Patent Publication Number: US-8966907-B2

Title: Turbine combustor system having aerodynamic feed cap

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to turbine combustors, and, more particularly, to a system for creating aerodynamic flow within a turbine combustor head end chamber. 
     A gas turbine engine combusts a fuel-air mixture in a combustion chamber of a turbine combustor, and then drives one or more turbines with the resulting hot combustion gases. In certain configurations, fuel and air are pre-mixed prior to ignition to reduce emissions and improve combustion. The gas turbine engine mixes the fuel and the air within one or more chambers, such as fuel nozzles. The fuel and air may travel together and/or separately through one or more paths through the turbine combustor. Unfortunately, the one or more paths may include sharp turns, recesses, and other obstructions that create recirculation zones, which may allow the flame holding and/or flashback. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In a first embodiment, a system includes a turbine combustor having a fuel nozzle with an inner shell and an outer shell and an feed cap disposed about the fuel nozzle having an outer wall and a back plate. The back plate joins respective upstream ends of the outer shell of the fuel nozzle and the outer wall of the feed cap. The turbine combustor is configured to flow a first pressurized air via a first air path extending along the outer wall of the feed cap, the back plate of the feed cap, and into the fuel nozzle. 
     In a second embodiment, a system includes a turbine combustor having: a combustion chamber, a head end chamber separated from the combustion chamber by a divider plate, and a pressurized chamber disposed in the head end chamber and about a fuel nozzle. The pressurized chamber includes a back plate that is joined to an upstream end of an outer shell of the fuel nozzle. 
     In a third embodiment, a system includes a turbine combustor having a combustion chamber, a head end chamber separated from the combustion chamber by a divider plate, and an air path disposed in the head end chamber and configured to flow a first pressurized air into a fuel nozzle. The air path includes a first segment disposed between a flow sleeve of the turbine combustor and an outer wall of an feed cap, and a second segment disposed downstream of the first segment and between a back plate of the feed cap and an end plate of the head end chamber. The second segment is substantially free of any flow-impeding surfaces between the back plate and the end plate. The air path also includes a third segment disposed downstream of the second segment and between inner and outer shells of the fuel nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic of an embodiment of a gas turbine system with a plurality of turbine combustors, each having a feed cap with a pressurized chamber configured to enable aerodynamic flow into a respective fuel nozzle; 
         FIG. 2  is a cross-sectional side view schematic of an embodiment of one of the turbine combustors of  FIG. 1 , illustrating an embodiment of the feed cap with the pressurized chamber having an aerodynamic back plate; 
         FIG. 3  is a cross-sectional side view schematic of an embodiment of a head end of the turbine combustor of  FIG. 2 , taken within  3 - 3 , illustrating air flow paths around and through the feed cap and the pressurized chamber; 
         FIG. 4  is a cross-sectional side view schematic of an embodiment of a head end of the turbine combustor of  FIG. 2 , taken within  3 - 3 , illustrating a plurality of pressurized chambers formed by the feed cap and a plurality of fuel nozzles; 
         FIG. 5  is a side view of an embodiment of the back plate of  FIGS. 3 and 4  having a generally straight shape that is substantially free of any flow-impeding surfaces; 
         FIG. 6  is a side view of an embodiment of the back plate of  FIGS. 3 and 4  having a curved shape that is convex with respect to the head end termini of the outer wall and the outer shell; and 
         FIG. 7  is a side view of an embodiment of the back plate of  FIGS. 3 and 4  in which the back plate is angled generally along a flow direction of the air flow path. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As noted above, a head end of a gas turbine combustor, which is upstream from a combustion chamber, include areas that are generally not aerodynamic, such as areas that create turbulent flow via one or more sharp turns or edges, areas having low flow conditions where pockets of compressed air and fuel can accumulate, and areas where mixing of fuel and air is undesirable. In other words, the head end of the gas turbine combustor may include recirculation zones, which may include zones in which a mixture of fuel and air has low flow or research relates such that a flame can hold or flash back. Any one or a combination of these conditions can lead to undesirable combustion (e.g., flame holding or flashback) upstream from the combustion chamber of the gas turbine combustor, such as within a head end region or a feed cap region of the gas turbine combustor. The present embodiments include an aerodynamic feed cap design and the head end of the gas turbine combustor to reduce or eliminate recirculation zones. The feed cap may be a one-piece design configured to reduce the possibility forming low-flow regions, no-flow regions, areas of undesired turbulence, recirculation, mixing of fuel and air, and the like. Accordingly, the present embodiments may provide enhanced reliability of gas turbine engines, which in turn may result in more reliable energy production and increased throughput in integrated gasification systems, such as integrated gasification combined cycle (IGCC) systems. Indeed, the present embodiments may be used in any context employing a turbine combustor having areas where low-flow, no-flow, turbulence, and/or recirculation may create undesirable situations (e.g., flashback or flame holding). 
