A novel application of flame hole dynamics to consistent turbulent nonpremixed combustion modeling

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Most systems used in transportation and energy conversion devices are powered through combustion of fuel in turbulent flows. Improvement of current and future versatile fuel feedstock, while reducing pollutant emissions, is a primary technological challenge. The chemical activity is typically concentrated within thin flames, usually called flamelets, which are embedded in an otherwise non-reacting turbulent flow field. These flamelets are one-dimensional models of the combustion process and are unable to account naturally for multi-dimensional phenomena appearing in flows where finite-rate-chemistry effects or preferential transport effects are important. Edge flames are two-dimensional structures consisting of curved edges that can arise at the boundary of a partially extinguished flame. In non-premixed flames, edge flames consist of premixed segments (lean and/or rich in fuel), with a diffusion flame that consumes the excess reactants trailing behind. Edge flames are relevant in turbulent flows where finite-rate chemical processes lead to extinction/reignition of the combustion; for example, near holes poked in turbulent diffusion flames at high strains (regions of strong turbulence activity). They should be incorporated in numerical simulations, where necessary, for a comprehensive and physically realistic description. The objective of the project is to develop, verify, and validate structural flame models for turbulent combustion that couple consistently real gas chemistry, flame structure, and hydrodynamics. Furthermore, the proposal pursues an extension of these flame-abstraction ideas to large-eddy simulation (LES) by developing a formalism and closure that can be applied with generality in turbulent simulations. The project involves integrating flamelet and edge flame structures and their dynamics in a methodology for accurate and efficient numerical simulations. The proposed program focuses on reducing empiricism and improving predictability through first principles in the modeling closures. The proposed research will make a significant impact in the understanding and in the capability to predictively model turbulent combusting flows with generality in terms of flow conditions and fuel chemistry. Enhancements in the prediction of these flows have numerous industrial and societal benefits: energy conversion efficiency has a direct effect on cost and pollution. Moreover, the advances and innovations in computational thinking and in the use of cyber resources will enhance the engineering and applied mathematics education, which in turn helps extend the national human-resources base for science and technology. The broader impact of the accomplished work is through publications and presentations in the technical and scientific community, and by educating and training students and young scientists for future careers in academia and industry.