Patent Publication Number: US-7905093-B2

Title: Apparatus to facilitate decreasing combustor acoustics

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to combustors and more particularly, to methods and apparatus to facilitate decreasing combustor acoustics. 
     During the combustion of natural gas, pollutants such as, but not limited to, carbon monoxide (“CO”), unburned hydrocarbons (“UHC”), and nitrogen oxides (“NOx”) 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 attempting to suppress the formation of such emissions. 
     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, or combustion instabilities, that can limit the overall operability and performance of a combustion system including a known DLE combustor. Over time, the magnitude of the combustion instabilities may increase to a level that may cause damage to the combustion system. As a result, operability, emissions, maintenance cost, and useful life of combustor components may be negatively affected. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method for operating a gas turbine engine including a compressor and a combustor is provided. The method comprises channeling between about 0% to about 8% of the total airflow discharged from a compressor towards a pilot swirler coupled within the burner, wherein the burner includes a main swirler and an annular centerbody extending between the pilot and main swirlers, injecting a portion of the total fuel flow supplied to the burner through a plurality of apertures defined in a hollow pilot centerbody coupled within the pilot swirler, channeling the remaining airflow discharged from the compressor towards the main swirler, and injecting the remaining fuel flow supplied to the burner through at least one main swirler vane coupled within the main swirler, such that the portion of fuel is pre-mixed with a portion of the total airflow. 
     In another aspect, a combustor for use in a gas turbine engine comprising is provided. The combustor comprises a main swirler, and a pilot swirler coupled to the main swirler. The pilot swirler comprises a hollow pilot centerbody, an inner swirler, an outer swirler, and an annular splitter extending between the inner and outer swirlers such that a pilot combustion zone is defined within the annular splitter. The hollow pilot centerbody comprises a plurality of apertures extending therethrough. The pilot swirler is operable with only between about 0% and about 8% of the total airflow entering the combustor. 
     In a further aspect, a gas turbine engine comprising a combustor and a compressor is provided. The gas turbine engine comprises a main swirler and a pilot swirler coupled to the main swirler, the pilot swirler comprising a hollow pilot centerbody, an inner swirler, an outer swirler, and an annular splitter extending between the inner and outer swirlers such that a pilot combustion zone is defined within the annular splitter, the hollow pilot centerbody comprising a plurality of apertures extending therethrough, the pilot swirler operable with only between about 0% and about 8% of the total airflow entering the combustor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary gas turbine engine including a combustor. 
         FIG. 2  is a schematic cross-sectional view of an exemplary combustor including two exemplary burners that each include an exemplary pilot swirler assembly that may be used with the gas turbine engine shown in  FIG. 1 . 
         FIG. 3  is a perspective view of an exemplary pilot swirler that may be used with the burner shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It should be appreciated that the term “forward” is used throughout this application to refer to directions and positions located axially upstream towards a fuel/air intake side of a combustion system, for the ease of understanding. It should also be appreciated that the term “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. 
       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  is a LM6000 engine commercially available from General Electric Company, Cincinnati, Ohio. 
     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 . 
       FIG. 2  is a schematic cross-sectional view of exemplary combustor  20  that may be used with a gas turbine engine, such as gas turbine engine  10  (shown in  FIG. 1 ). In the exemplary embodiment, combustor  20  includes an outer burner  21  and an inner burner  23 . Each burner  21  and  23  includes a pilot swirler assembly  100  and a main swirler assembly  40 . Generally, in the exemplary embodiment, combustor  20  includes main swirler assembly  40 , pilot swirler  100 , and an annular centerbody  44  extending therebetween. Specifically, in the exemplary embodiment, annular centerbody  44  is positioned radially outward from pilot swirler  100  and extends circumferentially about pilot swirler  100 . Moreover, annular centerbody  44  includes a radially inner surface  56  and a radially outer surface  54  that are connected at a trailing end  58 . More specifically, annular centerbody  44  is substantially co-axially aligned with a central axis  102  of pilot swirler  100  and defines a centerbody cavity  46 . 
     In the exemplary embodiment, main swirler  40  includes an annular main swirler housing  50  that is spaced radially outward from pilot swirler  100  and centerbody  44 , such that an annular main swirler cavity  52  is defined between housing  50  and radially outer surface  54  of centerbody  44 . In the exemplary embodiment, main swirler  40  also includes a plurality of main swirler vanes  42  that extends between annular centerbody  44  and housing  50 . Vanes  42  are spaced circumferentially within main swirler  40 . Moreover, in the exemplary embodiment, main swirler  40  is substantially co-axially aligned with respect to central axis  102  and extends circumferentially about pilot swirler  100 . A main swirler combustion zone  60  is defined downstream from main swirler  40  and pilot swirler  100 . Main swirler combustion zone  60  is defined by an annular combustor liner  61 . 
