Patent Publication Number: US-8522554-B2

Title: Fuel nozzle for a turbine engine with a passive purge air passageway

Description:
BACKGROUND OF THE INVENTION 
     Turbine engines that are used in the electric power generation industry typically include a plurality of combustors which are arranged concentrically around an input to the turbine section. A typical combustor assembly is shown in  FIG. 1 . The combustor assembly includes both primary and secondary fuel nozzles. 
     The primary and secondary fuel nozzles inject fuel into a flow of compressed air received from the compressor side of the turbine. The fuel is mixed with the air, and the fuel-air mixture is then ignited downstream from the fuel injectors in one or more combustion zones. Ideally, the combustion takes place at a location that is located downstream from the distal ends of the fuel nozzles so that the nozzles themselves are not subjected to extremely high temperatures. In addition, it is common for fuel nozzles to include purge air passageways which conduct a flow of the compressed air that is designed to cool the nozzles. 
     During some turbine operational conditions, the purge air passageways of the fuel nozzle are temporarily prevented from conducting a flow of cooling air. In those instances, portions of the fuel nozzles adjacent the combustion zones can be subjected to extremely high temperatures that can damage the fuel nozzles. Typically, the downstream ends of the nozzles are subjected to the highest temperatures, and are therefore most likely to be damaged. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, the invention may be embodied in a fuel nozzle for a turbine engine that includes an elongated housing having a central longitudinal axis, at least one fuel delivery passageway that extends down at least a portion of the housing, and an active purge air passageway that extends down the housing and that delivers purge air to at least one active purge air discharge opening that is located at the discharge end of the nozzle. The fuel nozzle also includes a passive purge air passageway having an inlet that admits air from a position outside the nozzle and adjacent an upstream end of the housing, and having an outlet that is located at the discharge end of the nozzle. 
     In another aspect, the invention may be embodied in a fuel nozzle for a turbine engine that includes an elongated housing having a central longitudinal axis, a fuel delivery passageway that extends along at least a portion of the housing, and an active purge air passageway that extends along the housing and that delivers purge air to at least one active purge air discharge opening that is located at a downstream end of the nozzle. The fuel nozzle also includes a passive purge air passageway having an inlet that admits air from a position outside the nozzle and adjacent an upstream end of the housing, and having an outlet that is located at the downstream end of the nozzle, wherein when the fuel nozzle is in use in an operational turbine engine, a pressure differential between air outside the nozzle and adjacent the upstream end of the housing and air located at the outlet of the passive purge air passageway causes purge air to flow along the passive purge air passageway from the inlet to the outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating elements of a typical combustor of a turbine engine; 
         FIG. 2  is a cross-sectional view of a secondary fuel injection nozzle used in a combustor of a turbine engine; 
         FIG. 3  is a cross-sectional view of another embodiment of a secondary fuel nozzle; and 
         FIG. 4  is a diagram illustrating the temperatures which exist at a tip of two different fuel nozzles during a fuel transfer procedure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical combustor assembly for a turbine engine is illustrated in  FIG. 1 . As shown therein, the combustor includes a transition duct  20  which routes combustion gases into the turbine section. The transition duct  20  is attached to a combustor liner  40 . A flow sleeve  30  surrounds the exterior of the combustor liner  40 . 
     Compressed air from the compressor section of the turbine is routed into the annular space between the combustor liner  40  and the flow sleeve  30 . The arrows in  FIG. 1  illustrate the direction of movement of the compressed air. As shown in  FIG. 1 , the compressed air moves along the annular space between the combustor liner and the flow sleeve  30  to the upper end of the combustor. The compressed air then turns and enters the space inside the combustor liner  40 . 
     A plurality of fuel nozzles  60 ,  70  are located at the upstream end of the combustor. Multiple primary fuel nozzles  60  are mounted in an annular ring around a combustor cap  50 . In addition, at least one secondary fuel nozzle  70  is located in the center of the combustor. As shown in  FIG. 1 , the secondary fuel nozzle  70  typically extends a greater distance down the length of the combustor. 
