Vortex breakdown types and global modes in swirling combustor flows with axial injection

This paper presents the results of a combined experimental, numerical, and analytical study of the occurrence of different vortex breakdown types and helical instabilities in realistic swirling combustor flows with axial air injection through a centerbody. A parametric study of the isothermal flowfield inside the combustion chamber and in the mixing tube upstream of the combustor is carried out in a water-tunnel test facility. Selected configurations were further assessed under reacting conditions. Next, a large-eddy simulation was conducted and successfully validated with the experimental data. The isothermal and reacting results show a strong effect of the inflow parameters on the type of the vortex breakdown and the frequency, amplitude, and shape of the global mode. Linear local hydrodynamic stability analyses, carried out on the time-average measured and simulated velocity data, yield the absolutely unstable domain inside the flowfield. Axial injection is shown to impede a zone of absolute instability near the combustor inlet, whereas a second zone further downstream remains. An excellent agreement of the measured to the calculated frequencies of the global modes is achieved over the whole range of investigated axial injection rates. The findings of this paper help to understand the mechanisms that are involved in the occurrence of global modes in swirling combustor flows and how they may be controlled by small flowfield modifications. Furthermore, axial air injection is shown to provide a suitable flowfield for flashback-proof combustor operation.