Abstract:
A flame holding inhibitor includes a base portion and an upstanding support extending away from the base portion; at least one delta-wing-shaped flap on the upstanding support, each having a relatively pointed end and a relatively broad end.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to gas turbine combustors, and specifically, to a flame holding inhibitor for use with a lean pre-nozzle injector diffuser located upstream of the combustor fuel nozzles. 
       BACKGROUND OF THE INVENTION 
       [0002]    In certain land-based gas turbine multi-combustor configurations, the individual combustors are arranged in an annular array about the gas turbine casing, each combustor supplying combustion gases to the first stage of the turbine. Each combustor is supplied with air from a compressor in a manner such that the compressor air is reverse-flowed into an annular air passage located between radially inner and axially-aligned transition piece and combustion chamber liner on the one hand, and radially outer, axially-aligned flow sleeve on the other. The compressor air generally flows into the passage through impingement cooling holes provided in the flow sleeve, thus also providing cooling to the transition piece and combustor liner, before reversing flow at the inlet or head end of the combustor. 
         [0003]    In one low NOx combustor configuration, five radially-outer nozzles surround a sixth center nozzle. In this arrangement, three pre-mix manifolds stage fuel to the six burners while a fourth pre-mix manifold supplies fuel to a plurality of fuel pegs arranged in the air passage supplying combustion air to the combustor, upstream of the head end of the combustor that supports the six nozzles. While there is no intentional combustion at the fuel pegs, flame holding in this lean pre-nozzle fuel injection peg diffuser remains a problem when the fuel pegs are in operation. Flame holding occurrence in the diffuser is mainly caused by a locally-rich fuel air mixture which is created by unsatisfactory mixing and local flow separation around the trailing edges of the airfoil shaped fuel pegs, especially under large angles of attack. It would therefore be desirable to eliminate the flow separation by introducing secondary flow into the fuel/air jet mixing zone to eliminate the wake region along the trailing edge of the fuel pegs and to boost local air/fuel mixing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In accordance with a first exemplary but nonlimiting embodiment, the invention provides a flame holding inhibitor comprising a base portion and an upstanding support extending away from the base portion; and at least one delta-wing-shaped flap on the upstanding support having a relatively pointed end and a relatively broad end. 
         [0005]    In another exemplary but nonlimiting aspect, the present invention provides turbine fuel system incorporating one or more combustors, each combustor comprising a combustor liner having a head end supporting a plurality of nozzles and an aft end adapted for connection to a transition piece which, in use, carries hot combustion gases in a first direction to a first turbine stage; sleeve surrounding the combustor liner defining an annular flow path for compressor air that, in use, flows along the annular flow path in a second, opposite direction and then reverses to the first direction at the head end and flows into the combustor liner; a plurality of fuel pegs located in the annular flow path radially between the combustor liner and the flow sleeve, adjacent and upstream of the head end; and a plurality of flame holding inhibitors located upstream and in proximity to the fuel pegs. 
         [0006]    In still another exemplary but nonlimiting aspect, the present invention provides a method for a method of enhancing flame holding margin and fuel/air premixing in a combustor that includes plural, radially-oriented fuel pegs in an air passage supplying combustion air to the combustor, where the plural, radially-oriented fuel pegs are located upstream of fuel nozzles supported in an end cover of the combustor, the method comprising (a) providing a flame inhibitor adjacent and upstream of each of said plural, radially-oriented fuel pegs; (b) aligning the flame inhibitor relative to fuel delivery holes in each of said plural, radially-oriented fuel pegs, such that vortices are created in the combustion air sufficient to insure premixing of the fuel issued from the fuel delivery holes, and to prevent fuel from adhering to exterior surfaces of each of said plural, radially-oriented fuel pegs. 
         [0007]    The invention will now be described in greater detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a simplified cross-section through a known gas turbine combustor; 
           [0009]      FIG. 2  is a simplified forward end view of the combustor arrangement of  FIG. 1 ; 
           [0010]      FIG. 3  is a partial perspective view of quaternary fuel pegs and flame holding inhibitors in accordance with an exemplary but non-limiting embodiment of the invention; 
           [0011]      FIG. 4  is an enlarged perspective view of a flame holding inhibitor device taken from  FIG. 3 ; 
           [0012]      FIG. 5  is a simplified flow diagram of a fuel peg without an adjacent flame holding inhibitor; and 
           [0013]      FIG. 6  is a simplified flow diagram similar to  FIG. 5  but illustrating flow when a flame holding inhibitor is located upstream and adjacent the quaternary fuel peg. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring initially to  FIGS. 1 and 2 , a gas turbine engine  10  includes a compressor  12 , a combustor  14 , and a turbine  16 . Only a first stage nozzle  18  of turbine  16  is shown in  FIG. 1 . In the exemplary embodiment, turbine  16  is drivingly coupled to compressor  12  with rotors (not shown) that are connected by a single common shaft (not shown). Compressor  12  pressurizes inlet air  20  which is then channeled to an array of combustors  14  (one shown) where it cools the combustor  14  and provides air to the combustion process. More specifically, air  22  channeled to combustor flows in a direction generally opposite to the flow of air through engine  10 . In the exemplary embodiment, gas turbine engine  10  includes a plurality of combustors  14  oriented circumferentially about engine casing  24 . More specifically, in the exemplary embodiment, combustors  14  are, for example, but are not limited to a so-called “canannular” arrangement of combustors. 
