Particle size distribution modification during and after electrical charging: Comparison between a corona ionizer and a radioactive neutr alizer

Abstract

This article presents the results of an experimental investigation carried out to examine the changes of the size distribution of ultrafine particles that take place in aerosol electrical chargers. Two types of commercially available chargers have been studied: an unipolar corona discharge ionizer, and a bipolar radioactive neutralizer. Particle diffusion losses to the walls modify the size distribution in both chargers to approximately the same extent. Additionally, particles can grow by Brownian coagulation in the radioactive neutralizer if the aerosol particle number concentration is above about 107/cm 3. In contrast, the extremely small volume of the corona ionizer used in the experiments, 2 cm3, prevents coagulation even when the aerosol concentration is very high. For particle number concentration above 107/cm3 aerosol coagulation takes place downstream of any of these two chargers. The experimental results have been compared with those obtained by numerical integration of the population balance equations including charging, coagulation and diffusion losses. In spite that the coagulation rate between charged particles of opposite polarity is one to two orders of magnitude larger than the corresponding rate for uncharged particles, the numerical calculations have shown that the expected electrostatic enhancement of coagulation is negligible, probably because the fraction of charged particles in the radioactive neutralizer is too low for the particle size range studied.