Hydrogen, Methane, Ethylene and Propylene Blending on the Ignition Delay Time of n-Heptane/Toluene Mixtures under Homogeneous and Nonpremixed Counterflowing Conditions

Effects of hydrogen (H-2), methane (CH4), ethylene (C2H4) and propylene (C3H6) blending on the ignition delay time of n-heptane/toluene mixtures were numerically studied under both homogeneous and nonpremixed counterflowing conditions. In both configurations, results reveal that the addition of H-2 and C2H4 is beneficial to promote ignition, while the addition of CH4 and C3H6 inhibits ignition at atmospheric pressure. The blending effects of H-2, CH4 and C2H4 at elevated pressures are similar to that at atmospheric pressure. However, the inhibiting effect of C3H6 becomes weaker under higher pressures. At the same mole blending ratio, in a homogeneous system, C2H4 blending has stronger promotion effect than H-2 blending, but in the nonpremixed counterflowing configuration, the results are opposite. The effect of light gas blending is barely caused by the changes of the thermophysical properties; instead, it is dominated by the changes of chemical kinetics, including the changes of the rates of the main endothermal branching reactions, main heat production reactions, and the radical formation reactions. Furthermore, for the homogeneous mixtures, H-2 addition directly enhances the main branching reaction H +?O-2 O +?OH, while C2H4 addition directly enhances the main exothermic reactions C2H4?+?OH C2H3 + H2O. CH4 addition consumes OH and produces CH3 to inhibit ignition. C3H6 addition consumes H and OH to inhibit ignition. The ignition of a stretched nonpremixed flame begins on the high-temperature oxidizer side. Due to the stronger mass diffusivity of H-2, the blended H-2 permeates to the oxidizer side with a higher temperature than the blended C2H4 does. A small amount of H-2 blending results in much higher H, OH, CH3 radicals, higher heat product rate, and thereby shorter ignition delay time than the C2H4 blending. The effect is enhanced with increasing strain rate. In addition, the results reveal that the ignition delay time is only sensitive to the molecular transport of the externally-added light gas components, rather than that of the gas molecules generated by the fuel decomposition.

Automated summary

The effects of blending hydrogen and ethylene are beneficial for promoting ignition, while blending methane and propylene inhibits ignition at atmospheric pressure. At elevated pressures, the effects of hydrogen, methane, and ethylene blending remain similar to those at atmospheric pressure, but the inhibiting effect of propylene weakens. The blending effects are mainly influenced by changes in chemical kinetics rather than thermophysical properties.