Large Eddy Simulation of a 5 x 5 rod bundle: Impacts of a central control rod thimble tube

Rod bundle flows are prevalent in nuclear engineering for both light water reactors (LWR) and advanced reactor concepts. Unlike canonical channel flow, the flow in rod bundles presents some unique characteristics, notably due to the inhomogeneous cross section which can present different local conditions of turbulence as well as localized effects characteristic of external flows. Despite the ubiquity of rod-bundle flows and the decades of experimental and numerical knowledge acquired in this field, high-fidelity datasets of multiple-pin configurations are still relatively sparse due to the computational cost involved in generating them at relevant Reynolds numbers. These datasets are of great value as they allow for assessment of the reliability of various turbulence models, as well as allow for a deeper understanding of the flow physics. We present Large Eddy Simulations (LES) of the flow at Reynolds number 19,000 in a square 5 x 5 rod bundle representative of LWR fuel generated by the flow solver Nek5000. We consider standard configurations as well as configurations where the central pin is replaced with a control rod guide thimble, and analyze the flow fields in detail. The accuracy of the LES of the base 5 x 5 configuration is established by comparing against Direct Numerical Simulation (DNS) results. We compare the LES results from Nek5000 with an advanced Reynolds Stress model from STAR-CCM+, which shows good agreement in the wide gaps with larger discrepancies in the narrow gaps. In particular, evidence of gap vortex streets and changes in the turbulence structure are seen in the edge subchannels and in the narrow thimble gaps in LES, but are not predicted by STAR-CCM+.