Improved Turbulence Modeling for Performance Evaluation of Novel Fuel Designs

An improved quadratic nonlinear eddy viscosity model (NLEVM) is introduced that respects the constraints of realizability for calculation of detailed coolant velocity and temperature distributions inside tight lattice fuel bundles. The model adopts an optimized low-Reynolds formulation based on direct numerical simulation data, combined with an enhanced nonlinear stress-strain relationship to correctly capture the anisotropy of the flow up to the solid wall. The capabilities of the model are first assessed on the prediction of fully developed flow inside triangular lattice bundles; it is shown how the ability to correctly reproduce the turbulent-driven secondary flow enables the model to accurately reproduce wall shear stress and velocity distributions inside the bundle. The model is applied to the evaluation of the thermal-hydraulic performances of novel fuel designs, discussing potential advantages and limitations of the newly proposed solutions.