     Turning now to the drawings,  FIG. 1  illustrates a block diagram of an embodiment of a gas turbine system  10 , which may utilize an aerodynamic feed cap in accordance with present embodiments. The system  10  includes a compressor  12 , turbine combustors  14 , and a turbine  16 . The turbine combustors  14  include fuel nozzles  18  which route a liquid fuel and/or gas fuel, such as natural gas or syngas, into the turbine combustors  14 . As shown, each turbine combustor  14  may have multiple fuel nozzles  18 . More specifically, the turbine combustors  14  may each include a primary fuel injection system having primary fuel nozzles  20  and a secondary fuel injection system having secondary fuel nozzles  22 . As described in detail below, each turbine combustor  14  may also include a feed cap configured to reduce undesirable no-flow, low-flow, recirculation, or other undesirable air flow situations. Furthermore, the aerodynamic feed cap of each turbine combustor  14  may mitigate acoustic waves and suppress pressure fluctuations (i.e., reduce the occurrence of dynamics) in the turbine combustor  14 . Indeed, such a feed cap design may be desirable to mitigate the possibility of retaining a combustible mixture of fuel and air in a recirculation zone, such as a low velocity region. For example, in a recirculation zone, a flame can hold in this region and/or travel upstream to this region, which may be undesirable. 
     The turbine combustors  14  ignite and combust an air-fuel mixture, and then pass hot pressurized combustion gasses  24  (e.g., exhaust) into the turbine  16 . Turbine blades are coupled to a shaft  26 , which is also coupled to several other components throughout the turbine system  10 . As the combustion gases  24  pass through the turbine blades in the turbine  16 , the turbine  16  is driven into rotation, which causes the shaft  26  to rotate. Eventually, the combustion gases  24  exit the turbine system  10  via an exhaust outlet  28 . Further, the shaft  26  may be coupled to a load  30 , which is powered via rotation of the shaft  26 . For example, the load  30  may be any suitable device that may generate power via the rotational output of the turbine system  10 , such as an electrical generator, a propeller of an airplane, and so forth. 
     Compressor blades are included as components of the compressor  12 . The blades within the compressor  12  are coupled to the shaft  26 , and will rotate as the shaft  26  is driven to rotate by the turbine  16 , as described above. The rotation of the blades within the compressor  12  compress air from an air intake  32  into pressurized air  34 . The pressurized air  34  is then fed into the fuel nozzles  18  of the turbine combustors  14 . The fuel nozzles  18  mix the pressurized air  34  and fuel to produce a suitable mixture ratio for combustion (e.g., a combustion that causes the fuel to more completely burn) so as not to waste fuel or cause excess emissions. As discussed below, the compressed air may pass through and/or around the feed cap in each combustor  14  upstream from fuel injection. 
       FIG. 2  is a schematic of an embodiment of one of the turbine combustors  14  of  FIG. 1 , illustrating a feed cap  50  disposed within a head end  52  of the turbine combustor  14  and about a single fuel nozzle  20 . As described above, the compressor  12  receives air from the air intake  32 , compresses the air, and produces the flow of pressurized air  34  for use in the combustion process within the turbine combustor  14 . As shown in the illustrated embodiment, the pressurized air  34  is received by a compressor discharge  54  that is operatively coupled to the turbine combustor  14 . As indicated by arrows  56 , the pressurized air  34  flows from the compressor discharge  54  toward the head end  52  of the turbine combustor  14 . More specifically, the pressurized air  34  flows through an annulus  60  between a liner  62  and a flow sleeve  64  of the turbine combustor  14  to reach the head end  52 . 
