Abstract:
To meet emissions standards, many gas turbine engines use some form of lean, pre-mixed combustion system. The lean nature of the fuel may lead to combustion oscillations or other instabilities. A fuel injector having an inner and outer cylinder receives pilot fuel and a portion of pre-mixed fuel-air into an annular space defined by the inner and outer cylinders. The amount of pilot fuel mixed with the pre-mixed fuel-air can be modulated based on sensed conditions within the turbine engine. Continuous modulation of the pilot fuel to adapt to the sensed conditions improves main burner flames and response to engine transients.

Description:
TECHNICAL FIELD  
       [0001]     The present invention is directed to an apparatus, system, and method for burning a mixture of fuel and air. More particularly, the present invention is directed to an apparatus, system, and method for burning a mixture of fuel and air in a gas turbine engine.  
       BACKGROUND  
       [0002]     Internal combustion engines, including diesel engines, gaseous-fueled engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds, which may include nitrous oxides (NOx). Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of NOx emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.  
         [0003]     It has been established that a well-distributed, low temperature flame can reduce NOx production. One way to generate a well-distributed, low temperature flame is to premix fuel and air to a predetermined lean fuel-to-air ratio. However, at lean fuel-to-air ratios, combustion instabilities may occur, such as combustion pressure oscillations, for example.  
         [0004]     Burners for gas turbine engines have become more sophisticated to overcome these instabilities while still providing low NOx emissions. For example, U.S. Pat. No. 6,971,242 to Boardman, dated Dec. 6, 2005, teaches a burner that uses offset orifices on radially positioned first and second cylinders to stabilize flame propagation within a combustion chamber. Specifically, the unique arrangement of orifices helps reduce combustion oscillations, which allows the combustor to run at conditions that result in low NOx.  
         [0005]     Although this unique arrangement helps reduce NOx, the pilot fuel remains rich, and when used to provide a steady flame, increases the amount of NOx that the engine produces.  
         [0006]     The disclosed burner is directed to overcoming one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0007]     In one aspect, the present disclosure is directed to a fuel injector for a turbine engine. The fuel injector includes a first cylinder, and a second cylinder positioned radially outward from the first cylinder. The first cylinder includes at least one of a first orifice and communicates a main fuel to a combustion chamber. The first and second cylinders form an annular space there between, which is in communication with the at least one of a first orifice. The annular space is adapted to receive both pilot fuel and main fuel from the first cylinder. A combined main fuel/pilot fuel mixture exits through at least one of a second orifice in the second cylinder to the combustion chamber to form a stable pilot flame.  
         [0008]     In another aspect, the present invention is directed to a combustion system for a turbine engine having a combustor liner, a source of main fuel, a source of pilot fuel, and at least one fuel injector positioned within the combustor liner. The fuel injector includes a first cylinder having at least one of a first orifice, and in communication with the source of main fuel, and a second cylinder positioned radially outward from the first cylinder. The first and second cylinders form a space there between and communicate with the source of main fuel through the at least one of a first orifice and the source of pilot fuel. The second cylinder also includes at least one of a second orifice in communication with the space and communicates a main fuel/pilot fuel mixture to the combustion chamber.  
         [0009]     In yet another aspect, the present invention is directed to a method of burning a fuel in a turbine engine. The method includes the steps of supplying a main fuel to a first cylinder, supplying a pilot fuel to a space between the first cylinder and the second cylinder, flowing a portion of the main fuel through at least one of a first orifice to the space between the first cylinder and the second cylinder, mixing the pilot fuel with the portion of the main fuel within the space, and passing the mixed main fuel/pilot fuel through at least one of a second orifice into a combustion chamber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic representation of a gas turbine engine including an exemplary embodiment of the present invention;  
         [0011]      FIG. 2  is a side view illustration of an exemplary fuel injector according to one embodiment of the present invention; and  
         [0012]      FIG. 3  is a side cross-sectional illustration of a burner of the fuel injector of  FIG. 2  according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows a turbine engine  10 . The turbine engine  10  may be associated with a stationary or mobile work machine configured to accomplish a predetermined task. For example, the turbine engine  10  may embody the primary power source of a generator set that produces an electrical power output or of a pumping mechanism that performs a fluid pumping operation. The turbine engine  10  may alternatively embody the prime mover of an earth-moving machine, a passenger vehicle, a marine vessel, or any other mobile machine known in the art.  
         [0014]     The turbine engine  10  includes a compressor section  12 , a combustion system  14 , and a turbine section  16 . The compressor section  12  may include components rotatable to compress inlet air. Specifically, the compressor section  12  may include a series of rotatable compressor blades  18  fixedly connected about a central shaft  20 . As the central shaft  20  rotates, the compressor blades  18  draw air into the turbine engine  10  and pressurize the air. This pressurized air may then be directed toward the combustion system  14  for mixture with a liquid and/or gaseous fuel. It is contemplated that the compressor section  12  may further include compressor blades  22  that are separate from the central shaft  20  that remain stationary during operation of the turbine engine  10 .  
         [0015]     The combustor section  14  may mix fuel with the compressed air from the compressor section  12  and combust the mixture to create a mechanical work output. Specifically, the combustor section  14  may include an annular combustion chamber  24 , a fuel supply line  26 , a dome  28 , and a plurality of fuel injectors  30  annularly arranged about the central shaft  20 . The fuel supply line feeds main fuel into the fuel injectors  30 .  
         [0016]     Each fuel injector  30  may inject one or both of liquid and gaseous fuel into the flow of compressed air from the compressor section  12  for ignition within the combustion chamber  24 . As the fuel/air mixture combusts, the heated molecules expand and move at high speed into the turbine section  16 .  
