Abstract:
A method for operating a combustion system is provided. The method includes coupling the main swirler to the pilot swirler such that the main swirler substantially circumscribes the pilot swirler, supplying fuel to a first fuel circuit defined in the main swirler, and inducing swirling to the supplied fuel via a first set of swirler vanes positioned within the main swirler. The method also includes supplying fuel to a second fuel circuit defined in the main swirler, inducing swirling to the supplied fuel via a second set of swirler vanes positioned within the main swirler, each of the second set of swirler vanes comprising at least one second fuel passage defined therein, and coupling a shroud in flow communication to at least one of the first set of swirler vanes and the second set of swirler vanes, the shroud comprising at least one third fuel passage defined therein.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to combustors and more particularly, to methods and apparatus to facilitate decreasing combustor acoustics. 
         [0002]    During the combustion of natural gas, pollutants such as, but not limited to, carbon monoxide (“CO 2 ”), unburned hydrocarbons (“UHC”), and nitrogen oxides (“NO x ”) may be formed and emitted into an ambient atmosphere. At least some known emission sources include devices such as, but not limited to, gas turbine engines and other combustion systems. Because of stringent emission control standards, it is desirable to control emissions of such pollutants by the suppressing formation of such emissions. 
         [0003]    At least some known combustion systems implement combustion modification control technologies such as, but not limited to, Dry-Low-Emissions (“DLE”) combustors and other lean pre-mixed combustors to facilitate reducing emissions of pollutants from the combustion system by using pre-mixed fuel injection. For example, at least some known DLE combustors attempt to reduce the formation of pollutants by lowering a combustor flame temperature using lean fuel-air mixtures and/or pre-mixed combustion. However, at least some known DLE combustors experience combustion acoustics that can limit the operability and performance of a combustion system that includes such known DLE combustor. 
         [0004]    Known strategies employed in an effort to reduce combustion acoustics include the following: (1) passive damping of pressure fluctuations with quarter-wave tubes, resonators, acoustic liners/baffles, and/or other acoustic damping devices; (2) incorporating design features into premixers to facilitate desensitizing a fuel-air mixing with respect to pressure fluctuations from a combustion chamber; (3) operating the combustor with significant variation in flame temperatures between individual domes of multidome combustors or individual premixers of singular annular combustors; (4) open-loop active control to introduce off-resonant fluctuations in fuel and/or air flows to facilitate weakening resonant modes; and/or (5) closed-loop active control methods that respond in real time to facilitate disturbing fuel and/or air flows in such a manner as to decouple physical processes responsible for feedback between pressure oscillations and heat release. 
         [0005]    At least some known DLE combustors include both passive and active control features to facilitate suppressing combustion acoustics such as, but not limited to, combustion-inducing acoustic waves and combustion-inducing pressure oscillations that may be formed as a result of combustion instabilities that may be generated when a pre-mixed fuel and compressed air ignite. For example, quarter wave tubes have been used to passively damp pressure fluctuations adjacent to premixer inlets. Also, supplemental fuel circuits such as Enhanced Lean Blow-Out (“ELBO”) fuel circuits have been used in known pilot swirlers to actively inject smaller amounts of fuel into the combustor at a different location than a primary fuel injection location. 
         [0006]    Compared to primary fuel circuits, ELBO fuel circuits generally require a shorter convective timescale for an ELBO fuel-air mixture to travel from a point of injection to a flame front where heat release occurs. As such, an acoustic frequency interacts differently with the ELBO fuel-air mixing at an ELBO fuel injection location as compared to primary fuel-air mixing at a primary injection location. As a result, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to pressure fluctuations in the combustor are generated to facilitate reducing combustion acoustics by reducing an amplitude of pressure fluctuations in the DLE combustor. 
         [0007]    However, combustion of lean fuel-air mixtures generates heat temperatures that are sensitive to any variation in the fuel-air ratio of the fuel-air mixture. Such variations in the fuel-air ratio may be caused by fluctuations in a flow rate of the fuel and/or a flow rate of the compressed air. Because fuel flow and/or compressed air flow through known DLE combustors may be turbulent, fluctuations in the fuel and/or compressed air flow rates may cause pressure disturbances in a combustion chamber/zone of such DLE combustors. If such pressure disturbances interact with a fuel-air mixing process, any heat being released may also fluctuate to reinforce an initial pressure disturbance. Over time, the increased amplitude of pressure disturbances may cause damage to portions of the DLE combustor. As a result, operability, emissions, maintenance cost, and life of combustor components may be negatively affected. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    In one aspect, a method for operating a combustion system including at least one premixer assembly that includes a pilot swirler and a main swirler is provided. The method includes coupling the main swirler to the pilot swirler such that the main swirler substantially circumscribes the pilot swirler, supplying fuel to a first fuel circuit defined in the main swirler, and inducing swirling to the fuel supplied to the first fuel circuit via a first set of swirler vanes positioned within the main swirler. Each of the first set of swirler vanes include at least one first fuel passage defined therein. The method also includes supplying fuel to a second fuel circuit defined in the main swirler and inducing swirling to the fuel supplied to the second fuel circuit via a second set of swirler vanes positioned within the main swirler. Each of the second set of swirler vanes includes at least one second fuel passage defined therein. The method further includes coupling a shroud in flow communication to at least one of the first set of swirler vanes and the second set of swirler vanes. The shroud includes at least one third fuel passage defined therein. 
