Abstract:
A combustor for a gas turbine engine is disclosed which is able to operate with high combustion efficiency, and low nitrous oxide emissions during gas turbine operations. The combustor consists of a can-type configuration which combusts fuel premixed with air and delivers the hot gases to a turbine. Fuel is premixed with air through a swirler and is delivered to the combustor with a high degree of swirl motion about a central axis. This swirling mixture of reactants is conveyed downstream through a flow path that expands; the mixture reacts, and establishes an upstream central recirculation flow along the central axis. A cooling assembly is located on the swirler co-linear with the central axis in which cooler air is conveyed into the prechamber between the recirculation flow and the swirler surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a system and apparatus for controlling temperatures within a combustor. More particularly, the present invention relates to a system and method for controlling the temperature of a swirler within the combustor. 
       BACKGROUND 
       [0002]    Typical combustors are arranged to create a toroidal flow reversal that entrains and recirculates a portion of hot combustion products upstream towards the swirler, which serves as a continuous ignition source for an incoming unburned fuel/air mixture. This process helps to maintain proper combustion stability. However, since the hot reversal flow impinges on the swirler surface, it can create a high temperature spot at the center of the swirler and generate an uneven temperature distribution across the swirler which can lead to thermal stress. 
       SUMMARY 
       [0003]    In one embodiment, the invention provides a combustor for combusting a mixture of fuel and air. The combustor includes a swirler for receiving a flow of air and a flow of fuel, the fuel and air being mixed together under the influence of the swirler, the swirler imparting a swirling flow to the fuel/air mixture. The swirler also has a central channel therethrough. A prechamber is in fluid communication with the swirler for receiving the swirling fuel/air mixture, the prechamber being a cylindrical member oriented along a central axis, the prechamber imparting an axial flow to the swirling fuel/air mixture in a downstream direction along the central axis, thereby creating a vortex flow of the fuel/air mixture having a low pressure region along the central axis. A combustion chamber is in fluid communication with and downstream of the prechamber, the combustion chamber having a greater flow area than the flow area of the prechamber, thereby permitting the vortex to expand radially and create a recirculation zone in which combustion products from combustion of the fuel/air within the combustion chamber are drawn upstream along the central axis back into the prechamber. The combustor also includes a cooling assembly received in the channel, the cooling assembly defining an axis that is co-linear with the central axis of the prechamber. The cooling assembly is in fluid communication with a source of air that is cooler than the recirculation flow and directs the cooler air in a downstream direction into the prechamber thereby creating a cooling flow. 
         [0004]    In another embodiment, the invention provides a swirler for use with a combustor for combusting a mixture of fuel and air. The swirler includes a body having an outer side and an inner side and a plurality of flow guides on the inner side of the swirler body. The flow guides define flow paths between adjacent flow guides for guiding air in a swirling motion about a centerline of the swirler body. A first annular chamber is formed within the swirler body and is in fluid communication with guide tubes located adjacent to the entrances of the flow paths. A second annular chamber is formed within the swirler body and is in fluid communication with apertures located adjacent exits of the flow paths. A channel at the centerline of the body extends from the outer side to the inner side. A cooling assembly is received in the channel and is approximately flush with the body at the inner side. 
         [0005]    In another embodiment, the invention provides a method of combusting fuel and air in a gas turbine engine. Fuel and air is premixed to a relatively uniform mixture adjacent a swirler surface at a front portion of a combustor. The fuel/air mixture is injected into a prechamber cylinder in a swirling motion about a centerline of the prechamber, thereby creating a vortex flow having a swirling and axial motion and having a low pressure region at the centerline. The vortex flow is conveyed axially in a downstream direction into a combustion cylinder having greater flow area than a flow area of the prechamber. The vortex flow is expanded into the combustion cylinder, wherein chemical reaction of the fuel and air occurs to form hot products of combustion. As a result of said expansion, a recirculation flow is formed at the centerline wherein the hot products are drawn upstream into the prechamber. Air is conveyed through the swirler at the centerline in a downstream direction into the prechamber, said conveyed air being cooler than the recirculation flow. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic illustration of a recuperated, two-spool gas turbine engine including a combustor for use with an embodiment of the invention. 
