Abstract:
A fuel nozzle assembly for a combustor of a gas turbine including: a nozzle body having a front and an inner tube defining a fuel passage extending through the nozzle body, wherein the front proximate to a combustion section of the combustor; an outer casing around the inner tube, wherein an air passage is defined between the outer casing and the inner tube; a gas conduit arranged in the air passage and having an outlet proximate to the front of the nozzle body, wherein fuel starts flowing through the expandable conduit only after a flashback condition occurs in the combustor, and a premix fuel passage and port discharging fuel to a premix section of the combustor, wherein the gas conduit has an inlet open to the premix fuel passage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to gas turbine combustion systems and, specifically, to a fuel nozzle design which minimizes combustor damage during a combustion flame flashback or flame holding event. 
     A gas turbine combustor mixes large quantities of fuel and compressed air and burns the resulting mixture. Conventional combustors for industrial gas turbines typically include an annular array of cylindrical combustion “cans” in which air and fuel are mixed and combustion occurs. Compressed air from an axial compressor flows into the combustor. Fuel is injected through fuel nozzle assemblies that extend into each can. The mixture of fuel and air burns in a combustion chamber of each can. The combustion gases discharge from each can into a duct that leads to the turbine. 
     Combustion cans, designed for low emissions, include premix chambers and combustion chambers. Fuel nozzle assemblies in each combustion can inject fuel and air into the chambers of the can. A portion of the fuel from the nozzle assembly is discharged into the premix chamber of the can, where air is added to and premixed with the fuel. Premixing air and fuel in the premix chamber promotes rapid and efficient combustion in the combustion chamber of each can, and low emissions from the combustion. The mixture of air and fuel flows downstream from the premix chamber to the combustion chamber which supports combustion and under some conditions receives additional fuel discharged by the front of the fuel nozzle assembly. The additional fuel provides a means of stabilizing the flame for low power operation, and may be completely shut off at high power conditions. 
     A flashback or flame holding condition may occur in combustion cans having premix chambers. The premix chambers are not intended to support combustion. Flashback occurs when flame propagates into the premix chamber from the downstream combustion chamber, typically caused by momentary transient conditions. Flame holding occurs when a flame is initiated in the premixing zone, possibly by an external source such as a spark or hot foreign object ejected by the compressor, and the flame then stabilizes in a recirculation zone or weak boundary layer zone immediately downstream of the portion of the fuel nozzle assembly discharging fuel into the premix chamber. The damage resulting from flashback or flame holding may include burning combustor components not intended to be subjected to the heat of combustion. The damage caused by burning these combustor components may cause the components to malfunction and break up. If broken sections of the combustor flow into the combustion gas stream, they potentially may damage the hot gas path, e.g., turbine in the gas turbine. 
     Fuses in fuel nozzle assemblies prevent flame holding by diverting fuel away from the fuel nozzles for the premix chamber. The diversion of fuel from the premix chamber causes the abnormal flame to burn out and prevents further combustion in the premix chamber. However, conventional fuse designs, such as disclosed in U.S. Pat. No. 5,685,139, are not suited to all types of fuel nozzle assemblies. Accordingly, there is a need for novel designs of fuses. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A fuel nozzle assembly for a combustor of a gas turbine has been developed comprising: a nozzle body having a front and an inner tube defining a fuel passage extending through the nozzle body; an outer tube around the inner tube and defining an air passage between the outer tube and the inner tube; a weakened region of the outer tube which burns through in event of a flashback thereby causing a portion of premix fuel to bypass the injectors and to be discharged from the weakened region; an expandable conduit arranged in the air passage and having an outlet adjacent the weakened region, wherein fuel flows through the expandable conduit when the weakened region of the outer tube burns through and the fuel flow is discharged from the conduit, through the weakened region and towards the front of the nozzle body, and a collar attached to the nozzle body, the collar including a premix fuel passage and ports discharging fuel from the collar, wherein the expandable conduit has an inlet open to the premix fuel passage. 
     A method has been developed for quenching a flashback condition in a combustor of a gas turbine, the method comprising: injecting fuel and compressed air from a fuel injector assembly to a premix chamber of the combustor, wherein the injected fuel and compressor air does not normally combust in the premix chamber; combusting the fuel and compressed in a combustion chamber downstream of the premix chamber in the combustor; providing air to the combustion chamber from a front of the injector assembly through an air passage extending through a nozzle body of the fuel injector; injecting fuel to the combustion chamber from a fuel passage having an outlet at the front of the injector assembly; opening an outlet of a conduit in response to a flashback condition adjacent the fuel injector assembly, wherein the outlet is proximate the front of the injector assembly and the conduit extends through the air passage; diverting fuel from the premix chamber to the conduit by the opening of the outlet, and quenching flames of the flashback condition by the diversion of fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing in partial cross-section a conventional combustion can of a gas turbine. 
