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This report deals with an experimental and analytical study of the occurrence of self-excited hydrodynamic instabilities in swirling combustor flows. Different flow field types are measured for varying combustor operating conditions using stereoscopic particle image velocimetry. The properties of the flow field are connected to the flame shapes encountered. It is shown that the appearance of a helical flow instability, featuring a precessing vortex core, is strongly dependent on the flame shape. Flames attached to the burner exit may suppress the precessing vortex core if the flame features a sufficient length, while suppression is not observed for very short attached and all detached flames. Linear hydrodynamic stability analysis is carried out on the time-averaged flow, yielding an excellent prediction of the instability frequency and the wavemaker location and proving that the self-excited oscillations are driven by a hydrodynamic instability. A parametric model study investigating the role of the backflow velocity, the swirl intensity, and the density distribution is carried out, revealing that the backflow on the jet axis and the density gradient in the shear layer are essentially the parameters that determine the suppression or excitation of the instability. |
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