Stabilization of gas-fluidized beds of magnetic powders by a cross-flow magnetic field

Abstract

In this paper we present an experimental study of the stabilization of gas-fluidized beds of magnetic powders by application of a cross-flow magnetic field. The powders tested consist of magnetite and steel powders in a range of particle size dp between 35 and 110 μm, allowing us to investigate the effect of particle size and material properties on magnetic stabilization. In the operation mode employed by us the magnetic field is applied to the unstable bubbling bed and the gas velocity is slowly decreased. According to our observations, the bed is stabilized at a critical gas velocity by the jamming of particle chains formed during bubbling because of the attractive forces induced between the magnetized particles, which are thus responsible for stabilization. Although the magnetic field is applied in the horizontal direction, these chains are mechanically stable at orientations close to the gas flow direction, in agreement with the prediction of an unconfined chain model based on the balance between gas flow shear and interparticle magnetic force fm. Since fm is increased as dp is increased, the critical gas velocity at marginal stability vc for a fixed field strength B is seen to increase with dp. As the gas velocity v0 is decreased below vc, there is a rearrangement of the structure depending on particle size. Restructuring of the bed depends on particle size as derived from measurements of its permeability to the gas flow, which causes the yield stress to be a function of particle size. It is also inferred from our results that natural agglomeration of fine particles (in the absence of a magnetic field) due to van der Waals forces enhances the yield stress of the magnetically stabilized bed. From our experimental results it is concluded that structural effects, as affected by operating conditions and material properties, play a main role in the rheology of the stabilized magnetofluidized bed (MFB).