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 and is delivered to the combustor with a high degree of swirl motion. This swirling mixture of reactants is conveyed through a flowpath that expands; the mixture reacts, and establishes a central recirculation zone. An imperforate trapped vortex cavity is disposed proximal to the swirler apparatus which provides for a second reaction zone. Fresh fuel/air reactants are exchanged with burned products in the trapped vortex and a pilot flame is established in the trapped cavity. The imperforate trapped cavity is not supplied with either fuel or air, but is cooled on a backside of the cavity with a flow of cooling air. The cooling air is then conveyed to the combustion chamber so as to not interfere with the critical flame holding flow features of the combustor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a combustion system and a method of combusting fuels. 
       BACKGROUND 
       [0002]    Air pollution is a concern for apparatuses which burn hydrocarbon fuels with air. Gas turbines and, more specifically, microturbines, which are used to generate electricity and hot exhaust gases used in cogenerative applications are increasingly subject to air emission restrictions. These turbines are sometimes known as industrial gas turbine generators (ITGs). The air emission restrictions are imposed by governmental regulatory organizations such as the California Air Resources Board, the states of Texas and New Jersey, and other governmental bodies. These restrictions may regulate the emission of oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOC&#39;s). Gas turbine engine manufacturers are obliged to develop improved combustion methods and apparatus configurations to satisfy these restrictions while also satisfying turbine engine operation requirements at full-power and low-power operation. 
         [0003]    The low emissions combustors used in ITG engines combust a premixed combination of fuel and air. The mixture of fuel and air is typically lean of the stoichiometric apportionment of fuel and air in order to limit flame temperatures and reduce gaseous emissions. By “lean” it is meant that an excess amount of air is mixed with the fuel, and not all the oxygen in the air is consumed in the reaction. When the fuel is composed of a gaseous fuel, such as natural gas, digester gas, landfill gas, syngas derived from gasification or pyrolysis processes, or other hydrocarbon gas mixtures, the gas fuel is premixed with air prior to combustion. This is commonly referred to as “lean premixed” (LP) combustion. When the fuel is liquid, such as jet fuel, diesel, kerosene, or other liquid fuel, the fuel must be both vaporized and mixed prior to combustion. This method is referred to as “lean, premixed, prevaporized” (LPP) combustion. 
         [0004]    Both LP and LPP combustion methods are capable of combusting fuel with low levels of NOx, CO, and VOC&#39;s. The lean fuel/air mixture combusts at low gas temperatures, avoiding high-temperature regions that produce NOx. The LP or LPP are also typically designed to burn hot enough, and for sufficient residence-times, to fully oxidize carbon monoxide (CO) to carbon dioxide (CO2) and unburned hydrocarbons and other VOC&#39;s to water (H2O) and carbon dioxide (CO2). 
         [0005]    A typical LP and LPP combustor can burn fuel with low emissions over a limited range of fuel/air mixtures. The mixture must remain lean enough to avoid the production of NOx. This lean mixture is typically close to the lean flame extinction limit, also known as the lean blowout (LBO) limit. When gas turbines are required to produce less than full power (“part-power”), the combustor typically receives less fuel, which decreases the fuel/air ratio, inducing LBO. Gas turbine combustors sometimes include a second source of fuel (pilot) which is injected into the combustor without premixing the fuel with air. The pilot fuel burns in a “diffusion” mode, where the flame front is locally controlled by the diffusion of fuel and air (oxygen) together. Diffusion flames burn at higher temperatures and produce higher levels of NOx, but permit gas turbines to operate at part-power. 
       SUMMARY 
       [0006]    In one embodiment, the invention provides a combustor for combusting a mixture of fuel and air. The combustor includes a swirlerhead for receiving a flow of air and a flow of fuel, the fuel and air being mixed together under the influence of the swirlerhead, the swirlerhead imparting a swirling flow to the fuel/air mixture. A prechamber is in fluid communication with the swirlerhead 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 a 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. A trapped vortex chamber is disposed radially outwardly from the prechamber, the trapped vortex having an imperforate wall defining a cavity, the trapped vortex chamber receiving fuel/air from an outer perimeter of the vortex into the cavity and exhausting combustion products into the vortex. 
         [0007]    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. The fuel/air mixture is injected into a prechamber cylinder in a swirling motion about a centerline of the prechamber, thereby creating a vortex having a low pressure region at the centerline. The vortex is conveyed axially in a downstream direction into a combustion chamber having greater flow area than a flow area of the prechamber. The vortex is expanded into the combustion chamber, wherein chemical reaction of the fuel and air occurs to form hot products of combustion. The expansion forms a recirculation zone at the centerline wherein the hot products are drawn upstream into the prechamber. Fuel/air from an outer perimeter of the vortex is trapped within a trapped vortex chamber disposed radially outwardly from the prechamber upstream of the combustion chamber. The fuel/air within the trapped vortex chamber has about the same fuel/air ratio as the fuel/air mixture in the combustion chamber. A swirling flow is induced within the trapped vortex chamber, the swirling trapped vortex flow rotating about the centerline of the prechamber, wherein chemical reaction of the fuel and air within the trapped vortex chamber occurs to form hot products of combustion. A shear flow of hot products of combustion is provided along an inner perimeter of the vortex flow from the recirculation zone and along an outer perimeter of the vortex flow from the trapped vortex flow. 
         [0008]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      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. 
           [0010]      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. 
           [0011]      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. 
           [0012]      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. 
           [0013]      FIG. 5  is a schematic illustration of a swirlerhead, prechamber and combustion chamber according to an embodiment of the invention. 
           [0014]      FIG. 6  is an end view of a radial swirler for use with an embodiment of the invention. 
           [0015]      FIG. 7  is a graph of flame speed versus g-load. 
           [0016]      FIG. 8  is a schematic of a trapped vortex chamber according to another embodiment of the invention. 
           [0017]      FIG. 9  is an exploded view of the trapped vortex chamber according to an embodiment of the invention. 
           [0018]      FIG. 10  is a sectional view of a trapped vortex chamber according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    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. 
