Bubbles were provoked in a gas fluidised bed by injecting a pulse of air and were then observed by X-rays. For a given quiescent bed, the volume of bubble produced, VB by injecting a volume, Vi, was found to be described by, where A is a constant of proportionality. The initiating volume, Vi,, decreased with increasing bed flow rate whilst the constant A generally increased. With one of the two materials used (Ballotini), both constants, the bubble shape and, to a slight degree, velocity depended on the previous history of the bed. In spite of these differences it is concluded that, although it may be difficult to predict the size of bubble produced by injection, once generated it behaves in a very similar manner to a spontaneously formed one.
MANY STUDIES of bubbles in gas fluidised beds have been made by observing singly injected bubbles and their effects . This technique makes it possible to study single bubbles at will and has led to considerable understanding of their nature. In most important respects injected bubbles behave in a similar way to those that form spontaneously from excess overall gas flow but in some matters of detail, such as size, shape and velocity, they may be a little different. Certainly the size of bubble produced from a given injection can vary considerably depending upon the overall flow rate through the bed (Ref. [l]). It was the object of this investigation to examine some of the characteristics of injected bubbles, how they varied with the bed conditions and, where possible, to compare them with spontaneously formed bubbles.