Transient gravity-induced stratification of hydrogen–methane mixtures in pipeline systems: A coupled thermodynamic–transport and timescale analysis

Gravity-induced stratification of hydrogen–methane(H₂–CH₄) mixtures remains a debated safety concern in hydrogen-blended natural gas pipelines, particularly in vertical and quiescent sections. In this work, a unified thermodynamic–transport framework is developed to quantitatively evaluate both the equilibrium distribution and the transient timescale of H₂–CH₄ mixtures under gravity. The framework integrates(i) a minimum-energy model based on Helmholtz free energy minimization with the Peng–Robinson real-gas equation of state and(ii) a transient convection–diffusion model coupling gravitational drift with molecular diffusion. Model predictions agree with benchmark literature and experimental data within 2%. Results show that temperature accelerates diffusion, pressure delays equilibration without altering steady-state distributions, and pipe height primarily controls the stratification timescale rather than its magnitude. Even under extreme conditions (−10°C, 10 MPa, 30 vol% H₂, 1 km height), the steady-state top–bottom hydrogen volume fraction difference remains below 1.5%, while more than 2.7 × 10³ years are required to reach equilibrium. Under typical urban gas conditions, the concentration difference after 10 days is below 0.01%. These findings demonstrate that gravity-induced stratification of H₂–CH₄ mixtures is physically possible but engineering-irrelevant within practical pipeline lifetimes, providing a time-explicit and quantitatively justified basis for pipeline design and safety assessment.