Three-dimensional numerical simulations were conducted for the natural convection phenomena around a hot inner circular cylinder positioned in a cold cubic enclosure in the Rayleigh number range of 10(3) <= Ra <= 10(6) at the Prandtl number of Pr = 0.7. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the inner cylinder, based on the finite volume method (FVM). In this study, the transition of the flow regime from the steady state to the unsteady state and consequent three-dimensionality in the system induced by the increase in the flow instability were investigated. Detailed three-dimensional vortical structures of the convection cells at a relatively high Rayleigh number of Ra = 10(6) were analyzed using the visualization technique, and the heat transfer characteristics in the system resulting from the change in the vortical structures were addressed. (C) 2015 Elsevier Ltd. All rights reserved.