On the use of small-scale LES for full-length heated fuel rod simulations

An alternative methodology is proposed here to overcome the excessive cost of Large Eddy Simulations (LES) of full-length heated rod bundle calculations, while improving the inaccurate results typically obtained with Reynolds Averaged Navier-Stokes equations (RANS). While the cost of the full-length LES is generally too high, a LES simulation of a small section of a single rod is usually affordable. The idea presented here consists of using the information granted by the small LES calculation to determine the appropriate turbulence viscosity or turbulent thermal diffusivity that could be used to solve only for the temperature field in a pseudo-RANS approach. The study has been performed with single rod simulations with a P/D of 1.12 and 1.24, considering rod lengths that are representative of regular and tight lattice configurations respectively, for the cases of uniform heat flux and a more realistic cosine-like axial heat distribution. The Spectral Element Method code Nek5000 has been used for all LES, RANS and pseudo-RANS simulations. Prediction of thermal quantities are compared to classical linear and non-linear RANS models. LES simulations for the full-length rods have also been performed and are used as a reference. Results of the proposed method show significant improvements with respect to those obtained with RANS.