Simulation studies of deposition mechanisms for aerosol particles in fibrous filters including slip flow

A. Wiegmann, K. Schmidt, S. Rief, L. Cheng and A. Latz. Fraunhofer-Institut für Techno- und Wirtschaftsmathematik, Kaiserslautern, Germany

Abstract 

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 Ám [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 Ám, 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 Ám
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.

Keywords 

Solid-Gas Separation, Simulation, Slip Flow, Nanofibers, Nanofiltration.


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