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

ABSTRACT

Effects of hydrogen (H₂), 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 H₂ and C₂H₄ is beneficial to promote ignition, while the addition of CH₄ and C₃H₆ inhibits ignition at atmospheric pressure. The blending effects of H₂, 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 H₂ 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, H₂ addition directly enhances the main branching reaction H + O₂ <=> 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 H₂, the blended H₂ permeates to the oxidizer side with a higher temperature than the blended C₂H₄ does. A small amount of H₂ 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.

KEYWORDS:

• Ignition delay time
• liquid fuel
• light gas blending
• homogeneous ignition
• counterflowing nonpremixed flame

Additional information

Funding

This study was supported by the National Natural Science Foundation of China (NSFC 51706119) and the Strategic Priority Research Program of Chinese Academy of Sciences, Grant No. XDA 15012800.