Patent Publication Number: US-2013232979-A1

Title: System for enhancing mixing in a multi-tube fuel nozzle

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a turbine engine and, more specifically, to a system to increase fuel-air mixing in a multi-tube fuel nozzle. 
     A gas turbine engine combusts a mixture of fuel and air to generate hot combustion gases, which in turn drive one or more turbine stages. In particular, the hot combustion gases force turbine blades to rotate, thereby driving a shaft to rotate one or more loads, such as an electrical generator. The gas turbine engine includes a fuel nozzle to inject fuel and air into a combustor. If the mixture of fuel and air is not well-mixed, the consequences could include an unstable flame, incomplete combustion, and increased production of nitric oxides (NO x ) and other undesirable byproducts. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In accordance with a first embodiment, a system includes a multi-tube fuel nozzle including a fuel nozzle head and multiple tubes. The fuel nozzle head includes an outer wall surrounding a chamber, and the outer wall includes a downstream wall portion configured to face a combustion region. The multiple tubes extend through the chamber to the downstream wall portion, and each tube includes an air inlet into the tube, a fuel inlet including a protrusion extending radially into the tube in a crosswise direction relative to a longitudinal axis of the tube, and an outlet from the tube. 
     In accordance with a second embodiment, a system includes a premixing tube configured to mount in a multi-tube fuel nozzle. The premixing tube includes an air inlet into the premixing tube a fuel inlet, and an outlet from the premixing tube. The fuel inlet has a protrusion extending radially into the premixing tube in a crosswise direction relative to a longitudinal axis of the premixing tube. The air inlet is upstream from the fuel inlet, and the outlet is downstream from both the air inlet and the fuel inlet. 
     In accordance with a third embodiment, a system includes a turbine fuel nozzle. The turbine fuel nozzle includes a premixing tube with an air inlet into the premixing tube, a fuel inlet having a protrusion extending radially into the premixing tube in a crosswise direction relative to a longitudinal axis of the premixing tube, and an outlet from the premixing tube. The air inlet is upstream from the fuel inlet, and the outlet is downstream from both the air inlet and the fuel inlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of an embodiment of a turbine system that includes a system to increase fuel-air mixing in a multi-tube fuel nozzle; 
         FIG. 2  is a cross-sectional view of an embodiment of a combustor that includes a plurality of multi-tube fuel nozzles; 
         FIG. 3  is a front plan view of an embodiment of the combustor taken along line  3 - 3  of  FIG. 2 , illustrating a plurality of circular multi-tube fuel nozzles spaced apart from one another in cap; 
         FIG. 4  is a front plan view of an embodiment of the combustor taken along line  3 - 3  of  FIG. 2 , illustrating a plurality of wedge-shaped multi-tube fuel nozzles disposed directly adjacent to one another in a multi-sector arrangement; 
         FIG. 5  is a cross-sectional view of an embodiment of a multi-tube fuel nozzle having a plurality of premixing tubes with radially protruding fuel inlets; 
         FIG. 6  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating a radially protruding fuel inlet that is perpendicular to a longitudinal axis; 
         FIG. 7  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating a radially protruding fuel inlet that is crosswise to a longitudinal axis and forms an acute angle with the longitudinal axis; 
         FIG. 8  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets in a diametrically opposed configuration; 
         FIG. 9  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets in an axially staggered configuration; 
         FIG. 10  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets that vary in radial depth into the tube, vary in diameter, vary in tubular shape, and vary in configuration; 
         FIG. 11  is a partial cross-sectional side view of an embodiment of a single premixing tube taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets that vary in angles relative to a longitudinal axis, vary in radial depth into the tube, and vary in diameter; 
         FIG. 12  is a cross-sectional view of an embodiment of a single premixing tube with radially protruding fuel inlets with axes that converge directly toward a longitudinal axis; and 
         FIG. 13  is a cross-sectional view of an embodiment of a single premixing tube with radially protruding fuel inlets oriented at an angle configured to induce a swirling flow about a longitudinal axis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The present disclosure is directed towards systems to increase fuel-air mixing within a multi-tube fuel nozzle. The multi-tube fuel nozzle may have multiple premixing tubes that each has one or more radially protruding fuel inlets to inject fuel into a flow of air. As may be appreciated, fluid velocity is highest at the center of the premixing tube, and the fuel inlets increase jet penetration proximate to this high velocity region. As a result, the formation of combustion byproducts, such as nitric oxides, may be decreased. Further, the length of the premixing tube may be decreased, resulting in a shorter length fuel nozzle and combustor. 
