A fundamental study of aerosol filtration by fibrous filters in the transition flow regime

In this work, theoretical and experimental studies were carried out to investigate filtration mechanisms in the transition flow regime.; In the experimental study, the transition flow conditions were achieved by reducing pressures and making use of fine fiber filters with mean fiber size between 0.25 {dollar}mu{dollar}m and 0.83 {dollar}mu{dollar}m and packing densities between 0.053 and 0.08. Monodisperse DOP particles of 0.04 {dollar}mu{dollar}m to 0.45 {dollar}mu{dollar}m in diameter were generated using the atomization-electrostatic classification technique. Effects of multiply charged particles were taken into account by means of an analytical model. Filters made of fibers ranging from 0.15 {dollar}mu{dollar}m to 10 {dollar}mu{dollar}m were fabricated, and theoretical analysis was carried out to search for alternatives for improving the performance of fibrous filters. Experiments were made at reduced pressures, 0.1 atm {dollar}<{dollar} P {dollar}<{dollar} 1.0 atm. Results revealed that aerosol penetration through, and pressure drop across a fibrous filter are reduced significantly with decreasing pressure and fiber size, or increasing Knudsen number.; A new theoretical model was established using "flux/resistance" concepts. Single fiber efficiencies given by the flux/resistance theory are in general agreement with experimental data. The flux/resistance concept was also applied to determining the flow resistance of fibrous filters. The pressure drop given by our model was found to be slightly greater than that predicted by the Kisrch and Stechkina' semi-empirical formula, and in better agreement with experimental data.; A systematic study was carried out to examine the performance of the Condensation Nucleus Counters (CNC) at non-atmospheric pressure and non-standard flow rate. The theoretical model shows that the counting efficiency of the CNC varies as a function of pressure and flow rate. The response characteristics of the instrument was satisfactorily described by a dimensionless correlation. In this correlation, the counting efficiency was found to depend on a dimensionless parameter, {dollar}xispprime{dollar}, which combined the effects of pressure, flow rate and the geometrical dimensions of the condenser. Within a certain range of {dollar}xispprime{dollar}, the CNC performs with the maximal efficiency, which enables us to select the optimal design and operation parameters. A sensitivity study shows that the cut size of the instrument is most sensitive to the temperature difference between the saturator and condenser.