U.S. patent application number 09/906544 was filed with the patent office on 2002-01-31 for fuel injector with bifurcated recirculation zone.
Invention is credited to Crocker, David S., Nickolaus, Daniel A., Smith, Clifford E..
Application Number | 20020011064 09/906544 |
Document ID | / |
Family ID | 26872173 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011064 |
Kind Code |
A1 |
Crocker, David S. ; et
al. |
January 31, 2002 |
Fuel injector with bifurcated recirculation zone
Abstract
A gas turbine fuel injection system of the lean direct injector
type designed to reduce nitrous oxide (NOx) emissions is provided.
The configuration includes a pilot fuel injector for injecting a
pilot fuel stream, and a pilot swirler for providing a swirling
pilot air stream to atomize and entrain the pilot fuel stream. A
main airblast fuel injector is located concentrically about the
pilot fuel injector, for injecting a main fuel stream
concentrically about the pilot fuel stream. Inner and outer main
swirlers provide a swirling main air stream to atomize and entrain
the main fuel stream. An air splitter is located between the pilot
swirler and the main swirler. The air splitter is so arranged and
constructed as to divide the pilot air stream exiting the pilot
swirler and the air splitter, from the main air stream exiting the
inner main swirler, whereby a bifurcated recirculation zone is
created.
Inventors: |
Crocker, David S.;
(Huntsville, AL) ; Nickolaus, Daniel A.;
(Huntsville, AL) ; Smith, Clifford E.; (Madison,
AL) |
Correspondence
Address: |
WADDEY & PATTERSON
414 UNION STREET, SUITE 2020
BANK OF AMERICA PLAZA
NASHVILLE
TN
37219
|
Family ID: |
26872173 |
Appl. No.: |
09/906544 |
Filed: |
July 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09906544 |
Jul 16, 2001 |
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09649518 |
Aug 29, 2000 |
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6272840 |
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60176379 |
Jan 14, 2000 |
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Current U.S.
Class: |
60/776 ;
60/748 |
Current CPC
Class: |
Y02T 50/677 20130101;
F23R 3/343 20130101; Y02T 50/60 20130101; F05D 2270/082 20130101;
F02C 7/22 20130101; F23D 2900/00015 20130101; F23D 11/107 20130101;
F23D 2900/11101 20130101 |
Class at
Publication: |
60/39.06 ;
60/748 |
International
Class: |
F23R 003/14 |
Goverment Interests
[0001] The invention was made with U.S. government support under
Contract No. NAS3-37746 awarded by the National Aeronautics and
Space Administration under the Small Business Innovative Research
(SBIR) program. The government has certain rights in this
invention.
Claims
What is claimed is:
1. A fuel injection system for a gas turbine, comprising: a pilot
fuel injector; a pilot swirler for swirling air past the pilot fuel
injector; a main airblast fuel injector; inner and outer main
swirlers for swirling air past the main airblast fuel injector; and
an air splitter, located between the pilot swirler and the inner
main swirler, the air splitter being so arranged and constructed to
divide a pilot air stream exiting the pilot swirler and the air
splitter, from a main air stream exiting the inner main swirler,
whereby a bifurcated recirculation zone is created between the
pilot air stream and the main air stream.
2. The system of claim 1, further comprising: a fuel supply control
system for providing fuel only to the pilot fuel injector at lower
power conditions, and for providing fuel to both the pilot fuel
injector and the main airblast fuel injector at higher power
conditions.
3. The system of claim 1, wherein: the main airblast fuel injector
includes an atomizer filmer lip having an aft end; and the air
splitter has an aft end angled radially inboard and axially
positioned upstream of the aft end of the atomized filmer lip of
the main airblast fuel injector.
4. The system of claim 1, further comprising: an outwardly flared
wall downstream of the main airblast fuel injector, the flare
ending at an angle in the range of 45.degree. to 70.degree. to a
central axis of the main airblast fuel injector.
5. The system of claim 4, wherein: the outwardly flared outer wall
has a length from an aft end of the main airblast fuel injector to
an aft end of the outer wall sufficiently short to prevent
autoignition of fuel within the outer wall.
6. The system of claim 4, wherein: the outwardly flared outer wall
has a length from an aft end of the main airblast fuel injector to
an aft end of the outer wall sufficiently short to prevent main
fuel wetting of the flared outer wall.
7. The system of claim 1, wherein: the pilot fuel injector is a
pressure atomizer; and the pilot swirler surrounds the pressure
atomizer.
