Evaluation of a Semiglobal Approach for Modeling Methane/n-Heptane Dual-Fuel Ignition

Because of the relatively high autoignition temperature of natural gas, its use in conventional diesel engines requires a pilot fuel, typically diesel oil, to promote ignition. To conduct computational fluid dynamics (CFD) simulations of the combustion in such engines, there is a need for dual-fuel combustion models that achieve a balance between computational efficiency and accuracy. This study investigates a semiglobal approach for modeling ignition of n-heptane/methane mixtures. In the semiglobal approach, the heptane oxidation is modeled by using a four-step global scheme from Müller, Peters, and Liñán (MPL) (1992), while the methane chemistry is described by a skeletal mechanism with 22 species, derived from a detailed reaction mechanism. The resulting semiglobal model includes 25 species and 138 reactions. Both the global heptane mechanism and the merged semiglobal dual-fuel model are validated against a wide range of ignition delay data from shock tubes as well as through comparison with predictions of the detailed heptane mechanism by Zhang et al. (2016). The MPL model cannot fully capture the ignition delay across the negative temperature coefficient (NTC) region for an n-heptane/air mixture. Despite this shortcoming, the ignition delay at high pressure (38–55 atm) is predicted typically within a factor of 2 compared to experiments. Under dual-fuel conditions with methane as the main fuel, the NTC behavior is less pronounced. Methane ignition is promoted by heat release from the n-heptane oxidation rather than by any direct chemical interaction. Predictions of ignition delays using the combined global/skeletal model are in good agreement with the n-heptane/methane measurements reported by Schuh et al. (2019; 60 atm, 785–1284 K), while at the higher temperatures and lower pressures of the experiments of Liang et al. (2019; 10 atm, 1257–1763 K), predictions are accurate within a factor of 2 for methane contents of 90% and higher. The present results indicate that it is possible with a small combined model to predict the ignition delay for methane/n-heptane mixtures with sufficient accuracy for practical use. The semiglobal model approach can thus be employed in CFD simulations, facilitating development of sustainable dual-fuel engines as well as identification of optimal fuel compositions and conditions.