Abstract:
A premix secondary fuel nozzle for use in transferring a flame from a first combustion chamber to a second combustion chamber is disclosed. The secondary fuel nozzle includes multiple fuel circuits, each of which are fully premixed, and neither of which are injected in a manner to directly initiate or support a pilot flame, thereby lowering emissions. Multiple embodiments are disclosed for alternate configurations of a first fuel injector, including an annular manifold and a plurality of radially extending tubes. Alternate premix secondary fuel nozzles are also disclosed incorporating improved tip cooling schemes that reduce the amount of cooling flow and increase the local heat transfer effectiveness. Reduced cooling flow to the tip region helps to improve flame stability and lower combustion dynamics by eliminating unnecessary cooling air from the fuel nozzle recirculation zone.

Description:
[0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/324,949, filed Dec. 20, 2002 and assigned to the same assignee hereof. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a premix fuel nozzle for use in a gas turbine combustor and more specifically to a premix fuel nozzle that does not contain a fuel circuit dedicated to support a pilot flame.  
           [0004]    2. Description of Related Art  
           [0005]    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.  
           [0006]    A combined diffusion and premix fuel nozzle, which is shown in FIG. 2, has been used instead of the diffusion type fuel nozzle shown proximate throat region  27  in FIG. 1. Although an improvement was attained through premix nozzle  31 , this nozzle still contained a fuel circuit  32  that contained fuel that did not adequately mix with air prior to combusting and therefore 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. 3 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 feedholes  54 . In this pilot circuit 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. 3 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 . More specifically, in order to maintain emissions levels in acceptable ranges, premix pilot feed holes  54  had to be adjusted depending on the engine type, mass flow, and operating conditions. This required tedious modifications to each nozzle either during manufacturing or during assembly and flow testing, prior to installation on the engine.  
           [0007]    In order to simplify the fuel nozzle structure and further improve emissions, it is desirable to have a fuel nozzle that supports combustion in a second combustion chamber  26  without having a pilot circuit. Elimination of a pilot circuit, whether diffusion or premix, will further reduce emissions since the pilot circuit is always in operation whether or not it was actually needed to support combustion. Furthermore, eliminating the pilot circuit will simplify fuel nozzle design and manufacturing. The major concern with eliminating the pilot circuit is combustion stability in the second combustion chamber given the reduced amount of dedicated fuel flow to the secondary fuel nozzle. Experimental testing was conducted on a gas turbine combustor having first and second combustion chambers by blocking the premix pilot nozzle  53  of fuel nozzle  50  in accordance with FIG. 3. The combustor was run through its entire range of operating conditions and positive results were obtained for maintaining a stable flame in the second combustion chamber. Changes in combustion dynamics or pressure fluctuations associated with the elimination of the pilot fuel circuit were found to be minimal and insignificant for typical operating conditions.  
           [0008]    An additional concern with prior art fuel nozzles relates to the amount of cooling air directed to the nozzle tip. While providing air to cool the nozzle tip region is necessary to prevent damage from exposure to the elevated temperatures, too much air can adversely affect combustion dynamics. This is especially a concern for fuel nozzles not having a pilot fuel circuit.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0009]    An improved fully premixed secondary fuel nozzle for use in a gas turbine combustor having multiple combustion chambers, in which the products of premixed secondary fuel nozzle are injected into the second combustion chamber, is disclosed. The improvement includes the elimination of the pilot fuel circuit, which previously served to support ignition and combustion in the second combustion chamber. The improved premix secondary fuel nozzle includes a first injector extending radially outward from the fuel nozzle body for injecting a fuel to mix with compressed air prior to combustion, a second injector located at the tip region of the fuel nozzle for injecting an additional fluid, either fuel or air, depending on mode of operation, and an air cooled tip having a swirler. In the preferred embodiment, the first injector is an annular manifold extending radially outward from the fuel nozzle by a plurality of support members and contains a plurality of first injector holes. Also in the preferred embodiment, the second injector is in fluid communication with a plurality of transfer tubes that transfer a fluid to the second injector from around the region of the fuel nozzle that contains the cooling air. In an alternate embodiment of the present invention, the first injector comprises a plurality of radially extending tubes and the second injector is in fluid communication with a generally annular passage that transfers a fluid to the second injector from upstream of the first injector.  
