US20160265781A1 - Air shield for a fuel injector of a combustor - Google Patents

Abstract

An air shield for an injector of a combustor includes a first section that extends axially from a first end to a second end, and a channel defined by the air shield. The channel includes at least one inlet proximate to the second end. The at least one inlet is configured to receive a channel airflow that is a portion of a surrounding airflow. The channel is configured to control a distribution of the channel airflow to the injector.

Description

BACKGROUND
• The field of the disclosure relates generally to a fuel injector for a combustor of a rotary machine, and more particularly to an air shield to control air flow to a fuel injector.
• At least some known combustors used with rotary machines, such as gas turbines, include at least one secondary fuel injector, often referred to as a “late lean injector,” located downstream from a primary fuel nozzle. At least some known late lean injectors mix a fuel supply with a supply of air, such as from a compressor discharge casing. However, such a supply of air may not be as steady or uniform as is desired under some operating conditions, and a potential exists for small quantities of fuel to escape through the late lean injector to the outside of the combustor.
BRIEF DESCRIPTION
• In one aspect, an air shield for an injector of a combustor is provided. The air shield includes a first section that extends axially from a first end to a second end, and a channel defined by the air shield. The channel includes at least one inlet proximate to the second end. The at least one inlet is configured to receive a channel airflow that is a portion of a surrounding airflow. The channel is configured to control a distribution of the channel airflow to the injector.
• In another aspect, a combustor for a gas turbine is provided. The combustor includes a liner that defines a primary combustion zone, and a sleeve that substantially circumscribes the liner. The combustor also includes a secondary combustion zone downstream from, and in flow communication with, the first combustion zone, and an injector coupled to the sleeve upstream from the secondary combustion zone. The injector includes at least one transfer tube in flow communication with the primary combustion zone. The combustor further includes an air shield. The air shield includes a first section that extends axially from a first end to a second end, and a channel defined by the air shield. The channel includes at least one inlet proximate to the second end. The at least one inlet is configured to receive a channel airflow that is a portion of a surrounding airflow of the combustor. The channel is configured to control a distribution of the channel airflow to the injector.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic view of an exemplary gas turbine;
• FIG. 2 is a schematic section view of an exemplary combustor that may be used with the exemplary gas turbine ofFIG. 1;
• FIG. 3 is a perspective view of a first exemplary embodiment of an air shield coupled to the exemplary combustor ofFIG. 2;
• FIG. 4 is a schematic section view of a first embodiment of an injector covered by the first exemplary air shield ofFIG. 3;
• FIG. 5 is another perspective view of the first exemplary air shield shown inFIGS. 3 and 4;
• FIG. 6 is a perspective view of a second exemplary embodiment of an air shield coupled to the combustor shown inFIG. 2 and covering a second embodiment of an injector;
• FIG. 7 is a schematic section view of the second exemplary air shield covering the second exemplary injector shown inFIG. 6; and
• FIG. 8 is a flow diagram of an exemplary method of assembling a combustor for a gas turbine, such as the exemplary gas turbine shown inFIG. 1.
DETAILED DESCRIPTION
• The exemplary methods and systems described herein overcome at least some of the disadvantages associated with known late lean injectors for combustors of rotary machines. The embodiments described herein include an air shield configured to cover a late lean injector. The air shield defines a channel that controls a distribution of an airflow to the late lean injector. For example, the air shield may be shaped to distribute the air flow in the channel to facilitate symmetric flow into an inlet of the late lean injector, facilitating improved fuel/air mixing and flow uniformity in the late lean injector. Moreover, the air shield may enclose at least a portion of a fuel supply line to the late lean injector.
• FIG. 1 is a schematic view of anexemplary gas turbine10 in which embodiments of the air shield of the current disclosure may be used. In the exemplary embodiment,gas turbine10 includes anintake section12, acompressor section14 coupled downstream fromintake section12, acombustor section16 coupled downstream fromcompressor section14, and aturbine section18 coupled downstream fromcombustor section16.