         [0020]    The invention described herein can be used for burning various hydrocarbon fuels in a gas turbine. The combustion process comprises a method to burn LP and LPP fuel/air (F/A) mixtures such that the onset of LBO is delayed to leaner F/A mixtures. This enables lower gas turbine exhaust emissions (NOx, CO, VOC&#39;s) at a wider range of operating engine conditions. 
         [0021]    Referring now to the drawings, like numerals are used throughout to refer to like elements within a gas turbine and combustor. 
         [0022]      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). 
         [0023]    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. 
         [0024]      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). 
         [0025]    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. 
         [0026]      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). 
         [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 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. 
         [0028]      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. 
         [0029]      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. 
         [0030]    With continued reference to  FIG. 4 , the outlet manifold  58  contains a silo or tubular combustor  52  and a swirlerhead  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 swirlerhead  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 swirlerhead  60  is upstream of the combustor  52 . 
         [0031]      FIG. 5  shows a cross-sectional view of the swirlerhead  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 through swirlerhead  60  to prechamber  64 , and then to combustion chamber  66 , inside combustor  52 . Air flows into the prechamber  64  through a plurality of slots  67  in swirlerhead  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 flow slots  67 . 
         [0032]      FIG. 6  shows an end view of swirlerhead  60 . Air is driven from outside the swirlerhead  60 , through swirl slots  67 , as indicated by arrow  72  (see  FIG. 5 ), to the prechamber  64 . Swirl slots  67  are oriented to inject the air into the prechamber  64  with a high degree of swirl about a centerline or central axis A of the cylindrical prechamber  64 . In the illustrated embodiment, the prechamber  64  and the combustion chamber  66  are coaxial. Swirl slots  67  terminate with at least one slot wall tangent to a prechamber wall  70 . Gaseous or liquid fuel can be injected at location  72  with one or a number of apertures from the wall or from a fuel injection cylinder or tube  73  with holes in it (see  FIG. 5 ). By injecting the fuel at the entrance to the swirl slot  67 , the fuel and air have adequate time to thoroughly mix prior to exiting the slot  67 . This uniform mixture of F/A avoids 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 swirl slots  67  uniformly mixed. 
         [0033]    With continued reference to  FIG. 6 , an electronic ignitor, flame torch, or other ignition device  74  is located between the centerline A of the prechamber  42  and the inside diameter of the slot  67  exits. The ignitor  74  ignites the premixed F/A exiting slots  67 , but is not subjected to the high temperatures of an inner recirculation zone  86  ( FIG. 5 ). 
         [0034]    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 swirlerhead  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 . For example, an axial swirler could also convey the F/A mixture with a high degree of swirl. A swirler that has both radial and axial velocity components is also possible. 
         [0035]    With continued reference to  FIG. 5 , 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 motion, as 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. 
         [0036]    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  84  to expand radially outward and slow its axial and rotational or swirling movement, as indicated by arrow  84 . The expansion of the vortex  84  reduces the pressure difference between outside edge of the vortex  84  and the center. Thus, the centerline of the prechamber  64  is at a lower pressure than the centerline of the combustion chamber  66 . An inner recirculation zone 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 . 
         [0037]    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 zone  86  therefore is composed of hot products of combustion. The flow of the inner recirculation zone  86  is directionally opposed to the unburned F/A mixture of  82 , and an inner shear layer is established between the two. Hot gas products and combustion radicals, which are unstable electrically-charged molecules like OH—, O—, and CH+ are exchanged with the unburned F/A of flow  82 . Flow  86  serves as a continued ignition source for flow  82 . The chemical radicals also enhance the reactivity of the unburned mixture of flow  82 , enabling the F/A mixture of flow  82  to extinguish combustion at a lower F/A ratio than if flow  82  did not have the radicals from flow  86 . 
         [0038]    With continued reference to  FIG. 5 , the combustor  52  further includes a trapped vortex chamber  90  provided in the prechamber  64 . The trapped vortex chamber  90  is an annular recess or cavity disposed at a radial periphery of the prechamber  64 . An inner radius of the trapped vortex chamber  90  is open to the prechamber  64 . An outer periphery and sides of the trapped vortex chamber  90  are defined by an imperforate liner or wall  94 . 
         [0039]    As the F/A mixture in flow  82  is conveyed downstream from the swirlerhead  60  through the prechamber  64  in a swirling motion, the F/A mixture at a periphery of the prechamber  64  is trapped in the trapped vortex chamber  90 . The axial velocity component of flow  82  encourages a separate, annular processing vortex flow  92  within the trapped vortex chamber  90 . The F/A flow  92  in the trapped vortex chamber  90  is both rotating, as shown in  FIG. 5 , as well as swirling around the centerline A of the prechamber  64 . 
         [0040]    Combustion chemical reaction occurs within the trapped vortex chamber  90  resulting from autoignition of the F/A mixture  92 . Autoignition is the ignition of an F/A mixture at temperatures above the autoignition temperature for an F/A mixture to ignite. Autoignition includes the requirement to keep an F/A mixture above a specified temperature for a specific time (ignition delay time). Once ignited, combustion will continue in the trapped vortex chamber  90  due to continued circulation of the trapped vortex flow  92  about the centerline A of the prechamber  64 . The residence time of the gases  92  inside the trapped vortex chamber  90  are believed to be longer than needed for complete combustion. The ignited trapped vortex flow  92  serves as a pilot flame for igniting the vortex flow  82 . Alternately, or in combination, combustion chemical reaction occurs within the trapped vortex chamber  90  resulting from flame propagation from the ignitor  64  through vortex flow  82  and into the trapped vortex chamber  90 . 
         [0041]    With respect to combustion within the trapped vortex chamber  90 , chemical reactions can occur at higher rates within the trapped vortex chamber  90  due to high g-loading. The observed flame speed of an F/A mixture increases with increasing centrifugal force or “g-load” on an F/A mixture. A peak increase can occur with a g-load around 3500, above which flame speed can start to decrease.  FIG. 7  illustrates the relationship between g-load and flame speed. The trapped vortex chamber  90  leverages the high swirl created by the swirling flow  82  to increase flame speeds within the trapped vortex chamber  90 . The g-load is calculated from the following equation: 
         [0000]    
       
