Highly-resolved LES of turbulent convective flow along a PWR rod bundle

A detailed numerical analysis of the turbulent convective flow along the heated rods of an idealized Pressurized Water Reactor (PWR) sub-channel is investigated using the CFD code TransAT. The flow is pretty much similar to circular pipe flow. Turbulent effects are predicted using highly-resolved Large-Eddy Simulation (LES) with a grid resolution of up to 6 million cells, resolving the viscous-affected layer. The sub-grid scale (SGS) viscosity produced by the model is indeed found to be of marginal effect for the grid and Reynolds number employed. Only first-order turbulence statistics are presented here. The results are discussed in detail, in particular key features specific to rod bundles, including low-Re effects in the narrow gap zone and the strong secondary flow motion, which is shown to exceed the turbulence counterpart (through the shear stress) near the wall. The secondary-flow motion induced by the mean flow is shown to be responsible for a large portion of the wall-to-flow heat transfer. The comparison of the LES results with existing DNS of pipe flow shows a very good agreement as to first-order statistics; higher-order statistics (including energy budgets) of the fluctuating fields have not been explored. A data basis has been generated for turbulence model comparison. Like in turbulent pipe flow, a physical explanation for the observed differences can be routed in the transverse curvature effects of the bundle geometry.