Patent Publication Number: US-2023135396-A1

Title: Multitube pilot injector having a split airflow for a gas turbine engine

Description:
BACKGROUND 
     The present disclosure relates generally to gas turbine engines and, more particularly, to injectors used to inject a mixture of compressed air and fuel into a combustor in the gas turbine engines. 
     Gas turbine engines are used to generate mechanical energy by combusting a fuel/air mixture within a combustor. Fuel and compressed air are delivered to the combustor through one or more fuel injectors. In one type of gas turbine engine disclosed in U.S. Pat. No. 9,752,781, main fuel injectors are located radially outward of a combustion liner in a combustor and are spread in an annular array about the combustion liner. A hemispheric combustor dome assembly is positioned at an inlet end of the combustion liner and reverses the direction of flow of a fuel/air mixture from the main fuel injectors. The hemispheric combustor dome assembly then directs the fuel/air mixture flow into an inlet end of the combustion liner through a series of passageways. A pilot fuel nozzle is positioned along a center axis of the combustion liner is used to ignite, support and maintain one or more stages of the fuel/air mixture from the main fuel injectors within the combustion liner. While the gas turbine engine disclosed in U.S. Pat. No. 9,752,781 is advantageous in that it allows for greater control of the velocity of the fuel/air mixture entering the combustion liner, which can lead to greater control over the power generated and reductions in the production of undesired oxides of nitrogen and carbon monoxide, further improvements in the control of the flue/air mixture would be desirable. 
     BRIEF DESCRIPTION 
     This brief description is provided to introduce a selection of concepts in a simplified form that are further described in the detailed description below. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present disclosure will be apparent from the following detailed description of the embodiments and the accompanying figures. 
     Aspects of the disclosure relate to a gas turbine engine including a combustor having one or more fuel injectors. More particularly, aspects are directed to a multitube pilot injector that reduces thermal stresses and combustor emissions while improving combustor performance and efficiency. 
     In one aspect, the disclosure is directed to an injector for a combustor of a gas turbine engine comprising: a plurality of air/fuel mixing tubes including a radially outer subset of air/fuel mixing tubes and a radially inner subset of air/fuel mixing tubes; a first fuel manifold in fluid communication with the radially outer subset of air/fuel mixing tubes; and a second fuel manifold in fluid communication with the radially inner subset of air/fuel mixing tubes. Each of the air/fuel mixing tubes of the radially outer subset of air/fuel mixing tubes includes a substantially quadrilateral cross-sectional profile, and each of the air/fuel mixing tubes of the radially inner subset of air/fuel mixing tubes includes a substantially circular cross-sectional profile. 
     In another aspect, the disclosure is directed to a combustor for a gas turbine engine and comprising: a generally cylindrical flow sleeve; a generally cylindrical combustion liner positioned radially inward from the flow sleeve and defining a combustion zone; a first injector that is generally annularly shaped and surrounds the combustion liner and is positioned at a downstream end of the flow sleeve, and a second injector that is positioned radially inward of the combustion liner at an inlet end of the combustion zone to receive the compressed air from the radially outward openings in the first injector following the radially outward path. The first injector comprises: radially outward openings to allow passage of compressed air following a radially outward path; and radially inward openings to allow passage of compressed air following a radially inward path. The second injector comprises: a plurality of air/fuel mixing tubes including a radially outer subset of air/fuel mixing tubes and a radially inner subset of air/fuel mixing tubes; a first fuel manifold in fluid communication with the radially outer subset of air/fuel mixing tubes; and a second fuel manifold in fluid communication with the radially inner subset of air/fuel mixing tubes. Each of the air/fuel mixing tubes of the radially outer subset of air/fuel mixing tubes includes a substantially quadrilateral cross-sectional profile, and each of the air/fuel mixing tubes of the radially inner subset of air/fuel mixing tubes includes a substantially circular cross-sectional profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present technology is described in detail below with reference to the attached drawing figures, in which like numerals represent the same components, and wherein: 
