Simulation of fluid particle separation in
realistic three dimensional fibre structures

The dependence of the functionality of a filter (pressure drop,
permeability, filter efficiency, Filtration ratio) on the
microstructure of fibrous filter media is of considerable practical
importance. Contrary to the filter mechanisms which are present
already for a single fibre (inertia, interception, Brownian motion)
and depend only indirectly on the microstructure via the flow field,
the sieving effect is mainly determined by the complex microstructure
of the filter. For Nonwovens this structure is highly irregular and
cannot simply be represented by single numbers such as the
porosity. Thus, it is necessary to understand the three-dimensional
structure of the Nonwoven and to know about the influence of geometric
variations (e. g., weights per unit area, fibre orientation, form of
the fibre) on the properties of the final product. In cooperation with
appropriate partners, the Fraunhofer ITWM has developed methods for a
three-dimensional image acquisition of textiles, a three-dimensional
image analysis for the extraction of appropriate parameters and the
three-dimensional modelling of textiles. These realistic models are
used as geometries for fluid dynamic simulations with the ITWMs own
parallel Lattice Boltzmann code. As a result, the flow field, the
pressure drop across the filter and the permeability are obtained and
are found to agree with experiments. To study the filtration
properties, we used a Lagrangian formulation of particle transport in
the calculated complex flow field. Depending on the size of the
particles, the microstructure of the filter and the form of the
fibres, we study the efficiency of sieving for particles large
compared to the fibre thickness.  Since controlled variations of
structural parameters like fibre orientation, form of the fibres,
spatially varying pore size distribution or gradients in the fibre
density are easily achieved within the simulation, our results
constitute a systematic and quantitative approach for the calculation
of the fluid particle separation in fibrous filters.

Keywords: nonwovens, models for complex fibrous microstructures,
Lattice Boltzmann simulation, Simulation of sieving effect, filter
efficiencies of complex geometries