Patent Publication Number: US-8528338-B2

Title: Method for operating an air-staged diffusion nozzle

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to gas turbines and more specifically to air-staged diffusion nozzles for gas turbine combustors. 
     In a diffusion nozzle for a gas turbine combustor, the fuel begins mixing with air in swirl vanes and then flows and expands in a swirling motion within the burner tube space of the combustor for mixing. In the current diffusion nozzles, a low velocity region was observed in the burner tube at the center of diffusion nozzle. High carbon formation on the diffusion nozzle tip has been identified during the start up as well as part load operation. For a highly reactive fuel, higher temperature is observed on the nozzle tip due to proximity of the flame. Further, enhanced mixing of the gas-fuel and air in the burner tube can result in reduced emissions from the gas turbine. 
     Accordingly, there is a need for a diffusion nozzle with a gas-fuel that provides for cooling of the nozzle tip while at the same time improving mixing of the fuel and air. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates to an air-staged nozzle. Briefly in accordance with one aspect of the present invention an embodiment is provided for an air-staged diffusion nozzle disposed in a combustor of a gas turbine, including a gas-fuel source and a compressed air source where the gas-fuel nozzle discharges to a burner tube space of the combustor. The air-staged diffusion nozzle includes a nozzle body disposed along a longitudinal axis including a gas-fuel cavity, bounded downstream by an end closure wall, bounded upstream by a connection to a gas-fuel source, and bounded peripherally by an annular wall. An outer swirler with swirl vanes extends from a tip end of the annular wall forming a swirled axial passage to a downstream burner tube space. A space external to the annular wall of the gas-fuel cavity includes a compressed air source in fluid communication with the swirled axial passage of the outer swirler. Passages are provided for gas-fuel through the first annular wall from the gas-fuel cavity into the swirled axial annular passage of the outer swirler. The outer swirler delivers a swirling mixture of a gas-fuel and the compressed air to the downstream burner tube space of the combustor. A cooling air chamber is enclosed within the gas-fuel cavity and is surrounded with an outer peripheral wall. A portion of the outer peripheral wall, disposed in proximity to the downstream end of the gas-fuel cavity, extends axially through the end closure wall to the burner tube space of the combustor. Passages through the annular wall of the gas-fuel cavity from the external compressed air space are coupled in fluid communication with the cooling air chamber. Passages fluidly communicate compressed air through the downstream end of the peripheral wall of the cooling air chamber to the burner tube space of the combustor, providing cooling air for the tip and enhancing mixing of the gas-fuel and air in the burner tube space. 
     According to another aspect of the present invention, a gas turbine combustor is provided including a compressor, a turbine, combustors, and air-staged diffusion nozzles with a gas-fuel source and a compressed air source wherein the air-staged diffusion nozzle discharges to a burner tube space of the combustor. The air-staged diffusion nozzle includes a nozzle body disposed along a longitudinal axis including a gas-fuel cavity, bounded downstream by an end closure wall, bounded upstream by a connection to a gas-fuel source, and bounded peripherally by an annular wall. An outer swirler with swirl vanes extends from a tip end of the annular wall, forming a swirled axial passage to a downstream burner tube space. 
     A space external to the annular wall of the gas-fuel cavity includes a compressed air source in fluid communication with the swirled axial passage of the outer swirler and a plurality of passages through the first annular wall from the gas-fuel cavity into the swirled axial annular passage of the outer swirler. The outer swirler delivers a swirling mixture of a gas-fuel and the compressed air to the downstream burner tube space of the combustor. A cooling air chamber enclosed within the gas-fuel cavity includes an outer peripheral wall. The outer peripheral wall is disposed in proximity to the downstream end of the gas-fuel cavity extending axially through the end closure wall to the burner tube space of the combustor. Multiple passages through the annular wall from the external compressed air space are coupled fluidly with the cooling air chamber. Multiple passages fluidly couple compressed air through the downstream end of the peripheral wall of the cooling air chamber to the burner tube space of the combustor. 
