Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROIJND C)F THE INVENTION
This invention relates to gas turbine engines an~, more
particularly, to a simplified approach for reducing nitrous oxide emissions
through the technique of water injection.
In this era of environmental awareness it is anticipated that
regulations covering air pollution will become increasingly restrictive and
that compliance with industrial emission standards will become more difficult
to attain. These environmental considerations will have an impact upon the
development of industrial gas turbine engine power plants and may require the
reduction of exhaust emission levels consistent with available technology at
realistic costs. However, the trend in gas turbine engine development is
toward higher temperature engines which, though they are inherently more
efficient, also tend to produce higher emission levels of nitrous oxide (NOX).
It is generally accepted that NOX formation increases
exponentially with flame temperature, It has also been generally acknowledged
that NOX formation can be reduced by introducing water in the form of liquid
or steam into the combustion process to reduce the temperature to which the
air is heated by the combustion of fuel in the primary zone of combustion.
Because of the exponential increase of NVX formation with flame temperature,
relatively large reductions in NOX can be achieved with relatively low water
flow rates. Furthermore, the specific method of water injection in gas turbine
engines does not appear to be particularly important. Water has been injected
separately into the combustor through distinct water nozzles as a liquid and as
steam. It has also been injected into the combustor in the upstream, down-
stream and "side stream" directions through separate water passages in the
fuel injector. It has even been introduced through dual~flow nozzles wherein
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the water and fuel were injected coaxially into the combustor. However,
although these methods of injecting water have been successful in controlling
NOX, they have, on occasion, produced some problems with hardware life due
to local temperature gradients in the region where the water is being injected.
In fact, instrumented sector tests have demonstrated that in using the upstream
method of injecting water through the nozzle, combustor metal temperature
variations increase from a normal 500 F temperature variation with no water
injection to an 800 F variation with the amounts of water injection necessary
to achieve significant NOX reductions. While these temperature variations
are the measured results of one particular series of engine sector tests, they
are representative of the trend in temperature variations to be found in other
gas turbine engine combustors.
More recently, a concept for emulsifying the fuel and water
together and injecting the mixture through the normal (or enlarged) fuel nozzleshas been used successfully. This has considerable advantages over the systems
relying on separate injection of fuel and water since complexity is minimized,
separate nozzles may be eliminated, and costs reduced accordingly.
There is an old axiom that fuel and water won't mix. However,
they will--but only temporarily. They then separate at a rate that appears to
be a function of the specific gravity of the fuel. As the specific gravity
approache~ unity (where fuel has the same density as water), the separation
rate becomes much slower. To achieve satisfactory fuel-water emulsion,
current practice has been to process the two separate liquids through a homo-
genizer where each fuel is pressurized to a very high level and then sprayed
through extremely small orifice~ into impingement against a hard impact
block in a common mixing chamber. The impact breaks each fluid into
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extremely fine particles which become intimately mixed, or emulsified, into
one homogeneous fluid, The subsequent separation rate is apparently slowed
by the intimacy or fineness of the emulsion. This homogenizing equipment is,
of course, very bulky and costly.
Since water suppression of NOX is simply a function of water
concentration, the emulsion concept is only one means employed to assure
that each fuel nozzle is supplied with the same quantity of fuel and water as
are all the others. Since all nozzles are supplied by a common pressure
source (usually a fuel manifold), then all will flow the same rate of fluid, be it
fuel, water or a fuel-water emulsion. If separation occurs prior to combus-
tion, then some nozzles wiLl flow more fuel (or water) than others and
unacceptable temperature distributions will result inside the engine. In fact,
it has been found that fuel variations between nozzles in excess of 10 percent
are generally undesirable. In short, the fuel and water need be mixed or
emulsified only to the extent required to assure uniform distributions through-
out the manifolded fuel nozzles. Since state-of-the-art fuel-water emulsifiers
or homogenizers are inherently complex, heavy and costly, it would be advan-
tageous to develop a simple emulsifier which merely meets requirement of
uniform fuel distribution among the manifolded nozzles.
SUMMARY OF THE INVENTION
Accordingly, it is the primary object of the present invention
to provide an improved gas turbine engine wherein NOX formation is reduced
through a simplified concept of water injection.
It is another object of the present invention to provide a method
for reducing NOX emissions from gas turbine engine combustors through a
simplified concept of water injection.
