This study conducted two-dimensional numerical simulations of the natural convection phenomena in a cold square enclosure containing four hot elliptical cylinders with a rectangular array. Rayleigh numbers in the range of 10(4) <= Ra <= 10(6) and a Prandtl number of Pr = 0.7 were considered. The immersed boundary method (IBM) was used to capture the virtual wall boundary of the cylinders based on the finite volume method (FVM). The main parameters investigated include the Rayleigh number and aspect ratio of the cylinders. The aspect ratio of the cylinders was varied in the range of 0.25 <= AR <= 4.00. The flow and thermal structures and corresponding heat transfer characteristics were investigated with respect to the transition of the flow regime from steady state to unsteady state. As a result, ((Nu(CYL)) over bar) increased by about 4.38% and ((Nu(EN)) over bar) increased by about 5.24% at AR = 4.00 with increasing AR compared to AR = 1.00 (Circular cylinder). Thus, the variation in AR has a remarkable influence on the heat transfer characteristics at the cylinder surfaces and the walls of the enclosure. (C) 2018 Elsevier Ltd. All rights reserved.