Abstract:
A premixing injector for use in gas turbine engines assists in the lean premixed injection of a gaseous fuel/air mixture into the combustor of a gas turbine. The premixing injector is designed to mix fuel and air at high velocities to eliminate the occurrence of flashback of the combustion flame from the reaction zone into the premixing injector. The premixing injector includes choked gas ports, which allow the fuel supply to be decoupled from any type of combustion instability which may arise in the combustor of the gas turbine and internal passages to provide regenerative cooling to the device.

Description:
REFERENCE TO RELATED APPLICATION 
     Priority is hereby claimed to provisional application Ser. No. 60/810,083, filed Jun. 1, 2006, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This disclosure relates to the field of combustion turbine engines. Specifically, the described devices can be used as a means of efficiently utilizing an alternative fuel, e.g., hydrogen, gas turbines while keeping the generation and emissions of nitrogen oxides to very low levels. More specifically, the present invention is a fuel/air premixing fuel injector or “premixing injector” which supports combustion in gas turbines with control of nitrogen oxide production. 
     DESCRIPTION OF THE PRIOR ART 
     Hydrogen use as a fuel in gas turbine engines has many benefits. In addition to being a renewable fuel, there are no carbon emissions from hydrogen combustion. Of available gas turbine fuels, hydrogen allows the widest range of combustible fuel-air mixtures, thus providing a superior opportunity for reduced flame temperature lean combustion. 
     In a typical gas turbine engine, the combustion chamber, fuel delivery system, and control system are designed to ensure that the correct proportions of fuel and air are injected and mixed within one or more combustors, typically a metal container, or compartment, where the fuel and air are mixed and burned. With diffusion flames in the combustor, there is typically a set of localized zones where peak combustion temperatures are achieved. These peak temperatures may reach temperatures in the range of 4000-5000° F. 
     Typically, to prevent thermal distress or damage to these combustors, a significant amount of the compressor discharge air passes along and through the walls of the combustor for cooling, and to dilute the exhaust gases. The heated compressed air, which then drives the turbine, is a combined mix of the hot combustion gasses and the cooling air. The resulting hot gas yield, which is admitted to the inlet of the turbine, is delivered at a very high temperature. The resultant products and emissions from the hydrogen combustion process are water vapor and oxides of nitrogen (NO x ), a known pollutant, which is exhausted into the atmosphere. NO x  is a harmful product of combustion, and is regulated by environmental laws. Low NO x  emission is a goal, and in many cases, a requirement for both power generation and aero propulsion gas turbines. 
     One method for controlling NO x  formation in the combustion processes of gas turbine engines is to premix the compressor discharge air and the fuel in a premixing injector before they enter the combustor. In this manner, the medium entering the combustion chamber is a homogeneous mixture of the fuel and compressor discharge air. This will allow lean combustion, keeping the combustion product temperature low, which reduces NO x  formation. 
     Multiple efforts have been made for the design of premixing injectors for gaseous hydrocarbon fuels, but very few designs have been made for operation with hydrogen fuel. In addition to achieving optimal fuel/air mixture, the issue of premixed flame stabilization in the proper position is paramount to avoid structural damage to the premixing injector and combustor. Challenges of conventional premixing designs include prevention of flashback and design flow breakdown in the premixing injectors. The term “flashback,” as used in this disclosure refers to the ignition and combustion of the fuel-air mixture within the premixing injector discharge channel, rather than in the combustor. A sustained flashback event will damage the premixing injector. 
     SUMMARY OF THE INVENTION 
     The present invention involves a unique lean premixing injector for a gas turbine engine which provides stable hydrogen fuel combustion with low NO x  production to solve the aforementioned problems associated with existing technology. These premixing injectors incorporate:
         swirl for uniform fuel-air premixing and flame stabilization that supports low equivalence ratio combustion and low NO x  production;   choked fuel injection for isolation of combustion pressure oscillations from the fuel injection system;   geometry that provides no internal flame holding sites for fuel-air combustion, thus preventing flashback;   an integral bluff body flame holding site external to the injector, which is provided as a feature of the overall design concept; and   internal channel structure designed to create internal regenerative cooling to improve the lifespan and preserve the longevity of the premixing injector.       