           [0008]      FIG. 2  is a schematic illustration of a recuperated, single-spool gas turbine engine including a combustor for use with an embodiment of the invention. 
           [0009]      FIG. 3  is a schematic illustration of a simple-cycle, single-spool gas turbine engine including a combustor for use with an embodiment of the invention. 
           [0010]      FIG. 4  is a schematic illustration of a can- or silo-type combustor inside a recuperator for use with an embodiment of the present invention. 
           [0011]      FIG. 5  is a schematic illustration of a swirler, prechamber and combustion chamber according to an embodiment of the invention. 
           [0012]      FIG. 6A  is front perspective view of a radial swirler according to an embodiment of the invention. 
           [0013]      FIG. 6B  is an exploded view of the swirler of  FIG. 6A , a combustor flange and a combustor. 
           [0014]      FIG. 7  is rear perspective view of the radial swirler of  FIG. 6A . 
           [0015]      FIG. 8  is a cut-away view of the swirler of  FIG. 6A . 
           [0016]      FIG. 9  is a sectional view of the cooling assembly of  FIG. 8 . 
           [0017]      FIG. 10  is a front view of the distribution ring of  FIG. 9 . 
           [0018]      FIG. 11  is a sectional view of the distribution ring of  FIG. 10  taken along line X-X. 
           [0019]      FIG. 12  is a front view of the heat shield of  FIG. 9 . 
           [0020]      FIG. 13  is a sectional view of the heat shield of  FIG. 12  taken along line Y-Y. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0022]    The invention described herein can be used for burning various hydrocarbon fuels in a gas turbine. The combustion process comprises a method to burn lean premixed and lean pre-vaporized premixed fuel/air (F/A) mixtures. This enables lower gas turbine exhaust emissions (NOx, CO, VOC&#39;s) at a wide range of operating engine conditions. 
         [0023]    Referring now to the drawings, like numerals are used throughout to refer to like elements within a gas turbine and combustor. 
         [0024]      FIG. 1  schematically illustrates a recuperated gas turbine engine  10  having a two spool configuration used for generating electricity. The engine  10  includes a compressor  12 , a recuperator  13 , a combustion chamber  15 , a gasifier turbine  16 , a power turbine  17 , a gearbox  18 , and an electric generator  19 . The engine  10  communicates with an air source  20  upstream of compressor  12 . The air is compressed and routed into recuperator  13 . In recuperator  13 , the compressed air is preheated by exhaust gases from the power turbine  17  and routed into the combustion chamber  15 . Fuel  22  is then added to the combustion chamber  15  and the mixture is combusted (as described in greater detail below). 
         [0025]    The products of combustion from the combustion chamber  15  are routed into gasifier turbine  16 . The F/A ratio is regulated (i.e. the flow of fuel is regulated) to produce either a preset turbine inlet temperature or preset electrical power output from generator  19 . Turbine inlet temperature entering gasifier turbine  16  can range within practical limits between 1500 F and 2000 F. The hot gases are routed sequentially first through the gasifier turbine  16  and then through the power turbine  17 . Work is extracted from each turbine to respectively transfer power to the compressor  12  and the generator  19 , with shaft power transferred through gearbox  18 . The hot exhaust gases from the power turbine  17  are then conveyed through the recuperator  13 , where heat is transferred by means of thermal convection and conduction to the air entering the combustion chamber  15 . An optional heat capturing device  24  can be used to further capture the exhaust heat for productive commercial uses. Heat capturing device  24  can be used to supply hot water, steam, or other heated fluid to device  26  which uses said heat for a variety of purposes. 