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It is yet another object of the present invention to provide a
method of operating a gas turbine engine with water injection which minimizes
the variation in fuel percentage between a plurality of nozzles by prolonging
the fuel-water separation time.
These, and other objects and advantages, will be more clearly
understood from the following detailed descriptions, the drawings and specific
examples, all of which are intended to be typical of, rather than in any way
limiting on, the scope of the present invention,
Briefly stated, the above objectives are attained by introducing
separate fuel and water flows through the generally perpendicular inlets of a
common plumbing T-section whereh the two flows are generally normal at
their point of cor~luence to produce turbulent mixing. The resulting mixture
remains homogeneous for the short time required for it to be injected into the
combustor, particularly if the flow remains turbulent until injection. It has
~,~15 been discovered that at water-to-fuel ratios slightly in excess of 0. 7 (for diesel
fuel No, 2) the mixture begins to transform into a homogeneous cream having a
much slower rate of separation. Therefore, the variance in fuel percentage
among a plurality of fuel nozzles fed from a common manifold receiving the
mixture may be reduced. In~trumented sector tests have demonstrated that
instead of an increase in combustor temperature variation as experienced with
.~ the separate in~ection of fuel and water, utilization of the pre~ent fuel-water
emulsification approach significantly decreases the temperature variation,
thereby tending to prolong combustor life.
BRIEF DESCRIPTION OF THE DRAWI~GS
While the ~pecification concludes with claims particularly
pointing out and distinctly claiming the sub~ect matter which i~ regarded as
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part of the present invention, it is believed that the invention will be more fully
understood from the following description of the preferred embodiments which
are given by way of example with the accompanying drawings in which:
Figure 1 is a graphical representation of the variation in NOX
S emissions from a gas turbine engine with flame temperature;
Figure 2 is a partial cross-sectional schematic view of a gas
turbine engine incorporating the subject invention;
Figure 3 is an enlarged fragmentary view of the fuel-water
mixing apparatus of the subject invention showing its relationship to the
combustor fuel nozzles;
Figure 4 graphically compares the effect of water injection on
NOX emissions for separate injection of fuel and water and an emulsified
mixture of fuel and water; and
Figure 5 illustrates the phase changes of an emulsified fuel-
water mixture as the water-fuel ratio is varied.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawings wherein like numerals correspond to
like elements throughout, attention is fir~t directed to Figure 1 which is a
graphical repre~entation of the exponential relationship of NOX generation
(represented by a nondimensional NOX index) with name temperature in a gas
turbine engine. Line 10 represents the locus of test data relating to this
phenomenon. It is generally recognized that NOX control can be achieved by
injecting li~uid water or steam into the combustion process to lower peak
Mame temperatures and, since the relationship between NOX generation and
flame temperature is exponential, it can be appreciated from Figure l that
relatively small amount~ of water injection can produce large reductions in
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N0x. The invention soon to be described relates to the reduction of N0x in
gas turbine engines and embodies the N0x reduction-through-water-injection
concept depicted in Figure 1.
While it is recognized that gas turbine engines are, by now,
5 well understood in the art, a brief description of a representative engine will
enhance appreciation of the interrelationship of various components in light of
the invention soon to be described. To that end, attention is now directed to
Figure 2 wherein a gas turbine engine of the marine or industrial variety,
depicted generally at 12 and embodying the present invention, is diagrammati-
10 cally shown. This engine may be considered as comprising a core engine 14and an independent power turbine 16. The core engine includes an axial flow
compre6sor 18 having a bladed rotor 20. Air enters inlet 22, is compressed
by compre6sor 18 and is then discharged to a combustor 24 where it is normally
mixed with fuel and combusted to provide high energy combustion gases to
drive the core engine turbine 26. Turbine 26, in turn, drives rotor 20 through
shaft 28. The hot gases of combustion then pass through and drive the power
turbine 16 which, in turn, drives an energy-absorbing device (not shown)
through power shaft 30. Power is thus obtained by the action of the hot gases
of combustion driving power turbine 16. The products of combustion are then
20 collected and exh~stedthrough discharge nozzle 32 which, in some applications,
may be a propulsive nozzle. The above description is typical of many present-
day gas turbine engines of the industrial power generation or marine propulsion
variety and is not meant to be limiting on the present invention since it will
600n become readily apparent from the following description that the present
25 invention is capable of application to any gas turbomachine, whether of the
turbojet, turboprop or turbo6haft variety. The foregoing description of the
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operation of the engine of Figure 2 is, therefore, merely meant to be
illustrative of one type of application.
