Abstract:
The structure and operation of a fuel nozzle for a gas turbine combustor is disclosed where the fuel nozzle provides for flameholding protection and, more specifically, to such a nozzle that provides for nondestructive protection from flamebolding. The nozzle provides for differential thermal expansion between tubes forming fuel passages to allowing for the nondestructive venting of fuel during a flameholding condition. Upon extinguishing the flameholding condition, the nozzle returns to normal operating condition.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the invention disclosed herein relates generally to the structure and operation of a fuel nozzle in a gas turbine combustor that provides for flameholding protection and, more specifically, to such a fuel nozzle that provides for nondestructive protection from flameholding. 
       BACKGROUND OF THE INVENTION 
       [0002]    By way of background, a gas turbine combustor is essentially a device used for mixing large quantities of fuel and air and burning the resulting mixture. Typically, the gas turbine compressor pressurizes inlet air, which is then turned in direction or reverse flowed to the combustor where it is used to cool the combustor and also to provide air to the combustion process. The assignee of this invention utilizes multiple combustion chamber assemblies in its heavy duty gas turbines to achieve reliable and efficient turbine operation. Each combustion chamber assembly comprises a cylindrical combustor, a fuel injection system, and a transition piece that guides the flow of the hot gas from the combustor to the inlet of the turbine section. Gas turbines for which the present fuel nozzle design is to be utilized may include six, ten, fourteen, or eighteen combustors arranged in a circular array about the turbine rotor axis. 
         [0003]    In an effort to reduce the amount of NO x  in the exhaust gas of the gas turbine, fuel nozzles have been developed that substantially premix air and fuel prior to the combustion flame, such that the temperature at the flame is reduced relative to conventional diffusion flames. Normal operation of these premixing fuel nozzles requires that a flame be prevented from forming within the premixing chamber. Moreover, the premixing fuel nozzles are designed to be able to eject and extinguish a flame that may inadvertently form in the premixing chamber due to momentary upset conditions owing to, e.g., a sudden transient in the gas turbine or a momentary change in fuel supply conditions. 
         [0004]    Typically, the premixing chamber is not designed to endure the high temperatures encountered in the combustion chamber. However, a problem exists in that the combustor can be unintentionally operated so as to cause the flame to “flashback” from the burning chamber into the premixing chamber where the flame may continue to burn—a condition referred to as flameholding. Another problem that can lead to flameholding is the exposure of hydrogen or higher order hydrocarbons to gas turbines having premixing zones designed to normally run natural gas fuels. The presence of these components promotes flame speeds that are higher than methane and creates an environment where flashback is more possible and flameholding is more difficult to extinguish by the normal thermodynamics of a premixing zone designed to operate on methane. In either case, flashback and flameholding can each result in serious damage to combustor components from burning, as well as damage to the hot gas path of the turbine when burned combustor pads are liberated and passed through the turbine section. 
         [0005]    U.S. Pat. No. 5,685,139 describes a premix nozzle that uses fuse regions near the discharge end of the nozzle to address flashback. In the event of a combustion flashback, these fuse regions burn through due to the higher temperatures experienced when the flame attaches to the nozzle&#39;s radial fuel injectors. The burn through allows fuel to substantially bypass the radial fuel injectors and thereby terminate the flameholding event. Any molten metal released into the combustor by reason of the rupturing fuse regions will be substantially vaporized in the combustion chamber without further damage to the combustor or hot gas path. Simultaneously, the combustor switches over from a premix burning mode to a diffusion burning mode until repairs can be effected. While the turbine will now operate with higher NOx emissions, it will nevertheless operate satisfactorily, with minimum damage to the combustor and no damage to the turbine itself. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The present invention provides for an improved fuel nozzle structure and operation for flameholding protection. More specifically, the present invention provides for nondestructive protection from flameholding through a nozzle that, upon activation, operates to extinguish flameholding and then automatically returns to its original state without damage requiring repair to the nozzle or turbine. Additional aspects and advantages of the invention may be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
         [0007]    In one exemplary embodiment, a fuel nozzle for a gas turbine is provided that includes a nozzle body defining an exterior and an axial direction. The nozzle body also has a tip portion. An inner tube extends axially within the nozzle body and defines an inner passage. An intermediate tube extends axially within the nozzle body. The intermediate tube is concentrically arranged and radially spaced from the inner tube and defines an intermediate passage therebetween. An outer tube extends axially within the nozzle body. The outer tube is concentrically arranged and radially spaced from the intermediate tube and defines an outer passage therebetween. A plug is attached at the tip portion of the nozzle body. The plug defines a first port connected to the outer passage. 
