Numerical investigation of thermoacoustic instability in a model afterburner with a simplified model for observed lock-in phenomena

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 high-frequency thermo-acoustic oscillations in the afterburner chamber is observed at specific fuel flow rates. The flow field from the CFD shows the bluff-body vortex shedding frequency to excite the acoustics of the afterburner chamber during the thermo-acoustic oscillations. The synchronization of the bluff-body wake with the acoustics of the domain results 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 coupled oscillator model based on the van-der Pol and simple harmonic oscillators is attempted to reproduce the observed resonant oscillations. The oscillator model qualitatively captures the observed resonant oscillations in the chamber. This model could be extended to combustors with bluff-body wake in predicting thermoacoustic oscillations.