         [0017]     The combustor chamber  24  includes a hot side  32 , a cold side  34 , a first portion  36  and a second portion  38 . The hot side  32  defines a combustion zone, while the cold side  34 , along with a housing  40 , defines an air channel  41 . The dome  28  may attach to the hot side  32  proximate the first portion  36 .  
         [0018]     The turbine section  16  fluidly connects with the combustion system  14  and receives a mass of exhaust gas (not shown) from the combustion system  14 . The mass of exhaust gas expands through the turbine section  16 . The compressor section  12  and the turbine section  16  connect through the shaft  20  between the turbine section  16  and the compressor section  12 . Other conventional methods for transmitting a force may include a hydraulic accumulator/motor, electric motor/generator, and gear systems.  
         [0019]     As shown in  FIGS. 2 and 3 , the fuel injector  30  includes a mixing section  42  and a burner section  44 . The mixing section  42  includes a mixing conduit  46 , which includes a fluid mixing means  48  for mixing the fuel with the mass of compressed air. In the present embodiment, the fluid mixing means  48  may include a plurality of vortex generator tabs  50 , but may embody any type of mixing device known in the art, such as a swirler, for example.  
         [0020]     The fluid conduit  46  may also include mixing orifices  52  positioned along the mixing conduit  46 , upstream of the dome  28  to introduce a portion of the mass of compressed air. The mixing orifices  52  may introduce the portion of compressed air with a tangential component of velocity with respect to the incoming main fuel.  
         [0021]     The burner section  44  includes an inner cylinder  54  and an outer cylinder  56  positioned about a central axis  58 . Each of the inner and outer cylinders  54 ,  56  include a first open end portion  60  and a second end portion  62 , which may be open or closed. In the present application, the term cylinder means a vessel having a volume that may at least partially bound a fluid and may have an irregularly shaped profile other than a rectangle. In the illustrated embodiment, the outer cylinder  56  comprises two separate diameters  64 ,  66  with an angled surface  68  between them. Similarly, the inner cylinder  54  comprises two separate diameters  70 ,  72  with an angled surface  74  between them. The outer cylinder  56  is displaced radially outward from the inner cylinder  54 .  
         [0022]     The burner section  44  further includes a pilot section  76  and a main burner section  78 . The larger diameters  70 ,  64  of the inner and outer cylinders  54 ,  56  comprise the pilot section  76 , while the smaller diameters  72 ,  66  of the inner and outer cylinder  54 ,  56 , respectively comprise the main burner section  78 . Within the pilot section  76 , the inner cylinder  54  includes a first array of orifices  80  and the outer cylinder includes a second array of orifices  82 . The orifices  80 ,  82  may be offset from each other. Each of the inner and outer cylinders  54 ,  56  of the main burner section  78  may also include a multiplicity of orifices  84 .  
         [0023]     The inner and outer cylinders  54 ,  56  define therein between an annular space  86  in communication with the first and second arrays of orifices  80 ,  82  and a pilot fuel feed  88 . The annular space  86  is positioned within the pilot section  76 . A pilot fuel source (not shown) provides pilot fuel to the annular space  76  between the inner and outer cylinders  54 ,  56  through the pilot fuel feed  88 .  
         [0024]     The annular space  86  receives pilot fuel from the pilot fuel source and premixed fuel-air mixture from the fluid conduit  46 , which also feeds the main burner section  78 . Within the annular space  86 , the pilot fuel and premixed fuel-air mixture combine before passing through the second array of orifices  82  of the outer cylinder  56 .  
         [0025]     A control module (not shown) monitors conditions within the combustion system  14  to detect instabilities and irregularities, which may result from improper fuel/air ratio, oscillations, or other conditions that may create NOx or damage to the turbine engine  10 . Upon detection of these instabilities, the control module may modulate amounts of pilot fuel into the annular space  86  to change the pilot mixture. For example, if the control module detects oscillations, additional pilot fuel may be passed through the annular space  86  to provide a more stable pilot flame. Similarly, the amount of pilot fuel may be increased during start-up and reduced during steady-state operation. Preferably, the majority of the pilot flame uses the premixed fuel-air mixture, however, continuous operation of the pilot fuel improves stability of the main burner flames and response to engine transients.  
       INDUSTRIAL APPLICABILITY  
       [0026]     The disclosed fuel injector  30  may be applicable to any turbine engine  10  where reduced oscillations and emissions within the turbine engine are desired. Although particularly useful for low NOx-emitting engines, the disclosed fuel injector may be applicable to any turbine engine regardless of the emission output of the engine.  
         [0027]     Fuel enters through the mixing conduit  46 , where it may be atomized using one of numerous techniques, such as air blast atomization. As the fuel moves through the mixing conduit  46 , compressed air from the compressor section  12  enters through the array of mixing orifices  52  creating a swirling motion causing the fuel to become entrained in the swirling compressed air to create a mixture of fuel and air. The fuel-air mixture accelerates as it passes into the burner section  44  of the fuel injector  30 .  
         [0028]     Upon entering the burner section  44 , a portion of the fuel-air mixture passes through the first array of orifices  80  of the inner cylinder  54  into the annular space  86  and mixes with the pilot fuel. The pilot fuel and fuel-air mixture combine and pass through the second array of orifices  82  to provide a stable pilot flame. Continuous modulation of the pilot fuel, or continuous monitoring of conditions within the turbine engine  10 , improves stability of the main burner flames and better response to engine transients.  
         [0029]     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed fuel injector. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed fuel injector. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.