Abstract:
A trapped vortex combustor for gas turbine engines. An annular combustor housing is provided having a plurality of inlet centerbodies disposed along a helical axis. The inlet centerbodies include a leading edge structure, opposing sidewalls, a pressurizable cavity, and a rear wall. Inlet centerbodies cooperate with adjacent structure and an aft bluff body to define a trapped vortex cavity combustion chamber for mixing an inlet fluid and burning fuel to form hot combustion gases. Mixing is enhanced by utilizing struts adjacent to the rear wall to create eddies in the fluid flow, and by injecting fuel and/or air in opposition to swirl created by the bulk fluid flow. Hot combustion gases are utilized in a turbine for extraction of kinetic energy, or in heat exchange equipment for recovery of thermal energy. High combustion efficiencies and less than 10 ppm emissions of oxides of nitrogen and of carbon monoxide are achieved.

Description:
RELATED PATENT APPLICATIONS  
       [0001]    This patent application claims the benefit of priority from prior U.S. patent application Ser. No. 10/200,780, filed on Jul. 23, 2002. That application also claimed the benefit of priority from U.S. Provisional Patent Application Serial No. 60/386,195, filed Jul. 17, 2002, which was converted on Jul. 17, 2002 from originally filed U.S. Non-Provisional patent application Ser. No. 09/912,265 filed on Jul. 23, 2001, the disclosures of each of which is incorporated herein in their entirety by this reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to the field of combustion technology. More particularly, the invention relates to the design of a combustor especially suited for use in a gas turbine engine.  
         BACKGROUND  
         [0003]    Gas turbine engines have been widely utilized as prime movers in stationary power generation plants. However, improvements in combustion efficiency, which would be especially advantageous in order to reduce emissions, would be desirable. In various attempts to achieve such improvements, many different methods and structures have been tried, either experimentally or commercially. Some of such attempts have included the use of recirculation zones to provide a continuous ignition source by mixing hot combustion products with the incoming fuel and air mixture. Structural devices such as swirl vanes, bluff bodies, and rearward facing steps have commonly been employed to establish recirculation zones for flame stability. The challenge, however, has been in fuel introduction methodology, and the structure of a flame stabilizer that ensures performance (including acceptable emissions and acoustic stability) while reducing capital and operating costs.  
           [0004]    Flame stabilization criteria are even more important when operating at trans-sonic or supersonic inlet conditions. It would be especially desirable for burners operating under such conditions to have flame stabilizers that would be highly resistant to external flow field dynamics and/or perturbations.  
           [0005]    Consequently, it would be desirable to provide a combustor for a gas turbine engine, specifically including combustion chamber structure that enables the engine to maintain high combustion efficiency while reducing the emission of undesirable products of combustion such as (a) nitrogen oxides, (b) partially oxidized hydrocarbons, and (c) carbon monoxide.  
           [0006]    Moreover, it is beneficial in such devices that efficient and effective combustor cooling be achieved. Such a design approach increases combustor operating life (and thus achieves overall reduction in life cycle costs, while maximizing combustion efficiency.  
           [0007]    Depending upon the specific operating needs of a particular implementation, certain subsets of (or even all) of the foregoing can be implemented using various combinations of exemplary embodiments, or variations of certain aspects of such exemplary embodiments.  
         SUMMARY  
         [0008]    One embodiment of a novel combustor design for a gas turbine engine disclosed herein has a combustor configuration in which a flameholder is provided that extends between inner and outer combustion chamber walls. In one embodiment the inner and outer walls are each substantially cylindrical, and thus form an annular combustion chamber in which a plurality of radially extending flame holders are provided.  
           [0009]    One design utilizes an inlet centerbody in which compression is achieved at preselected inlet velocities by exploiting an oblique shock extending from a leading edge structure laterally outwardly to, at the design velocity, adjacent centerbody structures. In such case, the centerbodies and accompanying aft bluff bodies are affixed in a preselected, substantially matched orientation, so as to smoothly and continuously acquire clean compressed inlet air, and to efficiently and reliably discharge the resulting products of combustion. In one configuration, a plurality of centerbodies are oriented at a helical angle within an annular combustor housing.  
           [0010]    The annular combustor housing is simplified in that a rear wall of an inlet centerbody serves as an upstream wall of a combustion chamber, providing flame holding. By virtue of the rear wall of the inlet centerbody extending outward from the inner chamber wall to the outer chamber wall, and the use of a first bluff body likewise extending outward between the inner chamber wall and the outer combustor wall, then located a distance downstream a trapped vortex combustor cavity is defined to provide for thorough mixing of fuel and air, sufficient residence time for reaction of fuel with oxidant to minimize the escape of incomplete combustion products, and general flame stability.  