       FIG. 3  is a perspective view of an exemplary pilot swirler assembly  100  that may be used with combustor  20  (shown in  FIG. 2 ). In the exemplary embodiment, pilot swirler  100  includes a pilot centerbody  110 , a radially inner pilot swirler  114 , a radially outer pilot swirler  116 , and an annular splitter  118  extending between inner and outer pilot swirlers  114  and  116 , respectively. Specifically, in the exemplary embodiment, splitter  118  is positioned radially outward from pilot centerbody  110  and extends circumferentially about pilot centerbody  110 . Splitter  118  extends axially downstream from inner and outer swirlers  114  and  116 , respectively, and in the exemplary embodiment, splitter  118  is substantially co-axially aligned with axis  102 . A pilot splitter cavity  120  is defined by a radially inner surface  119  of splitter  118 . Moreover, in the exemplary embodiment, cavity  120  includes a venturi throat  122  that is defined by radially inner surface  119  of splitter  118 . A pilot combustion zone  121  is positioned downstream of splitter cavity  120 . 
     Further, in the exemplary embodiment, pilot centerbody  110  is hollow and defines a pilot centerbody chamber  108  therein. Chamber  108  is substantially centered within centerbody  110 , and includes a plurality of apertures  112  that couple chamber  108  in flow communication with splitter cavity  120 . Each aperture  112  extends from a radially inner surface  106  of centerbody  110  to a radially outer surface  107  of centerbody  110 . More specifically, in the exemplary embodiment, pilot centerbody  110  includes five apertures  112  spaced circumferentially about centerbody chamber  108 . In another embodiment, pilot centerbody  110  may include more or less than five apertures  112 . 
     In the exemplary embodiment, pilot inner swirler  114  includes an inner swirler cavity  115  that is defined by splitter  118  and by pilot centerbody  110 . Cavity  115  extends circumferentially about pilot centerbody  110 . A plurality of swirler vanes  124  extend generally radially across inner swirler cavity  115 . Vanes  124  are spaced circumferentially about pilot centerbody  110 . 
     Outer swirler  116  includes an outer swirler cavity  117  that is defined by splitter  118  and by radially inner surface  56  of annular centerbody  44 . Cavity  117  extends circumferentially about splitter  118 . A plurality of swirler vanes  126  extend substantially radially across outer swirler cavity  117 . Vanes  126  are spaced circumferentially about splitter  118 . 
     Each pilot swirler  100  within combustor  20  is sized and oriented to receive a pilot airflow  66  channeled downstream from a compressor, such as compressor  19  (shown in  FIG. 1 ), for example. In the exemplary embodiment, each pilot swirler  100  receives between about 0% and about 8% of the total airflow  62  (shown in  FIG. 2 ) entering combustor  20 . More preferably, each pilot swirler  100  receives between about 2% and about 7% of total airflow  62  entering combustor  20 . Most preferably, pilot swirler  100  receives about 5% of total airflow  62  entering combustor  20 . In the exemplary embodiment, a remaining main swirler airflow  64  (shown in  FIG. 2 ) of total airflow  62 , is channeled towards main swirler  40 . An airflow ratio of main swirler  40  to pilot swirler  100  is about 20. As a result, the exemplary embodiment is a super small non-premixed pilot. 
     In the exemplary embodiment, pilot swirler  100  is configured to discharge a fuel flow (not shown) into splitter cavity  120  that is between about 0% and about 5% of a total fuel flow (not shown) supplied to combustor  20 . More preferably, pilot swirler  100  is configured to discharge the fuel flow into splitter cavity  120  that is between about 1% and about 4% of the total fuel flow supplied to combustor  20 . Most preferably, pilot swirler  100  is configured to discharge about 2% of the total fuel flow supplied to combustor  20  into splitter cavity  120 . 
     During operation of combustor  20 , the total airflow  62  is channeled to combustor  20  from compressor  19 . In the exemplary embodiment, the main swirler airflow  64  is channeled towards main swirler  40  and pilot airflow  66  is delivered to pilot swirler  100 . Main airflow  64  enters main swirler  40  and mixes with main fuel (not shown) supplied to main swirler  40  via a main swirler manifold (not shown). Specifically, in the exemplary embodiment, fuel and air are pre-mixed in main swirler  40  before the resulting pre-mixed fuel-air mixture is channeled through main swirler cavity  52  into main swirler combustion zone  60 . More specifically, main swirler  40  facilitates providing a lean, well-dispersed fuel-air mixture to combustor  20  that facilitates reducing NOx and CO emissions from engine  10 . The fuel-air mixture is supplied to main swirler combustion zone  60  via main swirler cavity  52  wherein combustion occurs. 