     A further aspect of the present invention provides a method for cooling a tip end of gas-fuel air-staged diffusion nozzle disposed in a combustor of a gas turbine with a compressor and a turbine, where the nozzle is upstream from a burner tube of the combustor. The method includes providing a gas-fuel air-staged diffusion nozzle including a nozzle body with a gas-fuel cavity bounded by an outer peripheral wall disposed along a longitudinal axis of the nozzle; an end closure wall; a cooling air chamber disposed within the gas-fuel cavity; an outer swirler supplied by gas-fuel from the gas-fuel cavity and compressed air from an external space surrounding the nozzle body; and a forward projection of the cooling air chamber, extending through the peripheral wall within and projecting through an end closure wall of the nozzle body. The method further includes supplying gas-fuel to the gas-fuel cavity from an upstream gas-fuel source. Gas-fuel is diverted to flow through gas-fuel injection holes defined about a periphery of the end closure wall into swirl passages of the outer swirler. The gas-fuel is mixed with compressed air from the external space within the outer swirler and discharged with a rotational direction into a burner tube space downstream from the end closure wall of the nozzle body. The method further includes diverting compressed air from the external space surrounding the nozzle body through the cooling air chamber to the burner tube space downstream from the end closure wall of the nozzle body, promoting cooling of the nozzle tip and mixing of the gas-fuel and air in the burner tube space. 
     Yet another aspect of the present invention provides a method of operating a gas-fuel air-staged diffusion nozzle disposed in a combustor of a gas turbine with a compressor and a turbine, where the nozzle is upstream from a burner tube of the combustor. The method includes providing a gas-fuel air-staged diffusion nozzle comprising a nozzle body including a gas-fuel cavity bounded by an outer peripheral wall disposed along a longitudinal axis of the nozzle; an end closure wall; a cooling air chamber disposed within the gas-fuel cavity; an outer swirler supplied by gas-fuel from the gas-fuel cavity and compressed air from an external space surrounding the nozzle body; and an inner swirler at a downstream end of the nozzle. The method includes supplying a gas-fuel to the gas-fuel cavity from an upstream gas-fuel source. The gas-fuel is diverted to flow through gas injection holes defined about a periphery of the end closure wall into swirl passages of the outer swirler. The gas-fuel is mixed with compressed air from the external space entering within the outer swirler and discharged from the outer swirler with a rotational direction into a burner tube space downstream from the nozzle body. The method also includes diverting compressed air from the external space surrounding the nozzle body into the cooling air chamber. The method further includes swirling the compressed air in the cooling air chamber through an inner swirler at a center of the tip end of the nozzle into the burner tube space downstream from nozzle, thereby cooling the tip of the nozzle and enhancing mixing of the gas-fuel and air mixture in the burner tube space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       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  illustrates an isometric cutaway view of an embodiment of an inventive air-staged diffusion gas nozzle; 
         FIG. 2  illustrates an expanded cutaway side view illustrating cooling air flow through a swirler at the tip of an embodiment of an inventive air-staged diffusion gas nozzle; 
         FIG. 3  illustrates an external view of the tip of end of an embodiment of the air-staged diffusion nozzle; 
         FIG. 4  illustrates an isometric view of an embodiment for the cooling air chamber of the air-staged diffusion nozzle; 
         FIG. 5  illustrates an expanded view illustrating cooling air flow through cooling holes in an embodiment of cooling holes at the tip end of the inventive air-staged diffusion nozzle; 
         FIG. 6  illustrates an expanded view illustrating an alternative cooling air flow path through cooling holes at the tip end of the inventive air-staged diffusion nozzle providing a radial component to the discharge flow; 
         FIG. 7  illustrates an embodiment of the inventive air-staged diffusion nozzle with a burner tube; and 
         FIG. 8  illustrates a combustor for a gas turbine including embodiments of the inventive air-staged diffusion fuel nozzles organized around a center secondary fuel nozzle; 
         FIG. 9  illustrates the circular arrangement of the air-staged diffusion nozzles with outer swirler and inner swirler on end cover assembly fed from gas-fuel piping; and 
         FIG. 10  illustrates a flowchart for a method for cooling a tip of an air-staged diffusion nozzle for a gas turbine combustor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following embodiments of the present invention of an air-staged gas diffusion nozzle for a gas turbine combustor have many advantages, including enhancing the mixing of gas-fuel and air, thereby reducing gas turbine emissions and also reducing soot formation during startup. The air-staged diffusion nozzle will also extract air from the main air flow path and introduce an air flow at the center of the nozzle tip with a swirl. For highly reactive fuels in particular, elevated temperature is observed on the nozzle tip due to proximity of the flame. The introduction of this bypassed air will cool the nozzle tip, forming a film of cold air between the surface of the nozzle tip and the hot gases in the downstream burner tube. The air flow leaving the nozzle tip and the swirl motion imparted to the air flow acts to enhance the mixing of the gas-fuel with air. The inventive arrangement is desirable for Dry Low NOx (DLN) combustors with multiple diffusion nozzles and may also be used advantageously on single nozzle combustors. 