           [0011]    The foregoing combustion chamber configuration is designed for efficient mixing of fuel and air, especially at supersonic inlet inflow velocities. By the utilization of a rear wall of an inlet body, a separation streamline is formed dividing the incoming cold charge with a highly energetic locked vortex pair located between the fore and aft bodies. This arrangement provides for a more compact primary zone and for stable flameholding that is desirable over an extended operating range. Use of fuel and/or oxidizer injection opposite to the natural rotation sense of the trapped vortices has found to still further enhance the mixing of fuel and air in such combustion chambers. The above mentioned embodiment provides increased combustion intensity through improved fuel/air mixing, over conventional schemes. A still further feature is provided by an embodiment incorporating side struts extending at or adjacent the rear wall of the centerbody, so that some primary zone hot products are more easily convected into the unreacted inlet flow for increased performance. In another variation, multiple trapped vortex cavities are provided, by use of multiple aft bluff bodies, which may, but need not use secondary injection to enhance performance.  
           [0012]    Further, it is to be understood that although a combustor cavity having roughly a segmented annular shape and having a substantially rectangular cross-section at any selected station along the flow path is depicted, other designs utilizing an inlet body rear wall flameholder shape other than that just described are also possible (e.g., rectangular cross-sectional shape, or non-rectangular cross-sectional shape). However, by optimizing combustor volume, i.e., operating at desirable pressures and temperatures to optimize heat release while maintaining desirable temperature profile and emission results, the “hot section” components of an engine utilizing such trapped vortex combustors may be advantageously reduced.  
           [0013]    In another embodiment, a fuel/air pre-mixture may be supplied at high velocity via inlet fluid compression ducts adjacent to an inlet centerbody, so that flashback from the combustor may be reliably avoided even in the case of fuels that have a very high flame velocity. Such a high velocity inlet can also serve to acoustically decouple the upstream flow acoustics from combustion chamber acoustics. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    In order to enable the reader to attain a more complete appreciation of the invention, and of the novel features and the advantages thereof, attention is directed to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 shows a partially sectioned perspective view of a combustor section provided for mounting in a gas turbine engine, ready to receive compressed air through an inlet between inner and outer combustor walls, and also showing inner and outer annular passageways for receiving fuel/air premix, a plurality of combustors offset at an angle, and the use of an inlet centerbody and two aft bluff bodies, each having pilot fuel addition to assist in mixing and flame stabilization as taught herein.  
         [0016]    [0016]FIG. 2 shows a cross-sectional view of a novel combustor as taught herein, shown mounted for use in a gas turbine engine having a compressor with three axial and one centrifugal stage, with compressed air moving forward through the combustor and fuel inlets prior to the centerbody to prepare a fuel-air premix, and with exhaust gases driving a gas turbine affixed to a central shaft before exiting toward the front of the gas turbine.  
         [0017]    [0017]FIG. 3 is a simple schematic of a prior art trapped vortex combustor configuration, showing how the use of fuel and air injection to the combustion zone is designed to direct momentum of such added streams in the direction of the vortex resulting from bulk flow past the initial centerbody.  
         [0018]    [0018]FIG. 4 shows a simple schematic of one embodiment of the present invention, wherein fuel and/or air injection is designed to project momentum in opposition to the direction of swirl resulting from bulk flow past the initial centerbody with the consequence of opposing vortex rotation.  
         [0019]    [0019]FIG. 5 is a perspective view of one embodiment of the present invention, similar to the flow scheme just depicted in FIG. 4, but now additionally including the use of side struts projecting laterally from the initial centerbody, the side struts create small recirculation zones for additional mixing of hot primary zone products into the incoming channel flow.  
         [0020]    [0020]FIG. 6 illustrates a further embodiment of the present invention, now showing the use of an axially adjustable aft bluff body, wherein the bluff body can be adjusted in position with respect to the rear end of the main bluff body, to create a primary zone of desired size; also illustrated are the use of pilot fuel and cooling air outlets in the aft bluff body.  
         [0021]    In FIG. 7, a combustor design is provided utilizing a simple bluff body, enhanced with respect to required cooling load, shown using a combination of backside impingement and effusion cooling on the flameholder face.  
         [0022]    In FIG. 8, the combustor design is further enhanced via use of pilot fuel injectors to stabilize the primary combustion zone.  
         [0023]    In FIG. 9, the combustor efficiency is further enhanced via use of a trapped vortex design, which locks stationary vortices between the fore (or “centerbody”) and aft bodies, and enhances channel air and/or premix entrainment through the pumping of secondary airflow into the primary zone, while also adding fuel and/or air in opposition to the swirl direction of trapped vortex.  
         [0024]    [0024]FIG. 10 illustrates a combustor design that includes a plurality of fuel mixing ports upstream of a centerbody, and fuel addition ports through the centerbody, to provide a lean premixed fuel to a trapped vortex combustor design.  
         [0025]    [0025]FIG. 11 schematically illustrates the trapped vortex combustor design just disclosed in FIG. 10, now showing the combustor centerline offset from a gas turbine centerline, in a manner such as that first illustrated in FIG. 1 above, to provide a helical flow structure within an annular space to extend the burnout zone, to reduce CO production.  