           [0010]    In a second and third alternate embodiments of the present invention, a redesigned nozzle tip region is disclosed incorporating an improved cooling scheme that utilizes less cooling air such that combustion dynamics are reduced. This is accomplished by reducing the total airflow passing through the tip region and changing the means of introducing the cooling air to the combustion chamber. Two nozzle tip regions are disclosed incorporating this alternate cooling configuration. One configuration contains a plurality of cooling holes generally perpendicular to a tip plate while the other orients the cooling holes at an angle, thereby lengthening the cooling holes for enhanced heat transfer and introducing a swirl to the combustor.  
           [0011]    It is an object of the present invention to provide an improved premix secondary fuel nozzle for use in a gas turbine combustor having a plurality of combustion chambers that does not contain a fuel circuit dedicated to the initiation and support of a pilot flame.  
           [0012]    It is a further object of the present invention to provide a gas turbine combustor having stable combustion while producing lower NOx and CO emissions.  
           [0013]    It is yet another object of the present invention to provide an improved premix secondary fuel nozzle for use in a gas turbine combustor having reduced combustion dynamics and a more stable flame front.  
           [0014]    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  
       [0015]    [0015]FIG. 1 is a partial cross section view of a gas turbine combustor of the prior art having first and second combustion chambers.  
         [0016]    [0016]FIG. 2 is a partial cross section view of a secondary fuel nozzle of the prior art.  
         [0017]    [0017]FIG. 3 is a cross section view of a premix secondary fuel nozzle of the prior art.  
         [0018]    [0018]FIG. 4 is a partial cross section view of a premix secondary fuel nozzle in accordance with the preferred embodiment of the present invention.  
         [0019]    [0019]FIG. 5 is a partial cross section of a gas turbine combustor utilizing the preferred embodiment of the present invention.  
         [0020]    [0020]FIG. 6 is a cross section view of a premix secondary fuel nozzle in accordance with an alternate embodiment of the present invention.  
         [0021]    [0021]FIG. 7 is a perspective view of a premix secondary fuel nozzle in accordance with a second alternate embodiment of the present invention.  
         [0022]    [0022]FIG. 8 is a cross section view of a premix secondary fuel nozzle in accordance with a second alternate embodiment of the present invention.  
         [0023]    [0023]FIG. 9A is a partial cross section view of the tip region of a premix secondary fuel nozzle in accordance with a second alternate embodiment of the present invention.  
         [0024]    [0024]FIG. 9B is a partial end view of the tip region of a premix secondary fuel nozzle in accordance with a second alternate embodiment of the present invention.  
         [0025]    [0025]FIG. 10A is a partial cross section of the tip region of a premix secondary fuel nozzle in accordance with a third alternate embodiment of the present invention.  
         [0026]    [0026]FIG. 10B is a partial end view of the tip region of a premix secondary fuel nozzle in accordance with a third alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    The present invention will now be described in detail and is shown in FIGS. 4 through 6. Referring now to FIG. 4, which is the preferred embodiment, a premixed secondary fuel nozzle  70  is shown in cross section. Secondary fuel nozzle  70  is utilized primarily to support combustion in a second combustion chamber of a gas turbine combustor having a plurality of combustion chambers. Secondary fuel nozzle  70  is comprised of an elongated tube  71  having a first end  72 , an opposing second end  73 , a centerline A-A defined therethrough, and a tip region  74  proximate second end  73 . Fuel nozzle  70  also contains at least one first injector  75 , which extends radially away from and is fixed to elongated tube  71 . First injector  75  contains at least one first injector hole  76  for injecting a fuel into a combustor such that air surrounding fuel nozzle  70  mixes with the fuel to form a premixture. In the preferred embodiment, first injector  75  comprises an annular manifold  77  circumferentially disposed about elongated tube  71  and affixed to a plurality of support members  78  which are affixed to elongated tube  71 . In this embodiment, at least one first injector hole  76  comprises a plurality of holes situated about the periphery of annular manifold  77  and are oriented to inject fuel in a downstream direction with at least one first injector hole being circumferentially offset from support members  78 . Furthermore, in order to provide the appropriate fuel distribution from first injector holes  76 , at least one of first injectors holes  76  is angled relative to the downstream direction.  
         [0028]    Secondary fuel nozzle  70  also includes a central core  79  coaxial with centerline A-A and located radially within elongated tube  71  thereby forming a first passage  80  between central core  79  and elongated tube  71 . Central core  79  extends from proximate first opposing end  72  to proximate second opposing end  73  and contains a second passage  81 , which extends from proximate first opposing end  72  to proximate first injector  75  and is in fluid communication with first injector  75 . Located axially downstream from second passage  81 , contained within central core  79 , and extending to proximate second opposing end  73 , is a third passage  82 , which along with second passage  81  is coaxial with centerline A-A. Central core  79  also contains a plurality of airflow channels  83 , typically seven, which have an air flow inlet region  84 , an airflow exit region  85 , and are in fluid communication with third passage  82 . Due to the geometry of air flow channels  83  and positioning of air flow inlet region  84 , first passage  80  extends from proximate first opposing end  72  to a point upstream of air flow inlet region  84 .  
