Natural convection heat transfer characteristics of molten salt with internal heat generation

Molten salt is a promising heat transfer fluid with its high heat capacity, high boiling point, and chemical stability. In liquid fueled molten salt reactors and spent fuel reprocessing unit, molten salt carries heat generating fuel itself in a liquid state. To investigate the natural convection heat transfer performance of molten salt, a vertical heater vertical cooler rectangular loop filled with chloride salt containing liquid fuel, NaCl-KCl-(Pu, 238U)Cl3, is simulated using an open source CFD package, OpenFOAM under various heat input conditions; external heat flux, internal heat generation (IHG), and a combined heat input type condition. Different aspects of natural convection heat transfer according to heat input condition are found for same amount of total heat. The external heat flux condition generates a high temperature gradient near the heater wall with very thin thermal boundary layer due to the low thermal conductivity of molten salt. On the other hand, the IHG condition has a relatively flat temperature profile from its uniform volumetric heating. Due to the different behaviors of temperature evolution, local upward flow is formed only at the wall region under the heat flux condition, but global natural circulation flow is promoted under IHG condition. The combined type condition is largely dependent on how much heat exists in heat flux type, which strongly influences local heat and fluid phenomena in natural convection heat transfer. Molten salt has a high dependence on the heat input condition from its thermo-physical properties. Therefore, the distinct characteristics of molten salt and heat input conditions should be considered, when designing and analyzing the molten salt systems with both external and internal heating conditions.