Swaminathan N., Bray K.N.C. (eds.) Turbulent Premixed Flames
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4.2 Application of Lean Flames in Aero Gas Turbines 313
Table 4.5. Comparison of fuel–air mixing parameters
Parameter Diesel Gas turbine
Reaction-zone equivalence ratio 0.7 0.7
Combustion air mass flow (kg/s) 0.17 68
Fuel flow (kg/s) 0.008 3.18
Single injector flow rate (kg/s) 0.008 0.159
Approximate burning volume/inj L 0.42 1.2
Mixture mass flow per litre (kg/L) 0.43 3.0
Residence time (ms) 3 3
Injector pressure drop (bars) 1000+ 20
Mean pressure (BMEP) (bars) (18.5) 43
4.2.2 Scoping the Low-Emissions Combustor Design Problem
It is highly desirable that the aviation industry should contribute to the reduction
in global emissions. A great deal has been achieved in the reduction of both UHCs
and CO at low power and in eradicating visible smoke. In 2001 ACARE (Advisory
Council for Aeronautical Research in Europe) declared the goal to reduce NO
x
by 80% and to halve GHG emissions by 2020 c.e. [204]. The reductions are to be
achieved in stages defined in a series of Strategic Research Agenda, the latest of
which is the 2008 addendum. All these documents are freely available to download
from the ACARE website. These reductions include improvements in engine effi-
ciency, airframe weight, and drag reduction and improved traffic management, to
avoid delays in landing, for example. The reduction in NO
x
can best be achieved by
lean combustion, requiring dispersal of the fuel very rapidly t hrough up to 65% of
the total combustor airflow – ideally before reaction starts.
To gain an idea of the magnitude of this challenge, we can compare the mixing
problem with that in a diesel engine. We will compare a Volkswagen V6 2.5-L turbo
diesel developing 132 kW at 4000 rpm with an 18-deg sector of a 300-kN thrust gas
turbine having 20 fuel injectors. Although combustion is intermittent in the diesel
engine, by considering all cylinders operating in sequence, we can think of the fuel
flow as steady, treating the process as if it were taking place continuously in one
cylinder. It is a rough-and-ready comparison, and Table 4.5 shows that it is not
completely straightforward, but it brings out one point clearly. The quantity of fuel
and air to be mixed in each fuel injector in a 300-kN thrust gas turbine is 20 times that
of a 2.5-L high-speed diesel engine at full load, but the diesel fuel injector pressure
drop is 50 times greater. The time available for mixing and combustion is similar.
Several factors hinder the application of lean premixed combustion in aircraft
engines. In approximate order of influence these are as follows:
1. Airworthiness or safety – this obviously cannot be compromised and is the
subject of rigorous international regulation.
2. Operability – which interacts strongly wtih safety and includes fast response to
change in power demand, ability to cope with ingestion of large amounts of
water during descent in tropical storms, and relight at altitudes above 8000 m.
3. Fuel employed – aviation kerosene, the specification of which, though tightly
controlled in some respects, like viscosity and aromatic content, allows significant
314 Lean Flames in Practice
variation in its constituents. Auto-ignition delay, always difficult to measure, will
vary with fuel composition.
4. Size and weight.
5. OPR, which in large engines is likely to reach between 50:1 and 55:1 by 2020.
Emissions regulations, though strongly influencing design practice, are a conse-
quence of what is feasible, given the preceding constraints. As we are here considering
the application of new technology, we must take into account the changes likely in en-
gine cycle during the probable technology development time scale. Technology that
has not yet reached the stage of engine demonstration is unlikely to enter service
within 10 years. Lean burn direct injection (LBDI) technology (not prevaporised
premixed), capable of delivering an intermediate NO
x
reduction of around 40%, has
been under development for 10 years and has been demonstrated in engines. It is
undergoing further development and the first combustor of this type, the General
Electric (GE) TAPS (twin annular premixing swirler) combustor, will enter service
in 2010.
4.2.3 Emissions Requirements
There are two issues to be considered: local air quality around airports, in which
there is a high concentration both of air traffic and other sources of pollution, and
the impact of emissions at high altitude. At present only the first of these is regulated.
Emissions at altitude remain the subject of a discussion that began in the 1980s.
Emissions regulations are applied worldwide and form part of the airworthiness
certification procedure. Some European countries also apply a landing surcharge
based on emissions.
