Abstract:
Embodiments of this invention provide a premixed pilot assembly for use with a fuel nozzle for a turbine. The level of nitrogen oxide (NOx) emitted by the pilot is reduced by mixing the fuel and air fast, at the end of the pilot nozzle, thereby avoiding significant zones of rich fuel and air mixtures. The air is mixed with the fuel through the use of openings in a first cylinder and a second cylinder, one of which carries air and one of which carries pilot fuel. The openings can be configured as necessary to obtain a desired effect on the pilot.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to combustors for turbines. More particularly, the invention relates to a solution for using lean direct injection for premixed piloted combustion. 
     BACKGROUND OF THE INVENTION 
     Traditional gas turbine combustors use non-premixed (“diffusion”) flames in which fuel and air freely enter a combustion chamber separately. However, the diffusion flames burn at such a high temperature that unacceptable levels of nitrogen oxide (NOx) are emitted. 
     One method used to lower NOx emissions includes using lean premixed combustion in which fuel and air are premixed in a premixer section, and the fuel-air mixture is injected into a combustion chamber where it is burned. Lean premixed combustion can result in low NOx emissions for very uniform fuel air mixtures, however, such mixtures are typically prone to combustion instability. This instability can be overcome by the presence of a pilot. However, typically a pilot results in excessive NOx formation, which therefore reduces the benefit of using the lean premixed combustion method. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Embodiments of this invention provide a premixed pilot assembly for use with a fuel nozzle for a turbine. The level of NOx emitted by the pilot is reduced by mixing the fuel and air fast, at the end of the pilot nozzle, thereby avoiding significant zones of rich fuel and air mixtures. The air is mixed with the fuel through the use of openings in a first cylinder and a second cylinder, one of which carries air and one of which carries pilot fuel. The openings can be configured as necessary to obtain a desired effect on the pilot. 
     A first aspect of the disclosure provides a premixed pilot assembly configured to be coupled to a fuel nozzle for a turbine combustor, the premixed pilot assembly including: a first cylinder, a second cylinder, configured to hold a selected one of: pilot fuel or air, the second cylinder disposed within the first cylinder such that an annular area is formed between the first cylinder and the second cylinder, the annular area configured to allow the other one of the pilot fuel or air to flow through the annular area; a tip, coupled to the second cylinder, the tip configured to create at least one second cylinder opening to direct the selected one of the pilot fuel or the air flowing through the second cylinder radially outward; and an end piece, coupled to the first cylinder, the end piece configured to create at least one first cylinder opening to direct the other one of the pilot fuel or air flowing through the annular area radially inward towards the selected one of the pilot fuel or the air directed from the at least one second cylinder opening. 
     A second aspect of the disclosure provides a fuel nozzle comprising: a housing defining a main fuel nozzle; a centerbody tube, disposed within the housing; a premixed pilot assembly coupled to an end of the centerbody tube, the premixed pilot assembly including: a first cylinder, a second cylinder, configured to hold a selected one of pilot fuel or air, the second cylinder disposed within the first cylinder such that an annular area is formed between the first cylinder and the second cylinder, the annular area configured to allow the other one of the pilot fuel or air to flow through the annular area; a tip, coupled to the second cylinder, the tip configured to create at least one second cylinder opening to direct the selected one of the pilot fuel or the air flowing through the second cylinder radially outward; and an end piece, coupled to the first cylinder, the end piece configured to create at least one first cylinder opening to direct the other one of the pilot fuel or air flowing through the annular area radially inward towards the selected one of the pilot fuel or air directed out of the at least one second cylinder opening. 
     A third aspect of the disclosure provides a premixed pilot assembly configured to be coupled to a fuel nozzle for a turbine, the premixed pilot assembly including: a first passage for delivering one of pilot fuel or air; a second passage for delivering the other of pilot fuel or air, the first and second passages isolated from each other along a length of the passages; a first opening in the first passage to emit one of the pilot fuel or the air radially outward; and a second opening in the second passage to emit the other of the pilot fuel or the air radially inward towards one of the pilot fuel or the air emitted from the first opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a main fuel nozzle for a gas turbine as known in the art. 
         FIGS. 2-9  show cross-sectional views of various configurations for a premixed pilot according to embodiments of this invention. 
     
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Gas turbines typically include multiple chambers for combustion, and each chamber for combustion includes multiple main fuel nozzles. One such main fuel nozzle  10 , as known in the art, is shown in  FIG. 1 . Main fuel nozzle  10  includes a burner tube  18 , with turning vanes  16 , and an inlet flow conditioner (IFC)  20  through which main air enters nozzle  10 . Main fuel nozzle  10  further includes a centerbody  14 , which consists of a tube or cylinder disposed within main fuel nozzle  10 . A liquid fuel cartridge  12  is disposed within centerbody  14 . The area noted by the dotted lines in  FIG. 1 , at an end of centerbody  14  and liquid fuel cartridge  12 , is an area referred to as a diffusion tip  22  and is typically coupled to the end of fuel nozzle  10 , for example, by a weld. Embodiments of the invention discussed herein include modifying or replacing diffusion tip  22  to include a premixed pilot  100  ( FIGS. 2-5 ). 
     A cross-sectional view of premixed pilot  100  incorporated into a main fuel nozzle  101  according to embodiments of this invention is shown in  FIG. 2 . Premixed pilot  100  can be added to main fuel nozzle  101  through any known means. For example, premixed pilot  100  can replace diffusion tip  22  ( FIG. 1 ) or diffusion tip  22  can be modified as known in the art to include the elements of premixed pilot  100  discussed herein. For example, premixed pilot  100  can be added to an end of main fuel nozzle  10  shown in  FIG. 1  and one of skill in the art could modify main fuel nozzle  10  as necessary in order to route air and fuel through premixed pilot  100 , and to include liquid fuel cartridge  12  if desired. One such configuration to route air and fuel through premixed pilot  100  is shown in  FIG. 2 . 
