Numerical and experimental study of extinction limits of partially premixed counterflow flames*

Combustion involving fuel droplets is widely used in practical combustion systems such as internal combustion engines and aeronautical combustors. A robust and quantitative description of flame extinction in spray flames is crucial for optimal engine design and flame stabilization. One complicating factor in spray flame extinction is the spatially inhomogeneous fuel distribution resulting from multi-physical processes such as evaporation and turbulent fluctuation. In this study, we numerically investigate extinction limits for one-dimensional partially premixed laminar counterflow flames for various fuels, including methane, n-heptane, JetA, and n-dodecane. For a system with a specific global equivalence ratio, we define multiple heterogeneous flames and discuss their extinction limits. Obtained extinction limits for methane/air flames are compared with experimental data and model uncertainties are explored. For methane/air flames, we observe extended extinction limits in rich flames as becomes larger. On the other hand, decreased extinction limits were found in lean and stoichiometric partially premixed methane/air flames, although the reduction is not significant. For large hydrocarbons, although significant increase in the extinction limits was observed in lean flames, extinction limits of stoichiometric and rich flames had almost the same values with the homogeneous flames. Finally, multidimensional quasi-DNS simulations are performed for gaseous and spray flames near the extinction limits using an open-source AMR solver PeleLMeX and compared with the canonical one-dimensional flames.