Flame acceleration and transition to detonation: Effects of a composition gradient in a mixture of methane and air

The effects of a composition gradient on flame acceleration and transition to detonation in a mixture of methane and air were studied by numerically solving the unsteady, fully compressible, reactive Navier–Stokes equations. The specific problem addressed here is for ignition in a two-dimensional, obstructed channel where there is a spatial gradient of equivalence ratios perpendicular to the propagation direction of the reaction wave. The solution method uses a calibrated, optimized chemical-diffusive model that reproduces correct flame and detonation properties for methane–air mixtures over a range of equivalence ratios. Comparisons were made to a stoichiometric, homogeneous mixture in order to focus on the worst-case scenario for safety concerns. The results showed that the flame speed is smaller and the average total heat release are lower, but the maximum flame surface area is larger in the inhomogeneous mixture. This is because there is more unburned material between obstacles but less energy released from this increased flame surface area in the fuel-lean region, leading to the reduction of the total heat release. The transition to detonation is delayed in the inhomogeneous mixture, because the hot spot forms in the fuel-lean region and the strength of the Mach stem that hits the obstacle is weaker. The detonation front tends to decouple into a shock and a flame earlier in the inhomogeneous mixture, due to the incomplete mixing throughout the entire domain during the detonation propagation process.