A Numerical Analysis of the Descending Behaviors of Clusters at the Wall of the Circulating Fluidized Bed Riser

Particle clusters at the wall of the CFB riser have significant effects on the bed-to-wall heat transfer and abrasion, while their descending behaviors are not well understood because the entire descending process is difficult to track with experiments, due to the limitations of measurement technology. In this study, the gas–particle two-phase flow in the CFB riser is simulated using the LES-DSMC method. The entire descending process of the cluster is recognized using a method that involves identifying the continuity of periods in which clusters appear in the successive cells at the wall. Then, the transient velocity, drag force, and particle concentration of the descending cluster as a function of its traveling distance are obtained. The results show that the descending clusters at the wall of the CFB riser are dynamic collections of particles. Their lifetimes are in the range of 0.2~0.5 s. During the descending processes, they are accelerated, and their particle concentrations are continuously decreased. The variation in the particle concentration, velocity, and drag force of different descending clusters indicates that they travel highly similar distances and fluidization velocity has little effect on them.