Abstract This study presents velocity field measurements in a randomly packed pebble bed obtained through particle image velocimetry (PIV) experiments. The optically transparent borosilicate duct was loaded with 30 mm glass spheres, giving a bed-to-sphere diameter ratio of 4.84. This arrangement enabled non-intrusive interrogation of near-wall flow dynamics for both upward- and downward-directed isothermal flows. Using d-Limonene with matched index of refraction techniques, we captured high-resolution velocity maps and reconstructed the 3-D randomly packed pebble configuration. The PIV results produced first- and second-order statistical measures of the flow, including assessments of mean velocity magnitude, velocity fluctuations, Reynolds stresses, jet-like flow, recirculation, and bypass flow structures within the interstitial voids of the randomly packed pebble bed. These structures change in position and intensity as the bulk flow direction is reversed. Proper Orthogonal Decomposition (POD) and multi-scale vortex-identification algorithms further exposed turbulent coherent structures and regions of high vorticity. The resulting high-fidelity database will support future validation of computational fluid dynamics models for pebble bed reactor cores (PBR). Ultimately, the insights gained here help refine the design envelope of pebble-bed reactor cores and related energy storage systems.
Macias et al. (Mon,) studied this question.