Pergamon PIh S0021-8502(96)00462-4 J. Aerosol Sci. Vol. 28, No. 4, pp. 599-611, 1997 ~) 1997 Elsevier Science Lid Printed in Great Britain. All rights reserved 0021-8502/97 $17.00 + 0.00 COAGULATION OF SYMMETRIC AND ASYMMETRIC BIPOLAR AEROSOLS Srinivas Vemury,* Christian Janzen ~ and Sotiris E. Pratsinis * Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171, U.S.A (First received 25 March 1996; and in final form 1 August 1996) Abstraet~Coagulation of charged aerosols is investigated by developing a sectional numerical model for both particle size and charge distributions and accounting for electrostatic dispersion. Symmetric bipolar charging increases the coagulation rate when particles are highly charged. Asymmetric bipolar charging results in a faster decrease in the particle number concentration, but does not actually result in larger particles as the decrease in number concentration is dominated by the electrostatic dispersion and not by the coagulation of particles. The performance of a simpler model in the literature is evaluated by comparing its predictions against those of the above sectional model. The simple model is most accurate for initially mildly asymmetric bipolar aerosols at short residence times. © 1997 Elsevier Science Ltd. B Cc dp e kB Nf N+ N_ P,q t T W Greek Y,.j P Gg NOMENCLATURE mobility of particles of size i and charge p, g s- 1 Cunningham slip correction factor, dimensionless particle diameter, cm elementary charge, 4.8 x 10 -~°, statC Boltzmann constant 1.38 x 10-16, erg K- 1 number concentration of particle of size i and charge p, cm - 3 number concentration of positive charges, cm - 3 number concentration of negative charges, cm 3 number of charges per particle, dimensionless time, s temperature, K particle volume, cm 3 Fuchs stability function, dimensionless letters coagulation coefficient of charged particles, cm 3 s- 1 coagulation coefficient of neutral particles, cm 3 s- 1 viscosity of the surrounding gas medium, g cm s- geometric standard deviation, dimensionless INTRODUCTION Coagulation of charged particles affects mechanical dispersion of powders, atomization of solutions or burning of metals, as aerosols generated in these routes are highly charged (Adachi et al., 1981). The diffusion and field charging of aerosol particles has gained importance in recent years, particularly in connection with the design of electrostatic precipitators for the collection of fine particles (Adachi et al., 1985). In gas cleaning systems, unipolarly charged small particles can be easily captured by oppositely charged large particles (Eliasson et al., 1987; Eliasson and Egli, 1991). Another application of charging of particles is in the measurement of particle-size distributions. Electrical mobility analyzers exploit the differences in the electrical mobility of charged particles of different sizes for measuring their size distributions (Adachi et al., 1985). Gutsch and L6fIter (1994) charged NaC1 particles using a bipolar corona discharge and found that the agglomerate size * Currently at Lucent Technologies, Murray Hill, NJ 07974, U.S.A. * Currently at Lehrstuhl fur Verbrennung und Gasdynamik, University of Duisburg, Duisburg 4100, Germany. Author to whom all correspondence should be addressed. 599