Effect of particle size polydispersity on the yield stress of magnetofluidized beds as depending on the magnetic field orientation

Abstract

This work analyzes the influence of particle size polydispersity and shape, as well as orientation and strength of an externally imposed magnetic field, on the rheology of gas-fluidized beds of fine magnetizable particles. Samples consisted of nearly spherical magnetite beads and irregularly shaped steel particles of same average mean diameter but different particle size distributions and magnetization properties. The application of an external magnetic field to the unstable bubbling bed confers it a solid-plastic behavior suppressing the growth of large bubbles. The yield stress, the permeability to gas flow and the gas velocity at the jamming transition of the stabilized magnetofluidized beds (MFBs) have been measured. Steel MFBs have significantly larger values of the yield stress than magnetite MFBs particularly when the gas flow and magnetic field directions were parallel (co-flow configuration). Visual observations and permeability data shows that polydispersed steel particles arrange into more porous and expanded structures than magnetite beads by the application of co-flow magnetic fields. In the cross-flow configuration (when the external magnetic field is perpendicular to the vertical gas flow), it is observed just a moderate enhancement of the magnetic yield stress of steel MFBs as compared to magnetite, which is explained by a larger misalignment between the steel chained particles and the horizontal magnetic field. Finally, a theoretical model has been used to reproduce the observed trends of the magnetic yield stress by taking into account details on the interparticle magnetic forces and the microstructure arrangement.