Patent Publication Number: US-8991188-B2

Title: Fuel nozzle passive purge cap flow

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to gas turbines and, more particularly, to a fuel nozzle for a gas turbine engine including a cooling circuit that utilizes passive purge flow for fuel nozzle tips supplied from end cap cooling flow before quat fuel injection. 
     Conventional quat fuel injection systems utilize CdC air mixed with quat fuel for passive purge feeds. The presence of fuel in the passive purge feed elevates a risk of flame holding in the passive purge cavities and within the fuel nozzle tips. It would be desirable to use the end cap purge feed that is free of quat fuel to provide an alternate means to purge the fuel nozzle tips and eliminate the flame holding risk from the design. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment, a cooling circuit for a fuel nozzle in a gas turbine includes an annulus receiving compressor discharge air, a quat cap including a fuel passage through which quat fuel is injected toward the fuel nozzle, and an air passage formed in the quat cap and receiving the compressor discharge air from the annulus. The air passage is positioned upstream of the fuel passage such that the compressor discharge air is not mixed with quat fuel. Purge passages in the fuel nozzle receive the compressor discharge air from the air passage. The purge passages direct the compressor discharge air to the fuel nozzle for tip cooling. 
     In another exemplary embodiment, a method of cooling a fuel nozzle in a gas turbine includes the steps of (a) receiving compressor discharge air in an annulus; b) directing the compressor discharge air from the annulus to an air passage formed in a quat cap, where the air passage is positioned upstream of a quat fuel passage such that the compressor discharge air in the air passage is not mixed with quat fuel; and (c) receiving the compressor discharge air from the air passage in purge passages in the fuel nozzle, the purge passages directing the compressor discharge air to the fuel nozzle for tip cooling. 
     In yet another exemplary embodiment, a cooling circuit for a fuel nozzle in a gas turbine includes an end cap cavity receiving passive purge flow from a compressor of the turbine, and fuel nozzle swozzles disposed in a swozzle shroud that impart swirl to incoming fuel and air. Purge slots are formed in the swozzle shroud and through the fuel nozzle swozzles in fluid communication with the end cap cavity. The purge slots are positioned upstream of a quat fuel injection passage, and the passive purge flow enters fuel nozzle tip cavities of the fuel nozzle to provide tip cooling and tip purge volume without mixing the passive purge flow with quat fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-section of a gas turbine; 
         FIG. 2  is a sectional view showing the fuel nozzles of the combustor; 
         FIGS. 3 and 4  are sectional views of an outer fuel nozzle; and 
         FIGS. 5 and 6  are sectional views of a center fuel nozzle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a typical gas turbine  10 . As shown, the gas turbine  10  generally includes a compressor at the front, one or more combustors  14  around the middle, and a turbine  16  at the rear. The compressor  12  and the turbine  16  typically share a common rotor. The compressor  12  progressively compresses a working fluid and discharges the compressed working fluid to the combustors  14 . The combustors  14  inject fuel into the flow of compressed working fluid and ignite the mixture to produce combustion gases having a high temperature, pressure and velocity. The combustion gases exit the combustors  14  and flow to the turbine  16  where they expand to produce work. 
     A casing surrounds each combustor  14  to contain the compressed working fluid from the compressor  12 . Nozzles are arranged in an end cover, for example, with outer nozzles radially arranged around a center nozzle. The compressed working fluid from the compressor  12  flows between the casing and a liner to the outer and center nozzles, which mix fuel with the compressed working fluid, and the mixture flows from the outer and center nozzles into upstream and downstream chambers where combustion occurs. 
     As noted, prior designs have used quat mixed CdC air to feed passive purge for fuel nozzle tips. The presence of fuel in the passive purge feed, however, elevates a risk of flame holding in the passive purge cavities and within the fuel nozzle tips. With reference to  FIGS. 2-6 , the described embodiments utilize end cap purge feed that is free of quat fuel as an alternate means to purge the fuel nozzle tips. With the purge feed being free of quat fuel, a flame holding risk is eliminated from the design. 
       FIG. 2  is a cross-sectional view showing the outer and center fuel nozzles. The assembly includes a cooling circuit  20 . In use, parts of the nozzle including a nozzle tip end  22  must be cooled due to their exposure to hot combustion gas. The combustor includes an annulus  24  that receives compressor discharge air from the compressor. A quat cap  26  includes a fuel passage  27  through which quat fuel is injected toward the fuel nozzles. The quat fuel is injected into a swozzle assembly  28 , including a fuel nozzle swozzle disposed in a swozzle shroud. The swozzle assembly  28  imparts swirl to the incoming fuel and air. 
     The cooling circuit  20  includes an air passage  30  formed in the quat cap  26  that receives the compressor discharge air from the annulus  24 . As shown in  FIG. 2 , the air passage  30  is positioned upstream of the fuel passage  27 . As a consequence, the compressor discharge air in the air passage  30  is not mixed with quat fuel. Purge passages  32  in the fuel nozzle receive the compressor discharge air via the air passage  30 . The purge passages  32  direct the compressor discharge air to the fuel nozzle for tip cooling. 
     As shown, the purge passages  32  are formed in the swozzle assembly  28 . Preferably, the purge passages  32  comprise slots formed in the swozzle  28 . 
     In a typical construction, the combustor includes several outer nozzles circumferentially surrounding a center nozzle.  FIG. 2  is a sectional view through one of the outer fuel nozzles  2  and showing a relative position of the center fuel nozzle  4 .  FIGS. 3 and 4  are sectional views through an outer fuel nozzle, and  FIGS. 5 and 6  are sectional views through the center fuel nozzle. As shown, the purge passages  32  are formed in the swozzle  28 . 
     With continued reference to  FIG. 2 , a nozzle tip cooling passage  34  surrounds the fuel nozzle, and a portion of the pressure discharge air from the air passage  30  is directed to the nozzle tip cooling passage  34  for cooling the nozzle tip. 
     The flow path of the compressor discharge air is shown by arrows in  FIGS. 2 ,  4  and  6 . The compressor discharge air is received in the annulus  24  and is directed to the air passage  30  formed in the quat cap  26 . As noted previously, since the air passage  30  is positioned upstream of the quat fuel passage  28 , the compressor discharge air in the air passage  30  is not mixed with quat fuel. From the air passage  30 , the compressor discharge air is received in purge passages or slots  32  in the fuel nozzle. The purge passages  32  direct the compressor discharge air to the fuel nozzle for tip cooling. Additionally, a portion of the compressor discharge air from the air passage  30  is directed to the nozzle tip cooling passage  34  for cooling the blank cartridge and/or liquid cartridge tips housed inside the outer fuel nozzles. 
     With the described embodiments, the fuel nozzle swozzles have purge slots on the outside of the swozzle shroud to allow passive purge cooling air from the end cap cavity to enter into the fuel nozzle tip cavities and provide tip cooling and tip purge volume. The cap feed air is before quat injection, thereby reducing or eliminating the risk of a flame holding event caused by passive purge air mixed with fuel in prior designs. 
     The added purge slots eliminate the need to provide purge air from the end cover side of the combustion chamber for cooling, this air typically has been mixed with fuel. Additionally, the purge slots simplify the design, eliminating a need to take a feeder pipe in the compressor discharge circuits and feed each end cover on the back end, which would require additional circuitry to direct air to the nozzles. The design still further reduces the fuel nozzle complexity by simplifying the number of fluid circuits required at the flange interface allowing for improved durability and lower cost. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.