8. The system of claim 1, wherein: the swirlers are constructed
such that from 5 to 20% of total airflow is through the pilot
swirler, from 30 to 70% of total airflow through the swirlers is
through the inner main swirler, and the balance of total airflow is
through the outer main swirler.
9. The system of claim 1, wherein: the pilot fuel injector is a
pilot airblast fuel injector; and the pilot swirler includes inner
and outer pilot swirlers located inward and outward of the pilot
airblast fuel injector.
10. The system of claim 9, wherein: the swirlers are constructed
such that 1 to 10% of total airflow through the swirler is through
the inner pilot swirler, 2 to 15% of total airflow is through the
outer pilot swirler, 30 to 70% of total airflow is through the
inner main swirler, and the balance through the outer main
swirler.
11. The system of claim 1, wherein: the inner and outer main
swirlers each include axial swirl vanes.
12. The system of claim 11, wherein: the axial swirl vanes are
curved to reduce the Sauter Mean Diameter of a main fuel spray from
the main airblast injector.
13. The system of claim 11, wherein: the axial swirl vanes of the
inner main swirler have a swirl vane angle in the range of 25 to 60
degrees; and the axial swirl vanes of the outer main swirler have a
swirl vane angle in the range of 45 to 65 degrees.
14. A fuel injection system for a gas turbine, comprising: a pilot
fuel injector; a pilot swirler for swirling air past the pilot fuel
injector; a main airblast fuel injector; inner and outer main
swirlers for swirling air past the main airblast fuel injector; and
an air-splitter means, located between the pilot swirler and the
inner main swirler, for dividing an outer pilot air stream exiting
the pilot swirler from an inner main air stream exiting the inner
main swirler, and for thereby creating a bifurcated recirculation
zone.
15. The system of claim 14, wherein: the main airblast fuel
injector includes an atomizer filmer lip having an aft end; and the
air splitter means has an aft end angled radially inboard and
axially positioned upstream of the aft end of the atomized filmer
lip of the main airblast fuel injector.
16. A fuel injector apparatus for a gas turbine, comprising: a
primary fuel injector; a first swirler, nested about the primary
fuel injector; a second swirler nested about the first swirler; a
secondary fuel injector, nested about the second swirler; a third
swirler nested about the secondary fuel injector; and an air
splitter, nested between the first and the second swirler, and
having a radially inwardly tapered inner surface defining an outlet
opening, the outlet opening being disposed axially downstream of
the primary fuel injector for creating a bifurcated recirculation
zone.
17. The apparatus of claim 16, wherein: the secondary fuel injector
is an airblast, secondary injector having an annular fuel outlet
and a pre-filming surface for providing an annular film of fuel to
be entrained between the second and third swirlers.
18. The apparatus of claim 17, further comprising: a flared outlet
axially downstream of the secondary fuel injector.
19. The apparatus of claim 16, wherein: the primary fuel injector
is an airblast primary injector; and the apparatus further includes
a fourth swirler nested inside of the primary fuel injector.
20. The apparatus of claim 16, wherein: the primary fuel injector
is an axially located pressure atomizer.
21. A fuel injector apparatus for a gas turbine, comprising: an
axially located pressure atomizer first fuel injector; a first
swirler located concentrically about the pressure atomizer fuel
injector; a second swirler located concentrically about the first
swirler; a third swirler located concentrically about the second
swirler; an airblast fuel injector located concentrically between
the second and third swirlers; and an air splitter located
concentrically between the first and second swirlers.
22. A fuel injector apparatus for a gas turbine comprising: a first
swirler; a second swirler located concentrically about the first
swirler; a third swirler located concentrically about the second
swirler; a fourth swirler located concentrically about the third
swirler; an inner airblast fuel injector located concentrically
between the first and second swirlers; an outer airblast fuel
injector located concentrically between the third and fourth
swirlers; and an air splitter located concentrically between the
second and third swirlers.
23. A method of injecting fuel into a gas turbine, comprising: (a)
injecting a pilot fuel stream; (b) injecting a main fuel stream
concentrically about the pilot fuel stream; (c) providing a
swirling pilot air stream to entrain the pilot fuel stream; (d)
providing a swirling main air stream to entrain the main fuel
stream; and (e) splitting the pilot air stream from the main air
stream and creating a bifurcated recirculation zone between the
pilot air stream and the main air stream.
24. The method of claim 23, wherein step (e) further includes:
avoiding creation of a central recirculation zone.