         [0015]    In the exemplary embodiment, engine  10  includes a double-walled transition duct  26 . More specifically, in the exemplary embodiment, transition duct  26  extends between an outlet end  28  of each combustor  14  and the inlet end  30  of turbine  16  to channel combustion gases  32  into turbine  16 . Further, in the exemplary embodiment, each combustor  14  includes a substantially cylindrical combustor casing  34 . Combustor casing  34  is coupled at an open aft end  36  to engine casing  24 . Combustor casing  34  may be coupled to engine casing  24  using, for example, but not limited to using, bolts  38 , mechanical fasteners (not shown), welding, and/or any other suitable coupling means that enables engine  10  to function as described herein. In the exemplary embodiment, a forward end  40  of combustor casing  34  is coupled to an end cover assembly  42 . End cover assembly  42  includes, for example, supply tubes, manifolds, valves for channeling gaseous fuel, liquid fuel, air and/or water to the combustor, and/or any other components that enable engine  10  to function as described herein. In the exemplary embodiment, the components within end cover assembly  42  are coupled to a control system  44  for controlling at least the air and fuel entering combustor  14 . Control system  44  may be, for example, but is not limited to a computer system and/or any other system that enables combustor  14  to function as described herein. 
         [0016]    In the exemplary embodiment, a substantially cylindrical flow sleeve  46  is coupled within combustor casing  34  such that the flow sleeve  46  is substantially concentrically aligned with casing  34 . Flow sleeve  46  is coupled at an aft end  48  to an outer wall  50  of transition duct  26  and coupled at a forward end  52  to combustor casing  34 . More specifically, in the exemplary embodiment, forward end  52  is coupled to combustor casing  34  by, for example, coupling a radial flange  54  of sleeve  46  to combustor casing  34  at a butt joint  56  such that a forward section  58  and an aft section  60  of casing  34  are coupled against each other. Alternatively, sleeve  46  may be coupled to casing  34  and/or transition duct  26  using any other suitable coupling assembly that enables engine  10  to function as described herein. 
         [0017]    Flow sleeve  46 , in the exemplary embodiment, includes a combustion liner  62  coupled therein. Combustion liner  62  is aligned substantially concentrically within flow sleeve  46  such that an aft end  64  is coupled to an inner wall  66  of transition duct  26 , and such that a forward end  68  is coupled to a combustion liner cap assembly  70 . Combustion liner cap assembly  70  is secured within combustor casing  34  by a plurality of struts  72  and an associated mounting assembly (not shown). In the exemplary embodiment, an air passage  74  is defined between liner  62  and flow sleeve  46 , and between transition duct inner and outer walls  66  and  50  and between cap inner barrel  73  and the inner wall of forward casing  58 . Transition duct outer wall  50  includes a plurality of apertures  76  formed therein that enable compressed air  20  from compressor  12  to enter air passage  74 . In the exemplary embodiment, air  22  flows in a direction opposite to a direction of flow  20  from compressor  12  towards end cover assembly  42 . 
         [0018]    Further, in the exemplary embodiment, combustor  14  also includes a plurality of spark plugs  78  and a plurality of cross-fire tubes  80 . Spark plugs  78  and cross-fire tubes extend through ports (not shown) in liner  62  that are defined downstream from combustion liner cap assembly  70  within a combustion zone  82 . Spark plugs  78  and cross-fire tubes  80  ignite fuel and air within each combustor  14  to create combustion gases  32 . 
         [0019]    In the exemplary embodiment, a plurality of fuel nozzle assemblies are coupled to end cover assembly  42 . More specifically, in the exemplary embodiment, combustor includes six nozzle assemblies, including five outer nozzle assemblies  84  arranged about a center nozzle assembly  85  the center of which lies on the longitudinal axis A of the combustor. Alternatively, combustor  14  may include more or less than five fuel nozzle assemblies  400 . In the exemplary embodiment, outer fuel nozzle assemblies are arranged in a generally circular array about the center nozzle  85  and the centerline A of combustor  14 , best seen in  FIG. 2 . Alternatively, fuel nozzle assemblies  400  may be arranged in a non-circular array. 
         [0020]    Further, in the exemplary embodiment, combustor  14  includes a plurality of fuel pegs  86  that extend radially into the air passage  74  from combustor casing  34 , and substantially circumscribe fuel nozzle assemblies  84 . The fuel pegs  86  are thus located upstream of the head end of the combustor, and thus upstream of the location where the air reverses direction and flows into the nozzle air inlet ends  87 . 
         [0021]    Referring now to  FIG. 3 , there are shown a plurality of the quaternary fuel pegs  86  extending radially into the air passage  74  at circumferentially spaced locations. There may be as many as  16  or more pegs, each of which is of substantially symmetrically, airfoil shaped, with the leading edge facing upstream, i.e., in a direction opposite the flow of air in the passage  74 . Each fuel peg  86  may be formed with a pair of fuel delivery orifices  88  on each side of the peg. The orifices may be aligned radially, as shown in  FIG. 3 , such that fuel emitted from the orifices  88  flows into the passage  74  from each side of the peg, in directions transverse to the flow of air. 