         FIGS.  1 - 2    are cross-sectional views of a gas turbine combustor according to aspects of the disclosure; 
         FIG.  3    is a perspective view of a first embodiment of an injector for a gas turbine combustor, such as the combustor shown in  FIGS.  1 - 2   , according to aspects of the disclosure; 
         FIG.  4    is a cross-sectional plan view of the injector shown in  FIG.  3   ; 
         FIG.  5    is a close-up view of a portion of the cross-sectional view of the injector shown in  FIG.  4   ; 
         FIG.  6    is a close-up view of a perspective and cross-sectional view of the injector shown in  FIGS.  3 - 5   ; 
         FIG.  7    is an alternative close-up view of a perspective and cross-sectional view of the injector shown in  FIGS.  3 - 5   ; 
         FIGS.  8  and  9    are close-up views of a perspective and cross-sectional view of the injector shown in  FIGS.  3 - 7   ; 
         FIG.  10    is a perspective view of a second embodiment of an injector for a gas turbine combustor, such as the combustor shown in  FIGS.  1 - 2   , according to aspects of the disclosure; 
         FIG.  11    is a perspective view of a third embodiment of an injector for a gas turbine combustor, such as the combustor shown in  FIGS.  1 - 2   , according to aspects of the disclosure; 
         FIG.  12    is a perspective, cross-sectional view of the injector shown in  FIG.  11   ; 
         FIG.  13    is a partial side view of the injector shown in  FIGS.  11 - 12   ; 
         FIG.  14 A  is a close-up view of a perspective and cross-sectional view of a modified upstream portion of the injector shown in  FIGS.  11 - 13   ; 
         FIG.  14 B  is an airflow diagram show in the effect of a flow modifier shown in air/fuel mixing tubes of  FIG.  14 A ; 
         FIG.  15    is a close-up view of a perspective view of a fourth embodiment of an injector for a gas turbine combustor, such as the combustor shown in  FIGS.  1 - 2   , according to aspects of the disclosure; 
         FIG.  16    shows a perspective view of a fifth embodiment of an injector for a gas turbine combustor, such as the combustor shown in  FIGS.  1 - 2   , according to aspects of the disclosure; 
         FIGS.  17 A-D  show schematic plan views of additional embodiments of fuel manifold assemblies that can be used with injectors of the disclosure, such as the injectors shown in  FIGS.  1 - 16   ; and 
         FIGS.  18 A-C  show schematic, cross-sectional views of various air/fuel mixing tubes according to aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings in greater detail, and initially to  FIG.  1   , a combustor of a gas turbine engine according to aspects of the disclosure is designated generally by the numeral  100 . The combustor  100  generally includes a first, or radially outward, injector  102 , a second, or radially inward, injector  104 , a generally cylindrical flow sleeve  106 , and a generally cylindrical combustion liner  108  that is positioned radially inward from the flow sleeve  106  and generally defines a combustion zone  110 . The first injector  102  is generally annularly shaped and surrounds the combustion liner  108  and is positioned at a downstream end of the flow sleeve  106 . The second injector  104  is positioned radially inward of the combustion liner  108  at an inlet end of the combustion zone  110 . 
     As best understood with reference to  FIG.  2   , during use of the combustor  100  compressed air is presented to both the first and second injectors  102 ,  104 , where it mixes with a fuel source and then is ignited, supporting a flame within the combustion chamber  110 . Compressed air following a radially outer path  112  along a radially outer surface of the flow sleeve  106  passes through radially outward vanes and/or openings provided in the first injector  102  without being mixed with fuel therein, instead continuing downstream to a portion of the combustor  100  where the air turns and passes through the second injector  104 . Here, the compressed air mixes with a fuel source and is ignited to support a flame near a central axis of the combustor  100 . Compressed air following a radially inner path  114  between the flow sleeve  106  and the combustion liner  108  passes through different (i.e., radially inward) vanes and/or openings provided in the first injector  102 , where it is mixed with a fuel source. The fuel/air mixture then travels to the dome plate  130  or a similar structure where it turns substantially 180 degrees, entering the combustion chamber  110  where it is ignited to support a flame radially outward of the flame supported by the second injector  104 . In some embodiments, the first injector  102  is referred to as the “main” injector, while the second injector  104  is referred to as the “pilot” and/or “pilot tune” (or simply “p-tune”) injector. 
