Understanding the Nonlinear Reactivity Promoting Effect of n-Heptane Addition on the Binary Mixture From Low to Intermediate Temperature: A Case of Methane/n-Heptane Mixtures

To understand the effect of n-heptane (NC₇H₁₆) addition on the auto-ignition of methane (CH₄) at low to intermediate temperatures, the ignition delay times (IDTs) of stoichiometric CH₄/NC₇H₁₆ blends with varying NC₇H₁₆ concentrations were measured at temperatures from 600 to 1000 K, pressures of 20 and 40 bar. Detailed chemical kinetic mechanisms were validated against the newly measured IDTs. Adding NC₇H₁₆ in the binary mixture shows a nonlinear promoting effect on the IDTs: micro-addition of NC₇H₁₆ can significantly reduce the IDTs of the binary mixture when the NC₇H₁₆ is lower than 20%. However, the decrease of the IDTs becomes much slower when further increasing the NC₇H₁₆ addition. Affected by the negative temperature coefficient (NTC) behavior of NC₇H₁₆, this nonlinear effect is particularly notable at around 795 K, the low boundary of the NTC region. To reveal the nonlinear reactivity-promoting effect of NC₇H₁₆ addition on the binary mixture, reaction flux, ignition sensitivity, rate of production of the key radicals along heat production analyses were conducted. Apart from contributing more $O˙$H production through the low-temperature chain-branching reaction pathways of NC₇H₁₆, adding NC₇H₁₆ also promotes the pre-ignition heat release of the binary mixture. The heat release raises the system temperature and further promotes the mixture ignition, enhancing the nonlinear effect at low temperatures.