EFFECT OF THE ELECTRICAL CLAMPING FORCES ON THE MECHANICS OF PARTICULATE SOLIDS

The electrical clamping force plays a significant role in the mechanics of particulate solids, where an electric field is applied to a bed of particles in a dense-phase state, e.g., in the electrostatic precipitators, electrically enhanced fabric filters, electrofluidised beds, and the Electromechanical Valve for Solids. These forces are produced by the current flow through the bed, and are due to current constriction at contact points between the particles and between the particles and walls. In this paper, the mechanism of interactions between the electrical and mechanical forces is analysed. Various models of the electrical clamping force are used to transform the microscopic single-contact electrical force to its macroscopic representation in order to analyse the particle-wall and internal yield behaviour. Using the continuum mechanics theories, the electrical stress is superimposed on the mechanical stresses, while assuming that the yield locus is unaffected by the presence of the electric field. It is shown that this approach does not provide a satisfactory description of the bulk failure of the particulate solids, as the electrical stresses, obtained from the single-contact models, are substantially overpredicted when compared with the experimental data. A possible explanation for the discrepancy may be found in the underlying assumptions for the calculation of the electrical clamping force, where the deformation of the contact arising from the electric field is taken to be described by the Hertz analysis. Because of the sensitivity of the electrical force to the contact cap size, it is argued that Hertzian deformation gives rise to unrealistically large electrical clamping forces. Furthermore, based on the experimental observations of the formation of arches under the influence of the electric field, and the lack of any evidence for appreciable consolidation on the application of the electric field, it is suggested that the electrical clamping force modifies the contact frictional behaviour of the solids by increasing the cohesive/adhesive strength of the bulk, and hence its resistance to shear, without necessarily adding, to any appreciable extent, to the prevailing compressive stresses.