LOCAL AIR QUALITY. Regulation is concerned with activity within a radius of 30 miles
(48 km) from an airport and below an altitude of 3000 ft (914 m)[205]. So that engines
can be evaluated on a sea-level testbed, where accurate measurement is possible,
engine operation over a typical take-off and landing cycle within the sensitive zone
is characterised by four thrust settings. The basis of the calculation is presented in
Fig. 4.52.
NO
x
, CO, and UHCs are measured at four thrust settings. These measured values
weighted by time are summed and then normalised by dividing by engine sea-level
static (SLS) thrust. The four thrust settings and the corresponding times are given in
Fig. 4.52. Smoke is also measured, and the concentration is limited to ensure invisible
exhaust.
To account for variation from engine to engine, an allowance is made in the
emissions testing procedure. To avoid having to test a large number of engines to
establish an average, a certificate may be obtained from a small number of engines,
but the compliance limit is correspondingly tightened. For example, to pass on the
basis of a single engine test, measured NO
x
must be 14% below the regulatory
limit.
No engine operates at these four conditions in practice. Most engines idle at be-
low 7% thrust, and during approach the engine power cycles over a fairly wide range
to maintain the required flight path. The power required during take-off and climb
4.2 Application of Lean Flames in Aero Gas Turbines 315
Figure 4.52. The landing and take-off cycle and emissions regulations. (Courtesy of Rolls-
Royce plc)
depends on aircraft payload and runway length. To conserve engine life, pilots use
as little power as can safely be employed. NO
x
output depends on the ambient tem-
perature and humidity. The simplification is needed to enable a comparable rating of
engines. The combustion system must be designed for real operation, however, not
optimised for operation at these four power ratings. Fuel staging describes a method
of dividing the combustor volume into separately fuelled zones, the boundaries of
which may be defined by solid walls or by control of the airflow pattern. Distribution
of fuel between zones is thrust dependent. Engine control with fuel staging is much
more difficult in practice than the system that would be theoretically required for
certification testing. Thrust must vary smoothly with throttle position over the entire
power range and without lag beyond that imposed by the inertia of the turboma-
chinery. Certification emissions testing merely requires the engine to operate stably
at four thrust settings.
The regulatory limits imply that combustion inefficiency that is due to CO must
be less than 0.5% and less than 0.3% for UHCs at 7% thrust. Although, as already
described, the regulations consider the four operating conditions, today’s combustors
are 100% efficient during all but ground idle operation. This will not necessarily be
true when new lean-burn combustors enter service. The limit for NO
x
at take-off is
a function of engine thrust and pressure ratio, and is expressed in grams per 1000 kg
of thrust per landing and take-off (LTO) cycle. Corrected to 15% O
2
for a large
engine, the NO
x
concentration at take-off today would be around 400 vppm – 8
times the limit for ground-based engines in most parts of the world, and 20 times the
limit in California and Florida. Limits for land-based engines may be halved again
in the near future. These figures are simplified to permit a comparison with much
more stringent ground-based engine limits. The comparison reflects the limitations
on combustor design imposed by airworthiness requirements.
316 Lean Flames in Practice
GLOBAL ATMOSPHERIC POLLUTION. Ninety-five percent of fuel consumed in a long-
haul flight is burned during cruise. Aviation is the only direct source of emissions into
the upper atmosphere and contributes 2%–3% of the world’s CO
2
to the greenhouse
effect [206]. Aircraft engines also produce contrails by releasing water into the upper
atmosphere, the effect of which on radiative forcing of climate change is, according
to [206], uncertain and relatively small.
NO
x
creates ozone, which is a GHG in the troposphere, but reduces it in the
stratosphere, allowing increased UV radiation at the Earth’s surface. The impact of
NO
x
is complex and not yet fully understood. That it has an effect constitutes a risk.
Lean, premixed combustion raises the possibility of inefficiency at cruise because of
CO emission, because pressure and flame temperature are so reduced that the resi-
dence time in the combustor of around 6 to 7 ms is insufficient for complete CO oxi-
dation. This is economically unacceptable, so could not be permitted for that reason.
