Particle Size Distribution Modification During and After Electrical Charging: Comparison between a Corona Ionizer and a Radioactive Neutralizer

M Alonso,F.J Alguacil

doi:10.4209/aaqr.2008.07.0029

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 10 7 /cm 3 . In contrast, the extremely small volume of the corona ionizer used in the experiments, 2 cm 3 , prevents coagulation even when the aerosol concentration is very high. For particle number concentration above 10 7 /cm 3 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.

Highlights

It is important to quantify the changes in the aerosol particle size distribution that occur in an electric charger, specially when
The particle size growth rate in unipolar chargers should be smaller since the fraction of unipolarly charged particles can be very high; for instance, practically all the particles with diameter above 15 nm can acquire a charge of the same sign in the corona ionizer we have used in this work
In the case of the corona ionizer, losses are due to three mechanisms, namely, Brownian diffusion, direct electrostatic deposition, and electrostatic dispersion or mutual repulsion between the unipolarly charged particles that are been generated during the charging process

Summary

Introduction

It is important to quantify the changes in the aerosol particle size distribution that occur in an electric charger, specially when (space-charge), and, if there is an external electric field, by electrostatic deposition as well.The extent to which any of the above mentioned particle loss mechanisms occur depends on the type of charger. (It is assumed that the presence of a charge in one of the particles does not affect the coagulation rate.) In equation (5), cij (ci2 c2j )1/ 2 where c j 8kT / Sm j is the mean thermal speed of the particle of size Dp j

Results

Conclusion