Experimental Investigation of Thermoacoustic Coupling for Low-Swirl Lean Premixed Flames

Combustion instabilities are frequently encountered in gas turbine engines. The main physical mechanism that leads to combustion instability is thermoacoustic coupling, where energy released as part of the combustion process Is transferred into combustor acoustics, which can then affect the combustion heat release in a feedback process. This paper presents an experimental study of this coupling for a lean premixed methane-air low-swirl flame. Planar laser-induced fluorescence of the hydroxyl radical was employed to measure the behavior of this flame in an acoustically driven environment and the Rayleigh index was used as an indicator of the thermoacoustic coupling. The acoustic excitation ranged from 13 to 270 Hz. Experiments show that the coupling occurred within certain frequency subranges depending on the Reynolds and Strouhal number. The acoustic excitation can couple with the shear-layer vortices, which in turn wrinkle the flame with a periodicity similar to the acoustic driving. This effect, through the changes in local flame surface density, gives rise to intense toroidal structures in the Rayleigh index field.