PIV measurement and CFD analysis of the turbulent flow in a 3 × 3 rod bundle.

• The turbulent flow in a rod bundle was studied experimentally and numerically. • First- and second-order turbulence statistics were provided. • Large-scale quasi-periodic flow pulsation was detected by both PIV and CFD. • EB-RSM model showed acceptable performance in predicting the flow in rod bundles. The turbulent flow in a fuel bundle affects the heat transfer from the fuel rods to the coolant, which is a concerned issue in the design of fuel assemblies. In this paper, an experiment was performed using particle image velocimetry to measure the velocity distribution of water in a 3 × 3 rod bundle at various Reynolds numbers. The technique of matching index of refraction was utilized to fully visualize the flow field. Emphases were put on the flow inside a bare bundle that is free from the effects of grid spacer. It was found that the axial mean velocity in the subchannel is 30% higher than that in the gap center due to the cross-sectional nonuniform flow area. However, the axial root-mean-square velocity showed an inverse trend. The Reynolds shear stress exhibited a wavy variation in the lateral direction. A quasi-periodic large-scale flow pulsation was observed across the gap, and its wave length increases with Reynolds number. It was also found that the turbulent mixing coefficient was higher than that predicted by classical correlations. A numerical simulation was conducted signifying the acceptable performance of the EB-RSM model in predicting the flow behavior of water in the 3 × 3 rod bundle. Transient simulation with this turbulence model is able to capture the main flow phenomenon in the rod bundle. [ABSTRACT FROM AUTHOR]