In the present work, we combine experiments and numerical simulations of a planar jet with heated co-ﬂow with medium (air) and low-Prandtl (He-Xe gas mixture) ﬂuids. Jets are recognized as representative test cases to be investigated in large components of pool-type liquid metal-cooled nuclear systems, like the Multi-purpose hYbrid Research Reactor for High-tech Applications (MYRRHA), currently under design at SCK•CEN. The present planar jet conﬁguration mimics a closed wind tunnel that is designed and operated at VKI to generate an experimental database for velocity and temperature ﬁelds of a turbulent forced-convection ﬂow regime. The performed experiments combine the Particle Imaging Velocimetry (PIV) (in characteristic planes) and thermocouple (single point) measurements. In parallel with experiments, comprehensive numerical simulations have been performed within the RANS modeling framework. Next to the standard eddy-viscosity based two-equation k − ε model, an extended variant based on the low-Reynolds elliptic relaxation concept (so-called ζ − f model) has been applied too. To investigate the low-Prandtl eﬀects on the heat transfer, series of the turbulent heat transfer models have been applied, ranging from a conventional constant turbulent Prandtl number to a more elaborate kθ − εθ model. The combination of the low-Reynolds ζ − f and kθ − εθ models was explored for the ﬁrst time in the content of nuclear engineering applications. The focus of the numerical studies is to address in details the eﬀects of low-Prandtl ﬂuid in the strongly forced convection ﬂow (central planar cold jet) in presence of a strong shear (hot co- ﬂow). We demonstrate the importance of the proper speciﬁcation of the inlet boundary conditions in numerical simulations to mimic correctly experimentally observed asymmetrical distributions of the cross-wise proﬁles of stream-wise velocity, turbulent kinetic energy and temperature. Finally, the minor diﬀerences in results between the assumed constant turbulent Prandtl number and more advanced kθ − εθ model of the turbulent heat ﬂux conﬁrmed the overly dominant mechanisms of the strong convection and molecular diﬀusion in the present conﬁguration.