     In certain embodiments, the head end  52  includes an end plate  66  that may support the primary fuel nozzles  20  depicted in  FIG. 1 . In the illustrated embodiment, the head end  52  has the single primary fuel nozzle  20  and associated feed cap  50 . However, as discussed below, the head end  52  may include a plurality of fuel nozzles  20 , with one or more feed caps  50  surrounding the fuel nozzles  20 . In accordance with one embodiment, a single feed cap  50  may surround a plurality of the fuel nozzles  20 , such as between 2 and 10 fuel nozzles (e.g., between 2 and 8 or 4 and 6 fuel nozzles). 
     A fuel supply provides fuel  68  to the primary fuel nozzle  20 . Additionally, an air flow path  72  delivers the pressurized air  34  from the annulus  60  of the turbine combustor  14  through the primary fuel nozzle  20 . The primary fuel nozzle  20  combines the pressurized air  34  with the fuel  68  provided by the primary fuel supply  68  to form an fuel-air mixture. Specifically, the fuel  68  may be injected into the air flow path  72  by a plurality of swirl vanes  74  and, in some embodiments, additionally by one or more quaternary fuel injectors  97 . The fuel-air mixture flows from the air flow path  72  into a combustion chamber  76  where the fuel-air mixture is ignited and combusted to form combustion gases (e.g., exhaust). The combustion gases flow in a direction  78  toward a transition piece  80  of the turbine combustor  14 . The combustion gases pass through the transition piece  80 , as indicated by arrow  82 , toward the turbine  16 , where the combustion gases drive the rotation of the blades within the turbine  16 . 
     As noted above, the turbine combustor  14  includes regions where combustion is desired, such as the combustion chamber  76 , and regions where combustion is undesirable, such as a head end chamber  84  disposed between the end plate  66  and a divider plate  86  separating the head end  52  from the combustion chamber  76 . Combustion (e.g., flashback and/or flame holding) within the head end chamber  84  may be a result of turbulent air flow and fuel-air pockets along the air flow path  72 , where the flow recirculates and/or has a low or no velocity in an upstream region, such as upstream of a combustion chamber and/or upstream of a fuel injector (e.g., fuel injector  20 ), and downstream of quaternary fuel injectors  97 . Thus, in accordance with the present disclosure, it is now recognized that these and other undesirable flow conditions may be mitigated, at least in part, by providing an aerodynamic back plate  88  connecting an outer wall  90  of the feed cap  50  with an outer shell  92  of the fuel nozzle  20 . Specifically, as discussed in detail below, the back plate  88  connects the outer shell  92  of the fuel nozzle  20  with the outer wall  90  of the feed cap  50  in such a way that the pressurized air  34  is able to flow along the air flow path  72  without encountering substantial turbulence or pockets of low flow or no flow. Further, the configuration of the back plate  88  may also help reduce the occurrence of pressure waves, acoustic waves, and other oscillations referred to as combustion dynamics, produced by the combustion process. Combustion dynamics may cause performance degradation, structural stresses, and mechanical or thermal fatigue in the turbine combustor  14  (e.g., within the head end chamber  84 ). 
     The back plate  88 , the outer wall  90  of the feed cap  50 , the outer shell  92  of the fuel nozzle  20 , and the divider plate  86  together define a closed volume or chamber  94 . The chamber  94 , as illustrated, receives an influx of preconditioned air  96  from the set of quaternary fuel injectors  97  at a pressure that may be equal to or greater than a pressure of the pressurized air  34  flowing along the air path  72 . Therefore, relative to the air path  72  and head end chamber  84 , the chamber  94  may be considered to be a pressurized chamber. The chamber  94 , in some embodiments, receives the preconditioned air  96  at a pressure that is between approximately 1 and 20% higher than the pressure of the pressurized air  34  and/or the air/fuel mixture flowing along the air path  72 , such as between approximately 1 and 15%, 1 and 10%, 2 and 8%, 2 and 6%, or 3 and 5% (e.g., approximately 3%, 4%, or 5%) higher. Therefore, the chamber  94  may be sealed to the head end chamber  84 , which may prevent an influx of the fuel and/or the air/fuel mixture from entering the chamber  94 . In preventing such an influx, the chamber  94  may reduce the likelihood of premature combustion of the air/fuel mixture within the head end chamber  84  as a result of no flow or low flow of the air/fuel mixture. As discussed in further detail below, the chamber  94  may also enable cooling of the divider plate  86  by passing preconditioned air  96  into the combustion chamber  76 . The preconditioned air  96  may be the pressurized air  34 , or may be from another air source. As discussed below, the quaternary fuel injectors  97  may also inject fuel  86  into the air path  72  to form a fuel-air mixture. Accordingly, the configuration of the back plate  88 , and in particular its manner of connection with the outer wall  90  of the feed cap  50  and the outer shell  92  of the fuel nozzle  20 , reduces the possibility of flame holding and recirculation of the fuel-air mixture. 
