Experimental investigation of heat transfer and friction characteristics of high-pressure helium with variable properties in circular tubes

Helium, extensively utilized in high-temperature gas-cooled reactors and aerospace systems, exhibits significant thermophysical property variations under intense heating. These variations pose challenges for the accurate prediction of flow and heat transfer behavior. This study conducts an experimental investigation into high-pressure helium flow (2.7–3.1 MPa) within circular tubes with inner diameters of 5, 8, and 10 mm. The experiments span Reynolds numbers from 1870 to 43,370 and wall-to-bulk temperature ratios ( T _w / T _b ) ranging from 1.06 to 2.18. The results demonstrate that classical correlations significantly overpredict heat transfer performance under variable-property conditions, particularly at low Reynolds numbers ( Re < 10⁴) and high T _w / T _b . For friction factors, the Koo correlation agrees with experimental data within ±15 % for Re > 3000, while the Taylor correlation incorporating the wall Reynolds number shows better predictive performance under high T _w / T _b . Local heat transfer is characterized by piecewise correlations: a simplified formulation for Re > 10⁴ and segmented equations for Re < 10⁴ that incorporate T _w / T _b threshold effects, both demonstrating prediction errors within ±20 %. A newly developed correlation for the average Nusselt number achieves deviations within ±20 % across the parameter ranges 2000 < Re _ave < 30,000 and 1.19 < T _w / T _b < 2.01. These findings advance the understanding of variable-property helium flow and heat transfer, providing enhanced predictive tools for optimizing the design and performance of helium-cooled systems in high-temperature applications.