Structure and extinction of heptane/air partially premixed flames

I A numerical investigation of the structure and extinction of n-heptane/air partially premixed flames in a counter-flow configuration is reported. The flame is computed using a detailed reaction mechanism consisting of 275 elementary reactions and 41 species. The mechanism is validated by comparing its predictions with measurements for both the premixed and nonpremixed flames. The partially premixed flame structure is characterized by two-stage burning or two spatially distinct, but synergistically coupled, reaction zones, namely, a rich premixed zone on the fuel side and a nonpremixed zone on the air side. Two major pathways characterize the n-heptane/air chemistry in these two reaction zones. The fuel is completely consumed in the premixed reaction zone, with ethylene and acetylene being the major intermediate species. Interactions between the two zones involve the transport of heat from the nonpremixed to the premixed reaction zone and the transport of CO, H-2, and C2H2 from the premixed to the nonpremixed reaction zone. The flame response to variations in equivalence ratio and strain rate is characterized. Increasing the equivalence ratio and/or strain rate to a critical value leads to merging of the two reaction zones. Further increase in equivalence ratio leads to a nonpremixed flame, whereas that in strain rate leads to flame extinction. The partially premixed combustion regime is obtained by computing the critical strain rate for merging as a function of the equivalence ratio. The flame structure is characterized in terms of a modified mixture fraction (conserved scalar), and laminar flamelet profiles are provided. Stretch-induced extinction is also investigated. Results indicate that as the level of partial premixing is increased, the flame extinction occurs at increasingly higher strain rates, implying that partially premixed flames are less prone to extinction compared to nonpremixed flames.