Air Fuel Mixer for Gas Turbine Combustor

1. Field of the Invention

The present invention relates generally to an air fuel mixer for the combustor of a gas turbine engine and, in particular, to an air fuel mixer which uniformly mixes fuel and air so as to reduce NOx formed by the ignition of the fuel/air mixture and minimizes auto-ignition and flashback therein.

2. Description of Related Art

The present invention involves an air/fuel mixer for a gas turbine combustor which provides gaseous and/or liquid fuel to the mixing duct so as to be mixed with air to form a uniform air/fuel mixture. Other dual fuel mixers in the art include U.S. Pat. No. 5,351,477 to Joshi et al. and Ser. No. 08/304,341 now U.S. Pat. No. 5,511,375 to Joshi et al., both of which are owned by the assignee of the present invention. Each of these prior art air/fuel mixers, as well as the mixer of the present invention, includes a mixing duct, a set of inner and outer counter-rotating swirlers adjacent to the upstream end of the mixing duct, and a hub separating the inner and outer swirlers to allow independent rotation of the air flow therethrough.

It has been found, however, that these dual fuel mixer designs do not include features to adequately reduce fuel residence time in the mixing duct or otherwise prevent auto-ignition or flashback. Accordingly, a patent application entitled "Dual Fuel Mixer For Gas Turbine Combustor," having Ser. No. 08/581,817, now U.S. Pat. No. 5,680,766 was filed by the assignee of the present invention to address the problems of auto-ignition and flashback. The '817 patent application includes features which energize the boundary layer flow along the mixing duct wall and the centerbody. Nevertheless, it has been found at high pressure and temperature conditions, typical of aircraft engine operation, that liquid fuel can still be entrained into separate regions and remain there long enough to auto-ignite. This can occur through flow separation from the swirler vanes, as well as by flow separation which occurs downstream of the circular fuel jets and air-assist openings disclosed in the '817 application.

Another patent application entitled "Dual Fuel Mixer For Gas Turbine Combustor," having Ser. No. 08/581,818, was further filed by the assignee of the present invention. The mixer design of the '818 application includes features for improving liquid fuel atomization by impinging fuel jets. Once again, at high pressure and temperature conditions, the bulk residence time in the mixing duct has been found to be long enough in some instances to permit liquid fuel to mix with the air flow and auto-ignite. Thus, while improved liquid fuel atomization is desirable, fuel residence time in the mixing duct must be reduced to prevent auto-ignition and/or flashback from occurring at high power operating conditions.

In light of the foregoing, it would be desirable for an air fuel mixer to be developed which better addresses the problems of auto-ignition and flashback while maintaining an emphasis on uniformly mixing liquid and/or gaseous fuel with air so as to reduce NOx formed by the ignition of the air/fuel mixture.

In accordance with a first aspect of the present invention, an apparatus for premixing fuel and air prior to combustion in a gas turbine engine is disclosed as including: a linear mixing duct having a circular cross-section defined by a wall; a centerbody located along a central axis of the mixing duct and extending substantially the full length of the mixing duct, the centerbody having a plurality of orifices therein to inject fuel into the mixing duct; a fuel supply in flow communication with the centerbody orifices; an outer annular swirler located adjacent an upstream end of the mixing duct and including a plurality of circumferentially spaced vanes oriented so as to swirl air flowing therethrough in a first direction; an inner annular swirler located adjacent the mixing duct upstream end and including a plurality of circumferentially spaced vanes, the vanes having an outer radial portion with a leading edge and a trailing edge oriented so as to swirl air flowing therethrough in a second direction opposite the first swirl direction by the outer annular swirler vanes and an inner radial portion with a leading edge and a trailing edge oriented so as to provide a boundary layer of air substantially along the centerbody; and, a hub separating the inner and outer annular swirlers to permit independent rotation of an air stream therethrough. High pressure air is injected from a compressor into the mixing duct through the inner and outer annular swirlers and fuel is injected into the mixing duct so that the high pressure air and the fuel is uniformly mixed therein, whereby minimal formation of pollutants is produced when the fuel/air mixture is exhausted out the downstream end of the mixing duct into a combustor and ignited.

