Charging of cylindrical insulating particles in mono-ionized fields

The efficiency of many electrostatic technologies is correlated with the amount of charge acquired by particles of various shapes and sizes passing through ionized fields. Numerous papers have discussed the problem of ionic charging of spheres, which seem to approximate satisfactorily the actual particles in most industry applications. However, in certain technologies, such as separation and flocking, the particles are often cylindrical in shape, so that existing formulae cannot be used. The present paper addresses this problem from both a computational and an experimental point of view. The charge acquired by cylindrical particles of various dielectric constants was evaluated with an original computer program, based on the boundary element method of electric field analysis. The computed results show that the position of the particle respect to the electrodes changes the value of the saturation charge. The experiments were carried on a laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyethylene and polyvinyl chloride, when subjected to positive or negative corona generated between these electrodes and a rotating roll electrode connected to the ground. The experimental results, which were in good agreement with the theoretical predictions, put forward a particle self-discharge effect, at field intensities beyond a well-defined threshold. This kind of information may guide the design of the electrostatic technologies based on the corona charging of granular matter.