Transient computational fluid dynamics analysis of solidification in molten salt air-cooled heat exchangers

Molten salts are ideal for high-temperature applications, including pumped thermal energy storage, molten salt reactors, and industrial processes requiring continuous high-temperature operation and energy recovery. Compared to thermal oils, molten salt gas-cooled heat exchangers offer superior performance in high-temperature applications, thereby enhancing plant efficiency. However, cyclic operation of the heat exchanger may result in local salt solidification caused by rapid thermal excursions, which can pose technical and operational issues. This paper presents a numerical analysis of localized solidification within an air-cooled heat exchanger utilizing molten salt, employing a three-dimensional, pressure-based Newtonian solver coupled with a realizable $k-\epsilon$ turbulence model and an enthalpy-porosity approach. The operational limits of the working fluid (air) that trigger solidification within the molten salt air-cooled heat exchanger tube bundle are established. A two-step method, involving steady-state and transient analyses, is employed to evaluate the effect of air inlet temperature, pressure, air velocity, and initial molten salt temperatures on the air outlet temperature, overall heat transfer coefficient, and effectiveness. Subsequently, based on the results of the transient analysis, the onset of localized salt solidification is identified. The results of the steady-state analysis suggest that changes in air pressure and velocity significantly impact the effectiveness and likelihood of salt solidification, more so than do variations in inlet air and initial salt temperatures. The onset of salt solidification is accelerated at high air velocity (>2.0 m/s) and pressure (>50 bar) when both the inlet air and initial salt temperatures are at their lowest values, 473 K and 673 K, respectively. Furthermore, the results indicate that the upper tubes near to the nozzle inlet are particularly prone to solidification. Overall, the results serve as a benchmark for the modelling of salt solidification in heat exchangers, as well as supporting the design and optimization of freezing protection strategies for molten salt heat exchangers, resulting in effective and dependable systems for high-temperature applications.