There are no regulations limiting emissions at altitude. CO
2
and H
2
o vapour
emissions are functions of the engine cycle, not of the combustor design, and there
are powerful economic reasons to reduce fuel burn. The impact of NO
x
and particu-
lates is uncertain, though probably adverse. Legislation is hindered by the difficulty
of demonstrating compliance. Combustor conditions vary greatly during cruise, as
the aircraft weight and optimum flight altitude change during the flight. Cruise can-
not be simulated on a sea-level testbed because of the fivefold difference in inlet
pressure to the engine and emissions cannot be measured in flight. To operate a large
engine in an altitude test facility is very expensive, and only one facility exists that is
capable of doing this. Emission of NO
x
at cruise is not necessarily coupled strongly
with NO
x
emission in LTO. The trend to increase the bypass ratio to reduce noise,
for instance, greatly reduces NO
x
measured on a sea-level testbed through reduced
fuel burn, but the increased nacelle drag and engine weight negate this benefit at
cruise.
FUTURE THINKING. There seems little doubt that NO
x
regulations will become more
stringent and the limits under discussion are challenging. Figure 4.53 shows the lowest
projected future limits compared with their current production engine performance.
This diagram, with filled symbols representing current Rolls-Royce engines, as-
sumes the same form as current legislation, and the current limits are included.
However, the form of and limits in future legislation could change and some possi-
bilities are listed here:
r
The LTO parameter (Dp/Foo) may be replaced with parameters taking account
of whole aircraft performance (emissions/passenger kilometre).
r
Emissions during climb and cruise may be limited, calculating emissions by
combining aircraft and engine performance data with sea-level measurement.
r
Studies are ongoing into emissions reduction by better traffic management and
ground operation of aircraft.
r
Emissions charges and fuel tax are under consideration.
r
New regulations could be applied to certification of new aircraft or engines as
early as 2014.
r
The question of cruise emission control for supersonic transports is still under
discussion. There is at present no plan to build a supersonic transport. Supersonic
4.2 Application of Lean Flames in Aero Gas Turbines 317
Trent 700
Trent 900
RB211-524H-T
RB211-535E4/E4B low NOx
Emissions Parameter – Dp/Foo
Dp – total grammes of emission produced in LTO cycle
Foo – Sea level, static max thrust
140
120
100
80
60
40
20
0
10 15 20 25 30
OVERALL PRESSURE RATIO
35 40 45 50
NOx CHARACTERISTIC Dp/Foo
ICAO 1986
CAEP2
New types 1996,
All production 2000
CAEP4
New types 2004
CAEP4 Long Term
Technology Goals
Trent 800
2016 EIS
2026 EIS
CAEP6
New types 2008
Final
Approach
3000 ft
Climb-Out
Take-Off
Take-In
Taxi-Out
Trent 500
Figure 4.53. Current and future NO
x
limits (g/kN). The long-term goals are technology ob-
jectives. Legislation will be moderated if these cannot be met. (Courtesy of Rolls-Royce
plc)
business jets are under discussion, but their flying hours being less than 10% of
civil transports, and, the aircraft being small, emissions are more a matter of
public relations than of real environmental concern. The number of supersonic
business jets in service will be limited by their high cost of ownership.
4.2.4 Engine Design Trend and Effect of Engine Cycle on Emissions
Increasing fuel prices stimulate demand for more fuel-efficient engines and thus the
OPR continues to increase. Increasing traffic density and the drive to reduce cost are
leading to increased airframe size and weight and thus to a demand for more powerful
engines. Airframe noise during landing increases with aircraft weight, and engine
noise also tends to increase with thrust. To compensate, the engine manufacturers
are driven to reduce exhaust velocity by increasing the bypass ratio. The engine core
size decreases and, as the same total energy must be generated, the smaller core
must work harder. Combustor inlet pressure, inlet temperature, and temperature
rise increase. NO
x
increases by 1% for every Kelvin increase in compressor delivery
temperature. A third of this is allowed for in the regulations through the dependence
of the NO
x
limit on pressure ratio (see Fig. 4.50).
Altitude relight requires that the combustor volume scale wheares core flow,
whereas other dimensions in the flow path scale by cross-sectional area. The com-
bustor wall area per unit throughput thus increases and this further increases the
proportion of the air used for wall-cooling flow beyond that required to compensate
for the higher gas temperatures.
318 Lean Flames in Practice
Less air is thus available for combustion, and as overall combustor FAR has
increased, this trend in engine cycle leads to increased flame temperature, smoke,
and NO
x
. The manufacturers aim to compensate by developing more temperature-
resistant materials and more complex cooling systems, but it takes two decades
to make an advance of such significance that it allows component design to be
revolutionised.
