Numerical simulation and analysis of flow and heat transfer in a 5 x 5 vertical rod bundle with buoyancy effects

A numerical investigation of mixed and forced turbulent convection in a 5 x 5 rod bundle with spacer grids has been performed in this paper. Wall temperatures and Nusselt numbers calculated by a Realizable k-s model and a Reynolds stress model are both validated with the experimental data and the latter model shows a better performance. From the CFD results, it is found that under conditions of high Bo* numbers, the higher values of axial velocity appear in the vicinity regions of the heated rod rather than in the center of sub-channels. The buoyancy force also influences the turbulence intensity, and it seems that the fluid flow is harder to get fully developed under conditions of high Bo* numbers. In addition, based on the comparison of calculation results for different buoyancy parameters, the heat transfer behavior downstream of a spacer grid is considered to be related to the turbulence kinetic energy affected by the buoyancy force.