Effect of swirler-induced perturbations on the thermoacoustic source term

The contributions of salient flow structures to the thermoacoustic source term are investigated experimentally via open-loop control of swirler vane angle. Experiments are performed for methane-air, turbulent, premixed, and swirling flames stabilized inside a gas turbine model combustor. The mixture fuel-air equivalence ratio and the mean bulk flow velocity are 0.7 and 9.0 m/s, respectively. Two swirler vane angle control schemes are examined: an actuation from 30॰ to 60॰ and back to 30॰ as well as an actuation from 30॰ to 0॰ and back to 30॰. Simultaneous pressure and flame chemiluminescence along with stereoscopic particle image velocimetry are employed. The results show that the above actuation mechanisms change the pressure amplitude by about -44% and +22%, respectively. Facilitated via the above open-loop control, the role of salient flow structures on the dynamics of the thermoacoustic source term is studied. It is obtained that using the swirler vane actuation to increase the sizes of the precessing vortex core, shear layers, and outer recirculation zones reduces the thermoacoustic source term, while enlarging high velocity zones tends to increase the source term. The findings underline the importance of active flow control for mitigating the thermoacoustic oscillations.