A nonwoven fibrous filter media is modeled as a planar stochastic array of straight lines defining multiple polygons. The cumulative distribution and mean of the hydraulic diameter of these polygons is determined, and related theoretically to the "mean flow" pore diameter commonly measured by commercial partial flow testers. This model is tested against data obtained for a range of wetlaid glass microfiber handsheets and electrospun polymeric nanofiber webs. The results indicate that the simple two-layer model is inadequate. However, a statistically powerful correlation between "mean flow" pore diameter, total fiber length per unit area, and fiber diameter, is demonstrated to hold.

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