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UBC Theses and Dissertations

Fluid mechanics of high velocity fluidised beds Brereton, Clive


This thesis project studied a number of aspects relating to the fluid, mechanics of circulating fluidised beds. Studies of the macrostructure of a 9.3 m high x .15 m dia. riser showed a strong dependence of one important macroscopic descriptor, the density profile, upon the geometry of the gas/solids exit and the location of the solids return. It was found that abrupt exits promoted inertial solids separation from the conveying gas which generated strong internal circulation patterns and high slip velocities. Microstructural studies, in support of the macrostructural investigation, and using a needle capacitance probe, showed how the radial density profile develops with height causing a gradual density decay. The structure, characterised by an "intermittency index," was strongly radially non-uniform at all locations in the lower regions of the column with pronounced aggregation or clustering at the highest densities. However, the cluster-like structures present at the base rapidly gave way to a more dilute core-annular type flow slightly further up the column. This radially non-uniform structure was used to explain a number of macroscopic phenomena. These included the effects of exit type, solids return location, secondary air addition and gas mixing. The results of the various studies, drawn together, allow fast fluidisation to be defined tentatively with respect to its relationships to choking, pneumatic transport, and other fluidisation regimes. Separate studies were performed to examine gas mixing and the transition to turbulent fluidisation. The gas residence time distribution was found to be substantially different from plug flow and could be characterised crudely by a two-zone model. The turbulent transition was found to be gradual, but nonetheless a transition, although a developed turbulent zone did not exist until well beyond transport conditions.

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