25. The method of claim 23, further comprising: centrifuging the
pilot fuel stream into the bifurcated recirculation zone by means
of the swirling pilot air stream of step (c).
26. The method of claim 25, further comprising: anchoring a pilot
flame in the bifurcated recirculation zone.
27. The method of claim 23, wherein: step (a) includes injecting
the pilot fuel stream through an axially located pressure atomizer
injector; step (b) includes injecting the main fuel stream through
an airblast injector; and step (d) includes providing inner and
outer swirling main air streams inward and outward of the airblast
injector.
28. The method of claim 27, further comprising: s dividing a total
airflow between the pilot air stream and the inner and outer main
air streams such that: the pilot air stream includes from 5 to 20%
of total airflow; the inner main air stream includes from 30 to 70%
of total air flow; and the outer main air stream includes the
balance of total airflow.
29. The method of claim 23, wherein: step (a) includes injecting
the pilot fuel stream through a pilot airblast injector; step (b)
includes injecting the main fuel stream through a main airblast
injector; step (c) includes providing inner and outer swirling
pilot air streams; and step (d) includes providing inner and outer
swirling main air streams.
30. The method of claim 29, further comprising dividing a total air
flow between the air streams such that: the inner pilot air stream
includes from 1 to 10% of total air flow; the outer pilot air
stream includes from 2 to 15% of total airflow; the inner main air
stream includes from 30 to 70% of total air flow; and the outer
main air stream includes the balance of total air flow.
Description
REFERENCE TO PRIOR APPLICATIONS
[0002] This application claims benefit of our co-pending
provisional patent application Ser. No. 60/176,379 filed on or
about Jan. 13, 2000 entitled "METHOD AND APPARATUS FOR DECREASING
COMBUSTOR EMISSIONS".
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to fuel injection
assemblies for gas turbine engines, and more particularly, but not
by way of limitation, to fuel injectors of the general type known
as lean direct injectors (LDI) which are designed to reduce nitrous
oxide (NOx) emissions.
[0005] 2. Description of the Prior Art
[0006] There is a continuing need, driven by environmental concerns
and governmental regulations, for improving the efficiency of and
decreasing the emissions from gas turbine engines of the type
utilized to power jet aircraft or generate electricity.
Particularly, there is a continuing drive to reduce nitrous oxide
(NOx) emissions.
[0007] Advanced gas turbine combustors must meet these requirements
for lower NOx emissions under conditions in which the control of
NOx generation is very challenging. For example, the goal for the
Advanced Subsonic Technology (AST) gas turbine combustor research
being done by NASA is a 50 to 70 percent reduction in NOx emissions
and an 8 percent improvement in fuel efficiency compared to ICAO
1996 STANDARDS TECHNOLOGY. Realization of the fuel efficiency
objective will require an overall cycle pressure ratio as high as
60 to 1 and a peak cycle temperature of 3000.degree. F. or greater.
The severe combustor pressure and temperature conditions required
for improved fuel efficiency make the NOx emissions goal much more
difficult to achieve.
[0008] One approach to achieving low NOx emissions is via a class
of fuel injectors known as lean direct injectors (LDI). Lean direct
injection designs seek to rapidly mix the fuel and air to a lean
stoichiometry after injection into the combustor. If the mixing
occurs very rapidly, the opportunity for near stoichiometric
burning is limited, resulting in low NOx production.
[0009] A general summary of the various types of fuel injectors for
gas turbine engines is shown in the text of Lefebvre, Gas Turbine
Combustion (1983) at Chapter 10 thereof. FIG. 10.61 of the Lefebvre
text discloses the basic design features of a piloted airblast
atomizer, having a central pilot simplex pressure atomizer
surrounded by a main airblast atomizer.
[0010] U.S. Pat. No. 5,477,685 to Samuelson et al. discloses a lean
burn injector utilizing a ring of radial injection ports, which
inject fuel into a chamber where it mixes with swirling air.
[0011] U.S. Pat. No. 5,505,045 to Lee et al. discloses a dual
airblast injector having first and second concentrically located
airblast injectors.
[0012] U.S. Pat. No. 5,603,211 to Graves discloses an injector
having an axial fuel injector surrounded by three swirlers having
different swirl angles.
[0013] U.S. Pat. No. 5,816,050 to Sjunnesson et al. discloses a low
emission combustion chamber for gas turbine engines having an axial
pilot fuel injector having an associated first flow swirler, and
having a main fuel injector which injects into an annular chamber
divided from the pilot fuel injector by a divider wall.