         [0029]    Positioned proximate nozzle tip region  74  and fixed within third passage  82  is a swirler  86  that is used to impart a swirl to air from third passage  82  for cooling nozzle tip region  74 . Also located proximate nozzle tip region  74  at second opposing end  73  is a second injector  87  which contains a plurality of second injector holes  88  for injecting a fluid medium into a combustor. The fluid medium injected through second injector initiates in first passage  80  and is transferred to second injector  87 , in the preferred embodiment, by means of a plurality of transfer tubes  89 , typically seven, which have opposing ends and surround third passage  82 . Transfer tubes  89  extend from upstream of first injector  75  to an annular plenum  90 , which is adjacent second injector  87 . Depending on the mode of operation, first passage  80 , transfer tubes  89 , and annular plenum  90 , may contain either fuel or air. For a combustor having a first combustion chamber and a second combustion chamber, as shown in FIG. 5, fuel is supplied to first passage  80 , transfer tubes  89 , and annular plenum  90  and injected through second injector  87  in an effort to transfer the flame from a first combustion chamber to a second combustion chamber. In this type of combustion system  10  there is a first combustion chamber or primary combustion chamber  25  and at least one primary fuel nozzle  28  delivering fuel to primary combustion chamber  25  where initial combustion occurs. Adjacent to and downstream of primary combustion chamber  25  is a secondary combustion chamber  26  with the combustion chambers separated by a venturi  27 . Primary fuel nozzles  28  surround secondary fuel nozzle  70 , which injects fuel towards secondary combustion chamber  26  to support combustion downstream of venturi  27 . From FIG. 5 it can be seen that all fuel from premix secondary fuel nozzle  70  is injected such that it must premix with the surrounding air and pass through cap swirler  91  prior to entering secondary combustion chamber  26 . Prior art designs allowed fuel from secondary fuel nozzles to pass directly into secondary combustion chamber  26  without passing through cap swirler  91 , thereby directly initiating and supporting a pilot flame, which is typically a source of high emissions.  
         [0030]    Referring now to FIG. 6, an alternate embodiment of the present invention is shown in cross section. The alternate embodiment is similar to the preferred embodiment in structure and identical to the preferred embodiment in purpose and function. A premix secondary fuel nozzle  100  contains an elongated tube  101  having a first end  102  and an opposing second end  103 , a centerline B-B defined therethrough, and a tip region  104  proximate second end  103 . Extending radially away and fixed to elongated tube  101  is at least one first injector  105  having at least one first injector hole  106  for injecting a fuel into a combustor so that the surrounding air mixes with the fuel to form a premixture. In the alternate embodiment, at least one first injector comprises a plurality of radially extending tubes, with each of the tubes having at least one first injector hole  106  that injects fuel in the downstream direction. Fuel injection may be directly downstream or first injector holes maybe oriented at an angle relative to the downstream direction to improve fuel distribution in the surrounding air.  
         [0031]    Alternate premix secondary fuel nozzle  100  also contains a central core  107  coaxial with centerline B-B and located radially within elongated tube  101  to thereby form a first passage  108  between central core  107  and elongated tube  101 . Central core  107  extends from proximate first opposing end  102  to second opposing end  103  and contains a second passage  109  that extends from proximate first opposing end  102  to proximate first injector  105  and is in fluid communication with first injector  105 . Central core  107  also contains a third passage  110  that extends from downstream of first injector  105  to proximate second opposing end  103  such that third passage  110  and second passage  109  are both coaxial with centerline B-B. Another feature of central core  107  is the plurality of air flow channels  111  that are in fluid communication with third passage  110  and each having an air flow inlet region  112  and an air flow exit region  113 . Air passes from air flow channels  111 , through third passage  110 , and flows through a swirler  114 , which is fixed within third passage  110  for imparting a swirl to the air, in order to more effectively cool tip region  104 .  