In accordance with a second aspect of the present invention, an apparatus for premixing fuel and air prior to combustion in a gas turbine engine is disclosed as including: a linear mixing duct having a circular cross-section defined by a wall; a fuel supply in flow communication with said mixing duct; an inner annular swirler located adjacent an upstream end of the mixing duct and including a plurality of circumferentially spaced vanes oriented so as to swirl air flowing therethrough in a first direction; an outer annular swirler located adjacent the mixing duct upstream end and including a plurality of circumferentially spaced vanes, the vanes having an outer radial portion with a leading edge and a trailing edge oriented so as to provide a boundary layer of air substantially along the mixing duct wall and an inner radial portion having a leading edge and a trailing edge oriented so as to swirl air flowing therethrough in a second direction opposite the first swirl direction by the inner annular swirler vanes; and, a hub separating the inner and outer annular swirlers to permit independent rotation of an air stream therethrough. High pressure air from a compressor is injected into the mixing duct through the inner and outer annular swirlers and fuel is injected into the mixing duct so that the high pressure air and the fuel is uniformly mixed therein, whereby minimal formation of pollutants is produced when the fuel/air mixture is exhausted out the downstream end of the mixing duct into a combustor and ignited.

In accordance with a third aspect of the present invention, an apparatus for premixing fuel and air prior to combustion in a gas turbine engine is disclosed as including: a linear mixing duct having a circular cross-section defined by a wall; a set of inner and outer annular counterrotating swirlers adjacent an upstream end of the mixing duct; a hub separating the inner and outer annular swirlers to allow independent rotation of an air stream through the swirlers; a centerbody located along a central axis of the mixing duct and extending substantially the full length of the mixing duct, the centerbody having a plurality of orifices therein located downstream of the inner and outer annular swirlers to inject fuel into the mixing duct, each of the orifices being oriented so as to provide an axial velocity component to the injection of the fuel; and, a fuel supply in flow communication with the orifices. High pressure air is injected from a compressor into the mixing duct through the inner and outer annular swirlers and fuel is uniformly mixed therein, whereby minimal formation of pollutants is produced when the fuel/air mixture is exhausted out the downstream end of the mixing duct into a combustor and ignited.

In accordance with a fourth aspect of the present invention, an apparatus for premixing fuel and air prior to combustion in a gas turbine engine is disclosed as including: a linear mixing duct having a circular cross-section defined by a wall; a set of inner and outer annular counter-rotating swiriers adjacent an upstream end of the mixing duct; a hub separating the inner and outer annular swirlers to allow independent rotation of an air stream through the swirlers; a centerbody located along a central axis of the mixing duct and extending substantially the full length of the mixing duct, the centerbody including a plurality of fuel posts therein located downstream of the inner and outer annular swirlers to inject fuel into the mixing duct, an air cavity in flow communication with an air supply, and an aerodynamically-shaped air slot located concentrically about each said fuel post in flow communication with said air cavity, wherein air flows through said aerodynamically-shaped slots to assist atomization and break up of fuel injected into said mixing duct through said posts while minimizing any flow separated region forming along said centerbody; and, a fuel supply in flow communication with the orifices. High pressure air is injected from a compressor into the mixing duct through the inner and outer annular swirlers and fuel is uniformly mixed therein, whereby minimal formation of pollutants is produced when the fuel/air mixture is exhausted out the downstream end of the mixing duct into a combustor and ignited.

