Experimental investigation of fluid mixing inside a rod bundle using laser induced fluorescence

The flow behavior and the flow structures inside the fuel assembly are a critical issue in the thermal-hydraulic analysis of a nuclear reactor. A better understanding of the fluid dynamics in the rod bundles can enhance the geometry design of the fuel assembly to improve the safety and efficiency of the reactor. The aim of this work is to investigate fluid mixing in a rod bundle experimentally with flow visualized and whole-field measurement techniques. In this paper, laser induced fluorescence (LIF) technique is adopted, and a visual experiment facility has been designed to achieve a concentration measurement in the both transverse and longitudinal sections. Matching index of refraction (MIR) technique and a new calibration method are applied to guarantee the experimental accuracy. Based on the above technology, concentration distribution in the rod bundle is obtained at the Reynolds number of 10450. A comparison of concentration distribution in the bare rod bundle and the rod bundle with spacer grids is performed to assess the mixing performance of the spacer grid. In the longitudinal measurement, the characteristics of fluid mixing downstream the spacer grid are analyzed, and the effect of different resistance components on the spacer grid for concentration distribution is revealed. The movement and position of dye in the transverse sections are obtained, the movement characteristics of dye are also analyzed. Furthermore, the coefficient of variation (COV) is used to quantify the fluid mixing in the rod bundle. The experimental results benefit the design of fuel assembly, and the data acquired in this study can be used as benchmark data for validating CFD codes.