| In this dissertation, a number of aspects relating to the characteristics of particle clusters in a circulating fluidized bed (CFB) riser of 15 cm I.D. and 10.8 m height was studied. Experimental tests were conducted using 251 {dollar}mu{dollar}m sand particle, covering a range of operating air velocities and solid mass fluxes.; Measurements of the instantaneous fluctuations in the local solid fractions and time-average local solid fractions were obtained by means of a single needle capacitance probe. The local solid fractions were found to vary with radial and axial position, solid mass flux, and gas velocity. Probability distributions of solid fractions, obtained from the dynamic fraction measurements, were used to interpret the solid flow structure.; Applying the local solid fraction information, a criterion was developed which can reasonably and efficiently identify occurrence of the clusters. Time-average local solid fractions in clusters and their probability distributions, the time fraction of clusters, and cluster residence time were obtained by this single capacitance probe technique. The results indicate that solid fraction in clusters, the time fraction of clusters, and cluster residence times all increased with decreasing elevation and increasing radial distance from the centerline of CFB riser, increasing solid mass flux, and decreasing gas velocity, respectively.; Cluster velocity and cluster length measurements were obtained with two capacitance probes, by cross correlating the two probe signals of instantaneous solid fractions. The results indicated existence of a dilute upflowing core region with relatively high cluster velocities and short cluster lengths, and in a dense downflowing wall region with relatively low cluster velocities and large cluster lengths. Decreasing elevation, increasing solid mass flux, decreasing in the gas velocity individually decreased the cluster velocity and increased the cluster length. The local average cluster velocities were measured to be lower than the superficial gas velocity. The range of local average length is approximately 70-200 times diameter of the 251 {dollar}mu{dollar}m sand particles. |
