Intensive work on modeling and simulations at microscopic pore- and particle-scale, as well as on the macroscopic filter element scale, is carried out for various filtration processes in the Fraunhofer ITWM. The current talk will present some recent developments carried out at the micro-scale, namely, at the scale of the filter media. The combined effects of interception, inertia and diffusion are well known to govern the efficiency of fibrous filters. Their influence, however, varies with the geometric filter structure and with the process parameters. Even the most penetrating particle size, a simplified measure of filter efficiency, varies significantly around the generally assumed particle diameter of 0.3 µm [1]. The filtration simulation model [2] takes into account the filter geometry, flow velocity and the aforementioned filtration mechanisms. But previously, interception, inertia and diffusion were always seen as combined contributors. In order to study individual effects, the simulation software is now configured to disable one or several effects, and thus to measure the individual contributions in the simulation just as in the analytical studies in [1]. In the regime of fiber diameters below 1 µm, the standard no-slip boundary conditions on the fiber surfaces are not valid. We present the modification of the flow solver to allow for slip flow and compare it to the well-known Kuwabara model for cylinder arrays as well as to an analytic solution for channel flows. Finally, we illustrate how enhancing the simulation by slip flow and Cunningham correction for the Brownian motion of particles below 1 µm changes the predicted filter efficiency.
[1] A. Balazy and A. Podgorski, Theoretical and experimental study on the most penetrating particle size of aerosol particles in fibrous filters. Filtech 2007, Volume II, pp. II-192 - II-199, February 2007.
[2] A. Latz and A. Wiegmann, Simulation of fluid particle separation in realistic three dimensional fiber structures. Filtech Europa, Volume I, pp. I-353 - I-361, October 2003.