Heat transfer of hydrogen with variable properties in a heated tube

Hydrogen as a fuel or a coolant plays an essential role in carbon emission reduction and energy system transformation. There has been a long need for a better method of explaining the heat transfer mechanism for gases with variable physical properties in the past few decades. In this study, an experimental investigation was conducted to explore the heat transfer and flow friction of hydrogen in a horizontally heated tube for the following ranges: local Reynolds number from 4300 to 48600, local wall temperature up to 1200 K, local wall-to-bulk temperature ratio up to 2.37, and heat flux up to 0.57 MW/m². The average friction coefficient equations with heat addition are developed for 6000 < Re < 10⁵ and 1.20 < T_w/T_b < 1.85. It is found that the modified average heat transfer correlation by the acceleration parameter K_v is proposed for 4900 < Re < 44500 and 5 × 10⁻⁸ < K_v < 1 × 10⁻⁶ that can cover the laminar-turbulent transition and turbulence regime with an accuracy of ±10%. A new correlation evaluated at the bulk temperatures is proposed with the temperature and length correction to predict the local heat transfer of hydrogen, which is in good agreement with the experimental data in the range of 8000 < Re < 48000, 6.2 < x/D < 111, and 1.17 < T_w/T_b < 2.37. This investigation can facilitate improvements in thermal hydraulics analysis codes and the heat transport system with variable fluid properties.