Laminar flame propagation and ignition properties of premixed iso-octane/air with hydrogen addition

Abstract

Numerical simulations of one-dimensional planar and spherical flame propagation of iso-octane/hydrogen/air mixtures at different equivalence ratios and hydrogen blending levels are conducted considering detailed chemistry. Our focus is on the effects of hydrogen addition on laminar flame propagation and ignition of iso-octane/air mixtures. Specifically, the laminar flame speed, Markstein length, and minimum ignition energy of iso-octane/hydrogen binary fuel blends are investigated. The laminar flame speed is found to increase first slightly and then exponentially with the molar ratio of hydrogen in the iso-octane/hydrogen binary fuel blends. However, a nearly linear trend is observed when the mass ratio instead of molar ratio of hydrogen blending is considered. Similar trend holds for hydrogen addition to other hydrocarbon fuels such as methane and propane. The thermal and chemical effects involved in laminar flame speed enhancement by hydrogen addition are quantified and it is found that the chemical effect prevails over the thermal effect. Unlike the laminar flame speed, the Markstein length is found to change non-monotonically with hydrogen blending ratio. Blending hydrogen to iso-octane/air and blending iso-octane to hydrogen/air both promote diffusive-thermal instability. Moreover, the minimum ignition energy of iso-octane/air is shown to be reduced by a small amount of hydrogen addition, especially for the fuel lean case. When the hydrogen blending molar ratio is above 60%, the minimum ignition energy is found to be insensitive to hydrogen addition.