Analysis of electrodynamic fluidization

Abstract

Electrodynamic fluidization is a technique to generate suspensions of electrically conducting particles using electric forces to overcome their weight. An analysis of electrodynamic fluidization is presented for a monodisperse aerosol of non-coalescing particles of infinite electrical conductivity and negligible inertia suspended in a gas in the gap between two horizontal plate electrodes. A DC voltage is applied between the electrodes that charges the particles initially deposited on the lower electrode and leads to a vertical electric force that lifts the particles and pushes them upwards across the gap. The direction of this force reverses when the particles reach the upper electrode, pushing them downwards until they fall onto the lower electrode and repeat the cycle. Stationary distributions of particles are computed for given values of the applied voltage and the number of suspended particles per unit electrode area. Interparticle collisions play a role when the second of these parameters is of the order of the inverse of the particle cross-section or larger. The electric field induced by the charge of the particles opposes the field due to the applied voltage at the lower electrode and thus sets an upper bound to the number of particles that can be suspended for a given voltage. This bound is attained in the normal operation of a fluidization device, in which there is an excess of particles deposited at the lower electrode, and is computed as a function of the applied voltage. The predictions are compared to experimental results in the literature. A linear stability analysis for dilute aerosols with negligible collision effects shows that the stationary solution becomes unstable when the deposition threshold is approached with a number of suspended particles per unit electrode area larger than a certain critical value. A hydrodynamic instability appears near the lower electrode, where the electric force on a localized accumulation of charged particles leads to an upward gas flow that helps carrying the particles away from the electrode and increases the amplitude of the initial particle accumulation. The instability gives rise to electrohydrodynamic plumes whose dynamics involves collisions, mergers and generation of new plumes.