Cluster patterns in circulating fluidized beds predicted by numerical simulation (discrete particle model versus two-fluid model)

First, the discrete particle model used by the present authors is described, particularly the direct simulation Monte Carlo (DSMC) method which is a powerful means of calculating particle motion under the effects of interparticle collision. Second, calculated results based on the discrete particle model are compared with the results of Tsuo and Gidaspow (AIChEJ., 36 (6) (1990) 885) who used the two-fluid model to predict cluster patterns in the riser of a circulating fluidized bed. Following Tsuo and Gidaspow, parametric studies were conducted, that is, the effects of gas velocity, solids mass flux, particle size and duct size on flow patterns were studied, Quantitatively, a large difference is observed between the results of both models, For instance, the cluster population is much larger in the discrete particle model than in the two-fluid model. Qualitatively, both results exhibit the same tendencies in most cases,However, concerning the effect of duct size, even the qualitative results are different: the discrete particle model shows that as the duct size increases, clusters are formed not only near the wall but also in the central part of the duct, while the two-fluid model gives the result that clusters disappear in a wide duct as the duct size increases. The present authors neglect fluid viscosity and thus the results are not satisfactory near the wall. (C) 1998 Elsevier Science S.A.