• Turbine section18 is coupled tocompressor section14 via arotor shaft17. It should be noted that, as used herein, the term “couple” is not limited to a direct mechanical, electrical, and/or communication connection between components, but may also include an indirect mechanical, electrical, and/or communication connection between multiple components. During operation ofgas turbine10,intake section12 channels air towardscompressor section14.Compressor section14 compresses the air to a higher pressure and temperature and discharges the compressed air towardscombustor section16. Incombustor section16, the compressed air is mixed with fuel and ignited to generate combustion gases that are channeled towardsturbine section18. More specifically,combustor section16 includes at least onecombustor20, in which a fuel, for example, natural gas and/or fuel oil, is injected into the air flow, and the fuel-air mixture is ignited to generate high temperature combustion gases that are channeled towardsturbine section18.
• Turbine section18 converts the thermal energy from the combustion gas stream to mechanical rotational energy, as the combustion gases impart rotational energy to at least onerotor blade19 coupled torotor shaft17 withinturbine section18.Rotor shaft17 may be coupled to a load (not shown) such as, but not limited to, an electrical generator and/or a mechanical drive application. The exhausted combustion gasesexit turbine section18.
• FIG. 2 is a schematic section view of an exemplary embodiment ofcombustor20 that may be used withgas turbine10. Although embodiments of the present disclosure will be described with reference tocombustor20, in alternative embodiments,combustor20 may be any suitable combustor that enables embodiments of the present disclosure to function as described herein. In the illustrated embodiment,combustor20 includes ahead end22. Aliner24 extends axially, with respect to alongitudinal axis40 ofcombustor20, fromhead end22 to anopposite aft end46.Liner24 is substantially circumscribed by asleeve26. In addition, aforward portion45 ofsleeve26 proximate tohead end22 is circumscribed by asleeve housing30.Liner24 also extends circumferentially aboutlongitudinal axis40 to generally define aprimary combustion zone23. Asecondary combustion zone33 extends downstream from, and is in flow communication with,primary combustion zone23.
• Head end22 includes a plurality ofprimary fuel nozzles21 that are configured to mix fuel and air in any suitable fashion for combustion withinprimary combustion zone23. The combustion of the mixture of fuel and air inprimary combustion zone23 produces combustion gases that flow intosecondary combustion zone33 and are channeled towards turbine section18 (shown inFIG. 1).
• Combustor20 also includes at least one secondary, or late lean,injector32. In the illustrated embodiment, each at least one latelean injector32 is coupled tosleeve26 upstream fromsecondary combustion zone33. In certain embodiments, the at least one latelean injector32 is a plurality of latelean injectors32 that are spaced circumferentially aroundliner24. Each at least one latelean injector32 receives fuel from a correspondingfuel supply line29. In an embodiment, eachfuel supply line29 extends generally axially along a radially outer surface ofsleeve housing30 and a radially outer surface ofsleeve26 to the corresponding latelean injector32. In alternative embodiments,fuel supply line29 may be at least partially defined within at least one ofsleeve housing30 andsleeve26. Additionally or alternatively,fuel supply line29 may be at least partially offset radially outwardly from at least one ofsleeve housing30 andsleeve26.
• Each at least one latelean injector32 is configured to mix fuel delivered fromfuel supply line29 and air drawn from anairflow44 that surroundscombustor20. In certain embodiments, surroundingairflow44 is a compressed air flow supplied from compressor section14 (shown inFIG. 1). Moreover, each at least one latelean injector32 includes at least onetransfer tube34 that is in flow communication withprimary combustion zone23. The at least one latelean injector32 is configured to inject the mixed fuel and air through the at least onetransfer tube34 intoprimary combustion zone23. The fuel injected by the at least one latelean injector32 is combusted insecondary combustion zone33.
• Each at least one latelean injector32 may be of any suitable design to enablecombustor20 to function as described herein. For example, but not by way of limitation, the at least one latelean injector32 may be at least one of a bell-mouth injector, a tube-in-tube injector, a swirl injector, a rich catalytic injector, and a shower-head type multi-tube injector.
