A computational study on turbulent flow in rod bundle subchannel with plain spacer grids

A computational study has been carried out to investigate the turbulent flow structure in the rod bundle subchannel with plain spacer grid using FLUENT CFD code and the computation results are compared with 5x5 rod bundle experimental data generated by KAERI. The computation works are performed in liquid water with Reynolds number of 61,200 at the atmospheric pressure condition and the analysis region is from the top of spacer grid to 55 DH of downstream direction. The Reynolds stress equation model is employed to analyze the turbulent flow characteristics in the subchannel. The computation results show the same trend with the experimental data. The velocity profile of mainstream flow in a subchannel shows defected shape at the center of internal subchannel until 5 DH of downstream direction from the top of spacer grid due to the effect of spacer grid. The typical velocity profile of rod bundle subchannel comes out after 15 DH of downstream. The cross sectional velocities of the experimental data are somewhat widely distributed along the subchannels but the analysis results have nearly constant values after 10 DH of downstream. An anisotropy of turbulent flow exists in all subchannels due to the spacer grid because the value of mainstream component term of Reynolds stress is higher than that of cross sectional components. The spacer grid effects to Reynolds shear stress reaches 10 DH of downstream and Reynolds shear stress is affected by both spacer grid and rod bundle after 10 DH