Strained dissipation and reaction layer analyses of nonequilibrium chemistry in turbulent reacting flows

A physically based formulation relating the chemical state of nonequilibrium combustion in turbulent flows to the mixing state of a conserved scalar is presented. The present strained dissipation and reaction layer approach is motivated by results from detailed imaging studies of scalar mixing in turbulent flows, which show that essentially all the instantaneous scalar energy dissipation rate field is confined to locally one-dimensional layer-like structures. These dissipation layers are a direct consequence of the dynamics of scalar mixing in turbulent flows and are independent of the extent of chemical nonequilibrium in the flow. The presence of these layer-like scalar dissipation structures in turn indicates a locally one-dimensional structure in the underlying chemical species fields. The resulting strained dissipation and reaction layer model has certain similarities with the classical "flamelet" model; however, it is based on entirely different physical observations, derived from entirely different arguments, and limited by an entirely different and more widely applicable set of conditions. Moreover, the boundary conditions for solution of the local chemical state differ fundamentally from those used in flamelet models. Results for OH concentration fields obtained from experimental imaging measurements of conserved scalar fields in turbulent flows for conditions ranging from near equilibrium to deep nonequilibrium demonstrate remarkable resemblances to direct OH PLIF measurements under similar combustion conditions. In particular, species concentration and reaction rate fields obtained clearly show the predominance of thin (flamelet-like) reaction zones under conditions of relatively weak chemical nonequilibrium, and the natural emergence and dominance of broad (distributed) reaction zones for increasing equilibrium departures. Detailed nonequilibrium O-atom concentration fields obtained via this formulation, unattainable by direct experimental techniques, provide insights into Zel'dovich NO formation in turbulent jet diffusion flames.