Auto-ignitive deflagration speed of methane (CH4) blended dimethyl-ether (DME)/air mixtures at stratified conditions

Front propagation speeds from fully resolved unsteady one dimensional simulations with dimethyl-ether (DME)/methane (CH4)/air mixtures under engine relevant conditions are presented using complex kinetics and transport. Different time-scales of monochromatic inhomogeneities in DME concentration with varying DME/CH4 blending ratios are simulated to unravel the fundamental aspects of auto-ignition and flame propagation under the influence of reactivity stratification. To understand the influence of different stratification time-scales on the flame-ignition interaction, two sets of conditions are simulated such that low temperature chemistry is present in only one of them. For a given amplitude of stratification, it is found that the instantaneous propagation speed is significantly affected by the level of CH4 concentration in the binary fuel blend. Specifically, for cases with low temperature chemistry, at relatively smaller time-scales, the overall fluctuation in the instantaneous propagation speed is found to subside as the level of CH4 concentration in the mixture is increased. However, for both sets of conditions, at comparatively larger time-scales, a rapid change in the instantaneous propagation speed is observed with an increase in the level of CH4 concentration in the mixture. The intrinsic effects of stratification time-scales on the low temperature chemistry and the high temperature chemistry are further examined to assess the flame-ignition interaction. A displacement speed analysis is also carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.