During the design and testing process of swirl-stabilized combustors, it is often impractical to maintain identical outlet boundary conditions. Furthermore, it is a common practice to intentionally change the acoustic boundary conditions of the outlet in order to suppress thermoacoustic instabilities. In the presented work the susceptibility of the reacting flow field to downstream perturbations is assessed by the application of an area contraction at the outlet. Since combustion and fuel composition are shown to be important parameters for the influence of the boundary conditions on the flow field, highly steam diluted flames are investigated in addition to dry flames, at different equivalence ratios and degrees of swirl. The applied measurement techniques include Particle Image Velocimetry, Laser Doppler Velocimetry, and emission analysis. The results reveal a clear correlation of the susceptibility of the flow field to downstream perturbations to both, the inlet swirl number and the amount of dilatation caused by the flame. The concept of an effective swirl number downstream of the flame is applied to the results and is proven to be the dominating parameter. A theoretical explanation for the influence of this parameter is provided by the usage of the well known theory of subcritical and supercritical swirling flows, where perturbations can propagate upstream solely in subcritical flows via standing waves. Knowledge of the flow state is of particular importance for the evaluation of combustion tests with differing exit boundary conditions and the results emphasize the need for realistic exit boundary conditions for numerical simulations.
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