Investigating Instability Mitigation through Flame Oscillator Synchronization

Abstract

Stringent emission regulations for reducing NOx and soot emissions are driving gas turbine combustors manufacturers to adopt lean premixed combustion technology. However, the rich dynamics of turbulent, lean premixed combustion inside the gas turbine combustor could pose challenges like thermoacoustic instabilities, blow-off, and flashback. As such the unsteady heat release rate can couple in-phase with the combustor duct acoustics to generate large amplitude oscillations known as thermoacoustic instabilities. During such instability, the combustor structure undergoes multiple cycles of thermal and mechanical loading which could increase the chances of a premature fatigue failure. Furthermore, when the fuel flow rate drops or the air flow rate suddenly increases due to upstream dynamics, the premixed flame could experience a blow-off event where a part or in the worst case the entire flame extinguishes. Such events drastically affect the performance of the gas turbine combustors, leading to severe safety risks for aviation gas turbine engines. It remains essential to understand the sources and factors causing thermoacoustic instabilities and blow-off to design improved next-generation engines where such problems are mitigated. To that end in this thesis, we seek to deepen our understanding on the mitigation of the self excited thermoacoustic instabilities with our in-house designed and developed rotating swirler burner experimental setup. This combustor is novel in its implementation of rotating the otherwise static swirler meant to stabilize the lean premixed flame to mitigate the instability. In such a combustor with inherent, high-amplitude thermoacoustic instability is realized while the swirler is static. However, we show that increasing the swirler rotation upto certain x speeds can progressively reduce the instability amplitude. We use simultaneous high speed stereo Particle Image Velocimetry (sPIV), high speed chemiluminescence measurement in the vertical (r −y) plane, alongside time resolved pressure measurement to investigate the flow-flame dynamics as the combustion transitions from a thermoacoustically unstable condition to a stable state. During such transition, we observe the pressure amplitude does not decrease uniformly but instead bursts of large-amplitude oscillations appear more sparsely in the low-amplitude noisy data as the stable state is approached. This is known as intermittency, which is mostly observed while transitioning from stable to unstable state by varying flow Reynolds number. To model such intermittent dynamics phenomenologically, we discretize the swirling turbulent flame into an ensemble of flame oscillators arranged circumferentially around the center-body of the swirler, oscillating in the r −y plane. To simulate the synchronization between these oscillators we modify the Kuramoto model to emulate the flame oscillator dynamics. The proposed model can qualitatively reproduce the time-averaged and intermittent dynamics while transitioning from unstable to stable states. Next, we move on to high speed chemiluminescence imaging of the horizontal (r −θ) plane with simultaneous pressure measurement to identify the synchronization dynamics of flame oscillators in the combustor. Furthermore, we vary the tube lengths to excite the flame at different acoustic frequencies to test the proposed model at different conditions. As we change swirler rotational rate, the system goes through intermittency while transitioning from unstable to stable state. We observe the system reaches to the state of combustion noise at lower swirler rotation rate, as the tube length increases. At higher Reynolds number, the swirler rotation rate required for instability mitigation increases. From the Rayleigh index map we find the source of flame-acoustic coupling to be distributed in the r−θ plane. As the flame images are transformed into the r −θ co-ordinate, we observe the flame at different azimuthal location oscillate together during combustion instability. During combustion noise these oscillations becomes asynchronous, which proves the location of the oscillators in the azimuthal plane. With this evidence, we proceed to modify the synchronization model by incorporating the feedback mechanism between the duct acoustics and heat release rate oscillations. The heat release rate oscillations are modelled with Kuramoto model with flamelet oscillators. We used a linearized Helmholtz equation with heat source to model the duct acoustics of the setup. The feedback mechanism synchronizes the flamelet oscillators with the pressure oscillations in the Kuramoto model whereas the heat source term in the Helmholtz equation is generated from the ensemble of the flame oscillators. This model is used to predict the heat release rate oscillations for the different experimental conditions. It showed qualitative match with the experimental data, and reproduced the different states of the thermoacoustic systems with good accuracy. In the last part of the thesis, as an appendix, the preliminary investigations on blow-off precursors of a model gas turbine combustor with three interacting swirling premixed flame, is included. We focus on the origin of the flame extinction that leads to a complete flame blow-off. We use high speed OH* chemiluminescence to locate the origin of the flame extinction. Using low-speed simultaneous stereo Particle Image Velocimetry and Planar Laser Induced Fluorescence (sPIV-PLIF) measurements, we identify the reasons behind the flame extinction through strain rate measurements