CFD modeling of buoyancy-driven methane diffusion from buried pipelines into manholes: Quantifying concentration stratification and early-warning thresholds

Abstract

Methane leakage from buried pipelines into adjacent confined manholes poses significant explosion hazards. Understanding transient diffusion dynamics for concentration buildup is crucial for timely detection, yet quantitative analysis of key parameters remains limited. This study establishes a validated CFD model to investigate buoyancy-driven transient methane migration from soil into manholes, focusing on impacts of gas inlet velocity, crack characteristics (size, location), and venthole dimensions on spatiotemporal concentration evolution (especially vertical stratification) and multi-tiered early-warning thresholds. Numerical simulations reveal persistent vertical stratification with higher concentrations in upper regions due to buoyancy. Inlet velocity and crack size exert exponential and strong nonlinear influences, respectively, accelerating accumulation and reducing warning times (e.g., increasing inlet velocity from 10⁻⁵ m/s to 0.1 m/s shortened Level-III warning time by over 80 %; doubling crack diameter from 10 mm to 20 mm reduced alarm time by 84 %). Conversely, cracks near the base prolonged detection, while larger ventholes delayed thresholds via enhanced atmospheric exchange. These quantified relationships provide insights into mass transport in confined subsurface spaces, forming a basis for optimizing leak monitoring and risk assessment in urban infrastructure.