Experimental study on flow and heat transfer characteristics of very-high-temperature helium flow in round tube

Helium exhibits excellent flow and heat transfer performances, stable physicochemical characteristics, and high material compatibility, rendering it extensively utilized in advanced industrial applications. Particularly in high-temperature gas-cooled reactors (HTGR), helium can serve both as a coolant for heat removal and as the working fluid in Brayton cycle systems. In Generation IV reactors, such as the very-high-temperature reactor (VHTR), the coolant outlet temperature exceeds 950 °C, reaching over 1000 °C. Therefore, the applicability and accuracy of traditional correlations within this extreme thermal range require evaluation. This study established an experimental system to investigate the flow and heat transfer characteristics of helium under very-high-temperature conditions. The friction factor, as well as the local and average heat transfer coefficients, was measured as helium flowed through a vertically heated round tube. The outlet temperature ranged from 987 °C to 1010 °C, Reynolds numbers varied from 3,797 to 1.15 × 10⁴, and the maximum heat flux is 0.78 MW·m^-2. In this study, the uncertainties of the friction factor and Nusselt number are lower than 3.39% and 3.61%, respectively. A comparative analysis was conducted between the traditional correlations and experimental data. The results indicate that the Gnielinski correlation predicts the Nusselt number more accurately under very-high-temperature conditions, while both the Blasius and Serghides relations underestimate the friction factor. Based on the experimental data, correlations for flow and heat transfer of helium in round tube under very-high-temperature conditions were derived. The new correlations demonstrate relative deviations within ±5% compared to experimental data, confirming their reliability for engineering applications.