Shear Layer Flame Stabilization Sensitivities in a Swirling Flow

Abstract

A variety of different flame configurations and heat release distributions can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper focuses on the sensitivities of the outer shear layer stabilization point to bulkhead temperature, flow velocity, swirl number, preheat temperature, and fuel/air ratio. It also characterizes the hysteresis that is present in conditions where the outer shear layer locally re-attaches and blows off. The sensitivities to bulkhead temperature, preheat temperature and fuel/air ratio follow the expected trends. Moreover, the strong bulkhead temperature sensitivities show that computations must include heat transfer to combustor hardware in order to capture flame stabilization correctly. The preheat temperature and fuel/air ratio sensitivities are captured with detailed kinetics calculations of the extinction stretch rate of the mixture. Somewhat counter intuitively, there is little variation in transition conditions with swirl number for the Sm∼0.6 and 0.8 swirlers analyzed here. Finally, velocity sensitivities are in many cases much weaker than what would be predicted assuming that the fluid mechanic straining time scales as 1/u.