         [0009]    In another aspect, a combustion system is provided. The combustion system includes a pilot swirler and a main swirler coupled to the pilot swirler such that the main swirler substantially circumscribes the pilot swirler. The main swirler includes a first set of swirler vanes for inducing swirling to fuel supplied to a first fuel circuit defined in the main swirler. Each of the first set of swirler vanes includes at least one first fuel passage defined therein. The main swirler also includes a second set of swirler vanes for inducing swirling to fuel supplied to a second fuel circuit defined in the main swirler. Each of the second set of swirler vanes includes at least one second fuel passage defined therein. Further, the main swirler includes a shroud coupled in flow communication to at least one of the first set of swirler vanes and the second set of swirler vanes. The shroud includes at least one third fuel passage defined therein. 
         [0010]    In another aspect, a fuel delivery apparatus is provided. The fuel delivery system includes a first set of swirler vanes for inducing swirling to fuel supplied to a first fuel circuit defined in the main swirler. Each of the first set of swirler vanes includes at least one first fuel passage defined therein. The fuel delivery system also includes a second set of swirler vanes for inducing swirling to fuel supplied to a second fuel circuit defined in the main swirler. Each of the second set of swirler vanes includes at least one second fuel passage defined therein. Further, the fuel delivery system includes a shroud coupled in flow communication to at least one of the first set of swirler vanes and the second set of swirler vanes. The shroud includes at least one third fuel passage defined therein. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine including a combustor; 
           [0012]      FIG. 2  is a cross-sectional view of a portion of an exemplary known combustor including a premixer assembly that may be used with the gas turbine engine shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a perspective view of the portion of the known combustor shown in  FIG. 2 ; 
           [0014]      FIG. 4  is an enlarged cross-sectional view of an exemplary premixer assembly that may be used with the combustor shown in  FIGS. 2 and 3 ; 
           [0015]      FIG. 5  is an enlarged cross-sectional view of an alternative embodiment of a premixer assembly that may be used with the combustor shown in  FIGS. 2 and 3 ; and 
           [0016]      FIG. 6  is an enlarged cross-sectional view of another alternative embodiment of a premixer assembly that may be used with the combustor shown in  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The exemplary methods and apparatus described herein overcome the disadvantages of known combustors by forming an Enhanced Lean Blow-Out fuel (“ELBO”) fuel circuit that supplies ELBO fuel through a main swirler shroud to facilitate reducing combustion acoustics. 
         [0018]    It should be appreciated that “forward” is used throughout this application to refer to directions and positions located axially upstream toward an fuel/air intake side of a combustion system for the ease of understanding. It should also be appreciated that “aft” is used throughout this application to refer to directions and positions located axially downstream toward an exit plane of a main swirler for the ease of understanding. Moreover, it should be appreciated that the term “ELBO” is used throughout this application to refer to various components of an Enhanced Lean Blow-Out fuel circuit, which is a supplemental fuel circuit that injects ELBO fuel that represents a relatively small portion of fuel injected as compared to an amount of main fuel supplied to a primary main fuel injector positioned within the combustor at a different location than the injector(s) for use with the ELBO fuel. 
         [0019]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine  10  including an air intake side  12 , a fan assembly  14 , a core engine  18 , a high pressure turbine  22 , a low pressure turbine  24 , and an exhaust side  30 . Fan assembly  14  includes an array of fan blades  15  extending radially outward from a rotor disc  16 . Core engine  18  includes a high pressure compressor  19  and a combustor  20 . Fan assembly  14  and low pressure turbine  24  are coupled by a first rotor shaft  26 , and high pressure compressor  19  and high pressure turbine  22  are coupled by a second rotor shaft  28  such that fan assembly  14 , high pressure compressor  19 , high pressure turbine  22 , and low pressure turbine  24  are in serial flow communication and co-axially aligned with respect to a central rotational axis  32  of gas turbine engine  10 . In one exemplary embodiment, gas turbine engine  10  may be a GE90 engine commercially available from General Electric Company, Cincinnati, Ohio. 