[0014] Smith, et al., Journal of Propulsion and Power, Vol. 11, No.
2, Mar-Apr 1995, "Dual-Spray Airblast Fuel Nozzle for Advanced
Small Gas Turbine Combustors", p. 244-251, describes a dual
airblast nozzle.
[0015] Thus, it is seen that there is a continuing need for
improved designs in fuel injector systems, for gas turbine engines
and particularly lean direct injector systems, for providing
improved combustion efficiencies and reduced emissions of various
pollutants, particularly NOx.
SUMMARY OF THE INVENTION
[0016] The present invention provides a fuel injection system for a
gas turbine engine which includes a pilot fuel injector, a pilot
swirler for swirling air past the pilot fuel injector, a main
airblast fuel injector, inner and outer main swirlers for swirling
air past the main airblast injector, and an air splitter located
between the pilot swirler and the inner main swirler. For a dual
airblast configuration, there is an additional air swirler radially
inside the pilot fuel injector. The air splitter is so arranged and
constructed as to divide a pilot air stream exiting the pilot
swirler from a main air stream exiting the inner main swirler,
whereby a bifurcated recirculation zone is created between the
pilot air stream and the main air stream.
[0017] No central recirculation zone is created. The pilot fuel
stream is either injected (at high fuel pressure drops) or
centrifuged (at low fuel pressure drops) into the bifurcated
recirculation zone. The pilot flame is aerodynamically anchored and
separated from the main flame. This allows the pilot flame to
operate in a stable manner on its own during low power operation,
thus minimizing problems with lean blowout of the pilot.
[0018] A relatively large amount of air is introduced through the
main swirlers. This air can effectively prevaporize and partially
premix with the main fuel, resulting in a leaner and cooler high
power flame which produces less NOx emissions.
[0019] Thus, a lean direct injection (LDI) fuel nozzle is provided
which can achieve the desired low NOx emission goals while
maintaining acceptable lean blowout performance. The radially
staged injection of fuel and air between the pilot injector and the
main injector is key to this performance. The radial staging
accomplishes two important objectives. First, excellent fuel/air
distribution resulting in very low NOx is achieved at high power
conditions by injecting the fuel in the form of an inner pilot fuel
injection and an outer concentric ring. Second, the radial staging
of the two fuel injection streams provides good low power
operability by allowing operation of only the inner stage at low
power conditions such as engine idle conditions. The result is a
relatively rich and stable pilot flame at idle conditions.
[0020] It is therefore, a general object of the present invention
to provide an improved lean direct injector for gas turbine
engines.
[0021] Another object of the invention is the provision of fuel
injectors for gas turbine engines which result in low emissions of
pollutants, particularly low NOx emissions, and CO emissions at low
power conditions such as idle.
[0022] Another object of the present invention is the provision of
a fuel injector for a gas turbine engine which has superior lean
blowout performance.
[0023] Another object is that the fuel injector be designed to
operate at the high power conditions of advanced gas turbine
engines without thermal damage to the fuel injector itself.
[0024] Other and further objects, features and advantages of the
present invention will be readily apparent to those skilled in the
art upon a reading of the following disclosure when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a sectioned schematic view of a first embodiment
for a piloted airblast lean direct fuel injector. The embodiment of
FIG. 1 includes an airblast pilot injector and an airblast main
injector. A combustor chamber downstream of the fuel injector is
schematically illustrated as are the air streams and fuel spray
distribution patterns of the fuel/air mixture exiting the fuel
injector.
[0026] FIG. 2 illustrates an alternative embodiment of the
invention utilizing a simplex pressure atomizer for the pilot fuel,
and utilizing an airblast main injector. This embodiment utilizes
an axial vaned pilot swirler, an axial vaned inner main swirler,
and a radial injection outer main swirler.
[0027] FIG. 3 is a cross-sectional schematic view of another
alternative embodiment of the invention which, like the embodiment
of FIG. 2, has a simplex pressure atomizer pilot injector, and an
airblast main injector. The embodiment of FIG. 3 utilizes an axial
vaned outer main swirler. Also in FIG. 3, the flame pattern is
schematically illustrated and the legends in the figure denote the
typical flame colors contained in the flame pattern during high
power operation.
[0028] FIG. 4 is a schematic illustration of a fuel flow control
system utilized with the invention.