         FIG. 2  is a perspective view of a fuel nozzle assembly. 
         FIG. 3  is a perspective view of a fuse assembly that is incorporated in the fuel nozzle body of the fuel nozzle assembly. 
         FIG. 4  is a side, cross sectional view of the fuse assembly in the rear collar of the fuel nozzle assembly. 
         FIG. 5  is a side, cross-sectional view of a front portion of the nozzle body. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is side view, showing in partial cross section, a conventional combustor  10  of a gas turbine  12  that includes a compressor  13  (represented by compressor casing  14 ), and a turbine section  15  represented by a single turbine blade  16 . The combustor includes an annular array of combustion cans  18  arranged around the compressor casing  14 . The compressor  13  is driven by the turbine which is drivingly connected along a common axis to the compressor. 
     Pressurized air from the compressor enters each combustion can  18  the combustor  10  and flows (see air arrow  19 ) through an annular duct  20  formed between a cylindrical sleeve  22  and an inner cylindrical liner  24  of the can. Compressed air flows through the duct  20  towards the end cover assembly  26  of the can in a reverse flow direction to the combustion gases formed in the can (see combustion gas arrow  28 ). Air enters the combustion chamber  30  and premix chambers  32  in each can through various openings in the liner  24  and through the premixer inlets  25  in the fuel nozzle assemblies  34 . 
     A mixture of fuel and air is supplied to the premix chambers  32  and the combustion chamber by fuel nozzle assemblies  34  arranged at the front of the can and attached to the end cover. The fuel and compressed air mix in the premix chamber and flow to the combustion chamber  30 . The mixture burns in the combustion chamber and the resulting combustion gases flow (see combustion flow arrow  28 ) from the cans to a transition duct  36  that directs the combustion gases to the turbine blades  16 . 
     Each combustion can  18  includes a substantially cylindrical combustion casing  38  which is secured at an open aftward end to the compressor casing  14 . The forward end of the combustion can is closed by the end cover assembly  26  which may include conventional fuel supply tubes, manifolds and associated valves for feeding gas, liquid fuel and air (and water if desired) to the combustor can. The end cover assembly  26  supports multiple fuel nozzle assemblies  34  for each can. For example, fuel nozzle assemblies may be arranged in a circular array around a center nozzle assembly. These nozzle assemblies may be treated has having the same structure, at least for purposes of describing the fuse system. 
       FIG. 2  is a perspective view of a fuel nozzle assembly  34 . The nozzle assembly  34  includes a nozzle body  40 , a rear collar  42  and a rear section  44  that connects to the end cover assembly of a combustor can. Fuel and air is supplied to the end cover assembly which directs the fuel to the rear section of the fuel nozzle assembly. The rear collar  42  forms an outer ring of an annular air passage  48  that provides premix air to the premix chamber of the combustion can. Within the annular air passage  48  are radial vanes  50  that impart a spiral flow to the premix air flowing through the passage  48 . The vanes  50  contain fuel discharge ports  52  ( FIG. 4 ) through which fuel is discharged from the fuel nozzle assembly into the premix chamber, where it mixes with the air flowing in air passage  48 . One or more fuel gas passages and fuel discharge ports may be arranged in the vanes  50 . The front  46  of the nozzle body includes the forward fuel nozzle ports that deliver fuel directly to the combustion chamber in the combustor can. 
       FIG. 3  is a perspective view of a fuse assembly  54  that is incorporated in the fuel nozzle assembly and, specifically, in the collar and nozzle body. The fuse assembly  54  includes a cylindrical array of helical conduits  56  that extend from a cylindrical rear fuse base  58  mounted in the rear collar to a cylindrical front fuse and nozzle base  60  mounted in the front of the nozzle body. The conduits  56  may be brazed to the bases  58 ,  60 . The helical shape of the conduits  56  allows the conduits to expand or contract in an axial direction, such as due to thermal expansion. The rear fuse base  58  includes openings  61 ,  62  that are aligned with a fuel passage or fuel passages in the collar when the fuse base  58  is inserted in the rear collar. Arranging the openings  61 ,  62  in two or more rows (as shown in  FIG. 3 ) allows the multiple conduits  56  to receive fuel from multiple premix fuel passages in the collar  42 . The openings  61 ,  62  lead to respective passages in the fuse base  58  and the conduits  56 . 
