Aerodynamics and Kinetics Effects on the Dynamics of Steady/Unsteady Laminar, Stretched Flames

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Flame behavior in realistic situations is governed by the detailed kinetics of chemical reactions, the diffusion of heat and mass, and the aerodynamic processes of stretching, turbulence, and large-scale flow nonuniformity. In this program, the response of laminar premixed flames to stretching and unsteadiness will be experimentally determined in a counterflow burner configuration in both reduced and elevated pressure environments and numerically simulated using detailed flame and kinetics codes. Emphasis will be placed on processes governing extinction of steady stretched flames, the effects of unsteadiness on flame extinction, location, and detailed structure, the implementation of non-intrusive laser- based diagnostics to map velocity profiles, and the determination of complex kinetic mechanisms. The effects of aerodynamic stretching on laminar diffusion flame extinction will also be studied for both steady and unsteady flowfields; in this case, the experimental configuration will consist of an air laminar jet impinging on a laminar fuel jet coming from the opposite direction. From comparisons between experimental and numerical results the validity of existing proposed kinetic mechanisms will be assessed, and, at the extinction point, previously developed kinetic criteria of extinction will be tested. The influence of unsteadiness on diffusion flame extinction will be quantified and insight obtained on the relative significance of various time scales associated with the problem. Practically all realistic flames are subjected to aerodynamic stretching, manifested through flow nonuniformity, flame curvature, or flame/flow unsteadiness. In premixed flames, this influence is strong due to the inherent interdependence between the transport and reaction processes. Thus, understanding the properties of stretched premixed flames is of substantial practical importance as well as of fundamental interest. Examples include flame stabilization and the modeling of turbulent flames via the concept of stretched laminar flamelets. Diffusion flames can also be affected by the presence of stretch. The response of a diffusion flame to flowfield nonuniformities depends critically on reactant leakage which can lead to incomplete reaction and extinction. Furthermore, diffusion flames are more susceptible to extinction in that they are "losing" heat on both sides in a counterflow configuration whereas premixed flames are adiabatic on one side.