       FIG.  3    shows one embodiment of the second, or pilot, injector  104  used to mix fuel and air passing through the radially outer path  112 . The embodiments of the second injector discussed herein, including second injector  104  and others, may beneficially reduce emissions, particularly carbon monoxide emissions, during a startup regime of the combustor  100  and thus, more broadly, reduce emissions within a gas turbine engine in which the combustor  100  is employed. The injectors are also beneficially configured to be supplied with a variety of types of fuel including natural gas, hydrogen gas, and others. Put another way, the second injectors discussed herein improve flame stabilization and reduce startup emissions among a wide array of firing temperatures, including relatively low firing temperatures. 
     The second injector  104  generally includes an inlet portion  116  configured to receive compressed air flowing in the radially outer path  112 , an outlet portion  118  configured to supply a fuel/air mixture to the combustion zone  110  where it is ignited and supports a flame near the central axis of the combustor  100 , and an air/fuel mixing portion  120  generally extending between the inlet portion  116  and the outlet portion  118 . 
     As best seen in  FIG.  4   , which is a partial cross-sectional view of the second injector  104 , the air/fuel mixing portion  120  includes a plurality of air/fuel mixing tubes  122  extending from the inlet portion  116  to the outlet portion  118 . Each air/fuel mixing tube  122  includes an upstream portion  124 , a downstream portion  126 , and an elbow portion  128  fluidly connecting the upstream portion  124  to the downstream portion  126 . In some embodiments, the upstream portion  124  is disposed at an oblique angle with respect to the downstream portion  126  such as, e.g., a 45-degree angle. By maintaining the elbows  128  at an approximate 45-degree angle or similar angle, the second injector  104  may be additively manufactured without the need for external supports. Moreover, the elbow  128  (e.g., the 45-degree bend) improves mixing of the fuel and air in the air/fuel mixing tubes  122  as the mixture travels therein, turns, and becomes turbulent. 
     The second injector  104  also includes a first fuel manifold  130  and a second fuel manifold  132 , each in fluid communication with one or more of the plurality of air/fuel mixing tubes  122  via a corresponding one or more fuel feed tubes  134  and  136 . For example, in the depicted embodiment the first fuel manifold  130  is in fluid communication with a radially outwardly located subset of the plurality of air/fuel mixing tubes  122  (i.e., the “outer diameter” or “OD” subset of the air/fuel mixing tubes  122 ) via the fuel feed tube(s)  134 , while the second fuel manifold  132  is in fluid communication with a radially inwardly located subset of the plurality of air/fuel mixing tubes  122  (i.e., the “inner diameter” or “ID” subset of the air/fuel mixing tubes  122 ) via the fuel feed tube(s)  136 . In this regard, the combustor  100  can be staged by selectively injecting fuel into the OD or ID subset of the air/fuel mixing tubes  122  via the first and second fuel manifolds  130 ,  132 , respectively, which in turn will be ignited in the combustion chamber  110  forming two separately supported and localized flames (i.e., an annular outer flame surrounding a centrally located inner flame). 
     The second injector  104  may be manufactured by any desired means such as, in one non-limiting example, by additive manufacturing. In this regard, the second injector  104  would be built up layer by layer in the substantially vertical direction as it appears in  FIG.  4   . This is schematically illustrated by build arrow  138 . Each layer will include gaps of material that in turn form the internal, hollow passages of the second injector  104  such as the plurality of air/fuel mixing tubes  122 , first and second fuel manifolds  130 ,  132 , fuel feed tubes  134 ,  136 , and other features discussed herein. Again, because the upstream portion  124  and the downstream portion  126  of the air/fuel mixing tubes  122  are arranged at an approximately 45-degree angle relative to one another, the injector  104  can be additively manufactured without the need for external supports, etc. 
       FIGS.  5  and  6    show up-close, sectional views of upstream portions  124  of the air/fuel mixing tubes  122  of the second injector  104  near the inlet portion  116 . In this embodiment, each of the plurality of air/fuel mixing tubes  122  is thermally isolated from the corresponding first or second fuel manifold  130 ,  132  via an intermediate air manifold or plenum. Namely, the first subset of the plurality of air/fuel mixing tubes  122  that are in fluid communication with the first fuel manifold  130  are separated from the first fuel manifold  130  by a surrounding first static air plenum  140 , while the subset of the plurality of air/fuel mixing tubes  122  that are in fluid communication with the second fuel manifold  132  are separated from the second manifold  130  and the second fuel manifold  132  by a surrounding second static air plenum  142 . 
