This paper investigates the cooling process of water in a circular enclosure subjected to non-uniform boundary conditions. Half of the enclosure boundary is maintained at a constant temperature, while the other half is well insulated. The enclosure is cooled from its top, bottom, or left side. The problem is solved numerically using the finite element method. The examined Rayleigh numbers are 1 x 10(5), 1 x 10(6), and 1 x 10(7), and the effect of maximum density at 4 degrees C is considered. The results indicate that the natural convection flow plays a significant role on the cooling process of water. The effects of the cooling side and Rayleigh number on the cooling process are presented. For Ra = 1 x 10(5), the results indicate that the bottom cooling side has the lowest cooling time, but it has the highest cooling time for Ra = 1 x 10(7). For Ra = 1 x 10(6) and 1 x 10(7), the left cooling shows lowest cooling time. For Ra = 1 x 10(5), the bottom cooling case takes 52.97% less time than the top cooling case to complete the cooling process. The density inversion near 4 degrees C creates a unique natural convection flow, and high resolution capturing of temperature contours and natural convection flow is presented. (C) 2010 Elsevier Ltd. All rights reserved.