[0029] FIG. 5 illustrates the preferred construction of the curved
aerodynamic axial vanes utilized with the swirlers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS THE DUAL AIRBLAST
INJECTOR OF FIG. 1
[0030] Referring now to the drawings, and particularly to FIG. 1, a
fuel injection system is shown and generally designated by the
numeral 10. The fuel injection system 10 is mounted upon the dome
wall 12 of a combustor 14 of a gas turbine engine.
[0031] Fuel injector system 10 has a central axis 11. The fuel
injection system 10 includes a pilot fuel injector 16, and a pilot
swirler 18 for swirling air past the pilot fuel injector 16. The
pilot swirler 18 is schematically illustrated, and in the
illustrated embodiment it is an axial vane type pilot swirler 18.
In general, the pilot swirler 18, and any of the other swirlers,
can be either radial vaned or axial vaned swirlers.
[0032] When utilizing an airblast type pilot fuel injector 16 as
illustrated in FIG. 1, the pilot swirler 18 may be further
described as an outer pilot swirler 18, and the pilot airblast fuel
injector 16 will have associated therewith an inner pilot swirler
19. Inner pilot swirler 19 in the illustrated embodiment includes
tangentially oriented flow ports 21 and 23 leading into an interior
swirl chamber 25 which leads to the filmer lip 44 of the airblast
type pilot fuel injector 16.
[0033] The fuel injection system 10 further includes a main
airblast fuel injector 20 which is concentrically located about the
pilot fuel injector 16. Inner and outer main swirlers 22 and 24 are
located concentrically inward and outward of the main airblast fuel
injector 20. The pilot fuel injector 16 and main fuel injector 20
may also be described as a primary fuel injector 16 and a secondary
20 fuel injector, respectively.
[0034] As will be understood by those skilled in the art, an
airblast fuel injector such as injector 20 provides liquid fuel to
an annular outlet 27 which allows the fuel to flow in an annular
film along atomizer filmer lip 40 leading to the aft end 42 of the
injector. The annular film of liquid fuel is then entrained in the
much more rapidly moving and swirling air streams passing through
inner main swirler 22 and outer main swirler 24, which air streams
cause the annular film of liquid fuel to be atomized into small
droplets which are schematically illustrated and designated by the
numeral 29. Preferably, the design of the airblast main fuel
injector is such that the main fuel is entrained approximately
mid-stream between the air streams exiting the inner main swirler
and the outer main swirler.
[0035] The vane angles of the outer main swirler may be either
counter-swirl or coswirl with reference to the vane angles of the
inner main swirler. Likewise, the swirl angle of the inner pilot
airflow may be either counter-swirl or co-swirl with respect to the
swirl direction of the outer pilot airflow.
[0036] An air splitter 26 is located between the pilot swirler 18
and the inner main swirler 22. The air splitter 26 has an upstream
inlet end 28 and a downstream outlet end 30. The geometry of the
air splitter 26 includes a cylindrical upstream portion 32, a
radially inward tapered intermediate portion 34, and a further
radially tapered downstream portion 36. The downstream portion 36
terminates in the circular outlet 30 defined by trailing edge 37
and having a diameter indicated at 38.
[0037] The main airblast fuel injector 20 includes an atomizer
filmer 40 having an aft end 42. The pilot airblast atomizer 16 has
an atomizer filmer lip 44 having an aft end 46.
[0038] The outlet 30 of air splitter 26 is axially positioned
upstream of the aft end 42 of the main airblast fuel injector 20
and downstream of the aft end 46 of the pilot airblast fuel
injector 16.
[0039] The geometry of and location of the air splitter 26 is such
that the air splitter divides a pilot air stream exiting the pilot
swirler 18 and the outlet 38 of air splitter 26 from a main air
stream exiting the inner and outer main swirlers 18 and 22, whereby
a bifurcated recirculation zone is created between the pilot air
stream and the main air stream.
[0040] In FIG. 1, the outer edge of the pilot air stream is
schematically illustrated by arrows 48 and the inner edge of the
main air stream is schematically designated by arrows 50. The
bifurcated recirculation zone is generally indicated in the area at
52. It will be understood that the bifurcated recirculation zone 52
is a generally hollow conical aerodynamic structure which defines a
volume in which there is some axially rearward flow. This
bifurcated recirculation zone 52 separates the axially aft flow of
the pilot injector 16 as designated by arrows 48 from the axially
aft flow associated with main injector 20 as designated by the
arrows 50. It is noted that there is no central recirculation zone,
i.e. no reverse flow along the central axis 11 as would be found in
conventional fuel injectors.
