Flame Structure Interactions and State Relationships in an Unsteady Partially Premixed Flame

Abstract

In this investigation our objective is 1 ) to compare the similitude between an unsteady, two-dimensional (axisymmetric), partially premixed e ame and an analogous steady, partially premixed e amelet to determine if state relationshipsintermsofamodie edconservedscalarapplyand2 )toinvestigatee amestructureinteractionsbetween the various reaction zones contained in partially premixed e ames. This comparison is of fundamental importance to theunderstanding and modeling of turbulente amesbecause the axisymmetrice ameinvolvesrelatively complex e ow chemistry interactions resulting from differential diffusion, e 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 simulation methodology using a relatively detailed CH 4‐airchemical mechanism isemployed to model inverseaxisymmetric partially premixed e ames that are established by introducing a fuel-rich (CH4‐air) annular jet that is sandwiched between an air jet (on the inside)and coe owing air (on theoutside ). The e ame 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 e ame is synergistically coupled with an oxidation layer, and the upstream region of the nonpremixed e ame is contained downstream of the premixed e ame. The signie cant hydrocarbon chemistry occurs almost solely in the PF where fuel and radical consumption produce CO and H 2, which are then oxidized to form CO 2 and H2O in the NF. The nonpremixed e ame provides radicals to accelerate the upstream region of the premixed e ame. Comparison with an analogous e amelet reveals that transport effects in the axisymmetric e ame are signie cant on the rich side. The coannulare amescalar proe lesshow regions of both frozen e ow and burning. Thescalar distributions in thee ame, when compared with analogous e amelet proe 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 e ammability limit. The synergistic interactions between the inner and outer layers lead to the formation of complex composite e ames.