Abstract:
A premix fuel nozzle and method of operation for use in a gas turbine combustor is disclosed. The premix fuel nozzle utilizes a fin assembly comprising a plurality of radially extending fins for injection of fuel and compressed air in order to provide a more uniform injection pattern. The fuel and compressed air mixes upstream of the combustion chamber and flows into the combustion chamber as a homogeneous mixture. The premix fuel nozzle includes a plurality of coaxial passages, which provide fuel and compressed air to the fin assembly, as well as compressed air to cool the nozzle cap assembly. An alternate embodiment includes an additional fuel injection region located along a conically tapered portion of the premixed fuel nozzle, downstream of the fin assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a fuel and air injection apparatus and method of operation for use in a gas turbine combustor for power generation and more specifically to a device that reduces the emissions of nitrogen oxide (NOx) and other pollutants by injecting fuel into a combustor in a premix condition. 
     2. Description of Related Art 
     In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of emissions, especially nitrogen oxide (NOx) and carbon monoxide (CO). Lower combustion emissions can be attributed to a more efficient combustion process, with specific regard to fuel injectors and nozzles. Early combustion systems utilized diffusion type nozzles that produce a diffusion flame, which is a nozzle that injects fuel and air separately and mixing occurs by diffusion in the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature. An improvement over diffusion nozzles is the utilization of some form of premixing such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. Some examples of prior art found in combustion systems that utilize some form of premixing are shown in FIGS. 1 and 2. 
     Referring to FIG. 1, a fuel nozzle  10  of the prior art for injecting fuel and air is shown. This fuel nozzle includes a diffusion pilot tube  11  and a plurality of discrete pegs  12 , which are fed fuel from conduit  13 . Diffusion pilot tube  11  injects fuel at the nozzle tip directly into the combustion chamber through swirler  14  to form a stable pilot flame. Though this pilot flame is stable, it is extremely fuel rich and upon combustion with compressed air, this pilot flame is high in nitrogen oxide (NOx) emissions. 
     Another example of prior art fuel nozzle technology is the fuel nozzle  20  shown in FIG. 2, which includes a separate, annular manifold ring  21  and a diffusion pilot tube  22 . Fuel flows to the annular manifold ring  21  and diffusion pilot tube  22  from conduit  23 . Diffusion pilot tube  22  injects fuel at the nozzle tip directly into the combustion chamber through swirler  24 . Annular manifold ring  21  provides an improvement over the fuel nozzle of FIG. 1 by providing an improved fuel injection pattern and mixing via the annular manifold instead of through radial pegs. The fuel nozzle shown in FIG. 2 is described further in U.S. Pat. No. 6,282,904, assigned to the same assignee as the present invention. Though this fuel nozzle attempts to reduce pollutant emissions over the prior art, by providing an annular manifold to improve fuel and air mixing, further improvements are necessary regarding a significant source of emissions, the diffusion pilot tube  22 . The present invention seeks to overcome the shortfalls of the fuel nozzles described above by providing a fuel nozzle that is completely premixed, thus eliminating all sources of a diffusion flame, while still being capable of providing a stable pilot flame for a constant combustion process. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a premixed fuel nozzle for a gas turbine engine that reduces NOx and other air pollutants during operation. 
     It is another object of the present invention to provide a premixed fuel nozzle with an injector assembly comprising a plurality of radially extending fins to inject fuel and air into the combustor such that the fuel and air premixes, resulting in a more uniform injection profile for improved combustor performance. 
     It is yet another object of the present invention to provide, through fuel hole placement, an enriched fuel air shear layer to enhance combustor lean blowout margin in the downstream flame zone. 
     It is yet another object of the present invention to provide a premixed fuel nozzle with improved combustion stability through the use of a plurality of fuel injection orifices located along a conical surface of the premixed fuel nozzle. 
     In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross section view of a fuel injection nozzle of the prior art. 
     FIG. 2 is a cross section view of a fuel injection nozzle of the prior art. 
     FIG. 3 is a perspective view of the present invention. 
     FIG. 4 is a cross section view of the present invention. 
     FIG. 5 is a detail view in cross section of the injector assembly of the present invention. 
     FIG. 6 is an end elevation view of the nozzle tip of the present invention. 
     FIG. 7 is a cross section view of the present invention installed in a combustion chamber. 
     FIG. 8 is a perspective view of an alternate embodiment of the present invention. 