FUEL TEMPERATURE. The fuel enters the engine at about 400 K, having first been
used to cool the engine oil. The fuel injector feed arm crosses the compressor gas path
to inject the fuel on the combustor annulus centreline – a distance of up to 150 mm.
As OPR increases, heat transfer to the fuel increases. If the fuel temperature exceeds
460 K, the fuel undergoes thermal decomposition, called lacquering or coking, and
injector passages become blocked.
Injectors today are double skinned to reduce heat transfer, but there is a margin
of only a few degrees beyond the maximum fuel temperature attained. NO
x
reduction
technology, which either increases the number of fuel injectors or increases the
wetted area by increasing injector size, exacerbates this problem. As NO
x
reduction
is achieved by improving the dispersion of fuel in a large amount of air, it is impossible
to avoid the problem of thermal management of the fuel. Fuel injector airflow is
likely to increase more than fivefold during the next 10 years. Wetted area of the
fuel passages within the injector will more than double. Fuel staging in which fuel is
turned on and off within circuits in the injectors may result in stagnant fuel remaining
in hot passages unless avoided by efficient purging and careful design. There is only
a 4% pressure drop available to drive an air purge system. Weight precludes the
option of carrying bottled gas for this purpose.
DEVIATION OF TURBOMACHINERY FROM SPECIFICATION. It is possible that the com-
pressors or turbines may not fully meet their specification at entry into service. A
shortfall in efficiency of the turbomachinery relative to specification has three effects
on the combustor:
r
An increased fuel burn, arising from reduced engine efficiency, directly increases
NO
x
output.
r
The reaction zone is richer than designed, increasing smoke formation and NO
x
production. This cannot be remedied without an expensive change to the fuel
injector, which must be reversed when the turbomachinery problem is rectified.
r
Compressor delivery temperature, increased by inefficiency, increases NO
x
.
The more uniform the mixture of fuel and air prior to reaction, the greater the
impact of these effects on NO
x
. A 1% loss of compressor efficiency would produce
an increase in flame temperature of about 10 K. The corresponding increase in NO
x
from a lean premixed combustor would be about 11%–12%. A minor shortfall in
engine performance against specification can be managed. If it renders the engine
uncertifiable through its effect on NO
x
, it becomes a far more serious liability.
4.2.5 History of Emissions Research to A.D. 2000
To gain some idea of the likely pace of future combustion technology development,
it will be useful to review its history. In the early 1970s, GE and Pratt and Whitney
4.2 Application of Lean Flames in Aero Gas Turbines 319
Rich burning primary
combustion zone.
Rich combustion distinctly
separate from the main zone
Stable to low power
Lean burning primary
combustion zone.
Lean fuel air mixture to max power
Fuel staged out at lower powers
Full mains
Part mains
Pilot only
Figure 4.54. Lean DI combustor, showing on the right, circumferential staging (in which the
grey circles represent active main injectors), centre, on the left the flame pattern, and on the
top left, a lean, DI fuel injector beside an injector from a current Trent engine. (Courtesy of
Rolls-Royce plc.)
(P&W) (United Technologies) started research into fuel-staged combustors under
a NASA-funded contract. Fifteen fully annular combustors were evaluated, and
two, the P&W Vorbix, an axially staged design, and GE’s parallel double-annular
combustor emerged as practical concepts. In Europe Rolls-Royce (R-R) developed
and demonstrated a double-annular combustor. The engine demonstrations were
not entirely successful. Judging from published literature, P&W and R-R stopped
work on these concepts but GE continued, until in 1990 they launched the GE90
with a double-annular combustor. The technology was also incorporated into the
CFM56. Today, 37 years after research started, the performance of these combustors
in service does not justify their increased cost and complexity. Superficially, the
International Civil Aviation Organisation (ICAO) data may suggest otherwise, but
the reason for the low NO
x
of the GE90 is its low specific fuel consumption (SFC)
on a sea-level testbed, resulting from its high bypass ratio, rather than because of
its combustor. Dp/Foo includes the SFC through division by SLS thrust at take-
off. The performance of the GE90 double-annular combustor has been matched by
incremental development of conventional combustors.
