Characteristics of the laminar convective heat transfer of molten salt in concentric tube

•Laminar convective heat transfer of molten salt in concentric tube is studied.•Nu increases with increasing temperature for molten salt in laminar flow.•Nu of molten salt in laminar flow is higher than that of pure forced convection.•Natural convection can enhance heat transfer of molten salt in laminar flow.•Heat transfer correlation of molten salt in laminar flow regime is proposed. In this paper, laminar convective heat transfer performance of molten salt with Reynolds number of 300–2300 and Grashof number of 8.56×104–3.95×106 in a concentric tube heat exchanger is reported. The effects of temperature on the heat transfer coefficient of molten salt are experimentally studied with the temperature of 473K–573K. Results show that the measured Nusselt number is larger than that predicted by pure forced convective heat transfer correlations and the deviations decrease as Reynolds number increases. In addition, the experimental result displays a completely different dependence on the temperature from forced convective correlations. It is presumably that the radial density gradient gives rise to buoyancy motion resulting in the formation of natural convection and then strengthen the heat transfer performance in laminar region. By comparison, it is found that the experimental result follows the trend of laminar convective heat transfer correlations derived from traditional working medium, however, the traditional correlations cannot describe the molten salt adequately. Therefore, based on the experimental results and existing correlations, an improved correlation composed of forced convection term and pure natural convection term is proposed, and shows a good agreement with the experiment data with deviations less than ±15%.