Descent velocities of particle clusters at the wall of a circulating fluidized bed

The descent velocities of clusters of particles near the wall of a circulating fluidized bed have been measured and reported by many investigators. Curiously, the measured velocities all fall within the range 0.3-2.0 m/s, regardless of particle size, material density or gas velocity. In an attempt to explain this phenomenon, a model for drag on a cluster was developed. This model neglects the interaction between the particles and the wall as well as the forced flow of gas in the wall region, considering only the descent of a cluster in quiescent air at a terminal velocity. There are two elements to the drag force acting on a cluster in this model: one caused by how passing through the cluster of particles, the other caused by flow passing around the cluster. Simplification of terms in this model shows that the terminal velocity of a cluster, non-dimensionalized by the minimum fluidization velocity of the particles, can be related to the Archimedes number of the gas-solid flow. Good agreement is obtained between this simple model and the reported measurements of cluster velocities. (C) 2000 Elsevier Science Ltd. All rights reserved.