LEAN DIRECT INJECTION. The failure of double-row staged combustors to deliver
the sought-for improvement in performance led to a change of direction in the late
1990s. GE and R-R developed lean DI combustors based on large, airblast atomisers
passing more than 5 times the airflow of previous injectors. These devices require
a central pilot zone that provides flame stability for ignition and a fuel-rich zone
to achieve high efficiency at low power. Figure 4.54 illustrates an arrangement for
combustion of this type, which is provided by the courtesy of Rolls-Royce, although
other possibilities exist. The pilot flame needs protection from reaction quenching by
320 Lean Flames in Practice
the high airflow around it. Conflicting with this, preheating of the main air by the pilot
flame is desirable to aid combustion at the end of cruise, when the relatively low flame
temperature and pressure can cause inefficiency because of incomplete combustion
of CO. Avoidance of fuel overheating with consequent blockage of fuel passages in
these large injectors complicates design and manufacture considerably. Combustors
of this type are scheduled to enter service from 2010. Reductions of 30%–40% in
NO
x
can be expected, both in the LTO cycle and at cruise. The GE TAPS injector
(see Section 5.2) is described in U.S. patent. 7,464,553B2, but the detailed design of
such a component is commercially sensitive. The GE TAPS combustor will be the
first to enter service. Twin annular refers not to the combustor configuration, but to
the concentric swirlers in the injector. The premixing length in the injector is too
short to achieve complete prevaporisation and mixing, but any degree of premixing
is probably better than none, and NO
x
emissions achieved at certification, published
on the GE website, were 58% below the 2008 CAEP6 limit. All other pollutant
species were extremely low.
The four difficulties in successfully engineering an injector of this type are as
follows:
1. To achieve satisfactory fuel distribution over the entire flight envelope.
2. To avoid overheating the fuel.
3. To avoid CO emission at end of cruise.
4. To avoid combustion instability.
A solution successfully addressing all of the preceding issues is a great achievement.
A fuel control strategy to ease the operability problems of lean combustors is
circumferential fuel staging, whereby the main fuel circuit is split, enabling some
main injectors to be turned off under conditions in which the main flame becomes
too lean to completely consume the CO. This complicates the fuel system, and,
through the consequent non-uniformity of the circumferential temperature profile,
may reduce turbine efficiency and introduce stress and distortion into the turbine
casing. These problems may not prevent application but are likely to add to the cost
and time scale of development.
ULTRA-LOW NO
x
COMBUSTION. At the end of the 1980s three significant programmes
were launched to develop low-NO
x
combustors. By far the biggest of these was the
NASA Supersonic Transport programme, with the declared goal of achieving less
than 5 EI NO
x
at supersonic cruise. This programme continued for 10 years at an
approximate cost of U.S. $200 M.
In Europe, a series of research programmes was conducted, aimed at subsonic
application and involving about one quarter that of the U.S. investment. In both the
United States and Europe, lean premixed prevapourised (LPP) and rich–quench–
lean (RQL) combustion and multipoint fuel injection were investigated. Other na-
tional and company programmes were conducted in parallel with these collaborative
programmes.
The Japanese government funded a much smaller collaborative emissions re-
duction programme over the same time scale for supersonic transport application,
involving a partnership between Kawasaki Heavy Industries (KHI) and R-R, which
formed part of the a much larger multinational engine technology programme,
HYPR.
4.2 Application of Lean Flames in Aero Gas Turbines 321
Although all of these programmes successfully demonstrated very low NO
x
in high-pressure combustion rigs, none produced anything close to an airworthy
concept. Indeed, airworthiness issues were not addressed. The author knows of no
feature in a current production engine derived from these massive efforts.
INCREMENTAL IMPROVEMENT OF CONVENTIONAL COMBUSTORS. Over the same pe-
riod of about 37 years, visible smoke has been eliminated, combustion inefficiency
at idle has been reduced by a factor of 10, and NO
x
has been halved. However,
increases in engine pressure ratio over the same period, from around 28 to 43, have
eroded the net reduction in NO
x
in service to around 20%. P&W developed [207]
a combustor that they call TALON, which controls NO
x
by careful admission of air
and fuel to achieve good mixing, followed by rapid dilution with reduced residence
time in the secondary burning zone. This is an effective and low-risk approach to
NO
x
reduction. This so-called RQL approach has been the subject of research for
three decades. The difficulty with such a concept is to avoid smoke at high power and
CO at low power, because the secondary burning zone is the part of the combustor
in which these species are consumed. In reducing the formation time for NO
x
the
consumption time for these species is also reduced. P&W appear to have achieved
substantial improvement by careful management of air admission and mixing. In-
deed their claims match those made for lean Direct Injection (DI). It is neither clear
whether the technology is at the same stage of development nor whether the claimed
performance is enhanced because associated with the P&W prop–fan cycle, which
has very low SLS SFC. The stage of development is important, as the optimisation
of a concept to meet operational requirements in service usually results in some loss
of emissions capability (see Subsection 4.2.7). The GEnx engine is certified for flight
and should enter service during 2010.
