RHEOLOGY OF A MAGNETICALLY STABILIZED BED CONSISTING OF MIXTURES OF MAGNETIC AND NONMAGNETIC PARTICLES

Hydrodynamic fluidization characteristics of an admixture bed consisti ng of 10 mass % iron (1416 mu m) and 90 mass % copper (935 mu m) are i nvestigated 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 th e 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 H elmholtz electromagnet and its spatial uniformity is established by de tailed direct measurements. Bed-pressure-drop is measured as a functio n of increasing and decreasing U-g, over a range of H values. These da ta are employed to determine the minimum fluidization (U-mf) and bubbl ing (U-mb) velocities as a function of H; and for the calculation of g ross bed voidage (epsilon) as a function of U-g. epsilon is found to i ncrease 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 par ticular interest is the observed bed fluidization behavior and its str uctural changes as U-g is increased at a given H, and the changes in t he 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 thes e become unstable as U-g is increased. Increase in H increases stalagm ite stability, number and penetration in the bed. Iron particles gravi tate toward the central region, channels are formed in the bed, and ir on clusters are formed in the outer bed region at larger U-g values. A t H = 9000 A/m, stalagmites are replaced with clusters of constant siz e, and at still greater H values larger stalagmites appear; the majori ty of iron particles are consumed in this structure with copper partic les contributing to major agitation. The stability of the stalagmites increases with H, and at 22 000 A/m stalagmite formation wobbles in th e bed and fountain type bubbling occurs from the central bed region.