Experimental flow visualization and thermal-hydraulic characterization of local heating effects in a molten salt natural circulation loop

As the primary mechanism of passive heat removal in nuclear reactors, understanding natural circulation is paramount to their safe operation and shutdown conditions. Natural circulation in molten salt reactors is of particular relevance due to the fluid’s high Prandtl number as well as the phase of the fissile material in liquid-fueled reactors. Natural circulation loops are employed to study the thermal-hydraulic behavior of fluids when subject to thermal gradients and small flow disturbances. The objective of this work is to introduce and analyze the experimental effects of local heating conditions on the velocity profile and near-wall behavior of molten salts in a heated transparent test section. The study combines experimental PIV measurements with thermal-hydraulic characterization of the same natural-circulation operating states. Particle image velocimetry (PIV) was performed, and the boundary layer was analyzed for three different heating conditions. Those conditions were applied to the transparent test section: a cooling test condition, a thermally isolated test condition, and a heated test condition. In addition, the other thermal conditions of the loop were held constant. As the test section is heated, the peak velocity and slope of the velocity profile increase with test section heater power.