(29g) Onset of Bubbling in Fluidized Beds of Silica Sand Particles: Determination from Pressure Drop Fluctuations

Batches of silica sand with diameters that place them in the vicinity of the Geldart A/B boundary have been used in experiments where the fluctuations in bed pressure drop have been measured over a range of superficial velocities. Plots were made of the standard deviation of pressure drop against superficial velocity and, by inspection, it was apparent that the first transition from values that were zero or very close to it was rapid and took place over a very small velocity change. Thereafter, the increase in standard deviation was linear with inceasing superficial velocity. Supporting evidence for this is provided by an analysis that uses the ratio of total variance to high speed variance, {VTOTP/VHIP}, and in plots of the Hurst exponent obtained for the same pressure fluctuation data sets. One consequence of (apparent) rapid initial growth in standard deviation is that linear extrapolation of the standard deviation plot to zero underestimates the point at which bubbling actually begins. Results are presented for eight different mean particle sizes.