       FIG. 1  illustrates a cutaway isometric view of an embodiment for an inventive air-staged diffusion nozzle for a combustor of a gas turbine. The air-staged diffusion nozzle  100  may include a frusto-conical nozzle body  110  on a longitudinal axis  111 , bounded with a peripheral wall  115  and a downstream end closure wall  125  defining a gas-fuel cavity  130  within the nozzle body. The peripheral wall  115  may taper down in diameter from an upstream end  112  to a downstream tip end  113 . A gas-fuel source  120  is provided from the upstream end  112  supplying the gas-fuel cavity  130 . Compressed air  135  may be externally supplied from an external space  136  radially outward from the peripheral wall  115  and enclosed within the combustor ( FIG. 8 ). The compressed air  135  may be supplied by discharge air from the gas turbine air compressor ( FIG. 8 ). Swirl vanes  141  of outer swirler  140  may extend radially outward and downstream from the end closure wall  125  of the nozzle body  110  defining flow passages  142  to a downstream burner tube space  145 . A plurality of gas-fuel passages  150  may penetrate through the peripheral wall  115  to supply gas-fuel  151  from the gas-fuel cavity  130  into each of the passages  142  between the swirl vanes  141 . Gas-fuel flow and compressed air flow through each of the swirl vanes  141  initiate a swirling mix  143  of the gas-fuel and the compressed air that continues with the gas-fuel-compressed air mixture swirling in the burner tube  145  downstream from the nozzle  100 . 
     A cooling air chamber  160  may be provided within the downstream end of the gas-fuel cavity  130  in proximity to end closure wall  125 . The cooling air chamber  160  may include a peripheral wall  161  including a projecting portion  162  extending downstream through a center portion of the end closure wall  125  around the longitudinal axis  111 . The peripheral wall  161  may be generally cylindrical along the longitudinal axis and closed on the upstream end. The projecting portion  162  may be frusto-conical, with sidewalls  172  tapering at the downstream end. The projecting portion  162  may be formed integral to the end closure wall  125 . 
     The cooling air chamber  160  may be in flow communication with the external space  136  of compressed air  135 . The flow communication path  165  may include corresponding penetrations  116  of the peripheral wall  115  and penetrations  164  of the cooling air chamber  160  interconnected with hollow tubing members  170 . The number and size of penetrations  116 ,  164  and the number and diameter  171  of corresponding hollow tubing members  170  may be arranged to provide a sufficient volume of compressed air to the cooling air chamber  160  to supply needs for cooling the tip of the nozzle, limiting impingement of hot gases from the downstream burner tube space  145  onto the downstream surface of nozzle tip end  113 , and promoting mixing within the downstream burner tube space  145 . The hollow tubing  170  may be arranged radially between the peripheral wall  115  of the nozzle body  110  and the peripheral wall  161  of the cooling air chamber  160 . The hollow tubing  170  may also be arranged in circumferential symmetry around cooling air chamber  160 . 
     The downstream face  163  of the projecting portion  162  of cooling air cavity  160  may form a continuous flush surface with a downstream face  126  of the end closure wall  125 . The projecting portion  162  may include a plurality of cooling flow passages  165  between the inner face  166  and downstream face  163 . The cooling passages  165  may be arranged as an inner swirler  180  to provide discharges  195  forming rotational swirl of compressed air from the downstream face  163  into the burner tube  145 , as will be described in greater detail. 