         [0020]    During operation, air enters through air intake side  12  and flows through fan assembly  14  to high pressure compressor  19 . Compressed air is delivered to combustor  20 . Airflow from combustor  20  drives high pressure turbine  22  and low pressure turbine  24  prior to exiting gas turbine engine  10  through exhaust side  30 . 
         [0021]      FIG. 2  is a cross-sectional view of a portion of known combustor  20  including a premixer assembly  100  that may be used with a gas turbine engine, such as gas turbine engine  10  shown in  FIG. 1 .  FIG. 3  is a perspective view of the portion of known combustor  20  including premixer assembly  100 . In the exemplary embodiment, combustor  20  includes a combustion chamber/zone  40  that is defined by annular liners (not shown), at least one combustor dome  50  that defines an upstream end of combustion zone  40 , and a plurality of premixer assemblies  100  that are circumferentially-spaced about each combustor dome  50  to deliver a fuel/air mixture to combustion zone  40 . 
         [0022]    In the exemplary embodiment, each premixer assembly  100  includes a pilot swirler  110 , an annular centerbody  120 , and a main swirler  130 . Pilot swirler  110  includes a pilot centerbody  112  having a central rotational axis  113 , an inner annular swirler  114 , and a concentrically disposed outer annular swirler  116 . Inner annular swirler  114  is circumferentially disposed about pilot centerbody  112  and co-axially aligned with central rotational axis  113 . Outer annular swirler  116  is circumferentially disposed about pilot centerbody  112  and inner annular swirler  114 , and co-axially aligned with central rotational axis  113 . 
         [0023]    Annular centerbody  120  is circumferentially disposed about pilot centerbody  112 , inner annular swirler  114 , and outer annular swirler  116 . Annular centerbody  120  is also co-axially aligned with central rotational axis  113  and defines a centerbody cavity  122 . Further, annular centerbody  120  extends between pilot swirler  110  and main swirler  130 . Main swirler  130  includes a plurality of main swirler vanes  140  and an annular main swirler shroud  160  that defines an annular main swirler cavity  170 . Main swirler shroud  160  is coupled to, and extends aftward from, an aft end  141  of main swirler vanes  140 . 
         [0024]      FIG. 4  is an enlarged cross-sectional view of an exemplary premixer assembly  200  that may be used with the combustor  20  shown in  FIGS. 2 and 3 . In the exemplary embodiment, premixer assembly  200  includes a pilot swirler  210 , an annular centerbody  220 , and a main swirler  230 . Pilot swirler  210  includes a pilot centerbody  212  having a central rotational axis  213 , an inner annular swirler  214 , and a concentrically disposed outer annular swirler  216 . Inner annular swirler  214  includes a plurality of inner pilot vanes  215  circumferentially disposed about pilot centerbody  212 , and is co-axially aligned with central rotational axis  213 . Outer annular swirler  216  includes a plurality of outer pilot vanes  217  circumferentially disposed about pilot centerbody  212  and inner annular swirler  214 , and is co-axially aligned with central rotational axis  213 . 
         [0025]    Annular centerbody  220  is co-axially aligned with central rotational axis  213  and defines a centerbody cavity  222 . Annular centerbody  220  also includes a plurality of orifices  224  coupled, in flow communication, to centerbody cavity  222 . Moreover, annular centerbody  220  includes a forward end portion  226  defining an annular pilot swirler fuel manifold  227  and an annular main swirler fuel manifold  228 . Further, annular centerbody  220  extends between pilot swirler  210  and main swirler  230  to control fuel flow through premixer assembly  200 . 
         [0026]    Main swirler  230  includes a plurality of main swirler vanes  240  and an annular main swirler shroud  260  that both define an annular main swirler cavity  270 . Main swirler vanes  240  include aft ends  241  and are annularly arranged about annular centerbody  220 . Moreover, each main swirler vane  240  includes a plurality of fuel passages. 
         [0027]    In the exemplary embodiment, a first subset of main swirler vanes  240  each include a first primary fuel passage  242 , a plurality of injection orifices  244 , and a plurality of intermediate primary fuel/air passages  246 . Moreover, the first subset of main swirler vanes  240  each partially define an aft Enhanced Lean Blow-Out (“ELBO”) fuel manifold  249 . First primary fuel passage  242  is coupled, in flow communication, with main swirler  230  via injection orifices  244 . Because first primary fuel passage  242  does not extend across the entire length of main swirler vane  240 , first primary fuel passage  242  is not coupled, in flow communication to aft ELBO fuel manifold  249 . 