         [0008]    The outer tube also defines a second port connected to the exterior. The second port is located near the tip portion of the nozzle body at a position proximate to the first port such that during normal conditions the first port is closed by the outer tube while during flameholding conditions the outer tube slides relative to the plug so as to connect the second port with the first port and thereby connect the outer passage to the exterior of the nozzle body. As such, fuel from the outer passage can be vented in a non-destructive manner to the exterior of the nozzle during a flameholding condition. 
         [0009]    In another exemplary aspect of the present invention, a method of protecting a fuel nozzle of a gas turbine during flameholding conditions is provided. The fuel nozzle includes a nozzle body defining an exterior and a tip portion, an inner tube extending axially within the nozzle body and defining an inner passage, an intermediate tube extending axially within the nozzle body and defining an intermediate passage with the inner tube, and an outer tube extending axially within the nozzle body and defining an outer passage with the intermediate tube. The exemplary method includes the steps of providing fuel into the outer passage, providing curtain air or purge air to the intermediate passage, sliding the outer tube along axially relative to the intermediate tube during a flameholding condition so as to vent at least part of the fuel to the exterior of the nozzle body near the tip portion, and extinguishing the flameholding condition. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
           [0012]      FIG. 1  provides a perspective view of a known fuel nozzle for a gas turbine. 
           [0013]      FIG. 2  is a cross-sectional view of the fuel nozzle shown in  FIG. 1 . 
           [0014]      FIGS. 3 through 6  are cross-sectional views of exemplary embodiments of the tip portions of fuel nozzles constructed according to the subject matter of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
         [0016]      FIG. 1  is a perspective view of a known fuel nozzle  100  and  FIG. 2  is a cross-sectional view of fuel nozzle  100 . Nozzle  100  includes a nozzle body  105  connected to a rearward supply section  110 . At its tip portion, fuel nozzle  100  also includes a forward fuel/air delivery section at nozzle tip  115 . Also included is a collar  120  that defines an annular passage  125  between the collar  120  and the nozzle body  105 . Within this annular passage is an air swirler  130  upstream of a plurality of radial fuel injectors  135 , each of which is formed with a plurality of discharge orifices  145  for discharging fuel such as a premix gas into passage  125  within the premix chamber of a combustor. 
         [0017]    With specific reference to  FIG. 2 , fuel nozzle  100  includes an inner tube  150  that extends axially within nozzle body  105  and defines an inner passage  155 . Inner passage  155  may, for example, feed air to the combustion zone or can be configured for receipt of a liquid fuel delivery cartridge. An intermediate tube  160  also extends axially within nozzle body  105 . Intermediate tube  160  is positioned around the inner tube  150  in a concentric manner but with a larger diameter to create an intermediate passage  165 . Intermediate passage  165  provides for the flow of e.g., diffusion gas, curtain air, or purge air through orifice  166 . Similarly, an outer tube  170  extends axially along nozzle body  105 . Outer tube  170  is positioned around the intermediate tube  160  in a concentric manner but with a larger diameter to create an outer passage  175 . Outer passage  175  provides for carrying fuel such as a premix gas. During normal (non-flamehold) operation of fuel nozzle  100 , fuel is forced to discharge from outer passage  175  by exiting through discharge orifices  145  in radial fuel injectors  135 . 
