Assessment of RANS models for flow in a loosely spaced bare rod bundle with heat transfer in low Prandtl number fluid

In the present study, a wide range of steady-state and unsteady Reynolds Averaged Navier Stokes (RANS) simulations have been performed to reproduce the turbulent flow and heat transfer in a fuel assembly of the ALFRED reactor. The considered fuel assembly consists of a loosely spaced bare rod bundle with pitch to rod diameter ratio of P/D = 1.32. It is cooled by liquid lead, hence the Prandtl number of the working fluid is very low, i.e. Pr = 0.016. The obtained results are extensively compared with the available Large Eddy Simulation (LES) results. This article is divided in two main parts. In the first part, a thorough assessment of eddy viscosity (i.e. linear and non-linear) and Reynolds stress turbulence models have been performed against the reference LES results. Flow and thermal fields have been qualitatively and quantitatively analysed. In general, RANS results are in a good agreement with the LES, however, most of the models severely overpredict wall shear stress and the corresponding pressure drop in the rod bundle. In the second part, URANS simulations have been performed to reproduce a gap vortex street in the considered loosely spaced bare rod bundle. For this purpose, two turbulence models, that gave the best steady-state RANS predictions, i.e. the k-ε cubic and the elliptic blending Reynolds-stress model, are tested as well. However, the gap vortex street in this particular loosely spaced bare rod bundle turned out to be too weak to be predicted by the URANS approach.