Hydrogen's intrinsic flammability and explosiveness pose serious safety hazards. This study systematically investigates the coupled effects of pipe diameter and ammonia addition proportions on self-ignition during high-pressure hydrogen leaks. Utilizing computational fluid dynamics (CFD) techniques integrated with the Large Eddy Simulation (LES) and the Eddy Dissipation Concept (EDC) combustion model, the effects of ammonia addition proportions (0 %∼15 %) on shockwave propagation characteristics and critical self-ignition conditions are analyzed for a range of pipeline diameters (5 mm, 10 mm, and 15 mm). A constant volume zero-dimensional homogeneous adiabatic reactor model in Chemkin-Pro was utilized to analyze the generation and consumption rates of H₂ and O₂, together with the corresponding elementary reaction kinetics. Principal findings indicate that increasing the ammonia mixing ratio significantly reduces the generation and consumption rates of H₂ and O₂, suppresses elementary reaction intensities, and elevates the threshold for self-ignition. Notably, mild ammonia blending produces suppression effects comparable to pipeline diameter expansion. A 5 % NH₃ mixture decreases the critical release pressure for a 5 mm pipeline from 1.61 MPa to 1.42 MPa, equivalent to the suppression efficacy of increasing the pipeline to 15 mm in pure air conditions. Furthermore, the combined chemical-physical synergistic mechanism between ammonia addition and diameter scaling amplifies ignition inhibition. In a 15 mm pipeline with 15 % NH₃ blending, the intensity of the leading shockwave after hydrogen leakage decreases by 23.18 % compared to a 5 mm pure-air pipeline, while the critical release pressure for ignition increases by a factor of 2.57. This study clarifies the chemical-physical coupling mechanisms of ammonia and pipeline diameter in mitigating self-ignition caused by high-pressure hydrogen leakage, providing theoretical foundations for the safety design and risk assessment of hydrogen energy transport systems.
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