A three dimensional CFD model was developed for studying flow and heat transfer in fixed beds with cylindrical particles for low tube to particle diameter ratios. The packing structures were generated by means of the discrete element method (DEM), in which the rigorous cylindrical particle was adopted. The detailed packing structure features were obtained, including the particle position, orientation, porosity distribution and the effect on the flow and heat transfer process was analyzed. The wall effect was found to be obvious for fixed beds with low tube to particle diameter ratios. The cylindrical particles tend to be organized in circles, which is even more visible in the vicinity of walls. The radial porosity distribution showed that the obvious oscillation existed in near wall region. The detailed velocity and temperature distribution were investigated. The average axial velocity showed similar tendency with radial porosity distribution. Large porosity caused large velocity and temperature gradients in the wall region. The radial temperature distribution was also calculated by the improved two dimensional quasi-homogeneous model, in which the local porosity and axial average velocity distributions from CFD simulations were adopted and good consistencies for temperature distribution were achieved for different tube to particle diameter ratios. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.