Powder Technology, Vol.120, No.3, 199-215, 2001
Voidage profiles in magnetically fluidized beds
Voidage profiles in a fluidized bed of iron particles (230 mum) were investigated under the influence of an external uniform axial magnetic field. Passing a direct current through five solenoids generated uniform magnetic field. The five solenoids were arranged elaborately to get larger uniform magnetic space than that generated by Helmholtz electromagnet coils. A sensitive optical measuring system, based on detection of light reflected by particles, was used to measure local voidage in both dense and dilute phases. Local voidage was measured as a function of superficial fluidizing air velocity, magnetic field intensity and the position in the bed. At a given magnetic field intensity and at the same position in the bed, the voidage was constant for a low air velocity range (in a fixed bed). The local voidage changed irregularly with increasing air velocity for an intermediate air velocity range (in a magnetically stabilized fluidized bed, MSFB). The local voidage changed linearly with increasing air velocity for a slightly high air velocity range (in a magnetized bubbling fluidized bed, MBFB). A general correlation was developed to predict the local solids fraction at the arbitrary position in the bed: (1 - epsilon) = (1 - epsilon)(c) + [(1 - epsilon)(w) - (1 -epsilon)(c)](r/R)(B) where (1 - epsilon), (1 - epsilon)(c) and (1 - epsilon)(w) represent the local solids fraction at arbitrary position in the bed, at the bed center and on the bed wall, and B, (1 - epsilon)(c) and (1 - epsilon)(w) are the function of air velocity, distance from the distributor and magnetic field intensity.