Natural convection in a T-shaped cavity with two different sets of thermal boundary conditions is studied numerically with time-dependent governing equations. The effect of boundary conditions on the flow regime and heat transfer is established at various Rayleigh numbers. Computational simulations are also performed to investigate the effect of four different fluids: melted silicon, air, water, and ethylene glycol with Prandtl numbers of 0.054, 0.71, 6, and 204, respectively, on heat transfer characteristics. Results indicate that the thermal conditions of enclosure and the type of fluid have significant influence on the flow and temperature fields. Unsteady formulation predicts different regimes, including steady-state and periodic variations in flow and thermal fields.