The present invention provides a simplified means for
emulsifying fuel and water to reduce gas turbine engine NOX emissions and to
achieve a more uniform fuel-water distribution within the combustor of the
gas turbine engine. Directing attention now to Figure 3, there is depicted
schematically an apparatus which embodies the present inventive concept.
Surprisingly, it was discovered that when water was added to a gas turbine
engine fuel delivery system through a simple plumbing "T" 34, sufficient
homogenization occurred. In particular, T-section 34 is provided with a first
inlet 36 for receiving a flow of fuel and a second inlet 38 for receiving a flowof water. Pipes or tubes 40, 42 are connected to inlets 36, 38, respectively,
and comprise portions of the respective fuel and water delivery systems. The
water and fuel flows are pipe flows, not sprays or mists, and are generaUy
normal to each other at their point of confluence within the "T" to produce
turbulent mixing therein. A single outlet 44 discharges the mixture into a
pipe 46 by which the mixture is carried to a flow splitter 48. Each splitter
leg 50 communicates with a generally semicircular fuel manifold 52 feeding a
plurality of nozzles 54 disposed within the upstream end of combustor 24 (see
Figure 2) in the usual manner of a gas turbine engine.
When the fuel and water are introduced into the "T" at conditions
sufficient to produce turbulent mixing (i. e., at a turbulent Reynolds number),
sufficient homogenization occurs such that the proportions of fuel and water
delivered to each nozzle 54 are sufficiently uniform. In tests performed using
a mixing "T" having a, 375" diameter water inlet, a . 625" diameter fuel inlet
and die~el No, Z as the fuel, a 1% variation in fuel content was measured at
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the discharge of splitter legs 50. In one manifold section 54 having fifteen
nozzles, a 3. 5% fuel variation ~maximum minus minimum, divided by average)
was recorded at the nozzle discharge. With JP4 fuel, the fuel variation at the
nozzles was 10%. For yet unexplained reasons, when the water inlet diameter
5 was reduced to . 31", the percent fuel variation (for diesel No. 2) between
splitter legs 50 increased from 1% to as high as 7. 8%, yet still within accept-
able limits. Experimental accuracy may account for much of this difference.
The flow velocity rates in a gas turbine engine are such that the
elapsed time that it takes to travel from the "T" element (which, practically
10 speaking, is located just upstream of the fuel manifolds) to the most distant
nozzle 54 is only a few sec~nds, well within the "stay" time of the emulsion
(the time before significant separation occurs). Additionally, the turbulence
level in the manifolds tends to complement the stay time because the inherently
high fluid velocities promote mixing rather than settling. Furthermore, while
15 Figure 3 depicts a right angle "T", some angle other than 90 at the point of
confluence would be acqeptable providing that the mixing and the flow at the
point of confluence wa~ turbulent so as to homogenize the two dissimilar
liquids.
Test8 conducted utilizing the present invention showed that N0x
20 reductions attained with the use of the emulsified fuel-water mixtures were
eomewhat greater than those obtained with separately injected water. Figure
4 shows a comparison of the N0x emissions measured during these tests. For
example, a N0x reduction of 50% would require a water-to-fuel ratio of 0. 6
when the water is injected separately. The same N0x reduction could be
25 obtained with a ratio of 0. 4 when the mixture is emulsified in accordance with
the present invention.
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The present invention could be incorporated into a gas turbine
engine, such as the representative industrial-type gas turbine engine of Figure
2, having a fuel flow delivery system 56 comprising, in part, a tank 58 from
which fuel is pumped and routed through a fuel control 60 of a known variety
which is responsive to operator throttle input and which senses and compen-
sates for measured engine parameters. Fuel from such a control is routed to
the inlet side 36 of "T" 34. Water is pumped to T-inlet 38 from a tank 62 and
through a valve 64 which is interlocked with the output of the fuel control to
maintain the desired water-fuel mixture ratio. Such a control apparatus is
within the skill of those familiar with such control art and is beyond the scopeof the present invention.
