Objective In hydrogen-blended natural gas pipelines, inherent differences in inlet temperatures between hydrogen and natural gas lead to thermal fluctuations, which are identified as a factor contributing to reduced pipeline service life. Existing research on this topic primarily focuses on simulating mass transfer characteristics under conventional turbulence model conditions. Utilizing Large Eddy Simulation (LES) to reveal the mixing patterns within hydrogen-blended natural gas pipelines helps establish a theoretical foundation for optimizing pipeline design, as well as enhancing safety and mixing efficiency.

Methods A T-shaped blending pipeline model was constructed using ANSYS Fluent. LES was employed to accurately capture turbulence characteristics during the flow process. The k-ω turbulence model was utilized to obtain initialization data, followed by transient calculations to systematically analyze the mixing characteristics under various conditions. The study focused on the effects of hydrogen injection positions (horizontal, top vertical, bottom vertical), pipe diameter ratios, and hydrogen blending ratios on the mass transfer effect, as well as the impacts of hydrogen injection positions and blending flow patterns (wall jet, deflected jet, impinging jet) on heat transfer. By qualifying mixing uniformity based on the coefficient of variation (CoV), the thermal mixing effect and fatigue risk were evaluated, considering dimensionless temperatures in conjunction with a thermal stress model.

Results In terms of mass transfer, the bottom vertical injection method, benefiting from the buoyancy effect, can achieve uniform mixing over the shortest distance. The optimal mixing efficiency is attained with a diameter ratio of the main pipeline to the branch between 4:1 and 5:1. Additionally, a hydrogen blending ratio exceeding 15％ is considered to increase the risk of hydrogen embrittlement. Regarding heat transfer, the bottom vertical injection method also facilitates efficient thermal mixing. However, the significant thermal fluctuations near the pipe wall highlight the importance of protecting these areas. The impinging jet promotes temperature mixing but comes at the cost of a shortened service life for the pipeline. The deflected jet strikes a balance between safety and mixing efficiency, while the wall jet performs the worst in terms of thermal mixing effect.

Conclusion The study presents the following recommendations for engineering design: the adoption of bottom vertical injection; a pipe diameter ratio ranging from 4:1 to 5:1; a hydrogen blending ratio of 15％ or less; a mixing section length of at least 15D (D denotes the main pipeline diameter); the selection of a deflected jet as the preferred choice to reduce thermal stress; and ensuring the protection of the pipe wall at distances of 0.5D to 6D downstream of the mixing section. These research findings provide essential parameter guidance for the safe design and efficient operation of hydrogen-blended natural gas pipelines, highlighting their significant engineering importance for facilitating large-scale hydrogen transportation.