Random packed bed unit operations of mono-sized spheres are widely used in many chemical and biochemical processes. The influence of the confining walls on fluid flow cannot be ignored in packed beds with low tube-to-particle diameter ratios due to its uneven distribution of radial porosity. In present paper

the discrete element method (DEM) and the computational fluid dynamics (CFD) are coupled to investigate the characteristics of fluid flow and heat transfer in the composite packed beds of spheres with two kinds of diameters. Three kinds of composite packed models and a control group (randomly packed bed with mono-sized spheres) were constructed with about 200 spheres

including radially layered model (RLM)

axially layered model (ALM) and randomly composite packed model (RCM). Simulation results showed that

not a big difference was observed on radial porosity distribution in the near-wall region (up to half a sphere diameter from the solid wall in the wall-normal direction)

and wall effect significantly affects the velocity and temperature distribution in packed beds. Both the distributions of porosity and pore scale hydraulic diameter are critical to the velocity and temperature distribution in packed beds

and smaller difference of temperature in core and near-wall regions would be found in radially layered composite packed model. Furthermore

the overall heat transfer efficiency would be improved with radially layered composite packing form. These simulation results may be useful for the improvement and further optimization of the design of packed beds with low tube-to-particle diameter ratios.