     In order to illustrate some of the unique features of the invention, the following is a brief summary of the preferred versions of the injector. More specific details regarding the preferred version are found in the Detailed Description with further reference to the Drawings. The claims at the end of this document define the various versions of the invention in which exclusive rights are secured. 
     Reference is now made to the attached  FIGS. 1-6  for exemplary embodiments of the premixing injector of the present invention. The premixing injector, depicted in assembly, exploded, and sectional views as  10  in  FIGS. 1-3  and  10   a  in  FIGS. 4-6 , is shown in two embodiments with Embodiment 1 illustrated at  10  in  FIGS. 1-3  and Embodiment 2 illustrated at  10   a  in  FIGS. 4-6 . Similar structures in each embodiment will be referenced by the same reference numbers with the reference numbers in Embodiment 2 being followed by a lowercase “a.” 
     The premixing injector  10 ,  10   a  includes an outer casing  12 ,  12   a  having a first inlet end  14 ,  14   a  and a second outlet end  16 ,  16   a . The outer casing  12 ,  12   a  surrounds a center body  20 ,  20   a , which includes a first open end  22 ,  22   a  extending from the first end  14 ,  14   a  of the outer casing  12 ,  12   a , a second closed end  24 ,  24   a  at the second end  16 ,  16   a  of the outer casing  12 ,  12   a , an exterior wall  26 ,  26   a , an interior wall  28  (illustrated in  FIG. 3 ),  28   a  and an endcap  30 . An exterior annular mixing channel  40 ,  40   a  is defined by the exterior wall  26 ,  26   a  of the center body  20 ,  20   a  and the interior wall  18 ,  18   a  of the outer casing  12 ,  12   a . The mixing between the compressor discharge air and the fuel occurs in the exterior annular mixing channel  40 ,  40   a . The area of the exterior annular mixing channel  40 ,  40   a  is constant over the length of the premixing injector  10 ,  10   a  to discourage low velocity regions and thus flashback within the premixing injector. A unique feature of the present design is that there are no bluff bodies or flow separation zones within the premixing injector downstream of the fuel injection point to provide flame holding for a flashback. Thus, flashback is discouraged, and easy recovery is provided should a transient flashback occur. 
     The center body  20 ,  20   a  also includes a fuel inlet duct  42 ,  42   a  having a first inlet end  44 ,  44   a , a second outlet end  46 ,  46   a , and an open passageway  48 ,  48   a  extending from the first inlet end  44 ,  44   a  to the second outlet end  46 ,  46   a . The fuel inlet duct  42 ,  42   a  extends to the second end  24 ,  24   a  of the center body  20 ,  20   a.    
     As illustrated in  FIGS. 2 and 5 , the center body  20 ,  20   a  is further defined by an annular sleeve  23 ,  23   a  positioned on the center body  20 ,  20   a  at the first open end  22 ,  22   a . In Embodiment 1, the annular sleeve  23  is solid, as the airflow enters the bell mouth air inlet ducts  60 . Swirl is generated by the tangential velocity component of the air produced by the angled location of the air inlet(s). The fuel enters through choked fuel injector ports  54 ,  54   a.    
     In Embodiment 2, the annular sleeve  23   a  is hollow, allowing air to enter the swirler region  70   a , which generate the required swirl. Fuel is introduced downstream of the swirler region  70   a  through choked fuel injector ports  54   a . Referring specifically to  FIG. 5 , it is noted that the annular sleeve  23   a  is normally in a position covering the vanes  74  situated on the center body  20 . To allow disclosure of the vane  74  in  FIG. 5 , the annular sleeve has been positioned at the second end  46   a  of the center body  20   a .  FIG. 6  illustrates the correct located of annular sleeve  23   a.    
     As illustrated in  FIGS. 3 and 6  in the assembled version of the premixing injector  10 ,  10   a , the fuel inlet duct  42 ,  42   a  is positioned within the center body  20 ,  20   a  in such a manner as to form an interior fuel channel  50 ,  50   a  which is connected to the open passageway  48 ,  48   a  by a conduit  52 ,  52   a . The interior fuel channel  50 ,  50   a  extends longitudinally and in parallel alignment with the exterior annular channel  40 ,  40   a  from the conduit  52 ,  52   a  to a choked fuel injection port  54 ,  54   a . The choked fuel injection port  54 ,  54   a  allows the introduction of fuel to the exterior annular mixing channel  40 ,  40   a . In addition, the choked fuel injection port  54 ,  54   a  inhibits any backflow of fuel and/or air into the upstream portion of the premixing injector  10 ,  10   a.    
     In this manner, fuel is introduced into the premixing injector  10 ,  10   a  by way of the passageway  48 ,  48   a  of the fuel inlet duct  42 ,  42   a  at the inlet end  44 ,  44   a . The fuel is then directed to the interior fuel channel  50 ,  50   a  by way of the conduit  52 ,  52   a.    
