T. Penner, J. Meyer, A. Dittler

Journal of Aerosol Science, January 2021, volume 151, 105674, https://doi.org/10.1016/j.jaerosci.2020.105674 (open access)

Structural properties in nonwoven materials, such as porosity and layer thickness, are typically spatially distributed on a micro- and mesoscale. The mesoscale inhomogeneity is visually perceived, inter alia, as the cloudiness of the nonwoven fabric or as inclusions in the bulk material. Differential box counting, which is a fractal based image analysis method, was used to assess the inhomogeneity of several wet laid nonwoven glass fiber filter media. It has been shown that the subjectively perceived extent of media inhomogeneity strongly correlates with the lacunarity of the analyzed image.

The analyzed media were then tested in different configurations for their operating behavior in the filtration of oil mist. The inhomogeneity of a medium significantly affected the formation of liquid channels in the medium, more specifically the inhomogeneity of the media layer on the filter upstream side. As most droplets are deposited on the filter upstream side, liquid likely accumulates in regions of finer pore structure in the foremost media layer. Distinct oil channels that transport the coalesced liquid to the filter rear side subsequently form behind these regions. Homogeneous media that lack such regions show a depth-filter like increase of Δp before the formation of channels, i.e. the Δp curve shows an increasing gradient with filter loading, which then changes into a linear increase of Δp. When filtering at low velocities, the liquid distribution inside the media drastically changed from distinct channels to a conically tapering form towards the filter rear for a homogenous medium and a system of many interconnected channels in an inhomogeneous medium.