Modeling and Validation of Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger

Abstract

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.

Developments in Unipolar Charging of Airborne Particles: Theories, Simulations and Measurements

“…In spite of its simple design, the specific design allowed relatively small diffusion and electrostatic losses since the small effective charging volume in comparison with other typical designs. As a consequence, the extrinsic charging efficiency attainable was higher than published similar devices of Chen and Pui (1999), Tsai et al (2010), Chien et al (2011), etc. for particle size in the range 5-10 nm while for smaller particles the charging efficiencies was slightly lower than those attainable chargers with sheath air to reduce particle diffusion losses.…”

“…Towards the practical application of numerical techniques, Chien et al (2011Chien et al ( , 2013) developed a two-dimensional (2D) numerical model to predict the charging process in real unipolar chargers with complex geometries, which contains the fully coupled calculation of flow field, electric potential and ion concentration fields, and the charging dynamics of airborne particles. Methods for calculating the fields is based on the work of Lin and Tsai et al (2010), while the charging model developed by Marlow and Brock (1975) was adopted to calculate the particle charging efficiency, and good agreement between the predicted and experimental charging efficiencies was obtained.…”

Response to Comment on “Modeling and Validation of Nanoparticle Charging Efficiency of a Single-Wire Corona Unipolar Charger”

“…However, Fern andez-D ıaz and Domat (2014) argued in their Comment that the geometry of the discharge electrode of the charger was a needle but not a wire, and hence the boundary condition for the ionic charge density at the discharge wire surface used in the simulation was not completely correct. They proposed a methodology to replace Equation (9) in Chien et al (2011) for improving the simulated results.…”

“…For the irregular internal geometry of the charging chamber, no simple mathematical models were available to describe the electrical field between the tip of the discharge and grounding electrodes until the numerical work conducted by Chien et al (2011). For the breakdown voltage and breakdown electric field in point-to-plane corona discharge, the breakdown voltage, V 0 (V), is expressed as (Adamiak et al 2005;Le et al 2013)…”

“…In Chien et al (2011), a detailed two-dimensional (2D) numerical model was developed to predict the flow, electric potential, ion concentration, charged particle concentration fields, charged particle loss, and charging efficiency in a single-wire corona unipolar charger. The numerical model is able to predict the experimental nanoparticle charging efficiency very well for particles smaller than 20 nm in diameter.…”