Numerical investigation of thermo-acoustic instability in a model afterburner with a simplified model for observed lock-in Phenomena

Abstract

Thermoacoustic oscillations in a gas turbine afterburner are numerically investigated using CFD. A simplified 2-dimensional axisymmetric afterburner with bluff-body stabilized flame is considered in the investigation. Occurrences of both low and high-frequency thermo-acoustic oscillations in the afterburner chamber are observed at specific fuel flow rates. The flow field from the CFD shows the bluff-body vortex shedding frequency to lock-in with the acoustics of the chamber during the thermo-acoustic oscillations. The synchronization and lock-in of bluff-body wake with chamber acoustics happen with increase in fuel injection rates resulting in thermoacoustic coupling. The Proper Orthogonal Decomposition of the flow field revealed the presence of chamber acoustics in the pressure field confirming the coupling. Then a simplified mathematical model based on the van-der Pol oscillator is attempted to reproduce the observed lock-in behavior of the bluff-body wake. The chamber acoustic field is considered as the forcing term for the simplified oscillator. The oscillator model qualitatively captures the synchronization of the flame-holder wake oscillations with the chamber acoustics. This model could be extended to combustors with bluff-body wake in predicting the thermo-acoustic oscillations.