A comprehensive numerical analysis of NO generation routes and combustion characteristics with varying strain rates in methane/n-heptane dual fuel flames

Abstract

The influence of strain rate on the combustion characteristics and routes of NO formation in methane/n-heptane dual-fuel engines was scrutinized in this research, utilizing a counterflow flame model. The insights gleaned from this study provide valuable theoretical guidance for a more profound comprehension of the NO formation mechanism, thereby facilitating the enhancement and optimization of natural gas–diesel dual-fuel engines. The findings reveal that with an increasing strain rate, the peak flame temperature initially exhibits a slight increase followed by a decrease, while the heat release rate experiences a significant surge. Concurrently, the high-temperature region and the distribution areas of the main species and intermediate products are markedly reduced. The NO emission index undergoes initial fluctuations and then demonstrates a consistent decline with the increment of the strain rate, implying that a higher strain rate is conducive to curtailing NO formation. Relative to the enhancing effect of elevated O radical molar fractions on NO formation, the reduction in flame temperature exerts a more pronounced inhibitory influence on the generation of thermal NO. An elevated strain rate also intensifies the production of CH radicals, which in turn, bolsters the formation of prompt NO. Furthermore, an increase in strain rate accelerates the reaction between HCCO and CH₂ radicals with NO, augmenting NO consumption and enhancing the reduction effect of the reburn route on NO. The contributions of the N₂O intermediate route and the NNH intermediate route to NO generation are found to be negligible.