Numerical research of flow and heat transfer characteristics of high-Prandtl-number fluoride salt in the pebble bed channel

Fluoride-salt-cooled High-temperature Reactors (FHRs) offer promising energy options for high-temperature industrial applications and electricity generation. The thermal-hydraulic characteristics of fluoride salt in the pebble bed reactor core are complex due to the molten salt's high Prandtl number and pebble bed structure. The research on it is important for the design and safe operation of the FHR reactor core. This work investigates the flow and heat transfer of fluoride salt in the pebble bed channel through computational fluid dynamics, with the Reynolds number ranging from 50 to 600. The results reveal that the critical Reynolds numbers for the laminar-to-transitional and transitional-to-turbulent flow regimes are 100 and 180, respectively. The evolution of vortex structures significantly influences the flow characteristics, directly impacting the turbulence intensity distribution and, consequently, the flow transition process. In the laminar regime, the narrow gaps between pebbles severely hinder fluid flow, leading to localized hot spots and elevated fluid temperatures in the vicinity of the gaps. Increasing the Reynolds number mitigates this effect, resulting in substantially reduced temperature peaks and a more uniform temperature distribution. Existing pebble-bed flow and heat transfer correlations show considerable deviation from the present numerical results at lower Reynolds numbers, with the new correlations developed based on the simulation data. This work can provide important thermal-hydraulic data and empirical correlations for the FHR reactor design and operation.