The current paper studies the thermoacoustically unstable combustion, under elevated
mean pressure, of a commercial swirl stabilized gas turbine burner fitted with optically accessible windows. The considered measurements include particle image velocimetry (PIV),
OH∗
chemiluminescence imaging, high speed broadband flame imaging and dynamic pressure
signals. We study cases A and B, wherein natural gas flames at mean pressures equal to 3 bar
and 6 bar delivered thermal loads equal to 335 kW and 685 kW respectively. The flow field
demonstrated a typical vortex breakdown induced inner recirculation zone and a sudden step
expansion induced outer recirculation zone. In case A, high amplitude dynamic pressure bursts
were observed amidst a quiescent acoustic background. The flame was conical, it anchored on
the shear layers of the recirculation zone and it periodically expanded in the outer recirculation zone (ORZ). In case B, the flame was consistently thermoacoustically unstable with seldom
requiescent events, while at the same time expansion to the ORZ was suppressed. By applying
Dynamic Mode Decomposition on high speed images of case A, it was showed that this expansion
introduced an additional time scale, further to the fundamental acoustically related timescale.
The superposition of two timescales over a turbulent background established an intermittent
regime of thermoacoustic instabilities, wherein the dynamics transitioned between quiescent
and fully oscillatory. A physical mechanism is suggested to explain the differences between
the flame shapes on adjusting mean pressure. The mechanism considers that the premixture
is characterized by a Lewis number lower than unity, the laminar flame speed increases on
decreasing mean pressure and the flow imposed on the flame strain rate oscillated over a period
of thermoacoustic instability. This combination resulted in oscillatory heat release rate, in the
region of the outer shear layers. The phenomenon was more pronounced in case A than in case
B, because flow dilatation imposed-strain rates are higher for the former than for the latter
flame. The paper argues that for a given fuel, elevated mean pressure introduces time scales
that can significantly affect the dynamic regime the combustor operates in.