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characterized the blowout phenomenon using OH PLIF. They also reported the
same pulsation and extinction of
flame toward blowout. Norton (
2003
), in their
CFD study, observed oscillations near extinction. De Zilwa et al. (
2000
) investi-
gated
fl
flame dynamics close to blowout in dump combustors with and without low
swirl. They reported very low-frequency oscillations as the rich and lean extinction
limits were approached.
Muruganandam et al. (
2002
,
2003
,
2005
) and Muruganandam and Seitzman
(
2005
), at Georgia Tech., characterize LBO phenomenon in methane-fueled, swirl-
stabilized, laboratory-scale gas turbine combustor using optical (OH chemilumi-
nescence) signature of
fl
flame. They observed extinction and reignition events near
blowout regime, which are called precursor events. They reported that the fre-
quency of those precursor events increases monotonically as the extinction limit is
reached. During extinction event if the
fl
fl
flame packets are unable to reignite the
reactant, the
fl
ame
finally blows off. They also report that the power in low-
frequency
fluctuation increases dramatically toward LBO. The authors assert that
the reason behind increasing the low-frequency power is the combination of
localized extinction timescale (
fl
*
10 ms) and the mean time between such events
(
1 s). Similarly, the parallel study was carried out by Prakash et al. (
2005
),
Prakash (
2007
), on the same swirl-stabilized combustor where the focus was on
acoustic emission rather than the optical emission. Similar to Muruganandam
*
'
s
observation in optical signature, they also observed dominant low-frequency
oscillations in acoustic signal near LBO. They also found that the acoustic signal
exhibits the same pattern of localized extinction events of optical signal, which they
extracted using a set of wavelets whose shapes are similar to the pattern. Nair et al.
(
2004
) and Nair and Lieuwen (
2005
) extended the characterization of blowout
phenomenon in terms of acoustic emission on different con
gurations of com-
bustion setup such as natural gas-fueled simple piloted burner, laboratory-scale
swirl-stabilized dump combustor, and bluff-body-stabilized combustors, and their
findings in terms of dominance of low-frequency spectrum and increased presence
of time-localized and intermittent events in acoustic signal match with the earlier
observation reported by Prakash.
Recently, Chaudhuri and Cetegen (
2009
) and Chaudhuri et al. (
2010
) charac-
terized LBO phenomenon in a bluff-body-stabilized premixed combustor and
reported that there is a signi
ame
shape changes from conical to columnar and high local stretch rates that exceed the
extinction stretch rates are the main cause for
cant reduction in
fl
ame speed toward LBO and
fl
final blowoff.
1.2 Existing Lean Blowout Sensing Approaches
The advantage of lean premixed or partially premixed combustion technique in
practical systems and its susceptibility to blowout put a major challenge to the
designers to reduce the emission as well as to maintain the power and reliability of
engine. The development of strategies for early detection of imminent blowout and
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