Molten salt flow visualization to characterize boundary layer behavior and heat transfer in a natural circulation loop

Experimental data for the natural circulation of high-Prandtl number fluids in molten salt systems has not yet been sufficiently studied to ensure safe operation in the event of a loss of forced coolant. A natural circulation loop facility was designed to mitigate major challenges in flow visualization experiments of molten salt. Experiments were conducted with molten salt and water, and four experimental data sets were obtained for each working fluid at increased heater power. Particle image velocimetry (PIV) measurements were performed, and the boundary layer was analyzed as a function of the Prandtl number. Velocity peaks near the walls were found in results for molten salt due to the underdevelopment of the thermal boundary layers, in contrast to the parabolic velocity profile found in results for water. The overall system behavior was characterized using velocity and temperature measurements. Temperature distributions in the loop demonstrated the developing thermal boundary layers at high Prandtl numbers. The wall shear stress was obtained from near-wall PIV measurements to determine the friction factor and a non-dimensional analysis of the boundary layer showed the expected relationship for laminar flow in all test cases. The local and mean Nusselt numbers were calculated and compared between fluids. The local Nusselt number for water stabilized as expected for the laminar flow, while results for salt showed evidence of continually developing flow. Results for the Nusselt number and friction factor were compared with correlations, and the most accurate predictions were identified for the current application.