental studies found in literature and the results are in good agreement with each other.

Theoretical Model of Single Fiber Efficiency and the Effect of Microstructure on Fibrous Filtration Performance: A Review

Fibrous filters are widely used in manufacturing and separation industries to remove aerosol particles produced in the production process. However, due to the complex internal structure of fibrous ...

Effect of Filter Inhomogeneity on Deep-Bed Filtration Process – A CFD Investigation

Aerosol filtration in fibrous filters is one of the principal methods of removal of solid particles from the gas stream. The classical theory of depth filtration is based on the assumption of existing single fiber efficiency, which may be used to the recalculation of the overall efficiency of the entire filter. There are several reasons for inappropriate estimation of the single fiber efficiency including the assumption of the negligible effect of the presence of neighboring fibers and perpendicular orientation of the homogenous fibers. This work aims to investigate the influence of mesoscale inhomogeneity on fibrous filter performance using Computational Fluid Dynamics in models of filters differed by internal structure.

Effect of mesoscale inhomogeneity and fibers size distribution on the initial stage of deep-bed filtration process

Aerosol filtration in fibrous filters is one of the principal methods of accurate removal of particulate matter from gas stream. The classical theory of depth filtration in fibrous structures is based on the assumption of existence of single fiber efficiency, which may be used for calculation of overall efficiency of the filter. There are several reasons for inappropriate estimation of the single fiber efficiency including assumption of negligible effect of presence of neighbouring fibers and perpendicular orientation of the homogenous fibers in filter structure. The aim of this work is to investigate influence of mesoscale inhomogeneity on fibrous filters performance using Digital Fluid Dynamics in models of filters differed by internal structure and to compare obtained results with experimental data. The influence of both, irregularity of structure and fiber size distribution on macroscopic filter performance (initial fractional efficiency, initial pressure drop and their time evolutions) is presented. The neglection of fiber size distribution in theoretical models of filtration efficiency leads to the underestimation of both, filtration efficiency and pressure drop.

Design and evaluation of a sintered metal fiber filter for gasoline direct injection engine exhaust aftertreatment

A novel metal fiber gasoline particulate filter (GPF) is designed and evaluated as a potential improvement to the traditional automotive wall flow substrate. A procedure based on single fiber efficiency theory and the Kuwabara flow model for fibrous filter media is developed to optimize filter design. The prototype design is derived from two constraints, namely to maximize PM removal efficiency while minimizing filter backpressure. Metal fibers are chosen that tolerate gasoline engine exhaust temperatures and pleating is used to fulfill size limits dictated by vehicle space constraints. Two prototypes are evaluated by vehicle and engine dynamometer testing. These tests reveal PM filtration efficiencies of greater than 78% for both the FTP and US06 drive cycles, with an average backpressure of approximately 1 kPa over a US06 drive cycle on a 2.0 L GDI light duty vehicle. Measured PM removal efficiencies at constant exhaust temperature and flowrate agree well with model predictions. Backpressure predictions agree withs measurements after accounting for GPF entrance and exit effects. The metal fiber GPF performance is discussed relative to the current state of the art wall flow filters and the limited literature data on metal foam filters. Whereas the fibrous media offers excellent efficiency and backpressure penalty, achieving surface areas comparable to wall flow substrates remains a challenge for practical implementation.

Filter media performance under the oscillating flow condition

This work investigated the performance of two mechanical glass-fiber and one electret filter media under the oscillating flow conditions (i.e., cyclic flows in this study). DMA-classified particles of 30, 50, 100 and 150 nm, and oscillating flow conditions with the Average Suction Flow Rates (ASFRs) of 6.4, 25.6 L/min and oscillating frequency of 6, 24, 40 min−1 were chosen for the experiments. It is found in general the particle penetration of test media increases with the increase of oscillating frequency. The above-observed penetration enhancement is found to be more pronounced for small particles (i.e., 30 and 50 nm) as compared to that for large particles (i.e., 100 and 150 nm). At the flow oscillating frequency of 24 and 40 min−1, the particle penetration through filter media is also higher than it under the corresponding constant flow conditions. The increase of ASFR increases the particle penetration of filter media for all the selected particle sizes. The apparent single fiber efficiency for media A and B under cyclic flow conditions was lastly found. The obtained efficiency enables the single fiber theory to calculate the average penetration of diffusive particles through mechanical filter media under the cyclic flow conditions.

Experiments on filtering nano-aerosols from vehicular and atmospheric pollutants under dominant diffusion using nanofiber filter

Nano-aerosols of size 100 nm and below are manifested everywhere in various urban micro-environments. In outdoor, the vehicular emission during traffic jam can release nano-aerosols with concentration above 200 million/m3 (200/cm3). Superimposed on vehicular emission are smog particles, around 10–50 nm, generated from the photochemical reaction of hydrocarbon and NOx in the presence of sunlight. These nano-aerosols, by virtue of their small sizes, can be inhaled readily into our bodies leading possibly to various chronic diseases. Air-borne viruses from influenza to epidemic viruses, which can lead to acute sickness and even death, are also in the same size range of 100 nm.Despite microfibers are being commonly used today in filters, there has been limited studies on filtration of real aerosols using microfiber filters, let alone nanofibrous filters, as most studies used simulated aerosols (e.g. sodium chloride) or test dust. Also, in standard sodium chloride test, only monodispersed aerosol size is allowed to challenge the filter. On the other hand, in reality aerosols of all sizes challenge simultaneously the filter. In this study, for the first time we have used nanofiber filter in a portable test filter set-up to filter polydispersed aerosols from the micro-environment near busy traffic area where aerosols comprise of both vehicular and atmospheric pollutants with size range between 10 and 400 nm. Real-time measurements are carried out with the portable test filter. The test results are compared to the theoretical correlation from Payet with diffusion correction at small Peclet number (Pe). We have found good comparison between test results with theoretical correlations using equivalent aerodynamic diameter for the aerosols with face velocity from 1 to 11 cm/s. A new dimensionless parameter, specific filter resistance (also established independently by Buckingham-π approach), is defined for the first time. It measures the flow drag on fibers to the amount of fibers present in the filter, the lower is the specific filter resistance the better is the filter. For our test condition, it is approximately 1.0–1.2. We have also improved the filtration efficiency of the filter by increasing the fiber basis weight by stacking up two layers of nanofibers. We have demonstrated that the efficiency increases while both the quality factor and specific filter resistance remain constant. We have also investigated the single fiber efficiency due to diffusion and found that at low velocity and low Peclet number (Pe < 10), the test results agree well with the theoretical prediction. On the other hand, at higher face velocity (>0.07 m/s) and large Peclet number (>10), there is small deviation from the theory, which is probably due to the smaller aerosols collecting by the larger aerosols upstream of the filter with an airstream consisting of polydispersed aerosol distribution challenging the filter. This has not been realized previously as tests conducted were mostly using monodispersed aerosol size distribution for which aerosol-aerosol interaction was absent.