Identification of homogeneous chemical kinetic regimes of methane-air ignition

Abstract

Sensitivity analysis results for ignition delay time (IDT) may be very different depending on the initial temperature, pressure and equivalence ratio φ, but similar in some regions of these variables. This phenomenon was investigated systematically by carrying out ignition simulations and local sensitivity calculations of methane−air mixtures using the Aramco-II-2016 mechanism at 14,417 combinations of initial temperature (changed between 500 and 3000 K), initial pressure (0.05−500 atm) and φ (0.05−8.0) values. The cluster analysis of the sensitivity vectors identified five large kinetically homogeneous regions. Each region has well defined borders in the (T, p φ) space and can be characterized by different sets of important reactions. The related kinetic scheme is very different in each region. Regions 1 and 2 are dominated by catalytic cycles based on species CH3O2/CH3O2H and HO2/H2O2/CH3O, respectively. In regions 3, 4, and 5 the H atoms are converted to CH3 in an identical chain branching sequence, but the back conversion is via three different routes. Literature experimental data on the IDTs of methane−air mixtures were sorted according to these five regions. Regions 1 to 5 contain 214, 328, 3, 0, and 237 experimental data points, respectively. In regions 1, 2 and 5 the data points are well reproduced by the Aramco-II-2016 mechanism, but little or no experimental information is available about kinetic regions 3 and 4. Further experimental exploration of the ignition of methane−air mixtures may aim the study of these regions. A similar approach can be used for the characterization of other combustion systems and sorting the related experimental data.