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  ame and an analogous steady, partially premixed  amelet to determine if state relationshipsin terms ofamodiŽ ed conserved scalar applyand2) to investigate amestructure interactionsbetween the various reaction zones contained in partially premixed  ames. This comparison is of fundamental importance to the understandingandmodeling of turbulent  ames because the axisymmetric  ame involves relatively complex  ow chemistry interactions resulting from differential diffusion,  ame 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 simulationmethodology using a relatively detailed CH4–air chemical mechanism is employed to model inverse axisymmetric partially premixed  ames that are established by introducing a fuel-rich (CH4–air) annular jet that is sandwiched between an air jet (on the inside) and co owing air (on the outside). The  ame consists of distinct layers that include 1) an inner layer (PF) in which methane and O2 consumption occur and 2) an oxidation layer (NF). The broadened inner premixed  ame is synergistically coupled with an oxidation layer, and the upstream region of the nonpremixed  ame is contained downstream of the premixed  ame. The signiŽ cant hydrocarbon chemistry occurs almost solely in the PF where fuel and radical consumption produce CO and H2, which are then oxidized to form CO2 and H2O in the NF. The nonpremixed  ame provides radicals to accelerate the upstream region of the premixed  ame. Comparison with an analogous  amelet reveals that transport effects in the axisymmetric  ame are signiŽ cant on the rich side. The coannular  ame scalar proŽ les show regions of both frozen  ow and burning.The scalar distributions in the  ame, when compared with analogous  amelet proŽ les, 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 H2 and CO oxidation, and 3) in the lean region where methane consumption occurs despite the local equivalence ratios being well below the lean  ammability limit. The synergistic interactions between the inner and outer layers lead to the formation of complex composite  ames.