Porous media are typically capable to enhance heat transfer, at the cost of an increase of the pressure drop, mainly in view of the huge increase in the surface wetted by the fluid. In this work, a tubular receiver for CSP applications, partly filled with a porous medium constituted by a packed bed of copper Raschig Rings is investigated for the first time. The analysis, carried out numerically, aims at studying in detail the mechanisms of the heat transfer from the wall to the gaseous heat transfer fluid (air) through the porous metal matrix in symmetric and asymmetric heating conditions. The computed results are compared to what occurs in a smooth tube subjected to the same heating, to check the increase in the heat transfer. The investigation carried out in this work represents the first step in the optimization of the porous medium structure inside the tubular receiver.