     As best shown in  FIG.  6   , the fuel feed tubes  134 ,  136  are each in turn generally L-shaped so that the fuel in each first and second fuel manifold  130 ,  132  is configured to travel from a downstream end of the respective first or second fuel manifold  130 ,  132  before turning at a substantially 90-degree angle to meet the respective air/fuel mixing tube  122 , where it is introduced to the compressed air flowing therethrough and thus mixed with the compressed air, forming the air/fuel mixture to ultimately be ignited in the combustion chamber  110 . Put another way, a distal portion of the fuel feed tubes  134 ,  136  may be oriented approximately normal to the air/fuel mixing tubes  122  such that the fuel is injected cross-stream into the flowing compressed air, which optimizes fuel and air mixing within the air/fuel mixing tubes  122 . Although only one of the air/fuel mixing tubes  122  forming part of the subset of the plurality of air/fuel mixing tubes  122  that is in fluid communication with the first fuel manifold  130  is shown in detail in  FIG.  6    for convenience, the structure and operation of the air/fuel mixing tubes  122  in fluid communication with the second fuel manifold  132  would be substantially similar. Put another way, each air/fuel mixing tube  122  is completely isolated from the respective first or second fuel manifold  130 ,  132  via the intermediate first or second static air plenum  140 ,  142  without any fuel feed tubes  134 ,  136  (or any other components for that matter) extending through the first and second static air plenum  140 ,  142 . 
     Among other benefits, the first and second static air plenums  140 ,  142  may beneficially serve as a buffer between the hot compressed air flowing through the air/fuel mixing tubes  122  and the cool fuel provided in the first and second fuel manifolds  130 ,  132 . That is, the first and second static air plenums  140 ,  142  insulate the air/fuel mixing tubes  122  from the cold fuel, thereby improving the strength and stress resistance of the tubes near the cold first and second fuel manifolds  130 ,  132 . 
     In the embodiment shown in  FIG.  6   , the inlets to the first and second static air plenums  140 ,  142  are annular; that is, the annular cross-section of the first and second static air plenums  140 ,  142  extends completely to the inlet surface of the injector  104 . However, in other embodiments the inlets to the first and second static air plenums  140 ,  142  need not be annular or unbroken. For example, as shown in  FIG.  7   , in some embodiments the inlets will include a broken or dashed configuration. That is, the annular cross-section of the first and second static air plenums  140 ,  142  are interrupted by a series of ribs  144  at the inlet thereof that connect the surrounded air/fuel mixing tubes  122  with a body of the second injector  104  that surrounds the first and second static air plenums  140 ,  142 . The ribs  144  may add rigidity and structural integrity at the inlet surface without significantly reducing the cross-sectional area of the inlet to the first and second static air plenums  140 ,  142 , thereby allowing air to enter and exit the first and second static air plenums  140 ,  142 . 
     As shown in  FIGS.  8  and  9   , in some embodiments the first and second fuel manifolds  130 ,  132  may include one or more internal baffle plates  146 ,  148 . The baffle plates  146 ,  148 , in turn include a plurality of through-holes  150 ,  152  provided therein providing fluid communication from a first portion of the respective first or second fuel manifold  130 ,  132  to a second portion of the respective first or second fuel manifold  130 ,  132  by allowing fluid to flow through the baffle plates  146 ,  148 . The baffle plates  146 ,  148  may increase rigidity and strength of the injector  104  and, more particularly, the inlet portion  116  thereof without significantly interfering with fuel flow within the first and second fuel manifolds  130 ,  132  and to the respective air/fuel mixing tubes  122 . Additionally, the baffle plates may beneficially serve to evenly distribute fuel from the first and second fuel manifolds  130 ,  132  to each of the air/fuel mixing tubes  122 . In one non-limiting example, each first and second fuel manifold  130 ,  132  includes a respective baffle plate  146 ,  148 , with each baffle plate having three circumferential rows  36  of through-holes  150 ,  152 , each of the through-holes having a diameter of approximately 0.100 inches. 
