THE VELOCITY OF SOUND IN FLUIDIZED-BEDS

The velocity of sound us in fluidised beds of particles, at or near incipient fluidisation, was measured by two methods: (1) by observing the damped pressure fluctuations immediately after giving the bed a sudden vertical impulse; (2) by cross-correlating the signals from two pressure probes, one above the other in an incipiently fluidised bed, the signals originating from a bubbling section of the bed immediately above the incipiently fluidised section. The measured values of us-of order 10 m/s-agree well with single theory which assumes that the sound wave is transmitted by compressing the interstitial gas isothermally. The sound waves are strongly damped: the theory of damping is inadequate but it appears that damping is due to: (1) inter-particle collisions, and (2) relative motion between the interstitial gas and the particles. © 1990.