Abstract:
A fuel nozzle assembly for a gas turbine, the assembly including: a cylindrical center body; a cylindrical shroud coaxial with and extending around the center body, and a turning guide having an downstream edge extending in a passage between the center body and an inlet to the shroud, wherein the turning guide extends only partially around the center body.

Description:
The invention relates to fuel combustion in a gas turbine, and particularly relates to guiding compressed air to a combustion zone in a combustor. 
     BACKGROUND OF THE INVENTION 
     A gas turbine combustor mixes large quantities of fuel and compressed air, and burns the resulting air and fuel mixture. Conventional combustors for industrial gas turbines typically include an annular array of cylindrical combustion “cans” in which air and fuel are mixed and combustion occurs. Compressed air from an axial compressor flows into the combustor. Fuel is injected through fuel nozzle assemblies that extend into each can. The mixture of fuel and air burns in a combustion chamber of each can. The combustion gases discharge from each can into a duct that leads to the turbine. 
     Pressurized air from the compressor enters a combustion can at the back end of the can, which is the same end from which hot combustion gases flow from the can to the turbine. The compressed air flows through an annular duct formed between a cylindrical wall of the can and an inner cylindrical combustion liner. The relatively cool compressed air cools the wall of the liner as the hot combustion gas flows through the interior of the liner. The hot combustion gas flows in a generally opposite direction to the flow of the compressed air through the duct. 
     As the compressed air reaches the head-end of the combustor can, the air is turned 180 degrees to enter one of the fuel nozzles. To enter the outer fuel nozzles the compressor air makes a tight and quick reversal of flow direction. This abrupt turn can create low velocity flow zones in the air while other zones of the air flow are at significantly higher velocities. The occurrence of low velocity flows is most acute as the air enters the outer fuel nozzles which are closest to the double walled flow path in the combustion chamber for compressed air. 
     Uniform flow velocities through a fuel nozzle are desired to provide uniform mixing of the air and fuel, and uniform combustion. Zones of low velocity airflow in the fuel nozzle also pose a flame holding risk inside the nozzle as low velocity zones provide an area for a flame to anchor inside the fuel nozzle. A flame in the fuel nozzle can destroy the hardware of the nozzle. In addition, low velocity air flows can cause localized variations in the air and fuel mixture. These variations can include regions where the fuel and air mixture is too rich resulting in too high combustion temperatures and excessive generation of nitrous oxides (NOx). There is a long felt desire to hold a steady flame in a combustor can, reduce NOx emissions from combustion in a gas turbine and maintain uniform airflow velocities through the fuel nozzles. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A fuel nozzle assembly has been conceived for a gas turbine, the assembly including: a cylindrical center body; a cylindrical shroud coaxial with and extending around the center body, and a turning guide having an downstream edge extending into the inlet of a passage between the center body and the shroud, wherein the turning guide extends only partially around the center body. 
     The turning guide may be a thin sheet shaped to conform to an inlet region of the shroud. The turning guide may have a wide mouth curved inlet region and a generally straight outlet region. The turning guide may be mounted to the shroud or center body by a rib or post. The turning guide may extend in an arc around the fuel nozzle, and the arc may be in a range of 200 degrees to 35 degrees. The turning guide may be on a side of the shroud adjacent an outer doubled-walled annular flow duct through which compressor air passes and is turned radially inward towards the assembly. 
     A combustion chamber has been conceived for a gas turbine comprising: an annular flow duct through which pressurized air flows in a direction opposite to a flow of combustion gases formed in the chamber; an end cover assembly having an inside surface; a radially inward turn in the flow duct proximate to the inside surface of the end cover assembly; at least one fuel nozzle assembly including a cylindrical center body, a cylindrical shroud coaxial with and extending around the center body, and a turning guide having an downstream edge extending towards a passage between the center body and the shroud, wherein the turning guide extends only partially around the center body, and the turning guide is aligned and proximate to an outlet of the annular flow duct such that the turning guide directs air from the annular flow duct into the passage between the center body and the shroud. The turning guide may be on a side of the shroud adjacent the annular flow duct. 
     A method has been conceived to direct pressurized air into an air flow duct of a fuel nozzle assembly in a combustion chamber, the method comprising: moving pressurized air in a first direction through an annular duct in the combustion chamber and turning the air radially inward from the duct towards the fuel nozzle; the turned pressurized air flowing into a passage between a cylindrical shroud and a center body of the fuel nozzle assembly; as the turned pressurized air flows into the passage, the air is directed by a turning guide having an inlet edge aligned with the turned air flowing from the annular duct and an outlet edge aligned with the passage, wherein the turning guide extends only partially around the center body. 