     A unique aspect of this system is that the flow of fuel through the conduit allows the cooler fuel gas to cool the closed second end  24 ,  24   a  of the center body  20 ,  20   a . As can be seen in  FIGS. 3 and 6 , the passageway  48 ,  48   a  directs fuel to the endcap  30 ,  30   a  of the center body  20 ,  20   a  where heat radiated and convected from the combustion flame will be transferred from the endcap  30 ,  30   a  of the center body  20 ,  20   a  into the fuel gas. The fuel will then continue to flow by way of the conduit  52 ,  52   a  to the interior fuel channel  50 ,  50   a  and through the choked fuel injection ports  54 ,  54   a  where the fuel will be introduced into the exterior annular mixing channel  40 ,  40   a  through the choked fuel injection ports  54 ,  54   a . The mass flow of the gaseous fuel is used to cool the center body  20 ,  20   a  as a regenerative effect. 
     From the choked fuel injection port  54 ,  54   a , the fuel then enters the swirling region  70  of the exterior annular mixing channel  40 ,  40   a  through the choked fuel injection ports  54 ,  54   a  where the fuel is mixed with the passing compressor discharge air which enters the premixing injector  10  via the air inlet ports  60 ,  60   a . The choked fuel injection ports  54 ,  54   a  are by design choked, thereby decoupling the fuel delivery system from downstream pressure fluctuations. In Embodiment 1 ( FIGS. 1-3 ), the choked fuel injection ports  54  are oriented to inject the fuel in the axial outwardly direction. In Embodiment 2 ( FIGS. 4-6 ), the choked fuel injection ports  54   a  are oriented to inject the fuel in the radially outward direction. The choked fuel injection ports  54 ,  54   a  are designed to be aerodynamically choked during all modes of operation of the gas turbine engine. Advantageously, this eliminates the chance of combustion instabilities coupling to the fuel supply. 
     The air inlet ports of the premixing injector  10  of Embodiment 1 include at least one and preferably four air inlet ducts  60  for channeling compressor discharge air to the exterior annular mixing channel  40 . By design, the location of the air inlet duct  60  advantageously turns the external flow of air gradually into the premixing injector  10  in order to minimize pressure losses due to a sudden contraction. 
     In Embodiment 2, air inlet is accomplished with a single bell mouth-shaped air inlet duct  60   a  on the annular sleeve  23   a  and outer casing  12   a  which introduces the air well upstream of where the flow enters the guide vanes  74 . The annular sleeves  23   a  may be fabricated integrally with the center body  20   a  without change to the operating principles of the premixing injector  10   a.    
     Another significant feature of the premixing injector  10 ,  10   a  is that the closed second end  24 ,  24   a  of the center body  20 ,  20   a  ends in relatively the same plane as the second end  16 ,  16   a  of the outer casing  12 ,  12   a . This feature allows the flame within the combustor chamber  90  ( FIG. 7 ) to stabilize near the second end  24 ,  24   a  of the center body  20 ,  20   a  by providing a low-pressure wake region, which supports the flame holding vortex shear layer previously described. 
     The premixing injector  10 ,  10   a  also includes a swirler region  70 ,  70   a  for mixing the fuel and the compressor discharge air in the exterior annular mixing channel  40 ,  40   a , and an outlet  80 ,  80   a  for expelling the thoroughly swirled and mixed fuel and air to the combustor  90 . 
     Referring now to Embodiment 1, illustrated in  FIGS. 1-3 , the swirler region  70  is comprised of a series of air inlet ducts  60  extending from the outer casing  12  of the premixing injector  10  to the external annular mixing channel  40  downstream of the choked fuel injection ports  54 . 
     Referring to Embodiment 2, illustrated in  FIGS. 4-6 , the swirler region  70   a  is defined by a series of serpentine guide vanes  74  positioned within the swirler region  70   a  formed by the outer casing  12   a  and the center body  20   a  and extending axially to the exterior annular mixing channel  40   a . The trailing edge  77  of the guide vanes  74  includes a discharge angle preferably determined with respect to the axis of the outer casing  12   a . The guide vane discharge angles are defined by a selected radial equilibrium condition to be substantially 45-60 degrees with respect to the axis of the outer casing  12   a  of the premixing injector  10   a . The guide vanes  74  are intended to impart a tangential velocity component (swirl) to the incoming air and to provide structural support for the center body  20   a.    
