Nonlinear flame transfer function characteristics in a swirl-stabilized combustor

An understanding of the amplitude dependence of the flame response to acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the nonlinear response of a lean, premixed flame to imposed acoustic oscillations. Detailed measurements of the amplitude dependence of the flame response were obtained at approximately 100 test points, corresponding to different flow rates and forcing frequencies. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed. The phase between the flow oscillation and heat release is also seen to have substantial amplitude dependence. The nonlinear flame dynamics appear to be governed by different mechanisms in different frequency and flowrate regimes. These mechanisms were investigated using phase-locked, two- dimensional OH Planar laser-induced fluorescence imaging. From these images, two mechanisms, vortex rollup and unsteady flame liftoff, are identified as important in the saturation of the flame's response to large velocity oscillations. Both mechanisms appear to reduce the flame's area and thus its response at these high levels of driving.