• FIG. 3 is a perspective view of a first exemplary embodiment of anair shield100 coupled tocombustor20. It should be understood that the particular illustrated embodiment ofcombustor20 is used for purposes of example only, and thatair shield100 may be used with any suitable alternative combustor. In the illustrated embodiment, the at least one latelean injector32 is a plurality of circumferentially spaced latelean injectors32, and a corresponding plurality of circumferentially spaced air shields100 is coupled tocombustor20 such that eachair shield100 covers a corresponding latelean injector32. In the illustrated embodiment, eachair shield100 is formed from a partially transparent plastic material. In alternative embodiments,air shield100 may be formed from any suitable material.
• Eachair shield100 includes afirst section102 that extends axially from afirst end101, configured to be disposed proximate the corresponding latelean injector32, to asecond end103, configured to be disposedproximate sleeve housing30. In certain embodiments, eachair shield100 extends circumferentially alongcombustor20 for a maximum distance of about one times to about three times a diameter of the corresponding latelean injector32. In a particular embodiment, eachair shield100 extends circumferentially alongcombustor20 for a maximum distance of about two times the diameter of the corresponding latelean injector32. In alternative embodiments, eachair shield100 extends circumferentially alongcombustor20 for a maximum distance of greater than about three times the diameter of the corresponding latelean injector32.
• Air shield100 defines achannel112 whenair shield100 is coupled tocombustor20.Channel112 is configured to receive achannel airflow144 that is a portion of surroundingairflow44, and to distributechannel airflow144 to latelean injector32. Moreover,air shield100 defineschannel112 to control a distribution ofchannel airflow144 to latelean injector32 in a desired fashion.
• For example,channel112 is configured to receive a substantial portion ofchannel airflow144 from surroundingairflow44 proximatesecond end103, rather than from surroundingairflow44 proximate tofirst end101. In certain embodiments, surroundingairflow44 proximate tosecond end103 is relatively less dynamic as compared to surroundingairflow44 proximate tofirst end101. Thus, eachchannel112 is configured to distribute a relativelyuniform airflow144 to each of the corresponding plurality of circumferentially spaced latelean injectors32.
• In the illustrated embodiment,first section102 is coupled tosleeve26, andair shield100 also includes asecond section104 coupled tosleeve housing30.Second section104 is in flow communication withfirst section102. In alternative embodiments,second section104 may be omitted. Also in the illustrated embodiment,first section102 includes aneck106 proximate tosecond end103 and a pair ofshoulder regions108 that extend fromneck106.First section102 also includes anannular dome region110 proximatefirst end101, such thatannular dome region110 is configured to be disposed radially outwardly from latelean injector32.Neck106, pair ofshoulder regions108, andannular dome region110 ofair shield100 further definechannel112 to control the distribution ofchannel airflow144 to latelean injector32 in a desired fashion, as will be described with reference toFIGS. 4 and 5.
• FIG. 4 is a schematic section view of a first particular embodiment of latelean injector32 covered byair shield100, as shown inFIG. 3. In the illustrated embodiment, latelean injector32 includes a bell-mouth air inlet114, in addition to acentral spindle inlet146.Channel airflow144 approaches bell-mouth air inlet114 withinchannel112 fromsecond end103. If an effect ofannular dome region110 is disregarded, a disproportionate portion ofchannel airflow144 would tend to flow over a side of arim118 of bell-mouth air inlet114 that is closest tosecond end103, which would tend to produce an asymmetric air flow through latelean injector32. Such asymmetric air flow would tend to result in less effective mixing of fuel and air in latelean injector32.
• As can be seen inFIG. 4,annular dome region110 ofair shield100 further defineschannel112 to control distribution ofchannel airflow144 to latelean injector32. More specifically,annular dome region110 is substantially centered over latelean injector32, andannular dome region110 is sized such that apeak116 ofannular dome region110 is positioned overrim118 of bell-mouth air inlet114. Thus, a portion ofchannel112 defined byannular dome region110 is configured to distributechannel airflow144 into latelean injector32 substantially evenly around a perimeter of bell-mouth air inlet114 as compared to the late lean injector with noair shield100, producing a more symmetric airflow through latelean injector32. It should be understood thatair shield100 may be used with any suitable latelean injector32, and is not limited to use with the particular embodiment of latelean injector32 shown inFIG. 4. For example, althoughpeak116 and the perimeter ofrim118 are generally circular in the illustrated embodiment, it should be understood thatpeak116 and the perimeter ofrim118 may have other suitable shapes. For another example, although latelean injector32 includesspindle inlet146 in the illustrated embodiment, certain other embodiments of latelean injector32 do not includespindle inlet146.