         [0028]    A second subset of main swirler vanes  240  each include a second primary fuel passage  248 . Moreover, the second subset of main swirler vanes  240  each partially define aft ELBO fuel manifold  249 . Because second primary fuel passage  248  extends across the entire length of respective main swirler vane  240 , the second subset of main swirler vanes  240  are coupled, in flow communication, to aft ELBO fuel manifold  249 . In the exemplary embodiment, main swirler vanes  240  are circumferentially arranged about central rotational axis  213  such that each first subset main swirler vane  240  alternates with each second subset main swirler vane  240 . 
         [0029]    Annular main swirler shroud  260  is coupled to, and extends aftward from, aft ends  241  of main swirler vanes  240  to partially define each aft ELBO fuel manifold  249 . Moreover, annular main swirler shroud  260  includes main ELBO fuel passages  262  and a plurality of ELBO fuel openings  264 . Each ELBO fuel opening  264  is coupled, in flow communication, to a respective aft ELBO fuel manifold  249 . 
         [0030]    During operation of the associated combustor, such as DLE combustor  20  (shown in  FIGS. 1-3 ), a fuel delivery system uses a pilot fuel circuit and a main fuel circuit to supply fuel to a combustion zone, such as combustion zone  40  (shown in  FIGS. 1-3 ). The pilot fuel circuit supplies pilot fuel (not shown) to pilot swirler  210  via pilot swirler fuel manifold  227 . Fuel and air are mixed in inner and outer annular swirlers  214  and  216  respectively, and the fuel-air mixture is supplied through inner pilot vanes  215  and  217  to centerbody cavity  222 . Additionally, pilot fuel may also be supplied to pilot swirler  210  via orifices  224 . 
         [0031]    The main fuel circuit includes a main primary fuel circuit and a main ELBO fuel circuit that supply fuel to main swirler  230  via main swirler fuel manifold  228 . In the main primary fuel circuit, the first subset of main swirler vanes  240  each include first primary fuel passage  242  coupled, in flow communication, to intermediate primary fuel/air passages  246  via injection orifices  244 . As a result, main primary fuel (not shown) is supplied from main swirler fuel manifold  228  to a primary main fuel injection location. Specifically, main primary fuel is supplied to a portion of main swirler cavity  270  positioned forward of annular main swirler shroud  260 . 
         [0032]    In the main ELBO fuel circuit, the second subset of main swirler vanes  240  each include second primary fuel passage  248  coupled, in flow communication, to aft ELBO fuel manifold  249 . As a result, ELBO fuel (not shown) is supplied from main swirler fuel manifold  228  to a secondary main fuel injection location. More specifically, in the exemplary embodiment, ELBO fuel is supplied to a portion of main swirler cavity  270  positioned aft of the first and second subsets of main swirler vanes  240  and adjacent a fuel-air mixture injection exit plane of main swirler  230 . 
         [0033]    ELBO fuel is a relatively small portion of the main fuel that is supplied as supplemental fuel into a combustor as compared to an amount of main fuel supplied to a primary main fuel injection location. However, ELBO fuel is supplied into the combustor at a different location than the primary main fuel injection location. More specifically, in the exemplary embodiment, ELBO fuel is supplied downstream of the primary main fuel injection location. Because ELBO fuel is a relatively small portion of the main fuel, it is desirable to control an amount of ELBO fuel supplied by controlling an amount and/or size of second primary fuel passages  248 . 
         [0034]    In the exemplary premixer assembly  200 , compared to the primary fuel circuit, the ELBO fuel circuit requires a shorter convective timescale for an ELBO fuel-air mixture to travel from the secondary main fuel injection location to the combustion zone, such as combustion zone  40 , where heat release occurs. Therefore, an acoustic frequency interacts differently with ELBO fuel-air mixing at the secondary main fuel injection location as compared to the primary fuel-air mixing at primary main fuel injection location. Moreover, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to the pressure fluctuations in DLE combustors are generated. 
         [0035]    Because ELBO fuel circuit facilitates reducing, in a fuel-air mixture, any fuel-air ratio variation that may be caused by fluctuations in a flow rate of fuel and/or a flow rate of compressed air, ELBO fuel circuit facilitates reducing combustion acoustics by reducing an amplitude of pressure fluctuations in DLE combustors. Moreover, ELBO fuel circuit facilitates reducing pressure disturbances in a combustion chamber/zone, such as combustion zone  40 , of DLE combustors so that pressure disturbances do not interact with a fuel-air mixing process to reinforce an initial pressure disturbance. Therefore, ELBO fuel circuit facilitates reducing an amplitude of pressure disturbances that may damage portions of the DLE combustor. As a result, in the exemplary embodiment, ELBO fuel circuit facilitates increasing operability, reducing emissions, reducing maintenance cost, and increasing life of combustor components. 
