Convective heat transfer and flow resistance of high-temperature argon under variable thermal hydraulics conditions

Inert monatomic gases such as argon are widely employed owing to their excellent chemical stability and well-defined thermophysical properties. However, substantial property variations during high-temperature argon cooling limit the applicability of conventional constant-property flow and heat transfer theories. This study experimentally characterized argon cooling in a horizontal smooth circular tube using a closed-loop high-temperature, high-pressure facility. Tests covered gauge pressures of 0.5 and 1.0 MPa, argon inlet temperatures of 800 and 1000 K, mass flow rates of 1.0–32.0 g·s⁻¹, coolant inlet temperatures of 303–363 K, Reynolds numbers of 1905–60,050, and bulk-to-wall temperature ratios of 1.76–2.50. The experimental results demonstrate that pressure had little effect on heat transfer or flow resistance. In contrast, increasing the mass flow rate markedly enhanced convective heat transfer but increased the frictional pressure drop, while variations in inlet gas and wall temperatures altered the thermophysical properties of argon and thereby appreciably affected both heat transfer and flow resistance. Conventional correlations, developed mainly for heating conditions or low-molar-mass gases such as helium, were inadequate: the Gnielinski correlation produced mean Nu prediction errors exceeding 100%, while the Blasius friction correlation yielded errors approaching 40%. Accordingly, argon-cooling correlations incorporating a temperature-ratio correction were developed from typical heat transfer and friction formulations. Comparisons with experimental data showed mean absolute percentage errors below 5% for the Dittus-Boelter-based heat transfer correlation and below 3% for the Taylor-based friction correlation. These results provide guidance for heat exchanger optimization, system safety criteria, and thermodynamic-cycle efficiency improvement.