59113 - Relative Effects of Velocity- and Mixture-Coupling in a Thermoacoustically Unstable, Partially-Premixed Flame 

Combustion instability, which is the result of a resonant coupling between the combustor acoustic modes and the unsteady rate of heat release is a severely limiting factor in the operability and performance of modern gas turbine engines. This resonant coupling can occur through different coupling pathways such as flow field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermochemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, it is important to characterize the influences that each of these coupling pathways can have on the stability dynamics of a complex partially premixed, swirl stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser induced fluorescence, and acetone planar laser induced fluorescence were used to obtain information about the velocity fields, flame and fuel flow behavior respectively. Depending on the flow conditions, two thermoacoustic oscillation modes and a hydrodynamic mode, identified as the precessing vortex core (PVC), can be seen. The focus of this study is to characterize the impact that the interaction between the various oscillation modes in the combustor can have on the dispersion and mixing of fuel and air. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. These results show that, by altering the distribution of air between the two concentric nozzles, we can significantly change the mixing dynamics and in turn the flame stability. This insight can be used to design instability control mechanisms in real gas turbine engines.