         [0026]    [0026]FIG. 12 illustrates, in partial cross-section taken looking down on a combustor, one trapped vortex combustor configuration, showing the use of fuel injection from a forebody to enhance flame stability, and with the use of fuel and/or air injection from an aft body to impart momentum in opposition to the swirl of the trapped vortex, in order to enhance mixing, combustion intensity, and combustion efficiency.  
         [0027]    [0027]FIG. 13 illustrates, the combustor just shown in FIG. 12, but now adding a second aft bluff body, wherein the second aft bluff body utilizes injection of fuel and/or air to impart momentum in opposition to the swirl of the trapped vortex, in order to enhance mixing, combustion intensity, and combustion efficiency.  
         [0028]    [0028]FIG. 14 graphically illustrates the use of fuel and air injection in a manner wherein fuel and air are injected to impart momentum in support of the swirl of the trapped vortex.  
         [0029]    [0029]FIG. 15 shows one embodiment of a unique trapped vortex combustor, wherein a simple bluff forebody (or “centerbody”) is provided having an interchangeable body module location, and wherein an aft body is provided having fuel and air injection ports.  
         [0030]    [0030]FIG. 16 is an embodiment similar to FIG. 15, now showing the addition of side struts which project laterally from the initial bluff forebody to create small recirculation zones for additional mixing.  
         [0031]    [0031]FIG. 17 is a partially cut away perspective view of an exemplary interchangeable aft bluff body, showing the location of cooling air ports and fuel and air injection ports.  
         [0032]    [0032]FIG. 18 is a partial cross-sectional view, taken through line  18 - 18  of FIG. 17, now showing the fuel and air passageways, and cooling air outlets angled to enhance flow of cooling air circumferentially around the aft bluff body.  
         [0033]    [0033]FIG. 19 is a partial side view of the aft bluff body, now showing the orientation of cooling air outlets, as well as several pilot fuel outlets.  
         [0034]    [0034]FIG. 20 depicts yet another embodiment, similar to that shown in FIG. 16, including the use of side struts adjacent the forebody, but now additionally showing the use of a second aft body to provide a secondary trapped vortex combustor of desired length.  
         [0035]    [0035]FIG. 21 depicts the use of the forebody illustrated in FIG. 16 in a square combustor cross-sectional configuration.  
         [0036]    [0036]FIG. 22 illustrates one embodiment for a second aft body, wherein only cooling air holes are provided, i.e., no fuel and/or air (other than cooling air) is injected in this embodiment of the second aft body.  
         [0037]    [0037]FIG. 23 graphically illustrates the relationship of NOx performance versus the primary zone (front end) equivalence ratio, where φ fe =total fuel/front end air, for a tested baseline TVC configuration.  
         [0038]    [0038]FIG. 24 graphically illustrates the relationship of CO performance versus primary zone (front end) equivalence ratio (φ fe ) for  a tested baseline TVC configuration.  
         [0039]    [0039]FIG. 25 graphically illustrates the relationship of NOx vs. CO in summary fashion for baseline TVC tests which were conducted.  
         [0040]    [0040]FIG. 26 graphically illustrates the relationship of combustion efficiency versus severity parameter for baseline TVC tests which have been conducted.  
         [0041]    [0041]FIG. 27 graphically illustrates the relationship of NOx vs. CO for baseline TVC tests which were conducted, for the cases where CO was below 50 ppm.  
         [0042]    [0042]FIG. 28 graphically illustrates the relationship of combustion efficiency versus severity parameter for two different TVC configurations, namely a trapped vortex combustor without struts, and a trapped vortex combustor with struts.  
         [0043]    [0043]FIG. 29 graphically illustrates the relationship of NOx vs. CO in summary fashion for tests of a trapped vortex combustor with struts, as taught herein. 
     
    
       [0044]    The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual implementations depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements and parameters are also shown and briefly described to enable the reader to understand how various optional features may be utilized in order to provide an efficient, reliable combustor for a gas turbine engine.  
       DETAILED DESCRIPTION  
       [0045]    A detailed view of an exemplary embodiment of a trapped vortex combustor  30  for a gas turbine engine is provided in FIG. 1. Inlet fluid, normally compressed air A as indicated by reference letter A is supplied through inlet  34  defined between inner inlet wall  36  and outer inlet wall  38 . Downstream from inlet  34 , inlet fluid A is divided into three streams, namely an outer cooling air supply B, a combustion air supply C, and an inner cooling air supply D. Leading edges  40  and  42  split entering air supply A thusly. The outer air cooling air supply B is contained within an outer plenum  44  defined between a combustor outer wall  46  and an outer plenum wall  48 . The inner cooling air supply D is contained within an inner plenum  50  defined between a combustor inner wall  52  and an inner plenum wall  54 . As illustrated, each of walls  48 ,  46 ,  52 , and  54  are provided in substantial portion by tubular cylindrical sections of desirable diameter to fit other components in a gas turbine engine of desired size and power output. The location of combustor inner wall  52  and combustor outer wall  46  provide an annular combustor housing  60  within which a plurality of inlet centerbodies  62  (also called “forebodies”) are placed. These centerbodies  62  extend from wall  52  to wall  46  and as shown, are offset at a helical angle alpha (α) with respect to the longitudinal axial centerline  64  of the combustor housing  60 . One suitable angle alpha (α) is about 30 degrees (30°).  
