VOID FORMATION IN STATIONARY AND MOVING BEDS

Cavities can be created when a fluid is injected through a pipe into e ither a static or moving bed of solids. Here air injection into beds o f particulate solids of a number of sizes and densities has shown that a variety of cavity shapes can exist. The three types of behaviour fo und can be categorised in a manner similar to that provided by Geldart for fluidisation. For a static bed of particles, cavities formed by a n increase in gas flow were smaller than those formed by a decrease in flow. When the bed was moving, the cavity size was unaffected by the direction of change of the air how and was similar to that found for a decreasing flow with a static bed. The many physical parameters and c omplicated physics render a theoretical description difficult but some properties of the equations help to interpret behaviour. (C) 1997 Els evier Science Ltd.