     In some embodiments, the plurality of air/fuel mixing tubes used to mix and deliver fuel and air may be structured and configured differently than that shown in  FIGS.  4 - 9    in order to, e.g., improve mixing of the fuel and air therein. For example, in some embodiments the plurality of air/fuel mixture and delivery tubes may be twisted or swirled in order to improve the mixture of fuel and air therein. This may be more readily understood with reference to  FIG.  10   . 
     More particularly,  FIG.  10    shows another embodiment of a second (e.g., pilot) injector  204 , with some of the internal air/fuel mixing tubes  222  therein shown in phantom to illustrate the specific configuration thereof. In the embodiments described herein, unless described otherwise, components with like-numbered trailing numerals operate in a substantially similar manner as the components discussed in connection with the first embodiment of the second injector  104 , and thus for simplicity will not be discussed again in detail. For example, the inlet portion  216  of this embodiment is configured and operates in a substantially similar manner to the inlet portion  116  of the second injector  104 , and thus will not be discussed again in detail. 
     In the  FIG.  10    embodiment, at least a subset of the air/fuel mixing tubes  222 , which are shown in phantom in  FIG.  10    as a radially inner (ID) subset, are twisted or swirled, creating a substantially helical path for the air/fuel mixture to follow. This in turn may lead to a more homogenous air/fuel fixture resulting in more efficient burning and reduced emissions. Although such a swirled configuration would be difficult to accomplish with traditional manufacturing techniques, these complex geometries can readily be implemented when the injector  204  is formed from additive manufacturing, as discussed above. 
     Although the embodiments described thus far have included air/fuel mixing tubes  122 ,  222  having a substantially circular cross-sectional area, embodiments are not so limited and in other embodiments the air-fuel mixture tubes may have other cross-sectional area configurations without departing from the scope of the disclosure. For example,  FIGS.  11 - 13    show an embodiment of a second injector  304  in which a first subset  322   a  of the plurality of air/fuel mixing tubes  322  includes a substantially quadrilateral cross-sectional profile, while a second subset  322   b  of the plurality of air/fuel mixing tubes  322  are twisted or swirled and each includes a substantially circular cross-sectional profile. In this embodiment, the quadrilateral profile of the first subset of air/fuel mixing tubes  322   a , which is shown as an outer diameter (OD) subset, allows a greater flowrate of the air/fuel mixture to flow through the air/fuel mixing tubes  322  as compared to the second subset of air/fuel mixing tubes  322   b , which is shown as an radially inner (ID) subset, having a circular cross-sectional profile. Additionally, or alternatively, the inlets to each air/fuel mixing tube  322  could similarly be any desired shape including quadrilateral, such as the first subset  322   a  of air/fuel mixing tubes shown in  FIG.  13   . Again, this increases the amount of compressed airflow within the mixing tubes  322  as compared to mixing tubes including a circular cross-sectional area. 
     In some embodiments, one or more flow modifiers may be included along the length of the air/fuel mixture mixing tubes  122 , such as in the upstream portion  124  prior to the 45-degree bend, in order to, e.g., create turbulence and improve air and fuel mixing within the respective tube. This may be more readily understood with respect to  FIG.  14 A , which shows one embodiment of a flow modifier  354  being implemented in the second injector  304  (and more particularly the first subset  322   a  of air/fuel mixing tubes  322  of the second injector  304 ), however, similar modifiers could be implemented in any of the injectors contemplated herein. 
     In this embodiment the flow modifier  354  includes a substantially saw-tooth type pattern, with a main wedge portion  356  and a serrated end portion  358 . As compressed air passes over the wedge portion  356 , it speeds up and is directed toward a central portion of the respective air/fuel mixing tube  322   a . As it travels over the serrated end portion  356 , the sawtooth pattern results in the airflow creating trapped vortices near the serrated end portion  358 , as illustrated by the airflow diagram  360  in  FIG.  14 B . These trapped vortices promote air and fuel mixing. Put another way, the flow modifier  354  creates a wake upstream of the fuel injection points, thereby improving air and fuel mixing at the fuel injection point. 