     The turning guide may be adjacent the outlet of the annular duct and directs air entering the passage at a location on a side of the center body opposite to the annular duct. The turning guide may be proximate to the inlet to the shroud and the directed air is air flowing near the inlet to the shroud. The turning guide may increase the velocity of air flowing into a radially outward portion of the passage. The turning guide may direct the turned air into a narrow gap between the turning guide and an inlet portion of the shroud, wherein the inlet portion has a wide mouth and the turning guide directs the turned air into the narrow gap between the turning guide and the wide mouth of the shroud. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a conventional combustion chamber in an industrial gas turbine, wherein the gas turbine is shown in cross-section. 
         FIG. 2  is a cross-sectional diagram of a portion of a combustion chamber showing the flow path of combustion air through the double-wall of the combustion chamber and turning into an outer fuel nozzle assembly. 
         FIG. 3  is a perspective view of an annular array of fuel nozzle assemblies, arranged around a center fuel nozzle assembly. 
         FIG. 4  is a perspective view of the side of an outer fuel nozzle assembly with a portion of the shroud is transparent to show the turning guide. 
         FIGS. 5 and 6  are front and rear perspective views of the turning guide mounted to a center body of a fuel nozzle assembly. 
         FIG. 7  is view of an array of fuel nozzle assemblies to show the orientation of the turning guides on the outer fuel nozzle assemblies. 
         FIG. 8  is a perspective view of the side and back of a fuel nozzle assembly with a turning guide attached to a shroud. 
         FIG. 9  is a cross-sectional view of the fuel nozzle assembly shown in  FIG. 8 , wherein the cross-section is along a plane perpendicular to an axis of the cross body. 
         FIGS. 10 and 11  are schematic diagrams showing, in cross-section, a turning guide on shrouds with and without a bell-mouth inlet. 
         FIGS. 12 and 13  are views of the air flow through the duct with and without a turning guide. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is side view, showing in partial cross section, a conventional gas turbine engine  2  including an axial turbine  4 , an annular array of combustion chambers  6 , and an axial compressor  8  which generates compressed air  10  ducted to the combustion chambers. Fuel  12  is injected into the combustion chambers and mixes with the compressed air. The air fuel mixture combusts in the combustion chambers and hot combustion gases  14  flow from the chambers to the turbine to drive the turbine buckets  16  to rotate the turbine  4 . The rotation of the turbine turns the compressor via the shaft  18  connecting the turbine and compressor. The rotation of the compressor generates the compressed air for the combustion chambers. 
       FIG. 2  is a cross sectional drawing of a portion of a combustion chamber  6  to show a fuel nozzle assemblies  20 . Each combustion chamber  6 , also referred to as a “can”, includes a substantially cylindrical sleeve  22  secured to the casing  24  of the gas turbine near the discharge end of the compressor. The forward end of the combustion can is closed by an end cover assembly  26  which may be coupled to fuel supply tubes, manifolds and associated valves  28  for feeding gas or liquid fuel  12  to the fuel nozzles of each combustion chamber. The end cover assembly  26  supports a circular array of the fuel nozzle assemblies  20  around a center fuel nozzle assembly  30  housed within the cylindrical sleeve  22 . 
     Pressurized air  10  enters an end of the combustion chamber  6  and flows (see arrow  32 ) through an annular duct  34  formed between a cylindrical sleeve  22  and an inner cylindrical liner  36  of the chamber  6 . The pressurized air  32  flows through the duct  34  towards the end cover assembly  26  in a flow direction opposite to the flow of combustion gases formed in the chamber. The pressurized air is turned by an annular portion of the duct  34  which may be U-shaped  38  in cross-section. 
     To assist in the turning of the air flow, a turning guide  42  is positioned on each of the fuel nozzle assemblies  20  and near the outlet of the U-shaped portion  38  of the air duct  34 . The turning guide  42  may be mounted to be proximate to a rear collar  44  of the fuel nozzle. 
