Numerical Analysis of Solidification in Molten Salt-Air Shell-and-Tube Heat Exchangers

Abstract The cyclic operation of molten salt heat exchangers raises concern regarding the potential freezing of salts due to their high freezing point. Therefore, pinpointing the initiation of salt solidification and forecasting the phase change behavior in high-temperature heat exchange processes is necessary, as these factors influence the design and operation of the heat exchangers. This paper presents a three-dimensional, transient computational fluid dynamics analysis of a pilot-scale molten salt-air-cooled exchanger with air on the shell-side and molten salt on the tube-side. We aim to replicate the conditions the molten salt experiences during operation in a concentrating solar power plant or pumped thermal energy systems. Consequently, the study explores the influence of the initial molten salt temperature and the baffle configuration on the air outlet temperature, pressure drop and onset of salt solidification. A realizable k-ε turbulence model was adopted to capture the turbulent viscosity and dissipation rate, and a segregated solver algorithm was used to iteratively solve the coupling between the pressure and velocity fields. The numerical model was validated using previous results of a shell-and-tube heat exchanger employing pure water as the shell-side fluid. The main novel contribution of this work is to estimate the time it takes until the molten salt begins to solidify within the tube bundle when air acts as the working fluid on the shell-side of a single segmented baffled shell-and-tube-type heat exchanger. Additionally, it aims to ascertain the impact of inclusion of a flow diverter on delaying the salt solidification. The findings indicate that varying the baffle configurations has little to no effect on the air outlet temperature nor on the onset of molten salt solidification for all tested configurations without a flow diverter. Concurrently, inclusion of flow diverter helps redistribute the flow near the shell inlet nozzle, thereby delaying the onset of solidification. Furthermore, the results suggest that about one-third of the total heat exchanger length proves to be ineffective in contributing to the heat transfer, due to the creation of recirculation zones. These results serve as a benchmark for future heat transfer analyses of the dynamic operation of the molten salt-air-cooled heat exchangers.