Investigation of flame propagation in autoignitive blends of n-heptane and methane fuel

Abstract

The effects of pre-ignition chemistry on laminar flame speed in methane/n-heptane fuel blends are investigated numerically, leading to flame speed modelling accounting for these effects. The laminar flame speeds of fuel blends are important input parameters for turbulent combustion models needed to support design of dual-fuel engines. At the autoignitive conditions found in engines, pre-ignition reactions cause the speed of the reaction front to increase. Fuels that exhibit two-stage ignition behaviour, such as n-heptane, also exhibit a two-stage increase in the speed of the reaction front as the reactant residence time increases. There is a corresponding reduction in the flame thickness until the residence time approaches the ignition delay time, whereupon the deflagrative scaling of flame thickness breaks down. The analysis shows that the increase in flame speed is due to distinct contributions of heat release, reactant consumption, and enhanced reactivity ahead of the flame. Addition of methane to n-heptane–air mixtures retards and reduces the first-stage increase in flame speed, in part due to dilution of the more-reactive n-heptane fuel, and in part due to consumption of radical species by the methane chemistry. The effect of methane/n-heptane fuel blending on flame speed is described adequately by a linear mixing rule. The effect of pre-ignition chemistry can then be modelled as a linear function of the progress variable ahead of the flame – accounting for heat release, reactant consumption, and enhanced reactivity ahead of the flame. The flame speed model accurately describes the variation of flame speed across the full range of methane/n-heptane blends at engine-relevant conditions, up to the deflagration/ignition transition.