An X-ray study of bubbles in fluidised beds

Gas-fluidised beds are characterised by the formation of bubbles which rise through them and determine the pattern of particle movement and gas flow within. The basic nature and shape of the bubbles are of interest not only in predicting the overall behaviour of the bed (from a gas-solids contacting point of view, for example) but also when considering the hydrodynamic characteristics of fluidised systems. In this paper the previous methods of studying bubbles are briefly discussed and the results are presented of observations of bubbles in cylindrical beds using the technique of X-ray cine photography. Measurements of the shapes, sizes, and velocities of bubbles in beds of spherical ballotini, irregular natural sand, and several other materials are given. It is observed that stable bubbles in all these materials have an essentially spherical form with the particle-wake region occupying the lower part of the sphere. Both the size of the particle wake and the bubble velocity depend upon the size and shape of the fluidised particles. Cine sequences showing the events (splitting and coalescence, for example) experienced by bubbles as they rise through the bed are presented and some statistical evidence for the frequency of these events is given. The size of bubble produced by a given volumetric injection of gas was measured. Only under special circumstances were bubble and injection volume equal.