EFFECT OF MEASUREMENT METHOD ON THE VELOCITIES USED TO DEMARCATE THE ONSET OF TURBULENT FLUIDIZATION

The transition velocity U-c corresponding to the maximum rms amplitude of pressure fluctuations in a 102 mm diameter fluidized bed was found to be a strong function of measurement method, while the other commonly used transition velocity U-k depends on the solids return system and is not a well defined parameter. Different results and trends are obtained for U-c depending on whether one uses absolute or differential pressure fluctuations and whether or not one normalizes by the time-mean local or differential pressure. For differential pressure fluctuation measurements, a higher U-c was obtained from the dimensionless standard deviation normalized by the local pressure drop, and the value decreased with height. For local voidage fluctuation measurements using an optical fibre probe, a maximum point also appeared in the standard deviation curve, although the curve was much flatter than corresponding pressure fluctuation curves. U-c corresponding to zero skewness of voidage fluctuations on the axis of the column gave a value close to U-c from differential pressure fluctuation measurements at the same level.