Mathematical analysis of characteristic pressure fluctuations in a bubbling fluidized bed

Abstract

Mathematical analysis methods including statistical, power spectral, and chaos analysis were used to analyze the pressure fluctuations in a bubbling fluidized bed. Different characteristic indicators of pressure fluctuations were calculated such as average pressure, standard deviation, skewness, kurtosis, dominant frequency, and Kolmogorov entropy. The effects of the superficial gas velocity, particle size, and position of the pressure probe on the flow behavior in terms of these characteristic indicators were investigated. Short videos were recorded using a digital video camera to capture the bubble behavior in the bed. It was shown that the bed was locally fluidized in a narrow range of gas velocity just above the minimum fluidization velocity (Umf). The relative order of the amplitudes of the pressure fluctuations at different axial positions varied with the gas velocity but the amplitudes of the pressures at the axial positions about 10 cm below the static bed surface were very close for all gas velocities. The skewness of the pressure fluctuations increased almost linearly with the gas velocity from a negative to a positive value in the fully fluidized state but decreased with the particle size. The pressure near the bed surface showed a bimodal probability density function (PDF) at gas velocities where the skewness was near zero and the kurtosis was near its minimum. The Kolmogorov entropy changed in a complex manner with the gas velocity; it was smaller at a higher axial position and decreased with the particle size. The Kolmogorov entropy of the pressures just above the distributor was insensitive to the gas velocity in the fully fluidized state. The results present a multi-perspective understanding of the pressure fluctuations in bubbling fluidized beds.