       FIG. 1  is a block diagram of an embodiment of a turbine system  16  with a fuel nozzle  26  (e.g., multi-tube fuel nozzle) equipped with multiple premixing tubes  68 , each having one or more radially protruding fuel inlets  11  to increase fuel-air mixing. Throughout the discussion, a set of axes will be referenced. These axes are based on a cylindrical coordinate system and point in an axial direction  10 , a radial direction  12 , and a circumferential direction  14 . For example, the axial direction  10  extends along the length (or longitudinal axis) of the premixing tubes, the radial direction  12  extends away from the longitudinal axis, and the circumferential direction  14  extends around the longitudinal axis. 
     The turbine system  16  includes a compressor  18 , a combustor  20 , and a turbine  22 . The compressor  18  receives air from an intake  24  and compresses the air for delivery to the combustor  20 . The combustor  20  also receives fuel from fuel nozzles  26 . The air and fuel are fed to the combustor  20  in a specified ratio suitable for optimum combustion, emissions, fuel consumption, and power output. The air and fuel mix and react to form combustion products. If the air and fuel are not well-mixed, undesirable combustion byproducts, such as nitric oxides, can form. Certain embodiments of turbine system  16  include systems for increasing fuel-air mixing to reduce the amount of combustion byproducts that are formed, particularly nitric oxides. The hot combustion products are fed into the turbine  22 , which causes a shaft  28  to rotate. The shaft  28  is also coupled to the compressor  18  and a load  30 . The rotating shaft  28  provides the energy for the compressor  18  to compress air, as described previously. The load  30  can be an electric generator or any device capable of utilizing the mechanical energy of the shaft  28 . Finally, the combustion products exit the turbine  22  and are discharged through to an exhaust outlet  32 . 
       FIG. 2  is a cross-sectional side view of an embodiment of the combustor  20  including multi-tube fuel nozzles  26 , each having premixing tubes  68  with one or more radially protruding fuel inlets  11  to enhance fuel-air mixing. The combustor  20  includes a flow sleeve or outer casing  44 , an end cover  46 , and a cap member or divider wall  94  and/or an outer wall  48  of the fuel nozzles  26 . The outer casing  44  has air inlets  50 , which allow air to flow into an annular space  49  between the casing  44  and a combustor liner  51 . The cap member  94  and/or the outer wall  48  has a downstream wall portion  52  that faces a combustion region  54 . The cap member  94  and/or the outer wall  48  separates the combustor internals from the combustion region  54 . Multiple fuel nozzles  26  are mounted within the combustor  20 . Each fuel nozzle  26  includes a fuel conduit  56  and a fuel nozzle head  58 . Each fuel conduit  56  is oriented in the axial direction  10  through a head end  60  of the combustor  20  and from an upstream end portion  62  to a downstream end portion  64 . The end cover  46  is disposed at the upstream end portion  62  and the fuel nozzle head  58  is disposed at the downstream end portion  64 . The fuel nozzle head  58  includes the outer wall  48 , which surrounds a fuel chamber  66  coupled to the fuel conduit  56 . Premixing tubes  68  of each multi-tube fuel nozzle  26  extend through the chamber  66  from an upstream wall portion  70  to the downstream wall portion  52 . Tubes  68  are arranged circumferentially  14  around the downstream portion of fuel conduit  56 . In certain embodiments, each multi-tube fuel nozzle  26  may include approximately 1 to 1000, 10 to 500, or 20 to 100 premixing tubes  68 , each having one or more radially  12  protruding fuel inlets to enhance fuel-air mixing. 
     In the arrangement shown, air flows along a path  72  through the air inlets  50  into the annular space  49  and then flows along a path  74  into the head end  60 . The air then flows along a path  76  into the premixing tubes  68 . Fuel enters fuel conduits  56  from the fuel supply and follows path  80  into fuel chamber  66 . In the embodiment shown, fuel chamber  66  also includes a baffle  82 , which forces the fuel to flow around the baffle  82  to reach the radially protruding fuel inlets of the premixing tubes  68 . Fuel enters the radially protruding fuel inlets and mixes with air within the tubes  68 . The fuel-air mixture flows through the premixing tubes  68  and enters combustion region  54 , where the mixture is converted into hot combustion products. 
