An experimental study of subcritical transition into thermoacoustic oscillations in a swirl stabilized model gas turbine combustor

Development of gas turbine combustors operating on the lean premixed mode has been hindered by the establishment of thermoacoustic instabilities that may prove challenging to the structural integrity of the engine. Transitions between a quiescent state of thermoacoustic operation towards an oscillatory one occur via a Hopf bifurcation, namely via the emergence of periodic oscillations amidst broadband fluctuations. Hopf bifurcations can be distinguished between supercritical and subcritical, with the later type introducing especially undesirable effects due the abrupt and intermittent nature of their demonstration. Subcritical Hopf bifurcations in a model swirl stabilized gas turbine combustor are investigated in the current work. The system is driven towards the intermittently unstable regime namely close to the subcritical Hopf bifurcation point via gradual hydrogen enrichment of close to blow-off methane mixtures, at a constant equivalence ratio of 0.55. High speed (\SI{3}{kHz}) particle image velocimetry (PIV) measurements were employed to capture the flowfield structure during the transition from the stable to the unstable state. Phase conditioned low speed (\SI{10}{Hz}) OH planar laser induced fluorescence (PLIF) measurements provided information with regards to the flame structure. The transition from the low amplitude state towards the high amplitude one is always preceded by a precessing motion of the inner recirculation zone (IRZ) that is followed by a complete collapse of the IRZ. The collapse is caused by a disturbance of high positive axial velocity, convected downstream by the mean flow acting as the triggering event of the subcritical Hopf bifurcation. The PIV measurements show that the mean and standard deviation of the spatial distribution of the turbulent intensity increase, thus the flame interacts with a greater range of length scales during the intermittent bursts. This is also exemplified by the PDFs of the curvature distribution of the flame front quantified via PLIF. The analysis shows that transitional flames are more wrinkled than their quiescent counterparts, as evidenced by the higher curvature standard deviations and the lower curvature kurtosis values