         [0022]    In the exemplary but nonlimiting embodiment, and with additional references to  FIG. 4 , a lean pre-nozzle fuel injection diffuser (also referred to as a flame holding inhibitor or vortex generator)  90  is located upstream (but proximate to) each of the fuel pegs  86 . Since the flame holding inhibitors are substantially identical, only one need be described in detail. With particular reference to  FIG. 4 , the flame holding inhibitor  90  may be constructed from sheet metal and includes at least one and preferably two substantially identical, radially aligned triangle plates, or delta wings,  92 ,  94  that are angled toward each other such that the sharp leading ends  96 ,  98  nearly touch, while the blunt or wider trailing ends  100 ,  102 , are radially spaced. The delta wings  92 ,  94  are cut (by laser cutting, for example) and bent from a single piece of plate stock  104  which forms an upstanding support for the delta wings. More specifically, a horizontal cut is made in the plate, extending from a hole  106  located between fore and aft edges  108 ,  110  of the plate that facilitates the cutting and bending process. Vertical (or radial) cuts along and within the thickness of the plate allow flaps of material to be the “peeled back” and bent in opposite directions to form the delta wings  92 ,  94 . The radial distance between the delta wings at the trailing edge, is determined by the angle of divergence therebetween, is dependent on the location of the fuel delivery orifices  88  in the downstream adjacent fuel peg  86 . An additional cut at the lower for radially inner) end of the plate allows the bending of two additional flaps  112 ,  114  to be bent in opposite directions to form a base  116  by which the flame holding inhibitor is attached to the combustor liner  62  or cap inner barrel  73  by, for example, welding or other suitable means. Note that the radially outer edges  118  of the flame holding inhibitors need not extend to the flow sleeve. More important is the location of the delta wings  92 ,  94  relative to the fuel delivery orifices  88  as explained further below. 
         [0023]    In an alternative arrangement, the inhibitors  90  may be rotated 180° so as to face in an opposite direction relative to the orientation of  FIG. 3 . In other words, for this alternative arrangement, the pointed or sharp leading ends  96 ,  98  of the inhibitor  90  will face in the downstream direction. Further adjustment of the location of the fuel delivery orifices  88  may be required to optimize air/fuel mixing and to prevent fuel from stagnating in the center of the vortex created by the inhibitor. 
         [0024]    it will be appreciated that the flame holding inhibitor  90  may also be formed in other ways and may include more than one component part. As noted above, for example, the inhibitor  90  may be formed with one, rather than a pair of delta wings. 
         [0025]    Installed as shown in  FIG. 3 , the delta wings  92 ,  94  point in the upstream direction, (i.e., with the pointed or sharp leading ends  96 ,  98  facing upstream) and as noted above, the location of the leading ends  96 ,  98  of the delta wings  92 ,  94  can be adjusted relative to the location of the fuel delivery orifices  88  in the fuel pegs  86  to achieve optimum fuel/air mixing and minimization if not elimination of the flow separation zone adherent to the peg, as described further below. 
         [0026]      FIG. 5  is a schematic diagram of a fuel peg  86  and the flow of air in the passage  74  impinging on the leading edge  118  at an angle of attack of about 20°. With no flame inhibitor in place, the flow separates along the trailing edge portion of the peg, creating a flow separation zone in a wake region or “bubble” area  120  that distorts and traps the fuel mass fraction on the surface of the peg. Should an unintended flame event occur, the locally rich fuel/air mixture flame in the bubble area could be anchored or held on the peg.  FIG. 6  is a similar view but shows the modification of the flow across the peg  86  when a flame holding inhibitor  90  is installed upstream of the peg  86 . Now, the flow separation zone in the wake region or bubble area  120  is essentially eliminated by the secondary flows or vortices generated by the delta wings  92 ,  94  of the flame holding inhibitor  90 . In addition, because the fuel entering the path  74  from the fuel delivery orifices  88  is aligned with the incoming flow generated by the delta wings  92 ,  94 , the local fuel-rich stream is washed away with enhanced local fuel/air mixing. 
         [0027]    As incoming air flows by the inhibitors, secondary flow (flows on the planes normal to the bulk flow direction) forms vortices, eliminates the wake regions and enhances local mixing. 
         [0028]    It will be appreciated therefore that the benefits of flame holding inhibitor as described herein are twofold: 1. The flame holding margin of existing quaternary fuel pegs can be improved through the elimination of the near-peg flow separation zone; and 2. efficient fuel/air mixing is boosted, providing the potential for further reductions in NOx emissions by mixing a large fraction of total fuel with incoming air upstream of the combustor fuel nozzles. 
         [0029]    It will also be appreciated that the flame holding inhibitor design could also be utilized elsewhere, for example, in the jet mixing zone of the combustor, and that the inhibitor may be of other shapes that perform in similar manner to achieve similar results. 
         [0030]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.