       FIG. 3  is a front plan view of an embodiment of the combustor  20  taken along line  3 - 3  of  FIG. 2 , illustrating a plurality of circular multi-tube fuel nozzles  26  (e.g.,  96 ,  98 ) spaced apart from one another in a cap member  94 . As illustrated, the combustor  20  includes a central fuel nozzle  96  centrally located within the cap member  94  of the combustor  20 . The combustor  20  also includes multiple outer fuel nozzles  98  disposed circumferentially about the center fuel nozzle  96 . As illustrated, six outer fuel nozzles  98  surround the center fuel nozzle  96 . Each fuel nozzle  26  includes the plurality of tubes  68 . As illustrated, the plurality of tubes  68  of each fuel nozzle  26  is arranged in multiple rings  100  and  101 . The rings  100  and  101  have a concentric arrangement about a central axis  102  of each fuel nozzle  26 . In certain embodiments, the number of rings  100  and  101 , number of tubes  68  per ring  100  and  101 , and arrangement of the plurality of tubes  68  may vary. Again, each tube  68  may include one or more (e.g. 1 to 50) radially protruding fuel inlets  11  to enhance fuel-air mixing in each tube  68 . 
       FIG. 4  is arrangement front plan view of an embodiment of the combustor  20  taken along line  3 - 3  of  FIG. 2 , illustrating a plurality of wedge-shaped multi-tube fuel nozzles  26  (e.g.,  116 ,  118 ) disposed directly adjacent to one another in a multi-sector arrangement. The combustor  20  includes an outer support structure  114  extending circumferentially  14  about the fuel nozzles  26 . As illustrated, the combustor  20  includes a center fuel nozzle  116  and multiple outer fuel nozzles  118  disposed circumferentially about the center fuel nozzle  116 . Six outer fuel nozzles  118  surround the center fuel nozzle  116 . However, in certain embodiments, the number of fuel nozzles  26  as well as the arrangement of the fuel nozzles  26  may vary. For example, the number of outer fuel nozzles  118  may be 1 to 20, 1 to 10, or any other number. For simplicity, only some of the tubes  68  are shown in the outer fuel nozzles  118  and the central fuel nozzle  116 . However, each fuel nozzle  26  includes multiple premixing tubes  68 . 
     Each outer fuel nozzle  118  includes a non-circular perimeter  120 . As illustrated, the perimeter  120  includes a wedge shape or truncated pie shape with opposing sides  122  and  124  and opposing sides  126  and  128 . The sides  122  and  124  are arcuate shaped sides that are radially  12  offset from one another. The sides  126  and  128  are linear and generally converge toward one another from side  122  to side  124 . However, in certain embodiments, the perimeter  120  of the outer fuel nozzles  118  may include other shapes, e.g., a pie shape with three sides. Regardless of the shape, each outer fuel nozzle  118  is a multi-tube fuel nozzle  26  with a plurality of premixing tubes  68 , each having one or more (e.g.,  1  to  50 ) radially protruding fuel inlets  11  to enhance fuel-air mixing in the tubes  68 . Similarly, the center fuel nozzle  116  is a multi-tube fuel nozzle  26  with a plurality of the premixing tubes  68 , each having one or more (e.g.,  1  to  50 ) radially protruding fuel inlets  11  to enhance fuel-air mixing in the tubes  68 . The center fuel nozzle  116  includes a perimeter  130  (e.g., circular perimeter). In certain embodiments, the perimeter  130  may include other shapes, e.g., a square, hexagon, triangle, or other polygon. The perimeter  130  of the center fuel nozzle  116  may be coaxial with a central axis  132  of the combustor  20  a may include concentric rings  134  of the premixing tubes  68 . 
       FIG. 5  is a cross-sectional view of an embodiment of the multi-tube fuel nozzle  26  (e.g., fuel nozzles  96 ,  98 ,  116 , and  118 ) with premixing tubes  68 , each having one or more radially protruding fuel inlets  11 ,  154  with respective protrusions  146  to increase fuel-air mixing. Each tube  68  is cylindrical about a centerline  150  in the axial direction  10 . Each tube  68  has an air inlet  152  into the tube, a radially  12  protruding fuel inlet  11 ,  154  into the tube, and an outlet  156  out of the tube. As shown, air inlet  152  extends axially  10  in the direction of centerline  150  at upstream end portion  148  of the tube  68 . Fuel inlet  154  comprises a protrusion  146  (e.g., a hollow protrusion) extending into the tube  68  in a crosswise direction (e.g., a radial direction  12 ) relative to a longitudinal axis (e.g., centerline  150 ) of the tube  68 . Both air inlet  152  and the outlet  156  are external to the chamber  66 . 
