Abstract:
A combustor dome for use in a combustor has an igniter mounted in a central bore. Moving outward from the bore is a concentric pilot fuel passageway. Concentric about the pilot fuel passageway is an air passageway. Lastly, concentric about the air passageway is a premix passageway. The outlets of the pilot fuel passageway, the air passageway, and the premix passageway are generally coplanar.

Description:
TECHNICAL FIELD 
     This invention relates generally to combustors used in gas turbine engines and in particular to a low emissions combustor that burns gaseous fuel. 
     BACKGROUND OF THE INVENTION 
     Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions for both industrial and power generation applications burning either liquid or gaseous fuel. 
     Sjunnesson et al, International Publication No. WO 96/02796 discloses a low-emission combustor for a gas turbine engine having an outer casing with an upstream end wall with a pilot fuel injector, a first radial flow swirler, an igniter for initiating a stable diffusion frame in a pilot zone, a second coaxial swirler, main fuel injectors, secondary air inlets, and a main combustion zone. Importantly, the pilot zone is confined radially outwardly by a surrounding wall which constitutes the radially inner confinement of an axial outlet portion of a radial vaporization channel extending from the second swirler and a third radial flow swirler is adapted to supply the secondary air in a rotary motion opposite to that of the main flow of fuel and air. 
     One disadvantage with having a confined or recessed pilot zone is that the walls surrounding the zone are exposed to very high temperatures and as a consequence need to be cooled. Typically, cooling air from other parts of the gas turbine engine are brought to these walls for this purpose. However, the extraction of the cooling air from the engine results in a reduction in the engine&#39;s performance, increases carbon monoxide emissions and produces inferior engine operability and starting. Another disadvantage to the combustor disclosed in the &#39;050 patent is that it requires three radial swirlers which adds expense and complexity to the design. Prior examples of combustors, therefore, are not as economical and robust as desired for use in small power generation systems. 
     Accordingly, there is a need for a low emissions natural gas combustor that does not have a confined pilot zone and is a simpler and more economic design than prior combustor designs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a combustor dome for use in a combustor having an igniter mounted in a central bore. Moving outward from the bore is a concentric pilot fuel passageway having an outlet with a plurality of holes for expelling the pilot fuel at an outward angle away from the igniter tip. Concentric about pilot fuel passageway is an air passageway that has an outlet with a nozzle. Lastly, concentric about the air passageway is a premix passageway. The outlets of the pilot fuel passageway, the air passageway, and the premix passageway are all approximately coplanar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a low emissions natural gas combustor contemplated by the present invention. 
     FIG. 2 is an enlarged, cross-sectional view of the dome of the combustor of FIG.  1 . 
     FIG. 3 is an enlarged, cross-sectional view of a portion of the combustor of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a low emissions natural gas combustor is generally denoted by reference numeral  10 . The combustor  10  includes an axially extending, annular heat shield  12  surrounding and radially spaced from an annular combustor liner  14  to define an air passage  16  therebetween. The combustor liner  14 , in turn, defines a combustion chamber  18 . It is within this chamber  18  that most of the combustion process occurs. The heat shield  12  is closed at one axial end by a dome  50 . At the opposite axial end, the heat shield  12  has an open annulus  20  and a plurality of holes (not shown) through which pressurized air from a gas turbine engine enters into passage  16 . The combustor  10  is symmetric about an axial centerline  22 . Turbulators  13  may be mounted on the outer surface of combustor liner  14  to improve convective cooling. 
     Referring to FIG. 2, the dome  50  is preferably a single cast piece. Alternatively, the dome  50  can be fabricated from separate machined parts welded for tight dimensional control. The dome  50  includes an annular outer wall  52  and annular inner wall  54 , and a radially extending wall  56  connecting the wall  52  to the wall  54  at one axial end. At the opposite axial end, the outer wall  52  is attached (e.g., welded, brazed and/or bolted) to the heat shield  12 . The inner wall  54  defines an axial bore extending from the wall  56  toward the combustion chamber  18 . An igniter  57  is mounted in the bore so that its tip  59  is aligned with the centerline  22 . Coaxial about the inner wall  54  is an igniter ring  58 . The ring  58  has a radial portion that is attached to radial wall  56  and an axial extending portion. This axial extending portion is radially spaced from the inner wall  54  to define a pilot fuel passage  60 . At the end of the axial extending portion is a shroud  62  that connects to the inner wall  54  thereby closing the passage  60 . Within the shroud  62  are a plurality of fuel holes  64  slanted at an angle so that the fuel exiting these holes moves away from the igniter tip  59  at an angle in the range of about 30 to 60 degrees relative to the center line  22 . The shroud  62  also has angled air holes  66  to provide cooling air to the igniter  57 . 