         [0018]    Still referring to the nozzle shown in  FIGS. 1 and 2 , nozzle  100  includes a plug  195  located at nozzle tip  115 . Plug  195  is sized to engage the nozzle body  105  and is typically welded thereto at interface  180 . Plug  195  is formed with an interior, annular shoulder  185  ( FIG. 2 ) that receives the forward edge of intermediate tube  160 , and which is welded or brazed at this forward edge. At or near shoulder  185  is also where the forward or downstream end of the intermediate passage  165  is closed. 
         [0019]    As described in U.S. Pat. No. 5,685,139, the wall thickness of the plug  195  along the longitudinally-oriented cylindrical wall  190 , which forms the forward or downstream part of the outer passage  175 , is thinned at a plurality of fuse regions  140  ( FIG. 2 ) that are spaced circumferentially about nozzle tip  115 . In the event of a combustion flashback into the premix zone, one or more of the fuse regions  140  created by thinned walls  190  will burn through as a result of the higher temperature experienced at the fuse regions  140  when the flame attaches at the radial fuel injectors  135 . The burn through allows fuel to substantially bypass radial fuel injectors  135  and exit directly into the combustion zone through the burned out wall area. While some fuel may continue to flow out of the radial fuel injectors  135 , the flow will be insufficient to sustain a flame, thereby causing the flamehold to terminate. The combustor containing nozzle  100  will switch over from a premix burning mode to a diffusion burning mode until repairs to fuse regions  140  can be effected. 
         [0020]      FIGS. 3 through 6  represent exemplary embodiments of nozzle tips  315 ,  415 ,  515 , and  615  as may be used on nozzles that are the subject of the present invention. For example, these tips may be used on fuel nozzle  100  or a fuel nozzle of alternate construction instead of nozzle tip  115 . Nozzle tips  315 ,  415 ,  515 , and  615  are provided by way of example, and not limitation, of the present invention. 
         [0021]    Referring now to  FIG. 3 , the plug  395  of nozzle tip  315  defines a first port  341  that connects to outer passage  375  containing fuel. First port  341  is created, for example, by a plurality of holes  342  located circumferentially about plug  395  and connected to an annular groove  343  machined into the radially outer surface of plug  395 . In addition, holes  342  are at an angle to the longitudinal axis (i.e., axial direction) of fuel nozzle body  105 . Outer tube  370  defines a second port  344  that connects to the exterior of the nozzle tip  315 . As shown in  FIG. 3 , second port  344  is created, for example, by a plurality of holes or openings extending through the wall of outer tube  370  and positioned about the circumference of outer tube  370 . Plug  395  defines a third port  366 , which provides for the flow of e.g., diffusion gas, curtain air, or purge air to the exterior of nozzle tip  315 . Third port  366  is created, for example, by a plurality of holes circumferentially spaced about plug  395 . 
         [0022]    Notably, plug  395  is attached to the intermediate tube  360  and may be attached to inner tube  350 . However, plug  395  is not attached to outer tube  370 , which is free to move or slide relative to plug  395  as shown by arrow A. The outer tube  370  and the intermediate tube  360  are fixed relative to each other at their upstream or forward ends at a position that may be upstream of or near the radial fuel injectors  135  ( FIG. 1 ). 
         [0023]    During a flameholding condition, the heat of a flame burning in the premixing zone adjacent to outer tube  370  will rapidly heat outer tube  370 . For example, during normal operating conditions, outer tube  370  might reach a temperature of about 425° C. During flamehold conditions, the outer tube  370  can reach a temperature of about 815° C. as the flame temperature can reach as high as about 1650° C., However, whether nozzle tip  315  is experiencing normal or flamehold conditions, the temperature of intermediate tube  360  will remain relatively constant and at about the same temperature as the fuel in outer passage  375  (e.g., about 200° C.). 
