DECOMPRESSION WAVES IN FLUIDIZED-BEDS

A packed bed of particles may be held against the permeable roof of a fluidized column by a fluid flux that is several times the minimum flu idization flux. When the fluid flux is sufficiently reduced, particles rain down and a decompression wave propagates into the packed bed. We report new data of the velocity of this wave, summarize previous work and compare with new analyses. The speed of the decompression wave ca nnot be predicted from continuum theories that contain a mutual drag f orce dependent only on the relative velocity and void fraction. Severa l hypotheses about additional forces are used to derive theoretical va lues of the decompression wave velocity which are compared with data. The three most successful hypotheses, which are shown to be roughly eq uivalent at the higher wave speeds, include: a force proportional to t he second derivative of void fraction; a discrete averaging method ove r distances scaled by particle size; and a modification to the drag fo rce using the geometrical relationship between area fraction and numbe r density.