While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed the same will be better understood from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a partial cross-sectional view through a single annular combustor structure including an air/fuel mixer in accordance with the present invention;

FIG. 2 is an enlarged, partial cross-sectional view of the air/fuel mixer and combustor dome portion depicted in FIG. 1;

FIG. 3 is an aft perspective view of the inner annular swirler for the air/fuel mixer depicted in FIGS. 1 and 2;

FIG. 4 is a perspective view of an inner annular swirler vane depicted in FIG. 3, wherein a plurality of separate cross-sections at different radial heights is shown;

FIG. 5A is a diagrammatic side view of the root portions for a pair of adjacent swirler vanes from the inner annular swirler of FIG. 3;

FIG. 5B is a diagrammatic side view of the tip portions for a pair of adjacent swirler vanes from the inner annular swirler of FIG. 3;

FIG. 5C is a graph schematically depicting the change in angles at the leading and trailing edges between the inner and outer radial portions of the inner annular swirler vanes shown in FIGS. 3-5B;

FIG. 6 is an aft perspective view of the outer annular swirler for the air/fuel mixer depicted in FIGS. 1 and 2;

FIG. 7 is a perspective view of an outer swirler vane depicted in FIG. 6, wherein a plurality of separate cross-sections at different radial heights is shown;

FIG. 8A is a diagrammatic side view of the tip portions for a pair of adjacent swirler vanes from the outer annular swirler of FIG. 6;

FIG. 8B is a diagrammatic side view of the root portions for a pair of adjacent swirler vanes from the outer annular swirler of FIG. 6;

FIG. 8C is a graph schematically depicting the change in angles at the leading and trailing edges between the inner and outer radial portions of the outer annular swirler vanes shown in FIGS. 6-8B;

FIG. 9 is an aft view of the inner and outer annular swirlers depicted in FIGS. 1-3 and 6;

FIG. 10 is a partial radial view of the air/fuel mixer taken along line 10--10 of FIG. 2 where an aerodynamic air-assist slot is shown;

FIG. 11 is a partial radial view of an alternative air-assist slot configuration as would be seen along line 10--10 of FIG. 2;

FIG. 12 is a partial cross-sectional view of the air/fuel mixer depicted in FIGS. 1 and 2 in which the centerbody has an alternative fuel post design; and

FIG. 13 is a partial radial view of the air/fuel mixer taken along line 13--13 in FIG. 12.

Referring now to the drawings in detail, wherein identical numerals indicate the same elements throughout the figures, FIG. 1 depicts a partial cross-sectional view of a continuous burning combustion apparatus 10 of the type suitable for use in a gas turbine engine and comprises a hollow body 12 which defines a combustion chamber 14 therein. Hollow body 12 is generally annular in form and is comprised of an outer liner 16, an inner liner 18, and a domed end or dome 20. The domed end 20 of hollow body 12 includes a swirl cup 22, having disposed therein a mixer 24 to promote the uniform mixing of fuel and air therein and the subsequent introduction of the fuel/air mixture into combustion chamber 14 with the minimal formation of pollutants caused by the ignition thereof.

It will be seen that air fuel mixer 24 includes an inner annular swirler 26 and an outer annular swirler 28 which are brazed or otherwise set in swirl cup 22. Inner and outer annular swirlers 26 and 28 are configured with vanes 32 and 34, respectively, so as to promote counter-rotation to an air flow provided thereto (see FIGS. 3, 6 and 9). A hub 30 is utilized to separate inner and outer annular swirlers 26 and 28, which allows them to be co-annular and still separately rotate air entering the upstream ends thereof.

As appreciated by a review of FIGS. 3-5C, inner annular swirler vanes 32 preferably have been modified from previous designs to include an outer radial portion 70 (i.e., toward the blade tip) which provides swirl to an air stream flowing therethrough in a direction opposite the swirl provided by outer annular swirler vanes 34, as well as an inner radial portion 72 (i.e., toward the blade root) which provides air substantially along the outer surface of a centerbody 42 (centerbody 42 is to be discussed in greater detail hereinafter). In order to provide the desired effects on the air stream entering mixing duct 40, inner radial portion 72 of vane 32 preferably has a leading edge 74 oriented at an angle .alpha.i.sub.inner approximately 0-30.degree. with respect to an axis 76 oriented radially thereto and a trailing edge 78 oriented at an angle .beta.i.sub.inner approximately -10.degree. to +10.degree. with respect to such axis 76 (see FIGS. 5A and 5C). Correspondingly, outer radial portion 70 of vane 32 preferably has a leading edge 80 oriented at an angle .alpha.i.sub.outer approximately +10.degree. to -10.degree. with respect to axis 76 and a trailing edge 82 oriented at an angle .beta.i.sub.outer approximately 50-60.degree. with respect to axis 76 (see FIGS. 5B and 5C). It will be appreciated that the respective leading and trailing edge angles for inner and outer portions 72 and 70 of inner annular swirler vanes 32 are best seen schematically in the graph of FIG. 5C.

