Investigations on heat transfer characteristic of molten salt flow in helical annular duct

• A molten salt double tube helical heat exchanger was built.
• Nusselt number of molten salt flow in helical annular duct was experimental obtained.
• The molten salt flow regimes could be distinguished by Darcy friction factor.
• Higher heat transfer enhancement ratio was found with decreasing molten salt temperature.
• Smaller inner–outer-pipe diameter ratio leads to higher heat transfer enhancement ratio.

Molten salt is a preferable heat storage and heat transfer media, which can be employed in the high temperature industrial applications. A novel double tube helical heat exchanger (DTHHE) was developed in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF) using molten salt as the hot fluid flowing through the outer annular duct, to supply the heat for subcritical and supercritical water. In this work, the heat transfer characteristics of molten salt flow from laminar flow regime to turbulent flow regime were experimentally investigated. Heat transfer enhancement was detected in helical annular duct, and the enhancement ratio was found different from that in helical circular pipe. The heat transfer results were correlated separately in various flow regime, and the divergences between correlated equation and the classical heat transfer correlations of helical circular tube were analysed. The effects of inner tube surface were found pronounced on the flow and heat transfer characteristics. The experimental effort and results will benefit the design and operation of heat exchanger process between molten salt flow and water.