The study of premixed flames in a turbulent environment is of great interest in many industrial applications. At the same time turbulent combustion is a formidable challenge due to its complexity, mainly arising from the strong coupling between chemistry and turbulence. Research in the last half century has focused on the determination of a universal model for the propagation of a premixed flame in a turbulent flow and, in particular, a model for the turbulent flame speed. Experimental data exhibit a wide scatter due to accuracy of the methods and operating conditions, and modeling and simulation efforts have invariably relied on ad-hoc closure assumptions and empirically determined coefficients. Direct numerical simulations that faithfully represent the physico-chemical processes on all scales, small and large, without invoking any turbulence and/or other reduction models are currently unassailable due to the prohibitively high computational cost.
Intellectual merit: The proposed work will address the complex dynamics that result from flame interaction with turbulence using a simplified hydrodynamic model derived systematically from the full conservation laws of mass, momentum and energy. In the context of the hydrodynamic theory the flame is represented by a surface separating burned from unburned gas which propagates into the fresh mixture according to a law that, together with the conditions across the flame front, mimic the influences of diffusion and chemical reaction occurring within the flame zone. The mathematical formulation involves a nonlinear, free-boundary problem that is quite challenging, but is more easily tractable by existing computational means. An appropriate methodology will be developed for the implementation of the hydrodynamic model in two and three-spatial dimensions. Since the flame surface is determined unambiguously, all pertinent information to its propagation will be directly contained in the flame topology and in the flow field at the same location. This permits the independent or concurrent analysis of the dependence of the turbulent flame speed on turbulence intensity and turbulence scale, as well as on other local flame and flow properties, such as flame front curvature, hydrodynamic strain, heat release by chemical reactions and gas thermal expansion. The effect of instabilities on flame propagation and their role on the overall burning process, which have been invariably neglected in previous studies involving turbulent flows, will also be studied. Suppression of combustion instabilities within engines is of major importance in the design of combustor chambers. The transformational nature of the proposed research is in addressing the complex dynamics of multi-dimensional flames and their interaction with the underlying turbulence by accessible means, and in extending fundamental understanding of combustion phenomena with predictive capabilities that are based on physical first principles.
Broader impact: The proposed work falls within the flamelet regime of turbulent combustion, which encompasses many practical applications, including spark-ignition engines and ramjets. Deeper understanding of flame propagation in this regime will lead to better design capabilities and, in turn, will have an effect on improving combustion technologies. The broader impact of the proposed work will occur through publications and presentations in the technical and scientific community and by educating and training students and young scientists. This will serve extending the national human-resources base for science and technology. Results from the proposed activity will be integrated into teaching, primarily at the graduate level, and in developing models used in the classroom and for pedagogy.
