With the Computational Particle Fluid Dynamic (CPFD), which is based on the scheme of Multi-Phase Particle-In-Cell (MP-PIC), the present work studied the flow hydrodynamics and solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. It was found that the flow hydrodynamics and solids back-mixing behaviors are quite different in the riser with different exit geometries. While particles can uniformly flow out of the riser with smooth exits with relatively lower residence time, part of the particles tend to be reflected back into the riser with abrupt exits. Different mechanisms were proposed to explain the distinct solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. The moderate solids back-mixing in the riser with smooth exits is mainly due to the dynamic formation and dissolution of particle meso-scale structures/particle-clusters at the bottom part of the riser, while the intense solids back-mixing in the riser with abrupt exit geometries is the combined results of the particle downward flow from the riser top and the dynamic formation and dissolution of particle meso-scale structures at the riser bottom. The present work implied that the riser exit geometry should be carefully specified in the modeling study of the CFB riser. (C) 2015 Elsevier B.V. All rights reserved.