Full-scale pipeline experimental study on combustion flame characteristics of hydrogen-blended natural gas crack leakage

Transporting hydrogen via existing natural gas pipelines represents an economical pathway for hydrogen utilization; however, the associated risk of leakage and combustion, particularly from crack-induced failures due to hydrogen embrittlement, requires thorough assessment. This study conducts full-scale experiments to examine the combustion characteristics of hydrogen-blended natural gas (HBNG) leaking from a high-aspect-ratio crack (AR = 80) and compares them with conventional circular hole leakage. Results demonstrate that crack leakage produces significantly taller flames (exceeding 9 % greater height) and higher oscillation frequencies compared to equivalent-area hole leaks, leading to non-conservative risk underestimation if circular orifice models are applied. Operating pressure (0.2–1.6 MPa) exhibits a dominant positive correlation with flame height, oscillation velocity, and frequency, with a 76.5 % increase in maximum flame height for cracks as pressure rises from 0.8 to 1.6 MPa. Introducing 20 vol% hydrogen reduces methane flame height by 9.4–14.5 % but alters combustion dynamics. This work highlights the critical influence of leakage geometry and system pressure on fire hazards and provides essential experimental data for developing accurate risk assessment models for HBNG pipeline safety.