PROPAGATION OF PRESSURE WAVES AND FORCED-OSCILLATIONS IN GAS-SOLID FLUIDIZED-BEDS AND THEIR INFLUENCE ON DIAGNOSTICS OF LOCAL HYDRODYNAMICS

Experiments were conducted in a 50-mm diameter gas-fluidized bed to investigate the origin and propagation behaviour of pressure waves. The attenuation and amplification of pressure waves during propagation away from their sources are explained by the interaction between particles and the fact that forced oscillations of fluidized beds are coupled with propagating pressure waves. Both the pseudo-homogeneous compressible wave theory and the separated flow compressible wave theory are shown to give good predictions of the propagation velocity of pressure waves in gas-fluidized beds. The dramatic increase in wave velocity when the superficial gas velocity is decreased below the minimum fluidization velocity is attributed to a change in the forms of waves. Due to the existence of pressure waves from many locations in gas-fluidized beds, absolute pressure probes are not suitable for determining local bubble behaviour. Closely-spaced differential pressure probes filter out most pressure waves so that their signals mainly reflect local void and particle behaviour. However, a differential pressure probe does not provide the same local measurement as an optical fibre probe due to its large measurement volume and distortion from nearby bubbles.