Non-linearities in the thermoacoustic response of a premixed swirl burner

Abstract

Lean premixed combustion remains one of the simplest and most effective methods of reducing NOx emissions in industrial gas turbines. Lean premixed flames are however prone to an undesirable side effect known as combustion instability, reducing lifetime or in severe cases causing irreversible damage to the turbine. Previous studies on this subject mostly concentrated on the prediction and control of linear instabilities, whereas the current study pays particular attention to the non-linear response. In this work, scaled axial and radial swirl burners were used under atmospheric conditions to investigate the characteristics of the Flame Transfer Function (FTF) between the heat release from methane/air flames and the imposed velocity fluctuations. The velocity fluctuations imposed upon the air flow of the burners encompassed frequencies of 40 to 200 Hz, each with stepwise increase of velocity amplitude, until blow-off occurred. The work was carried out with non-intrusive, phase-locked optical diagnostic techniques, such as Particle Image Velocimetry (PIV) for flow field visualisation and an Intensified Charged Couple Device (ICCD) for analysis of the OH* chemiluminescent intensity distribution of the flame. It is concluded that there are two dominant mechanisms responsible for the non-linear response of the flame for both swirler geometries at low (below 140 Hz) and high (above 140 Hz) frequencies of excitation. At low frequencies the flame response is governed by equivalence ratio fluctuations due to the 'stiff' fuel system and volumetric fluctuations of the input air caused by the forcing. At high frequencies the flame response is governed by the flow features such as vortex roll-up, stretching the flame over the high speed annular jet, and in some cases, causing some flame extinction.