         [0036]    In the exemplary embodiment, the first and second subsets of main swirler vanes  240  are respectively coupled, in flow communication, to primary and secondary main fuel injection locations. As a result, every main swirler vane  240  cannot be used to inject main fuel and ELBO fuel into primary main fuel injection location of main swirler cavity  270 . Therefore, premixer assembly  200  does not facilitate optimizing a level of fuel-air mixing in primary main fuel injection location to control pollutant formation and combustion acoustics. However, only one fuel manifold, such as main swirler fuel manifold  228 , is required to supply fuel to each of main primary fuel circuit and main ELBO fuel circuit. As a result, such arrangement facilitates distributing a fixed percentage of ELBO fuel to the secondary main fuel injection location. 
         [0037]      FIG. 5  is an enlarged cross-sectional view of an alternative embodiment of a premixer assembly  300  that may be used with the combustor  20  shown in  FIGS. 2 and 3 . In the exemplary embodiment, premixer assembly  300  includes a pilot swirler  310 , an annular centerbody  320 , and a main swirler  330 . Pilot swirler  310  includes a pilot centerbody  312  having a central rotational axis, an inner annular swirler  314 , and a concentrically disposed outer annular swirler  316 . Inner annular swirler  314  includes a plurality of inner pilot vanes  315  circumferentially disposed about pilot centerbody  312 , and is co-axially aligned with the central rotational axis. Outer annular swirler  316  includes a plurality of outer pilot vanes  317  circumferentially disposed about pilot centerbody  312  and inner annular swirler  314 , and is co-axially aligned with the central rotational axis. 
         [0038]    Annular centerbody  320  is co-axially aligned with the central rotational axis and defines a centerbody cavity  322 . Annular centerbody  320  also includes a plurality of orifices  324  coupled, in flow communication, to centerbody cavity  322 . Moreover, annular centerbody  320  includes a forward end portion  326  defining an annular pilot swirler fuel manifold  327  and an annular main swirler fuel manifold  328 . Further, annular centerbody  320  extends between pilot swirler  310  and main swirler  330  to control fuel flow through premixer assembly  300 . 
         [0039]    Main swirler  330  includes a plurality of main swirler vanes  340  and an annular main swirler shroud  360  that both define an annular main swirler cavity  370 . Main swirler vanes  340  include aft ends  341  and are annularly arranged about centerbody  320 . Moreover, each main swirler vane  340  includes a plurality of fuel passages. 
         [0040]    In the exemplary embodiment, main swirler vanes  340  each include a first primary fuel passage  342 , a plurality of injection orifices  344 , a plurality of intermediate primary fuel/air passages  346 , and an intermediate ELBO fuel passage  347 . Moreover, main swirler vanes  340  each partially define an aft ELBO fuel manifold  349 . First primary fuel passage  342  is coupled, in flow communication, with main swirler  330  via injection orifices  344 . Because first primary fuel passage  342  extends across the entire length of respective main swirler vane  340 , each main swirler vane  340  is also coupled, in flow communication, to aft ELBO fuel manifold  349  via intermediate ELBO fuel passage  347 . 
         [0041]    Annular main swirler shroud  360  is coupled to, and extends aftward from, aft ends  341  of main swirler vanes  340  to partially define each aft ELBO fuel manifold  349 . Additionally, annular main swirler shroud  360  includes main ELBO fuel passages  362  and a plurality of ELBO fuel openings  364 . Each ELBO fuel opening  364  is coupled, in flow communication, to a respective aft ELBO fuel manifold  349 . 
         [0042]    During operation of the associated combustor, such as DLE combustor  20  (shown in  FIGS. 1-3 ), a fuel delivery system uses a pilot fuel circuit and a main fuel circuit to supply fuel to a combustion zone, such as combustion zone  40  (shown in  FIGS. 1-3 ). The pilot fuel circuit supplies pilot fuel to pilot swirler  310  via pilot swirler fuel manifold  327 . Fuel and air are mixed in inner and outer annular swirlers  314  and  316  respectively, and the fuel-air mixture is supplied through respective pilot vanes  315  and  317  to centerbody cavity  322 . Additionally, pilot fuel may also be supplied to pilot swirler  310  via orifices  324 . 