       FIG. 3  is a perspective view of an annular array of fuel nozzle assemblies  20 , referred to as the outer fuel nozzle assemblies, arranged around a center fuel nozzle assembly  30 . The fuel nozzle assemblies  20 ,  30  are attached at their rear collars  44  to flanges  27 . The flanges are mounted to the end cover assembly  26  For each of the outer fuel nozzle assemblies  20 , a turning guide  42  is positioned between its fuel nozzle assembly and the U-shaped end  38  of the annular duct  34  shown in  FIG. 2 . As shown in  FIG. 3 , the turning guides are generally positioned at the periphery of a circle formed by the arrangement of outer fuel nozzle assemblies  20  on the end cover assembly  26 . 
       FIG. 4  is a side view of an outer fuel nozzle assembly  20  with a portion of the shroud  46  transparent to provide a better view of the turning guide  42 . The turning guide and center body are show in dotted lines. The turning guide  42  is mounted adjacent the collar  44  of the fuel nozzle assembly. The shroud may have an annular wide-mouth inlet  56 . The turning guide  42  may fit partially in the wide-mouth inlet of the shroud. The inlet of the turning guide extends axially out of the shroud inlet and radially outward such that the outer peripheral rim  58  of the wide-mouth inlet  56  is substantially the same radial distance from the axis of the fuel nozzle assembly as the inlet rim  60  of the turning guide. 
     The rear collar  44  connects the fuel nozzle assembly to a flange  27  which is attached to the end cover assembly  26 . The collar may be brazed or welded to a flange  27 . The flange  27  may be bolted to the end cover  26 . 
     The turning guide may  42  have a cross-sectional shape conforming to the end of the U-shaped portion  38  of the annular duct. The turning guide  42  may extend in an arc partially around the circumference of the collar  44 , such as 180 degrees around the collar. The arc of the turning guide may be in a range of 35 to 200 degrees. The upstream end of the turning guide  42  may extend, at least partially, into the U-shaped portion  38  of the flow duct. The downstream end of the turning guide may be aligned with the inlet of the annular duct  52  between the cylindrical shroud  46  and center body  50 . The turning guide may extend partially into the annular duct  52 . The downstream end of the turning guide may be radially inward of the shroud  46  such that a gap  53  exits between the shroud and the downstream end of the turning guide. The gap is at the radially outer region of the annular duct  52 . Air flowing on the radially outer surface of the turning guide moves into the gap to ensure an air velocity at the radially outer region of the annular duct. 
     The turning guide  42  assists in providing a uniform flow of the pressurized air being turned into the fuel nozzle assemblies and cylindrical liner  36 . The turning guide forms a flow path that increases the velocity of the pressurize air flow near the radially outer part of the shroud  46 . The increase in the air velocity due to the turning guide suppresses the tendency of relatively low velocity air flows forming at the outer portion of the shroud. Using the turning guide to increase the flow velocity at the radially outer portion of the annular duct  52  creates a more uniform flow velocity through the entire fuel nozzle. 
     Air flow having a uniform velocity in the fuel nozzle promotes uniform fuel air mixing and promotes flame holding resistance in the fuel nozzle. 
     The air flowing through the annular duct  52  mixes with fuel entering the duct from the swirl vanes  54 . The air-fuel mixture passing through the annular duct  52  is swirled by swirl vanes  54 . The swirl vanes may be a generally cylindrical device mounted between the center body and shroud. The spiral flow induced by the swirl vanes promotes mixing of air and fuel in the duct  52 . The mixture of fuel and air flows from the end of the duct  52  to the combustion zone  55  of the combustion chamber. The mixture of fuel and compressed air combust in the combustion zone and the combustion gases flow (see combustion flow arrow  14  in  FIG. 1 ) from the combustion chamber to the buckets  16  in the turbine  4 . 
       FIGS. 5 and 6  are a perspective view and a front view of a turning guide  42  mounted to the center body  50  of a fuel nozzle assembly. Support brackets  62  extend between the center body  50  and the turning guide  42 . The support brackets may be pairs of legs arranged in a trapezoid. The legs may be planar and aligned with the air flowing between the turning guide and center body, such as an alignment with the axis of the fuel nozzle assembly. The rib support brackets  62  structurally support the turning guide in the duct  52 . 
     The turning guide  42  may include an inlet portion  68  in the outlet region that is curved radially outward to conform to a desired flow path of air coming from the U-turn  38  shown in  FIG. 2 . The radially outer perimeter  60  of the inlet section may be at or radially beyond the same radial dimension as the inlet rim  58  of the shroud  46 . The inlet portion  68  extends radially inward and joins a cylindrical outlet region  68  of the turning guide. The outlet region  68  extends in a direction parallel to the axis of the center body. The outlet region  68  may extend to and, optionally, into the shroud  46 . 
