Mechanisms of fibrous filtration

This thesis is concerned with the removal of particulate material from gases using fibrous filters. described under three headings:- (a) The Collision Efficiency The particle fibre collision efficiency is calculated by computing trajectories in the Davies and Kuwabara flow fields. Electrostatic and gravitational field forces are taken into account. The influence of fibre Knudsen number and Reynolds number on inertial interception is predicted. A model is described which takes into account a log normal distribution of fibre spacing in a filter. It is used to predict the pressure drop across a random fibre mat and its mean efficiency of inertial interception both of which are a factor of two or three less than predicted by the simple Kuwabara model. (b) Particle Retention Mechanisms' It is shown that bounce is the only significant mechanism responsible for particle non-adhesion in fibrous filters. An equilibrium model is used to predict the critical particle size above which adhesion fails. The behaviour of filters in the low adhesion region is examined by measuring the collection efficiency of model filters using narrow sized fractions of dust. The efficiency is a decreasing function of particle size and velocity, trends which agree with the equilibrium model. (c) Non-stationary Filtration The behaviour of filters under load is examined experimentally. The efficiency may either increase or decrease initially with loading, the characteristics depending on the same factors which influence the single fibre efficiency.