Ab initio kinetic study on the abstraction reactions of methylcyclohexane and implications for high-temperature ignition simulations from shock tube experiment

Abstract

Methylcyclohexane (MCH) is the simplest alkylated cyclohexane, and has been widely employed in surrogate models to represent the cycloalkanes in real fuels. Thus, extensive experimental and kinetic modeling studies have been performed to understanding the combustion chemistry of MCH. However, through a detailed literature analysis, there still lack a systematic theoretical study on the abstraction reactions of MCH, which are the main initial oxidation pathway of MCH. Herein, this work reports a systematic ab initio chemical kinetic study on the abstraction reactions of MCH with different radicals/species. Specifically, reaction rate constants of 30 abstraction reactions of MCH with H/O/OH/O₂/HO₂/CH₃ at different sites are computed using transition state theory (TST) by using quantum chemistry calculation results at DLPNO-CCSD(T)/CBS//M06-2X/cc-pVTZ level. The computed results are incorporated into a detailed mechanism to simulate newly measured ignition delay times (IDTs) of MCH in this work at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 2 and 5 bar, temperatures ranging from 1140 to 1640 K. The updated detailed mechanism demonstrates improvement in the prediction of IDTs, especially at fuel-rich conditions. The fuel concentration and dilution effect on the IDTs are discussed, and a general Arrhenius expression is adopted to fit the IDTs from both this work and literature work. This work should be valuable for further optimization of detailed kinetic mechanisms and also for gaining insight into the combustion chemistry of MCH.