Heat transfer characteristics of supercritical CH4-H2 mixed working fluid based on Rankine cycle in horizontal tubes

Based on Rankine cycle, the heat transfer characteristics of supercritical CH4-H2 mixture flowing through horizontal pipes were analyzed. The Re-Normalization Group k-ε turbulence model was adopted for numerical simulation and theoretical analysis to explore the effects of different hydrogen proportion (5%–15%) and pressures (6–10 MPa) on the physical properties of the working fluid. The influences of hydrogen proportion (0%–15%), mass fluxes (150–250 kg/(m2 · s)), and heat fluxes (150–250 kW/m2) on the heat transfer characteristics were analyzed. The results showed that as the hydrogen proportion increased, the pseudo-critical pressure rose, while the pseudo-critical temperature initially increased slightly and then decreased. When the pressure rose, the pseudo-critical temperature shifted to the right, the hydrogen proportion increased, and the overall thermophysical properties decreased. Increasing the proportion of hydrogen, mass flow rate and heat flux density all enhanced heat transfer coefficient. In the Rankine cycle system, hydrogen-doped working fluid as a mixed refrigerant effectively reduced the logarithmic mean temperature difference and improved the heat exchange efficiency of the heat exchanger in the cycle. Furthermore, a Nusselt number correlation formula based on the concentration weight factor (ηα) applicable to a certain working condition range (α = 0%–15%, Tin=130–170 K, G = 150–250 kg/(m2 · s)) was proposed, and the prediction error was within ±10%. This research provided a technical support and theoretical basis for the optimal design of hydrogen-blended natural gas transmission and power cycle systems.