Abstract:
A fuel nozzle having a dedicated circuit to cool the diffusion tip with lower part count and reduced complexity. More specifically, the proposed design uses an independent circuit to cool the tip with diffusion fuel or purge air. An impingement plate may be provided to augment the cooling effect.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a diffusion tip for a fuel nozzle for use in gas turbines. More particularly, the invention relates to a diffusion tip configuration and adaptations for cooling the same. 
     In a gas turbine, fuel nozzles are used to mix air and fuel for later combustion downstream. A diffusion mode is used for stable combustion during start up until premixed mode can be used to reduce NOx emissions. The diffusion tip of the nozzle must provide a mechanism for generating the diffusion flame during start up and remain cool enough to resist damage from hot combustion gases during premixed mode. Current designs use air diverted from the main path to cool the diffusion tip resulting in an uncertain air proportion of cooling versus main flow and a complicated flow path. 
     More specifically, a conventional diffusion tip  10  is illustrated in  FIG. 1 . As illustrated therein, the current design splits curtain air  12  into burner tube cooling air  14 , diffusion air  16  and shower head air  18 . As understood from the phantom line passage depiction at  20 , the diffusion purge does not flow to the shower head portion  22 . The flow split and therefore effective cooling in the three circuits  14 ,  16 ,  18  can vary based on input conditions and the cooling of the tip (from shower head air effusion cooling) cannot be independently modified. As illustrated, the configuration of  FIG. 1  uses a plurality of holes  24  to accomplish the effusion cooling in the diffusion tip. For instances of high thermal and/or structural loading, these holes can act as stress intensification sites, reducing life to crack initiation. In addition, these holes may allow combustion gas entry into the diffusion cooling circuit if the pressure of the combustion gas is locally higher than the pressure in the diffusion cooling circuit. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention proposes to use a dedicated circuit to cool the diffusion tip with lower part count and reduced complexity. More specifically, the proposed design uses an independent circuit to cool the tip with diffusion fuel or purge air. An impingement plate may be provided to augment the cooling effect. Thus, the invention may be embodied in a fuel nozzle, comprising: a burner tube component; a center body assembly concentrically disposed within said burner tube component; a premix flow passage defined between said burner tube component and said nozzle center body; a diffusion tip, said diffusion tip comprising a peripheral wall mounted to said center body assembly, a substantially imperforate end wall at a distal axial end of said peripheral wall, at least one orifice defined in said peripheral wall adjacent said axial end wall, and a diffusion tip shroud disposed in surrounding relation to said peripheral wall and mounted to said center body so as to define a cooling air flow passage therebetween, said at least one orifice being in flow communication with at least one of said cooling air flow passage and a recirculation zone downstream of said diffusion tip; and a diffusion fuel passage defined within said center body assembly and terminating distally at an inner surface of said substantially imperforate end wall. 
     The invention may also be embodied in a diffusion tip for a fuel nozzle, comprising: a peripheral wall, a substantially imperforate end wall at a distal axial end of said peripheral wall, at least one orifice defined in said peripheral wall adjacent said axial end wall, and a diffusion tip shroud disposed in surrounding relation to said peripheral wall so as to define a cooling air flow passage therebetween, said at least one orifice being in flow communication with at least one of said cooling air flow passage and a recirculation zone downstream of said diffusion tip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred example embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a conventional diffusion tip; 
         FIG. 2  is a schematic cross-sectional view of a diffusion tip embodying the invention; 
         FIG. 3  is an exploded perspective view of a diffusion tip and shroud; 
         FIG. 4  is an enlarged perspective view illustrating assembly of the distal end of the shroud to the diffusion tip; and 
         FIG. 5  is an enlarged perspective view illustrating the assembled shroud and diffusion tip assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an assembly of machined and cast parts that allow injection of fuel into the gas turbine during diffusion operation. During premix operation, the unique arrangement of features of the inventive diffusion tip allows it to be effectively cooled and thus maintain a high level of reliability. 