     Air from the head end  60  flows into each premixing tube  68  via air inlet  152 . Fuel from the fuel supply travels though fuel conduit  56  and into chamber  66  through a flow path  158 . The fuel encounters the baffle  82 , which forces the fuel to follow a path  160  through the chamber  66  to help uniformly distribute the fuel to the fuel inlets  154  of the plurality of premixing tubes  68 . The fuel then enters premixing tubes  68  through fuel inlets  154 . Within each premixing tube  68 , the air and fuel contact each other, mix, and exit the tube  68  through the outlet  156  into the combustion region  54  with a well-mixed composition. The protrusion  146  helps the fuel penetrate further into each tube  68  (e.g., in radial direction  12 ), thereby enhancing fuel-air mixing in the tube  68 . The protrusion  146  also may enhance mixing by disturbing the flow, inducing turbulence, inducing swirling flow, inducing vortices, or any combination thereof. As discussed in detail below, each tube  68  may include 1 to 100 (e.g., 1, 2, 3, 4, 5, or more) fuel inlets  154  with protrusions  146 , and each protrusion  146  may have a common or different diameter, radial  12  height, shape, angle relative to the axis  150 , or any combination thereof. 
       FIG. 6  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating a radially  12  protruding fuel inlet  11 ,  154  that is perpendicular to the longitudinal axis  150 . The premixing tube  68  is symmetric about the centerline  150  in the axial direction  10  and has an outer diameter  172 , an inner diameter  174  (e.g., internal diameter), an outer surface  176 , an inner surface  178 , and a tubular shape  180 . The cross-sectional area available for fluid flow, or flowing area  182 , is a function of the inner diameter  174 . Tube  68  has the air inlet  152 , the fuel inlet  154 , and the outlet  156 . Additionally, the fuel inlet  154  is offset from the air inlet  152  (e.g., end  153 ) and the outlet  156  (e.g., end  157 ), such that air inlet end  153  is upstream of the fuel inlet  154 , and the outlet end  157  is downstream from both the air inlet  152  and the fuel inlet  154 . The premixing tube  68  has a length  184  between the inlet end  153  and the outlet end  157 . In certain embodiments of the turbine system  16 , it may be desirable to shorten the length  184  of premixing tube  68  to decrease the size of the fuel nozzle  26  and/or the combustor  20 . 
     The protrusion  146  is disposed at the fuel inlet  154  to inject fuel nearer the centerline  150  of premixing tube  68 . Protrusion  146  may include an insert  147  that is coupled to an opening  186  in the tube  68 . For example, insert  147  may be coupled to the opening  186  at a joint  187 , such as a weld, braze, or other fixed or removable joint. Alternatively, protrusion  146  may be integrally formed with tube  68  as a one-piece structure. In the case of a one-piece structure, tube  68  could be formed by casting. Thus, the protrusion  146  (e.g., hollow protrusion) may be formed via casting, deformation, punching, or another technique. 
     The protrusion  146  of the radially protruding fuel inlet  154  is configured to increase fuel-air mixing in the premixing tube  68 . The degree of mixing of the fuel-air mixture when it exits the premixing tube  68  through the outlet end  156  is also affected by the fluid velocity. The velocity of the fluid flowing through the premix tube  68  depends on the flow rate and the offset from the tube centerline  150  in radial direction  12 . A fluid, such as air, may have a maximum velocity at the tube centerline  150 , while having a minimum velocity along the tube wall (e.g. tube inner surface  178 ). Flow of the air in contact with the wall  178  is essentially zero and increases as the radial  12  offset from tube centerline  150  approaches zero. The protrusion  146  delivers fuel into a region of higher air velocity, which results in improved mixing. Furthermore, the protrusion  146  may induce turbulence, swirl, and/or formation of large scale vortices and small scale eddies to enhance fuel-air mixing within the tube  68 . In other words, the protrusion  146  may generally disturb the flow, while also increasing radial  12  penetration of the fuel into the air flow. In this manner, the protrusion  146  of the radially protruding fuel inlet  154  may provide a more uniform distribution of fuel in the air, thereby improving the fuel-air distribution (i.e., more uniform) exiting each tube  68 . 
     The tubular shape of the protrusion  146  could be cylindrical, conical, polyhedral, or another geometry suitable for delivering fuel to the premixing tube  68 . The protrusion  146  has a centerline  188  in the radial direction  12 , an outer diameter  190 , an inner diameter  192 , and a radial depth  194 . Depending on the dimensions of the tube  68 , the inner diameter  192  of the protrusion  146  may be approximately 25 to 500, 50 to 250, 75 to 125, or less than approximately 100 mils The protrusion  146  injects fuel at radial depth  194 , which is measured from the tube inner surface  178 . The radial depth  194  may range from 1 percent to 50 percent, or 5 percent to 40 percent, or 10 percent to 30 percent of the tube inner diameter  174 . For example, the radial depth  194  may be greater than approximately 5, 10, 15, 20, 25, 30, 35, or 40 percent of the inner diameter  174 . Generally, for a single protrusion  146 , the degree of fuel penetration increases as the depth  194  approaches the tube centerline  150 . The radial depth  194  also may gradually increase flow disturbance (e.g., turbulence) and mixing as it increases. 