       FIG. 2  illustrates a cross-sectional cutaway view of the air-staged diffusion nozzle.  FIG. 3  illustrates an external view of the tip of end of the air-staged diffusion nozzle. More specifically, the passages  165  may be arranged within an inner swirler  180  between swirl vanes  181  that impart a discharge velocity  195  to the compressed air discharging into the burner tube  145 . The discharge velocity  195  may include an axial velocity  183  and a circumferential velocity  184 . The swirl vanes  181  and passages  165  to the burner tube may be arranged to impart a circumferential (rotational) velocity  184  in the same rotational direction  196  or in an opposite rotational direction  197  as that rotational direction  144  imparted to the gas-fuel mixture by the outer swirler  140 . The rotational direction of the compressed air flow through projecting portion  162  relative to the rotational direction of the gas-air mixture from the outer swirler will influence mixing of the gas-fuel and air in the burner tube. The discharging air also tends to cool the tip and will form a thin film of cool air  190  on downstream surface  163 . Further, the axial component  183  of velocity of the compressed air entering the burner tube  145  may discourage the rotational flow of hot gases in the burner tube impinging on the nozzle tip. The swirl vanes  181  may further be formed to add a radial velocity component  186  to the gas-air mixture, further influencing mixing within the burner tube space. 
     Consequently, the volume of compressed air flow, the axial velocity of compressed air flow, the rotational velocity of compressed air flow, and the rotational direction of compressed air flow relative to the rotational flow of the fuel-air mixture from the outer swirler provide adjustable design parameters that improve mixing of fuel and air in the burner tube, thereby promoting reduced emissions and reduced soot formation in startup. Further by creating a cool air film and forcing the rotational flow of hot gases away from the tip of the nozzle, the compressed air flow will cool the tip of the nozzle. 
       FIG. 4  illustrates an isometric view of an embodiment for the cooling air chamber  160  of the air-staged diffusion nozzle. The cooling air chamber  160  includes a peripheral wall  161  forming a generally cylindrical body closed on the upstream end  177  around cooling air cavity within. A projecting portion  162  of frusto-conical shape extends downstream including an inner swirler  180  for the nozzle (not shown) at the downstream end. A plurality of tube members  170  for receiving compressed air into the cooling air cavity  178  extend radially from the peripheral wall  161 , preferably in a symmetrical arrangement. The inner diameter  171  of the tubes may be established to provide a sufficient volume of compressed air for the inner swirler  180 . The inner swirler  180  may include a plurality of swirl passages  165  that discharge through downstream surface  163  and whose arrangement and flow properties were previously described. The number, shape, size and orientation of the swirl passages  165  may be selected to provide an appropriate volume and flow of compressed air for promoting cooling and mixing in the burner tube space. 
       FIG. 4  illustrates the downstream face  163  of the projecting portion  162  of an embodiment for the cooling air chamber  160  of the air-staged diffusion nozzle, including tip cooling holes  187 . The tip cooling holes  180  may form a circular pattern on the downstream face  163  and on the inner face  166  ( FIG. 3 ) of wall  163  the projecting portion  162  of the cooling air cavity  160 . The circular patterns of tip cooling holes on the respective faces  163 ,  166  may be angularly displaced with respect to the longitudinal axis  111  defining a passage  192  through the projecting portion  152  such that the discharge  193  from the downstream face  163  will include both an axial flow component  198  and a circumferential flow component  199 . The angular displacement of the tip cooling holes  180  on respective sides may be alternatively arranged, allowing the circumferential flow component to be reversed, thus allowing the circumferential flow to be in a same rotational direction  196  or an opposed rotational direction  197  to that created by the outer swirler  140  ( FIG. 4 ). Further as shown in  FIG. 5 , the tip holes  180  may further be arranged to provide a radial displacement between the inner face  166  ( FIG. 3 ) and the downstream face  163  of the downstream wall adding a radial flow component exiting the downstream face  163 . While a circular configuration of holes has been illustrated, it should be understood that alternative patterns, shapes, sizes and numbers of holes and discharge direction promoting gas-fuel with air mixing in the burner tube and cooling of the nozzle tip should be considered within the spirit of the present invention. 
       FIG. 7  illustrates an expanded view for an embodiment of the inventive air-staged diffusion nozzle with a burner tube. The nozzle  100  receives a gas-fuel from gas-fuel source  112  mounted at upstream end the nozzle body  110  through ports  117  of fuel plate  114 . Compressed air is provided at the nozzle body  110  through external space  136 . The compressed air passes through the peripheral wall penetrations  164  and then through tube members  170  to the cooling air chamber  160 , and past swirler wall extension  148  to outer swirler  140 . The burner tube  146  is joined to the nozzle body  110  at nozzle body-burner tube joint  147 . Gas-fuel and air mixture  143  from flow passages  142  of outer swirler  140  discharge into burner tube space  145  with rotational swirl and downstream velocity. Compressed air flows through cooling air chamber  160  through swirl passages  165  of inner swirler  180  into burner tube space  145  of burner tube  146  with rotational swirl. The rotational swirl of the flow from the inner swirler passages  180  into the burner tube space  145  may be in the same rotational direction or an opposed rotational direction to the swirl from the outer swirler  140 . 
