Simulation models of gas-solid flows are classified into two kinds; one is the continuum model and the other is the discrete particle model. The present principal investigator has been using the discrete particle model for predicting gas-solid flows from dilute to dense phase. The purpose of this report is to compare results based on the discrete particle model with those based on the continuum model concerning flows in the riser of circulating fluidized bed. Trajectories of particles are calculated by the Newtonian equations of motion in the discrete particle model. In this report, methods and techniques used by the present principal investigator are described in detail, particularly about the DSMC (Direct Simulation Monte Carlo) method which is a powerful means for calculating particle motion under the effects of collision. Calculation based on a small number of sampled particles is possible owing to the DSMC method.
Results for comparison are those calculated by Tsuo and Gidaspow( 1990 ) who used the two fluid model. Following Tsuo and Gidaspow, parametric studies were conducted, and effects of gas velocity, solids mass flux, particle size and duct size on flow patterns were studied. Quantitatively large difference is observed between the results of the discrete particle model and the two fluid model. For instance, cluster population is much larger in the discrete particle model than the two fluid model. Qualitatively both results show the same tendencies in most cases. However, concerning the effect of duct size, results are different even qualitatively; the discrete particle model shows that as the duct size increases, clusters are formed not only near the wall but also in the center part of the duct, while the two fluid model shows that clusters disappear in the wide duct. The present principal investigator neglects fluid viscosity and thus the results are not satisfactory near the wall. Fluid viscosity must be taken into account in the future work.