     As shown, the protrusion  146  is oriented crosswise (e.g., perpendicular) to the tube centerline  150 . The protrusion centerline  188  is offset from the air inlet end  153  by a distance  196 . Certain embodiments may position the protrusion  146  to be proximate to the air inlet end  153  to maximize the residence time for fuel-air mixing within tube  68 . In another embodiment, the fuel inlet  154  may be disposed directly at or adjacent the air inlet end  153 , while still having a crosswise orientation to the tube centerline  150 . For example, the distance  196  could be approximately 0 to 75, 1 to 50, 5 to 25, or 10 to 15 percent of the length  184 . In certain embodiments, the axis  188  of the protrusion  146  may be oriented at an angle  189  relative to the centerline  150 , wherein the angle  189  may be approximately 5 to 90, 10 to 80, 20 to 70, 30 to 60, 40 to 50, 30, 45, 60, or 90 degrees relative to the centerline  150 . The angle  189  may be oriented in the upstream axial  10  direction, downstream axial  10  direction, clockwise circumferential  14  direction, or counterclockwise circumferential  14  direction. 
     Air enters the air inlet  152  and flows in the axial direction  10  along the premixing tube  68  toward outlet  156 . At position  196 , fuel enters the fuel inlet  154  and begins to mix with air at a contact area  198  (e.g., central region), as indicated by fuel path  200 . The fuel-air continues to mix as the mixture flows in a primarily axial direction  10  along the tube  68 . An improved fuel-air distribution is achieved when the mixture exits tube  68  through outlet end  156 . Generally, the degree of mixedness of the fuel-air mixture increases along the pipe length  184 , from a minimum mixedness at contact area  198  to a maximum mixedness at outlet end  156 . By increasing the degree of flow disturbance and fuel penetration (e.g., radial depth  194 ), the protrusion  146  enables a shorter premixing tube  68  to achieve the same degree of mixedness as a longer premixing tube  68  without the protrusion  146 . Similarly, the degree of mixedness of the fuel-air mixture is increased for a tube  68  with the protrusion  146  compared to that of a tube  68  of identical length  184  without the protrusion  146 . 
       FIG. 7  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating a radially protruding fuel inlet  11 ,  154  that is crosswise to the centerline  150  and forms an acute angle  212  with the centerline  150 . The premixing tube  68  and the protrusion  146  are structurally similar to the tube and the protrusion described in  FIG. 6 . The protrusion centerline  188  forms the acute angle  212  with the longitudinal axis in the axial direction  10  (e.g. tube centerline  150 ). The protrusion  146  may be axially  10  angled, such that the acute angle  212  is oriented in an upstream flow direction  214  or a downstream flow direction  216  (as shown) relative to the tube centerline  150 . The protrusion  146  may also be circumferentially  14  angled at the acute angle  212  configured to induce a swirling flow about the tube centerline  150 . In such a case, the protrusion centerline  188  is skew with tube centerline  150 , and the angle  212  is defined by protrusion centerline  188  and a longitudinal axis parallel to (but radially  12  offset from) tube centerline  150 . Certain embodiments may select the acute angle  212  to maximize the degree of mixedness at the outlet end  157 . Additionally, other embodiments may include more than one angled protrusion  146  (e.g., 2 to 100 angled protrusion  146 ), which may include uniformly or differently angled protrusions  146  (e.g., 30, 45, 60, 75 and/or 90 degree angled protrusion  146 ). 
     Air enters the air inlet  152  and flows in the axial direction  10  along the premixing tube  68  toward outlet  156 . At position  196 , fuel enters the fuel inlet  154  and begins to mix with air at a contact area  198  (e.g., central region), as indicated by fuel path  200 . The fuel-air continues to mix as the mixture flows in a primarily axial direction  10  along the tube  68 . An improved fuel-air distribution is achieved when the mixture exits tube  68  through outlet end  156 . Specifically, the acute angle  212  may further increase the turbulence, swirl, and/or formation of large scale vortices and small scale eddies to enhance fuel-air mixing within the tube  68 . For example, if the acute angle  212  is oriented in the upstream flow direction  214 , the residence time for fuel-air mixing within the tube  68  may be increased. Additionally, if the acute angle  212  is oriented in the downstream flow direction  216 , the velocity of the fuel-air mixture through the tube  68  may be increased, which may increase the turbulence of the fuel-air mixture. 