• FIG. 5 is another perspective view ofair shield100 in which at least one inlet120 to channel112 is illustrated. The at least one inlet120 is configured to receive a portion of surroundingairflow44 as channel airflow144 (shown inFIG. 3). Each at least one inlet120 is located generally proximatesecond end103. In the illustrated embodiment, each at least one inlet120 is located on at least one ofneck106 andshoulder regions108.
• In the illustrated embodiment, the at least one inlet120 includes side windows122. Each side window122 is defined through a side wall offirst section102 ofair shield100 along acorresponding shoulder region108. The at least one inlet120 also may include at least one top window124 defined through a top wall ofneck106. Additionally, the at least one inlet120 may include a plurality of apertures126 defined through a top wall of eachshoulder region108, and may include a plurality of apertures128 defined through the top wall ofneck106. The at least one inlet120 further may include an aperture or window130 defined through a wall ofsecond section104. For example, in the illustrated embodiment, aperture130 is defined around an opening through whichfuel supply line29 extends intochannel112. Additionally or alternatively, the at least one inlet120 may include any other suitable window, aperture, channel, or other type of inlet intochannel112.
• It should be understood that any type or position of inlet120 may be used in combination with any other type or position of inlet120 without departing from the scope of this disclosure. For example, in a particular embodiment, the at least one inlet120 includes side windows122 and top window124, and does not include apertures126,128, and130. For another example, in an alternative embodiment, the at least one inlet120 includes side windows122 and apertures126 and128, and does not include top window124 and aperture130. In general, a type and number of inlets120 may be chosen to further control a distribution ofchannel airflow144 to late lean injector32 (shown inFIG. 3) in a desired fashion. For example, apertures126 may be used to inputadditional channel airflow144 nearshoulder regions108 to compensate for a tendency ofchannel airflow144 to separate nearshoulder regions108. Similarly, at least one of top window124 and apertures128 may be used to pushchannel airflow144 closer to the side walls ofair shield100. For another example, a size of side windows122 relative to a size of at least one of top window124 and apertures128 can be chosen to reduce non-axial components ofchannel airflow144 as it approaches latelean injector32.
• In certain embodiments,air shield100 is configured to capture any fuel that escapes from latelean injector32. More specifically,channel112 is configured such thatchannel airflow144 develops a velocity towards latelean injector32 sufficient to sweep the escaped fuel back through the latelean injector32 into theprimary combustion zone23. The velocity ofchannel airflow144 prevents the escaped fuel from exitingchannel112 through the at least one inlet120.
• In the illustrated embodiment,first section102 includes atelescoping portion134 atsecond end103 that is configured to extend at least partially oversecond section104. More specifically,telescoping portion134 is configured for sliding movement oversecond section104 in a direction generally parallel to longitudinal axis40 (shown inFIG. 2), such thatair shield100 accommodates relative motion parallel tolongitudinal axis40 betweensleeve26 andsleeve housing30. For example, in certain embodiments, upon initiation of operation ofgas turbine10,sleeve26 expands axially towardshead end22 relative tosleeve housing30. Becausefirst section102 is coupled tosleeve26,first section102 moves towardssecond section104.Telescoping portion134 slides oversecond section104 towardshead end22 to maintain an integrity ofchannel112. Upon cessation of operation ofgas turbine10,sleeve26 retracts axially fromsleeve housing30, andtelescoping portion134 slides oversecond section104 away fromhead end22 to maintain an integrity ofchannel112. In alternative embodiments,first section102 does not includetelescoping portion134.
• With reference toFIGS. 3 and 5, in the illustrated embodiment,air shield100 is configured to enclose at least a portion of the correspondingfuel supply line29. In certain embodiments,air shield100 is configured to protectfuel supply line29 from damage during at least one of shipping, installation, and maintenance of the combustor. For example,air shield100 may have a suitable strength and stiffness to absorb accidental impacts that otherwise potentially could damagefuel supply line29. In alternative embodiments,air shield100 is not configured to enclose at least a portion of the correspondingfuel supply line29.
