LIF study of temporal and spatial fluid mixing in an annular downcomer

Abstract

In possible accident scenarios of the reactor coolant system, the convection and mixing characteristics of borated water have a great influence on reactivity variations in the reactor, and then impact the reactor safety. The mixing and diffusion characteristics of the boric acid solution in the annular downcomer is studied by laser-induced fluorescence (LIF). An aqueous solution of Rhodamine B (RhB) fluorescence dye, which is used to simulate boron solution, was injected in the coolant inlet to visualize the fluid mixing process in the annular volume. Laser sheet was formed by optical system to emit the fluorescence, a high-speed camera was used to capture the fluorescence information for reconstructing the concentration distribution of dye. In order to guarantee the experimental accuracy, LIF image processing and a calibration technology are applied. The concentration profiles were implemented to illuminate the fluid mixing between dye solution and diluted water in the downcomer. The coefficient of variation (COV) was conducted to quantify the spatial and temporal transport of dye in the annular downcomer. The results indicate that a higher flow improves the transport ability of dye in the transverse direction, and shortens the distance and time to uniform. The mass force can accelerate the dye transport in the vertical direction. The whole-field measurement technique implemented in this study is expected to provide an insight in the research of flow mixing in the complex geometry. The experimental results can provide a better understanding of mixing phenomena in annular downcomer, and it is also useful for the annular downcomer CFD validation.