     Referring to  FIG. 2 , as compared to  FIG. 1 , the plurality of holes conventionally provided to accomplish effusion cooling of the diffusion tip are omitted according to the invention so that the proposed design does not have such holes as a source of stress intensification, and backflow is substantially precluded. Instead, the central portion  122  of the diffusion tip  110  is imperforate and orifices  124  are provided to flow diffusion purge air or diffusion fuel, according to nozzle operation, to join the curtain air flowing initially at  112  within the diffusion tip shroud  128  and at  116  at the diffusion tip. It should be noted that the orifices  124  are essentially the same as the orifices provided in the structure of  FIG. 1  for the diffusion purge to join the curtain air at  16 . 
     In the illustrated example embodiment, an impingement plate  130  is mounted in spaced parallel relation to the imperforate central portion  122  of the end wall of the diffusion tip  110 . The impingement plate  130  comprises one or more impingement orifices  132  for impingement flow of e.g., diffusion purge air toward and against the inner surface of the central portion  122 . 
     As also illustrated in  FIG. 2 , this example embodiment has a cooling enhancement feature in the impingement-cooled diffusion tip. More specifically, a rippled, wave-like back side surface is provided as illustrated at  134 . This feature enhances cooling by increasing the surface area of the back side surface and/or turbulates the post-impingement coolant flow. Rather than the rippled wave-like back side illustrated, cooling may be enhanced by ribs, fins, pins or the like. As noted above, a plurality of orifices  124  are defined peripherally of the impingement cooled inner surface for diffusion purge to join the curtain air flowing concentrically thereto. 
     In an example embodiment, a layer of thermal barrier coating  136  is also added to the front face of the diffusion tip as schematically illustrated in  FIG. 2 . A class B TBC coating protects the tip from temperature gradients and increases back side cooling effectiveness. 
     The conventional design illustrated in  FIG. 1  consists of three parts machined from Hast-X bar stock, then brazed together. The invention illustrated, e.g., in  FIG. 2  uses only one part machined from Hast-X bar stock and uses a single full penetration weld instead of multiple brazes. In this way, a diffusion tip assembly embodying the invention reduces parts and braze joints, and allows swirl holes to have fillets. 
     As will be understood, the simplified diffusion tip design and flow paths provided according to the invention as illustrated in the example embodiment of  FIG. 2  gives the same flow geometry as the current diffusion tip design for diffusion operation. However, rather than devoting a portion of the curtain air to flow through a perforated diffusion tip end face as in the  FIG. 1  design, the tip end face is impingement cooled on the backside with diffusion purge air during premix and all curtain air  112  is flowed for diffusion  116  and burner tube cooling  114 . The diffusion tip design also uses diffusion fuel to back side cool the diffusion tip so that diffusion mode and piloted premix metal average temperature is very cool, e.g., only 100° F. hotter than diffusion fuel temperature. 
     According to a further feature of the invention, the shroud  128  and tip redundantly retain each other forward and aft. More specifically,  FIG. 3  illustrates the shroud exploded away from the remainder of the diffusion tip. According to the retention feature, a plurality of wedges  160  are defined adjacent but spaced from the distal end of the shroud  128 . Although a plurality of wedges are included in the illustrated embodiment, manufacturing optimization will likely result in fewer wedges than shown, perhaps 3 to 6 on the full 360 degree part. As illustrated, the periphery of the distal end  122  of the diffusion tip has a plurality of grooves  162  defined therein and the wedges  160  are spaced to slide through the respective groove when the shroud is telescopingly received on the diffusion tip as illustrated in  FIG. 4 . Once the shroud is fully inserted to engage the nozzle, as illustrated in  FIG. 5 , the wedges are disposed just forward of the outer periphery of tip end  122 . Rotation of the shroud as shown by arrow R then displaces the wedges  160  with respect to the grooves  162  so as to be aligned with the diffusion tip structure to provide forward retention. Meanwhile, in this example embodiment, the distal end of the shroud is wedged as at  164  to provide aft retention. The parts are then brazed at their forward interface  166 . 
     The diffusion tip  110  embodying the invention is not dependent upon particulars of the design of the balance of the fuel nozzle and, thus, may be incorporated in any of a variety of fuel nozzles of the type including a burner tube, a center body assembly concentrically disposed within the burner tube, a premix flow passage defined between the burner tube and the nozzle center body, and a diffusion fuel passage defined within the center body. In an example embodiment, the diffusion tip may be provided in a fuel nozzle of the type illustrated in U.S. Pat. No. 6,438,961, the disclosure of which is incorporated herein by this reference. 
     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.