Charging of one or several cylindrical particles in monoionized electric fields

In many electrostatic processes (precipitation of dust, separation of granular mixtures, spraying of powders) insulating particles are subjected to unipolar charging in the presence of other bodies. The present paper addresses this problem from both a computational and an experimental point of view, with the aim to extend an original numerical method of unipolar charge computation to the case of two or more particles, relatively close to each other. The charge acquired by cylinders of various dielectric constants was evaluated with a computer program based on the boundary element method of field analysis. A group of simulations concerned two infinite-length parallel cylinders spaced at various distances and with their axes defining a plane either parallel or normal to the direction of the electric field generated between two parallel plate electrodes. In another group of simulations, an insulating cylinder was situated between two equally spaced conducting cylinders, in a plane normal to the applied electric field. The experimental set-up simulated the charging conditions in a roll-type electrostatic separator. The unipolar space charge was generated by a wire-type electrode. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyvinyl chloride on a rotating roll electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward that the relative spacing between the particles changes the value of the saturation charge, an effect, which is stronger for particles of higher dielectric constant. These facts should be taken into account in the design of any electrostatic technology based on the corona charging of granular matter.