Numerical Simulation of Hydrogen Mixing Process in T-Junction Natural Gas Pipeline

As a cost-effective transitional strategy, the integrated utilization and transportation of hydrogen and natural gas have gained significant attention as a viable pathway toward carbon neutrality. However, hydrogen's low density, viscosity, and calorific value cause upward migration and accumulation in pipelines, raising embrittlement risks. Its high diffusion and leakage rates also pose significant safety challenges. To address hydrogen-natural gas blending challenges, achieving uniform mixing is crucial. This study systematically examines hydrogen-methane mixing in T-junction pipelines via numerical simulations, analyzing hydrogen mixing ratios (HMR: 10-25%) and methane flow rates (4-10 m/s) to assess flow and mixing dynamics. The coefficient of variation (COV) quantifies mixing uniformity with spatial and temporal analyses, optimizing hydrogen injection for rapid, homogeneous mixing. The key findings are as follows: (1) The uniform mixing length (the minimum axial distance required for the first pipeline cross-section to achieve 95% mixing uniformity) decreases inversely with the HMR, from 100 D to 20.875 D (D represents the pipeline diameter) as the HMR rises from 10% to 25%. (2) Analysis of initial uniform mixing time (defined as the duration required for the first pipeline cross-section to achieve 95% mixing uniformity) shows significant reduction with increasing HMR. While methane flow rate has a less pronounced effect, it nevertheless contributes to reducing the outlet uniform mixing time (defined as the time required to attain 95% mixing uniformity at the pipeline outlet). (3) A fundamental trade-off in engineering applications is established: increasing the HMR reduces mixing length but extends overall mixing time (difference between outlet and initial mixing times), while higher methane flow rates shorten overall mixing time at the cost of increased mixing length. The primary objective of this research is to elucidate the fundamental fluid dynamics of hydrogen-methane mixtures in T-junction pipelines, providing scientific insights for the safe and efficient operation of hydrogen-blended natural gas pipeline systems.