     Although the sawtooth pattern is illustrated in  FIG.  14 A , in other embodiments the modifier could be alternatively structured and configured such as, e.g., by including a wavy or sinusoidal shaped distal end. Moreover, although the depicted embodiment is shown with two flow modifiers  358  per air/fuel mixing tube  322   a , in other embodiments more or fewer flow modifiers  358  could be implemented without departing from the scope of the invention including, e.g., one flow modifier  358  per air/fuel mixing tube  322   a  or three or more flow modifiers  358  per tube air/fuel mixing  322   a.    
     In some embodiments, particularly embodiments in which the second injector includes air/fuel mixing tubes having substantially quadrilateral cross-sectional areas, an exit profile of the air/fuel mixing tube may be modified to include one or more surfaces to encourage flame anchoring at the exit face of the injector. This will be more readily understood with reference to  FIG.  15   . In  FIG.  15   , the radially outer (OD) subset  422   a  of the air/fuel mixing tubes  422  have a substantially quadrilateral cross-sectional area along most of the length of the air/fuel mixing tube  422 , however, at the outlet portion  418 , each air/fuel mixing tube  422   a  includes a blocker or arcuate portion  456   a - d  at one of the four corners thereof. Neighboring ones of the blockers or arcuate portions  456   a - d  are disposed proximate one another to create a substantially circular flame anchoring surface  458 . That is, when the fuel/air mixture exiting the OD tubes  422   a  is ignited, the resulting flame may stay anchored near the outlet portion  418 , and, more particularly, at or on the flame anchoring surfaces  458 , because of the recirculation zone created by the blockers  456   a ,  456   b ,  456   c ,  456   d  and flame anchoring surfaces  458 . Any other shape blocker could be implemented (i.e., substantially triangular, half-moon shaped blockers, rectangular blockers, etc.) without departing from the disclosure, so long as the blocker provides a flame anchoring surface and/or increased webbing at the outlet portion  418  of the injector  404  to create recirculation zones and thus encourage flame anchoring. Moreover, more than one blocker could be implemented on some of the air/fuel mixing tubes  422   a  without departing from the scope of the disclosure. 
       FIG.  16    shows an alternative design of a second injector  504  that is contemplated according to aspects of the disclosure. In the alternative design, each of the air/fuel mixing tubes  522  in a radially outer (OD) subset  522   a  of the air/fuel mixing tubes  522  is twisted or swirled, creating a substantially helical path for the air/fuel mixture to follow, and each of the air/fuel mixing tubes  522  in a radially inner (ID) subset  522   b  of the air/fuel mixing tubes  522  lacks the twist or swirl. This in turn may lead to a more homogenous air/fuel fixture resulting in more efficient burning and reduced emissions. Although such a swirled configuration would be difficult to accomplish with traditional manufacturing techniques, these complex geometries can readily be implemented when the injector  504  is formed from additive manufacturing, as discussed above. The injector  504  may include a radial array of outer channels  560  used to channel air along the outwardly facing surface of the injector  504 , thereby cooling the hot outer face of the injector  504 , particularly portions located below a hula seal or similar seal that may otherwise become very hot during use of the injector  504 . 
       FIG.  17 A-D  show schematic representations of a second injector  804 , which may be any of the second injectors described above, in which the fuel manifold assemblies have their fuel circuits stacked in different ways to provide different ways to stage the second injectors and thereby vary the burner flame characteristics for improved emissions, dynamics and/or performance. In  FIGS.  17 A-D , a plurality of air/fuel mixing tubes  822  in the second injector  602  are divided into a radially outer (OD) subset  822   a , a radially inner (ID) subset  822   b , and a radially intermediate subset  822   c  of the plurality of air/fuel mixing tubes  822 . 
     In  FIG.  17 A , a first fuel manifold  830  is in fluid communication with all three subsets  822   a - c  of the air/fuel mixing tubes  822 , a second fuel manifold  832  is in fluid communication with only the radially inner subset  822   b  of air/fuel mixing tubes  822 , and a third fuel manifold  833  is in fluid communication with only the radially inner and the radially intermediate subsets  822   b - c  of the air/fuel mixing tubes  822 . 
