The gas-solid flow and mixing behavior in two dimensional downers with different entrance structures were simulated by using a CFD-DEM method.A gas-solid catalytic reaction model was incorporated in the governing equations to investigate the contact efficiency between phases.The simulations showed the dynamic and time-averaged flow structures characterized by the distributions of solids fraction

particle velocity and the reaction product in the whole downer as well as the residence time distributions (RTDs) of gas and solids

which were all heavily influenced by the design of the entrance structure.Non-ideal initial contact between gas and solids would cause wide RTDs of gas and worsen the reactor performance.