Impact of Shear Flow Instabilities on the Magnitude and Saturation of the Flame Response

Abstract

Amplitudedependent flame transfer functions, also denoted as flame describing functions, are valuable tools for the prediction of limitcycle amplitudes of thermoacoustic instabilities. However, the effects that govern the transfer function magnitude at low and high amplitudes are not yet fully understood. It is shown in the present work that the flame response at perfectly premixed conditions is strongly influenced by the growth rate of vortical structures in the shear layers. An experimental study in a generic swirlstabilized combustor was conducted in order to measure the amplitudedependent flame transfer function and the corresponding flow fields subjected to acoustic forcing. The applied measurement techniques included the multimicrophonemethod, highspeed OH*chemiluminescence measurements, and highspeed particle image velocimetry. The flame response and the corresponding flow fields are assessed for three different swirl numbers at 196 Hz forcing frequency. The results show that forcing leads to significant changes in the timeaveraged reacting flow fields and flame shapes. A triple decomposition is applied to the timeresolved data, which reveals that coherent velocity fluctuations at the forcing frequency are amplified considerably stronger in the shear layers at low forcing amplitudes than at high amplitudes, which is an indicator for a nonlinear saturation process. The strongest saturation is found for the lowest swirl number, where the forcing additionally detached the flame. For the highest swirl number, the saturation of the vortex amplitude is weaker. Overall, the amplitudedependent vortex amplification resembles the characteristics of the flame response very well. An application of a linear stability analysis to the timeaveraged flow fields at increasing forcing amplitudes yields the decreasing growth rates of shear flow instabilities at the forcing frequency. It therefore successfully predicts a saturation at high forcing amplitudes and demonstrates that the mean flow field and its modifications are of utmost importance for the growth of vortices in the shear layers. Moreover, the results clearly show that the amplification of vortices in the shear layers is an important driver for heat release fluctuations and their saturation.