[0041] The creation of the bifurcated recirculation zone which
aerodynamically isolates the pilot flame from the main flame
benefits the lean blowout stability of the fuel injector. The pilot
fuel stays nearer to the axial centerline and evaporates there,
thus providing a richer burning zone for the pilot flame than is
the case for the main flame. The fuel/air ratio for the pilot flame
remains significantly richer than that for the main flame over a
wide range of operating conditions. Most of the NOx formation
occurs in this richer pilot flame, and even that can be further
reduced by minimizing the proportion of total fuel going to the
pilot flame.
[0042] The selection of design parameters to create the bifurcated
recirculation zone 52 includes consideration of both the diameter
of the outlet 30 of air splitter 26, and the angle of swirl
imparted to the airflow flowing therethrough. As will be understood
by those skilled in the art, the greater the angle of swirl, the
greater the centrifugal effect, and thus increasing swirl angle
will tend to throw the pilot airflow further radially outward. The
tapered design of the air splitter, on the other hand, tends to
direct the pilot airflow mixture radially inward. The combination
of these two will determine whether the desired bifurcated
recirculation zone is created. Also, the amount of pilot airflow
through the fuel injector is controlled mainly by the diameter of
the outlet 30 and the angle of swirl through the outlet. If the
percentage of pilot airflow is too low (less than four percent),
the main airflow will dominate and produce a central recirculation
zone. If the outlet opening 30 is too small or if too great a swirl
angle is provided to the pilot air flow, then the pilot airflow
will be thrown too far radially outward so that it merges with the
main fuel air flow, which will in turn create a conventional
central recirculation rather than the desired bifurcated
recirculation. In general, for designs like those illustrated, the
swirl angle of the pilot air stream should be less than about 30
degrees.
[0043] To further describe the various flow regimes within the
combustor 14, the radial outer aft flow stream lines of the flow
from the main airblast injector 20 are designated by arrows 54.
Also, there are corner recirculation zones in the forward corners
of combustor 14 indicated by arrows 56.
[0044] The outer flow streamline of the fuel and air flowing from
the main airblast injector 20 and inner and outer main swirlers 22
and 24 is further affected by the presence of an aft flared wall 58
downstream of the main airblast fuel injector 20. The flare of aft
flared wall 58 ends at an angle 60 to the longitudinal axis 11
which is preferably in the range of from 45.degree. to
70.degree..
[0045] The outwardly flared outer wall 58 has a length 62 from the
aft end 42 of main airblast injector 20 to an aft end 64 of the
outer wall 58 sufficiently short to prevent autoignition of fuel
within the outer wall 58. The length 62 may also be described as
being sufficiently short to prevent fuel from the main fuel
injector 20 from wetting the flared outer wall 58. In a typical
embodiment of the invention, the length 62 will be on the order 0.2
to 0.3 inch.
[0046] The short residence time in the flared exit precludes
autoignition within the nozzle. Significant evaporation and mixing
does occur within the flared outlet, even for such a short
residence time. The partial pre-mixing improves fuel/air
distribution and reduces NOx. The extension combined with the
flared exit also results in a larger stronger bifurcated
recirculation zone 52.
[0047] As noted, the swirlers 18, 22 and 24 are schematically
illustrated in FIG. 1. In one embodiment, the swirlers each include
axial swirl vanes which are curved in a manner such as illustrated
in FIG. 5. The curved axial swirl vanes are provided to reduce the
Sauter Mean Diameter of the main fuel spray from the main airblast
injector 20 as compared to the Sauter Mean Diameter that would be
created when utilizing straight vanes.
[0048] The axial swirl vanes of the inner main swirler 22
preferably have a swirl vane angle in the range of from 25.degree.
to 60.degree.. The axial swirl vanes of the outer main swirler 24
preferably have a swirl vane angle in the range of from 45.degree.
to 65.degree..
[0049] For vanes having a constant vane angle, the ranges set forth
above apply literally. For helically curved vanes having a changing
vane angle, the ranges set forth above are to be interpreted as the
average swirl vane angle of the helical swirl vanes.
[0050] It will be appreciated that in a typical fuel injection
system 10, all three swirlers 18, 22 and 24 are fed from a common
air supply system, and the relative volumes of air which flow
through each of the swirlers are dependent upon the sizing and
geometry of the swirlers and their associated air passages, and the
fluid flow restriction to flow through those passages which is
provided by the swirlers and the associated geometry of the air
passages. In one preferred embodiment of the invention of FIG. 1,
swirlers are constructed such that from one to ten percent of total
air flow through the swirlers is through the inner pilot swirler
19, two to fifteen percent of total air flow is through the outer
pilot swirler 18, thirty to seventy percent of total air flow is
through the inner main swirler 22, and the balance of total air
flow is through the outer main swirler 24.
