High-resolution velocity measurements in the vicinity of a customized spacer grid with rod bundle using matching index of refraction and particle image velocimetry

In Pressurized Water Reactors (PWRs), the fuel rods are held together by spacer grids, which also have the purpose of improving convective heat exchange by increasing turbulence mixing in the sub-channels. The flow in rod bundles with spacer grids has motivated many research studies over the past decades, and one of the applications of experimental data obtained for this kind of flow is the need of computational code validation. Recently, the fast increase of computational power has led to the possibility of a deeper understanding of small scale phenomena and therefore there is now a need of highly resolved experimental data and uncertainty quantification. Most of the experiments performed with spacer grids use commercial geometries, which details cannot be widely distributed. In this study, a customized noncommercial spacer grid was developed with the objective of allowing results comparison by different research groups. Experiments were performed in a 5x5 rod bundle facility with the use of a recently developed combination of Matching Index of Refraction (MIR) materials for the rods and fluid. An experimental procedure combining Particle Image Velocimetry (PIV) and MIR techniques was employed to obtain high resolution velocity measurements in the vicinity of the new spacer grid. This paper presents and discusses the results for velocities, Reynolds stresses and uncertainties obtained with the experiments using the non-commercial spacer grid.