Vortex Breakdown and Global Modes in Swirling Combustor Flows with Axial Air Injection

摘要

Strongly swirling flows are used in the vast majority of gas turbine combustors. The complex flow field downstream of the vortex breakdown provides good flame stabilization but is also prone to self-excited large scale hydrodynamic instabilities. The role of these global modes, which usually manifest in a Precessing Vortex Core (PVC), for the combustion process is still an open question and influences on mixing processes and ther-moacoustic oscillations have been proposed. In the current study the effect of axial air injection through a truncated center body on the type of the Vortex Breakdown (VB) and the global hydrodynamic mode is investigated using a combined experimental, numerical, and analytical approach. A parametric study of the isothermal flow field 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 using methane fuel. Next, a Large Eddy Simulation (LES) was conducted and successfully validated with the experimental data. All results show a strong effect of the inflow parameters (axial injection rate and inlet swirl number) on the type of the vortex-breakdown and the frequency, amplitude, and shape of the global mode. The reacting cases show very similar results as the isothermal cases, proving the relevance of the isothermal investigation. Linear local hydrodynamic stability analyses, carried out on the time-average measured velocity data and the numerically obtained data, yield the absolutely unstable domain inside the flow field. Axial injection is shown to impede a zone of absolute instability near the combustor inlet while a 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 into the occurrence of global modes in swirling combustor flows and how they may be controlled by small flow field modifications. Furthermore, axial air injection is shown to provide a suitable flow field for flashback-proof combustor operation.