[0051] In FIG. 1, a pilot fuel supply line 66 is shown providing
fuel to the pilot fuel injector 16, and a main fuel supply line 68
is shown providing fuel to the main airblast injector 20.
[0052] FIG. 4 schematically illustrates a fuel supply control
system 70 utilized with the fuel injector like the fuel injector
system 10 of FIG. 1. The fuel supply control system 70 includes
control valves 72 and 74 disposed in the pilot and main fuel supply
lines 66 and 68, which supply lines lead from a fuel source 76. A
microprocessor based controller 78 sends control signals over
communication lines 80 and 82 to the control valves 72 and 74 to
control the flow of fuel to pilot fuel injector 16 and main fuel
injector 20 in response to various inputs to the controller and to
the pre-programmed instructions contained in the controller. In
general, during low power operation of the gas turbine associated
with the fuel injection system 10, fuel will be directed only to
the pilot fuel injector 16, and at higher power operating
conditions, fuel will be provided both to the pilot fuel injector
16 and the main airblast fuel injector 20.
[0053] The pilot fuel is entrained between the inner air flow from
the inner pilot swirler 19 and the outer air flow from the outer
pilot swirler 18, which air flows cause the annular film of pilot
fuel to be atomized into small droplets which are schematically
illustrated and designated by the numeral 45.
[0054] During low power operation of the fuel injector 10, fuel is
provided only to the pilot fuel injector 16 via the pilot fuel
supply line 66. The fuel is atomized into the small droplets 45.
The swirling motion of the air streams from inner and outer pilot
swirlers 19 and 18 cause the pilot fuel droplets 45 to be
centrifuged radially outwardly so that many of them are entrained
within the bifurcated recirculating flow zone 52. This causes the
pilot flame to be anchored within the bifurcated recirculation zone
52.
[0055] At higher power operation of the fuel injector 10, fuel is
also injected into the main airblast injector 20 via the main fuel
line 68. The main fuel droplets 29 are entrained within the air
flow between air stream lines 50 and 54 which represent the inner
and outer flow boundaries of the main air stream which flows
through inner and outer main swirlers 22 and 24.
[0056] The air flow which flows through the swirlers 19, 18, 22 and
24 preferably is divided in the proportions previously described.
As this air flow flows past the air splitter 26, the main air flow
50, 54 passing through main swirlers 22 and 24 is split away from
the pilot air flow which flows through swirlers 19 and 18 and which
must flow through the air splitter 26 and exit the outlet 30
thereof past trailing edge 37, thus creating the bifurcated
recirculation zone 52 which separates the main air flow from the
pilot air flow within the combustor 14.
THE SIMPLEX PILOTED AIRBLAST INJECTOR OF FIG. 2
[0057] FIG. 2 illustrates an alternative fuel injection assembly
generally designated by the numeral 100. The related components
such as the fuel supply lines, combustor and the various air flow
regimes are substantially identical to those of the embodiment of
FIG. 1 and are designated by the same numerals.
[0058] The primary modification of the fuel injector system 100 of
FIG. 2, as compared to the fuel injector system 10 of FIG. 1, is
that the fuel injector system 100 utilizes a pilot fuel injector
102 of the type commonly referred to as a simplex pressure atomizer
fuel injector. As will be understood by those skilled in the art,
the simplex pressure atomizer atomizes fuel based upon a pressure
differential placed across the fuel, rather than atomizing fuel
with a rapidly moving air stream as do the airblast atomizers
described above with regard to FIG. 1. Another modification in the
embodiment of FIG. 2 is that the outer main swirler 104 is of the
radial injection type.
[0059] The fuel injection system 100 includes a pilot swirler 106
and an inner main swirler 108. Fuel injection system 100 includes
an airblast type main fuel injector 110. An air splitter 112
separates the pilot air stream from the main air streams to again
create the bifurcated recirculation zone 52 previously
described.