     In  FIG.  17 B , each of the first, second and third fuel manifolds  830 ,  832 ,  833  is in fluid communication with each of the radially inner, radially outer, and radially intermediate subsets  822   a - c  of air/fuel mixing tubes  822 . 
     In  FIG.  17 C , the first fuel manifold  830  is in fluid communication with only the radially inner subset  822   b  of air/fuel mixing tubes  822 , the second fuel manifold  832  is in fluid communication with each of the radially inner, radially outer, and radially intermediate subsets  822   a - c  of air/fuel mixing tubes  822 , and the third fuel manifold  833  is in fluid communication with only the radially inner and radially intermediate subsets  822   b - c  of air/fuel mixing tubes  822 . 
     In  FIG.  17 D , the first fuel manifold  830  is in fluid communication with only the radially inner subset  822   b  of air/fuel mixing tubes  822 , the second fuel manifold  832  is in fluid communication with only the radially inner and radially intermediate subsets  822   b - c  of air/fuel mixing tubes  822 , and the third fuel manifold  833  is in fluid communication with each of the radially inner, radially outer, and radially intermediate subsets  822   a - c  of air/fuel mixing tubes  822 . 
     Although embodiments of the second injector discussed and shown herein include air/fuel mixing tubes that each include two fuel feed tubes in fluid communication with a respective fuel manifold, embodiments are not so limited. In other embodiments, each air/fuel mixing tube can include one fuel feed tube or else more than one fuel feed tube. For example,  FIGS.  18 A-C  show schematic cross-sectional profiles of alternative embodiments of air/fuel mixing tubes  922  that include two, three, and four fuel feed tubes  934  at three different exemplary arrangements. As seen in  FIGS.  18 A-C , an angle, alpha, of the fuel feed tubes  934  can be varied to provide enhanced performance, mixing, and reduced emission benefits. For example, in some embodiments the angle alpha will be approximately 180-degrees, 120-degrees or 90-degrees, while in other embodiments the angle alpha could be another oblique angle without departing from the scope of the invention. 
     From the foregoing, it will be seen that this disclosure is one well adapted to attain all the ends and objects set forth above, together with other advantages which are obvious and inherent to the system and method. It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. 
     ADDITIONAL CONSIDERATIONS 
     In this description, references to “one embodiment,” “an embodiment,” or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment,” “an embodiment,” or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments but is not necessarily included. Thus, the current technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     In the specification and claims, reference will be made to several terms, which shall be defined to have the following meanings. The singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and the claim, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Range limitations may be combined and/or interchanged. Such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     As used herein, the terms “axial” and “axially” refer to directions and orientations extending substantially parallel to a center longitudinal axis of the combustor. The terms “radial” and “radially” refer to directions and orientations extending substantially perpendicular to the central axis. Moreover, directional references, such as, “top,” “bottom,” “front,” “back,” “side,” and similar terms are used herein solely for convenience and should be understood only in relation to each other. For example, a component might in practice be oriented such that faces referred to herein as “top” and “bottom” are in practice sideways, angled, inverted, etc. relative to the chosen frame of reference. 
     The terms “coupled,” “fixed,” “attached to,” and the like refer to both direct coupling, fixing, or attaching, as well as indirect coupling, fixing, or attaching through one or more intermediate components or features, unless otherwise specified herein. 
     Although the present application sets forth a detailed description of numerous different embodiments, it should be understood that the legal scope of the description is defined by the words of the claims and equivalent language. The detailed description is to be construed as exemplary only and does not describe every possible embodiment because describing every possible embodiment would be impractical. Numerous alternative embodiments may be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims. 
     Throughout this specification, plural instances may implement components, operations, or structures described as a single instance. Although individual operations of one or more methods are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently, and nothing requires that the operations be performed in the order recited or illustrated. Structures and functionality presented as separate components in example configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements fall within the scope of the subject matter herein. The foregoing statements in this paragraph shall apply unless so stated in the description and/or except as will be readily apparent to those skilled in the art from the description. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 
     Although the disclosure has been described with reference to the embodiments illustrated in the attached figures, it is noted that equivalents may be employed, and substitutions made herein, without departing from the scope of the disclosure as recited in the claims.