INSTABILITIES AND THE FORMATION OF BUBBLES IN FLUIDIZED-BEDS

As is well known, most gas-fluidized beds of solid particles bubble; that is, they are traversed by rising regions containing few particles. Most liquid-fluidized beds, on the other hand, do not. The aim of the present paper is to investigate whether this distinction can be accounted for by certain equations of motion which have commonly been used to describe both types of bed. For the particular case of a bed of 200 mu m diameter glass beads fluidized by air at ambient conditions it is demonstrated, by direct numerical integration, that small perturbations of the uniform bed grow into structures resembling the bubbles observed in practice. When analogous computations are performed for a water-fluidized bed of 1 mm diameter glass beads, using the same equations, with parameters modified only to account for the greater density and viscosity of water and to secure the same bed expansion at minimum fluidization, it is found that bubble-like structures cannot be grown. The reasons for this difference in behaviour are discussed.