Abstract:
An improved secondary fuel distribution system having a readily adjustable premix pilot nozzle for use in a gas turbine combustor is disclosed. The secondary fuel nozzle assembly has a premix fuel nozzle comprising an annular tubular manifold and a premix pilot nozzle. Multiple embodiments of premix pilot nozzles are disclosed that have a fuel flow rate that can be controlled and adjusted as required, including a plate and feed hole combination, a pressfit premix pilot nozzle, and a premix pilot nozzle that is threaded into the secondary fuel nozzle assembly.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to a fuel nozzle for use in a gas turbine combustor and more specifically to a fuel nozzle having a premix pilot circuit that can be adjusted or modified to meet a desired flow rate. 
   2. Description of Related Art 
   The U.S. Government has enacted requirements for lowering pollution emissions from gas turbine combustion engines, especially nitrogen oxide (NOx) and carbon monoxide CO. These emissions are of particular concern for land based gas turbine engines that are used to generate electricity since these types of engines usually operate continuously and therefore emit steady amounts of NOx and CO. A variety of measures have been taken to reduce NOx and CO emissions including the use of catalysts, burning cleaner fuels such as natural gas, and improving combustion system efficiency. One of the more significant enhancements to land based gas turbine combustion technology has been the use of premixing fuel and compressed air prior to combustion. An example of this technology is shown in FIG.  1  and discussed further in U.S. Pat. No. 4,292,801.  FIG. 1  shows a dual stage dual mode combustor typically used in a gas turbine engine for generating electricity. Combustor  12  has first stage combustion chamber  25  and a second stage combustion chamber  26  interconnected by a throat region  27 , as well as a plurality of diffusion type fuel nozzles  29 . Depending on the mode of operation, combustion may occur in first stage combustion chamber  25 , second stage combustion chamber  26 , or both chambers. When combustion occurs in second chamber  26 , the fuel injected from nozzles  29  mixes with air in chamber  25  prior to ignition in second chamber  26 . As shown in  FIG. 1 , an identical fuel nozzle  29  is positioned proximate throat region  27  to aid in supporting combustion for second chamber  26 . While the overall premixing effect in first chamber  25  serves to reduce NOx and CO emissions from this type combustor, further enhancements have been made to the centermost fuel nozzle since fuel and air from this fuel nozzle undergo minimal mixing prior to combustion. 
   A combined diffusion and premix fuel nozzle replaced the diffusion type fuel nozzle shown proximate throat region  27  in FIG.  1 . Although an improvement, this nozzle still contained a diffusion fuel circuit that contributed to elevated levels of NOx and CO emissions. As a result, this fuel nozzle was modified such that all fuel that was injected into a combustor was premixed with compressed air prior to combustion to create a more homogeneous fuel/air mixture that would burn more completely and thereby result in lower emissions. This improved fully premixed fuel nozzle is shown in FIG.  2  and discussed further in U.S. Pat. No. 6,446,439. Fuel nozzle  50  contains a generally annular premix nozzle  51  having a plurality of injector holes  52  and a premix pilot nozzle  53  with a plurality of feed holes  54 . In this embodiment, fuel enters a premix passage  55  from premix pilot nozzle  53  and mixes with air from air flow channels  56  to form a premixture. Fuel nozzle  50  is typically utilized along the centerline of a combustor similar to that shown in FIG.  1  and aids combustion in second chamber  26 . 
   Although the fully premixed fuel nozzle disclosed in  FIG. 2  provides a more homogeneous fuel/air mixture prior to combustion than prior art fuel nozzles, disadvantages to the fully premixed fuel nozzle have been discovered, specifically relating to premix pilot nozzle  53 . Depending on the base load operating conditions, compressor air flow, and other factors, the amount of fuel required to be injected through holes  52  and  54  will vary from engine to engine, and therefore, producing a common fuel nozzle for different engines is not possible. This is especially a disadvantage with respect to premix pilot nozzle  53 , for which feed holes  54  must be machined prior to assembly of fuel nozzle  50 , since feed holes  54  are inaccessible once premix pilot nozzle  53  is installed in fuel nozzle  50 . Therefore, it is necessary to know fuel flow requirements of the fuel nozzle for each engine before fuel nozzle assembly  50  is assembled. As a result, this prohibits the storage of completed fuel nozzle assemblies for a wide variety of engines. Furthermore, having individual or “custom” flowing fuel nozzle designs prevents the engine operator from interchanging fuel nozzles between different flowing engines. In addition, from the manufacturer&#39;s perspective, it would be advantageous to have a uniform design assembled, which can be shipped to an engine operator on short notice. Therefore, what is desired, and is disclosed in the present invention, is a fully premixed fuel nozzle for a combustor, which can be fine-tuned through an interchangeable or adjustable premix pilot nozzle. A variety of alternate embodiments of the present invention are disclosed in detail. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   It is an object of the present invention to provide a fully premixed secondary fuel nozzle assembly having a premix pilot nozzle with a means for regulating the amount of fuel to the pilot nozzle. 
