Advanced optical investigation of near-wall flow and heat transfer in a randomly packed pebble bed

This study employs time-resolved particle image velocimetry (TR-PIV) and laser-induced fluorescence (LIF) to experimentally investigate near-wall flow and heat transfer dynamics in a randomly packed pebble bed under both isothermal and non-isothermal conditions. A cylindrical borosilicate glass test section is packed with glass pebbles at a bed-to-sphere diameter ratio of 4.84 and includes two stainless-steel pebbles heated using an induction coil. By matching the refractive index of the working fluid, D-Limonene, with that of the glass, high-fidelity velocity and temperature measurements are obtained. Three-dimensional reconstruction of the randomly packed geometry validates computational domain fidelity for future numerical simulations. The TR-PIV data reveal that heated spheres induce buoyancy-driven flow features, altering local turbulence, vortex formation, and thermal mixing in the voids between pebbles and the wall. The proper orthogonal decomposition and vortex identification techniques confirm the vortex and circulation in non-isothermal conditions. Complementary LIF measurements captured temperature distributions around the heated spheres, which showed buoyancy-driven effects that varied with Reynolds number and flow direction. Notably, higher Reynolds numbers dispersed heat more effectively throughout the interstitial pores, while lower Reynolds numbers showed localized heating near the spheres. These results furnish comprehensive datasets for the validation of computational fluid dynamics models and offer deeper insight into reactor core design considerations for next-generation high-temperature gas-cooled reactor systems.