         [0043]    The main fuel circuit includes a main primary fuel circuit and a main ELBO fuel circuit that supply fuel to main swirler  330  via main swirler fuel manifold  328 . In the main primary fuel circuit, main swirler vanes  340  each include primary fuel passage  342  coupled, in flow communication, to intermediate primary fuel/air passages  346  via injection orifices  344 . As a result, main primary fuel (not shown) is supplied from main swirler fuel manifold  328  to a primary main fuel injection location, Specifically, main primary fuel is supplied to a portion of main swirler cavity  370  positioned forward of annular main swirler shroud  360 . 
         [0044]    In the main ELBO fuel circuit, main swirler vanes  340  also include intermediate ELBO fuel passage  347  in addition to first primary fuel passage  342 . Therefore, each main swirler vanes  340  is also coupled, in flow communication, to intermediate primary fuel/air passages  346  via intermediate ELBO fuel passage  347 . As a result, ELBO fuel (not shown) is supplied from main swirler fuel manifold  328  to a secondary main fuel injection location. More specifically, in the exemplary embodiment, ELBO fuel is supplied to a portion of main swirler cavity  370  that is positioned aft of main swirler vanes  340  and adjacent a fuel-air mixture injection exit plane of main swirler  330 . 
         [0045]    ELBO fuel is a relatively small portion of the main fuel that is supplied as supplemental fuel into a combustor as compared to an amount of main fuel supplied to a primary main fuel injection location. However, ELBO fuel is supplied into the combustor at a different location than the primary main fuel injection location. More specifically, in the exemplary embodiment, ELBO fuel is supplied downstream of the primary main fuel injection location. Because ELBO fuel is a relatively small portion of the main fuel, it is desirable to control an amount of ELBO fuel supplied by controlling an amount and/or size of intermediate ELBO fuel passages  347 . 
         [0046]    In the exemplary premixer assembly  300 , compared to the primary fuel circuit, the ELBO fuel circuit requires a shorter convective timescale for an ELBO fuel-air mixture to travel from the secondary main fuel injection location to the combustion zone, such as combustion zone  40 , where heat release occurs. Therefore, an acoustic frequency interacts differently with ELBO fuel-air mixing at secondary main fuel injection location as compared to primary fuel-air mixing at primary main fuel injection location. Moreover, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to pressure fluctuations in DLE combustors are generated. 
         [0047]    Because ELBO fuel circuit facilitates reducing, in a fuel-air mixture, any fuel-air ratio variation that may be caused by fluctuations in a flow rate of fuel and/or a flow rate of compressed air, ELBO fuel circuit facilitates reducing combustion acoustics by reducing an amplitude of pressure fluctuations in DLE combustors. Moreover, ELBO fuel circuit facilitates reducing pressure disturbances in a combustion chamber/zone, such as combustion zone  40 , of DLE combustors so that pressure disturbances do not interact with a fuel-air mixing process to reinforce an initial pressure disturbance. Therefore, ELBO fuel circuit facilitates reducing an amplitude of pressure disturbances that may damage components of the DLE combustor. As a result, in the exemplary embodiment, ELBO fuel circuit facilitates increasing operability, reducing emissions, reducing maintenance cost, and increasing life of combustor components. 
         [0048]    In the exemplary embodiment, main swirler vanes  340  are each coupled, in flow communication, to primary and secondary main fuel injection locations. Therefore, only one fuel manifold such as, main swirler fuel manifold  328 , supplies fuel to each of main primary fuel circuit and main ELBO fuel circuit. As a result, main primary and ELBO fuels cannot be independently varied. Instead, a fuel flow split between primary and ELBO fuel circuits is controlled by effective areas of respective intermediate primary fuel/air passages  346  and intermediate ELBO fuel passage  347  diameters. However, every main swirler vane  340  facilitates supplying both main primary fuel and ELBO fuel into respective primary and secondary main fuel injection locations of main swirler cavity  370 . As a result, every main swirler vane  340  facilitates optimizing a level of fuel-air mixing in primary main fuel injection location. Therefore, such arrangement facilitates distributing a fixed percentage of ELBO fuel to the secondary main fuel injection location. 
         [0049]      FIG. 6  is an enlarged cross-sectional view of another alternative embodiment of a premixer assembly  400  that may be used with the combustor  20  shown in  FIGS. 2 and 3 . In the exemplary embodiment, premixer assembly  400  includes a pilot swirler  410 , an annular centerbody  420 , and a main swirler  430 . Pilot swirler  410  includes a pilot centerbody  412  having a central rotational axis, an inner annular swirler  414 , and a concentrically disposed outer annular swirler  416 . Inner annular swirler  414  includes a plurality of inner pilot vanes  415  circumferentially disposed about pilot centerbody  412 , and is co-axially aligned with the central rotational axis. Outer annular swirler  416  includes a plurality of outer pilot vanes  417  circumferentially disposed about pilot centerbody  412  and inner annular swirler  414 , and is co-axially aligned with the central rotational axis. 
