High fidelity simulations in support to assess and improve RANS for modeling turbulent heat transfer in liquid metals: The case of forced convection

Highlights: • Wall-Resolved Large eddy simulations of channel flows heat transfer. • Direct numerical simulation of impinging jet heat transfer. • New temperature-wall-function for low Prandtl fluids. • Comparison between high fidelity simulations and different RANS approaches. • Simple gradient diffusion assumption versus implicit and explicit algebraic heat flux models. This paper attempts to give a brief overview of the work conducted through some recent EU funded projects under the Euratom research for innovative nuclear systems (THINS, SESAME and MYRTE) concerning the use of high fidelity (HiFi) simulations, namely Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES), to adapt Reynolds Averaged Navier-Stokes (RANS) models for the computation of turbulent heat transfer in liquid metal flows.Here the focus is on forced convection only, that prevails in normal reactor operation, through some selected cases performed at UCLouvain, e.g. the channel flow and the impinging jet. The considered RANS approaches are those based on the simple gradient diffusion hypothesis or SGDH, and those based on the algebraic heat flux formulation (AHFM). Among the AHFM models the two possible formulations are assessed, i.e. the explicit form ($k-∊-k_{\theta }-{∊}_{\theta }$ of Manservisi and Menghini (2014) still based on the eddy diffusivity concept (gradient diffusion assumption), and the implicit form of the AHFM-NRG which is essentially a recalibration of the reference model of Kenjeres et al. (2005). The Results show the overall superiority of AHFM models, although SGDH-models using the Kay correlation for the turbulent Prandtl number and the dedicated thermal wall-function developed by Duponcheel et al. (2014) provide reasonable results at a much lower effort making them interesting for industrial applications. However further research is undoubtedly required to come closer to more universal models working for a wide range of Prandtl numbers and flow conditions.