Experimental And Numerical Investigation On Characteristics Of Flow In A Fuel Rod Bundle

The fuel assembly of a pressurized water reactor consists of a bundle of fuel rods. In a reactor core the coolant flows axially through the rod-bundle fuel assembly and removes the heat. The efficiency of heat removal is essential for the operation safety and economic performance of a nuclear reactor. Hence, more comprehensive understanding of flow and heat transfer characteristic in a rod bundle can help the design of fuel assembly. Recently, Computational Fluid Dynamics(CFD) is becoming an important tool for the design and performance analysis of fuel assemblies. However, it has been recognized that there still exist large uncertainties while applying the CFD tools to analyze the flow in the rod bundle due to various reasons, such as the irreality of inlet condition, imprecision of meshing and unreasonable of the choose of tuebulence models.. Therefore, more validation and assessment are needed based on the experimental data, in order to develop the best practice guideline for CFD calculation.In order to meet the demand of the validation and assessment of CFD analysis, we carried out a 6x6 rod bundle experiment in which the flow field is measured with the five-component three-dimensional laser Doppler velocimetry(LDV). In the experiment, the axial mean velocity and root mean square of axial velocity fluctuation have been measured at three cross sections downstream of a grid spacer. Three dimensional velocities and all the six components of Reynolds stress have been measured on the cross section near the outlet of the bundle. The influence of P/D ratio on the distribution of flow fields is also investigated. Based on the comparison of measured flow fields at different cross section, we studied the evolution of mean velocity and turbulence intensity in the downstream of grid spacer. It has been found that the larger local axial velocity in all the measured cross section tends to appear in the subchannels where P/D ratio is relative small. The small P/D ratio helps to maintain relatively high turbulence intensity, even in absence of any external mixing devices. In order to study the influence of Reynolds number, the experiments have been carried at the Reynolds numbers ranging from 6600 to 70000. And we found that the Reynolds number effect is relatively weak in the typical P/D ratio of a PWR.For the assessment of CFD calculation, three turbulence models, including the SST, Baseline Reynolds Stress Model(RSM) and SSG RSM were selected. Through comparison with experimental results, we found that the BSL RSM model shows the best prediction of the axial velocity, however a large discrepancy in Reynolds stress.