       FIG. 8  illustrates a cutaway view of an embodiment for a dry-low NOx (DLN) combustor for a gas turbine  300  that includes the inventive air-staged diffusion nozzle  100 . The combustor also includes a compressor  312  (partially shown), a plurality of combustors  314  (one shown for convenience and clarity), and a turbine  316  (represented by a single blade). Although not specifically shown, the turbine  316  is drivingly connected to the compressor  312  along a common axis. The compressor  312  pressurizes inlet air, which is then reverse flowed to the combustor  314  where it is used to cool the combustor  314  and to provide air to the combustion process. Although only one combustor  314  is shown, the gas turbine  300  includes a plurality of combustors  314  located about the periphery thereof. A transition duct  318  connects the outlet end of each combustor  314  with the inlet end of the turbine  316  to deliver the hot products of combustion to the turbine  316 . 
     Each combustor  314  includes a substantially cylindrical combustion casing  324  which is secured at an open forward end to a turbine casing  326  by means of bolts  328 . The rearward end of the combustion casing  324  is closed by an end cover assembly  330  which may include conventional supply tubes, manifolds and associated valves, etc. for feeding gas, liquid fuel and air (and water if desired) to the combustor  14 . Gas-fuel manifold  350  may supply gas-fuel for the air-staged diffusion nozzle  100 . The end cover assembly  330  receives a plurality (for example, six) of the inventive air-staged diffusion nozzle assemblies  100  (only one shown for purposes of convenience and clarity) arranged in a circular array about a longitudinal axis  331  of the combustor  314 .  FIG. 9  illustrates the circular arrangement of the air-staged diffusion nozzles  100  with outer swirler  140  and inner swirler  180 , where the nozzles are mounted on end cover assembly  330  and fed from gas-fuel piping  350 . 
     Again referring to  FIG. 8 , a secondary fuel nozzle  380  may be mounted at in a centerbody  381 . Each air-staged fuel nozzle  100  is supplied gas-fuel  120  from rearward supply section  352  and delivers a swirled gas and air mixture to burner tube space  145 . 
     Within the combustion casing  324 , there is mounted, in substantially concentric relation thereto, a substantially cylindrical flow sleeve  334  which connects at its forward end to the outer wall  336  of the transition duct  318 . The flow sleeve  334  is connected at its rearward end to the combustion casing  324  where fore and aft sections of the combustor casing  324  are joined. 
     Within the flow sleeve  334 , there is a concentrically arranged combustion liner  338 , which is connected at its forward end with the inner wall  340  of the transition duct  318 . The rearward end of the combustion liner  38  is supported by a combustion liner cap assembly  342 , which is, in turn, supported within the combustion casing  324 . It will be appreciated that the outer wall  336  of the transition duct  318 , as well as that portion of flow sleeve  334  extending forward of the location where the combustion casing  324  is bolted to the turbine casing  326 , may be formed with an array of apertures  344  over their respective peripheral surfaces to permit air to reverse flow from the compressor  312  through the apertures  344  into the annular space between the flow sleeve  334  and the liner  338  toward the upstream or rearward end of the combustor  314  (as indicated by the flow arrows  370 ). 
     The arrangement is such that air flowing in the annular space between the liner  338  and the flow sleeve  334  is forced to again reverse direction in the rearward end of the combustor  314  and to flow (See  FIG. 1 ) into space  136  external to the air-staged diffusion nozzle  100 , where it is made available for the outer swirler  140  of the nozzle and to the cooling air cavity  160  to flow through the inner swirlers  180 , and burner tube space  145 , before entering the burning zone or combustion chamber  390 . 
     For prior art diffusion nozzles with only an outer swirler, a recirculation bubble of hot gases may be formed within the burner tube and premixing tubes in response to the swirling fuel-air swirl mixture being discharged from the outer around an outer periphery of the burner tube. This downstream flow of fuel-air mixture encourages a circulation of hot gases from downstream to flow upstream along a center area of the burner tube, thereby bringing the hot gas into proximity of the nozzle tip end. The flow heats the tip end of the nozzle and promotes soot buildup on the tip end of the nozzle during startup and low power operation. With the swirled air from the inner swirler of the inventive air-staged nozzle, the stagnant recirculating hot gas is forced back and away from the tip end. Further, the flow of cool air through the tip end encourages a film of cool air on the tip. 
