Comparison of liquid-solid flow characteristics in upward and downward circulating fluidized beds by CFD approach

The flow behaviors of the particles in the upward and downward liquid-solid circulating fluidized beds were simulated by Eulerian-Eulerian approach incorporating the kinetic theory of granular flow. The Syamlal-O'Brien drag model was used to account for the interphase interaction between the liquid and solids phases. The numerical results agreed well with the experimental data obtained by other group members, successfully validating the numerical approach. The hydrodynamic characteristics of the particles with different densities and sizes in the upward and downward circulating fluidized beds were compared in detail under different liquid velocities and solids flow rates. The results indicated that there were many hydrodynamic similarities in the upward and downward circulating fluidized beds. However, due to the deviation of the standard drag curve, the drag force on the lighter particles in the downward liquid-solid circulating fluidized bed became different, leading to random and irregular particle flow behaviors. The predicted average solids holdups matched well with the results by an empirical model proposed by Sang and Zhu and the matching degree between the numerical results and the model was larger than 85%, further proving that the Sang and Zhu model is widely applicable.