     FIG. 9 is a detail view in cross section of an alternate embodiment of the injector assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A premix fuel nozzle  40  is shown in detail in FIGS. 3 through 6. Premix fuel nozzle  40  has a base  41  with three through holes  42  for bolting premix fuel nozzle  40  to a housing  75  (see FIG.  7 ). Extending from base  41  is a first tube  43  having a first outer diameter, a first inner diameter, a first thickness, and opposing first tube ends. Within premix fuel nozzle  40  is a second tube  44  having a second outer diameter, a second inner diameter, a second thickness, and opposing second tube ends. The second outer diameter of second tube  44  is smaller than the first inner diameter of first tube  43  thereby forming a first annular passage  45  between the first and second tubes,  43  and  44 , respectively. Premix fuel nozzle  40  further contains a third tube  46  having a third outer diameter, a third inner diameter, a third thickness, and opposing third tube ends. The third outer diameter of third tube  46  is smaller than said second inner diameter of second tube  44 , thereby forming a second annular passage  47  between the second and third tubes  44  and  46 , respectively. Third tube  46  contains a third passage  48  contained within the third inner diameter. 
     Premix nozzle  40  further comprises an injector assembly  49 , which is fixed to each of the first, second, and third tubes,  43 ,  44 , and  46 , respectively, at the tube ends thereof opposite base  41 . Injector assembly  49  includes a plurality of radially extending fins  50 , each of the fins having an outer surface, an axial length, a radial height, and a circumferential width. Each of fins  50  are angularly spaced apart by an angle α of at least 30 degrees and fins  50  further include a first radially extending slot  51  within fin  50  and a second radially extending slot  52  within fin  50 , a set of first injector holes  53  located in the outer surface of each of fins  50  and in fluid communication with first slot  51  therein. A set of second injector holes,  54  and  54 A are located in the outer surface of each of fins  50  and in fluid communication with second slot  52  therein. Fixed to the radially outermost portion of the outer surface of fins  50  to enclose slots  51  and  52  are fin caps  55 . Injector assembly  49  is fixed to premix nozzle  40  such that first slot  51  is in fluid communication with first passage  45  and second slot  52  is in fluid communication with second passage  47 . Premix nozzle  40  further includes a fourth tube  80  having a generally conical shape with a tapered outer surface  81 , a fourth inner diameter, and opposing fourth tube ends. Fourth tube  80  is fixed at fourth tube ends to injector assembly  49 , opposite first tube  43  and second tube  44 , and to third tube  46 . The fourth inner diameter of fourth tube  80  is greater in diameter than the third outer diameter of third tube  46 , thereby forming a fourth annular passage  82 , which is in fluid communication with second passage  47 . Premix fuel nozzle  40  further includes a cap assembly  56  fixed to the forward end of fourth tube  80  and includes an effusion plate  57  having an end surface including a set of third injector holes  58  therein. The use of a conical shaped tube as fourth tube  80  allows a smooth transition in flow path between injector assembly  49  and cap assembly  56  such that large zones of undesirable recirculation, downstream of fins  50 , are minimized. If the recirculation zones are not minimized, they can provide an opportunity for fuel and air to mix to the extent that combustion occurs and is sustainable upstream of the desired combustion zone. 
     The premix fuel nozzle  40 , in the present embodiment, injects fluids, such as natural gas and compressed air into a combustor of a gas turbine engine for the purposes of establishing a premixed pilot flame and supporting combustion downstream of the fuel nozzle. One operating embodiment for this type of fuel nozzle is in a dual stage, dual mode combustor similar to that shown in FIG. 7. A dual stage, dual mode combustor  70  includes a primary combustion chamber  71  and a secondary combustion chamber  72 , which is downstream of primary chamber  71  and separated by a venturi  73  of reduced diameter. Combustor  70  further includes an annular array of diffusion type nozzles  74  each containing a first annular swirler  76 . Premix fuel nozzle  40  of the present invention is located along center axis A—A of combustor  70 , upstream of second annular swirler  77 , and is utilized as a secondary fuel nozzle to provide a pilot flame to secondary combustion chamber  72  and to further support combustion in the secondary chamber. In operation, flame is first established in primary combustion chamber  71 , which is upstream of secondary combustion chamber  72 , by an array of diffusion-type fuel nozzles  74 , then a pilot flame is established in secondary combustion chamber  72 . Fuel flow is then increased to secondary fuel nozzle  40  to establish a more stable flame in secondary combustion chamber  72 , while flame is extinguished in primary combustion chamber  71 , by cutting off fuel flow to diffusion-type nozzles  74 . Once a stable flame is established in secondary combustion chamber  72  and flame is extinguished in primary combustion chamber  71 , fuel flow is restored to diffusion-type nozzles  74  and fuel flow is reduced to secondary fuel nozzle  40  such that primary combustion chamber  71  now serves as a premix chamber for fuel and air prior to entering secondary combustion chamber  72 . The present invention will now be described in detail with reference to the particular operating environment described above. 
