In this study,mixed convection heat transfer and fluid flow is numerically investigated in an open system enclosure containing a heated obstacle.Effects of the positions of both obstacles and fluid flow outlet on the flow field and heat transfer within the enclosure were studied.Richardson number ranging from 0.1 to 10 was examined for a constant Grashof number at different Reynolds numbers in both base fluid and fluid mixed with Cu nanoparticles (0 to 4％ of volume fractions).It is found that the obstacle and outlet position as well as the Richardson number greatly affect the flow field and heat transfer rate.The maximum heat transfer rate occurs for Richardson number 0.1,when the outlet and obstacle position are located at the bottom of the right side of the enclosure (P3) and near right side of the enclosure (8H),respectively.Also,the heat transfer rate is higher when the obstacle is placed close to the inlet (2H) and outlet (8H) compared to the center of the enclosure (5H).It is shown that although the change of volume fraction of nanofluid does not have significant effect on the flow pattern,however,it enhances the heat transfer rate through improving the effective thermo-physical properties of the base fluid.