An Investigation of Aerosol Filtration via Fibrous Filters

Abstract

The most common method of removing particles from a gas stream is via fibrous filters. However, most of the previous studies have been limited to systems consisting of rows of fibers (often in two-dimensional geometries) perpendicular to the flow direction. The current work is aiming to develop an understanding of the role of filter?s microstructure and manufacturing process.

In the first part of this study, pressure drop and nanoparticle collection efficiency of lightweight spun-bonded media are simulated by solving the Navier-Stokes equations inside three-dimensional geometries resembling the microstructure of such media. These pressure drop and collection efficiencies showed a perfect agreement with experimental data.

In the second part of this work, the influences of fiber length and compaction ratio of filter media on the pressure drop are discussed. Simulation data of staple fiber media have shown good agreement with Davies? empirical equation. Such an agreement indicates that, within the range of dimensions considered, the fiber length has no significant influence on the materials? through-plane permeability as long as the SVF remains constant. Our simulation results for nonwovens with different compaction ratios, together with our experimental data, indicate that pressure drop of the porous media increases with increasing the compaction ratio or temperature of the calender rolls.

In the third part of this work, we presented our approach for modeling permeability of fibrous filters with bimodal fiber size distributions (referred to as bimodal filters in this context). The three-dimensional microstructures resembling bimodal filter media with random in-plane fiber orientation distribution were generated to compute their permeability constants. These results were compared with the previous analytical and numerical models as well as our experimental data. Here we concluded that there exists an area-weighted equivalent average diameter for each bimodal filter that can be used in the existing expressions for calculating the permeability of unimodal filters.

The last part of this thesis is dedicated to studying the permeability woven fabrics. Concerned with the accuracy of the homogeneous anisotropic lumped model of Gebart (1992) for predicting the permeability of multifilament fabrics, we devised a series of numerical simulations conducted in full three-dimensional geometry of idealized multifilament woven fabrics wherein the filaments were packed in Hexagonal arrangements. While a relatively good agreement was obtained, our results indicate that Gebart?s model underestimates the permeability of multifilament fabrics at high yarn?s solid volume fractions. We also simulated the pressure drop of monofilament woven fabrics under tension where we observed a logarithmic relationship between the discharge coefficient and the Reynolds number of the flow.