It will be understood that inner radial portion 72 of vanes 32 is configured to provide a boundary layer 77 (see FIG. 2) of air along centerbody 42 in order to prevent flow separation from residing in such location. Since the flow area required for boundary layer 77 is minimal compared to the swirl area of mixing duct 40, inner radial portion 72 preferably will have a radial height hi.sub.inner only approximately 5-20% of the total radial height hi.sub.total of vane 32 (see FIG. 5C).

Further, it is desired that vanes 32 have a solidity in the range of 2.0-4.0 at inner radial portion 72 and in the range of 1.5-3.0 at outer radial portion 70. Solidity is defined as chord length l of vane 32 divided by circumferential spacing s between adjacent vanes. FIG. 5A depicts these parameters for inner radial portion 72 and FIG. 5B depicts such parameters for outer radial portion 70. It is also desired that vanes 32 have a thickness t.sub.i, as compared to chord length l.sub.i, so that wide angles of attack may be tolerated without flow separation from leading edges 74 and 80 thereof. In this regard, it has been found that a thickness-to-length ratio of approximately 0.18 or greater will be sufficient.

Although inner annular swirler vanes 32 preferably have a symmetrical airfoil shape when viewed in cross-section (see FIG. 4), it will be appreciated that such vanes 32 further include a transitional portion 84 located between outer radial portion 70 and inner radial portion 72. Transitional portion 84 has a leading edge 85 and a trailing edge 87 which functions to provide a gradual change between leading edges 74 and 80, as well as trailing edges 78 and 82, of inner and outer radial portions 72 and 70, respectively (see FIG. 5C). Transitional portion 84 also involves a twisting design (approximately 80.degree. to 100.degree. clockwise when forward looking aft) with respect to a longitudinal axis 46 of mixer 24 for effecting the gradual axial change between the leading edges and trailing edges of outer radial portion 70 and inner radial portion 72.

While typically not employed when fuel is supplied through passages therein, outer annular swirler vanes 34 also may be configured (in mirror image) like inner annular swirler vanes 32 described above and depicted in FIGS. 6-8C in order to provide a boundary layer 79 (see FIG. 2) of air along wall 41 of mixing duct 40. In such case, outer annular swirler vanes 34 will have an outer radial portion 86 to provide boundary layer 79 substantially along mixing duct wall 41 and an inner radial portion 88 for providing swirl to the air stream flowing therethrough (opposite the swirl direction provided by inner annular swirler vanes 32). Outer radial portion 86 will preferably have a leading edge 90 with an angle .alpha.o.sub.outer of approximately -10.degree. to +10.degree. with respect to an axis 92 (see FIGS. 8A and 8C) while inner radial portion 86 will preferably have a leading edge 94 with an angle .alpha.o.sub.inner approximately 0-30.degree. with respect to axis 92 (see FIGS. 8B and 8C). Although mixing duct 40 will typically be frusto-conical in shape, and mixing duct wall 41 likely will be oriented at an angle of approximately 10.degree. to 20.degree. to longitudinal axis 46 and thus to outer annular swirler 28 (as opposed to forward section 44 of centerbody 42 being substantially aligned or parallel to longitudinal axis 46 and inner annular swirler 26), trailing edge 96 for outer radial portion 86 will still preferably have an angle .beta.o.sub.outer approximately -10.degree. to 10.degree. with respect to axis 92 (see FIGS. 8A and 8C) while angle .beta.o.sub.inner for trailing edge 98 of inner radial portion 88 will be approximately -50.degree. to -60.degree. (see FIGS. 8B and 8C). It will be noted that the respective leading and trailing edge angles for inner and outer portions 88 and 86 of outer annular swirler vanes 34 are best seen schematically in the graph of FIG. 8C. The radial height ho.sub.outer of outer radial portion 86 will preferably be approximately 5-20% of the total radial height ho.sub.total of vane 34 since only a relatively small amount of flow area is required to provide boundary layer 79 along mixing duct wall 41 compared to the swirl area within mixing duct 40 (see FIG. 8C).

