Based on the micro-scale flow mechanism，a method combining computational fluid dynamics (CFD) and discrete element method (DEM) was established to simulate the gas-solid flow systems in the 2-dimensional riser and downer.Periodic boundary conditions were employed in the simulations

in which particles with mean diameter of 520 μm and density of 2620 kg·m-3 were used.The results qualitatively or semi-quantitatively disclosed the different particle clustering phenomena on the micro-scale in the co-current gas-solid flows against or along the gravity direction.In the riser

severe particle clustering existed and the particles in clusters showed evident backmixing.While in the downer

rather loose clustering was observed and the particles in clusters flowed in the same direction as the macro-flow.Backmixing of particles could hardly be found in the downer.Ensemble-averaged macro-flow behavior

including the distributions of gas and solids velocities and concentrations

were obtained statistically

which had qualitative agreement with the experimental data in the literature.