A single-wire corona unipolar aerosol charger with a sheath air to avoid particle loss was designed and experimental charging efficiencies were obtained at a fixed aerosol flow rate of 1 L/min using monodisperse silver nanoparticles of 2.5 to 20 nm in diameter. The charger has a cylindrical casing of 30 mm in inner diameter in which a gold wire of 50 µm in diameter and 2 mm in length is used as the discharge electrode. A two-dimensional (2-D) numerical model was developed to predict nanoparticle charging efficiency in the unipolar charger. Laminar flow field was solved by using the Semi-Implicit Method for Pressure Linked Equations (SIMPLER method), while electric potential and ion concentration fields were solved on the basis of Poisson and convection-diffusion equations, respectively. The charged particle concentration fields and charging efficiencies were then calculated on the basis of the convection-diffusion equation in which ion-particle combination coefficient was calculated by Fuchs diffusion charging theory (Fuchs, N. A. (1963). On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere. Geophys. Pura. Appl., 56:185-193). Good agreement between predicted and experimental extrinsic charging efficiencies was obtained. Numerical results showed the advantage of using sheath air to minimize charged particle loss and indicated the location where major charged particle loss occurred. It is expected that the present model can be used to facilitate the design of more efficient corona-wire unipolar charger in the future.

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The previous analytical solution for the drag coefficient (C d ) for a spherical particle attached on the flat surface, which was derived by O'Neill (1968), is only valid in the creeping flow conditions. It is important to extend O'Neill's formula to cover a wide range of particle Reynolds number (Re p ). In this study, the drag coefficient was calculated numerically to cover Re p from 0.1 to 250. For a particle suspended in the air, an empirical drag coefficient exists, which is defined as C d = f × 24/Re p , where f is a correction factor depending on Re p . The applicability of the correction factor f for O'Neill's analytical equation for the spherical particle attached on the flat surface for Re p = 0.1 to 250 was examined in this study.