Two-dimensional numerical simulations are conducted for natural convection in an enclosure with an hot inner cylinder located at the center of a cold enclosure for four Rayleigh numbers of 10(3), 10(4), 10(6), and 10(6). The study focuses on the effects of the locally heated bottom wall of the enclosure on thermal and flow structures of natural convection. The results indicate negligible changes in thermal and flow structures based on variations in the size of the local heating zone on the bottom wall at Ra = 10(3) and 10(4), although there is a small variation in the convection velocity in the enclosure. At Ra = 10(6), small inner vortices formed in the lower part of the cylinder show significant changes in their size with increases in the size of the local heating zone. At Ra = 10(6), secondary vortices are generated in the lower part of the cylinder because of flow separation from the side wall. The generation and dissolution of vortices are dependent mainly on the size of the local heating zone. (C) 2013 Elsevier Ltd. All rights reserved.