         [0026]      FIG. 2  schematically illustrates a recuperated gas turbine engine  10   a  used for generating electricity. Gas turbine  10   a  is similar to  FIG. 1 , with the exception that only a single turbine is used. The engine  10   a  includes a compressor  12 , a recuperator  13 , a combustion chamber  15 , a turbine  16 , a gearbox  18 , and an electric generator  19 . The engine  10   a  communicates with an air source  20  upstream of compressor  12 . The air is compressed and routed into recuperator  13 . In recuperator  13 , the compressed air is preheated by exhaust gases from turbine  16  and routed into the combustion chamber  15 . Fuel  22  is then added to the combustion chamber  15  and the mixture is combusted (as described in greater detail below). 
         [0027]    The products of combustion from the combustion chamber  15  are routed into turbine  16 . The F/A ratio is regulated (i.e. the flow of fuel is regulated) to produce either a preset turbine inlet temperature to turbine  16  or preset electrical power output from generator  19 . Turbine inlet temperature can range within practical limits between 1500 F and 2000 F. Work is extracted from the turbine to transfer power to both compressor  12  and the generator  19 , with shaft power transferred through gearbox  18 . The hot exhaust gases from turbine  16  are then conveyed through the recuperator  13 , where heat is transferred by means of thermal convection and conduction to the air entering the combustion chamber  15 . An optional heat capturing device  24  can be used to further capture the exhaust heat for productive commercial uses. Heat capturing device  24  can be used to supply hot water, steam, or other heated fluid to device  26  which uses the heat for a variety of purposes. 
         [0028]      FIG. 3  schematically illustrates a simple-cycle gas turbine engine  10   b  used for generating electricity. Gas turbine  10   b  is similar to  FIG. 2 , with the exception that no recuperator exists. The engine  10   b  includes a compressor  12 , a combustion chamber  15 , a turbine  16 , a gearbox  18 , and an electric generator  19 . The engine  10   b  communicates with an air source  20  upstream of compressor  12 . The air is compressed and routed into combustion chamber  15 . Fuel  22  is then added to the combustion chamber  15  and the mixture is combusted (as described in greater detail below). 
         [0029]    The products of combustion from the combustion chamber  15  are routed into turbine  16 . The F/A ratio is regulated (i.e. the flow of fuel is regulated) to produce either a preset turbine inlet temperature or preset electrical power output from generator  19 . Turbine inlet temperature to turbine  16  can range within practical limits between 1500 F and 2000 F. Work is extracted from the turbine  16  to transfer power to both compressor  12  and the generator  19 , with shaft power transferred through gearbox  18 . The hot exhaust gases from turbine  16  are then conveyed to either the exhaust, or an optional heat capturing device  24  can be used to further capture the exhaust heat for productive commercial uses. The heat capturing device  24  can be used to supply hot water, steam, or other heated fluid to device  26  which uses said heat for a variety of purposes. 
         [0030]      FIGS. 1-3  illustrate gas turbine component arrangements that can be used with various embodiments of the invention. A variety of other engine configurations (multiple spools, multiple compressor and turbine stages) could also be used in conjunction with the invention. For example, instead of using gearbox  18  and generator  19 , one could use a high-speed generator to generate a high-frequency alternating current (AC) power signal, and then use a frequency inverter to convert this to a direct current signal (DC). This DC power could then be converted back to an AC power supplied at a variety of typical frequencies (i.e. 60 Hz or 50 Hz). The invention is not limited to the gas turbine configurations of  FIGS. 1-3 , but includes other component combinations that rely on the Brayton cycle to produce electric power and hot exhaust gases useful for hot water generation, steam generation, absorption chillers, or other heat-driven devices. 