     Both the premixing injector  10 ,  10   a  of Embodiment 1 and Embodiment 2 are intended for injection of a lean premixed gaseous hydrogen fuel/air mixture into the combustor region  90  of a gas turbine engine; however, natural gas or any other gaseous fuel can be used with the premixing injectors of the present invention. The combustible mixture produced by both designs is predicted to have a uniformly distributed fuel-to-air mass ratio at the exit  80 ,  80   a  of the premixing injector  10 ,  10   a . The lean premixed combustion of the mixture produces lower combustion temperatures than diffusion combustion of the fuel and air. These lower temperatures produce low NO x  levels in the products of the combustion. The premixing injector  10 ,  10   a  is also designed to mix the fuel and air at high axial velocities to eliminate the occurrence of flashback of the reaction zone into the premixing injector  10 ,  10   a.    
     An additional unique aspect of the present invention is that the premixing injector  10 ,  10   a  has the feature of cooling the closed end  24 ,  24   a  of the center body  20 ,  20   a  as discussed previously. This feature reduces the thermal loading on the center body  20 ,  20   a , which will prolong the life of the premixing injector  10 ,  10   a.    
     An additional unique feature of the present invention is that the premixing injector  10 ,  10   a  is designed with choked fuel inlet ports  54 ,  54   a . This choked feature allows the fuel supply to be decoupled from any type of combustion instability which may arise in the combustor of the engine. 
     Another unique feature of the present invention is that the passage of the air from the air inlet duct  60 ,  60   a  to the exterior annular mixing channel  40 ,  40   a  has been designed to reduce pressure losses that may occur when air enters the exterior annular mixing channel  40 ,  40   a . For Embodiment 1 of the current invention, this is accomplished by a smooth flared air inlet duct  60 , which gradually accelerates the air flow. For Embodiment 2, this is accomplished with the annular sleeve  23   a  on the elongated center body  20   a  and a bell mouth-shaped rounded edge on the air inlet ducts  60   a  that extends in front of the swirl vanes  74 . 
     Another significant advantage of the premixing injector of the present invention is that the second closed end  24 ,  24   a  of the center body  20 ,  20   a  ends in the same plane as the second end  16 ,  16   a  of the outer casing  12 ,  12   a . This feature allows for a flame stabilization zone past the end of the premixing injector  10 ,  10   a.    
     Furthermore, the premixing injector  10   a  is designed with a mathematically specified radial equilibrium constraint on the guide vanes  74 . This feature alone allows for a large decrease in pressure losses through the premixing injector  10   a  and control of the axial velocity profile as compared to vanes without this constraint. This feature also creates a desirable axial velocity distribution across the exterior annular mixing channel  40   a.    
     Summarizing the invention, unique fuel/air premixing injectors have been conceived and developed for the purpose of supporting fuel and compressor discharge air injection as the medium for combustion, resulting in the production of single digit parts per million (ppm) levels of NO x  as a by-product, a wide range of stable operation, and suitability for integration into gas turbines. 
     In view of the foregoing, this disclosure relates to unique operation of the invention in the field of combustion in gas turbine engines. More specifically, the invention can be used as a means of utilizing alternative fuels that will perform in gas turbines while keeping emissions of nitrogen oxides below established target levels. 
     The features and advantages of the invention will be illustrated more fully in the following detailed description of the preferred embodiment of the invention made in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the overall design of the first embodiment of the premixing injector of the present invention (“Embodiment 1”). 
         FIG. 2  is an exploded view of the premixing injector of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the premixing injector of  FIG. 1  taken along lines  3 - 3  of  FIG. 1 . 
         FIG. 4  is a perspective view of a second embodiment of the premixing injector of the present invention (“Embodiment 2”). 
         FIG. 5  is an exploded view of the premixing injector of  FIG. 4 . 
         FIG. 6  is a cross-sectional view of the premixing injector of  FIG. 4  taken along lines  6 - 6  of  FIG. 4 . 
         FIG. 7  is a partial perspective view of a combustor illustrating the positioning of the premixing injectors of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As previously noted, Embodiment 1 of the present invention, referenced in  FIGS. 1-3 , uses a combination of an angled air jet producing tangential and axial velocity components in the mixing region to achieve the condition of a premixed swirl stabilized flame. It was determined that for the correct balance of pressure losses and mixing, a hybrid scheme of a jet in crossflow and a jet in coflow should be used. A jet in crossflow is defined as a seeder stream (generally fuel) being injected perpendicular to the bulk stream (generally air). A jet in coflow is defined as the seeder stream and the bulk stream in a coaxial configuration. The hybrid scheme means that the angle between the two streams is between 0 and 90 degrees. In this embodiment, the angle was set at 60 degrees. This design is well-suited for most gas turbine engines because it is adaptable to standard combustor designs and, with the predicted operation, will keep the production of NO x  low and produce a properly stabilized flame. 