It was also discovered that when the water-fuel ratio for diesel
No. 2 exceeds 0. 7, the consistency of the emulsion begins to change from
suspended water drops in fuel to a homogeneous cream, and the cream is fully
formed as the ratio approaches 1. 0. This is shown schematically by the solid
line path 64 of Figure 5. Whereas the normal mixture of suspended water drops
in fuel required less than several minutes to separate, the creamy homogeneous
mixtures required two to eight hours to ~eparate. Clearly, this reduced
tendency to separate means that a more uniform fuel distribution may be
obtained between the plurality of nozzles and that the mixing "T" can be locatedfurther from the nozzles. Tests performed with JP4 fuel showed that the
creamy mixture forms at a water-fuel ratio of 1. 4.
Interestingly, after the cream has been formed and the water
flow reduced slightly, there is no significant apparent change in consistency
until the water-fuel ratio is reduced from 0. 9 to 0. 65, as shown by the dottedline path 66 of Figure 5. As this point is approached, the creamy mixture
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becomes heavy, changing from the consistency of whipping cream to that of
paint, Decreasing the water-fuel ratio further returned the mixture to its
normal suspended state. Tests indicate that there is no significant increase in
manifold fuel pressure for the creamy emulsion at similar operating conditions.
5 While the water-fuel ratio at which conversion to the creamy mixture occurs
may be above the normal operating range for some engines, it may be advanta-
geous to initially run to high water-fuel ratios to get the cream to "start" and
then to reduce the ratio, staying on the dotted curve 66 of Figure 5 to take
advantage of the hysteresis-like phenomenon whereby a creamy texture can be
10 maintained at lower water-fuel ratios than on the solid line path of Figure 5,
The phenomenon causing the cream to form can probably be
explained by the large difference in surface tension of the two fluids. Applica-
tion of high shear stress to the two-phase mixture results in more subdivision
of the fuel pha~e than the water phase Initially, the fuel is a continuous phase
15 while the water is in relatively large drops that settle rapidly. As water
addition continues, a point is reached where a phase reversal occurs and the
water becomes the continuous phase having the fuel suspended therein as
relatively small drops with little opportunity to coalesce and separate. This is
the emulsion described herein as "cream, " probably a pseudoplastic fluid
20 rather than a Newtonian fluid ~such as the ~uel or water alone). The process
can be reversed by adding larg~ quantities of fuel to the mixture while the
shearing action continues.
It should become obvious to one skilled in the art that certain
changes can be made to the above-described invention without departing from
25 the broad inventive concepts thereof. For example, the present invention is
meant to embrace any arrangement whereby two pipe flows, one of fuel and
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one of water, are combined turbulently without necessity of
complex homogenizers. While a simple plumbing "T" section has
been demonstrated to be effective, this is merely one example
of a means for directly combining the two fluids through
turbulent mixing to produce a homogeneous mixture. Injecting
the flows at an angle other than 90 at the point of confluence
would also be acceptable providing that the mixing which occurred
was turbulent (i.e., occurred at a Reynolds number consistent
with turbulent flow for the direct combining means employed).
It is intended that the appended claims cover these and all
other such variations in the present invention's broader inventive
concepts.
SUPPLEMENTARY DISCLOSURE
It has been found that the distance of location of
the mixing zone from the combustor is not critical, due to the
extended "stay" times that suitable mixtures can achieve, in
excess of one minute. However, insofar as nozzle fuel percentage
variation is concerned, the best results for the subject system
are achieved if the transit time between mixing zone and com-
bustor nozzle does not exceed 30 seconds.
In the operation of the system, using a mixing device
such as the disclosed "T" arrangement, the homogeneous mixing
requires no emulsifying agents intentionally added to fuel or
- water, and the provision of turbulent mixing with a Reynolds
number of at least 1500 in the resulting mixture produces the
desired result. The ratio of water to fuel can operate in the
range 0.6 to 1.4 by weight.
What is provided is, in a simplified method for
reducing NO emissions from a gas turbine engine having a
combustor in which a flow of pressurized air is mixed with fuel
and combusted the steps of:
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a) providing a flow of fuel under pressure;
b) providing a flow of water under pressure;
c) mixing directly, the fuel flow and the water flow
in a water: fuel ratio in the range 0.6 and l.~ by weight and
wherein the conditions are sufficient to produce turbulent
mixing with a Reynolds number of at least 1500 in the resulting
mixture; and
d) delivering the resulting mixture to a plurality
of combustor nozzles for combustion,the combustion occurring
within thirty seconds from the time the fuel and water flows
are mixed.
In the case where the fuel is Dlesel Fuel No. 2 the
water: fuel ratio is in the range of 0.7 and 1.0 by weight.
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