| An on-line statistical study of the density and pressure fluctuations in gas-solid fluidized beds was conducted by using a mini-capacitance probe, pressure transducers, a correlation and probability analyzer, and a Fourier transform analyzer. The causes of the fluctuations were explored, and the effects of the gas velocity, bed height and particle size on the fluctuations were investigated.; The use of the mini-capacitance probe coupled with the on-line correlation technique has been found to be effective in locally or microscopically determining the jet penetration depth, dead zone height, bubble formation and bubbling frequency in a shallow gas-solid fluidized bed. Bubbling has been observed to be the major cause of density fluctuations and the technique employed has confirmed the known observation that a jet is formed at a distributor hole and penetrates into the bed with bubbles detaching from its end periodically. The results have also revealed that a shallow gas-solid fluidized bed can be roughly divided into three zones, i.e., the jet, bubble coalescence and bubble burst zones. The mean bubbling frequency decreases exponentially with respect to the probe position in the bubble coalescence zone, and it remains essentially constant with respect to the probe position in the bubble burst zone. The stochastic models proposed, namely, a renewal counting process and a pure death process, have been found to describe well the phenomena of bubble formation and bubble coalescence, respectively.; The use of the pressure transducers coupled with the on-line correlation technique has been found to be efficient with respect to computational effort in studying pressure fluctuations and in determining the rise velocities of bubbles, slugs and pressure waves and the onset of slugging in a gas-solid fluidized bed. The results have indicated that the motion of bubbles appears to be the major cause of pressure fluctuations in the upper portion of the bed.; The rise velocities measured using the present technique have been correlated with two models developed in this work and have been compared with the data obtained by other investigators. A semi-empirical model has been derived to predict the superficial gas velocity at the onset of slugging. The model appears to predict the present and available experimental results reasonably well. |