     Combustion within the combustor typically takes place in two different locations. There is a primary combustion zone  90  located at the far upstream end of the combustor and adjacent the discharge ends of the primary fuel nozzles  60 . In addition, there is a secondary combustion zone  100  located further down the length of the combustor and adjacent a discharge end of the secondary fuel nozzle  70 . In some combustors, a venturi is formed between the primary combustion zone  90  and the secondary combustion zone  100  by angled walls  55 . The angled walls  55  neck in to reduce an interior diameter of the combustor. The venturi formed by the angled walls  55  increases the speed of the air and fuel passing through this section of the combustor immediately before the air-fuel mixture enters the secondary combustion zone  100 . 
     During an initial start up procedure, fuel is delivered into the combustor through both the primary fuel nozzles  60  and the secondary fuel nozzle  70 . The air fuel mixture is ignited in both the primary combustion zone  90  and the secondary combustion zone  100 . The operating speed of the turbine is increased and a load, typically in the form of an electrical power generator, is placed on the turbine. 
     To achieve optimum emissions, it is desirable for combustion to take place only in the secondary combustion zone  100 . Thus, although it is necessary to initially have combustion occurring in both the primary combustion zone  90  and the secondary combustion zone  100 , at some point during the start up procedure it is necessary to eliminate combustion in the primary combustion zone  90 . 
     In order to eliminate combustion in the primary combustion zone  90 , it is necessary to temporarily cut off fuel to the primary fuel nozzles  60 . During this transition time period, fuel is still delivered into the secondary combustion zone  100  through the secondary fuel nozzle  70 . Once fuel has been cut to the primary fuel nozzles  60  for a period of time, combustion in the primary combustion zone  90  will cease, and combustion will only continue to take place in the secondary combustion zone  100 . 
     Because a load is placed on the turbine, and to ensure that the turbine maintains this load, one cannot simply cut fuel to the primary fuel nozzles. Instead, it is necessary for approximately the same amount of fuel to be continuously delivered into the combustor during the transition time period. Thus, in a typical transition sequence, when the fuel is cut to the primary fuel nozzles  60 , the same amount of fuel that was being delivered through the primary fuel nozzles  60  is instead delivered through passages of the secondary fuel nozzle  70 . This means that the secondary fuel nozzle  70  must deliver all of the fuel which was previously being delivered into the combustor through both the primary fuel nozzles  60  and the secondary fuel nozzle  70 . 
     Once combustion is no longer occurring in the primary combustion zone  90 , fuel can again be delivered through the primary fuel nozzles  60 . Fuel delivered through the primary nozzles  60  will swirl around the interior of the primary combustion zone to fully mix with the surrounding air, and as the air-fuel mixture moves into the secondary combustion zone  100  it would then be ignited. Thus, during steady state operations, it is desirable to deliver fuel through both the primary fuel nozzles  60 , and the secondary fuel nozzle  70 , and for all of the air and fuel to burn in the secondary combustion zone  100 . More details of this fuel transition procedure will be described below after a description of a typical secondary fuel nozzle  70  has been provided. 
       FIG. 2  is a functional diagram of a typical secondary fuel nozzle  100 . The secondary fuel nozzle includes multiple passageways which extend down the length of the housing of the nozzle. In the embodiment shown in  FIG. 2 , there is a central passageway  110  which can be used to deliver either fuel or air to the downstream end of the nozzle. A second passageway  120  concentrically surrounds the first passageway  110 . There is also a third passageway  130  which concentrically surrounds the second passageway  120 . Finally, there is a fourth passageway  140  which concentrically surrounds the third passageway  130 . At least one of these passageways would deliver fuel to a plurality of radially extending fuel injectors  145 . In some embodiments, the fourth passageway  140  might deliver fuel to the fuel injectors  145 . In other embodiments, the third passageway  130  might deliver fuel to the fuel injectors  145 , and the fourth passageway  140  might be used as a purge air passageway. 