     In the preferred embodiment, the premix nozzle  40  operates in a dual stage dual mode combustor  70 , where premix nozzle  40  serves as a secondary fuel nozzle. The purpose of the nozzle is to provide a source of flame for secondary combustion chamber  72  and to assist in transferring the flame from primary combustion chamber  71  to secondary combustion chamber  72 . In this role, the second passage  47 , second slot  52 , and second set of injector holes  54  and  54 A flow a fuel, such as natural gas into plenum  78  where it is mixed with compressed air prior to combusting in secondary combustion chamber  72 . During engine start-up, first passage  45 , first slot  51 , and first set of injector holes  53  flow compressed air into the combustor to mix with the fuel. In an effort to maintain machine load condition when the flame from primary combustion chamber  71  is transferred to secondary combustion chamber  72 , first passage  45 , first slot  51 , and first set of injector holes  53  flow fuel, such as natural gas, instead of air, to provide increased fuel flow to the established flame of secondary combustion chamber  72 . Once the flame is extinguished in primary combustion chamber  71  and securely established in secondary combustion chamber  72 , fuel flow through the first passage  45 , first slot  51 , and first set of injector holes  53  of premix nozzle  40  is slowly cut-off and replaced by compressed air, as during engine start-up. During this entire process, compressed air is flowing through third passage  48  and third set of injector holes  58  to provide adequate cooling to the nozzle cap assembly  56 . 
     NOx emissions are reduced through the use of this premix nozzle by ensuring that all fuel that is injected is thoroughly mixed with compressed air prior to reaching the flame front of the combustion zone. This is accomplished by the use of the fin assembly  49  and through proper sizing and positioning of injector holes  53 ,  54 , and  54 A. Thorough analysis has been completed regarding the sizing and positioning of the first and second set of injector holes, such that the injector holes provide a uniform fuel distribution. To accomplish this task, first set of injector holes  53 , having a diameter of at least 0.050 inches, are located in a radially extending pattern along the outer surfaces of fins  50  as shown in FIG.  3 . To facilitate manufacturing, first set of injector holes  53  have an injection angle relative to the fin outer surface such that fluids are injected upstream towards base  41 . Second set of injector holes, including holes  54  on the forward face of fins  50  and  54 A on outer surfaces of fin  50 , proximate fin cap  55 , are each at least 0.050 inches in diameter. Injector holes  54 A are generally perpendicular to injector holes  54 , and have a slightly larger flow area than injector holes  54 . Second set of injector holes  54  and  54 A are placed at strategic radial locations on fins  50  so as to obtain an ideal degree of mixing which both reduces emissions and provides a stable shear layer flame in secondary combustion chamber  72 . To further provide a uniform fuel injection pattern and to enhance the fuel and air mixing characteristics of the premix nozzle, all fuel injectors are located upstream of second annular swirler  77 . 
     In the preferred embodiment, compressed air flows through third set of injector holes  58  for cooling the cap assembly  56 . Cooling efficiency is enhanced when using effusion cooling due to the amount of material that is cooled for a given amount of air. That is, an angled cooling hole has a greater surface area of hot material that is cooled using the same amount of cooling air as other cooling methods. In order to provide an effective cooling scheme for the cap assembly, the third set of injector holes  58 , which are located in effusion plate  57 , have an injection axis that intersects the end surface of effusion plate  57  at an angle β up to 20 degrees relative to an axis perpendicular to the end surface of effusion plate  57 , and have a diameter of at least 0.020 inches. 
     An alternate embodiment of the present invention is shown in FIGS. 8 and 9. The alternate embodiment includes all of the elements of the preferred embodiment as well as a fourth set of injector holes  83 , which are in communication with fourth annular passage  82  of fourth tube  80 . These injector holes provide an additional source of fuel for combustion. The additional fuel from fourth set of injector holes  83  premixes with fuel and air, from injector assembly  49 , in passage  78  (see FIG. 7) to provide a more stable flame, through a more fuel rich premixture, in the shear layer of the downstream flame zone region  90 . Fourth set of injector holes  83  are placed about the conical surface  81  of fourth tube  80 , between injector assembly  49  and cap assembly  56 , and have a diameter of at least 0.025 inches. 
     While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that one skilled in the art of combustion and gas turbine technology would recognize 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 within the scope of the following claims.