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 a n analogous steady, partially premixed flamelet to determine if state relat ionships in terms of a modified conserved scalar apply and 2) to investigat e 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 axisymmet ric dame involves relatively complex how chemistry interactions resulting f rom differential diffusion, dame curvature, and spatiotemporally varying st rain rates, whereas the development of state relationships generally assume s negligible differential diffusion effects. A time-dependent, axisymmetric model based on a direct numerical simulation methodology using a relativel y detailed CH4-air chemical mechanism is employed to model inverse axisymme tric partially premixed dames that are established by introducing a fuel-ri ch (CH4-air) annular jet that is sandwiched between an air jet (on the insi de) and coflowing air (on the outside), The flame consists of distinct laye rs 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 o f 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 for m CO2 and H2O in the NE The nonpremixed flame provides radicals to accelera te the upstream region of the premixed flame. Comparison with an analogous flamelet reveals that transport effects in the axisymmetric dame are signif icant on the rich side. The coannular flame scalar profiles show regions of both frozen how and burning. The scalar distributions in the flame; when c ompared with analogous flamelet profiles, indicate that upstream interactio ns occur 1) in the rich region with the consequence of enhanced heat releas e, 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 oc curs despite the local equivalence ratios being well below the lean flammab ility limit. The synergistic interactions between the Inner and outer layer s lead to the formation of complex composite flames.