This work presents a numerical and experimental study of convection heat transfer in the unconfined air gap of a discoidal technology rotor-stator system. In particular, the rotor is cooled here by an eccentric air jet impingement. The cavity is characterized by a dimensionless spacing G = 0.02 and a low aspect ratio for the jet H/D= 0.25. the rotational Reynolds numbers from Reω = 2.38x105, to 5.44x10, and the jet Reynolds number between 16.5x103 and 49.5x103 are used. The experimental technique is based on using infrared thermography to access the rotor surface temperature measurement, and on the numerical resolution of the energy equation at steady state to evaluate the local convective coefficient. The numerical study is based on the turbulence model RSM (Reynolds Stress Model). The results of this study are described in terms of radial distributions of the mean velocity components, Reynolds stress tensor components and corresponding local and mean Nusselt numbers. Three flow regions have been identified: a region dominated by the air jet, a mixed zone at low radii and a zone dominated by outward rotation. A good agreement between the two approaches has been obtained, which confirms our choice of numerical turbulence model.