     Referring to  FIGS. 1-3 , the premixing injector  10  of the present invention is defined by the exterior annular mixing channel  40  comprising three main structures: the outer casing  12 , the center body  20  and the air inlet duct  60 . The center body  20  is nested within the outer casing  12 , and the air inlet duct  60  is nested within the center body  20  to form the premixing injector  10 . Preferably, the three parts to the premixing injector  10  are welded together to form the single unit premixing injector  10 . 
     As illustrated in  FIGS. 1-3 , the outer casing  12  is a generally cylindrical tube having a first inlet end  14 , a second outlet end  16 , and an external surface  17  and an internal surface  18 . The outer casing  12  includes a flange  19 , which allows the attachment of the premixing injector  10  to a gas turbine engine combustor liner  91  ( FIG. 7 ) by bolts or other means. It is within the scope of the present invention to have other means for attaching the premixing injector  10  to the gas turbine engine combustor line  91 . As illustrated in  FIG. 1 , the attachment flange  19  is situated near the second outlet end  16  of the outer casing  12 . 
     Embodiment 1 illustrates four tangential circular air inlet ducts  60 , which serve as the inlet stream of air (or other oxidizer), which is to be fed from a compressor or another source (not illustrated). The ends  62  of the air inlet ducts  60  are flared at an angle, preferably 45 degrees. The inner walls  64  of the flared ends  62  are rounded. These two features allow the airflow to accelerate gradually, thereby reducing the pressure losses and increasing the efficiency of the premixing injector  10 . The air inlet ducts  60  deliver the compressor air into the premixing injector  10 . The air inlet ducts  60  are also at an angle of preferably 60 degrees relative to the axial flow direction to reduce the pressure losses. 
     Referring now to  FIGS. 2 and 3 , the center body  20  is a generally cylindrical structure having a first open end  22  and a second closed end  24 . As illustrated in  FIG. 2 , the second end includes an endcap  30 . The center body  20  includes an exterior wall  26  and an interior wall  28 . The exterior annular mixing channel  40  is created between the internal surface  18  of the outer casing  12  and the exterior wall  26  of the center body  20  and forms the exterior annular mixing channel  40 . 
     As illustrated in  FIG. 2 , the first open end  22  of the center body  20  includes a sold annular sleeve  23  with a larger diameter such that the diameter of the annular sleeve  23  is approximately the same as the internal diameter of the outer casing  12 . This allows for a smooth press fit between the center body  20  and the outer casing  12 . 
     The transition zone between the exterior wall  26  and the choked fuel injection point  54  is known as the constant radius fillet  56 . The constant radius fillet  56  is necessary to reduce the pressure losses in the premixing injector  10 . The constant radius fillet  56  reduces the area for separation for the inlet air stream, and helps gradually turn the airflow. The air enters from the air inlet ducts  60  and the constant radius fillet  56  guides the flow axially. 
     The fuel enters from the choked fuel injection port  54  and enters the exterior annular mixing channel  40  at the area of the constant radius fillet  56 . The smoother this transition, the less pressure loss occurs. Therefore, a curved radius of the constant radius fillet  56  is preferable to a right angle. It allows smoother blending of the gas/air mixture. The choked fuel injector ports  54  are choked to eliminate the possibility of downstream pressure fluctuations from propagating upstream into the fuel delivery system. 
     The fully assembled premixing injector  10  contains a series of chambers within the premixing injector  10  including the open passageway  48  of the fuel inlet duct  42 , the interior fuel channel  50 , a plenum  62 , and the exterior annular mixing channel  40 . In addition, there is a choked fuel injection port  54  formed between the plenum  62  and the exterior annular mixing channel  40 . 
     The fuel system will be at an elevated pressure to satisfy the choked flow requirement in the fuel injection ports  60  and the overall fuel mass flow requirement. The plenum  62  is an open area designed to settle out velocity profiles of the fuel. 
     In the exterior annular mixing channel  40 , the fuel/air mixture has both a tangential and axial velocity component creating a swirling structure. The air inlet ducts  60  are positioned such that the air enters the exterior annular mixing channel  40  at an angle which forces the air and fuel mixture to propagate through the exterior annular mixing channel  40  in a helical fashion. The swirl of the air/fuel mixture and the fact that the mixture is premixed is important in keeping the flame shortened in the combustor  90 . 