       FIG. 8  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets  11 ,  154  in a diametrically opposed configuration. The protrusions  146  and  228  are in a diametrically opposed configuration at a common axial position  230  relative to tube centerline  150 . The protrusion  228  extends in the radial direction  12  into the tube  68  in a crosswise direction (e.g., radial  12  direction) relative to the tube centerline  150 . The premixing tube  68  is structurally similar to the premixing tube described in  FIG. 6  with the exception that the tube  68  has two radially  12  opposed fuel inlets  154  and  232 . According to other embodiments, the number of fuel inlets and protrusions may vary between approximately 2 to 100, 3 to 50, 4 to 25, or 5 to 10. The protrusions  146  and  228  direct fuel towards the tube centerline  150 . The protrusion  228  may be the same or different than protrusion  146 . For example, protrusions  146  and  228  may vary in radial depth  194 ,  234  into the tube  68 ; angle  189 ,  236  relative to the centerline  150 ; diameter  192 ,  238 ; tubular shape  239 ,  240 ; or any combination thereof. As illustrated, the protrusions  146  and  228  share the common axial position  230 , while being circumferentially  14  offset from one another (e.g., rotated 180 degrees in the circumferential direction  14  about the centerline  150 ). In other embodiments, the protrusions  146  and  228  may share the common axial position  230 , but may be circumferentially  14  offset from one another at a different angle, such as approximately 10 to 180, 30 to 150, or 45 to 135 degrees. As illustrated, the axial position  230  of the protrusions  146  and  228  are both offset from the air inlet end  153  by the axial distance  196 . In certain embodiments, the distance  196  could be chosen such that protrusions  146  and  228  are proximate to the air inlet end  1523  to maximize the residence time for fuel-air mixing within the tube  68 . In particular, the fuel inlets  154 ,  232  may be disposed along an upstream portion of the tube  68  and may be within the distance  196  that is approximately 0 to 75, 1 to 50, or 5 to 25 percent of the length  184 . Further, the protrusions  146  and  228  may be angled the same or different as discussed in  FIG. 7 . 
     Air enters the air inlet  152  and flows in the axial direction  10  along the premixing tube  68  toward outlet  156 . At position  196 , fuel enters the fuel inlets  154 ,  232  and begins to mix with air at contact areas  198 ,  242  (e.g., central regions), as indicated by fuel paths  200 ,  244 . In certain embodiments, the fuel inlets  154 ,  232  may share the contact area  198  (e.g., fuel jets directly impinge one another in area  198 ). The fuel-air mixture continues to mix as the mixture flows in a primarily axial direction  10  along the tube  68 . An improved fuel-air distribution is achieved when the mixture exits tube  68  through outlet end  156 . Specifically, the opposed fuel inlets  154 ,  232  may further increase the turbulence, swirl, and/or formation of large scale vortices and small scale eddies to enhance fuel-air mixing within the tube  68 . For example, the opposed fuel inlets  154 ,  232  may cause the fuel from each inlet  154 ,  232  to impinge onto one another other and increase the turbulence at the contact areas  198 ,  242 . Thus, the opposed fuel inlets  154 ,  232  may enhance fuel-air mixing within the tube  68  and enable the tube  68  to be shortened. 
       FIG. 9  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets  11  (e.g.,  154 ,  232 ) in an axially  10  staggered configuration  255  at different axial positions  230  and  256 . Premixing tube  68  is structurally similar to premixing tube described in  FIG. 8  with the exception that the fuel inlets  154  and  232  are in the staggered configuration  255 . The protrusion  228  may be the same or different than protrusion  146 . For example, protrusions  146  and  228  may vary in radial depth  194 ,  234  into the tube  68 ; angle  189 ,  236  relative to the centerline  150 ; diameter  192 ,  238 ; tubular shape  239 ,  240 ; or any combination thereof. As illustrated, the axial position  230  of the protrusion  146  is axially  10  offset from the air inlet end  153  by the distance  196 . Additionally, the axial position  256  of the protrusion  228  is axially  10  offset from the air inlet end  153  by a distance  258 . The distances  196  and  258  may be equal or different. However, as illustrated, the distances  196  and  258  are different to define an axial spacing or offset  260  between the fuel inlets  154  and  232  and the associated protrusions  146  and  228 . In certain embodiments, the spacing  260  may be approximately 0 to 75, 1 to 50, 5 to 25, or 10 to 15 percent of the length  184  of the tube  68 . In yet other embodiments, the spacing  260  may be approximately 1 to 1000, 10 to 150, or 20 to 90 percent of the inner diameter  174  of the tube  68 . As may be appreciated, the tube  68  may have any number (e.g., approximately 2 to 100, 5 to 50, 10 to 25) of fuel inlets ( 154  and  232 ) and associated protrusions (e.g.,  146  and  228 ) at various axial  10  positions, radial  12  depths, angles, circumferential  14  positions, or any combination thereof. 
