The interactions of gas and solids are important in determining the flow structures in the Circulating Fluidized Bed (CFB) downer. Proper description of the interphase drag model is thus crucial for simulating the downer. Considering that cluster with several numbers of particles can be present in the downer, an interphase drag model was developed by considering the dynamic equilibrium of the amount of particles in and out of the cluster in the downer, which was then incorporated into the Computational Particle Fluid Dynamics (CPFD) model. CPFD simulation results show that the predicted particle flow behaviors are in good agreements with experimental data under different operating conditions. Solids circulation rate and gas velocity can significantly affect particle flow behaviors especially in the acceleration region near the inlet. More amounts of particles flow faster in the center while a few part of particles flow slower near the wall, resulting that the particle residence time is shorter and uniform in the center and longer near the wall. The residence time distribution (RID) curve of particles is narrower with a higher peak in the downer than in the riser. Simulation results indicate that less back-mixing of particles is present in the downer, and the flow structure in the downer approximates the ideal plug flow. The present method considering the cluster effect is reasonable in predicting the flow behavior and RTD of particles in the downer. (C) 2019 Elsevier B.V. All rights reserved.