Filtration of Unipolarly Charged Aerosol Nanoparticles with an Initially Discharged Dielectric Screen

Abstract

This paper presents experimental results of penetration of nanometer-sized aerosol particles through an initially discharged dielectric screen. Experiments have been carried out with two types of monodisperse unipolarly charged particles having different dielectric constants (Ag and NaCl) and mobility-equivalent particle diameters between 2 and 10 nm (Peclet number between 4 and 80). At the very beginning of the process, the screen is uncharged and filtration is controlled by particle diffusion. As the number of charges on the screen increases due to the diffusional deposition of charged particles, an electric field of increasing strength is developed between the dielectric screen and the conductive metallic walls of the cylinder where the screen is placed. As a result, particles are also driven and lost to the walls and penetration decreases. This transient process can be well described by two dimensionless parameters, namely the Peclet number and a nondimensional number expressing the ratio of the particle drift velocity due to the field to the particle convective velocity due to the flowing medium (air).