Coagulation of bipolarly charged ultrafine aerosol particles

Particle charging during coagulational growth is widely used in material synthesis processes as well as with industrial particle removal equipment. Coagulation behavior of charged particles is significantly different from that of neutral particles. To calculate the change in size/charge distribution of particles undergoing bipolar coagulation, a two-dimensional sectional model has been usually used. This method, however, needs considerable computation time although it gives very accurate prediction. In this study, the moment model, to solve the bipolar coagulation problem in the free-molecule regime, was developed to provide a time-efficient tool. Simultaneous particle charging by bipolar ions was also considered in this study. The developed model is based on the assumption that particles cannot have more than one unit charge and the particle size distribution remains log normal. The developed model was compared to the two-dimensional sectional model, with good agreement being shown. Some characteristics of bipolar coagulation were investigated using the developed model. The bipolar coagulation with simultaneous bipolar diffusion charging was shown to significantly increase the coagulation rate compared to the neutral Brownian coagulation. It was also shown from the simulation results that if one needs a higher coagulation rate in the initial stage, bipolar coagulation without ions is recommended, while bipolar coagulation with simultaneous charging by bipolar ions should be used if one wants a high coagulation rate for a long time.