     Fuel from the fuel passage, that would normally flow to the premix chamber, flows through the rear fuse base  58  and the helical conduits  56  to the nozzle base  60  when the fuse is activated by a flashback event. After the fuse has been activated, the fuel flowing through the helical conduits  56  diverts fuel from the premix chamber(s) to prevent further combustion of fuel in that chamber(s). 
     Openings  63 ,  64  on the front fuse and nozzle base  60  allow the fuel from the helical conduits  56  to discharge through the front of the nozzle body and into the combustion chamber. The openings  63 ,  64  are normally blocked to prevent the flow of fuel through the helical conduits. When the openings  64  are not blocked, the flow of fuel through helical conduits diverts fuel from the premix chamber, so as to quench a flash back or flame holding condition. The front fuse and nozzle base also includes air nozzles  66  for air discharged from the front of the fuel nozzle. The discharged air forms an air curtain around the fuel flowing from the front  46  of the fuel nozzle. 
       FIG. 4  is a side, cross sectional view of the fuel nozzle assembly and, specifically, the rear collar  42  and rear section  44  of the fuel assembly. The rear fuse base  58  is mounted in the rear collar. A cylindrical gas passage  68  is defined by an inner tubular section  69  aligned with the axis of the fuel nozzle and extending through the rear section  44 , the rear collar  42  and the nozzle body  40  of the fuel assembly. An annular gas passage  70  is defined between the inner tube  69  and an outer wall of the passage. Fuel flows through the annular fuel gas passage  70  from the rear section  44  of the fuel assembly to the rear collar  42 . 
     As indicated by flow arrow  72 , the fuel gas flows from the gas passage  70 , through passages  71  in the rear fuse base  58 , the openings  61 ,  62  that lead to the radial vanes  50  of the rear collar, out the fuel ports  52  in the vanes and into the premix chamber. The gas flows as indicated by arrow  72 , unless the fuse has been activated. An single flow arrow  72  is shown to indicate a premix gas path through the rear collar  42  and passages in the vanes  50 . However, one or multiple premix gas paths may be in the rear collar and vanes. Each of the premix gas paths may be associated with a different one of the helical conduits  56 . Further each of the premix gas paths may be associated with one or more of the helical conduits. 
     When the fuse is activated, the gas flows from passage  70 , through the passages  71  in the rear fuse base  58  and to the helical conduits  56  as indicated by flow arrow  74 . The conduits  56  provide a flow path that diverts most of the fuel in passage  70  away from the vanes  50  and the fuel ports  52 . 
     The helical conduits  56  are arranged in an annular air passage  76  between the tube  69  of the gas passage  68  and an outer tubular casing  78  of the nozzle body  40 . Air enters through ports  77  in the rear collar  42  and flows into the air passage  76 . The air flows through the passage  76 , across outer surfaces of the helical conduits  56  and to the front fuse and nozzle base. The size and number of the conduits  56  are such that the air flowing through the passage  76  is sufficient for the curtain of air flow needed at the front of the fuel nozzle. Preferably, the helical conduits occupy less than one half of the volume of the passage  76 . 
       FIG. 5  is a side, cross-sectional view of a front portion of the nozzle body  40 . The helical conduits  56  are arranged in the annular air passage  76  defined between the inner cylindrical tube  69  of the gas passage  68  and the tubular casing  78  of the nozzle body  40 . The helical shape of the conduits  56  allows for axial expansion of the conduits. The front fuse and nozzle base  60  is seated between the wall of the gas passage  68  and the tubular casing  78 . 
     The openings  64  in the front fuse and nozzle base  60  are adjacent a weakened section  80 , e.g., a relatively thin annular section, of the casing  78 . The weakened sections  80  may be a segmented annular region of the casing  78  that has been machined to remove some of the thickness of the casing wall adjacent the openings  64  of the base  60 . The weakened sections  80  are susceptible to burning through in the event of a flashback. Once burned through, the opened weakened sections  80  allow fuel to flow out the openings  64  in the fuse and nozzle base  60  and flow through the helical conduits  56 . The flow of fuel through the helical conduits diverts fuel from the premix chamber and starves and quenches any flame occurring in the premix chamber to stop the flash back condition. 
     The inner cylindrical wall of the gas passage  68  has a front end that fits into a quasi-conical inner sleeve assembly  82  that supports the front nozzle  84 . The inner sleeve assembly allows for thermal expansion between the cylindrical wall of the gas passage and the front nozzle. Air from the annular passage  76  flows through the front fuse and nozzle base  60  and through swirl vanes  86  before being discharged around the front of the center fuel discharge nozzle ports  88  for the gas passage  68 . 
     While the invention has been described in connection with what is presently considered to be the most practical and 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 included within the spirit and scope of the appended claims.