Co. Paschereit et al., Excitation of thermoacoustic instabilities by interaction of acoustics andunstable swirling flow, AIAA J, 38(6), 2000, pp. 1025-1034
Unstable thermoacoustic modes were investigated and controlled in an experi
mental low-emission swirl stabilized combustor, in which the acoustic bound
ary conditions were modified to obtain combustion instability. The acoustic
boundary conditions of the exhaust system could be adjusted from almost an
echoic (reflection coefficient \r\ < 0.2) to open-end reflection. Several a
xisymmetric and helical unstable modes were identified for fully premixed a
nd diffusion-type combustion. These unstable modes were associated with Row
instabilities related to the recirculation wake-like region on the combust
or axis and shear-layer instabilities at the sudden expansion (dump plane).
The combustion structure associated with the different unstable modes was
visualized by phase-locked images of OH chemiluminescence. The axisymmetric
mode showed large variation of the heat release during one cycle, whereas
the helical modes showed variations in the radial location of maximal heat
release. The axisymmetric mode was the dominant one during unstable combust
ion. It was obtained by forcing a longitudinal low-frequency acoustic reson
ance. Helical modes could only be obtained when the axisymmetric mode was s
uppressed by using a nonreflecting boundary condition. A closed-loop active
control system was employed to suppress the thermoacoustic pressure oscill
ations and to reduce NOx and CO emissions. Microphones were used to monitor
the pressure oscillations during the combustion process and provide input
to the control system. An acoustic actuation was used to modulate the airfl
ow and thus affected the mixing process and the combustion. Upstream excita
tion modified the shear-layer structure and was shown to be superior to dow
nstream excitation, which combined less effective shear-layer excitation wi
th noise cancellation. Suppression levels of up to 5 dB in the pressure osc
illations and a concomitant 24% reduction of NOx emissions were obtained in
premixed combustion using an acoustic power of less than 0.002% of the com
bustion power. The control of the diffusion flame was less effective, and N
Ox emissions increased at the phase that was most effective in suppressing
the pressure oscillations. The differences between the behavior of the cont
rol system in the two combustion modes was caused by different levels of in
teraction between the combustion process and the shear layer.