         [0031]      FIG. 4  illustrates a recuperator  50 . Recuperator  50  can be similar to the recuperator disclosed in U.S. Pat. No. 5,983,992, issued Nov. 16, 1999, the entire contents of which are incorporated herein by reference. The recuperator  50  includes a plurality of stacked cells  54  that are open at each end to an inlet manifold  56  and an outlet manifold  58  and which route the flow of compressed air from the inlet manifold  56  to the outlet manifold  58 . Between the cells  54  are exhaust gas flow paths that guide the flow of hot exhaust gas between the cells  54 . There are fins in the cells  54  and in the exhaust gas flow paths to facilitate the transfer of heat from the hot exhaust gas to the cooler compressed air mixture. 
         [0032]    With continued reference to  FIG. 4 , the outlet manifold  58  contains a silo or tubular combustor  52  and a swirler  60 . Air entering outlet manifold  58  flows around the outside of the combustor  52 . The air then flows into the combustor  52  through a variety of orifices and slots in combustor  52  and swirler  60 , and exits the combustor  52  with a flow as indicated by arrow  62 . The overall flow  62  of the air in the combustor  52  can be considered to define an orientation of the combustor  52  with the flow  62  being oriented in a downstream direction, i.e., from left to right, such that the swirler  60  is upstream of the combustor  52 . 
         [0033]      FIG. 5  shows a cross-sectional view of the swirler  60  and a portion of the combustor  52 . The combustor  52  includes a prechamber  64  and a combustion chamber  66  that is downstream of the prechamber  64 . As illustrated, the prechamber  64  has a smaller diameter than the combustion chamber  66 . Compressed air from the outlet manifold  58  is conveyed sequentially downstream through the swirler  60  to the prechamber  64 , and then to combustion chamber  66 , inside combustor  52 . Air flows into the prechamber  64  through the swirler  60 . Air pressure in the outlet manifold  58  is higher than the air pressure inside the combustion chamber  66 , and this pressure difference provides the energy potential to convey air through the swirler  60 . 
         [0034]      FIGS. 6-8  show the swirler  60  according to an embodiment of the invention. The swirler  60  is disc-shaped and includes a body  135  and a cooling assembly  200 . The body  135  defines an inner annular chamber  137 , an outer annular chamber  139  and a plurality of flow guides  145 . The body  135  further includes a circumferential flange  150  that facilitates the attachment of the swirler  60  to the recuperator  50 . The flange  150  separates the swirler  60  into an outer portion or side  155  and an inner portion or side  160  that faces the prechamber  64 . The inner side  160  faces the combustion chamber  66 , while the outer portion  155  faces away. As illustrated herein, the swirler  60  is a separate component that attaches to the combustor  52 . In some embodiments, the swirler  60  forms a sealing engagement at the flange  150  with the recuperator  50 . However, other constructions employ a swirler head that is formed as part of the combustor  52 . In still other constructions, the swirler  60  is a separate component positioned away from the remainder of the combustor  52 . 
         [0035]    The outer chamber  139  is an annular chamber within the body  135  of the swirler  60 . A fuel inlet  165  can be coupled to the outer side  155  of the body  135  in fluid communication with the outer chamber  139  to deliver fuel into the outer chamber  139 . A plurality of bores between the outer chamber  139  and the inner side  160  of the swirler  60  permit fuel in the outer chamber  139  to flow through the swirler  60  into the prechamber  64 . Guide tubes  169  extending from the inner side  160  of the swirler  60  adjacent to the bores guide the flow of fuel into the prechamber  64 . 
         [0036]    The inner chamber  137  is disposed radially inwardly of the outer chamber  139 . A pilot fuel inlet  175  can be coupled to the outer side  155  of the body  135  in fluid communication with the inner chamber  137  to deliver pilot fuel into the inner chamber  137 . A plurality of bores  177  between the inner chamber  137  and the inner side  160  of the swirler  60  permit pilot fuel in the inner chamber  137  to flow through the swirler  60  into the prechamber  64 . The pilot fuel inlet  175  provides a flow of fuel through the swirler  60  that may be used to maintain the flame stability within the combustor  52  at low power settings or to initiate combustion within the combustor  52  during engine start. 
