A new model for nonequilibrium mixing-chemistry coupling in nonpremixed and partially premixed turbulent combustion

This work describes the development, implementation and assessment of a new model for nonequilibrium mixing-chemistry coupling in nonpremixed and partially premixed turbulent combustion. This model, termed the Strained Dissipation and Reaction Layer (SDRL) formulation, is a direct consequence of the layer-like fine scale structure of conserved scalar mixing in turbulent flows and provides a canonical physical picture of molecular mixing and nonequilibrium chemistry in turbulent combustion. Consistent with observations of passive scalar mixing in nonreacting turbulent flows, it has been determined that layer-like scalar dissipation structures are established and maintained in reacting turbulent flows as well. Furthermore, an investigation of classical laminar flamelet model universality has demonstrated the ability of a mixture fraction variable and its associated dissipation to uniquely describe the local state of combustion for small equilibrium chemistry departures only, suggesting that detailed local scalar mixing information is essential to making improved predictions of nonequilibrium chemistry in nonpremixed and partially premixed turbulent combustion. A DNS validation study of the SDRL formulation, classical laminar flamelet model, and equilibrium chemistry approximation clearly demonstrates the potential of the SDRL model to not only accurately predict species concentrations in the near equilibrium, large Damkohler number limit, but to also make substantially improved predictions over these more traditional approaches to turbulence-chemistry coupling for conditions involving significant equilibrium chemistry departures. Analyses of the SDRL formulation when applied to turbulent jet mixing measurements reveal thin (flamelet-like) concentration fields at relatively far upstream locations in turbulent jet diffusion flames and under conditions of relatively weak chemical nonequilibrium and the natural emergence and dominance of broad (distributed) reaction zones for increasing equilibrium departures and at farther downstream locations, in strong qualitative agreement with OH PLIF measurements. Results for combustion temperature and OH radical predictions by the SDRL model in turbulent jet diffusion flames via an assumed-shape pdf implementation reveal the SDRL model's ability to accurately capture both near equilibrium and significant nonequilibrium reaction chemistry present within turbulent jet diffusion flames. SDRL model predictions of NO concentration in turbulent jet diffusion flames are presented, and limitations of the SDRL model within the context of these results are discussed.