       FIG. 3  is a schematic of an embodiment of the head end  52  of the turbine combustor  14 , taken within line  3 - 3  of  FIG. 2 , illustrating the chamber  94 , the quaternary fuel injectors  97 , and the primary fuel nozzle  20  disposed within the head end chamber  84 . As noted, the back plate  88  joins the outer wall  90  of the feed cap  50  with the outer shell  92  of the fuel nozzle  20 . Specifically, the back plate  88  directly connects a head end terminus  98  of the outer wall  90  with a head end terminus  100  of the outer shell  92 . The back plate  88  may take on any aerodynamic form, wherein a main portion  102  of the back plate  88  may be curved (e.g., concave or convex), sinuous, angled, or substantially parallel with respect to the end plate  66  and/or the divider plate  86 , as discussed below with respect to  FIGS. 5-7 . In certain embodiments, the back plate  88  may extend in a substantially straight line from the head end terminus  98  to the head end terminus  100 , and may be substantially free of flow-impeding surfaces such as recesses or concave shapes (e.g., with respect to the termini  98 ,  100 ). In one embodiment, the back plate  88  is substantially parallel with respect to the end plate  66  and/or the divider plate  86 . As defined herein, substantially parallel includes configurations where the entirety of the first portion  102  is oriented to within approximately 2° of parallel with respect to either or both of the end plate  66  and the divider plate  86 , accounting for manufacturing tolerances. However, configurations where the main portion  102  is angled with respect to the end plate  66  and/or the divider plate  86  is also presently contemplated, such as angled to between approximately 2 and 30°, 2 and 20°, 2 and 15°, 3 and 10°, or 4 and 8°. As illustrated, the main portion  102  is substantially parallel with respect to both the end plate  66  and the divider plate  86 , and is rounded at its first and second edges  104 ,  106  where it connects to the outer wall  90  and the outer shell  92 , respectively. In accordance with present embodiments, the first and second edges  104 ,  106  may assume any edge configuration, including rounded edges, straight edges, beveled edges, protruding edges, or any edge that does not create shear forces on the pressurized air  34 . 
     Indeed, the configuration of the back plate  88 , as discussed herein, facilitates flow of the pressurized air  34  and the fuel-air mixture along the air flow path  72 . As mentioned above, the air flow path  72  receives the pressurized air  34  from the annulus  60  of the turbine combustor  14 . Additionally, in certain embodiments, the quaternary fuel injectors  97  inject fuel  68  into the flow path  72  to form the fuel-air mixture, which may also flow along the air flow path  72 . The air flow path  72  includes a first portion  120 , a second portion  122 , and a third portion  123 . The first portion  120 , the second portion  122 , and the third portion  123  are operatively coupled. The first portion  120  of the air flow path  72  is defined by an outer wall  124 , which may be a head end casing or the flow sleeve  64 , and the outer wall  90  of the feed cap  50 . The second portion  122  of the air flow path  72  is defined by the end plate  66  of the head end chamber  84  and the back plate  88  of the feed cap  50 . The outer shell  92  and an inner shell  130  of the fuel nozzle  20  define the third portion  123 . In other words, the flow of the pressurized air  34  and/or the fuel-air mixture flows along an outer surface of the pressurized chamber  94  including a first outer surface of the feed cap  50  disposed around the fuel nozzle  20 , an outer surface of the back plate  88 , and an outer surface of the outer shell  92  of the fuel nozzle  20 . As illustrated, the back plate  88  is disposed at the juncture of the first and second portions  120 ,  122  and the juncture of the second and third portions  122 ,  123 . In accordance with present embodiments, the shape and positioning of the back plate  88  may facilitate flow of the pressurized air  34  between each portion. 
