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

Abstract

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.

Oxidation of Methane/n-Heptane Mixtures in a High-Pressure Flow Reactor

“…However, for other conditions investigated here, the predictions of n -heptane and CH 4 mole fractions are in good agreement with experiment. It is interesting to note that chemical coupling between n -heptane and methane in the NTC region apparently does not affect ignition delays for the mixture at high pressure, according to the recent findings of Thorsen et al…”

“…The model by Zhang et al, which has been validated at high pressure, ,,, is used in this study to predict the methane/heptane oxidation. The flow reactor experiments are simulated in ANSYS Chemkin-Pro, assuming ideal plug-flow and employing the full temperature profile.…”

“…To simplify the analysis, fuel surrogates are investigated instead of the real multicomponent fuels. n -Heptane is known to have combustion properties (cetane number and burning velocity) similar to heavy fuel oil. , Even though the combustion properties of natural gas are affected by the presence of higher hydrocarbons, methane, its major component, is a useful surrogate for natural gas when it is combusted together with n -heptane …”

“…Oxidation of n -heptane is also widely investigated in the literature . Its ignition behavior has been characterized in rapid compression machines (RCM) − and shock tubes, ,− and data are available from flame speed experiments. , Species profiles have been reported from flow reactors − and jet-stirred reactors. − Thorsen and co-workers , have shown that the model by Zhang et al provides the best agreement with measured high-pressure ignition delay times and laminar flow reactor data for n -heptane oxidation.…”

“…3,4 Even though the combustion properties of natural gas are affected by the presence of higher hydrocarbons, 5 methane, its major component, is a useful surrogate for natural gas when it is combusted together with n-heptane. 6 Oxidation of neat methane has been widely investigated under engine-relevant conditions. Low-temperature (T < 1000 K) measurements have been obtained in pressurized flow reactors, 7−11 jet-stirred reactors, 12 and static reactors.…”