     Turning to  FIG. 3 , an enlarged cross-sectional view of premixed pilot  100  is shown. Premixed pilot  100  includes a first cylinder  106  and a second cylinder  104 , within first cylinder  106 . As such, an annular area  105  is formed between first cylinder  106  and second cylinder  104 . Cylinder  104  carries fuel while annular area  105  carries air. It is noted that while cylinders  104  and  106  are shown  FIGS. 2-9  as being cylindrical in shape, one of skill in the art would understand that any suitable sized or shaped passage could be used. It is also noted that while cylinder  104  is shown in  FIGS. 2-9  as being a hollow cylinder carrying fuel, one of skill in the art would understand that additional elements may be included within cylinder  104 , such as additional passages or mechanical elements, in order to assist premixed pilot  100  in interacting with fuel nozzle  10 . 
     First cylinder  106  further includes an end piece  103  which is configured to create at least one first cylinder opening  108  (also referred to as air openings) to direct the air flowing through annular area  105  radially inward. Premixed pilot  100  further includes a tip  107  at one end of second cylinder  104 . Tip  107  is configured to create at least one second cylinder opening  110  (also referred to as fuel openings) to direct the pilot fuel flowing through second cylinder  104  radially outward. As such, pilot fuel emerges from second cylinder  104 , through openings  110 , sufficient to maintain a pilot flame  109 . As discussed herein, one or more air openings  108  allow the air in annular area  105  to be injected directly at the pilot fuel emerging from fuel openings  110  in second cylinder  104 . This air flow from air openings  108  is shown by the arrows marked “a” while the pilot fuel flow from fuel openings  110  is shown by the arrows marked “f.” The air is injected radially inward directly at the fuel as it exits second cylinder  104 . As such, the air and fuel are isolated until just as the fuel emerges from second cylinder  104 , thereby avoiding significant zones of rich fuel and air mixtures. Once the air and fuel have mixed, the air and fuel mixture (shown by arrows marked “f+a”) flows downstream, i.e., away from second cylinder  104  and tip  107 , and will start burning approximately where pilot flames  109  are shown in  FIG. 3 . As such, vigorous mixing of air and fuel will occur in a mixing zone, within a short axial dimension from fuel tip  107 . In addition, injecting the air at the fuel according to embodiments of this invention produces a supportive zone for stable combustion for pilot flame  109 . 
     Openings  108 ,  110  may be of any shape or size to achieve a mixing of air and fuel at the desired intensity and inclination. For example, in one embodiment, shown in the three-dimensional view in  FIGS. 4-6 , fuel openings  110  and air openings  108  can comprise one or more discontinuous separate openings, displaced along tip  107  and second cylinder  104 , respectively, each opening acting as a discrete jet to inject either the air or fuel. In another embodiment (not shown), fuel openings  110  and air openings  108  can comprise one or more continuous, annular openings around entire tip  107  and entire second cylinder  104 , respectively. It is understood that any configuration of annular openings or discontinuous separate openings can be used. For example, both fuel opening  110  and air opening  108  can be continuous annular openings, or both openings  108 ,  110  can be discontinuous separate openings. Alternatively, fuel opening  110  can be a continuous annular ring while air openings  108  can be discontinuous separate openings, and vice versa. Regardless of whether the openings  108 ,  110  are continuous (annular) or discontinuous separate, openings  108 ,  110  can also be of any shape or geometric cross-section desired. 
     Openings  108 ,  110  can be configured in second cylinder  104  and tip  107  to achieve a desired level of mixing of air and fuel. For example, if openings  108 ,  110  are discontinuous separate openings, openings  108  can be configured to be aligned with a corresponding opening  110 , as shown in  FIG. 7 , such that a flow of fuel directly impinges on a flow of air. Alternatively, as shown in  FIG. 8 , openings  108  can be configured to be staggered with respect to openings  110 , such that a flow of fuel does not directly impinge on a flow of air. 
     While openings  108 ,  110  are shown in  FIGS. 2-8  as directing the flow of air at the fuel substantially perpendicularly, openings  108 ,  110  can also be angled to achieve mixing of the air and fuel at a desired angle. For example, openings  108 ,  110  can be angled to direct an angled flow of air at an angled flow of fuel, or only one of the openings  108 ,  110  can be angled while the other opening is substantially perpendicular to cylinders  104 ,  106 . For example, as shown in the simplified schematic in  FIG. 9 , angle α is the angle between two surfaces of tip  107  that include fuel openings  110 , and can be in the range of approximately 0° to approximately 60°. Angle θ is the angle between a horizontal plane of cylinder  106  and a surface of air openings  108 . Angle θ can be in the range of approximately 120° to approximately 180°. Although these angle ranges are provided as one example of how openings  108 ,  110  can be angled to achieve a desired mixing of air and fuel, it is understood that the angles provided are not intended to limit the invention disclosed herein, as any angle can be used for either openings  108  or  110  to achieve a desired result. In addition, although  FIGS. 2-9  show air being injected at the fuel substantially perpendicularly or slightly downstream (i.e., away from premixed pilot  100  toward flame  109 ), air may also be injected upstream at the fuel. 
     While the embodiments discussed herein refer to second cylinder  104  holding fuel and annular area  105  formed by first cylinder  106  holding air, it is understood that the reverse is also disclosed. Second cylinder  104  can hold air, and therefore air can be directed radially outward through openings  110 , while annular area  105  can hold fuel and therefore fuel can be directed radially inward through openings  108  in first cylinder  106 . 
     The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). 
     While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.