Effect of the Fibre Diameter Polydispersity on the Permeability of Nonwoven Filter Media

This paper studies the permeability values of air filter media obtained by 3-D simulations using the GeoDict (R) code. 3-D fibrous structures with different specific characteristics that can be encountered in air filtration (0.03 <= solid volume fraction [alpha] <= 0.25, monodisperse fibres [1 mu m <= df <= 8 mu m], or polydisperse fibres) were generated. For monodisperse fibres, the permeability values obtained were compared with various correlations identified in the literature. After confirming that Davies' or Jackson and James' relations allowed a good estimate of the permeability, it is shown that the modified Happel's correlation provides a better prediction. In the case of normal (standard deviation: sigma <= 1.5) or lognormal fibre size distribution (geometric standard deviation: sigma G <= 2), this modified Happel's correlation, in which the fibre diameter is replaced by an equivalent fibre diameter, leads to a relative deviation of less than +/- 8% and +/- 4% for lognormal and normal fibre distributions, respectively. The comparison with experimental permeability values obtained on real media provides quite encouraging results.