Mean field coupling mechanisms explaining the impact of the precessing vortex core on the flame transfer function

Abstract

The flame transfer functions (FTF) is a key quantity to assess the thermoacoustic properties of combustion systems. It is known to depend on the hydrodynamic instabilities of the combustor flow. This work investigates how the FTF is affected by a global flow instability known as the precessing vortex core (PVC) which is often observed in swirl flames. To study the exclusive effect of the PVC on the FTF, a perfectly premixed swirl flame is considered where the PVC mode is damped with a close to zero growth rate. An active flow control system is applied in the region of high receptivity to excite the PVC at precisely controlled amplitudes. The conducted experiments show that the excited mode corresponds to the least stable global mode predicted from mean field stability analysis and that the most responsive frequency is equal to the predicted global mode frequency, which brings credibility to the control approach. FTF measurements conducted at different PVC actuation amplitudes show that the FTF gain decreases significantly with increasing PVC amplitude while the FTF phase remains unchanged. The FTF gain reduction is explained by two mechanisms: the reduction in gain of the Kelvin–Helmholtz instability through mean field modifications and the upstream movement of the flames center of mass due to enhanced coherent fluctuations at the flame root. The unchanged FTF phase is traced back to an unchanged location of the most influential heat release rate fluctuations at the flame tip. This study suggests that the control of the PVC is an effective way to avoid or mitigate thermoacoustic instabilities. The control is thereby very efficient as it exploits the natural global hydrodynamic instability of the flow.