Mechanism development for larger alkanes by auto-generation and rate rule optimization: A case study of the pentane isomers

The core chemistry and thermodynamic data of large alkanes in NUIGMech1.3 were recently updated. In the present work, the set of rate rules for large alkanes is optimized against experimental data to improve the predictive capability of the mechanism. As an initial step in developing a consistent set of rate rules for any larger alkane, we optimized the mechanism of the pentane isomers, whose mechanisms were generated based on 185 rate rules in 24 reaction classes using the MAMOX++ code. The final combined mechanism contains 1427 species and 6676 reactions. For the efficient optimization of such a large mechanism, the Optima++ code was extended to rate rules and was linked with the Zero-RK simulation code. As reference data, first-stage and total ignition delay times measured in shock tubes and rapid compression machines, and species concentrations measured in jet-stirred reactors were collected in wide ranges of conditions. The prior uncertainties of the Arrhenius equations of the 185 rate rules were determined based on a review of alkane fuel rate constant studies. The PCA-SUE method was used for the selection of the influential rate rules. This method identified 94 important rate rules, whose Arrhenius parameters were subsequently optimized within their prior uncertainty ranges using Optima++ against a representative subset of the data collection with a moderate computational effort. The optimization notably improved the accuracy of the mechanism, which now performs significantly better than the Bugler et al. mechanism (PROCI, 2017). The present study demonstrates the effectiveness of the proposed methodology, thereby paving the way to the optimization of a complete set of rate rules that can be used for the generation of a reliable combustion mechanism for any larger alkane, and with some extensions even for unsaturated fuels or oxygenated fuels such as biodiesels.