Post-corona unipolar chargers with tuneable aerosol size-charge relations: Parameters affecting ion dispersion and particle trajectories for charger designs

TL;DR

This study investigates how various parameters influence ion dispersion and particle charging in unipolar aerosol chargers, enabling tunable size-charge relations for improved aerosol control and measurement.

Contribution

It introduces methods to control and optimize size-charge relations in unipolar aerosol chargers by analyzing flow dynamics and electric field effects.

Findings

Discrepancies in size-charge relations due to ion-aerosol mixing conditions.

Control of particle trajectory enables tuning of size-charge relations.

Operating conditions for constant mean charge on particles larger than 200 nm.

Abstract

This paper focusses on the mean charge per particle of monodisperse submicron aerosol, charged by diffusion of unipolar ions in post-corona discharge. It aims to confirm and discuss the limits of considering a single value of Ni$\times$t to describe aerosol charging and then to present methods to control the size-charge relation. Three aerosol chargers, with different mixings of ion and aerosol flows are investigated. Despite comparable ion sources with discharge currents of a few tens of $\mu$A, the size-charge relations differs from one charger to another due to different ion-aerosol mixing conditions and subsequent ion density along particles trajectories. Discrepancies are even more noticeable as the particle size increases. Discharge current, velocities of ion and aerosol flows and electric field control post-discharge ion density in each point of the charging volume. The control of particle trajectory in expanding unipolar ion cloud, leads to tuneable size-charge relations. Aerosol inertia and charging dynamics, that both depends on particle size, affects the Ni$\times$t experienced by the particle and thus the final charge of the particle. Operating conditions to reach a constant mean charge for particles larger than 200 nm are reported. Conclusions provide a basis to design aerosol chargers devoted to electric mobility selection for aerosol deposition, separation or electrical measurements especially to overcome the limits of mobility-to-size data inversion due to multiple charge ambiguity using diffusion chargers.