Deployment of Time-Resolved Particle Image Velocimetry between two PWR surrogate bundles

Highlights: • Time Resolved PIV has been for the first time deployed in between two PWR sub-bundles. • Design rules used to overcome optical challenges are illustrated in the paper. • Velocity field data prove the success of the set-up and set the base for further phenomenological analysis. There is a need to expand detailed experimental data on PWR thermal hydraulics to capture larger scale features than at the sub-bundle level. However, measuring fluid velocity between PWR fuel bundles is particularly challenging due to narrow gap geometries; to date, only point-wise data are available in the literature. Here, Time-Resolved Particle Image Velocimetry (TR-PIV) is performed in this intricate configuration. To prevent glare and scattering between laser beam and rods in the test section, a very careful optimization of the optical setup is necessary. The employed design rules are summarized in this manuscript. Sample velocity fields illustrate the successful design; they correlate well with bundle motion recorded independently with motion sensors. Captured flow structures ($\mathrm{Re}\approx {10}^4$) between bundles will set the foundation to improve the physical understanding of hydrodynamic coupling between bundles.