Numerical study on small hole leakage and diffusion of hydrogen in buried pipelines

In this paper, a numerical simulation method is systematically employed to study the diffusion behavior of pure hydrogen and hydrogen-blended natural gas during small-hole leakage from buried pipelines. The study focuses on the quantitative effects of operating parameters, leakage orifice characteristics, burial depth, and soil properties on leakage dynamics and surface hazard time. The results indicate that increasing soil porosity and particle size significantly accelerates the diffusion rate, reducing the time required for hydrogen to reach the Lower Explosive Limit (LEL, 4%) at the ground surface from 130 s (Porosity 0.43) to 75 s (Porosity 0.55). Upward-oriented leakage is confirmed to be the highest risk scenario, leading to the fastest concentration saturation due to buoyant forces. Furthermore, the diffusion performance of the leakage orifice is ranked as axial rectangular slit > radial rectangular slit > circular hole, demonstrating that shape and aspect ratio critically influence the diffusion range and surface ignition risk. This research provides a quantitative foundation and dimensionless correlations for hazard assessment and safety management of low-pressure hydrogen energy and hydrogen-blended natural gas distribution pipelines.