Aerosol filtration in homogeneous and heterogeneous filters.

摘要

A theoretical analysis for the prediction of the performance of fibrous filters in removing submicron aerosol particles is developed. The method of local volume averaging is applied to the particle equation of motion to form the Darcy-scale transport equation. The closed form of the latter is obtained by solving numerically two closure problems in a unit cell of spatially periodic model of a fibrous filter. In order to predict the most penetrating particle size of the filter, the first correction term to the Smoluchowski equation is retained to take into account the particle inertia effects. Numerical simulations were carried out for a wide range of particle size (0.01--1 mum), air velocity (0.01--0.1 m/s), and filter solid fraction (0.04--0.15). The collection efficiency results from the present work compared favorably with other existing theories, and were in very good agreement with laboratory experiments. In the second part of this study, the impact of the structure inhomogeneity on the efficiency of fibrous filters is examined. The method of large-scale volume averaging is applied to the Darcy-scale transport equation, and a solution to a set of closure problems leads to the determination of the large-scale effective coefficients. Numerical simulations revealed that the internal structure heterogeneity engenders a diminution in the collection efficiency of the filter. Furthermore, the amplitude of this diminution depends strongly on the degree of inhomogeneity of the filtering medium. The results obtained showed that the thickness of a heterogeneous filter may have to be increased by as much as 70%, in order to compensate for the performance decline induced by the inhomogeneities.