     Operation 
     The fuel, generally pressurized gaseous fuel, enters the fuel inlet duct  42  of the premixing injector  10  via the fuel inlet duct  48 . The fuel then travels the length of the passageway  48  to the conduit  52  where the fuel provides back wall cooling to the endcap  30  of the center body  20 . Backwall cooling reduces the thermal load on the center body  20 . This prolongs the life of the premixing injector  10 . Another term for this process is “regenerative cooling.” 
     Once the fuel reaches the endcap  30 , it is channeled from the passageway  48  to the interior fuel channel  50  via the conduit  52  and toward the plenum  62 , thereby increasing the heat transfer to the fuel, and conditioning internal velocity profiles. 
     At the plenum  62  area, the fuel flows through the choked fuel injection ports  54  into the exterior annular mixing channel  40  at the area of the constant radius fillet  56  where the compressor discharge air entering through the air inlet ducts  60  is mixed with the fuel. The choked fuel injection port  54  eliminates the possibility that downstream pressure fluctuations will affect the fuel delivery flow rate. Additionally, the high-speed fuel jet penetrates farther into the incoming air stream because the momentum ratio (fuel jet/air) is high. This enhances the mixing between the two streams. 
     At this point, the fuel air mixture propagates in a helical vortex structure around the exterior surface  26  of the center body  20  in the exterior annular mixing channel  40  toward the exit end  80  of the premixing injector  10  where it is passed into the engine. This feature is important for flame placement. The design velocities are such that flashback is eliminated. Finally, the fuel/air mixture, now fully premixed and swirling, enters the combustion region through the exit end. 
     The premixing injector  10  provides a swirling and well-mixed reactant stream of fuel and air to the combustor. The premixing injector  10  produces stable combustion and low NO x  emissions. The current design was sized to accommodate hydrogen as a fuel; however it is within the scope of the present invention to consider other forms of gas, such as natural gas with or without hydrogen, gas mixtures resulting from coal gasification, ethylene, propane and other forms of gaseous fuel with this design. 
     Reference is now made to  FIGS. 4-6  for an alternative Embodiment 2 of the premixing injector of the present invention. Referring to  FIG. 4 , the premixing injector  10   a  is comprised of an outer casing  12   a , a center body  20   a  having an exterior wall  26   a , between which is defined the exterior annular mixing channel  40   a , and a fuel inlet duct  42   a.    
     A plurality of air guide vanes  74  are securely affixed to the center body  20   a  and extend radially outward from the center body  20   a  toward the outer casing  12   a . Each vane  74  has an inner end  75  and an outer end  76 . The inner end  75  is proximate to the center body  20   a  relative to the outer end  76 . Each vane  74  includes a leading edge  78  and a trailing edge  77 . The leading edge  78  is upstream of the flow path relative to the trailing edge  77 , which is downstream of the leading edge  78 . The vane  74  is radially arranged with respect to the center body  20   a  to facilitate manufacturing and produce the required flow. Each vane  74  is curved in the same direction. 
     The purpose of these guide vanes  74  is to add structural support to the premixing injector  10   a  as well as to provide the desired tangential and axial velocity components to the air entering the premixing injector  10   a . The vanes  74  are designed to produce a specific radial equilibrium condition to control the swirling velocity distribution and minimize flow losses. Air enters the exterior annular mixing channel  40   a  upstream of the guide vanes  74  at the swirl region  70   a  and, following mixing with the injected fuel, exits the premixing injector  10   a  at the downstream end  16   a  of the exterior annular mixing channel  40   a.    
     Gaseous fuel enters the premixing injector  10   a  through the passageway  48   a  within the fuel inlet duct  42   a  and is introduced to the exterior annular mixing channel  40   a  through choked radial fuel ports  54   a , initiating mixing with the passing air stream. Before the gaseous fuel reaches the passing air stream, it will be accelerated to sonic velocities through the radial fuel ports  54   a . The gaseous fuel is introduced into the airflow downstream of the guide vanes  74  to eliminate the possibility of flame stabilization inside the premixing injector  10   a.    