         [0046]    In situations where environmental concerns are significant, in order to obtain the proper conditions for combustion while producing low-pollution products of combustion, a fuel supply system provides a pre-mixing stage in which the fuel and combustion air are premixed prior to flow past the centerbodies  62 . In this stage, as better illustrated in FIG. 10, fuel injectors  70  add the fuel F to an inlet fluid, normally air A or other fuel free oxidant containing stream (but which may contain some high value fuel such as hydrogen, or some low value fuel, such as coal bed methane, coal mine purge gas, landfill methane, biomass produced fuel gas, sub-quality natural gas, or other low grade fuels). In order to carry out the actual combustion step in an operationally reliable manner, the velocity of the compressed inlet fuel air pre-mix should preferably be high at the intermixing point between the combustion chamber  72  and the delivery point of the combustible fuel/air mixture, so that flashback of the flame front from the combustion chamber  72  toward the fuel injectors  70  is reduced or avoided. In the exemplary engine design described herein, when operated at the suitable conditions described herein, the residence time in the diffuser portion  74  is too short to initiate an auto-ignition process in the prescribed residence time. Further, the aerodynamics of the diffuser  74  design and of the inlet sections  76  and  78  are not conducive to flame holding.  
         [0047]    In order to stabilize the combustion process downstream of the rear wall  104  of inlet centerbody  80 , the velocity of gases through the combustion chamber  72  is reduced by providing a combustion chamber  72  having substantially larger cross-sectional flow area than provided by the inlet ducts  76  and  78  thereto. As seen in FIG. 10, localized recirculation zones are provided to confine trapped vortices V in order to have an adequate residence time to substantially minimize creation of carbon monoxide in the combustor  72 , and a suitable length of burnout zone LBz is provided in order to bring the remaining CO in the exiting combustion gases to an environmentally acceptable low residual level.  
         [0048]    At the outset, we determined that the length of a primary mixing zone “LPz” in a combustor can be shortened considerably by adopting the configuration illustrated in FIGS. 7 and 8, even in the absence of trapped vortex cavities. These figures are views taken looking inward along a combustor, such as if radially mounted in the manner first set forth in FIG. 1 above, looking down on the exemplary combustor  100  taught herein. Combustor  100  is situated between a first wall  122  and a second wall  124 , behind rear wall  126  of centerbody  128 . In one embodiment, rear wall  126  of the centerbody  128  includes provision by way of a perforated wall  130  operably communicating with cooling gas source (for example, by pressurization of the interior space  133  of centerbody  128 ), for impingement cooling of rear wall  126  by the use of perforations  138  in interior rear wall  140 , as indicated by reference arrows  142  in FIG. 7. Alternately, or in addition, effusion cooling is provided for rear wall  126 , as indicated by cooling air flow  144  through perforations  146  in rear wall  126 . The enhanced with the combination of impingement and effusion cooling on the rear wall/flameholder  126  of centerbody  128 , utilizing an interior rear wall  140  with coolant passageways  138  therethrough, is useful in many configurations as indicated in other figures herein. Although this simple, robust bluff body combustor design allows easy sizing for the required inlet velocity, the combustion efficiency can be expected to be less than optimum since the length of primary zone is not as compact as it could be, and perhaps would encounter unsteady vortex shedding and reduced primary zone combustion intensity, and thus, would not be an optimum design solution for many commercially important combustor designs. Thus, combustion efficiency might be less than optimal. Therefore reduced combustion intensity would be expected, especially as compared to the novel trapped vortex combustors as explained below.  
         [0049]    Turning now to FIG. 9, an exemplary combustor  200  has been developed. Combustor  200  is provided using an aft body  202  for provision of trapped vortices  204  and  206 . This combustor  200  configuration has lower pressure drop through the combustor  200  relative to the simple bluff body configuration illustrated in FIGS. 7 and 8. Additionally, flame stability is improved through the locking of downstream vortices  204  and  206  between the rear wall  126  of centerbody (or “forebody”)  128  and the aft body  202 . And, by providing for sufficient space rearward (flow-wise) of the aft body  202 , the burnout zone of length LBz is provided. Thus, a compact primary zone Pz having a relatively high combustor efficiency is provided. Note that this and other designs provided herein can advantageously utilize the combination of impingement and effusion cooling methods first discussed in connection with FIGS. 7 and 8 above.  
         [0050]    In FIG. 9, the efficient combustion and high heat release per unit of combustor volume is made possible with a highly turbulent primary zone. Importantly, the jet impingement, and pumping action, as is further illustrated in FIG. 10, may increase combustor efficiency to at least 99% or more, and more preferably, to at least 99.5% or more.  
