Stability Models for Augmentor Design Tools and Technology Assessment

Augmentors, increase the thrust of a gas turbine engine by burning additional fuel with hot engine exhaust gases. Maintaining flame stability over a wide range of operating conditions is desired. An understanding of the effects of geometry and combustion processes on the flame stability is required to design advanced, efficient, and stable augmentors. Key factors affecting flame stability are vitiation level of combustion products entering the augmentor, and flame holder vortex shedding. The effect of vitiated air on flame stability has not been analyzed. Flame holders stabilize the flame by creating recirculation wakes and low speed eddies, allowing more fuel to burn. A detailed computational fluid dynamics (CFD) analysis with state-of-the-art physical and chemical models in required for meaningful numerical simulation of the complex physics encountered which involves rapid mixing, recirculation around flame holders, vortex shedding downstream of the bluff bodies and corresponding instabilities. Another major factor in achieving flame stabilization of vitiated air combustion is the competition of auto-ignition and flame propagation. Therefore, in predictive modeling of augmentors, turbulent partially premixed combustion and propagation by auto-ignition fronts, have to be considered. Appropriate large-eddy simulation (LES) tools will be developed and will be validated against carefully designed experiments.