         [0050]    Annular centerbody  420  is co-axially aligned with the central rotational axis and defines a centerbody cavity  422 . Annular centerbody  420  also includes a plurality of orifices  424  coupled, in flow communication, to centerbody cavity  422 . Moreover, annular centerbody  420  includes a forward end portion  426  defining an annular pilot swirler fuel manifold  427 , an annular main swirler fuel manifold  428 , and an annular forward ELBO fuel manifold  429 . Further, annular centerbody  420  extends between pilot swirler  410  and main swirler  430  to control fuel flow through premixer assembly  400 . 
         [0051]    Main swirler  430  includes a plurality of main swirler vanes  440  and an annular main swirler shroud  460  that both define an annular main swirler cavity  470 . Main swirler vanes  440  include aft ends  441  of main swirler vanes  440  and are annularly arranged about annular centerbody  420 . Moreover, each main swirler vanes  440  includes a plurality of fuel passages. 
         [0052]    In the exemplary embodiment, a first subset of main swirler vanes  440  each include a first primary fuel passage  442 , a plurality of injection orifices  444 , and a plurality of intermediate primary fuel/air passages  446 . Moreover, the first subset of main swirler vanes  440  each partially define an aft ELBO fuel manifold  449 . First primary fuel passage  442  is coupled, in flow communication, with main swirler  430  via injection orifices  444 . Because first primary fuel passage  242  does not extend across entire length of main swirler vane  440 , first primary fuel passage is not coupled, in flow communication, to aft ELBO fuel manifold  449 . 
         [0053]    A second subset of main swirler vanes  440  each include a second primary fuel passage  448 . Moreover, the second subset of main swirler vanes  440  each partially define aft ELBO fuel manifold  449 . Because second primary fuel passage  448  extends across the entire length of respective main swirler vane  440 , the second subset of main swirler vanes  440  is coupled, in flow communication, to aft ELBO fuel manifold  449 . In the exemplary embodiment, main swirler vanes  440  are arranged about a central rotational axis such that each first subset main swirler vane  440  alternates with each second subset main swirler vane  440 . 
         [0054]    Annular main swirler shroud  460  is coupled to, and extends aftward from, aft ends  441  of main swirler vanes  440  to partially define each aft ELBO fuel manifold  449 . Additionally, annular main swirler shroud  460  includes main ELBO fuel passages  462  and a plurality of ELBO fuel openings  464 . Each ELBO fuel opening  464  is coupled, in flow communication, to a respective ELBO fuel manifold  449 . 
         [0055]    During operation of the associated combustor, such as DLE combustor  20  (shown in  FIGS. 1-3 ), a fuel delivery system uses a pilot fuel circuit and a main fuel circuit to supply fuel to a combustion zone, such as combustion zone  40  (shown in  FIGS. 1-3 ). The pilot fuel circuit supplies pilot fuel (not shown) to pilot swirler  410  via pilot swirler fuel manifold  427 . Fuel and air are mixed in inner and outer annular swirlers  414  and  416  respectively, and the fuel-air mixture is supplied through respective pilot vanes  415  and  417  to centerbody cavity  422 . Additionally, pilot fuel may also be supplied to pilot swirler  410  via orifices  424 . 
         [0056]    The main fuel circuit includes a main primary fuel circuit and a main ELBO fuel circuit that supply fuel to main swirler  430  via main swirler fuel manifold  428  and forward ELBO fuel manifold  429 , respectively. In the main primary fuel circuit, the first subset of main swirler vanes  440  each include first primary fuel passage  442  coupled, in flow communication, to intermediate primary fuel/air passages  446  via injection orifices  444 . As a result, main primary fuel (not shown) is supplied from main swirler fuel manifold  428  to a primary main fuel injection location. Specifically, main primary fuel is supplied to a portion of main swirler cavity  470  positioned forward of annular main swirler shroud  460 . 
         [0057]    In the main ELBO fuel circuit, the second subset of main swirler vanes  440  each include second primary fuel passage  448  coupled, in flow communication, to aft ELBO fuel manifold  449 . As a result, ELBO fuel (not shown) is supplied from forward ELBO fuel manifold  429  to a secondary main fuel injection location. More specifically, ELBO fuel is supplied to a portion of main swirler cavity  470  positioned aft of the first and second subsets of main swirler vanes  440  and adjacent a fuel-air mixture injection exit plane of main swirler  430 . 
