Deposition mechanisms and the efficiency of fibrous filters

Fibrous filters generally consist of many threadlike fibres oriented more or less normal to the fluid flow which passes through them. Particles carried by the fluid flow may impact upon the fibres and become removed from the flow. The collected particles accumulate forming complex structures which influence the fluid flow and further deposition. Understanding the process of particle deposition and its effects upon further deposition are crucial in understanding the performances of fibrous filters. We have been developing a numerical model of fibrous filtration aimed at investigating deposition due to various mechanisms and the effect filter properties and particle characteristics have upon it (Dunnett and Clement 2006, 2009). We have shown that deposit porosity does not influence further deposition for small particles where diffusion is dominant, but that deposition by interception can be strongly enhanced at high porosity. In this paper we examine which deposition mechanism is expected to dominate for a given particle size and flow velocity, and calculate the deposition efficiency as the deposit builds up.