     For example, the first edge  104  of the back plate  88 , which couples the back plate  88  with the outer wall  90  of the feed cap  50 , may be rounded so as to prevent turbulent, recirculating, and/or low-velocity flow as the pressurized air  34  and/or fuel-air mixture flows from the first portion  120  to the second portion  122 . Likewise, the second edge  106  of the back plate  88 , which couples the back plate  88  with the outer shell  92  of the fuel nozzle  20 , may be rounded so as to prevent turbulent, recirculating, and/or low-velocity flow as the pressurized air and/or fuel-air mixture flows from the second portion  122  to the third portion  123 . The main portion  102  of the back plate  88  prevents the air and/or fuel-air flow from stalling. In other words, the main portion  102  prevents pockets of pressurized air  34  and/or the air/fuel mixture from becoming trapped in the second portion  122  by enabling continuous flow of the pressurized air  34  and/or the fuel-air mixture. Additionally, the main portion  102  is shaped to prevent areas where a crosswise flow of air is formed in the second portion  122 . Generally, the back plate  88  will not have any surfaces that create flow shearing, such as protrusions, obstructions, recesses, and so on. Indeed, the outer wall  90 , the back plate  88 , and the outer shell  92  are configured such that the air path  72  is substantially free of no flow or low flow regions in which a flow of the pressurized air and/or fuel-air mixture is impeded, halted, or otherwise sheared. That is, the back plate  88  may be a substantially smooth, continuous surface. 
     As indicated by arrows  132 , the pressurized air  34  flows from the annulus  60 , first through the first portion  120  of the air flow path  72 , through the second portion  122  of the air flow path  72 , and then through the third portion  123 . As noted, the pressurized air  34  may mix with the fuel  68 , forming a fuel-air mixture. Therefore, in the first, second, and third portions  120 ,  122 ,  123 , the arrows  132  may also represent the fuel-air mixture. The pressurized air  34  and/or fuel-air mixture also flows around the swirl vanes  74 . As discussed above, the fuel  68  is released into the pressurized air  34  through the swirl vanes  74 . Specifically, the fuel  68  flows down a fuel path  134  within the inner shell  130  of the fuel nozzle  20 , as represented by arrows  136 . The fuel  68  passes into the swirl vanes  74  from the fuel path  134 , as represented by arrows  138 , and exits the swirl vanes  74  through fuel ports  140  in the swirl vanes  74 , as represented by arrows  142 . The fuel  68  mixes with the pressurized air  34  to create an air/fuel mixture. The air/fuel mixture flows downstream, as indicated by arrows  144 , toward the combustion chamber  76 . In the illustrated embodiment, the divider plate  86  includes one or more openings  146  that operatively join the head end chamber  84  and the combustion chamber  76 . 
     As mentioned above, the head end  52  of the turbine combustor  14  includes the chamber  94 , which receives preconditioned air  96 . Specifically, the preconditioned air  96  enters the chamber  94  through a preconditioned air inlet  148 , while a flow of the fuel  86  enters the first portion  120  of the air flow path  72  through a series of fuel inlets  149 . For example, the preconditioned air  96  may be supplied by the compressor discharge  54 . While the illustrated embodiment shows two preconditioned air inlets  148 , other embodiments may include fewer or more preconditioned air inlets  148 . For example, the turbine combustor  14  may have 1, 3, 4, 5, 6, 7, 8, or more preconditioned air inlets  148 . The chamber  94  receives preconditioned air  96  from the preconditioned air inlet  148  and fills with the preconditioned air  96 , as indicated by arrows  150 . Additionally, the preconditioned air  96  may be directed toward apertures  152  in the divider plate  86 , as indicated by arrows  154 . In certain embodiments, the apertures  152  may be straight or angled holes. The preconditioned air  96  may pass through the apertures  152 , thereby cooling the divider plate  86  and entering the combustion chamber  76 . As noted, the preconditioned air  96  is provided to the chamber  94  at a pressure sufficient to prevent the influx of the fuel-air mixture produced at the fuel nozzle  20  into the chamber  94 . That is, the fuel-air mixture may be at a first pressure, the preconditioned air  96  within the chamber  94  may be at a second pressure, and the second pressure may be greater than the first pressure. Again, the preconditioned air  96  may be between approximately 1 and 15%, 1 and 10%, 2 and 8%, 2 and 6%, or 3 and 5% (e.g., approximately 3%, 4%, or 5%) higher than the pressurized air  34  and/or the fuel-air mixture. Additionally, in certain embodiments, the pressure within the chamber  94  may be at a level sufficient to prevent the influx of the combustion products produced within the combustion chamber  76 . 