• FIG. 6 is a perspective view, andFIG. 7 is a schematic section view, of a second exemplary embodiment of anair shield200 coupled tocombustor20 and covering a second particular embodiment of latelean injector32. As described above, the at least one latelean injector32 may be a plurality of circumferentially spaced latelean injectors32, and a corresponding plurality of circumferentially spaced air shields200 may be coupled tocombustor20 such that eachair shield200 covers a corresponding latelean injector32. Eachair shield200 generally has an axial and circumferential extent similar to that described forair shield100.
• With reference toFIGS. 6 and 7,air shield200 is substantially similar toair shield100 in several respects, and similar features will be given the same reference numbers. For example,air shield200 extends fromfirst end101 tosecond end103, defineschannel112 configured to receivechannel airflow144 that is a portion of surroundingairflow44, and is configured to receive a substantial portion ofchannel airflow144 from surroundingairflow44 proximatesecond end103, rather than from proximatefirst end101. In addition,air shield200 includesfirst section102 coupled tosleeve26 and, optionally,second section104 coupled tosleeve housing30. In the illustrated embodiment,first section102 includesneck106 proximate tosecond end103, pair ofshoulder regions108 that extend fromneck106, at least one inlet120, and, optionally,telescoping portion134. In the illustrated embodiment, the at least one inlet120 includes side windows122 and top window124, although in alternative embodiments, any suitable combination of inlets120 may be used, as described above with regard toair shield100. Similarly, in the illustrated embodiment,air shield200 is configured to enclose at least a portion of the correspondingfuel supply line29, although in alternative embodiments,air shield200 is not configured to enclose at least a portion of the correspondingfuel supply line29.
• As described above, latelean injector32 is configured to inject mixed fuel and air through the at least onetransfer tube34 into primary combustion zone23 (shown inFIG. 2). In the illustrated embodiment ofFIGS. 6 and 7, latelean injector32 includes aswirler inlet214, in addition to aspindle inlet246.Swirler inlet214 includes a plurality ofvanes216 circumferentially spaced about acentral axis220 ofswirler inlet214.Central axis220 is defined to be normal to a surface ofsleeve26 whenair shield200 is coupled tocombustor20. Eachvane216 is oriented at avane angle226 with respect to aradial line222 extending fromcentral axis220 through the vane, such thatswirler inlet214 is configured to impart a swirl aboutcentral axis220 to air received fromchannel airflow144. In certain embodiments, the swirl imparted byswirler inlet214 improves a mixing of fuel and air by latelean injector32.
• In the illustrated embodiment,air shield200 includes ascroll region232 proximatefirst end101, such thatscroll region232 is configured to be disposed radially outwardly from latelean injector32.Air shield200 also includes atransition region230 disposed betweenscroll region232 andsecond end103. Scrollregion232 is defined by aradius234 measured from acentral point236 that is configured to lie oncentral axis220 whenair shield200 is coupled tocombustor20.Radius234 generally decreases along an arcuate path aboutswirler inlet214, as illustrated inFIG. 6 at several representative locations. In the illustrated embodiment,radius234 generally decreases from a maximum value proximate a location at which scrollregion232 intersectstransition region230. Scrollregion232,transition region230,neck106, and pair ofshoulder regions108 are in flow communication and definechannel112 to control the distribution ofchannel airflow144 to latelean injector32 in a desired fashion, as will be described herein.
• Channel airflow144 approachesswirler inlet214 withinchannel112 fromsecond end103. If an effect ofscroll region232 is disregarded, a disproportionate portion ofchannel airflow144 would tend to impinge certain ones of the plurality ofvanes216 at a range of angles that vary significantly with respect tovane angle226, which would tend to produce a significant variation in inlet velocities around a perimeter ofswirler inlet214 and produce an asymmetric air flow through latelean injector32. Such asymmetric air flow would tend to result in less effective mixing of fuel and air in latelean injector32.
• As can be seen inFIG. 6, the general decrease ofradius234 along the arcuate path aboutswirler inlet214 tends to impart a pre-swirl to channelairflow144. Thus, scrollregion232 is shaped to decrease a variation in the angle at which airflow144 impinges eachvane216. Moreover, in the illustrated embodiment,transition region230 is shaped to transitionchannel airflow144 from a generally axial velocity proximatesecond end103 to a velocity that approaches latelean injector32 generally tangential toswirler inlet214. Thus,transition region230 cooperates withscroll region232 to decrease the variation in the angle at which airflow144 impinges eachvane216.
