Velocity and pressure fluctuations downstream analytical spacer grids: Structure and transport

In Pressurized Water Reactors (PWR), fluid–structure interaction provokes vibrations of the fuel rods leading to grid-to-rod fretting. To explore the origins of the local excitations of the rods by the flow, analytical experiments are performed within a 5 × 5 rod bundle maintained by spacer grids. Experiments are performed using two sets of grids, i.e. configurations with and without mixing vanes, over a wide range of Reynolds number, i.e. from about 10,000 to 120,000. Particle Image Velocity measurements near the central instrumented rod reveal different structures of velocity fields and their fluctuations for the two configurations. Mean fields are in U inverted shape without mixing vanes and in λ shape with mixing vanes. Fields of velocity fluctuations are in butterfly shape without mixing vanes and in Y (low Re) and K (high Re) shapes with mixing vanes. Near the grid, ${u^{\prime }}_{\mathrm{rms}}/U_{\mathrm{flow}}$ increases by about 30% for ${\mathrm{Re}}_{\mathrm{Dh}}$ varying from 10,000 to 120,000. The presence of eddies is highlighted by visualisations of the fields of the velocity fluctuations. The spacing of these eddies and their sizes are found to be in agreement with the periodic length scales (obtained from frequency peaks) and the integral length-scales measured. Frequencies of the observed frequency peaks are found to be the same for both velocity and pressure fluctuations. Consequently, a new Strouhal versus Reynolds map is built over all existing data for Reynolds numbers between 10,000 and 120,000 and for the two grids configurations. The integral length-scales are found to be about the same for velocity and pressure fluctuations near the grid. It is shown that events of pressure fluctuations are persistent and transported with a speed close to the one of the mean flow. Moreover, this transport is correlated with the displacement of velocity events. The coherence between measured turbulence statistics, the observed eddies and cross-correlations of pressure and velocity fluctuations support the conclusion that the large-scale eddies (about 0.2 $D_h$) are at the origin of main pressure fluctuations and their transport. To the authors’ knowledge, this is the first experimental evidence of the origin and transport of pressure fluctuations downstream analytical grids with different geometries relevant to PWR fuel assemblies.