A mechanistic evaluation of Soret diffusion in heptane/air flames

The influence of Soret diffusion on the structure and response of n-heptane/air flames is investigated numerically with detailed reaction mechanism and transport. Unstretched freely-propagating planar premixed flames as well as stretched counterflow premixed and diffusion flames are studied, with emphasis on the separate and combined Soret effects of heptane and its oppositely-directed decomposition species H ₂ and H, as well as those of the major species O ₂, N ₂, CO ₂ and H ₂O. Results show that, in the unstretched case for which the flame temperature remains at its adiabatic value, Soret diffusion primarily affects the chemical kinetics embedded in the flame structure and the net effect is small; while in the stretched cases, its impact is mainly through those of heptane and the secondary fuel H ₂ in modifying the flame temperature, with substantial effects, while H ₂ and H also affect the chemical kinetics especially when the active reaction zone is localized. The dilution/enrichment of the reactant concentrations in the flame through the Soret diffusion of the major species O ₂, N ₂, CO ₂ and H ₂O further exert finite effects on the flame burning intensity.