Low exhaust emissions lean premixed gas turbine combustors have been hindered by thermoacoustic instabilities. Transitions from a quiescent state to an oscillatory state 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 latter introducing undesirable abrupt and intermittent behaviour. The current work investigates subcritical Hopf bifurcations in a model swirl stabilized gas turbine combustor which occur via gradual hydrogen enrichment of close to blow-off methane mixtures, at equivalence ratio of 0.55. The temporal evolution of the flowfield structure during the transition from the stable to the unstable state was measured using high speed (3 kHz) particle image velocimetry (PIV). Phase averaged low speed (10 Hz) OH planar laser induced fluorescence (PLIF) measurements quantified the change in the local flame structure, in terms of curvature characteristics. The results show that the transition from the quiescent towards the oscillatory state one is always preceded by a precessing motion of the inner recirculation zone (IRZ) that is followed by a complete collapse of the IRZ. This collapse is caused by a disturbance of the high positive axial velocity that is convected downstream by the mean flow, triggering the subcritical Hopf bifurcation.
The PIV measurements show that the mean and standard deviation of the spatial distribution of the turbulent intensity increases during the intermittent bursts, implying flame interacts with a wider range of flow length scales. 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.