Combined effects of ignition position and hydrogen ratio on vented CH4/H2/air explosions

Abstract

Experiments on the effects of ignition position and hydrogen ratio (χ) on the explosions of CH4/H2/air mixtures were experimentally investigated in a vented cylindrical vessel. Relevant experiments were conducted utilizing ignition sources situated at the rear (rear ignition), the center of the vessel (central ignition), or near the vent (front ignition) with χ varying from 0 to 1.0. Two types of cellular structures on the flame surface, owing to the diffusional-thermal instability and acoustically enhanced combustion, respectively, were observed, and the latter resulted in acoustic oscillations of the overpressure within the vessel and a unique overpressure peak p2 with amplitude sensitive to both ignition position and χ. The effects of ignition position and χ on the build-up of the internal pressure were not significant when χ ≤ 0.15. The maximum explosion overpressure in the vessel (pmax) under the explosions of rear and central ignitions (RI and CI) increased monotonically as χ was increased from 0.3 to 1.0, but a nonmonotonic trend was found in the explosions of front ignition (FI). CI could be regarded as the worst-case scenario when 0.45≤χ ≤ 1.0 except for χ = 0.7, because FI resulted in the highest pmax at χ = 0.7. A pressure peak outside the vent (pext) caused by the combustion expansion of the combustible cloud could be distinguished when χ ≥ 0.45 in the explosions of RI and CI. The amplitude of pext increased with an increase in χ. Rear ignition always led to the highest pext when χ > 0.6.