Flame structure interactions and state relationships in an unsteady partially premixed flame

In this investigation our objective is 1) to compare the similitude between an unsteady, two-dimensional (axisymmetric), partially premixed dame and an analogous steady, partially premixed flamelet to determine if state relationships in terms of a modified conserved scalar apply and 2) to investigate flame structure interactions between the various reaction zones contained in partially premixed dames, This comparison is of fundamental importance to the understanding and modeling of turbulent flames because the axisymmetric dame involves relatively complex how chemistry interactions resulting from differential diffusion, dame curvature, and spatiotemporally varying strain rates, whereas the development of state relationships generally assumes negligible differential diffusion effects. A time-dependent, axisymmetric model based on a direct numerical simulation methodology using a relatively detailed CH4-air chemical mechanism is employed to model inverse axisymmetric partially premixed dames that are established by introducing a fuel-rich (CH4-air) annular jet that is sandwiched between an air jet (on the inside) and coflowing air (on the outside), The flame consists of distinct layers that include 1) an inner layer (PF) in which methane and O-2 consumption occur and 2) an oxidation layer (NF). The broadened inner premixed dame is synergistically coupled with an oxidation Layer, and the upstream region of the nonpremixed flame is contained downstream of the premixed flame. The significant hydrocarbon chemistry occurs almost solely in the PF where fuel and radical consumption produce CO and H-2, which are then oxidized to form CO2 and H2O in the NE The nonpremixed flame provides radicals to accelerate the upstream region of the premixed flame. Comparison with an analogous flamelet reveals that transport effects in the axisymmetric dame are significant on the rich side. The coannular flame scalar profiles show regions of both frozen how and burning. The scalar distributions in the flame; when compared with analogous flamelet profiles, indicate that upstream interactions occur 1) in the rich region with the consequence of enhanced heat release, 2) at the nonpremixed interface leading to higher heat release through H-2 and CO oxidation, and 3) in the lean region where methane consumption occurs despite the local equivalence ratios being well below the lean flammability limit. The synergistic interactions between the Inner and outer layers lead to the formation of complex composite flames.