COMBUSTION INSTABILITY. All attempts to reduce NO
x
have increased the propensity
of the combustor to generate self-excited pressure oscillations, even in conventional
diffusion flame combustors. An amplitude above 15 kPa peak to peak is considered
hazardous, and a higher pressure amplitude results in rapid failure of fuel pipes, oil
pipes, or the combustion liner. The problem has not so far proved insoluble, but as
azimuthal modes occur round the annulus they do not appear until relatively late
in the development of a new combustor concept. By the time HP annular testing is
being conducted, changes in injector configuration become very expensive and time
consuming. Full sets are required rather than the one to four injectors needed for
sector testing. Design and manufacture of a set typically take from 6 to 9 months. Cost
of experimental components of this complexity is probably in the region of £100k
per set, and test costs exceed that figure. Combustion instability, though audible, is
not a significant contributor to engine noise. It is often encountered in production
engines during acceleration. If it were audible under continuous load, when noise is
measured, then the amplitude under acceleration would be considerably higher, and
rapid mechanical failure would result.
4.2.6 Operability
The following aspects of engine operation are combustor dependent. In this sub-
section, variable geometry in the gas path is not included in the discussion. The
322 Lean Flames in Practice
associated increase in complexity and concern about reliability renders it impracti-
cal. It is discussed in Subsection 4.2.9
1. Thrust variation from full power to deceleration to flight idle during descent
must be accommodated. Overall combustor AFR at take-off is around 35 and
could in future reduce to 33. To achieve low NO
x
at full power, the reaction-zone
equivalence ratio must not exceed 0.65 (23 AFR) so about 65% of the combustor
airflow must participate in combustion at full load. Efficiency must be greater
than 99% at idle, when combustor overall AFR is around 100. To achieve this,
the equivalence ratio in t he reaction zone must be at least 0.6. A threefold
change in overall AFR is required with scarcely any change in reaction-zone
stoichiometry, so fuel staging is essential.
2. AFR during descent is between 100 and 150, depending on the engine design. A
turn-down ratio
1
without extinction of greater than 3 is required. Lean extinction
for premixed reactants at high pressure and temperature, relevant for gas turbine
operation, occurs at 40 AFR. At low temperature and pressure it may occur
at 30 AFR. Allowing a margin for safety, 35 AFR is probably the maximum
allowable in the reaction zone at high power. The available turn-down ratio for
a premixed combustor while operating at high inlet temperature is thus about
1.5, sufficient to cover the range of operation from take-off to end of cruise, but
not the descent phase.
3. Altitude relight. AFR is adjustable, but combustor inlet pressure and tempera-
ture are 0.2 bar and 250 K. A pilot zone is needed to provide adequate reaction
time for ignition and acceleration to flight idle.
4. Water ingestion. During descent in a tropical storm, water ingestion may be
5 times the fuel flow. It is good practice to allow a 100% weak extinction margin
over the descent phase of the flight envelope to allow for this. This rule has
proved adequate with combustors in which the reaction zone admits about a
third of the combustor airflow. Doubling this figure may have an adverse effect
on water ingestion performance, but no evidence of this has yet been reported.
It may be that the water is diverted around the outside of the combustor and
has little effect, but this cannot be assumed and needs to be demonstrated. The
GE TAPS combustor has satisfactorily passed the water ingestion test required
for flight certification.
The implication is that any combustor that employs lean burn with very good fuel–air
mixing will need a pilot zone that confers operability similar to combustors of today.
This zone will compromise NO
x
emissions at high power.
Implications of Fuel Staging
Fuel staging has five consequences:
1. Thermal management of the lean-burn stage raises concern over injector block-
age. There are three factors involved – the large wetted surface area, the difficulty
of completely purging fuel from the fuel passages when the main injectors are
not in use, and, if purging is not used, maintaining stagnant fuel below 460 K. The
1
Turn-down ratio is the ratio of the FAR at maximum power to the FAR at minimum power.