Impact of a Central Pilot Jet on the Stability of a Swirling Flow

Abstract Swirling flows are a preferred flame stabilization mechanism for many lean premixed combustors used in gas turbines for both power generation and aircraft applications. The vortex breakdown zone of a swirling jet produces a large inner recirculation zone and regions of high shear to support flame stabilization. While these flows are successful at stabilizing flames over a large range of operating conditions, issues related to engine turndown and thermoacoustic instability can lead to the loss of both static and dynamic stability of the flame. The presence of a pilot flame in the vicinity of the main flame often helps improve static stability near lean blowoff and suppress combustion instabilities. Prior work showed how a central pilot flame enhances static and dynamic stability through thermochemical back-support of the main flame. However, the impact of the central jet on the hydrodynamic stability of the flow, and hence its receptivity to velocity-coupled instability, was not considered. This study investigates the impact of the central pilot jet on the hydrodynamic stability of a swirling flow in a configuration relevant to power generation and industrial gas turbine engines. Large-eddy simulations are done at a range of central pilot jet flow rates. Results are first analyzed using spectral proper orthogonal decomposition, which shows the presence of a precessing vortex core (PVC) at all pilot flow rates. A wavelet proper orthogonal decomposition shows that the PVC oscillation is intermittent at three of the four flow rates. The intermittency and change in PVC energy is connected back to the structure of the inner recirculation zone, which includes both the wake of the centerbody and the vortex breakdown bubble, as well as the incoming turbulence field. The central pilot jet flow rate changes the structure of this region, which in turn changes the stability of the flow and the PVC oscillation. The implications of the results in the context of velocity-coupled combustion instability are discussed.