• In the illustrated embodiment,vanes216 have avane angle226 oriented such thatswirler inlet214 is configured to impart a counterclockwise swirl aboutcentral axis220, andradius234 decreases along a correspondingly counterclockwise path aboutswirler inlet214 to impart a correspondingly counterclockwise pre-swirl tochannel airflow144. Moreover,transition region230 is oriented to facilitate transitioningchannel airflow144 to a counterclockwise tangential velocity. In an alternative embodiment (not shown),vanes216 have an oppositely orientedvane angle226 such thatswirler inlet214 is configured to impart a clockwise swirl aboutcentral axis220,radius234 decreases along a correspondingly clockwise path aboutswirler inlet214 to impart a correspondingly clockwise pre-swirl tochannel airflow144, andtransition region230 is oriented to facilitate transitioningchannel airflow144 to a clockwise tangential velocity.
• Thus, a portion ofchannel112 defined byscroll region232, and optionally also bytransition region230, is configured to distributechannel airflow144 into latelean injector32 substantially evenly around a perimeter ofswirler inlet214 as compared to the late lean injector with noair shield200, producing a more symmetric airflow through latelean injector32. It should be understood thatair shield200 may be used with any suitable latelean injector32, and is not limited to use with the particular embodiment of latelean injector32 shown inFIGS. 6 and 7.
• Anexemplary method800 of assembling a combustor, such ascombustor20, for a gas turbine, such asgas turbine10, is illustrated inFIG. 8. With reference also toFIGS. 1-7,method800 includes disposing802 a first end, such asfirst end101, of an air shield, such asair shield100 orair shield200, proximate an injector, such as latelean injector32.Method800 also includes disposing804 a second end, such assecond end103, of the air shield upstream of the first end.Method800 further includescoupling806 the air shield to a sleeve, such assleeve26, such that a channel, such aschannel112, is defined. The channel is configured to control a distribution of a channel airflow, such aschannel airflow144, to the injector. The channel has at least one inlet, such as at least one inlet120, proximate to the second end. The at least one inlet is configured to receive a portion of a surrounding airflow, such as surroundingairflow44, of the combustor as the channel airflow.
• In certain embodiments, coupling806 the air shield to a sleeve further includescoupling808 the air shield such that the channel is further configured to distribute the channel airflow substantially evenly around a perimeter of an inlet, such as bell-mouth air inlet114 orswirler inlet214, of the injector. The air shield may have an annular dome region, such asannular dome region110, proximate the first end, andmethod800 may further include positioning810 a peak, such aspeak116, of the annular dome region over a rim, such asrim118, of the inlet of the injector. Alternatively or additionally, the air shield may include a scroll region, such asscroll region232, proximate the first end, andcoupling806 the air shield to a sleeve may further include coupling812 the air shield such that a radius of the scroll region generally decreases along an arcuate path about the inlet of the injector. In certain embodiments, coupling806 the air shield to a sleeve further includes enclosing814 at least a portion of a fuel supply line to the injector, such asfuel supply line29, within the air shield.
• Exemplary embodiments of an air shield configured to cover a late lean injector of a combustor are described above in detail. The embodiments provide an advantage in controlling a distribution of an airflow to the late lean injector. For example, the air shield may be shaped to facilitate symmetric flow into an inlet of the late lean injector, facilitating improved fuel/air mixing and flow uniformity in the late lean injector. The embodiments also provide an advantage in that the air shield may enclose at least a portion of a fuel supply line to facilitate protecting the fuel supply line during, for example, shipping, installation, and maintenance of the combustor.
• The methods and systems described herein are not limited to the specific embodiments described herein. For example, components of each system and/or steps of each method may be used and/or practiced independently and separately from other components and/or steps described herein. In addition, each component and/or step may also be used and/or practiced with other assemblies and methods.
• While the disclosure has been described in terms of various specific embodiments, those skilled in the art will recognize that the disclosure can be practiced with modification within the spirit and scope of the claims. Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience only. Moreover, references to “one embodiment” in the above description are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. In accordance with the principles of the disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

