Extinction strain rate suppression of the precessing vortex core in a swirl stabilised combustor and consequences for thermoacoustic oscillations

Abstract

In the current paper, time resolved high speed optical Particle Image Velocimetry and CH* chemiluminescence measurements were performed, to study self-excited limit cycle combustion instabilities in a swirl stabilized model gas turbine combustor operating at atmospheric pressure with choked propane and air flow supplies. The combustor was operated under a constant Reynolds number (Re=22,000) and four equivalence ratios, namely ϕ=0.50 for operation susceptible to extinction, and ϕ=0.55, ϕ=0.60 and ϕ=0.65 for operation under a thermoacoustically unstable combustion regime, to encounter two limit cycle dynamic states. The period-1 limit cycle was driven by thermoacoustic coupling between the acoustic and the thermal field at a fundamental timescale dictated by an acoustic eigenmode of the combustor. The period-2 limit cycle, further to the fundamental acoustic timescale featured a subharmonic aerodynamic signature in the heat release rate and dynamic pressure spectra caused by the helical coherent structure of a Precessing Vortex Core (PVC). Previous studies have shown that the PVC in the limit cycle regime may be suppressed by the temperature stratification at the inlet of the combustor. A mechanism is suggested to interpret the flame anchoring locations which effectively regulated whether PVC was excited or suppressed. It is showed that the conditions under which the flame attached to the centerbody and suppressed the PVC can be explained by the spatial distribution of the relative ratio of the flow imposed to the mixture extinction strain rate. The PVC was excited due to local extinction by aerodynamic straining at the inlet of the combustor, at the phase angle of maximum dynamic pressure. On increasing the equivalence ratio, the flame became robust to aerodynamic straining and flashed back at the phase angle of maximum dynamic pressure. The PVC was then suppressed due to the relative ratio of the flow imposed to the extinction strain rate, which allowed the establishment of swirl-damping temperature gradients at the combustor inlet. The paper underlines the importance of quantifying the relation between the flow imposed and extinction strain rate, as it largely dictates the eventual combustor limit cycle dynamic state and its resonant frequencies.