As with inner annular swirler vanes 32 described above, it is desired that outer annular swirler vanes 34 have a solidity in the range of 1.5-3.0 at outer radial portion 86 and 2.0-4.0 at inner radial portion 88. Further, vanes 34 will preferably have a thickness t.sub.o, when compared to the chord length l.sub.0, that will tolerate a wide angle of attack without flow separation from leading edges 90 and 94 thereof (approximately 0.18 or greater).

Outer annular swirler vanes 34 will also preferably have a symmetrical airfoil shape when viewed in cross-section (see FIG. 7), but will include a transitional portion 100 with a leading edge 101 and a trailing edge 102 located between outer and inner radial portions 86 and 88, respectively, to provide a gradual change between leading edges 90 and 94 and trailing edges 96 and 98 thereof (see FIG. 8C). Transitional portion 100 also includes a twisting design with respect to longitudinal axis 46 (approximately 80.degree. to 100.degree. counter-clockwise when viewed forward looking aft) for effecting the gradual change between the leading and trailing edges of outer radial portion 86 and inner radial portion 88.

A shroud 36 is provided which surrounds mixer 24 at the upstream end thereof with a fuel manifold 38 contained therein. Downstream of inner and outer annular swirlers 26 and 28 is an annular mixing duct 40 as defined by an annular wall 41. In at least one embodiment, fuel manifold 38 may be in flow communication with vanes 34 of outer swirler 28 where it is metered by an appropriate fuel supply and control mechanism depicted schematically by box 25 in FIG. 1. Vanes 34 of outer swirler 28 are then preferably of a hollow design, as shown and described in FIGS. 4a and 4b of U.S. Pat. No. 5,251,447, with internal cavities in flow communication with fuel manifold 38 and fuel passages in flow communication with the internal cavities. It will be seen in FIG. 1 that a purge air supply 27 is also preferably associated with manifold 38 so that air may be supplied to a purge manifold (not shown) and the internal cavities and vane passages when fuel is not injected therethrough. This purge air prevents hot air in combustion chamber 14 from recirculating into such fuel passages.

A centerbody 42 is provided in mixer 24 which, contrary to prior designs, preferably has a forward section 44 which is substantially parallel to longitudinal axis 46 through mixer 24 and an aft section 48 which converges substantially uniformly to a downstream tip 50 of centerbody 42. It will be noted that forward centerbody section 44 extends from an upstream end adjacent inner and outer annular swirlers 26 and 28 downstream to a point so that it has an axial length l.sub.1. Centerbody aft section 48 then extends from the downstream end of centerbody forward section 44 to tip 50 so as to have an axial length 1.sub.2. It will be appreciated that axial length 1.sub.2 of centerbody aft section 48 will generally be greater than axial length 1.sub.1, of centerbody forward section 44 since an angle of convergence .theta. for centerbody aft section 48 is preferably less than approximately 20.degree.. Otherwise, given the total axial length l.sub.total of mixing duct 40, the separation of flow between centerbody forward and aft sections 44 and 48, respectively, has a tendency to increase.

Centerbody 42 is preferably cast within mixer 24 and is sized so as to terminate immediately prior to a downstream end 52 of mixing duct 40 in order to address a distress problem at centerbody tip 50, which occurs at high pressures due to flame stabilization at this location. Centerbody 42 preferably includes a passage 54 through centerbody tip 50 in order to admit air of a relatively high axial velocity into combustion chamber 14 adjacent centerbody tip 50. This design decreases the local fuel/air ratio to help push the flame downstream of centerbody tip 50.

