Interference mechanisms of acoustic/convective disturbances in a swirl-stabilized lean-premixed combustor

Abstract

Interference mechanisms of acoustic/convective disturbances were experimentally investigated in a swirl-stabilized lean-premixed gas turbine combustor operated with natural gas fuel and air at atmospheric pressure and elevated temperature. Interference between azimuthal and acoustic velocity disturbances at high-amplitude limit cycle oscillations is characterized in detail as a function of axial swirler location, oscillation frequency, and mean nozzle velocity. We show that both the frequency and the intensity of self-excited instabilities in a model gas turbine combustor are correlated with axial swirler position, which indicates that a vorticity wave generated at the swirl vanes is a primary source of convective disturbances in the absence of equivalence ratio nonuniformities. Flame transfer function measurements confirm that the linear/nonlinear heat release response is a strong function of axial swirler location, even when unforced flame structures remain unchanged. The key parameter controlling this phenomenon is the phase difference between the azimuthal and acoustic velocity perturbations at the combustor dump plane; the phase difference is affected by swirler location, frequency, mean velocity, and the speed of sound. It was found that out-of-phase interference between azimuthal and acoustic velocity disturbances at the combustor inlet yields large flame angle fluctuations in relation to swirl number fluctuations, and therefore the formation of a coherent structure is hindered due to high kinematic viscosity within the vortex formation region. In-phase interference mechanisms, on the other hand, lead to high-amplitude limit cycle oscillations. This interference mechanism is then explored in the presence of temporal equivalence ratio nonuniformities, in which two different sources of convective mechanisms should be considered simultaneously in connection with acoustic velocity perturbations and the vortex dynamics. Results reveal that equivalence ratio oscillation has a significant effect on the strength of combustion-acoustic interactions. Strong self-excited instabilities of partially premixed flames are produced by in-phase interactions between acoustic velocity and equivalence ratio oscillations, which are governed by fuel injection location, frequency, mean nozzle velocity, and fuel injector impedance. At this phase condition, unburned reactants with high equivalence ratio impinge on the flame front with high inlet velocity, potentially causing large fluctuations of heat release rate.