         [0024]    Accordingly, during a flamehold condition, outer tube  370  will experience a thermal expansion along the axial direction as shown by arrow A in  FIG. 3  while intermediate tube  360  will experience either no expansion or much less than that experienced by outer tube  370 . Because plug  395  is fixed to intermediate tube  360 , this differential thermal growth will cause outer tube  370  to slide in the direction of arrow A relative to intermediate tube  360  and plug  395 . As a result, second port  344  in outer tube  370  will connect with the first port  341  in plug  395  and thereby connect the outer passage  375  to the exterior of nozzle body  105 . Fuel in outer passage  375  will now vent to the exterior of the fuel nozzle  100  and thereby reduce the flow of fuel that normally flows from the outer passage  375 , through radial fuel injectors  135 , and then out through discharge orifices  145  ( FIG. 1 ). 
         [0025]    The sizing of the effective cross-sectional flow area for the first and second ports  341  and  344  is such that the reduction of fuel flowing from discharge orifices  145  will starve the flame within the premix chamber adjacent to the nozzle body  105  and thereby extinguish the flameholding condition. For example, the effective cross-sectional flow area when the first and second ports  341  and  344  are aligned could be sized to a magnitude similar to the flow area from discharge orifices  145 . In such case, during a flame holding condition, the quantity of fuel flowing from discharge orifices  145  would be about half the amount flowing during normal operation. This reduction should be sufficient to extinguish the flameholding condition. 
         [0026]    Consequently, upon extinguishing the flameholding condition, outer tube  370  will begin to cool and return to its original size and position. More specifically, as outer tube  370  cools it will slide along the axial direction in manner opposite to that shown by arrow A. As a result, first port  341  and second port  344  will eventually be disconnected as the nozzle tip  315  returns its normal conditions of operation. The flow of fuel to discharge orifices  145  will then be restored to its original operating flow. Because the flameholding condition is extinguished before damage occurs, fuel nozzle  100  can now continue operation without requiring repair to nozzle tip  315  and can react to another flamehold condition if required. In addition, with nozzle tip  315 , nozzle  100  is more capable of being used with natural gas fuel that may contain certain amounts of hydrogen or higher order hydrocarbons. 
         [0027]    In order to increase the thermal responsiveness of nozzle tip  315  to flamehold conditions, the wall thickness of outer tube  370  can be reduced relative to that of the intermediate tube  360 . Reducing the wall thickness will allow the outer tube  370  to heat more rapidly and thereby slide in the direction of arrow A more quickly upon a flameholding condition. As an alternative or in addition thereto, outer tube  370  can be constructed from a material having a coefficient of thermal expansion that is larger than the coefficient for the material used in construction of intermediate tube  360 . 
         [0028]    As stated previously, second port  344  can be constructed from a plurality of openings or holes positioned about the circumference of outer tube  370 .  FIG. 4  illustrates an alternative exemplary embodiment of the invention that may be used to reduce the number and increase the diameter of holes necessary to create second port  344 . More specifically, nozzle tip  415  is constructed and operates in a manner similar to that of tip  315 . However, outer tube  470  is provided with an annular groove  446  extending circumferentially about the radially-inner surface of outer tube  470 . Annular groove  446  acts as a reservoir connecting each of the circumferentially-spaced holes that create second port  444  about the circumference of outer tube  470 . The annular gap created between annular grooves  443  and  446  results in a larger area being opened to flow by the motion of outer tube  470  relative to plug  495  than may be feasible with the design shown in  FIG. 3 . As such, annular groove  446  allows more fuel to be vented from first port  441  into second port  444  while having a smaller number of holes of larger diameter located about the circumference of outer tube  470  than required with the exemplary embodiment of  FIG. 3 . 
         [0029]      FIG. 5  provides another exemplary alternative embodiment of a nozzle tip  515 . As with previous embodiments, outer tube  570  is configured to slide relative to plug  595 , which is fixed to intermediate tube  560 . Outer tube  570  defines a fourth port  577  located radially adjacent to plug  595 . Fourth port  577  is created, for example, by an annular groove along the inside surface of outer tube  570  and a plurality of axial holes  579  that are circumferentially-spaced about the end of outer tube  570 . Outer tube  570  also defines a second port  544  that connects to the exterior of fuel nozzle  100  by conduit  584 , which is in turn connected to the annular groove of fourth port  577 . 
