Patent Publication Number: US-8528336-B2

Title: Fuel nozzle spring support for shifting a natural frequency

Description:
TECHNICAL FIELD 
     The present application relates generally to gas turbine engines and more particularly relates to a spring support used to position a fuel nozzle within a cap assembly of a turbine combustor. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines generally include a combustor with a number of fuel nozzles positioned therein in various configurations. For example, a DLN2.6+ (“Dry Low NOx”) combustion system offered by General Electric Corporation of Schenectady, N.Y. provides a six fuel nozzle configuration with a center fuel nozzle surrounded by five outer fuel nozzles. Such a combustion system mixes one or more fuel streams and air streams before entry into a reaction or a combustion zone. Such premixing tends to reduce overall combustion temperatures as well as undesirable emissions such as nitrogen oxides (NOx). 
     As is known, the fuel nozzles generally include a number of fuel and air tubes mounted onto a flange. In the DLN2.6+ combustion system, the fuel nozzles may be positioned within a cap assembly in a somewhat cantilevered fashion. The combination of the cantilevered structure and the natural frequency of the center fuel nozzles, however, have caused somewhat high amplitude resonance that has resulted in issues with respect to a braised joint between the flange and one of the outer premixed tubes. 
     Although the design of the fuel nozzle and the cap assembly may be revised to eliminate the issue with the joint, there is a considerable amount of equipment currently operating in the field. There is a desire therefore for systems and methods to dampen or at least to shift the natural frequency of the center fuel tube so as to avoid any issues that may arise with high amplitude resonance. The systems and methods preferably can dampen or shift the natural frequency of the fuel nozzle without extensive equipment replacement or modification costs. 
     SUMMARY OF THE INVENTION 
     The present application thus provides a fuel nozzle spring support system. The fuel nozzle spring support system may include a fuel nozzle, a cap assembly, and a spring support positioned between the fuel nozzle and the cap assembly. 
     The present application further provides a method of operating a combustor having a fuel nozzle and a cap assembly. The method may include the steps of sizing a spring support to alter the natural frequency of the fuel nozzle, positioning the spring support between the fuel nozzle and the cap assembly, and operating the fuel nozzle at the altered natural frequency. 
     The present application further provides a fuel nozzle spring support system. The fuel nozzle spring support system may include a fuel nozzle, a cap assembly, and a spring support positioned between the fuel nozzle and the cap assembly. The spring support may include a hula seal and a collar. 
     These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a gas turbine engine. 
         FIG. 2  is a perspective view of a known fuel nozzle and cap assembly. 
         FIG. 3  is a side cross-sectional view of the fuel nozzle and cap assembly of  FIG. 2 . 
         FIG. 4  is a side cross-sectional view of a machined ring of the fuel nozzle and a floating collar of the cap assembly of the  FIG. 2 . 
         FIG. 5  is a perspective view of a fuel nozzle spring support as is described herein. 
         FIG. 6  is a perspective view of a fuel nozzle spring support system as is described herein with a fuel nozzle and a cap assembly. 
         FIG. 7  is a side cross-sectional view of a machined ring of the fuel nozzle, the spring support, and the cap assembly of the  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numbers refer to like elements throughout the several views,  FIG. 1  shows a schematic view of a gas turbine engine  10 . As is known, the gas turbine engine  10  may include a compressor  20  to compress an incoming flow of air. The compressor  20  delivers the compressed flow of air to a combustor  30 . The combustor  30  mixes the compressed flow of air with a compressed flow of fuel and ignites the mixture. (Although a single combustor  30  is shown, the gas turbine engine  10  may include any number of combustors  30 .) The hot combustion gases are in turn delivered to a turbine  40 . The hot combustion gases drive the turbine  40  so as to produce mechanical work. Mechanical work produced by the turbine  40  drives the compressor  20  and an external load  50  such as an electrical generator and the like. The gas turbine engine  10  may use natural gas, various types of syngas, and other types of fuels. The gas turbine engine  10  may have other configurations and may use other types of components herein. 
       FIGS. 2 through 4  show an existing fuel nozzle  60 . Specifically, a 9FBA center fuel nozzle  60  is shown. The fuel nozzle  60  is positioned within a cap assembly  65 . The cap assembly  65  may be part of the DLN2.6+ combustion system. As is shown, the DLN2.6+ combustion system uses a five around one nozzle configuration. Specifically, the nozzle  60  is held within the cap assembly  65  via a floating collar  70  riding along a machined ring  75  on the fuel nozzle  60 . The 9FBA center fuel nozzle  60  operates at about zero margin to 3/rev rotor speed. As described above, high amplitude resonance has resulted in issues between a flange  80  and an outer premixer tube  85  of the fuel nozzle  60 . 
       FIGS. 5 through 7  show a fuel nozzle spring system  100  as is described herein. The fuel nozzle spring support system  100  includes a spring support  110  positioned between the fuel nozzle  60  and the cap assembly  65 . As is shown in  FIG. 5 , the spring support  110  includes a hula seal  120  positioned within an outer collar  130 . As described in, for example, commonly owned U.S. Pat. No. 6,334,310, the hula seal  120  is defined as a system of leaf springs formed into a round loop. The hula seal  120  generally is used to seal a sliding interface joint or annular cap between two concentric ducts. 
     The hula seal  120  provides spring stiffness and dampening to the fuel nozzle spring system  100 . As is shown in  FIGS. 6 and 7 , the hula seal  120  may be positioned against the machined ring  70  of the fuel nozzle  60  instead of the use of the floating collar  70 . The hula seal  120  supports the fuel nozzle  60  at a full 360 degrees around. The spring support  110  may use a number of hula seals  120  therein. In addition to providing stiffness, frictional losses in the hula seal  120  may provide mechanical damping to reduce vibration amplitudes. 
     The use of the hula seal  120  at the mid-span of the fuel nozzle  60  thus may increase the natural frequency of the nozzle  60 . Specifically, the hula seal  120  may raise the first natural frequency of the nozzle  60  from about 150 Hz to above about 230 Hz. Based upon the available space, the hula seal  120  may increase the natural frequency by about four times or more. The hula seal  120  and the stiffness of the seal may be sized to move the natural frequency of the fuel nozzle to a desired range. The hula seal  120  preferably has a stiffness of about 70 klb/in and may range from about 30 klb/in to over about 150 klb/in. The hula seal  120  may be made out of Inconel X750 (a Nickel-Chromium alloy made precipitation hardenable by additions of Aluminum and Titanium, having creep-rupture strength at high temperatures to about 700° C. (1290° F.)) or similar types of materials. 
     The use of the spring support  110  thus avoids costly retrofitting of the center fuel nozzle  60  and the cap assembly  65 . Moreover, the use of the spring support  110  may be retrofitted on site. The spring support  110  likewise may increase the useful lifetime of the fuel nozzle  60 . 
     It should be apparent that the foregoing relates only to certain embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.