Study Of Fluid Mixing In Rod Bundle Using Laser Diagnostic Technology

Experimental and theoretical studies of single-phase water fluid mixing characteristics in rod bundles with spacer grids were conducted in this paper.A test facility with excellent visualization performance has been designed and constructed, and the single-phase water fluid mixing tests based on laser induced fluorescence technique were conducted in rod bundles with 3×3 and 5×5 spacer grids respectively. The test facility contained a test loop, a test platform, rod bundle assemblies, the optical and imaging system, and relevant instruments and equipments. The rod bundle assemblies were formed by Plexiglas, and had a square cross section with a side length of 40.9mm or 65mm. The rods were made by FEP with a diameter of 9.5mm,and the pitch of the rod bundle was 12.6mm.Fluorescent dye tracer was injected into the sub-channels in the upstream of a spacer grid to visualize the fluid mixing process. Laser sheet was formed horizontally or vertically by the optics system to emit the fluorescence, and the imaging system was used to record the fluorescence information. Then a post-processing program, including image reshaping and online calibration, was made to reconstitute the instantaneous and time-averaged concentration fields of each test pictures. In addition, a velocity field measurement methods based on the LIF technique was proposed.Different flow rate and inject position tests were conducted in a 3x3 rod bundle, and the instantaneous and time-averaged concentration fields of different horizontal and vertical cross sections were reconstituted, then statistics analysis was conducted to quantify the fluid mixing. The qualitative and quantitative results show that the mixing vanes can induce crossflow and turbulent fluctuation, which lead to fluid mixing, and the crossflow is the major factor of flow mixing 0-3 hydraulic diameter (Dh) behind the spacer grid, while turbulent fluctuation is the dominant factor 3Dh behind the spacer grid. The statistics analysis indicate that the fluid mixing phenomenon in the subchannel is more intense than in the rod gaps behind the spacer grid, and the fluid mixing decay gradually 5-6Dh behind the spacer grid.Different flow rate and frame rate tests were conducted in a 5×5 rod bundle, and the instantaneous and time-averaged concentration fields were reconstituted, and an image fractal measurement code was compiled. Then fractal dimension of the experimental pictures were analyzed by the fractal measurement code. The analysis results indicate that the turbulent flow in the downstream of the spacer grid was fully developed. And the fractal dimension of the fully developed turbulent flow is 2.676, which does not change with frame rate, distance and Reynolds number.Comparison analysis indicate that laser induced fluorescence technique can be competent in the measurement of fuel rod bundle. Qualitative and quantitative analysis of fluid mixing can be achieved,which is useful for the development and evaluation of spacer grid, and the development of subchannel codes, as well as the improvement of turbulence models. What's more, it can provide dates for the V&V of CFD codes.