FILTECH 2022 in Cologne / Germany (March 8-10, 2022)

When the air in motion contacts with a solid surface, usually the air is thought to be approaching to a complete stop at the surface, and the velocity is zero relative to the solid surface. However, when the solid is so small that when the air passes the solid, only a fraction of the air molecules contact the surface. As the result, only those molecules have their velocities changed. The remaining air molecules remain their bulk flow motion. When we still consider the air as a continuum, the air velocity near the solid surface is not zero. That is the phenomenon called as slip flow.

The slip flow effect explains the low pressure drop for nanofiber filters because fewer air molecules exchange momentum with the fiber, there is less air drag on the fiber leading to less energy loss. Also because of the slip flow, the flow streamline is closer to the fiber surface and the single fiber efficiency of small particles is increased on nanofiber [1]. Therefore, the slip flow is a very important factor for nanofiber filtration. It must be considered when simulating the nanofiber filtration.

The slip flow was studied historically either with empirical equations or for very simple 1D or 2D structures, which cannot provide detailed information for a complicated 3D fibrous material structure. Cheng et al. [2] presented the approach to implement slip flow for 3D voxelized structures. In this approach, the expression of the slip velocity, which assumes the slip velocity proportional to the shear stress at the surface, is reformulated for a locally quadratic velocity profile instead of the standard linear profile, and reimplemented in our flow solver. The direct simulation of the slip flow is then possible, and it can be validated with analytic solutions.

The simulation results are compared to the measurement data for nanofiber filters. The influence of slip flow in nanofiber structures then is studied and the results of flow and filtration simulations are presented. Very Good agreement was observed between the experimental and simulation results.

[1]    Brown, R.C., Air Filtration: An Integrated Approach to the Theory and Applications of Fibrous Filters, Pergamon Press, Oxford, 1993
[2]    L. Cheng, S. Linden, A. Wiegmann, The influence of slip flow on Filtration simulations on the nano scale, Proceedings of Filtech 2019