| dc.description.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 | en_US |
