Experimental study on convective heat transfer characteristics of fluoride salt outside the tube bundle in hairpin heat exchanger

Convective heat transfer performances of fluoride-salt flowing outside U-tubes with 4871 < Re < 12298 and 11 < Pr < 17 in hairpin heat exchanger are experimentally investigated. The effects of temperature on the heat transfer coefficient of fluoride-salt are studied with the temperature 540–598.7 °C. Results show that the measured Nusselt number is in line with the empirical heat transfer correlation, but it is slightly larger than that predicted by conventional correlations and the deviations increases as Prandtl numbers increase. It is presumably that the special geometric construction with densely packed bundles and U-bend in hairpin heat exchanger disrupt the boundary layer resulting in the formation of back-flow zone and then strengthen the heat transfer. Therefore, previous correlations are insufficient in describing the fluoride-salt performance. Finally, based on the model of Sieder-Tate correlation, an improved correlation considering a comprehensive heat transfer enhance is proposed, and shows a good agreement with the experiment data with deviation less than 4.8 %.