To investigate the means for improving molten salt heat transfer components, helically rifled tubes, a passive heat transfer enhancement, are investigated in this study. The investigation focused on predictions of thermal performance relevant to molten salts used in fission, fusion, and concentrated solar power plants. Currently, there are limited systematic studies for helical rifling heat transfer enhancements as opposed to corrugated, internal finned, or repeating ribbed tubes. For this study, the computational fluid dynamics (CFD) code Nek5000/NekRS was used to simulate different convective flow regimes for different molten salts (i.e. fluorides, chlorides, and nitrate salts).

The outcomes from the CFD investigations involved both frictional pressure drop (friction factor) and heat transfer coefficient (Nusselt number) predictions for a range of turbulent Reynolds numbers and Prandtl numbers spanning from unity “1” to 25. The friction factor results were observed to have increased by a percent difference of 35% to 48% compared to plain tube data. This is consistent with boiler tube results from groups such as Lam et al. and Pan et al.

The CFD predictions resulted in the Nusselt number (heat transfer) increasing substantially with increasing Prandtl numbers for helically rifled tubes. It was also found that that the Nusselt number was observed to have minimal increases for all Reynolds numbers at Prandtl numbers close to unity. Using the predicted thermal performance information (friction factor and Nusselt number), the thermal performance factor of the helically rifled tubing was calculated as compared to plain tubes. It was found that the thermal performance factor was higher for Prandtl numbers larger than unity. However, it was determined that the thermal performance decreased for all Reynolds numbers when fluids with Prandtl numbers at or below unity were analyzed. Overall, this study presents a useful starting point for future investigations of helically rifled tubing in molten salt applications.