Mixed convection heat transfer in a channel filled with porous medium and containing an isolated heat source at the bottom wall is studied in this work. The porous medium is assumed to be made of circular cylinders and is placed only on the heater surface. Three different configurations of porous medium are considered in this study. Pore-scale numerical simulations are carried out using the exact geometry of porous medium. The same configuration is also investigated using the volume-averaged approximation. The temperature distribution of the heater surface obtained from the pore-scale numerical simulation is compared with the results obtained from the volume-averaged numerical simulation. Parametric studies are carried out by varying the material of the cylinders, the porosity, and the height of the porous medium. The effects of Grashof number and Reynolds number of the flow are also studied as part of this investigation. The results obtained from the pore-scale numerical simulations show that the presence of the porous medium leads to reduction in heat transfer, while the results obtained from the volume-averaged numerical simulations show an enhancement of heat transfer due to the presence of the porous medium on the heater surface. However, the pore-scale numerical simulation results show that the heat transfer enhancement is only possible if the channel height is completely filled with the selected porous medium.