   It is a further object of the present invention to provide a fully premixed secondary fuel nozzle assembly in which the regulated fuel flow to a premix pilot nozzle can be adjusted. 
   It is yet another object of the present invention to provide a fully premixed secondary fuel nozzle assembly in which the premix pilot nozzle is removable. 
   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 dual stage dual mode gas turbine combustor of the prior art. 
       FIG. 2  is a cross section view of a fuel nozzle of the prior art. 
       FIG. 3  is a cross section view of the present invention. 
       FIG. 4  is a detailed cross section view of the preferred embodiment of the present invention. 
       FIG. 5   a  is a detailed cross section view of a first alternate embodiment of the present invention. 
       FIG. 5   b  is a detailed cross section view of a second alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention, an improved secondary fuel distribution system having an annular premix nozzle and a separate premix pilot nozzle with means for fuel flow rate adjustment, is disclosed and shown in detail in  FIGS. 3-5   b . The improved fuel distribution system  60  comprises a housing  61  for receiving fuel from a supply source and delivering it to a secondary fuel nozzle assembly  62 , which is attached to housing  61 . Secondary fuel nozzle assembly  62  comprises an elongated tube  63  having a first end  64 , an opposing second end  65 , and a centerline A—A defined therethrough as well as a nozzle tip region  66  located proximate elongated tube second end  65 . Furthermore, secondary fuel nozzle assembly  62  also contains a premix fuel nozzle  67  comprising an annular tubular manifold  68  circumferentially disposed around elongated tube  63  by a set of support members  69  which are fixed to and extend radially outwards from elongated tube  63 . Annular tubular manifold  68  is in fluid communication with support members  69  and has a plurality of first holes  70  situated about its periphery and facing in a downstream direction, towards second end  65 , for dispersing fuel wherein at least one first hole  70  is offset circumferentially from a support member  69 . In operation, compressed air passes outside of elongated tube  63  and around premix fuel nozzle  67  such that fuel injected through first holes  70  mixes with the compressed air to form a fuel/air mixture. 
   Secondary fuel nozzle assembly  62  further includes a central core  71  coaxial to centerline A—A and located radially within elongated tube  63  such that a first passage  72  is formed between central core  71  and elongated tube  63  and extends from proximate first opposing end  64  of elongated tube  63  to second opposing end  65 . Contained within central core  71  is a second passage  73 , which extends from proximate first opposing end  64  of elongated tube  63  to premix fuel nozzle  67  and serves to supply fuel to at least premix fuel nozzle  67 . Central core  71  also contains a third passage  74 , which extends from downstream of premix fuel nozzle  67  to proximate second opposing end  65  and includes a swirler  79  for inducing a swirl to the fluids passing through third passage  74  prior to injection into a combustor. Furthermore, central core  71  contains a plurality of air flow channels  75  which are in fluid communication with third passage  74  and are arranged in an annular array about centerline A—A. Compressed air is drawn in to air flow channels  75  from outside secondary fuel nozzle assembly  62  through air flow inlet regions  76  and exits channels  75  into third passage  74  at exit regions  77 . Due to the geometry of air flow passages  75 , first passage  72  extends from proximate first opposing end  64  to proximate air flow inlet region  76 . 
   Secondary fuel nozzle assembly  62  also contains a means for transferring fuel from first passage  72  to nozzle tip region  66 . In the preferred embodiment, this is accomplished by a plurality of tubes  78  arranged in an annular array about centerline A—A, radially between central core  71  and elongated tube  63 . Tubes  80  extend axially from first passage  72  to nozzle tip region  66 . 