     The combustion zone is expected to stabilize downstream of the exterior annular mixing channel  40   a . With the combustion zone close to the exterior annular mixing channel  40   a , the endcap  30   a  of the center body  20   a  will experience high temperatures. To counter this effect, the premixing injector  10   a  is designed to transfer heat from the endcap  30   a  of the center body  20   a  to the incoming gaseous fuel. After the fuel enters the core of the center body  20   a , it is directed toward the endcap  30   a  of the center body  20   a  where heat transfer occurs. This provides a form of regenerative cooling for the second closed end  24  of the center body  20   a.    
     Reference is now made to  FIG. 7  which illustrates a gas turbine engine pressure casing  89 . The pressure casing  89  encompasses the combustor  90  which contains the annular combustion liner  91 . The combustion liner  91  is conventional in design and will not be described in detail except to note that the combustion liner  91  may be modified to ensure that the desired amount of the compressor discharge air flows through the premixing injectors  10  and the combustion liner  91  once the premixing injectors  10  are installed. Two premixing injectors  10  are shown in  FIG. 7 . However it is within the scope of the present invention to include one or a plurality of premixing injectors  10  depending on the requirements of the gas turbine engine. Multiple combustion chambers  90  can also be provided, if necessary or desirable. In addition, while the premixing injector  10  will be described with respect to the combustion liner  91 , it should also be understood that premixing injector  10   a  can also be provided with the combustion chamber  91 . 
     The combustion liner  91  is generally defined as a sheet metal object which is generally annular in shape that has a domed end  92  with circular openings  94  of a size and shape to receive the premixing injector  10 . The combustion liner  91  must be matched with the premixing injectors  10 . For example, the openings  94  can be slightly larger than the outer diameter of the premixing injector  10  to allow a small amount of cooling compressor discharge air to flow around the outer casing  12  of the premixing injector  10  to allow for management of the combustion liner  91  temperature. This could take advantage of the fact that a premixed flame utilizing gaseous fuel is much shorter than a diffusion flame. The premixing injectors  10  will remain in the correct orientation through the use of two locator pins (not illustrated) per premixing injector  10 . 
     Opposite the domed end  92  on the combustion liner  91 , there is an open end  95  which allows the combustion products exiting the combustion liner  91  to enter the turbine guide vanes (not illustrated). If desired, dilution air inlets  96  are present in the combustion liner  91  to introduce additional compressor discharge air to prevent the excessive heating of the combustion liner  91  itself due to the combustion process, and to cool the combustion products sufficiently so as not to destroy the turbine vanes and blades. 
     In operation, the fuel, e.g., hydrogen, enters a fuel manifold port  98 . Each fuel manifold port  98  is connected to a hydrogen or fuel source (not illustrated). Each fuel manifold port  98  is in turn connected to the fuel inlet duct  42  of the premixing injector to admit the fuel through the premixing injector  10  and allow mixing with the compressor discharge air entering through the air inlet ducts  60  as described above. The thoroughly mixed and swirling fuel/air mixture exits the premixing injectors  10  through the second end  16  within the openings  94  in the domed end  92  of the combustion liner  91  wherein it is diverted via a series of baffles (not illustrated) known to the art through the combustion chamber  90  to the turbine inlet. 
     Example 
     The following Example is included solely for the purpose of providing a more complete and consistent understanding of the invention disclosed and claimed herein. The Example does not limit the scope of the invention in any fashion. 
     The design specifications for the premixing injector  10  enabled its use in a Pratt and Whitney PT6-20 turboprop engine. Since varying operating conditions of the engine (take off, cruise, and full power) are possible, there are multiple possible optimizations for the injector. The cruise condition was chosen for the optimization due to the normal high percentage of operational time at cruise. Table 1 shows the overall design constraints and the constraints per nozzle for the cruise condition of the engine. The fuel flow rate was determined by the equivalent energy flow rate based on lower heating value of hydrogen and kerosene. The number of premixing injectors  10  was chosen to ensure relative spatial uniformity in the engine liner. The equivalence ratio constraint is from a desire to have low emissions. These constraints define the flow rates of both the fuel and air to each premixing injector  10 . Using the aforementioned tangential entry swirl design concept, a prototype was developed. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Overall design Constraints for the Premixing Injector 10 
               