Centerbody 42 further includes a plurality of orifices 56 positioned preferably immediately upstream of centerbody aft section 48 from which fuel also can be injected into mixing duct 40. Centerbody fuel orifices 56 are spaced circumferentially about centerbody forward section 44 and while the number and size of such orifices 56 is dependent on the amount of fuel supplied thereto, the pressure of the fuel, and the number and particular design of swirlers 26 and 28, it has been found that 4 to 12 orifices work adequately. Fuel is supplied to centerbody orifices 56 by means of a fuel passage 58 within an upstream portion of centerbody 42. Fuel passage 58 is in turn in flow communication with a fuel supply and control mechanism 60, such as by means of a fuel nozzle entering the upstream portion of centerbody 42 or a fuel line 59 in flow communication with a separate fuel manifold in shroud 36 (shown in FIG. 2). It will be understood that if gaseous and liquid fuel are to be injected within mixer 24, the gas fuel will preferably be injected through passages in outer swirler 28 and the liquid fuel will be injected through centerbody fuel orifices 56. Further, fuel passage 58 is also associated with a purge air supply 62 so that air may be used to purge fuel from fuel passage 58 and orifices 56 when fuel is not injected into mixing duct 40 therethrough. Accordingly, it will be understood that the change of fuel types may be accomplished "on the fly" by ramping the amount of fuel injected through the outer swirler passages or centerbody orifices 56 up while correspondingly ramping down the fuel injected by the other.

More specifically, fuel orifices 56 are oriented with respect to mixing duct 40 (preferably 15-60.degree. with respect to a radial axis 64) so as to impart an velocity component in the axial direction (i.e., along longitudinal axis 46), thereby reducing the residence time for such fuel within mixing duct 40. This is accomplished via fuel passage 58 in centerbody 42 which is in flow communication with fuel supply 60 and preferably a plurality of circumferentially spaced posts 68 with a fuel hole 69 in flow communication with fuel passage 58. It will be appreciated that posts 68 may be configured to inject a fuel jet or a fan spray of fuel (see FIGS. 12 and 13) into mixing duct 40.

In order to assist in atomization and break up of fuel injected into mixing duct 40 through posts 68, an air cavity 71 is provided in centerbody 42. Air cavity 71 is in flow communication with purge air supply 62 and provides air to slots 73 located concentrically about each post 68 in addition to air passage 54. While air slots 73 may be circular in shape as shown in FIG. 11, it is preferred that they have an aerodynamic shape as seen in FIG. 10. This is because a small recirculation zone 75 (see FIG. 11) has a tendency to form downstream of slots 73, which is due to the shape of such slots. By changing the shape of slots 73 to be aerodynamic, the flow separation along centerbody aft section 48 (and thus the recirculation zone formed thereabout) can be minimized. In fact, it will be appreciated that slots 73 having an aerodynamic shape may be utilized regardless of the orientation of fuel posts 68 (may be substantially radial to axis 46) and the design of centerbody 42 (may be substantially converging throughout). Another way to positively affect this circumstance is to align slots 73 with the residual swirl component along centerbody 42 by angling slots 73 approximately 10-20.degree. with respect to longitudinal axis 46.

In operation, compressed air from a compressor (not shown) is injected into the upstream end of mixer 24 where it passes through inner and outer swirlers 26 and 28 and enters mixing duct 40. Fuel is injected into an air flow stream exiting swirlers 26 and 28 (which includes intense shear layers in the middle area of mixing duct 40 and boundary layers 77 and 79 along centerbody 42 and mixing duct wall 41, respectively) from passages within vanes 34 and /or fuel orifices 56 in centerbody 42. At the downstream end of mixing duct 40, the premixed fuel/air flow is supplied into a mixing region of combustor chamber 14 which is bounded by inner and outer liners 18 and 16. The premixed fuel/air flow is then mixed with recirculating hot burnt gases in combustion chamber 14. In light of the improvements by the inventive mixer described herein, however, where flow separations are minimized at high power operating conditions, the concerns of eliminating flashback and auto-ignition within mixing duct 40 are met.

Having shown and described the preferred embodiment of the present invention, further adaptations of the air fuel mixer can be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the invention. Accordingly, the manner in which fuel is provided to mixing duct 40 is not imperative in order to obtain the benefits of the inner and outer swirler vanes described herein.