PIV investigations of flow structures in the fluidised bed freeboard region

The whole field velocimetric technique particle image velocimetry (PIV) has been used to produce vector maps of the gas-phase flow in the freeboard region of fluidised beds of square and round cross-sections. A characteristic toroidal vortex is generated as a single bubble erupts. In freely bubbling beds, there is a highly chaotic region immediately above the bed surface. At low fluidising velocities, the flow higher above the bed surface is characterised by a high velocity wall region and a near-stagnant core. The velocity profile slowly becomes more uniform at greater heights above the bed. At higher fluidising velocities, the velocity profile is more uniform from the outset. An expansion in freeboard bed area delays the onset of this pattern by concentrating flow in the core region.