INSTABILITIES IN FLUIDIZED BEDS

This paper reviews recent advances in our understanding of the origin and hierarchy of organized flow structures in fluidized beds, distinction between bubbling and nonbubbling systems, and stages of bubble evolution. Experimental data and theory suggest that, at high particle concentrations, the particle-phase pressure arising from flow-induced velocity fluctuations decreases with increasing concentration of particles. This, in turn, implies that nonhydrodynamic stresses must be present to impart stability to a uniformly fluidized bed at very high particle concentrations. There is ample evidence to support an argument that, in commonly encountered gas-fluidized beds, yield stresses associated with enduring particle networks are present in the window of stable bed expansion, where the particles are essentially immobile until bubbling commences. However, some recent data on gas-fluidized beds of agglomerates of cohesive particles suggest that there exists a window of bed expansion where the bed does manifest a smooth appearance to the naked eye and the particles are mobile; at higher gas velocities the bed bubbles visibly. The mechanics of such beds remain to be fully explained.