In this study a three-dimensional model was employed to numerically investigate the natural convection inside a partially-open cubic cavity with internal heat source. The boundary conditions are set so that two opposite vertical walls are maintained at different constant temperatures while the others walls are considered adiabatic. An opening occupying half of the entire area is placed in the right cold wall, allowing the fluid input and output of the cavity. The temperature difference between the non-adiabatic walls was defined using the external Rayleigh number within the range Ra-e = 10(3)-10(5). The intensity of the internal heat source was evaluated through the relation R = Ra-i/Ra-e, investigated between 0 and 2000. The results show a significant influence of three-dimensional effects, in special for high Ra-e and R values, affecting the energy and flow distribution inside the cavity. In many cases the heat transfer between the fluid and the walls, measured by the Nusselt number, shows a rather different pattern even at the central part of the cavity when compared with results obtained with a two-dimensional model. Moreover, different Prandtl numbers were analyzed to consider the effect of the thermal and momentum diffusion proprieties of the fluid. It was observed that the main effect of the Prandtl number is on the heat exchange at the hot wall, while the maximum dimensionless temperature and the heat exchange at the cold wall are less dependent on the Prandtl number. (C) 2013 Elsevier Ltd. All rights reserved.