         [0030]    Plug  595  also defines a third port  566  connected to intermediate passage  565 , which provides for the flow of e.g., curtain air, or purge air. However, unlike previous embodiments, third port  566  is at an angle with respect to the axial direction (i.e., longitudinal axis) of nozzle body  105 . In addition, instead of connecting to the exterior of fuel nozzle  100 , third port  566  connects intermediate passage  565  to the fourth port  577  to allow air flow to exit through the same. The fourth port  577  is positioned and sized so that regardless of the movement of the outer tube  570  relative to intermediate tube  560 , connection with third port  566  is maintained to allow for the flow of air from intermediate passage  565  regardless of whether fuel nozzle  100  is operating normally or experiencing a flamehold condition. 
         [0031]    Plug  515  also defines a first port  541  connected to the outer passage  575  containing fuel. First port  541  is created, for example, from a plurality of axially-oriented conduits connecting to an annular groove  543  that is machined into the radially-outer surface of plug  595 . 
         [0032]    During a flamehold condition, outer tube  570  will experience a thermal expansion along the axial direction as shown by arrow A while intermediate tube  560  will experience either no expansion or much less than that experienced by outer tube  570 . Because plug  595  is fixed to intermediate tube  560 , this differential thermal growth will cause outer tube  570  to slide in the direction of arrow A relative to intermediate tube  560  and plug  595 . As a result, second port  544  in outer tube  570  will connect with the first port  541  in plug  595  and thereby connect the outer passage  575  to the exterior of nozzle body  105  via conduit  584  and fourth port  577 . Fuel in outer passage  575  will now vent to the exterior of the fuel nozzle  100  and thereby reduce the flow of fuel through discharge orifices  145  ( FIG. 1 ). However, before discharge to the exterior, the fuel will mix with air from third port  566  to help minimize NO x  formation when the fuel is subsequently burned. The flow of air through third port  566  also helps to cool plug  595 . 
         [0033]    Once the flamehold condition is extinguished, outer tube  570  will begin to cool and return to its original size and position by sliding along the axial direction in manner opposite to that shown by arrow A. As a result, first port  541  and second port  544  will eventually be disconnected as the nozzle tip  515  returns to its normal conditions of operation. The flow of fuel to discharge orifices  145  will then be restored to its original operating flow. Because the flameholding condition is extinguished before damage occurs, fuel nozzle  100  can now continue operation without requiring repair to nozzle tip  515 . In addition, as with previous embodiments nozzle tip  515  allows nozzle  100  to perform more desirably when natural gas containing hydrogen or higher order hydrocarbons is burned. 
         [0034]    It should also be understood that because of the sliding fit between outer tube  570  and plug  595 , a small leakage of fuel from first port  541  to second port  544  and/or fourth port  577  may occur during normal operating conditions. More specifically, even though first port  541  is disconnected from these other ports during normal operation, some fuel may leak through the movable interface between the outer tube  570  and plug  595 . However, by arranging third port  566  to vent curtain or purge air into fourth port  577  as shown in  FIG. 5 , the formation of undesirable NO x  will be minimized as the leaking fuel will be mixed with such air before combustion. 
         [0035]      FIG. 6  illustrates another exemplary embodiment of the present invention with a structure and operation similar to that described for the embodiment of  FIG. 5 . However, nozzle tip  615  includes a pair of beveled edges  682  and  683  that are configured to meet during normal operation and separate during flamehold conditions. More specifically, plug  695  provides a beveled edge  682  adapted to meet with A complementary beveled edge  683  formed by outer tube  670 . Movement of the outer tube  670  during flamehold operation will separate edges  682  and  683  so as to vent fuel from outer passage  675  and extinguish the flamehold condition as previously described. After extinguishment, edges  682  and  683  will return to the closed position shown in  FIG. 6 . Accordingly, the exemplary embodiment of  FIG. 6  provides a “poppet style” valve seat to provide a positive closing force during normal operation conditions. 
         [0036]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.