     A plurality of fuel delivery apertures  146  are formed on the radially extending fuel injectors  145 . As a result, fuel delivered to the fuel injectors exits through the fuel delivery apertures  146 . During normal operations, compressed air is flowing down the length of the exterior of the fuel nozzle. Thus, the fuel exiting the fuel delivery apertures  146  mixes with the air passing down the length of the fuel nozzle to create an air-fuel mixture which can then be ignited. Although not shown, a variety of different swirler devices can be located upstream and/or downstream of the radially extending fuel injectors to increase the swirling and mixing action of the air, to thereby better mix the air with the fuel being delivered through the fuel delivery apertures  146 . 
     Fuel being delivered through the fuel injectors  145  forms one fuel delivery mechanism of the fuel nozzle. However, fuel is also typically delivered through one or more of the internal passageways. For instance, fuel might be delivered through the second passageway  120 . This fuel delivery circuit is often referred to as a pilot fuel circuit. The fuel being delivered through the second or pilot fuel passageway  120  exits the downstream end of the fuel nozzle and is also ignited. The flame produced by the fuel passing through the pilot or secondary passageway  120  is often referred to as a pilot flame. The pilot flame is quite stable and is not typically subjected to flame out. 
     The third passageway  130  typically carries purge air and/or fuel. The purge air being delivered through the third passageway  130  is used to cool the outer nozzle tip. In particular, the purge air cools the downstream end of the fuel nozzle, which is typically subjected to the highest temperatures due to the combustion zone located just downstream of the discharge end of the fuel nozzle. In addition, purge air is frequently delivered through the first passageway  110  located at the center of the fuel injection nozzle. Here again, the purge air passing through the first passageway  110  is designed to cool the nozzle, and in particular, the downstream end of the nozzle. 
     A header or manifold  150  is formed at the upstream end of the nozzle. The header  150  includes a variety of passageways which are designed to deliver fuel and air into the first, second, third and fourth passageways inside the fuel nozzle. The header  150  would typically be connected to a fuel delivery line  162  and to a purge air line  164 . The purge air line  164  would be connected to a source of compressed air, which is typically tapped from the compressor section of the turbine. Thus, the line  164  delivering compressed air would typically run to a tap on the compressor section of the turbine or a compressor discharge plenum. 
     In addition, a transition fuel delivery line  166  is also connected to the manifold  150 . The transition fuel delivery line  166  conveys fuel to the secondary fuel nozzle  100  which would otherwise be carried by and delivered from the primary fuel nozzles. Thus, during a fuel transition procedure as described above, when fuel is cut off from the primary fuel nozzles, that fuel would be delivered to the secondary fuel nozzle  100  through the transition fuel delivery line  166 . 
     As explained above, when it is time to cease combustion in the primary combustion zone  90  of the combustor, the fuel to the primary fuel nozzles  60  is cut off, and the fuel that would otherwise be delivered through the primary fuel nozzles  60  is instead routed to the secondary fuel nozzle  70 . As also explained above, that fuel must be delivered into the secondary combustion zone  100  through the secondary fuel nozzle  70 . 
     Because one of the passageways is already delivering fuel through the radially extending fuel injectors  145 , and because the pilot fuel passageway  120  is already delivering fuel through the secondary fuel nozzle, the only other portions of the secondary fuel nozzle which are available to deliver fuel into the secondary combustion zone are the passageways within the nozzle that are carrying purge air. Accordingly, during the fuel transition procedure, the fuel delivered to the secondary fuel nozzle via the transition fuel delivery line  166  is typically routed into purge air passageways. This means that the purge air normally carried through these passageways must be cut off, and fuel is instead delivered through these passageways. And during the time required to switch off the purge air and route fuel into the purge air passageways, no fuel or purge air is flowing through the purge air passageways. 