     Air enters the air inlet  152  and flows in the axial direction  10  along the premixing tube  68  toward outlet  156 . At positions  196  and  258 , fuel enters the fuel inlets  154 ,  232  and begins to mix with air at contact areas  198 ,  242  (e.g., central regions), as indicated by fuel paths  200 ,  244 . The fuel-air mixture continues to mix as the mixture flows in a primarily axial direction  10  along the tube  68 . An improved fuel-air distribution is achieved when the mixture exits tube  68  through outlet end  156 . Specifically, the staggered fuel inlets  154 ,  232  may further increase the turbulence, swirl, and/or formation of large scale vortices and small scale eddies to enhance fuel-air mixing within the tube  68 . For example, the staggered fuel inlets  154 ,  232  may cause the fuel from each inlet  154 ,  232  to impinge onto opposite sides of the tube inner surface  178  and increase the turbulence at the contact areas  198 ,  242 . Thus, the opposed fuel inlets  154 ,  232  may enhance fuel-air mixing within the tube  68  and enable the tube  68  to be shortened. 
       FIG. 10  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets  11  that vary in radial  12  depth into the tube, vary in diameter, vary in tubular shape, and vary in configuration. In particular, the premixing tube  68  has protrusions  146 ,  228 ,  272 ,  274 ,  276 ,  278 ,  280 ,  282 ,  284 , and  286  associated with the fuel inlets  11  that vary in radial  12  depth into the tube, vary in diameter, vary in tubular shape, and vary in configuration.  FIG. 10  depicts many variations and combinations of the protrusion characteristics above. It should be understood that  FIG. 10  is intended to show that any of the features disclosed herein are capable of use together, and thus are not mutually exclusive. 
     The protrusions  146 ,  228  of the fuel inlets  11  have the radial depth  194 ; the protrusions  272 ,  274  of the fuel inlets  11  have a radial depth  287 ; and protrusions  276 ,  278  of the fuel inlets  11  have a radial depth  288 . The radial depths  194 ,  287 ,  288  are different from one another and progressively increase in the downstream flow direction  216 . In other embodiments, the radial depths  194 ,  287 ,  288  may progressively decrease or both increase and decrease in the downstream flow direction  216 . As illustrated, the protrusions  146 ,  228 ,  272 ,  274 ,  276 , and  278  of the fuel inlets  11  have the tubular shape  240  (e.g., cylindrical), while the protrusions  280 ,  282 ,  284 , and  286  have a different tubular shape  290  (e.g., conical). As shown, the conical protrusions  280 ,  282 ,  284 , and  286  each converge at an angle  300  relative to a central axis  302  of the respective protrusion. In general, the angle  300  may be approximately 1 to 40, 2 to 30, 3 to 20, or 4 to 10 degrees. Furthermore, the protrusions  280 ,  282 ,  284 , and  286  may have equal or different angles  300 . 
     In addition, the protrusions  146 ,  228  have a diameter  292 ; the protrusions  272 ,  274  have a diameter  293 , and the protrusions  276 ,  278  have a diameter  294 . The diameters,  292 ,  293 ,  294  are different from one another and progressively decrease in the downstream flow direction  216 . In other embodiments, the diameters  292 ,  293 ,  294  may progressively increase or may both increase and decrease in the downstream flow direction  216 . As illustrated, the protrusion  146  is in an opposed configuration relative to the protrusion  228 , the protrusion  272  is in an opposed configuration relative to the protrusion  274 , and the protrusion  276  is in an opposed configuration relative to the protrusion  278 . Further, each set of opposed protrusions has common features (e.g., diameter, radial depth), but has different features compared to other sets. 
     Further, the protrusions  282  and  284  are arranged in a staggered configuration at different axial positions  296  and  298 . Similarly, the protrusions  280  and  286  are in a staggered configuration. Still further, the protrusions  146 ,  228  are staggered relative to protrusions  272 ,  274 ,  276 ,  278 ,  280 ,  282 ,  284 , and  286 . As may be appreciated, the protrusions may be staggered on the same or opposite sides of the tube  68 . As shown in  FIG. 10 , any of the protrusions  146 ,  228 ,  272 ,  274 ,  276 ,  278 ,  280 ,  282 ,  284 ,  286  may be integrally formed with the tube  68  or may be an insert  147  coupled to the tube  68  via the joint  187  as discussed previously. 
