The effect of bed diameter on near-wall hydrodynamics in scale-model circulating fluidized beds

Previous research has shown that the diameter of a circulating fluidized bed (CFB) has a significant effect on the heat transfer rates to peripheral walls, a phenomenon important for the application of CFBs as combustors and boilers. In order to better understand the effect of bed diameter on heat transfer, two laboratory-sized scale models were built and run at room temperature. The two units were of the same height and were run at the same operating conditions with the same particles; the only difference was that the diameter of the second unit was 50% larger than that of the first. The two scale models are designed to simulate the hydrodynamic behavior in full-sized pressurized CFB combustors that are 14.3 m tall with diameter of 0.33 and 0.50 m, respectively. To compare the effect of bed diameter on hydrodynamics, the solid-fraction profiles and the fraction of the wall covered by clusters of particles were measured. The coverage of the wall by clusters was determined by analyzing images of visual data recorded with a digital high-speed video camera through the transparent bed wall. The results show that distinctly different solid-fraction profiles exist in the different-sized beds and that a 50% increase in bed diameter can nearly double the fraction of the wall covered by clusters. In addition, statistical analysis of rime-resolved measurements of the wall coverage by clusters indicates that the presence of clusters follows a spatial Poisson process, i.e. the arrival of a cluster is an event that is independent from the arrival of other clusters. (C) 2000 Elsevier Science Ltd. All rights reserved.