THE PRESSURE DROP ACROSS FIBROUS FILTER MEDIA IN THE PRESENCE OF ELECTRIC FIELDS (AEROSOL, POROUS MEDIA, PERMEABILITY)

Previous investigators of aerosol filtration using fibrous filter media have observed that the application of an electric field reduces the pressure drop across the dust-laden filter by 50% or more. The aim of this work was to (i) establish the mechanisms behind this pressure drop reduction, and (ii) develop a mathematical model which incorporates these mechanisms to predict the pressure drop across a fabric filter in the presence of an electric field.; The first goal was accomplished through a series of experiments which show that the electric field increases the permeability of the dust-laden filter by redistribution the dust along the surface of the fabric, by shifting the dust upstream to regions of higher porosity, and by changing the morphology of the dust cake.; Since fibrous materials are structurally anisotropic, and the presence of dust makes them more so, the effect of anisotropy on the permeability was studied theoretically and experimentally. A rigorous mathematical model was developed from which the permeability tensor for slow viscous flow through a dilute, anisotropic assemblage of cylinders can be computed, given the cylinder orientation distribution. The bulk equations of motion, to leading order in c (the volume fraction of cylinders), were shown to have the form of a singular perturbation problem with an "inner" region governed by the Stokes equation and an "outer" region described by the anisotropic Brinkman equation. An exact solution for the permeability was obtained for small c using the method of matched asymptotic expansions. This model: (i) shows that as c approaches zero the drag on a test cylinder within the array behaves as (ln(l/SQRT.(c)))('-1), (ii) discriminates between orientation distributions other models cannot, and (iii) predicts values for the permeability in good agreement with experimental results.; Finally, a comprehensive mathematical model was developed which, by representing the dust deposits as cylindrical resistance units, predicts the pressure drop across a fabric filter given the dust distribution, fiber distribution, and dust cake structure. The model shows that the changes in the dust deposition caused by the electric field result in a reduction of the pressure drop comparable to what is observed experimentally.