         [0058]    ELBO fuel is a relatively small portion of the main fuel that is supplied as supplemental fuel into a combustor as compared to an amount of main fuel supplied to a primary main fuel injection location. However, ELBO fuel is supplied into the combustor at a different location than the primary main fuel injection location. More specifically, in the exemplary embodiment, ELBO fuel is supplied downstream of the primary main fuel injection location. Because ELBO fuel is a relatively small portion of the main fuel, it is desirable to control an amount of ELBO fuel supplied by controlling an amount and/or size of secondary primary fuel passages  448 . 
         [0059]    In the exemplary premixer assembly  400 , compared to the primary fuel circuit, the ELBO fuel circuit requires a shorter convective timescale for an ELBO fuel-air mixture to travel from the secondary main fuel injection location to the combustion zone, such as combustion zone  40 , where heat release occurs. Therefore, an acoustic frequency interacts differently with ELBO fuel-air mixing at secondary main fuel injection location as compared to primary fuel-air mixing at primary main fuel injection location. Moreover, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to pressure fluctuations in DLE combustors are generated. 
         [0060]    Because ELBO fuel circuit facilitates reducing, in a fuel-air mixture, any fuel-air ratio variation that may be caused by fluctuations in a flow rate of fuel and/or a flow rate of compressed air, ELBO fuel circuit facilitates reducing combustion acoustics by reducing an amplitude of pressure fluctuations in DLE combustors. Moreover, ELBO fuel circuit facilitates reducing pressure disturbances in a combustion chamber/zone, such as combustion zone  40 , of DLE combustors so that pressure disturbances do not interact with a fuel-air mixing process to reinforce an initial pressure disturbance. Therefore, ELBO fuel circuit facilitates reducing an amplitude of pressure disturbances that may damage components of the DLE combustor. As a result, in the exemplary embodiment, ELBO fuel circuit facilitates increasing operability, reducing emissions, reducing maintenance cost, and increasing life of combustor components. 
         [0061]    In the exemplary embodiment, the first and second subsets of main swirler vanes  440  are respectively coupled, in flow communication, to primary and secondary main fuel injection locations. As a result, every main swirler vane  440  cannot be used to inject main fuel and ELBO fuel into primary main fuel injection location of main swirler cavity  470 . Therefore, premixer assembly  400  does not facilitate optimizing a level of fuel-air mixing in primary main fuel injection location to control pollutant formation and combustion acoustics. However, main swirler fuel manifold  428  supplies main primary fuel to main primary fuel circuit and forward ELBO manifold  429  separately supplies ELBO fuel to main ELBO fuel circuit. As a result, main primary and ELBO fuels can be independently varied. Therefore, such arrangement facilitates distributing a variable percentage of ELBO fuel to the secondary main fuel injection location. Moreover, such arrangement facilitates increasing combustor operability. 
         [0062]    In each exemplary embodiment, the above-described main swirlers includes ELBO fuel circuits having fuel passages that extend across entire length of a respective main swirler vane. Such fuel passages are coupled, in flow communication, to an aft ELBO fuel manifold. Each aft ELBO fuel manifold is coupled, in flow communication, to main ELBO fuel passages and a plurality of ELBO fuel openings of an annular main swirler shroud. 
         [0063]    As a result, ELBO fuel is supplied to a secondary main fuel injection location, which is a portion of a main swirler cavity that is positioned aft of main swirler vanes and adjacent to a fuel-air mixture exit plane of the main swirler. Therefore, fuel-air mixture fluctuations that are out-of-phase with respect to each other and at least one fuel-air mixture fluctuation that is out-of-phase with respect to pressure fluctuations in the combustor are generated to facilitate reducing combustion acoustics by reducing an amplitude of pressure fluctuations in the DLE combustor. Moreover, fluctuations in the fuel and/or compressed air flow rates may be controlled to facilitate reducing an amplitude of pressure disturbances. Further, increasing operability, reducing emissions, reducing maintenance cost, and increasing life of components may be facilitated. 
         [0064]    Exemplary embodiments of combustor fuel circuits are described in detail above. The fuel circuits are not limited to use with the combustor described herein, but rather, the fuel circuits can be utilized independently and separately from other combustor components described herein. Moreover, the invention is not limited to the embodiments of the combustor fuel circuits described above in detail. Rather, other variations of the combustor fuel circuits may be utilized within the spirit and scope of the claims. 
         [0065]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.