     As used herein, “extending” (in the context of one wall or other component “extending” from another) means contiguously passing, abutting, adjoining, or connecting. As used herein, “mounted” (in the context “mounted in the bore”) includes removably or permanently fixed in the bore and/or relative to the bore. 
     Continuing with reference to FIG. 2, radially spaced and concentric with the igniter ring  58  is a premixer inner wall  68 . Like the igniter ring  58 , the premixer inner wall  68  has a radial portion and axial portion. Both axial portions of the igniter ring  58  and the premixer inner wall  68  end at substantially the same axial distance from a common point such as the wall  56 . That is, the axial ends are substantially coplanar as the ends of both of these walls abut an imaginary radial-facing plane represented by line  70 . Substantially, as used in this application, means within assembly and manufacturing tolerances acceptable to those skilled in the art. Together, the premixer inner wall  68  and the igniter ring  58  define an air passageway  90  that ends at nozzle  92 . As used herein, “nozzle” means a device, component, or combination of components used to pass (either actively or passively), inject, or expel fluid. Disposed in the air passage  90  at its radial outer end is a plurality of circumferentially spaced-apart struts  84 . Concentric with the premixer inner wall  68  and radially spaced therefrom is a premixer outer wall  76 . The premixer outer wall  76  and inner wall  68  define a premix passageway  94 . The exits of the premix passageway  94 , air passageway  90  and fuel holes  64  are substantially coplanar with respect to an axial facing plane represented by dashed line  70 . 
     The premixer outer wall  76  has an upstream radial portion connected to the radial portion of the premixer inner wall  68  by a plurality of circumferentially spaced apart radial swirler vanes  78 . The premixer outer wall  76  also has an axial portion extending from the upstream radial portion to a downstream radial portion that is attached (e.g., welded, brazed and/or bolted) to the combustor liner  14 . The extension of this axial portion beyond dashed line  70  improves starting and stability. The downstream radial portion has a plurality of circumferentially spaced apart, axially extending dilution air holes  80 . Disposed between the premixer outer wall  76  and the dome outer wall  52  is an plenum  82  in fluid communication with air passage  16 . 
     During the start of the gas turbine engine in which the combustor  10  is mounted, gaseous pilot fuel, such as natural gas, flows through tube  96  into pilot fuel passage  60 . The term “pilot fuel” as used herein means the fuel used to initiate the combustion process. At the same time, air flows through passage  16  into plenum  82 . From the plenum  82 , generally unswirled air flows through passage  90  and swirled air flows through passage  94 . These air flows and pilot fuel flows mix just downstream of the igniter tip  59 , which ignites the air-fuel mixture to form a swirling hot gas referred to as a pilot zone, roughly represented by circle  100 . The pilot zone by its presence in the combustion chamber  18  sustains the combustion process by assisting in both mixing and igniting as more air and fuel enter the chamber. It should be appreciated that during an engine start, relatively little air is likely to be available from the engine and therefore the mixture of fuel and air in the pilot zone  100  tends to be fuel rich. To avoid NOx generation, it is important to avoid high concentrations of fuel. The Applicants have found that this novel arrangement of fuel and air passages results in a more uniform fuel-to-air ratio in the pilot zone and hence lowers NOx emissions. 
     Once the engine reaches above 70 to 80 percent of its operating speed, additional fuel is added through a primary fuel inlet  102 . Referring to FIG. 3, the fuel entering inlet  102  flows through holes  104  in the struts  84  into the spaces between the swirler vanes  78 . The fuel and air are then mixed in the premix passageway  94  so that when the mixture comes into contact with the pilot zone  100  it does not disrupt the uniformity of the mixture in the pilot zone, thus maintaining reduced NOx emissions. 
     Various modifications and alterations to the above-described preferred embodiment will be apparent to those skilled in the art. For example, the present invention can be used with any type of combustor and other types of fuel such as a liquid fuel. Accordingly, these descriptions of the invention should be considered exemplary and not as limiting the scope of the invention as set forth in the following claims.