Investigation of turbulence characteristics of the flow in a 1x3 array fuel rod bundle with spacer grids by using computational fluid dynamics method

In nuclear reactor analysis, understanding the effects of spacer grids on flow have an important place in determining the reactor thermal-hydraulic performance and ensuring that reactor works under reliable operating conditions. Computational Fluid Dynamics (CFD) methods which could reflect the physics of the real flow are capable of presenting precise analysis results. In this study effects of simple support grids, independently of the mixing vanes, on flow through an 1 x 3 array rod bundle has been investigated with CFD methodology and the most appropriate turbulence model reflecting physics of the flow has been determined. Experimental studies in the flow laboratories of Penn State University has been referenced in comparison of the simulation results where, single phase flow in subject rod bundle was examined [1]. In the first part, mesh convergence study has been carried out on tetra, hybrid and poly type mesh in order to determine the most appropriate type and density. In this section, only k-ε Standard and RSM LPS turbulence models have been utilized. In the second part of the study, using the mesh determined in the first section, comparison has been made covering all turbulence models examined in the study. Velocity distribution in the center of the rods, velocity and turbulance intensity contour plots on the upstream and downstream of the spacer grid at –3dh, +3dh and +40dh locations have been examined and compared with the experimental data. The results of the study reveal that the effects of the mesh type on calculations where, hybrid mesh having the most structured elements exhibit better performance over the others. Comparison between numerical and experimental results of channel central velocities shows an overall agreement with all turbulence models while, complex models and exceptionally k- ε RNG present better results than other models. As a result of this study, flow through a simple support grid has been examined and the most appropriate turbulence model reflecting the physics of the flow has been determined.