Claims (20)

What is claimed is:

1. An air shield for an injector of a combustor, said air shield comprising:

a first section that extends axially from a first end to a second end; and

a channel defined by said air shield, said channel comprises at least one inlet proximate to said second end, said at least one inlet is configured to receive a channel airflow that is a portion of a surrounding airflow, said channel is configured to control a distribution of the channel airflow to the injector.

2. The air shield ofclaim 1, wherein said first section includes a neck proximate to said second end and a pair of shoulder regions that extend from said neck, said at least one inlet is located on at least one of said neck and said shoulder regions.

3. The air shield ofclaim 2, wherein said at least one inlet includes a respective aperture near each of said shoulder regions.

4. The air shield ofclaim 1, wherein said channel is further configured to distribute the channel airflow substantially evenly around a perimeter of an inlet of the injector.

5. The air shield ofclaim 4, wherein said air shield further comprises an annular dome region proximate said first end, said annular dome region comprises a peak configured to be positioned over a rim of the inlet of the injector.

6. The air shield ofclaim 5, wherein said air shield further comprises a scroll region proximate said first end, said scroll region is defined by a radius that generally decreases along an arcuate path configured to lie about the inlet of the injector.

7. The air shield ofclaim 6, wherein said air shield further comprises a transition region shaped to transition the channel airflow to a velocity that approaches the injector generally tangential to the inlet of the injector.

8. The air shield ofclaim 1, wherein said air shield is configured to enclose at least a portion of a fuel supply line to the injector.

9. The air shield ofclaim 1, wherein said air shield further comprises a second section, said first section comprises a telescoping portion at said second end that is configured to extend at least partially over said second section.

10. The air shield ofclaim 1, wherein said air shield is configured to extend circumferentially along the combustor for a maximum distance of about one times to about three times a diameter of the injector.

11. A combustor for a gas turbine, said combustor comprising:

a liner that defines a primary combustion zone;

a sleeve that substantially circumscribes said liner;

a secondary combustion zone downstream from, and in flow communication with, said first combustion zone;

an injector coupled to said sleeve upstream from said secondary combustion zone, said injector comprises at least one transfer tube in flow communication with said primary combustion zone; and

an air shield comprising:

a first section that extends axially from a first end to a second end; and

a channel defined by said air shield, said channel comprises at least one inlet proximate to said second end, said at least one inlet is configured to receive a channel airflow that is a portion of a surrounding airflow of said combustor, said channel is configured to control a distribution of the channel airflow to said injector.

12. The combustor ofclaim 11, wherein said first section includes a neck proximate to said second end and a pair of shoulder regions that extend from said neck, said at least one inlet is located on at least one of said neck and said shoulder regions.

13. The combustor ofclaim 12, wherein said at least one inlet includes a respective aperture near each of said shoulder regions.

14. The combustor ofclaim 11, wherein said channel is further configured to distribute the channel airflow substantially evenly around a perimeter of an inlet of said injector.

15. The combustor ofclaim 14, wherein said air shield further comprises an annular dome region proximate said first end, said annular dome region comprises a peak positioned over a rim of said inlet of said injector.

16. The combustor ofclaim 14, wherein said air shield further comprises a scroll region proximate said first end, said scroll region is defined by a radius that generally decreases along an arcuate path about said inlet of said injector.

17. The combustor ofclaim 16, wherein said air shield further comprises a transition region shaped to transition the channel airflow to a velocity that approaches said injector generally tangential to said inlet of said injector.

18. The combustor ofclaim 11, wherein said air shield is configured to enclose at least a portion of a fuel supply line to said injector.

19. The combustor ofclaim 11, wherein said air shield further comprises a second section, said first section comprises a telescoping portion at said second end that is configured to extend at least partially over said second section.

20. The combustor ofclaim 11, wherein said air shield extends circumferentially along said combustor for a maximum distance of about one times to about three times a diameter of said injector.

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