In this paper we investigate the natural convective heat transfer process inside a ventilated rectangular cavity with a projected heat source. The heat source block is mounted on the bottom wall and a horizontal vent is provided on the top wall of the rectangular cavity. The flow is induced due to the density difference which arises due to the variations in temperature between the heat source block and the surrounding ambient fluid. A FORTRAN 90 CFD solver is developed to simulate the natural convection phenomena by solving the Navier-stokes equation, energy equation coupled with Realizable k-ε turbulence model. The transient flow behavior inside the cavity is simulated by varying the heat source aspect ratios, Grashof number and the heat source locations. It is found that the heat source aspect ratio and its locations significantly influences the flow and heat transfer characteristics inside the cavity. The bidirectional exchange rate across the horizontal opening increases linearly with Grashof number and heat source aspect ratio. A chaotic flow behavior pattern is observed across the opening and the strength of the instabilities increases linearly with heat source aspect ratio. It is identified that by varying the aspect ratio 0.1 ≤ β ≤ 3, the average Nusselt number and mass flow rates are increased by 28% and 43% respectively.