     Still further, the tube  68  has a spacing  304  between the protrusions  146 ,  228  and the protrusions  272 ,  274  and a spacing  306  between the protrusions  272 ,  274  and the protrusions  276 ,  278 . As shown, the spacings  304 ,  306  gradually decrease along the length  184  of the tube  68  in the downstream flow direction  216 . In other embodiments, the spacings  304 ,  306  may gradually increase or may be random along the length  184  of the tube  68 . 
       FIG. 11  is a partial cross-sectional side view of an embodiment of the single premixing tube  68  taken within line  6 - 6  of  FIG. 5 , illustrating radially protruding fuel inlets  11  that vary in angles relative to the centerline  150 , vary in radial  12  depth into the tube, and vary in diameter. It should be understood that  FIG. 11  is intended to show that any of the features disclosed herein are capable of use together, and thus are not mutually exclusive. 
     As illustrated, the protrusions  146 ,  228  (e.g., centerline  188 ) are both oriented at an acute angle  212  with the tube centerline  150  in the downstream flow direction  216 . In addition, protrusions  276 ,  278  (e.g., centerline  312 ) are both oriented at an acute angle  314  with tube centerline  150  in the upstream flow direction  214 . In general, the acute angles  212 ,  314  may be the same or different from one another, e.g., approximately 1 to 89, 5 to 85, 20 to 70, or 35 to 55 degrees. As shown, the protrusions  146 ,  228  have the radial depth  194 ; the protrusions  272 ,  274  have a radial depth  287 ; and protrusions  276 ,  278  have the radial depth  288 . The radial depths  194 ,  287 ,  288  are different from one another and progressively decrease in the downstream flow direction  216 . In other embodiments, the radial depths  194 ,  287 ,  288  may progressively increase or both increase and decrease in the downstream flow direction  216 . In addition, the protrusions  146 ,  228  have the diameter  292 ; the protrusions  272 ,  274  have the diameter  293 , and the protrusions  276 ,  278  have the diameter  294 . The diameters,  292 ,  293 ,  294  are different from one another and both increase and decrease along the length  184  of the tube  68 . In other embodiments, the diameters  292 ,  293 ,  294  may progressively decrease or progressively increase in the downstream flow direction  216 . 
       FIG. 12  is a cross-sectional view of an embodiment of the single premixing tube  68  with radially  12  protruding fuel inlets  11  with axes that converge directly toward a longitudinal axis (e.g., centerline  150 ). The tube  68  includes the protrusions  146 ,  228 ,  272 , and  274  that do not induce a swirling flow about the tube centerline  150 , as each protrusion centerline  188 ,  326 ,  328 ,  330  intersects tube centerline  150 . In other embodiments, the protrusions  146 ,  228 ,  272 ,  274  may be uniformly or differently spaced circumferentially  14  about the centerline  150 . 
       FIG. 13  is a cross-sectional view of an embodiment of the single premixing tube  68  with radially protruding fuel inlets  11  (e.g., protrusions  146   228 ,  272 , and  274 ) oriented at an angle configured to induce a swirling flow about the tube centerline  150 . In particular, each fuel inlet  11  (e.g., protrusions  146 ,  228 ,  272 ,  274 ) is oriented at an angle  350  relative to a radius or radial line  352 . For example, the angle  350  may be defined at the intersection between the tube  68 , the radial line  352 , and the axis  188 ,  326 ,  328 , and  330  of each respective protrusion  146 . The angle  350  may be the same or different from one protrusion to another. Furthermore, the angle  350  may be approximately 1 to 90, 5 to 60, 10 to 45, or 20 to 30 degrees. The arrangement shown induces a swirling flow in a counterclockwise direction  342 . A different arrangement could produce a swirling flow in a clockwise direction. Further, in certain embodiments, the protrusions  146 ,  228 ,  272 ,  274  may be uniformly or differently spaced circumferentially  14  about the centerline  150 . 
     Technical effects of the disclosed embodiments include a system to increase fuel-air mixing in a combustor with multi-tube fuel nozzles. A protrusion disposed at the fuel inlet on a premixing tube increases the jet penetration of the fuel. Fluid velocity is highest at the center of the tube, and the protrusion allows fuel to be injected proximate to this high velocity region. The formation of combustion byproducts, such as nitric oxides, correlate directly to the poor mixing of air and fuel. Thus, a protrusion disposed at a fuel inlet on a premixing tube decreases nitric oxide emissions for the premixing tube. The protrusion also creates a flow disturbance, which further enhances fuel-air mixing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.