       FIG. 7  is an end view of a portion of an array of fuel nozzle assemblies  20 ,  30  in a combustion chamber showing the turning guides  42  at the inlet of the shrouds of the outer fuel nozzle assemblies  20 . The half-circle turning guides  42  are mounted to the wide-mouth inlets  56  of the outer fuel nozzle assemblies  20 . The turning guides  42  are oriented on each of the fuel nozzle assemblies  20  to face the U-shaped exit from which pressurized air exits the annular duct after having gone through a reversal of flow direction. 
       FIGS. 8 and 9  are a perspective view and a front view, respectively, of a turning guide  70  mounted to the inlet of a shroud  72 . The turning guide  70  is similar to the turning guide  42  except that the turning guide  70  is mounted to the shroud  72 . The turning guide  70  is between the shroud  72 , on the one side, and the rear collar  44  and center body  50  on the other side. The turning guide  70  may be attached and mounted to the wide mouth inlet  56  of the cylindrical shroud  72 . The turning guide  70  and wide mouth  56  may be aligned with the junction between the collar  44  and the center body  50 . The turning guide and wide mouth may be upstream of and slightly radially outward of the swirl vanes  54  between the center body and the shroud. 
     The turning guide may extend partially around the wide mouth inlet  56  as an arc, half-circle or other portion of circle. As illustrated in  FIGS. 5 to 8 , the turning guide  42 ,  70  extends half-way, e.g., 180 degrees, around the inside surface of the wide mouth. The turning guide may extend in an arc in a range of, for example, 200 degrees to 35 degrees. 
     The turning guide  70  may be formed of a ceramic or metal, and may be an integral component. The turning guide  70  may have an inlet section  66  that curves radially inwardly to the axis of the center body, and a cylindrical outlet section  68  that is straight along the axis. 
     The turning guide  70  may be attached to the shroud  72  by ribs  74  and posts  76  extending from the wide mount shroud inlet  56 , through the gap  53  and to the curved inlet  66  of the turning guide. The rib may be aligned to be parallel to the axis of the center body to reduce air flow resistance through the gap  53 . The rib  74  may be at the center of the turning guide and the posts  76  may be near the sides of the turning guide. 
     The turning guide  70  may be shaped to conform to the wide mouth inlet  56 . The gap  64  formed between the turning guide  70  and the wide mouth inlet  56  may have a uniform width and be proximate to the radially outer region of the duct between the turning guide and wide mouth. The inlet to the gap may extend generally radially inward and turn axial at the discharge of the gap. The gap is the guided flow passage for a portion of the pressurized air entering the annular air passage between the shroud and the collar and center body. 
       FIGS. 10 and 11  are cross-sectional schematic diagrams showing a turning guide  76  associated with a shroud  78  having a wide-mouth inlet  80  ( FIG. 10 ) and a shroud  82  having a straight, cylindrical inlet. The curved inlet  66  of the turning guide conforms to the shape of the wide mouth inlet  80  for shroud  78 , and does not conform to the cylindrical inlet of the shroud  82 . The curved shape of the turning guide is intended to force the compressed air flowing from the U-turn in the doubled wall duct  36  towards the gap  53  and the radially outer region of duct  52 . By forcing the air through the gap and towards the radially outer region of duct  52 , the turning guide assists in making the flow velocity in duct  52  more uniform. 
       FIGS. 12 and 13  are views of the air flow through the duct  52  with ( FIG. 13 ) and without ( FIG. 11 ) a turning guide. The curved arrows  102  represent the air being turned by the turning guide  76  as the air enters the duct  52 . The curved arrows  104  represent the air flowing into the duct  52  without being guided by a turning guide. 
     An air velocity profile  106  illustrates the generally uniform velocity of the air flow through the duct when a turning guide is at the inlet to the duct. The air velocity profile  108  shows the large variation in air velocity when a turning guide is not present. In particular, the air near the shroud  50  moves substantially slower than the air near the center body  78 . As shown in  FIGS. 12 to 14 , the turning guide increases the air speed through radially outer region of the duct and thereby makes the airflow more uniform through duct. 
     The more uniform air velocity through the duct  52  resulting from the turning guide may provide advantages such as reduced NOx emissions from the combustion chamber, and an increase in steady flame performance of the chamber. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.