DNS analysis of partially premixed combustion in spray and gaseous turbulent flame-bases stabilized in hot air

Direct numerical simulations of weakly turbulent-lifted flame bases are examined in the case of both gaseous and spray fuel jet injection. Simplified transport properties and an adjustable single-step chemistry that matches the flame response to equivalence ratio are used. The flames are stabilized within a coflowing stream of heated air. The properties of the zone where burning starts are found to strongly depend on the type of fuel injection. The gaseous flame base is essentially composed of an edge flame, with a large contribution of partially premixed combustion. This partially premixed flame takes two different forms, a nearly stoichiometric propagating kernel and a rich trailing flame whose burning rate is diffusion controlled. The rich premixed flame is parallel to the stoichiometric line, along which a diffusion flame burns the fuel left by this rich trailing flame, up to the very leading edge of the flame base. In the spray case, a nonnegligible amount of oxidizer is entrained within the dilute spray, also leading to an important contribution of partially premixed burning. However, diffusion and premixed burning are found more distributed in space in the spray case than with gaseous injection. A progress variable that is generalized to partially premixed combustion is discussed and the relative contributions of the terms of its balance equation are analyzed from the DNS. A flame partitioning into premixed and diffusion types is then examined and the stabilization zone is decomposed into basic flame prototypes. A subgrid scale flame decomposition is further discussed from a direct filtering of DNS and some a priori tests of subgrid scale modeling are reported.