Abstract
Sudden changes of flame shape are an undesired, yet poorly understood feature of swirl combustors used in gas turbines. The present work studies flame shape transition mechanisms of a bistable turbulent swirl flame in a gas turbine model combustor, which alternates intermittently between an attached V-form and a lifted M-form. Time-resolved velocity fields and 2D flame structures were measured simultaneously using high-speed stereo-PIV and OH-PLIF at 10 kHz. The data analysis is performed using two novel methods that are well adapted to the study of transient flame shape transitions: Firstly, the linear stability analysis (LSA) of a time-varying mean flow and secondly the recently proposed spectral proper orthogonal decomposition (SPOD). The results show that the transitions are governed by two types of instability, namely a hydrodynamic instability in the form of a precessing vortex core (PVC) and a thermoacoustic (TA) instability. The LSA shows that the V-M transition implies the transient formation of a PVC as the result of a self-amplification process. The V-M transition, on the other hand, is induced by the appearance of a TA instability that suppresses the PVC and thereby modifies the flow field such that the flame re-attaches at the nozzle. In summary these results provide novel insights into the complex interactions of TA and hydrodynamic instabilities that govern the shape of turbulent swirl-stabilized flames.
Highlights
Modern gas turbines are required to run at low NOx emissions and over a wide range of operating conditions
The flame alternates intermittently between an M-shape featuring a precessing vortex core (PVC) and a V-shape without PVC, which implies that the PVC is repeatedly formed and suppressed
The process of detachement, which is induced by the precessing vortex core (PVC), is no longer sustained and the flame
Summary
Modern gas turbines are required to run at low NOx emissions and over a wide range of operating conditions. [6] it was found that the M-flame features a large region of absolute instability near the inlet In this case weakly nonparallel stability theory predicts that the global mode wavemaker is located at the inlet, and the growth rate σ is given by the imaginary part of the absolute frequency at this point, reading σ = I {ω0(x = 0)}. From the spectrum we identify a few discrete modes that clearly stick out due to their energy content or due to their spectral coherence The shapes of these modes are displayed in the frames above the SPOD spectrum showing the fluctuating transversal velocity component with superimposed streamlines of the mean flow field. The attached V-flame is TA-stable and the oscillations die out
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