         [0037]    Also visible on the outer side  155  of the swirler  60  is a hole  190  in the swirler  60  for receiving an ignition device  195 . The ignition device  195  provides a flame, spark, hot surface or other ignition source to initiate combustion during engine start-up or at any other time when a flame is desired but not present. 
         [0038]    The flow guides  145  are generally raised triangular blocks on the inner side  160  of the body  135 . Each flow guide  145  has two planar surfaces  180  and an arcuate outer surface  183 . The planar surfaces  180  of each flow guide  145  are arranged such that they are substantially parallel to the planar surfaces  180  of the adjacent flow guides  145 . Using this arrangement, a plurality of flow paths  185  are defined between adjacent flow guides  145  extending inwardly. The flow paths  185  are oriented to inject the premixed fuel and air into the prechamber  64  with a high degree of swirl about a centerline or central axis A (see  FIG. 5 ) of the cylindrical prechamber  64 . Many different arrangements are possible to direct fuel and air into the prechamber  64 . As such, the invention should not be limited to the aforementioned example. 
         [0039]    The flow guides  145  are disposed radially between the inner chamber  137  and the outer chamber  139 . Thus, the guide tubes  169  communicating with the outer chamber  139  are located at an outer end or entrance  186  of the flow paths  185  and the bores  177  communicating with the inner chamber  137  are located at an inner end or exit  187  of the flow paths  185  (see  FIG. 6A ). Referring now to  FIG. 6B , an annular combustor flange  153  is mounted to flow guides  145  with fasteners (not shown) at aligned openings  154   a,    154   b.  The combustor flange  153  partially encloses the flow paths  185  to facilitate the flow of air and fuel from the entrances  186  to the exits  187 . The combustor flange  153  can also be secured to the combustor  52  to facilitate securing the swirler  60  to the combustor  52 . 
         [0040]    By injecting the fuel at the entrance  186  to the flow path  185 , the fuel and air have adequate time to thoroughly mix prior to exiting the flow path  185  at the exit  187 . This uniform mixture of F/A reduces the likelihood of fuel-rich burning in combustion chamber  66 , which could lead to high levels of NOx. In other embodiments, fuel could be injected at a plurality of other locations also, so as to ensure the F/A mixture leaving the flow paths  185  uniformly mixed. 
         [0041]    The hole  190  for the ignition device  195  is located between the centerline A of the prechamber  64  and an inside “diameter” defined by the flow path exits  187 . The ignition device  195  can ignite the premixed F/A exiting the flow paths  185  and can ignite the pilot fuel exiting the holes  177 , but is not subjected to and/or is less subjected to the high temperatures of an inner recirculation zone  86  (see discussion below with regard to  FIG. 5 ). 
         [0042]    As shown in  FIG. 5 , premixed F/A is injected into the prechamber  64  with a swirling flow path or directionality under the influence of the action of the swirler  60 , as indicated by arrow  80 . Other structures may be provided to impart a swirl to the F/A mixture and introduce it to the prechamber  64 . The swirling F/A mixture  80  is conveyed in a downstream direction through the prechamber  64  and exits the prechamber  64  into the combustion chamber  66 . This axial motion is combined with a swirling motion about the centerline axis A of the combustion chamber  66 , producing a vortex, indicated by arrow  82 . This vortex  82  creates a pressure difference between the center of the vortex  82 , located at the centerline A, and the inner perimeter of the prechamber  64 . The centerline of the vortex  82  is at a lower pressure than the outside edge of the vortex  82 , similar to the low pressure experienced at the center of a hurricane. 
