Numerical Simulation Of The Turbulent Flow Of Water In A 3×3 Rod Bundle

At the downstream of the positioning grid of PWR fuel assembly,the local disturbance gradually attenuates along the flow direction,and the flow field finally tends to be stable.The turbulent flow and mixing characteristics of the coolant in the smooth rod bundle region are one of the important factors affecting the reactor economy and safety,so it is necessary to study them in depth.In this thesis,the turbulent flow characteristics of the water in the rod bundle are studied by numerical simulation.The geometry used is a 3 × 3 rod bundle.The outer diameter of each rod is 9.5 mm,the spacing between adjacent rods is 12.6 mm,and the grid distance ratio(P/D)is 1.33,which is consistent with the grid distance of PWR fuel assembly.The cross-sectional area of the square tube around the bundle is 37.8 × 37.8 mm2,and the length is 200 mm.ANSYS software CFX is used for steady and unsteady state simulation.Firstly,eight turbulence models(k-epsilon,SST-k-ω,SSG,RNG k-epsilon,BSL Reynolds stress,LRR Reynolds stress,QI-Reynolds stress and Omega Reynolds stress)are selected for steady-state simulation,and the results are compared with the experimental data to select the best turbulence model for development.The effects of the inlet velocity and the ratio of bar to grid on the flow characteristics are studied by numerical simulation.The range of inlet velocity is 0.5 m/s to 2 m/s,and the ratio of pitch to diameter is 1.18 to 1.42.The results show that QI-Reynolds stress model is better than other turbulence models,and the predicted results are most close to the experimental data.The velocity of the central sub-channel is obviously higher than that of the rod gap region,and the deviation between them is related to the grid distance ratio.Secondly,the QI-Reynolds stress model is used to simulate the unsteady state.It is found that there is large-scale flow pulsation in the dense rod cluster,and the pulsation intensity increases with the decrease of grid distance ratio.The three components of the velocity fluctuated with time,and the fluctuation is more stronger in the stream wise direction than in the lateral direction.This research work contributes to understand the coolant flow behavior in the rod bundle.