             
          
           
               
                   
                 Design Constraint 
                 Value 
               
               
                   
                   
               
             
          
           
               
                   
                 Power 
                 410 
                 kW 
               
               
                   
                 Fuel flow rate 
                 14.5 
                 g/s 
               
             
          
           
               
                   
                 Equivalence Ratio 
                 0.4 
               
               
                   
                 Number of Nozzles 
                 18   
               
             
          
           
               
                   
                 Upstream Pressure 
                 537 
                 kPa 
               
               
                   
                   
               
             
          
         
       
     
     The engineering design process needed both the listed quantities above and additional design constraints. The constraints that were added include the following: the axial velocity within the premixing injectors  10  must exceed 100 m/s, the swirl number must be above 0.8 for a “high swirl” injector, pressure losses must not exceed 10%, and there must not be any instability in the operational range of the injector. The high swirl number and the high velocity requirement were set such that the flame will stabilize outside the nozzle in the shear layer between the vortices and not within the injector. The pressure loss requirement is present because pressure losses are parasitic to the engine efficiency and must be minimized. Finally, the instability requirement is present because in the presence of instabilities pressure forces can damage hardware, the increased convection and radiation has the potential of melting the hardware, and local regions with high equivalence ratios are formed, raising emissions, and the overall combustion efficiency decreases. 
     Referring to  FIGS. 1-3 , the four air inlet ports  60  are designed such that the fabrication would necessitate standard ¼″ tubing. The minor diameter of the air inlet ports  60  is 4.57 mm and has a 45° rounded bell mouth opening  64 . The reason for the opening  64  is to accelerate the compressor discharge air flow gradually and reduce the pressure losses associated with the air inlet ports  60 . The fuel inlet duct  42  is oriented in the axial direction, located at the upstream end of the premixing injector  10 . 
     Another feature that reduces the pressure loss is located inside the swirler region  70 , illustrated in  FIG. 3 . Early simulations showed that the area near the first inlet end  14  of the premixing injector  10  at the center body  20  caused a significant separation zone and a void where fuel/hydrogen accumulation was possible. A 6.25 mm constant radius fillet  56  was placed on the center body  20  to fill the void and gradually turn the mixing flow into the swirler region  70 . 
     The swirler region  70  has an outer diameter of 21.18 mm and an inner diameter of 15.24 mm, yielding an exit area of 0.0001699 m 2 . Using the mass flow rate and the area, the area average velocity is approximately 113 m/s based on ideal gas behavior. This high velocity is good flashback prevention because the turbulent flame speed will not approach such a high value. 
     The fuel side design decisions were made as precautions to address failures and problems typically seen in premixing injectors  10 . With the flame zone for a premixing injector  10  being close to the end cap  30  of the center body  20 , there is potential for the thermal failure of the endcap  30 . To alleviate this problem the hydrogen fuel provides convective back wall cooling before it is introduced into the exterior annular mixing channel  40 . To achieve this, the fuel is routed from the open passageway  48  of the fuel inlet duct  42  to conduit  52  located at the endcap  30  of the center body  20 . Here, the fuel provides the back wall cooling to the endcap  30  and is routed to the plenum  62  of the premixing injector  62 , and finally through the exterior annular mixing channel  40  to the downstream end  16  of the premixing injector  10 . 
     To circumvent thermoacoustic instabilities in the combustor  90  caused by equivalence ratio perturbations associated with acoustic wave propagated upstream through the fuel delivery system, the premixing injector  10  is provided with choked radial fuel ports  54  (Mach=1). Choking the radial fuel ports  54  eliminates the possibility for equivalence ratio perturbations, but mixing perturbations can still exist leading to instabilities. It is however important that the bulk mixing qualities remain constant, which are determined in part by the momentum flux ratio defined as 
             j   =         ρ   f     ⁢     V   f   2           ρ   a     ⁢     V   a   2               
where the subscripts a and f refer to the air and fuel respectively. In a choked passage the mass flow rate is determined by the pressure, which positively correlates to the density. It is important that the fuel stream does not over penetrate into the air crossflow, thus disrupting the mixing processes. Therefore the area of the choked radial fuel ports  54  was chosen to be the largest area in which the passage remained choked during the idle condition of the gas turbine engine. The idle condition of the gas turbine engine is the lowest mass flow rate of fuel that is required. The calculated choked radial fuel port size is 0.406 mm. The diameter ratio between the air inlet ports  60  and the choked radial fuel ports  54  is 11.24, which is relatively small. A benefit for making the choked radial fuel ports  54  larger is that the surface area on the windward side of the fuel jet becomes large, aiding in the fuel shedding and mixing process. An additional benefit of maximizing the choked radial fuel ports  54  is that the fuel inlet pressure is minimized. This could potentially be a parasitic loss on the engine power, depending on the storage method of the hydrogen.
 
     In summary, the design choices for the premixing injector  10  were all derived from the gas turbine engine requirements. The power desired at cruise needed determined the design flow rate of hydrogen/fuel. The equivalence ratio specification to reduce NO x  determined the air flow rate and thus the exterior annular mixing channel  40 ,  40   a  cross-sectional area. 
     It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims. Thus, the invention encompasses all different versions that fall literally or equivalently within the scope of the claims.