         [0032]    A second injector  115  is positioned at second end  103 , proximate nozzle tip region  104 , and contains a plurality of second injector holes  116  for injecting a fluid medium into a combustor. The fluid medium injected through second injector  115  initiates in first passage  108  and flows around central core  107  through a generally annular passageway  117  while being transferred to second injector. Depending on the mode of operation, first passage  108  and annular passage  117  may contain either fuel or air. For a combustor having a first combustion chamber and a second combustion chamber, and as shown in FIG. 5, fuel is supplied to first passage  108 , annular passage  117 , and injected through second injector  115  in an effort to transfer the flame from a first combustion chamber  25  to a second combustion chamber  26 . As with the preferred embodiment, all fuel for combustion from the alternate embodiment secondary fuel nozzle is injected radially outward of and upstream of swirler  114  such that the fuel is injected in a manner that must premix with the surrounding air and pass through cap swirler  91  prior to entering secondary combustion chamber  26 .  
         [0033]    Referring now to FIGS.  7 - 10 B, second and third alternate embodiments of the present invention are shown in detail. In each of these alternate embodiments, the tip region of the premix fuel nozzle is modified to reduce the amount of air required to sufficiently cool the nozzle tip, and thereby injected into the recirculation zone. As a result, flame stability improves and combustion dynamics are decreased. The preferred embodiment of the present invention discloses a pilotless fuel nozzle configuration that utilizes cooling air from third passage  82  and directs it through swirler  86  for cooling nozzle tip region  74 . It has been determined that in a pilotless fuel nozzle configuration of this geometry, lesser amounts of air are actually required to cool the nozzle tip than previously thought. Without a pilot fuel circuit, the air passing through third passage  82  and swirler  86  provided a dilution effect to the recirculation zone created by cap swirler  91  thereby reducing the combustion stability and raising combustion dynamics. By reducing the amount of cooling air flow and changing the nozzle tip geometry to utilize the reduced cooling flow more efficiently, combustion dynamics are reduced and a more stable flame front is established. The nozzle tip geometry can be altered to maintain sufficient tip cooling while utilizing less cooling air through the use of effusion cooling, comprising a plurality of holes arranged in an array about a thicker plate of material, thereby maximizing the cooling capability of the air throughout the plate thickness.  
         [0034]    Referring to FIG. 7, a premix secondary fuel nozzle  270  in accordance with a second alternate embodiment is shown in perspective view. The focal point of the second and third alternate embodiments are located at tip region  274  with all other features of the premix secondary fuel nozzle identical to those disclosed in the preferred embodiment. Therefore, only the new matter will be discussed further. Referring now to FIG. 8, premix secondary fuel nozzle  270  is shown in cross section view with tip region  274  detailed in FIGS. 9A and 9B. Premix secondary fuel nozzle  270  includes a tip plate  275  fixed to central core  79  proximate tip region  274  having a first surface  276 , a second surface  277 , and a plate thickness  278  therebetween. For the second alternate embodiment, the preferred plate thickness  278  is at least 0.125 inches. Tip plate  275  also contains a plurality of cooling holes  279  extending from first surface  276  to second surface  277  such that cooling holes  279  have a hole length L and a diameter D ranging from 0.020 inches to 0.070 inches. In the second alternate embodiment, cooling holes  279  are generally perpendicular to second surface  277  such that hole length L is equal to plate thickness  278 . For example, in the second alternate embodiment shown in FIG. 9A, tip region has a plate thickness of 0.312 inches and contains cooling holes having a diameter D of 0.040 inches, thereby resulting in a L/D ratio of slightly less than eight. For most applications, the L/D ratio will be approximately 6-8, but could vary depending on fuel nozzle and combustor conditions.  
         [0035]    A tip region  374  for a third alternate embodiment of the present invention is shown in detail in FIGS. 10A and 10B. In this third alternate embodiment a tip plate  375  has a first surface  376 , a second surface  377 , and a plate thickness  378  therebetween. The preferred plate thickness  378  for the third alternate embodiment is the same as for the second alternate embodiment, at least 0.125 inches. Tip plate  375  also contains a plurality of cooling holes  379  extending from first surface  376  to second surface  377  with cooling holes  379  oriented at an angle α with respect to second surface  377 , having a diameter D ranging from 0.020 inches to 0.070 inches, and having a length L. Angling cooling holes  379  allows for a longer hole to be placed in the same thickness material as a straight hole would, thereby increasing the heat transfer effect of the cooling air as well introducing a swirl to the flow exiting tip plate  375 . It is preferred that angle arange between 25 and 45 degrees. As a result of angle α, hole length L of cooling holes  379  is greater than plate thickness  378 .  
         [0036]    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.