The forced convective heat transfer of a molten nitrate salt in a circular smooth tube and a spirally grooved tube is experimentally investigated using a water-cooled induction heater. The novel experimental setup is first validated using water as the main fluid, as its heat transfer is well-known. The final experiments for both tubes are carried out with the so-called nitrate salt “Solar Salt” (60% NaNO3 40% KNO3) at fluid bulk temperatures ranging between 300 °C and 550 °C. Consequently, Prandtl numbers range from 10 to 3.7. For the smooth tube experiments, the salt mass flow is varied to cover Reynolds numbers ranging from 14000 up to 222000. Furthermore, the experiments are conducted with different heat fluxes ranging between 330 kW/m² up to 930 kW/m². To investigate the impact of local overheating of the salt above its chemical stability limit (i.e. 600 °C), the mean Nusselt number is evaluated for inner wall temperatures up to 633 °C. Mean Nusselt numbers are reported and compared to the well-known Gnielinski correlation. The measurements for the spirally grooved tube are carried out for a tube with a relative groove height e/di=0.017, a relative groove pitch p/di =0.913 and groove angle α=73.8° using the same water-cooled induction heater setup. The data is provided for Reynolds numbers ranging between 11000 up to 285000 and for flux densities between 330 and 930 kW/m². The setup achieves inner wall temperatures up to 633 °C, thus providing data on the behaviour of the forced convective heat transfer as a function of inner wall temperature. The mean Nusselt number and friction factor as a function of Reynolds and Prandtl number are reported and compared to the measurement results yielded with the circular smooth tube as well as correlations and data from the literature.
Cathy Martine Lina Frantz (Wed,) studied this question.