Environmental Engineering Reference
In-Depth Information
combustion products from the pilot zone which is separated by a center body (CB)
so that its primary zone combustion process is shielded from the colder nonreacting
air of the main combustor. The fuel nozzle has two tips, namely pilot and main tips
identi
ed as PF and MF in Fig.
22
. The height of the pilot and main primary zone
combined is obviously bigger than its counterpart SAC because the nozzle cir-
cumferential spacing is generally closer to the primary zone height. In order to
distribute combustion air through the pilot and main, the preferred diffuser
(D) should be a dual-passage which may not be easy to incorporate as retro
tin
product engines with SAC. The fuel
flow split between the pilot and main changes
from 100 % with pilot only operation to lower values at higher power setting,
normally decided by NO
x
and/or combustor exit pro
fl
le considerations. One has to
develop a more complex control system in addition to requiring more hardware
including the number of fuel manifolds compared to SAC. If the main fuel nozzles
are not allowed to dribble during low-power setting in order to avoid tip coking,
cooled main nozzle tip with pilot fuel and the associated plumbing will be required.
The mixing of the pilot and main streams to provide combustor exit pro
le along
with low pattern factor pose a big design challenge especially when the dilution and
trimming air fractions go down signi
cantly compared to SAC, because the main
mixer is designed to operate lean in contrast to rich mixers in SAC
'
s. Bahr and
'
Gleason (
1973
) were fully aware of DAC
s challenges; but its lower NO
x
emission
potentials relative to the first generation rich dome SAC provided the motivation to
develop DAC technology in order to address its de
ciencies as described in
Gleason et al. (
1975
), Gleason and Bahr (
1979
), Stearns et al. (
1982
) and Burrus
et al. (
1984
).
DAC technology successfully transitioned into product as retro
ts for the
CFM56-5 and CFM56-7 engine models (ICAO engine emissions tested during
1994 and 1997) and in a new centerline engine, the GE90 emissions tested in
February 1995. The CFM56 engine models gave an average margin of 50 % from
the CAEP2 regulatory standard with the average takeoff pressure ratio of 28.
However, the corresponding margin for the GE90 was 25 % at the average takeoff
pressure ratio of 38; attributed primarily to the use of less-effective liner cooling
than the segmented structure used in the Energy Ef
cient Engine demonstration
program (Stearns et al.
1982
). It was possible to pursue the use of advanced liner
cooling with attendant increase in the combustion and lower NO
x
emissions in
combination with enhancing fuel/air mixing by improving the mixer design
(Mongia
2013e
). However, seeing no support for advancing cooling technology, we
pursued formulation and development of a new low-NO
x
technology with the long-
term plan for achieving premixed combustion NO
x
entitlement in addition to
addressing ALL combustion system design requirements including durability;
pro
ameout;
combustion dynamics; and fewer number of lower cost nozzles fabricated by Direct
Metal Laser Sintering (DMLS) spearheaded by Marie McMasters. The next section
describes this approach.
le and pattern factor; low-power CO and HC; ignition, relight, and
fl
Search WWH ::
Custom Search