     As explained above, fuel would be delivered through the purge air passageways until combustion ceases in the primary combustion zone  90 . During this period of time, fuel is passing through the purge air passageways, and the fuel acts to cool the downstream end of the secondary fuel nozzle. As a result, the temperature at the downstream end of the secondary fuel nozzle remains relatively low while fuel is running through the purge air passageways. 
     Once combustion ceases in the primary combustion zone  90 , the fuel is transitioned back to the primary fuel nozzles, and purge air can again be delivered through the purge air passageways of the nozzle. However, the switch over procedure takes a certain amount of time. Typically, the fuel would be immediately switched back to the primary fuel nozzles  60 . However, for a short period of time after that switch over, fuel will still reside in the purge air passageways of the nozzle. 
     When purge air is again introduced into the purge air passageways, the purge air will force the fuel remaining in these passageways out the downstream end of the nozzle. If the purge air were immediately switched on to its normal flow level, this would rapidly inject a large amount of fuel into the secondary combustion zone  110  at the same time that the primary fuel nozzles are also already delivering fuel, which would result in a surge in the combustion zone and possibly a resulting surge in the load output of the turbine. For these reasons, once fuel has been switched back to the primary fuel nozzles, the purge air is gradually and slowly introduced back into the purge air passageways so that any fuel in those passageways is gradually pushed out into the secondary combustion zone. And this further delays the time before purge air is flowing normally to cool the downstream end of the nozzle. 
     As explained above, during a fuel transition procedure the temperature at the downstream end of the nozzle can raise to extremely high temperatures at two points in time. The upper half of  FIG. 4  shows a diagram of the temperature of the tip of the nozzle during a typical fuel transition procedure. As shown in  FIG. 4 , fuel will be cut to the primary nozzles, and then the purge air to the secondary nozzle would be closed off. At that point in time, the temperature at the downstream tip of the fuel nozzle begins to rise quite rapidly. The temperature at the downstream end of the nozzle will continue to rise until fuel is running though the purge air passageways. 
     During normal steady state conditions, when purge air is being sent through the nozzle, the tip of the nozzle remains at approximately 300° F. Once the purge air is stopped, the temperature rapidly rises past 1000° F. Once the transfer fuel (which was previously being delivered into the combustor through the primary nozzles) begins to flow through the purge air passageways of the secondary nozzle, the temperature quickly returns to a temperature close to that of the fuel temperature. 
     During the time that fuel is being sent through the purge air passageways of the secondary nozzle, the temperature remains at relatively low, safe temperatures. However, once the transfer fuel supply is stopped and fuel is again delivered into the combustor through the primary nozzles, the temperature at the tip of the secondary nozzle again begins to rise. The temperature will continue to rise until purge air is gradually introduced into the purge air passageways, as explained above. Once the purge air is again flowing through the purge air passageways of the secondary nozzle, the temperature again returns back to normal. 
     As shown in the upper half of  FIG. 4 , the temperature at the tip of the secondary nozzle tends to peak at two different points in time during the fuel transfer procedure. The first peak occurs when the purge air has been closed off and no fuel is yet flowing through the purge air passageways of the secondary nozzle. The second peak occurs when the transfer fuel is shut off and before purge air is again flowing normally through the purge air passageways. During both these events, the temperature at the tip of the nozzle can rise as high as 1500° F. These temperatures are potentially quite damaging to the material of the nozzle tip and can lead to permanent damage. 
       FIG. 3  shows a modified version of the secondary fuel nozzle. This embodiment is similar to the one shown in  FIG. 2 , however, the central purge air passageway  110  has been modified to communicate with an aperture formed on the exterior of the fuel nozzle at the upstream side of the nozzle. As shown in  FIG. 3 , a first passageway  170  is coupled to the inlet of the central purge air passageway  110 . The first passageway  170  extends radially and is coupled to a second passageway  160  which leads to an aperture  161  on the header  150  of the nozzle. In addition, in this embodiment a swirler plate  115  is located at a downstream end of the central purge air passageway  110 . Swirler plates  125  are also located in the pilot fuel passageway  120 . 
