In-situ tracking of mixture fraction gradient trajectories and unsteady flamelet analysis in turbulent non-premixed combustion

Based on in-situ tracking of gradient trajectories in a direct numerical simulation (DNS), this work combines a structural analysis of a turbulent, temporally evolving syngas jet with an extended flamelet model. This flamelet formulation accounts for curvature-induced flame-tangential transport effects and is formulated in a Lagrangian manner. Using the in-situ trajectory tracking algorithm flamelets are tracked in time and space, and for the first time complete unsteady flamelet histories are reconstructed for further analysis. By extracting all relevant flamelet parameters from the DNS, solutions for the flamelet equations in mixture fraction space with and without flame-tangential transport effects are studied and discussed together with budgets of the equations. Although the overall flamelet structure is compliant with laminar flamelet theory, significant departures from classical flamelet realizations are observed for the scalar dissipation rate. Thus, in addition to flamelets in the classical flamelet regime (no significant flame-tangential transport), additional flamelet structures with non-negligible multi-dimensional effects are observed and both types are analyzed in detail. It is shown how the regime classification is influenced by the relative magnitude of scalar dissipation rate and curvature, two quantities related to the topology of mixture fraction isosurfaces. These surfaces exhibit both regions of high curvature and sheet-like structures. The dynamic interplay of compressive and extensive strain, curvature, and scalar dissipation rate is further studied with a formulation of the scalar dissipation rate equation which accounts for variable thermo-chemical properties. The results illustrate how the topological structure of the mixture fraction field interacts with the flamelet structure.