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 fluidization 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 cannot be predicted from continuum theories that contain a mutual drag force dependent only on the relative velocity and void fraction. Several hypotheses about additional forces are used to derive theoretical values of the decompression wave velocity which are compared with data. The three most successful hypotheses, which are shown to be roughly equivalent at the higher wave speeds, include: a force proportional to the second derivative of void fraction; a discrete averaging method over distances scaled by particle size; and a modification to the drag force using the geometrical relationship between area fraction and number density.