[0060] The alternative embodiment of FIG. 2 utilizing the simplex
pressure atomizer pilot fuel injector has been generally found to
be more suitable for somewhat smaller gas turbine engines than is
the embodiment of FIG. 1, because the overall radial dimension of
the fuel injector can be minimized, which is important for
minimizing the hole diameter through the engine case for insertion
of the fuel injector. For example, the simplex piloted airblast
fuel injector of FIG. 2 may be utilized with smaller jet aircrafts
such as those utilized for regional jet service. However, there is
nothing precluding the use of the simplex piloted airblast fuel
injector of FIG. 2 being utilized in larger jet aircrafts, and
recent experimental tests have shown superior lean blowout was
obtained with the embodiment compared to the airblast piloted
version 10.
[0061] In the system of FIG. 2, the relative volumes of air flow
through the pilot swirler 106 and the inner and outer main swirlers
108 and 104 is somewhat different from the embodiment of FIG. 1. In
one version of the system 100, the swirlers and passage heights are
constructed such that from 5 to 20 percent of total swirler air
flow is through the pilot swirler 106, from 30 to 70 percent of
total air flow is through the inner main swirler 108 and the
balance of total air flow is through the outer main swirler
104.
[0062] When utilizing the simplex pressure atomizer pilot fuel
injector, the atomizer should be selected with a high spray angle
to inject spray into the bifurcated recirculation zone, but not so
high as to impinge onto the air splitter 26.
THE EMBODIMENT OF FIG. 3
[0063] FIG. 3 illustrates a third embodiment of the fuel injection
system of the present invention which is shown and generally
designated by the numeral 200. The fuel injector system 200 is a
simplex pressure atomizer piloted system similar to that of FIG. 2.
The system 200 includes a simplex pressure atomizer pilot fuel
injector 202, a pilot swirler 204, air splitter 206, an inner main
swirler 208, and airblast main fuel injector 210, an outer main
swirler 212, and a flared aft outlet wall 214. Differences as
compared to the fuel injector system 100 of FIG. 2 include a
slightly different geometry of the air splitter 206, and the use of
an axial vaned outer main swirler 212 rather than the radial
swirler of FIG. 2.
[0064] FIG. 3 also includes a schematic representation of the shape
and color of both a pilot flame 216 and a main flame 218 at full
power conditions and a 10/90 pilot/main fuel flow split. As
previously noted, the pilot flame 216 is anchored by and generally
contained within the bifurcated recirculation zone 52. The pilot
flame is generally yellow in its radial and axially aft extremities
and has a generally blue axially forward axial portion. The main
flame 218 is generally blue in color. In general, blue flames are
fuel-lean flames, and are a necessary, but not sufficient,
condition of low NOx emissions. This is because lean flames can
still have local stoichiometry (fuel-to-air ratio) that approaches
stoichiometric values and the hottest possible temperatures. The
ideal situation (for lowest NOx emissions) would be for the main
fuel to entirely prevaporize and premix with the main airflow
before reaction occurs, thus producing a uniform stoichiometry and
lowest possible flame temperatures. Although fuel/air uniformity is
desired, many factors can influence how closely uniform
stoichiometry is achieved in the real application, e.g.
circumferential fuel uniformity, vane wakes from the swirlers,
airfeed uniformity into the swirlers, etc.
[0065] Yellow flames are always indicative of fuel-rich flames, and
stoichiometric flames somewhere in the flowfield. This type of
flame is to be expected (and desired) for the pilot flame in order
to minimize the fuel-to-air ratio of the fuel injector at lean
blowout. Since only approximately 10 percent of the total fuelflow
enters the pilot at full power conditions, the amount of NOx
produced by the pilot flame is somewhat limited. If possible, the
amount of pilot fuel should be reduced at fill power conditions to
minimize NOx emissions; however, at low pilot fuelfilows, one must
be concerned about carbon deposition within the pilot fuel circuit.
For minimum full power NOx, pilot fuelfilow can be eliminated if
purging is performed.
[0066] As seen in FIG. 3, the air splitter 206 may have small
diameter holes 207, in the range of 0.010 to 0.060 inch diameter
placed around the tapered end portion, and spaced from 2 to 8 hole
diameters apart, to improve durability of the splitter 206 and to
eliminate carbon formation on the downstream face 209 of the
splitter.
[0067] Thus, it is seen that the methods and apparatus of the
present invention readily achieve the ends and advantages mentioned
as well as those inherent therein. While certain preferred
embodiments of the invention have been illustrated and described
for purposes of the present disclosure, numerous changes in parts
and steps may be made by those skilled in the art, which changes
are encompassed within the scope and spirit of the present
invention as defined by the appended claims.
* * * * *