Rheology of a magnetically stabilized bed consisting of mixtures of magnetic and non-magnetic particles

Abstract

Hydrodynamic fluidization characteristics of an admixture bed consisting of 10 mass % iron (1416 μm) and 90 mass % copper (935 μm) are investigated under the influence of an external uniform magnetic field collinear with the gas velocity vector. The experiments are conducted at ambient conditions for superficial air velocity ( U g) varying in the range 0–4.0 m/s, and for ten values of the magnetic-field intensity ( H) in the range 4000-22 000 A/m. The magnetic field is created by a Helmholtz electromagnet and its spatial uniformity is established by detailed direct measurements. Bed-pressure-drop is measured as a function of increasing and decreasing U g, over a range of H values. These data are employed to determine the minimum fluidization ( U mf) and bubbling ( U mb) velocities as a function of H; and for the calculation of gross bed voidage (ϵ) as a function of U g. ϵ is found to increase from approximately 0.4 to 0.8 as U g is increased but does not exhibit any pronounced dependence on H. U mf is independent of H, but U mb is a weak function of H and increases with increase in H. Of particular interest is the observed bed fluidization behavior and its structural changes as U g is increased at a given H, and the changes in the nature of bed response as H is increased. At low H (5400 A/m) and U g values, iron particles form stalagmites at the bed surface but these become unstable as U g is increased. Increase in H increases stalagmite stability, number and penetration in the bed. Iron particles gravitate toward the central region, channels are formed in the bed, and iron clusters are formed in the outer bed region at larger U g values. At H = 9000 A/m, stalagmites are replaced with clusters of constant size, and at still greater H values larger stalagmites appear; the majority of iron particles are consumed in this structure with copper particles contributing to major agitation. The stability of the stalagmites increases with H, and at 22 000 A/m stalagmite formation wobbles in the bed and fountain type bubbling occurs from the central bed region.