Effect of cross-sectional area variation and methane blending on flame-turbulence and pressure characteristics of hydrogen explosions

Geometric discontinuities such as sudden expansions or contractions are common in pipelines, ventilation, and storage–transport systems, where even minor perturbations can trigger flame acceleration and pressure amplification. However, existing studies have paid limited attention to their underlying mechanisms and the regulatory effects of methane blending. This study employs large eddy simulation (LES) to investigate how cross-sectional area variations and methane blending affect the evolution mechanisms of locally premixed hydrogen explosions in partially confined spaces. The results show that sudden contraction induces strong jet flow at the flame front, generating “vortex ring” structures that significantly enhance flame propagation and overpressure. The maximum flame speed reaches 308.49 m/s, representing a 235 % increase compared to the continuous geometry, while the maximum overpressure rises by 214 % to 105.72 kPa. In contrast, sudden expansion triggers the formation of large-scale annular vortices in the buffer zone, leading to a 33-fold increase in the maximum turbulent kinetic energy dissipation rate (18.37 W), along with a 76.07 % increase in maximum flame speed (162.00 m/s) and a 96.41 % rise in maximum overpressure (66.21 kPa). Methane blending weakens the explosion intensity by enhancing the consumption pathways of O, H, and OH radicals. When the blending ratio increases from 10 % to 15 %, the combined effect of reduced fuel reactivity and insufficient turbulence in the continuous geometry results in a 75.57 % and 86.53 % decrease in flame speed and overpressure, respectively. These findings provide theoretical guidance for structural optimization and hydrogen explosion prevention in engineering applications.