         [0043]    The flow area in the combustion chamber  66  has a larger cross-sectional area than the flow area in the prechamber  64  (i.e., the combustion chamber  66  has a greater inner diameter than the prechamber  64 ). When the axially processing vortex  82  enters the combustion chamber  66 , the increase in flow area causes the vortex  82  to expand radially and slow its axial and rotational or swirling movement, as indicated by arrow  84 . The expanded vortex  84  has a reduced pressure difference between the outside edge of the vortex  84  and the center. Thus, the centerline A of the prechamber  64  at the vortex  82  is at a lower pressure than the centerline of the combustion chamber  66  at the vortex  84 . An inner recirculation flow, as indicated by arrow  86 , is established which pulls a portion of the gases from the combustion chamber  66  back into the prechamber  64  in an upstream direction, i.e., from right to left. This process is referred to herein as a “vortex breakdown” structure and stabilizes the flame in the combustion chamber  66 . 
         [0044]    The F/A mixture conveyed from the prechamber  64  to the combustion chamber  66  chemically reacts in a combustion flame. The products of combustion are hotter than the reactants introduced into the prechamber  64  (i.e., the premixed F/A at flow  80 ). The inner recirculation flow  86  therefore is composed of hot products of combustion. The inner recirculation flow  86  is directionally opposed to the unburned F/A mixture of vortex  82 , and an inner shear layer is established between the two. Hot gas products and combustion radicals in the recirculation flow  86 , which are unstable electrically-charged molecules like OH—, O—, and CH+are exchanged with the unburned F/A of vortex flow  82 . Recirculation flow  86  serves as a continued ignition source for vortex flow  82 . The chemical radicals also enhance the reactivity of the unburned mixture of vortex flow  82 , enabling the F/A mixture of vortex flow  82  to extinguish combustion at a lower F/A ratio than if vortex flow  82  did not have the radicals from recirculation flow  86 . 
         [0045]      FIGS. 8 and 9  illustrate the cooling assembly  200 . Air, including recuperated air, can be injected through the cooling assembly  200  into the prechamber  64 . The cooling assembly  200  is provided to reduce any temperature differential across the inner surface  160  of the swirler  60  that may be generated by the hot recirculation flow  86  at the centerline A. 
         [0046]    The cooling assembly  200  resides in a channel  202  extending through the swirler  60  at the centerline A. In general, the channel  202  and the cooling assembly define a central axis that is co-linear with the central axis A of the prechamber  64 . The channel  202  has sloped sides, so that a channel opening  203  on the inner side  160  is larger than a channel opening  204  on the outer side  155  (see  FIGS. 8-9 ). The outer channel opening  204  can be coupled to an air inlet  205  so that the channel  202  is in fluid communication with a source of cooling air. In the illustrated embodiment, the air inlet  205  receives air from the recuperator  50 . Specifically, the air inlet  205  is coupled to an opening  151  in the flange  150  that is in fluid communication with the recuperator  52  (see  FIG. 8 ). However, any source of air that is cooler than the recirculation flow  86  will suffice. 
         [0047]    As shown in  FIGS. 8-11 , the cooling assembly  200  includes a distributor ring  206  and a perforated shield  210 . The distributor ring  206  is located within the channel  202  downstream of the air inlet  205 . The ring  206  includes a plurality of apertures  207  for receiving air therethrough from the air inlet  205 . In some embodiments, the apertures  207  are angled outwardly to direct air flowing therethrough uniformly onto the shield  210 . 
         [0048]    Downstream of the distributor ring  206 , the shield  210  covers the inner opening  203  of the channel  202  (see  FIGS. 8-9 ). The shield  210  includes a plurality of apertures  214  for permitting air flow through the shield  210 . In the illustrated embodiment, the apertures  214  are in the form of nozzles. In some embodiments, the shield  210  is approximately flush with the inner side  160  of the swirler  60 . 
