Presented is a combined experimental and modeling study of the kinetics of the reactions of H and CH3 with n-butane, a representative aliphatic fuel. Abstraction of H from n-alkane fuels creates alkyl radicals that rapidly decompose at high temperatures to alkenes and daughter radicals. In combustion and pyrolysis, the branching ratio for attack on primary and secondary hydrogens is a key determinant of the initial olefin and radical pool, and results propagate through the chemistry of ignition, combustion, and by-product formation. Experiments to determine relative and absolute rate constants for attack of H and CH3 have been carried out in a shock tube between 859 K and 1136 K for methyl radicals and 890 K to 1146 K for H atoms. Pressures ranged from 140 kPa to 410 kPa. Appropriate precursors are used to thermally generate H and CH3 in separate experiments under dilute and well-defined conditions. A mathematical design algorithm has been applied to select the optimum experimental conditions. In conjunction with post-shock product analyses, a network analysis based on the detailed chemical kinetic combustion model JetSurf 2.0 has been applied. Polynomial chaos expansion techniques and Monte Carlo methods are used to analyze the data and assess uncertainties. The present results provide the first experimental measurements of the branching ratios for attack of H and CH3 on primary and secondary hydrogens at temperatures near 1000 K. Results from the literature are reviewed and combined with the present data to generate rate expressions for n-butane covering 300 K to 2000 K. Values for generic n-alkanes and related hydrocarbons are also recommended. The present experiments and network analysis further demonstrate that that C-H bond scission channels in butyl radicals are an order of magnitude less important than currently indicated by JetSurf 2.0. Updated rate expressions for butyl radical fragmentation reactions are provided.
Journal of Physical Chemistry A
shock tube, kinetics, methyl radicals, H atoms, n-butane, reaction networks