An interparticle force model for ac–dc electric fields in powders

An interparticle force model is presented for ac–dc current flow in a semi-insulating powder. The model applies to contacting particles as in packed and fluidized beds as well as to particle stringers. The electrostatic model is based on simple lumped parameter circuit theory and correctly predicts the field–frequency trend observed for decreased bubble formation and increased bed height in a fluidized bed. Particle–particle and self-particle electrostatic interactions give two characteristic time constants resulting from the resistance–capacitance effects. The time constants are interpreted from single-particle and continuum relaxation theories. A second empirical force equation is presented by curve-fitting data over variable frequency and field strength for the demarcation between a frozen and fluidized bed. The two force models (theory and empirical) are shown to be of the same form in field–frequency dependence and predict bulk relaxation times that are comparable in magnitude. Numerical estimates for the time constants and other parameters for the powder bed are given.