     When a secondary fuel nozzle as illustrated in  FIG. 3  is in operation inside a combustor, the pressure adjacent the downstream end of the nozzle will be lower than a pressure adjacent the upstream end of the nozzle. Because a swirler plate causes a pressure drop, the pressure differential between the upstream and downstream ends can be increased though the addition of a swirler plate  115  within the central purge air passageway  110 . Because the pressure of the air at the upstream end of the fuel nozzle is higher than at the downstream end, during steady state operation of the nozzle, air will be drawn in through the aperture  161  and it will flow through the first and second passageways  170 ,  160  into the inlet of the purge air passageway  110 , and then down the central purge air passageway  110 . The flow will remain continuous so long as normal operations are occurring within the combustor. 
     When a nozzle as illustrated in  FIG. 3  is used, during a fuel transition procedure as described above, the transition fuel supply line  166  will be coupled only to one or more of the second, third or fourth passageways. During the fuel transition procedure, purge air will continuously run through the central purge air passageway  110 . This continuous supply of purge air ensures that the temperature of the tip of the nozzle at the downstream end will remain relatively constant, even during the fuel transition procedure. In addition, because the flow of the purge air through the central purge air passageway  110  is generated due to a pressure differential between the air surrounding the downstream end of the nozzle and the air at the upstream end of the nozzle, there is no need to provide a separate supply of compressed purge air from a compressor section of the turbine. Instead, the purge air running though the central purge air passageway  110  is simply drawn from the compressed air already surrounding the upstream end of the nozzle. 
     In the embodiment illustrated in  FIG. 3 , the secondary flame detector sight hole of the nozzle is used as the second passageway  160  for delivering air to the central purge air passageway  110 . As a result, it is only necessary to modify the first embodiment shown in  FIG. 2  through the addition of the radially extending first passageway  170  that connects the secondary flame detector sight hole and the inlet to the central purge air passageway  110 . Thus, a very simple modification to the original secondary nozzle can insure that purge air is always supplied, even during a fuel transition procedure. 
     In addition, in the embodiment shown in  FIG. 3 , the central purge air passageway  110  is coupled to the passageway  170  leading out to a position outside the upstream end of the nozzle. In alternate embodiments, different passageways other than the central passageway  110  might be connected to the passageway  170 . In still other embodiments, the passageway  170  might be connected to multiple ones of the passageways inside the nozzle. 
     The lower portion of  FIG. 4  illustrates the temperature at the downstream end of a nozzle as shown in  FIG. 3  during a fuel transition procedure. As shown therein, the temperature of the downstream tip of the fuel nozzle remains relatively constant, even during the fuel transition procedure. This prevents the downstream end of the nozzle from being damaged by high temperatures during the fuel transition procedure. 
     In the foregoing description, the embodiments shown in  FIGS. 2 and 3  were used as an example of how a passive purge air circuit could be provided on a secondary fuel nozzle to ensure a constant supply of purge air, even during a fuel transition procedure. The embodiments shown in  FIGS. 2 and 3  illustrate a secondary nozzle having two purge air passageways, a pilot fuel passageway, and a main fuel passageway. In alternate embodiments, the passageways could be configured in different orientations and the pilot fuel passageway could be eliminated. In addition, one fuel delivery passageway could deliver fuel to the radially extending fuel injectors, and the same or a different fuel delivery passageways could also deliver fuel through apertures located at the downstream end of the nozzle. The configurations shown in  FIGS. 2 and 3  are only intended to be illustrative, and not limiting. A secondary fuel nozzle could be configured in a variety of different ways depending on the requirements of a particular turbine. 
     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.