         [0051]    By comparison of FIG. 4 to FIG. 9, one of the improvements in the art can be easily noted with respect to the design illustrated in FIG. 4. In FIG. 4, a trapped vortex combustor  300  for a gas turbine engine is provided. An air supply system provides inlet air A to the combustor  300 . A fuel supply system is provided for the supply of gaseous fuel F. The fuel supply system has a premix stage wherein fuel F from the fuel supply system is ejected from fuel supply structures  302  and mixed with inlet air A to provide a lean pre-mixture  304  upstream of a first  306  of one or more trapped vortex cavities. Here, a first trapped vortex cavity  303  is provided rearward of rear wall  306  of forebody  310 . A first aft bluff body  310  with a front wall  312  is provided to define the rear or the first  303  trapped vortex cavity. Due to the bulk flow of the premix past the forebody  308 , a predetermined inward bulk fluid swirl direction in the manner indicated by reference arrows  320  and  322  is provided. At least one pilot fuel stage is provided, using one or more pilot injectors  330  in fluid communication with the fuel supply system. Each of the least one of the one or more pilot injectors  330  are configured to inject fuel into a first  306  of said one or more trapped vortex cavities. As indicated in FIG. 4, the pilot injectors are configured to inject fuel as indicated by reference arrows  332  and  334  into the first  306  of the one or more trapped vortex cavities in a direction oriented to provide a jet from the pilot injectors  330  to provide momentum of fuel and burning gases in opposition to the bulk fluid swirls  320  and  322  of predetermined direction. Here, two pilot injectors  330  are provided, although this number may be adjusted as necessary to provide the necessary mixedness and to achieve the combustion efficiencies desired, as well as to provide the proper quantity of supplemental fuel. For example, it has been found that providing about 95% of the necessary gaseous fuel in the bulk premix is acceptable, and in such cases about 5% of the necessary gaseous fuel is then provided by the pilot fuel injector stage of the fuel supply system.  
         [0052]    [0052]FIG. 5 provides a partially broken away perspective view of the exemplary trapped vortex combustor just illustrated in FIG. 4, now more clearly showing the use of first  340  and second  342  struts which are provided in partial airfoil shaped configuration with a rear wall  344  that is substantially co-planar with the rear wall  306  if forebody  308 . The struts  340  and  342  introduce yet additional vortex locations for trapping vortexes  350  and  352 . Note that the flow field will be three dimensional, not just in the transverse direction as indicated here for purposes of illustration and explanation. Alternately, a cylindrical, dowel shaped strut  360  can be provided. The cylindrical strut  360  is shown spaced upstream a length S L  from rear wall  306  of forebody  308 , and, if desired, struts  340  and  342  can likewise be located upstream from rear wall  306 . As shown, struts, whether  340 ,  342 , or  360 , extend outward from first  362  or second  364  sidewalls of forebody  308  toward, and as configured in FIG. 5, to exterior sidewalls  370  and  372  of combustor  300 . It should be understood that the combustor shown in FIG. 4 can advantageously utilize the cooling techniques for rear wall  306  as taught above with respect to FIGS. 7, 8 and  9 .  
         [0053]    Attention is again directed to FIG. 5, where In combustor  300 , one or more duct passageways  380  are provided adjacent to the forebody  308 . Such passageways are in this configuration defined between walls  364  and  372  on one side, and walls  362  and  370  on the other side, and between floor  380  and roof  382  of the combustor on both sides. The forebody rear wall  306  defines a dump plane having a cross-sectional area defined between the floor  380  and roof  382 , and sidewalls  370  and  372 . The cross-sectional area of the dump plane is the sum of (i) the cross-sectional area of the forebody rear wall  306  and (i) the cross-sectional area of all of the one or more duct passageways adjacent to the forebody rear wall  306 . A blockage ratio determined by dividing the cross-sectional area of the forebody rear wall  306  by the total cross-sectional area of the dump plane is in excess of 60 percent. In one embodiment, the blockage ratio is approximately 63 percent.  
         [0054]    Attention is now directed to FIG. 6, where a combustor  400  is provided with a unique trapped vortex cavity  403  with volume adjustment mechanism provided. This embodiment is shown as if taken through line  6 - 6  of FIG. 6, but with the addition of an adjustable aft bluff body  410 . Aft bluff body  410  is provided with a substantially I-beam shape in which an upper flange  412  having a lower sealing surface  414  is provided for sealing against upper roof sealing surface  416 . A lower flange  422  is provided having an upper sealing surface  424  for sealing against floor lower sealing surface  426 . Roof  430  and floor  432  have gaps therein defined by edgewalls  434  and  435  with respect to the roof, and  436  and  437  with respect to the floor, and with respect to which the exact position of aft bluff body  410  is adjustable forward in the direction of reference arrow  440 , or rearward in the direction of reference arrow  442 . Adjustment is provided by action of servo motor  448  and appropriate gearing such as worm gear  450  acting on grooved teeth  452  in the upper flange  412 . This configuration allows tuning of the vortex cavity  303  size for combustion efficiency.  