     Further, in the exemplary embodiment, pilot airflow  66  is channeled towards pilot swirler  100 . Pilot airflow  66  is separated into an inner pilot swirler cavity flow  68  and an outer pilot swirler cavity flow  70 . Outer flow  70  is channeled through outer swirler  116  and is channeled to main swirler combustion zone  60  past inner surface  56  of annular centerbody  44 . In the exemplary embodiment, outer flow  70  is not mixed with fuel before being channeled towards main swirler combustion zone  60 . Moreover, in the exemplary embodiment, outer flow  70  facilitates cooling annular centerbody  44 . Inner flow  68  is channeled through inner swirler  114  and mixes in splitter cavity  120  with pilot fuel injected from chamber  108  via apertures  112  to form a non-pre-mixed pilot fuel-air mixture. The resulting non-pre-mixed pilot fuel-air mixture is ignited to generate a pilot flame which extends adjacent to inner surface  119  of splitter  118  and extends downstream from pilot swirler  100 . As such, the pilot flame is substantially sheltered from outer swirler cavity flow  70 . Moreover, centerbody  44  facilitates sheltering outer flow  70  and the pilot flame from the main fuel-air mixture discharged through main swirler cavity  52 . 
     Combustor  20  has naturally occurring acoustic frequencies that may be experienced during operation of engine  10 . Specifically, when operated under lean conditions, high combustion acoustics, or combustion instabilities, can be produced in combustor  20 . High combustion instability magnitudes may produce dangerous levels of vibrations. In the exemplary embodiment, as described in more detail below, pilot swirler  100  facilitates suppressing combustion acoustics during combustion of the lean fuel-air mixture while maintaining low NOx and CO emissions. 
     Pilot swirler  100  facilitates suppressing combustion instabilities using the pilot flame generated by the pilot fuel-air mixture. In the absence of the pilot flame, the main swirler flame is unstable. During operation of pilot swirler  100 , the pilot flame generates a hot gaseous flow that extends downstream from splitter cavity  120  and pilot swirler combustion zone  121 . The flow mixes with the main swirler flame at trailing end  58  where combustion instabilities may occur. The pilot flame, extending from pilot swirler combustion zone  121 , facilitates stabilizing and positioning the main swirler flame around trailing end  58 . As a result, the stabilized main swirler flame positioned at trailing end  58  suppresses the overall combustion instabilities of combustor  20 . 
     In the exemplary embodiment, testing has shown that when pilot swirler  100  receives about 5% of total airflow  62 , combustion instabilities were suppressed by using fuel flows between about 0% and about 5% of the total fuel flow to combustor  20 . Moreover, in the exemplary embodiment, testing has shown that use of pilot swirler  100  with fuel flows between about 0% and about 5% did not substantially increase the emissions of NOx and CO. For example, in a first exemplary pilot fuel test, about 0% of the fuel was discharged by pilot swirler  100 . In the first test, the NOx and CO emissions generated were about 11 parts per million (“ppm”) and about 10 ppm, respectively. In a second exemplary pilot fuel test, about 2% of the total fuel flow was discharged by pilot swirler  100 . The NOx emissions, in the second exemplary test, increased between about 1 ppm to about 2 ppm, however, the CO emissions did not increase. 
     As described above, in the exemplary embodiment, pilot swirler  100  facilitates limiting emissions of NOx and CO levels and controlling combustion instabilities in combustor  20 . Specifically, main swirler  40  facilitates providing a lean fuel-air mixture by pre-mixing fuel with main swirler airflow  64 . The resulting main swirler flame has a lower temperature than a non-lean flame and facilitates reducing an amount of NOx emissions produced during combustion. The low flame temperature, however, facilitates increasing combustion instabilities of combustor  20 . In the exemplary embodiment, pilot swirler  100  facilitates suppressing the combustion instabilities of combustor  20  by providing a non-lean and non-pre-mixed fuel-air mixture using about 0% to about 5% of the total fuel flow supplied to combustor  20 . More specifically, as described above, the pilot flame generates a hot gaseous flow that suppresses combustion instability. As a result, in the exemplary embodiment, pilot swirler  100  facilitates reducing the combustion instabilities and further facilitates limiting emissions of NOx and CO. 
     In the exemplary embodiment a combustor includes two co-axial swirlers, a pilot swirler and a main swirler with an annular centerbody extending between the pilot and main swirlers. The pilot swirler is sized to receive about 5% of a total airflow entering the combustor. Moreover, the pilot swirler is configured to discharge about 0% to about 5% of a total fuel flow to the burner. The pilot swirler, in the exemplary embodiment, facilitates suppressing combustion instabilities occurring within the combustor. Moreover, in the exemplary embodiment, the pilot swirler facilitates limiting emissions of NOx and CO. As a result, pilot swirler  100 , maintains low emission levels of NOx and CO while increasing the life of combustor components. 
     Exemplary embodiments of combustor pilot swirlers are described in detail above. The pilot swirler is not limited to use with the combustor described herein, but rather, the pilot swirler can be utilized independently and separately from other combustor components described herein. Moreover, the invention is not limited to the embodiments of the combustor pilot swirlers described above in detail. Rather, other variations of the combustor pilot swirlers may be utilized within the spirit and scope of the claims. 
     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.