Maximizing the singly charged fraction of sub-micrometer particles using a unipolar charger

Particle charging via the mixing of aerosols with unipolar ions typically results in multiple charges on particles. Particle classification and sizing, based on the electrical mobility, ideally requires all the particles being singly charged to the performance enhancement. In this study, we explored the feasibility of maximizing the singly charged fraction of particles via the control of the N(i)t product in a unipolar charger. The feasibility was first investigated by modeling unipolar diffusion charging. It was found that the singly charged fraction of monodisperse particles could be maximized by the control of the N(i)t product. A corona-based unipolar charger was also constructed to study the maximization of the singly charged fraction of monodisperse particles. It was found that a wider range of ion concentration in the charging zone could be obtained by the variation of ion-driving voltage compared to that by changing the corona-discharge current. The maximum singly charged fraction of monodisperse particles in various sizes was characterized when the charger was operated at the flow rates of 1.5 and 3.0 lpm. It was evidenced that the current charger could be conditioned to achieve a higher singly charged fraction of particles than that by bipolar chargers in the particle size range of 20-200 nm, particularly in the ultrafine particle size range. The control of N(i)t product in the charging zone of a unipolar charger offers a simple and effective means to enhance the singly charged fraction of particles in a given size range. Copyright (c) 2019 American Association for Aerosol Research