         [0055]    [0055]FIG. 11 illustrates the use of multiple inlet centerbodies  500  and  502  between sidewalls  506  and  508 , offset at a spiral or helical angle alpha (α) with respect to the centerline  504  of a gas turbine or other engine. The helical angle in a gas turbine engine can be up to about 30 degrees, or more. As illustrated, aft bodies  510  and  512  are provided rearward of flameholding rear walls  514  and  516  of centerbodies  500  and  502 , in order to provide for trapping of a vortex between the rear wall  514  and aft body  510 , and between rear wall  516  and aft body  512 .  
         [0056]    In FIG. 12, an exemplary enhanced double wall flameholding rear wall  600  with both impingement cooling passageways  602  in a first wall  603  and effusion cooling passageways  604  in a second wall  605  is provided for inlet centerbody  610 . Also, pilot fuel ports  612  and  614  are provided. Additionally, an aft body  620  with pilot fuel ports  622  and  624  provide for trapped vortex operation of combustor  630 .  
         [0057]    In FIG. 13, an exemplary double bluff body configuration is shown. Here both a first aft bluff body  620  and a second aft bluff body  700  are illustrated. This exemplary embodiment may utilize the double wall flameholding rear wall  600  configuration as just illustrated in FIG. 12 above. However, a second bluff body  700  is provided for creating both a second recirculation zone  710 , in addition to the first recirculation zone  720  as illustrated in FIG. 12 above. This configuration allows further enhancement of combustion efficiency.  
         [0058]    For background and reference, FIG. 14 graphically illustrates one prior art trapped vortex combustor design which uses the basic concept of providing fuel and air injection in a manner wherein fuel and air are directed so that the injection fluid jet imparts momentum to the swirling gases in a direction which is supporting of, rather than in opposition to, the direction of swirl of the trapped vortex. However, in the present invention, it has been discovered that improved combustion intensity and efficiency can be achieved by proving fuel and/or air injection directed so that the injection fluid imparts momentum to the swirling gases in a direction which is in opposition to, rather than supporting of, the direction of swirl of the trapped vortex.  
         [0059]    [0059]FIG. 15 shows one embodiment of a unique trapped vortex combustor  800 , wherein a simple bluff forebody (or “centerbody”)  802  is provided having an interchangeable body module location  804  for the supply of an interchangeable rear wall portion  805 . The centerbody  802  has sidewalls  804  and  806  which define, together with combustor sidewalls  810  and  814 , duct passageways  816 . A first aft body  820  is provided having fuel passageways  830  and fuel injectors  832 . Either a portion of burner fuel, or pilot fuel may be provided at injectors  832 . Also provided in first aft body  820  are cooling air passageways  840  connected to a cooling air supply system, and cooling air injection ports  842  which deliver cooling air for film cooling of the aft bluff body  820 . A plurality of perforations  842  are in fluid communication with the cooling air supply system, so that the plurality of perforations provide effusion cooling to the walls, especially including the forward wall  821 , of the first aft bluff body  820 . In the embodiment further indicated in FIG. 19, the perforations  842  are provided having a passageway  843  upstream of the outlet configured at a selected pitch angle (i.e. up and down with respect to the longitudinal flow axis indicated as centerline  850  in FIG. 15) and yaw angle (i.e., side to side with respect to the longitudinal flow axis indicated as centerline  850  in FIG. 15) with respect so said forward wall of said first aft bluff body. In one embodiment a pitch angle of the perforations is about 30 degrees upward in the upstream direction. In such an embodiment, it is also useful to provide the perforations in a yaw angle of about 30 degrees to one side or the other, so that a cooling film circulates around the bluff body  820 , either clockwise or counterclockwise. Normally, such perforations are provided with uniform pitch and yaw angles, so as to create a uniform cooling air film that sweeps across said forward wall of said first aft bluff body. However, as further indicated in FIG. 18, the uniform pitch and yaw angles can be reversed on first  860  and second  862  sides (i.e., up on one side and down on the other) to create a swirling action for the film cooling. In such cases an upward angle omega (Ω) may be provided equal to a downward pitch angle beta (β), but preferably, the swirl direction is maintained uniformly.  
         [0060]    Turning now to FIG. 16, this embodiment similar to FIG. 15, but now shows an improved combustor  801  which utilizes the addition of side struts  870  and  872  which project laterally from the initial bluff forebody to create small recirculation zones for additional mixing of hot burning gases, to thereby improve combustion efficiency. As illustrated, such struts  870  and  872  are similar to struts  340  and  342  depicted in FIG. 5.  
         [0061]    [0061]FIG. 17 is a partially cut away perspective view of an exemplary interchangeable aft bluff body  820 , showing the location of cooling air ejection ports  842  and fuel injection ports  832 . FIG. 18 is a partial cross-sectional view, taken through line  18 - 18  of FIG. 17, now showing the fuel  830  and air  840  passageways, as well as further details of the cooling air outlets angled to enhance flow of cooling air circumferentially around the aft bluff body, as just discussed above. FIG. 19 is a partial side view of the aft bluff body  820 , now showing the orientation of cooling air outlets  842 , as well as several pilot fuel outlets  832 .  
