Study on the Effect of Nitrogen on the Explosion Characteristics of Methane-Hydrogen-Air Premixed Gases

Employing a 1 m3 spherical explosion vessel, this study investigates the suppression of hydrogen-enriched methane–air premixed explosions through systematic nitrogen dilution across equivalence ratios (Φ) from 0.6 to 1.4. Nitrogen is introduced at volumetric fractions of 0%, 5%, 10%, 15% and 20%, and its inerting efficacy is rigorously assessed via high-speed imaging and pressure diagnostics. The equivalence ratio is identified as the primary determinant of flame morphology: under lean conditions (Φ=0.6) the initially lenticular flame front evolves into a centrally concave ellipsoid, whereas at Φ=1.0 and richer conditions the same front transitions to a near-spherical envelope. Flame propagation velocities decrease monotonically in the order Φ=1.0>1.2>1.4>0.8>0.6; at fixed Φ=0.6 and a hydrogen volumetric fraction of 70% the burning velocity rises by 25% relative to the undiluted case. Maximum explosion overpressure and the peak rate of pressure rise both scale positively with hydrogen content and attain their maxima at Φ=1.0. Progressive nitrogen addition distorts the spherical flame into a cairn-like configuration; at≥15% N2 flame wrinkling is suppressed by more than 50%, the central luminosity diminishes by approximately 60%, and the propagation velocity is reduced by up to 40%. For any given nitrogen concentration, higher hydrogen fractions progressively weaken the suppression efficiency. Nitrogen also attenuates explosion severity: ΔPmax and (dP/dt)max decline by 36–50%, and at a hydrogen fraction of 70% the introduction of 25% N2 markedly retards the arrival of peak overpressure, signalling a definitive transition from explosive to deflagrative behaviour.