Large-eddy simulation of combustion dynamics in swirling flows

The impact of premixer swirl number, S, and overall fuel equivalence ratio, Φ, on the stability of a model swirl-stabilized, lean-premixed gas turbine combustor has been numerically investigated using a massively-parallel Large-Eddy Simulations Combustion Dynamics model.; Through the use of a premixed combustion model, unsteady vortex-flame and acoustic-flame interactions are captured. It is observed that for flows with swirl intensity high enough to form Vortex-Breakdown (i.e., a phenomena associated with a large region of reverse or recirculating flow along the axis of rotation), the measured rms pressure amplitude (p′) are attenuated significantly (over 6.6 dB reduction) compared to flows without this phenomena. The reduced p′ amplitudes are accompanied by reduced longitudinal flame-front oscillations and reduced coherence in the shed vortices. Similar p′ reduction levels are achieved through changes in the operating equivalence ratio, Φ. Compared to the leanest equivalence ratio simulated (Φ = 0.52), p′ at a stoichiometric mixture is reduced by 6.0 dB.; Methodologies for active control based on modulation of the inlet Swirl number (S, a measure of the intensity of swirl) and Φ are also investigated. Open-loop control through S variation is demonstrated for a lean mixture with a significant reduction in the fluctuating mass-flow-rate and p′ after a convective time-delay. A partially-premixed combustion model, which allows for variations in the local Φ, is used to model both temporal and spatial variations in Φ. It is found that the response time to changes in Φ are much faster than those for changes in S. Also, it is shown that spatial variations in Φ (or unmixedness) actually lead to p′ attenuation in the current combustor configuration.