With advantage of higher heat transfer area per unit mass, a pebble bed is usually adopted as an essential component for design of energy production systems and thermal energy storage (TES) systems. The majority of this paper investigates the sphere blockage ratio (beta) on the thermal-hydraulic characteristics of a pebble with 14 spheres using a three-dimensional (3-D) computational fluid dynamics (CFD) model with the (v(2)) over bar -f turbulence model. In a previous work, this model has been validated against measured distributions of the heat transfer coefficient on the selected spheres. The measured data are obtained using the transient liquid-crystal technique. According to the simulation results, the thermal-hydraulic characteristics in the sphere array can be captured reasonably with the present CFD model, including flow stagnation, flow separation, vortex formation and anisotropic characteristics of the heat transfer on the sphere surface. Comparisons of the simulation results for the sphere arrays with different blockage ratios show that the flow and turbulent intensity distributions are similar in most regions of a sphere array, except the portions between the pebbles. The heat transfer coefficient for the upstream spheres increases slightly as the blockage ratio decreases. However, a lower heat transfer coefficient is predicted for the downstream sphere if beta is less than 0.75. In addition, the heat transfer coefficient around the front of a downstream sphere would not be influenced by the upstream spheres until beta <= 0.75. Similar results are also revealed in the dependence of the heat transfer coefficient on the zenith angle of the spheres for the different blockage ratios. (C) 2014 Elsevier Ltd. All rights reserved.