         [0062]    [0062]FIG. 20 depicts yet another embodiment for a combustor  803 , similar to that shown in FIG. 16, including the use of side struts  870  and  872  adjacent the forebody  802 , but now additionally showing the use of a second aft body  880  having a forward wall  882  to provide a secondary trapped vortex cavity  882  between wall  882  and rear wall  884  of first aft body  820 , for providing a trapped vortex combustor of desired length.  
         [0063]    As previously mentioned the TVC combustion concept relies on inhibiting vortex shedding from the bluff body which would otherwise destabilize the primary zone and hence prematurely limit the system&#39;s operating envelop. Wide operating envelopes are highly desired for most combustion systems, especially for land based ones due to part power requirements. Furthermore, the intense combustion activity existing between the fore and aft bodies (see FIG.  16 ) should serve as a mechanism to facilitate the interaction between the cold charge and hot combustion products, thereby encouraging competitive emissions levels. For evaluation of performance, the values of the air-to-fuel (A/F) ratio are normalized by the stoichiometric air-to-fuel (A/F) ratio, to obtain the equivalence ratio, φ=(A/F) stoichiometric/(ANF) measured. With this definition, φ&gt;1 indicates fuel rich operation, and φ&lt;1 indicates fuel lean operation. In general, while maximum efficiency occurs when the mixture is lean, unfortunately NOx production is enhanced over certain lean mixture ranges, although as combustion temperatures further decrease with increasingly lean mixtures, the NOx production also decreases. Thus, an improved TVC burner design as taught herein which results in efficient combustion of lean mixtures while decreasing undesirable emissions is an important improvement in the art.  
         [0064]    The discussion which follows details testing which was conducted on two TVC configurations: one as shown in FIG. 15, and another as shown in FIG. 16 with side struts located at the dump plane to further enhance mixing.  
         [0065]    First, with respect to the TVC depicted in FIG. 15, two tests were completed with the basic TVC and bluff body installation. Fueling was effectuated using the premixed fuel systems and TVC fuel legs. No bluff body fueling (partial premixing) or diffusion piloting were utilized. All testing centered on 5% (3.5% total air) front end air loading to the TVC and 95% front end air to the channel. Three distinct primary zone (front end) equivalence ratio (φ fe ) points were tested (0.51, 0.55, 0.60), with variations in the TVC to main fueling fractions for the prescribed primary zone (front end) equivalence ratio (φ fe ). The liner cooling air load was maintained at 25% (overall air flow percentage) throughout all TVC (FIG. 16 struts and FIG. 15 no strut configuration) tests. Attempts were made at increasing the air loading to the TVC to 10% front end air loading but were unsuccessful due to blow out and/or hardware overtemping (liner and/or IE module thermocouples). It should be noted that throughout all excursions, liner temperatures were not uniform along the length of the combustor, ranging from 800 to 1700° F., depending on the location of the heat release and/or fuel loading (equivalence ratio). Full combustion pressure (275 psia) and preheat temperatures (761° F.) were attained.  
         [0066]    [0066]FIGS. 23 and 24 denotes the NOx and CO emissions, respectively, versus primary zone (front end) equivalence ratio (φ fe ) for all data acquired under this configuration, irrespective of the TVC/channel fuel splits. Note that emissions were corrected to a 15% O2 standard. The NOx emissions demonstrate the typical behavior with increasing primary zone (front end) equivalence ratio (φ fe ) or front end flame temperature: increasing primary zone flame temperature increases NOx emissions due to the primary NOx generation mechanism. The CO emissions, conversely, demonstrate the inverse trend which is again, typical for kinetic CO production. Apparently, the equilibrium CO production mechanism whereby CO levels begin to increase with temperature was not attained with excursions during testing. Note the use of a logarithmic scale on the y-axis is due to the rapid jump in CO levels with leaner mixtures.  
         [0067]    [0067]FIG. 25 compiles the three excursions in main equivalence ration values (051, 0.55 and 0.60) with a plot of NOx versus CO emissions. At each channel based equivalence ratio, where φ main =channel fuel/channel air, the excursions in TVC based equivalence ratio, φ tvc =TVC fuel/TVC air, were undertaken in attempts at holding total fuel levels constant per the design table. This lead to wide excursions in TVC equivalence ratios: 0.51-1.5. Apparently the lower equivalence ratio setting has a dramatic effect on the CO production in that high levels (hundreds of ppm) were produced over the excursion. On richening the overall, front end equivalence ration, sizeable drops in CO were achieved as the mixture effectively “burnt out” or converted the CO to CO2.  
         [0068]    The optimal fueling split achieved was with a main equivalence ratio, φ main =0.6 whereby a simultaneous NOx/CO values of 28/28 ppm corrected to 15% O2 were recorded. See FIG. 27 below which is an enlargement of the previous figure. Note that the main equivalence ratio, φ main =0.51 curve, has been excluded from the data set for clarity.  
