Experimental investigation and theoretical modeling of inhomogeneity in filter media

Theoretical modeling of filter behavior is required for predicting media performance and for improving filter designs. Initial efforts to model the filtration process assumed the filter media to be homogeneous, i.e ., all fibers sized and spaced uniformly. These simplified theories are seen to be inadequate in predicting the media filtration characteristics and the inherent filter inhomogeneities have to be taken into consideration.;The deviation of the experimental filter efficiency values from the theoretical predictions are seen to be a function of the operating conditions and media type. This is not accounted for in the classical approach to model inhomogeneity. The different inhomogeneities in a media---i.e., non-uniform packing density distribution, polydisperse fiber diameters, and three-dimensional fiber structure---must be taken into consideration in developing a filter model for inhomogeneous media.;The packing densities in a commercial filter media are seen to be non-uniformly distributed. These variations in packing density affect the local velocities and hence the filtration characteristics. A filter scanner has been built to study the effect of packing density variations on local filter efficiencies as a function of the operating conditions. The particle counts are obtained over small regions of a filter and the count distribution is obtained by translating the filter along the different directions. Based on the observed packing density distributions, a simplistic statistical theoretical model is developed. The effect of the packing density distribution on local velocity and efficiency distributions are obtained and the total efficiency of a inhomogeneous media is calculated based on the filter macroscale properties.;The fiber drag values and a monodisperse estimate of the fiber diameter distribution in the filter are required for use with the theoretical model. The inhomogeneity in the media result in experimental fiber drag values lower than the theoretical predictions. To obtain an accurate estimate of the filter fiber drag, an asymmetric three-dimensional model is developed. The fiber drag values are calculated as a function of the inter-fiber distance ratio, packing density distribution and fiber diameter polydispersity.;These fiber drag values are seen to be closer to the experimental predictions for real filter media. A drag-equivalent fiber diameter is defined based on the calculated fiber drag expressions and measured pressure drop values. Comparisons with experimental results indicate that the calculated diameter values are close to the experimental observations.;The calculated fiber drag and drag-equivalent fiber diameter values are used with the statistical theoretical model to obtain the distribution of local efficiencies. The total filter efficiency values are then calculated and seen to correlate well with the total efficiency measurements of the different media.