An experimental and kinetic modeling study of n-hexane oxidation

Abstract

Ignition delay times for n-hexane oxidation have been measured in a rapid compression machine (RCM) at stoichiometric conditions and at 15 bar. Due to the high reactivity of n-hexane and non-ideal experimental effects associated with measuring short ignition delay times in the RCM (i.e. under 5 ms), further experiments were performed in a high-pressure shock tube for multiple fuel mixtures at equivalence ratios of φ = 1 and φ = 2 over the temperature range of 627–1365 K at pressures of 15 and 32 bar. To further study the concentration of intermediate species during the oxidation process, experiments have also been carried out in a jet-stirred reactor over a wide temperature range of 530–1160 K at 10 atm pressure and at equivalence ratios of φ = 0.5, 1.0 and 2.0. Species which include reactants, intermediates and products were identified and quantified as a function of temperature. These experimental results were used to aid the development and validation of a detailed kinetic model. The low-temperature chemistry of n-hexane has been refined by adopting alternative isomerization reactions for peroxyl alkylhydroperoxide (O˙2QOOH) radicals, leading to more detailed chemistry for this type of intermediate with multiple product channels. This mechanism has adopted a series of new reaction rates and rate rules mostly from recently reported high-level calculations. Slight modifications have been made to the suggested reaction rates and rate rules within their reported uncertainty ranges to achieve better agreement with the experimental results for both ignition delay times and speciation measurements. The new model has been validated against the experimental data presented here, with an overall good agreement compared to the experimental results. The molecular structure of n-hexane is more representative of normal alkanes that may be found in transportation relevant fuels (e.g. gasoline) compared to those with shorter carbon chains which is important in developing a robust sub-mechanism of base chemistry for larger, more practical fuels. Since the modified reaction rate rules presented in this work have shown to successfully predict the oxidation kinetics of n-hexane, these rate rules could be the basis for the development of mechanisms for even larger normal alkanes that are more representative of diesel and jet fuels. As a further demonstration of the utility of the rate rules they are shown to predict well ignition delay times and species concentrations measured at low temperature for n-heptane oxidation from previous studies.