   Referring now to  FIG. 4 , a further component of secondary fuel nozzle assembly  62  is a premix pilot nozzle  80  that is fixed to central core  71  at an axial position proximate premix fuel nozzle  67  such that premix pilot nozzle  80  is positioned within third passage  74  and coaxial to centerline A—A. Premix pilot nozzle  80  has a first end  81 , an opposing second end  82 , an outer surface  83 , and an internal passage  84  in fluid communication with second passage  73 , and is preferably circular in cross section such that internal passage  84  has a passage diameter D1. One skilled in the art of fuel nozzle design will understand that although this cross section is the preferred embodiment, other cross sectional shapes maybe necessary depending on fuel nozzle assembly structure, operation, and flow requirements. Located proximate second end  82  are a plurality of second holes  85 , typically having a common diameter or effective flow area, for dispersing fuel to third passage  74 . The improvement comprises the addition of a plate  86  that has at least one feed hole  87  for regulating fuel flow rate to premix pilot nozzle  80 . Plate  86  is typically fixed to premix pilot nozzle  80  by a means such as brazing or welding such that the plate does not come loose during operation. In order to regulate the amount of fuel flow to premix pilot nozzle  80  at first end  81 , after secondary fuel nozzle assembly  62  has been manufactured, a feed hole  87  is drilled in plate  86 . This will allow completed fuel nozzle assemblies to be prepared with the exception of determining the size of feed hole  87 . By utilizing a common size or flow rate for second holes  85 , overall effective flow rate for premix pilot nozzle  80  can be regulated by plate  86  and feed hole  87 . More specifically, it is desired that feed hole  87  is sized such that it has a greater effective flow area than the combined effective flow area of second holes  85 . As a result, feed hole  87  while restricting the fuel flow to premix pilot nozzle  80 , will maintain a higher fuel pressure in second passage  73  than the fuel pressure in internal passage  84 . 
   Referring now to  FIGS. 5   a  and  5   b , alternate embodiments of the present invention are shown in detail cross sections. In each of the alternate embodiments, the present invention is identical in all aspects except for the premix pilot nozzle interface with the central core. In the first alternate embodiment and shown in  FIG. 5   a , premix pilot nozzle  180  has a first end  181 , a second opposing end  182 , an outer surface  183 , and an internal passage  184  that is in fluid communication with second passage  73  and is preferably circular in cross section such that internal passage  184  has a passage diameter D1. Premix pilot nozzle  180  has a plurality of second holes  185  proximate second end  182  for dispersing fuel to third passage  74 . Unlike the preferred embodiment of the present invention, premix pilot nozzle  180  is removable from secondary fuel nozzle assembly  62 , due to the addition of mating threads  186 A and  186 B. Threads  186 A are integral to outer surface  183  of premix pilot nozzle  180  and located proximate first opposing end  181 . Threads  186 A engage corresponding threads  186 B that are integral to central core  171 , which are located proximate second passage  73 , thereby fixing premix pilot nozzle  180  to central core  171  such that internal passage  184  is in fluid communication with second passage  73  and therefore fuel discharging from second holes  185  is directed into third passage  74  to mix with air from air flow channels  75 . In this first alternate embodiment, second holes  185  can be machined into premix pilot nozzle  180  after secondary fuel nozzle assembly  62  is manufactured and then premix pilot nozzle  180  can be installed in secondary fuel nozzle assembly  62 . Alternatively, due to corresponding threads  186 A and  186 B, premix pilot nozzle  180  can be removed, second holes  185  adjusted, or premix pilot nozzle  180  replaced with an alternate configuration. As a result of premix pilot nozzle  180  having the capability of being removed and second holes  185  machined later and to a custom size, plate  86  with feed hole  87  of the preferred embodiment would no longer be necessary. 
   In a second alternate embodiment shown in  FIG. 5   b , a premix pilot nozzle  280  has a first end  281 , a second opposing end  282 , an outer surface  283 , and an internal passage  284  that is in fluid communication with second passage  73  and is preferably circular in cross section such that internal passage  284  has a passage diameter D1. Premix pilot nozzle  280  has a plurality of second holes  285  proximate second end  282  for dispersing fuel to third passage  74 . Unlike the preferred embodiment of the present invention, premix pilot nozzle  280  may be installed in secondary fuel nozzle assembly  62  after manufacturing, due to a pressfit feature between premix pilot nozzle  280  and central core  271 . In this second alternate embodiment, premix pilot nozzle  280  is pressed into central core  271  along region  290 , such that diameter D2 of premix pilot nozzle  280  causes an interference fit along region  290  of central core  271 . In this second alternate embodiment, since premix pilot nozzle  280  can be installed after secondary fuel nozzle assembly  62  is manufactured, second holes  285  can be machined in premix pilot nozzle  280  at a later time allowing second holes  285  to be machined to a desired size and flow rate. As a result, plate  86  with feed hole  87  of the preferred embodiment would no longer be necessary. 
   While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements within the scope of the following claims.