THE CLUSTER-SIZE DISTRIBUTION AND MOTION BEHAVIOR IN A FAST FLUIDIZED-BED

The cluster size distribution and motion behavior in a fast fluidized bed have been measured systematically by using two optical fiber probe s and a computer data acquisition system. The measurement results show ed that (1) although clusters behave in a transient nature, sometimes aggregating and sometimes dispersing, the local time-averaged cluster lengths remain almost constant radially in the core region of both the upper dilute section and the lower dense section, but the local time- averaged cluster lengths in lower dense section are larger than those in the upper dilute section. (2) Cluster motion velocities are in dire ct proportion to the superficial gas velocity. In the lower dense sect ion the probabilities of upward motion and downward motion are basical ly equal; but in the upper dilute section, most of the clusters move u pwards. However, near the wall region of both the upper and lower sect ion, the probability of downward motion is bigger than that of upward motion. (3) The fast fluidized bed is a bed of fluidized clusters. The re are two phases in fast fluidized bed: a dispersed phase and a clust er phase. In the dispersed phase the particles present essentially as individuals, while in the cluster phase the particles agglomerate with one another and are enmeshed in the dispersed phase. The motion behav ior of clusters in the lower dense section and the upper dilute sectio n are similar to the motion behavior of particles in the lower dense r egion and the upper freeboard region of a bubbling fluidized bed, resp ectively.