Numerical simulations are an established tool for investigating the interaction between air flow and the transport and deposition of aerosols in filter nonwovens. The representation of the microstructure of the nonwoven fabric (pore spaces and fibers) in the computational grid makes it possible to assess the filter performance (flow resistance, fractional efficiency, lifetime) and to obtain valuable information for improving the design of the filter material.

Most simulation models assume that the nonwoven can be represented as a porous microstructure formed by individual fibers with (usually) different diameters. In reality, however, a certain proportion of the fibers forms bundles (Figure 1). This effect is process-related (e.g., meltblown process) and cannot be completely avoided. The formation of fiber bundles has a positive effect on the material properties: The bundles improve mechanical stability and increase the air permeability of the nonwoven fabric, since for a given fiber volume fraction, there are larger pore spaces than in the case of a microstructure consisting exclusively of individual fibers. On the other hand, larger pore spaces have a negative effect on filter efficiency.

The present work is devoted to a systematic study of the influence of the presence of fiber bundles on the filter performance of nonwoven media for air filtration. First simulations conducted using GeoDict software reveal, as expected, that the presence of bundles in fibrous structures leads to increased permeability and reduced filtration efficiency in comparison to a structure based on individual fibers.

In the simulated cases, the relative change of permeability varies from a few percent to several tens of percent, depending on bundle size (i.e., the number of agglomerated fibers) and the proportion of bundles within the structure (for a constant overall packing density). However, the impact on filtration efficiency appears to be ...