Wu, Keng-Tung; (1997) Bubble coalescence from multiple entry nozzles in fluidized beds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The phenomenon of bubble coalescence in fluidized beds is a significant factor and dominates the distribution of bubble sizes and positions in the bed; it also affects the transfer of gas between bubbles and the emulsion phase. Although the mechanism of bubble coalescence has been investigated for around thirty years, few studies have reported the interaction between bubbles from two or more nozzles. In order to understand the processes of bubble coalescence, this study was carried out using X-ray attenuation to measure the bubble interactions in gas discharging from two nozzles at varying separation distances submerged in a fluidized bed and a defluidized bed at ambient pressure. The measurements of the positions of coalesced bubbles were obtained by analysing X-ray images. Generally, the bubbles issuing from the entry point coalesce immediately when the nozzles are sufficiently close together so that the contact between the gas and the solids will be restrained initially. Alternatively, the bubbles coalesce at higher levels in the bed when the nozzle separation distances are greater. Correlations for predicting the height of coalescence were found to be a function of the distance of separation of the nozzles, the nozzle gas velocity and Archimedes number due to different flow patterns. Moreover, the patterns of coalescence for gas streams or bubbles occurring in a defluidized bed were found to be dependent on the separation distance of the nozzles. In addition, deterministic chaos, a new tool was employed to investigate the processes of bubble coalescence from three nozzles. The chaotic time series measured from analysis of multiple bubble interactions was characterized by the Hurst exponent, the correlation dimension, the Lyapunov exponent and the Kolmogorov entropy of its chaotic attractor. The results provided a new criterion for the nature of bubble coalescence in a fluidized bed.

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