Response of a swirl-stabilized flame to simultaneous perturbations in equivalence ratio and velocity at high oscillation amplitudes

The flame describing function is a valuable tool for the limit-cycle prediction of thermoacoustic instabilities, which are frequently observed in modern gas turbines. In these applications, flames are of lean partially premixed type, and their unsteady response originates from a combination of mechanisms related to perturbations in velocity and equivalence ratio. The interference between these two mechanisms at high forcing amplitudes is the focus of the present study. Well-defined variations of the degree of unmixedness of a practically relevant swirl flame are analyzed to obtain the nonlinear response of the flame to equivalence ratio fluctuations. A lean premixed swirl-stabilized flame is investigated experimentally at atmospheric conditions for a Reynolds number of approximately 35,000. The flow field and the flame dynamics are investigated for perfectly and partially premixed conditions at various frequencies and excitation amplitudes using high-speed particle image velocimetry and OH*-chemiluminescence imaging. The multi microphone method is applied at different degrees of unmixedness of fuel and air to obtain the nonlinear flame response. By a change in the fuel split between two injection positions the unmixedness is controlled. The nonlinear flame response of the different partially premixed flames and the corresponding premixed flame are analyzed in this study. A decomposition approach is applied to emphasize the trends caused by the contribution of the equivalence ratio perturbations to the flame describing function. Flow field measurements indicate that the flow field dynamics are very similar for the investigated premixed and partially premixed flames. The results of the flame describing function decomposition support the assumption of a superposition of equivalence ratio perturbations and velocity fluctuations effects at low and high forcing amplitudes for most of the investigated frequencies. The decomposition reveals a saturation of the equivalence ratio contribution to the flame response already at small acoustic forcing amplitudes. A conceptual reasoning is presented with the help of a simplified flame model, which explains the observed saturation in the response of partially premixed flames. Increased mixing at high forcing amplitudes is proposed as a new saturation mechanism. Especially at small and intermediate forcing amplitudes as well as higher frequencies, increased turbulence production and changes in the mean flow field lead to an increase of the damping of equivalence ratio fluctuations. An experimentally observed growth of the turbulent shear stresses with increasing acoustic forcing amplitude supports this explanation. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.