Numerical Investigation on the Thermal-Hydraulic Performance of Molten Salt in Internal Helically Finned Tubes

Molten salts have attracted considerable attention as heat transfer and thermal storage media in high-temperature energy systems, including concentrating solar power systems, advanced heat exchangers, and nuclear-related thermal systems. However, their relatively low thermal conductivity, high Prandtl number, and strong temperature-dependent viscosity may limit convective heat transfer performance and increase the difficulty of thermal-hydraulic design. Internally helically finned tubes have been widely used as passive heat transfer enhancement structures in conventional thermal systems, but their applicability to high-temperature molten-salt flows remains insufficiently understood. In this study, a three-dimensional numerical model was developed to investigate the thermal-hydraulic performance of a low-melting-point quaternary nitrate salt flowing through internally helically finned tubes. The effects of fin pitch, fin height, and helix angle were systematically examined over a Reynolds number range of 14,000–26,000. The results show that decreasing the fin pitch and increasing the helix angle enhance near-wall flow disturbance and improve convective heat transfer, while also increasing the pressure drop. The fin height exhibits a more pronounced trade-off effect: although larger fins increase the heat transfer coefficient, excessive fin height causes a substantial hydraulic penalty and weakens the overall performance. Based on the Performance Evaluation Criterion, the configuration with a fin pitch of 1.4 mm, fin height of 0.4 mm, and helix angle of 30° achieves the best overall thermal-hydraulic performance within the investigated parameter range, with a maximum PEC of 2.14. Compared with representative enhanced-tube configurations reported for molten-salt heat transfer, the internally helically finned tube shows competitive comprehensive performance. This work provides a numerical assessment of the feasibility and design sensitivity of internally helically finned tubes for molten-salt heat exchangers and offers useful guidance for the optimization of high-temperature thermal energy systems.