         [0049]    The shield  210  includes a sleeve  216  for threadedly coupling the shield  210  to the distributor ring  206 . A portion of the swirler body  135  adjacent to the channel  202  is clamped between the shield  210  and the distributor ring  206  to secure the cooling assembly  200  to the swirler  60 . This arrangement permits some expansion and contraction of the shield  210  relative to the swirler  60 . In other embodiments (not shown), the distributor ring  206  is snap-fit, bolted, adhesively bonded or otherwise coupled to the shield  210 . In other embodiments (not shown), the shield  210  and/or the distributor ring  206  are coupled to the swirler  60  through a threaded coupling or a snap-fit coupling at the channel  202 , can be bolted to the swirler  60 , and can be adhesively coupled to the swirler  60 . In still other embodiments, all or a portion of the cooling assembly  200  is integrally formed with the swirler  60 . 
         [0050]    Air from the cooling air inlet  205  flows through the apertures  207  in the distributor ring  206  into the channel  202 . Heat is conducted from the swirler  60  to the cooling assembly  200  while still within the channel  202 , then transferred by convection to the air flowing through the channel  202 . The air flowing through the channel  202  flows through the apertures  214  in the shield  210  and into the prechamber, generating a cooling flow, indicated at arrow  212 . The heat transferred from the swirler to the cooling assembly  200  is removed from the swirler  60  as the cooling flow  212  exits the channel  202  and flows into the prechamber  64 . This can facilitate reducing the temperature of the swirler  60  adjacent to the cooling assembly  200  and of the cooling assembly  200  itself. 
         [0051]    Referring to  FIG. 9 , the cooling flow  212  flows opposite to and meets with the recirculation flow  86  to generate a stagnation plane, indicated at  218 , between the swirler inner side  160  and the recirculation flow  86  (see also  FIG. 5 ). The cooling flow  212  as well as the stagnation plane  218  form an air layer separating the swirler inner side  160  from the hot recirculation flow  86 . This air layer provides a thermal barrier to heat transfer from the recirculation flow  86  to the swirler  60 . Any heat transfer from the recirculation flow  86  to the swirler  60  passes through the air layer via conduction rather than convection. 
         [0052]    The cooling assembly  200  can be formed of a different material than the swirler  60 . For example, the cooling assembly  210  can be formed of one or more materials having a different resistance to thermal transfer and/or coefficient of thermal expansion than the material of the swirler  60 . In other embodiments, all or a portion of the cooling assembly  200  is formed of the same material(s) as the swirler  60 . 
         [0053]    The cooling assembly  200  inhibits the forming of a “hot spot” on the swirler inner side  160  at the centerline A due to impingement of the hot recirculation zone  86 . This provides for a more radially uniform swirler temperature during use. Radial temperature uniformity can reduce nonuniform thermal stresses on the swirler  60  (such as, for example, increased thermal expansion at the centerline A in relation to thermal expansion closer to the flange  150 ), thereby increasing the life of the swirler  60 . In addition, the cooling assembly  200  can be formed of a material that has a greater resistance to thermal expansion than the remainder of the swirler  60 , regardless of the operation of the cooling flow  212 . Furthermore, the cooling assembly  200  can be formed separately from the swirler  60 , so that some or all of the thermal stresses on the cooling assembly  200  are not mechanically transferred to the remainder of the swirler  60 . For example, the cooling assembly  200  can be allowed to undergo thermal expansion and contraction separately from the remainder of the swirler  60 . 
         [0054]    In addition to a single can combustor, can-annular combustor arrangements are commonly used, where multiple single combustor cans are oriented upstream of an annular combustor liner. Transition hardware is used to convey the combustion gases from the individual cans to the annular portion of the combustor. The annular portion of the combustor then conveys hot gases to a turbine, typically with the use of turbine nozzles or turbine vanes. The invention disclosed herein is applicable to can-annular combustors, applying to the upstream portion where fuel and air are injected and flow stabilization occurs. 
         [0055]    Thus, the invention provides, among other things, a method and apparatus to inhibit circumferentially non-uniform thermal stresses on the swirler surface. Various features and advantages of the invention are set forth in the following claims.