     The flow of air from the inner swirler further reduces the fuel mass fraction near the tip end of the nozzle, promoting a uniform unmixed profile with the air-staged nozzle. The low velocity region occurring in the center of the tip end in prior art is altered, as described above, by the swirling discharge of the inner swirler. A high axial velocity at the periphery of the burner tube is also reduced with the air-staged nozzle by due to the inner swirler. Further, the fuel mass fraction becomes more uniform at the burner tube exit relative to prior art and the unmixedness is reduced at the burner tube exit. Here, the improved mixing positively impacts emissions from the gas turbine. 
     According to another aspect of the present invention, a method is provided for cooling the tip end of an air-staged diffusion nozzle disposed in a combustor of gas turbine with a compressor and turbine, where the nozzle is disposed upstream from a burner tube of the combustor.  FIG. 10  illustrates a flowchart for the method for cooling the nozzle tip of the air-staged diffusion nozzle and mixing gas-fuel and air in burner tube section. 
     Step  410  provides a gas-fuel air-staged diffusion nozzle where the nozzle includes a nozzle body including a gas-fuel cavity bounded by an outer peripheral wall disposed along a longitudinal axis of the nozzle; an end closure wall, a cooling air chamber disposed within the gas-fuel cavity; an outer swirler supplied by gas-fuel from the gas-fuel cavity and compressed air from an external space surrounding the nozzle body; and a forward projection of the cooling air chamber, extending through the peripheral wall within and projecting through an end closure wall of the central fuel chamber. Step  415  supplies a gas-fuel to the gas-fuel cavity from an upstream gas-fuel source. Step  420  diverts gas-fuel to flow through gas injection holes defined about a periphery of the end closure wall into swirl passages of the outer swirler. Step  425  mixes the gas-fuel with compressed air from the external space within the outer swirler. Step  430  discharges the swirled gas-fuel and compressed air with a rotational direction into a burner tube space downstream from the end closure wall of the nozzle body. 
     In step  440  the step of diverting the compressed air provides for flowing compressed air from the external space to the cooling air chamber with tubes fluidly connected through the outer peripheral wall of the gas-fuel cavity to the compressed air chamber and fluidly connected through a peripheral wall of the cooling air chamber to a cooling air cavity within. The sizing of the tubes and penetrations through the peripheral walls of the nozzle body and cooling air chamber may be established to provide sufficient compressed air flow for cooling a tip of the nozzle. The sizing of the tubes and penetrations through the peripheral walls of the nozzle body and cooling air chamber may further be established to provide sufficient compressed air flow for promoting mixing of swirled gas-fuel and air within the burner space from the outer swirler. In step  445 , diverting the compressed air may further include passing the compressed air through an inner swirler on a forward projection of the peripheral wall of the cooling air chamber on a tip of the nozzle to a space of the burner tube downsteam from the nozzle. Here, sizing of the swirl vane passages and orienting the swirl vane passages are arranged for cooling of the nozzle tip. The sizing of the swirl vane passages and orienting the swirl vane passages may be arranged for mixing of gas-fuel and air within the burner tube space. Step  450  provides alternately for flowing the compressed air through a plurality of tip holes within the forward projection of the peripheral wall of the cooling air chamber. Here, the sizing the of the tip holes and orientation of the tip holes may be arranged for cooling of the nozzle tip or promoting mixing of gas-fuel and air within the burner space or for both functions. 
     The method may include other arrangements of swirl vanes and tip holes and may further include combinations of swirl vanes and tip holes. A discharge of the compressed air from the nozzle tip may apply a downstream axial velocity and a rotational velocity to the compressed air relative to the longitudinal axis of the nozzle. The rotational velocity applied to the compressed discharged from the nozzle tip air, in step  460 , may be in a same direction as a direction of swirl from the outer swirler or for step  465  in an opposite direction to a direction of swirl from the outer swirler. In step  470 , the discharge provides cooling for the nozzle tip. In step  480 , the discharge provides mixing of the gas-fuel and air from the outer swirler in the burner tube space, where the improved mixing promotes reduced emissions from the gas turbine. 
     While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.