The gas-solid flow in a riser has strong inherent nonuniformities both in the flow structure and in the dynamic phases. The flow structure in a riser can be altered with the implementation of different solids feeding devices, and hence may affect the reactor performance. This paper is aimed at investigating the effect of various riser entrances on the overall flow structure and its stability at different operation conditions. Three riser entrances are selected to simulate the common solids feeding devices of risers. namely, the J-bend feeder, the L-valve feeder with a distributor, and the L-valve feeder after a. taper section. The Study is first focused on the flow dynamics in the riser entrance region. This is to identify the characteristic height of the entrance region as well as to obtain the radial distributions of phase transport properties at the end of the entrance region. These radial profiles are then used as the flow inlet conditions in the mechanistic model for the study of overall flow structure and stability in the riser main region. The study shows that the flow structure in the entrance region can be strongly affected by the selection of solids feeding patterns but weakly dependent upon the operation conditions. The flow structure in the main riser region, however, is weakly dependent upon the selection of solids feeding patterns but strongly affected by the operation conditions. The riser characteristic length of the entrance region is nearly independent of the gas inlet velocity and solids mass flow rate; however, it is moderately influenced by the solid feeding pattern. As part of model validation, some simulation results are directly compared with available experimental measurements, with reasonably good agreement.