Stereoscopic particle image velocimetry measurements of flow in a rod bundle with a spacer grid and mixing vanes at a low Reynolds number

In this study, we experimentally investigate the flow field characteristics in a fuel rod bundle with a spacer grid and split-type mixing vanes at a low Reynolds number of 14, 000. Time-resolved stereoscopic particle image velocimetry (TR-SPIV) measurements were performed in a matched-index-of-refraction facility, and taken downstream of the mixing vanes. Velocity measurements were performed along the vertical-horizontal planes in the inter-channels between the fuel rods. From the obtained TR-SPIV velocity vector fields, the full-field flow statistics such as the mean velocity and Reynolds stresses were computed. Moreover, spectral analysis, squared coherent functions and spatial-temporal velocity cross correlations were performed on the TR-SPIV velocity snapshots. A variety of integral length scales estimated from the two-point spatial cross-correlations have shown that the flow downstream of the mixing vanes was strongly anisotropic. Vortex shedding frequencies were found to contribute to the peaks of coherent functions computed from the fluctuating velocity. This work is the first experimental evidence confirming the findings and discussions from previous numerical studies about the anisotropy flow and a wide range of turbulence length scales generated by the mixing vanes.