The yield stress of jammed magnetofluidized beds

Abstract

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during