         [0069]    The final figure in the set (FIG. 26) denotes the emissions based combustion efficiency versus the Severity Parameter (SP) which was defined by Roquemore et al., in 2001, as follows:  
       SP   =         (       P   comb     /   14.7     )     0.26808     *     exp        (       T   inlet     /   257.693     )                  *              0.0581              *     φ   oa     *                  (       φ   tvc     /     φ   oa       )     0.291096                             
 
         [0070]    In essence, the Severity Parameter mimics a loading parameter which effectively standardizes various fuel/air scheduling studies. Clearly shown is the increase in efficiency as the loading parameter increases from 7.0 to 8.0, where-after it levels off. The increase is expected due to the intensification of turbulence and hence combustion activity within the TVC region (region between the bluff body and aft body) under the action of increasing fuel loading. More important, however, is the high levels of efficiency (&gt;99.5%) demonstrated. This is a tribute to the superior nature of the novel design provided herein as a whole relative to other flame stabilizer concepts such as axial and/or radial inflow premixers.  
         [0071]    With respect to the design depicted in FIG. 16, i.e., a TVC with struts, the data confirms that further augmentation of the interaction of the primary and highly energetic core flow with the cold, co-flowing channel flow is required to fully exploit the benefits of the TVC concept. When the hot and turbulent combustion products between the fore and aft body are distributed more effectively within the channel flow, higher combustion intensity and hence efficiency results. Furthermore, this heightened interaction reduces CO emissions since ignition is commenced earlier within the liner, thereby allowing more time for burnout. In the test, four, 0.25 inch diameter rods were installed as struts at the dump plane, spanning equal flow areas to serve as conduits for the hot TVC combustion products into the channel flow. A simple row of cooling holes  842  (see FIG. 21, for similar configuration) was employed along the axis of the rods to ensure reliability of the part at the combustor temperature. Table 1 details the exact data points run for this hardware configuration. A combustion pressure of approximately 270 psia and TVC equivalence ration (φ tvc ) of 0.75 were maintained for all points. After establishing these baseline conditions, excursions in front end equivalence ratio were conducted through adjustments of the channel or main equivalence ratio (φ main ).  
         [0072]    [0072]FIG. 28 below compares the results of such a modification to the baseline TVC results shown earlier. Again, combustion efficiency is plotted versus the Severity Parameter, SP. Clearly shown is the greater breadth of near 100% combustion efficiency relative to the previous runs; the region of near perfect combustion efficiency extends to lower Severity Parameter values than before.  
         [0073]    Perhaps the greatest proof of this configuration&#39;s performance gains can be demonstrated by a plot of CO versus NOx (corrected to 15% O2) were achieved over a sizeable stoichiometric range, far superior to all prior configurations (TVC or bluff body). Furthermore, the 10 ppm NOx/10 ppm CO threshold was even achieved and exceeded. The curve demonstrates both CO generation mechanisms: equilibrium production for richer mixtures and kinetic production for the leaner mixtures. The importance of encouraging the interaction between the cold and hot charge has been clearly demonstrated.  
         [0074]    Combustion testing on three hardware configurations has been completed. Testing commenced on the simplest flame stabilizer (bluff body, i.e. FIG. 7) with a 63% blockage ratio with expected levels of success. Combustion efficiencies above 99% were routinely achieved but competitive combustion exhaust gas emissions were not. The second test series evaluated the TVC concept which relies on locking a vortex pair between a fore and aft body (i.e., FIG. 15). This configuration resulted in dropping emissions levels from the hundreds (characteristic of the bluff body configuration) to the 20-30 ppm range, with resulting gains is overall combustion efficiency (&gt;99%). Knowledge that promoting the interaction between the highly turbulent TVC and channel flow is favorable for efficiency gains, lead to the evaluation of a third concept which utilized struts within the channel itself (FIGS. 4, 5 and  16 ). For testing purposes, rods were used as struts, as noted in FIG. 4. These sheltered conduits were shown to be effective in increasing the interaction of the incoming cold premix stream and the hot circulating burning gases. The resulting emissions levels (below 10 ppm NOX/CO and high combustion efficiencies (&gt;99.9%) have proven the success of this configuration for possible implementation into an industrial gas turbine system.  
         [0075]    It is to be appreciated that the various aspects and embodiments of the combustion chamber designs described herein are an important improvement in the state of the art of trapped vortex combustors. Although only a few exemplary embodiments have been described in detail, various details are sufficiently set forth in the drawings and in the specification provided herein to enable one of ordinary skill in the art to make and use the invention(s), which need not be further described by additional writing in this detailed description. Importantly, the aspects and embodiments described and claimed herein may be modified from those shown without materially departing from the novel teachings and advantages provided by this invention, and may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Therefore, the embodiments presented herein are to be considered in all respects as illustrative and not restrictive. This disclosure is intended to cover the structures described herein and not only structural equivalents thereof, but also equivalent structures. Numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention(s) may be practiced otherwise than as specifically described herein. Thus, the scope of the invention(s), as set forth in the appended claims, and as indicated by the drawing and by the foregoing description, is intended to include variations from the embodiments provided which are nevertheless described by the broad interpretation and range properly afforded to the plain meaning of the claims set forth below.