       FIG. 4  is a schematic of an embodiment of the head end  52  of the turbine combustor  14 , taken along line  3 - 3  of  FIG. 2 , illustrating the feed cap  50  disposed about a plurality of fuel nozzles  20 . For example, the turbine combustor  14  may include a central fuel nozzle and a plurality of surrounding fuel nozzles (e.g., 2 to 10). In the illustrated embodiment, the feed cap  50  surrounds first and second outer fuel nozzles  160 ,  162  and a central fuel nozzle  164 . In present embodiments, the outer wall  90  of the feed cap  50  acts as a main wall that surrounds all of the fuel nozzles. Additionally, it should be noted that the fuel nozzles  20  are illustrated in a linear configuration to facilitate discussion, and may be in other configurations, such as in an annular arrangement where the fuel nozzles  20  are disposed in a barrel-like configuration. Accordingly, the outer wall  90  of the feed cap  50  is illustrated as only being disposed proximate the first and second outer fuel nozzles  160 ,  162 . Furthermore, the feed cap  50  includes a plurality of openings corresponding to each of the fuel nozzles  160 ,  162 ,  164 , wherein the respective outer shells  92 ,  166  of each of the nozzles  160 ,  162 ,  164  defines each opening. 
     As discussed above with respect to  FIG. 3 , the joining of the outer wall  90  of the feed cap  50  with the outer shell  92  of the fuel nozzle  20  by the back plate  88  and the divider plate  86  forms chamber  94 . In a similar manner, the respective outer shells  92  of the first and second outer fuel nozzles  160 ,  162  and the outer shell  166  of the central fuel nozzle  164  are joined by the back plate  88  and the divider plate  86  to form a volume  168 . That is, the back plate  88  of the feed cap  50  joins the head end termini  100  of the outer shells  92  of the first and second outer fuel nozzles  160 ,  162  with a head end terminus  170  of the outer shell  166  of the central fuel nozzle  164 . In this way, the air flow path  72  in the area of the central fuel nozzle  164  may be substantially free of areas of low flow or no flow. Therefore, the illustrated configuration is adapted to reduce the possibility of undesirable flow situations, such as recirculation flow, low-velocity flow, flame holding, and so on, within the head end chamber  84 . Additionally, like the chamber  94 , the volume  168  may be filled with the preconditioned air  96 . 
     For example, it will be appreciated that the central fuel nozzle  164  is not disposed proximate the outer wall  90  of the feed cap  50 . Rather, the first and second outer fuel nozzles  160 ,  162  are positioned between the first and second central fuel nozzles  164 ,  166  and the outer wall  90 . Thus, the preconditioned air  96  is not directly injected into the volume  168 . Instead, the preconditioned air  96  is first directly injected into the chamber  94 , and flows toward the central region of the head end  52 , which includes the central fuel nozzle  164  and the volume  168 . The preconditioned air  96  then fills the volume  168 . Thus, the volume  168  and the chamber  94  are in direct flow communication, and may have the same pressure. Indeed, the volume  168  may have a pressure of preconditioned air  96  that is greater than a pressure of an air fuel mixture flowing through the central nozzle  164 . For example, the pressure of the preconditioned air  96  within the volume  168  may be between approximately 1 and 15%, 1 and 10%, 2 and 8%, 2 and 6%, or 3 and 5% (e.g., approximately 3%, 4%, or 5%) higher than the pressurized air  34  and/or the fuel-air mixture. 
     As noted above, the back plate  88  may take on any aerodynamic form that connects the outer wall  90  of the feed cap  50  with the outer shell  92  of the fuel nozzle  20 . That is, the back plate  88  is configured to maintain sufficient flow along all boundary surfaces so as to prevent recirculation of the fuel-air mixture and/or the pressurized air  34 . Examples of such configurations are illustrated in  FIGS. 5-7 . Specifically, in  FIG. 5 , the back plate  88  may have a generally straight shape that is substantially free of any flow-impeding surfaces. Alternatively, the back plate  88  may be bent and/or angled. For example, as illustrated in  FIG. 6 , the back plate  88  may have a curved shape that is convex (i.e., bows outward) with respect to the head end termini of the outer wall  90  and the outer shell  92 . Alternatively or additionally, the back plate  88  may be angled, as illustrated in  FIG. 7 . In  FIG. 7 , the back plate  88  is illustrated as being angled generally along a flow direction of the flow path  72 . Indeed, any general shape and configuration of the back plate  88  that is configured to reduce the possibility of flame holding, flashback, low-velocity flow, no flow, and similar undesirable flow conditions, are presently contemplated. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.