Turbulence Model Evaluation in 5x5 Pwr Rod Bundle Numerical Simulations with a Split Vane Spacer Grid

This work presents results of flow simulations performed with the commercial code CFX 11.0 in a PWR 5x5 rod bundle segment with a split vane spacer grid. The geometrical configuration and flow conditions used in the experimental studies performed by Karoutas were assumed in the simulations. To make the simulation possible with a limited computational capacity and acceptable mesh refinement, the computational domain was divided in 7 subdomains. The subdomains were simulated sequentially applying the outlet results of a previous subdomain as inlet condition for the next. In this study 3 turbulence models were evaluated: k-ε, BSLRS (Baseline Reynolds Stress) and SST (Shear Stress Transport k-ω). The results show that all models perform similarly up to ~100 mm downstream the spacer grid showing an reasenably accurate prediction when compared to the experimental axial and lateral velocity profiles. Away from the grid appreciable differences between the axial velocity profiles predicted by the turbulence models were observed. In these cases the SST model showed a slightly better qualitative agreement with the experiment. Measured and predicted swirl factor indicates also a good qualitative agreement. The three turbulence models predict a similar tendency of reduction of the swirl with the distance from the spacer grid. Numerical pressure loss predictions were compared to a semi-empirical formulation and showed good agreement for the total and spacer grid losses along the rod bundle for all evaluated models except the k-ε model that for the grid pressure loss predicted a value 10.9% higher than the formulation, indicating a poorer predictability of pressure behavior of this model for the spacer grid simulation. These results confirmed that the assumed numerical procedure of dividing in several parts the numerical domain with a better quality mesh is a feasible approach for rod bundle simulation. In general the results obtained with the evaluated turbulence models show little differences that indicate that SST and BSLRS models are more suited for spacer grid simulation.