CFD methodologies for A PWR fuel rod assembly
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Copyright (2015) by American Nuclear Society All rights reserved. Although numerous applications of computational fluid dynamics (CFD) codes to predict turbulent flows and heat transfer in a pressurized water reactor (PWR) fuel rod assembly have been performed, most of them have not been fully validated mainly due to lack of high fidelity experimental data. However, NESTOR experiment performed by CEA-EDF-EPRI provided accurate data for various variables (mean and RMS axial velocities, and pressure drops) over a broad range in a 5 5 PWR fuel rod assembly with and without mixing vane grids (MVGs). This study considered the steady Reynolds Averaged Navier- Stokes (RANS) turbulence models because of its feasibility in application to the real PWR fuel rod bundle. Mesh size sensitivity studies investigated global base and axial mesh perturbations, with respect to a selected mesh refinement. Isotropic and anisotropic turbulence models with high y+ wall function were examined. Furthermore, we investigated the importance of the anisotropic turbulence model in the typical PWR rod bundle by removing a SSG in-between two MVG spans. Axial and global mesh size sensitivity studies identified reasonable mesh refinements for various regions of the SSG and MVG span types. Examined turbulence models over predicted and slightly under predicted MVG and SSG pressure loss ratios, respectively. CFD-calculations produced comparable mean axial velocity (MVG and SSG spans) and RMS axial velocity fluctuation (MVG span) profiles with respect to experimental measurements. The SSG's main purpose in the MANIVEL-MVG bundle is to stabilize the 5X5 rod array between consecutive MVGs. Unintended or undesirable consequences of the SSG (in this problem) include; the observed presence of secondary flow structures in the SSG far wake region, the sudden reduction of cross-flow upon passing the SSG, and the re-distribution of wall shear stresses upon passing the SSG. Thus, the mere inclusion of SSGs in the MANIVEL MVG bundle introduces a basic limitation for validating CFD methodologies to a general PWR MVG bundle. Without the SSG between consecutive MVGs, cross-flow engendered by mixing-vanes was more dominant than the turbulence anisotropy- driven secondary flow along the MVG's wake region. This reduces the anisotropic turbulence model's relative importance in application to the actual PWR MVG bundle.