         
           
             g 
             = 
             
               
                 V 
                 tan 
                 2 
               
               
                 
                   g 
                   c 
                 
                  
                 
                   r 
                   trap 
                 
               
             
           
         
       
     
         [0042]    Where V tan  is the velocity of the air circling about the prechamber centerline A, r trap  is the radius of the inner edge of the trapped vortex chamber  90 , and g c  is the acceleration of gravity. 
         [0043]    Flame speed is an inherent measure of ability of a chemical reaction to release heat. The higher flame speeds in the trapped vortex chamber  90 , enabled by the high g-loads, serve to complete the combustion reactions faster and also enables the F/A mixture of flow  92  to stay lit at leaner (lower F/A) conditions. 
         [0044]    Unburned F/A leaving swirl slots  67  at flow  80  turbulently diffuses across a shear layer established between the trapped vortex flow  92  and the prechamber flow  82 . Air and fresh fuel reactants in the air are exchanged across the shear layer between  82  and  92 . Burned combustion products and chemical radicals also exit the trapped vortex chamber  90  and mix with flow  82 . 
         [0045]    Note that exchange of fresh F/A from flow  82  to flow  92  is also encouraged by the density gradient between the two flows. The unburned, colder F/A mixture of flow  82  has a higher density than that of flow  92 , which is at a higher temperature. The swirling motion within the prechamber  64  establishes an unstable flow pattern, where higher density gases are swirling inside of a lower density flow in the trapped vortex chamber  90 . This density differential promotes gaseous exchange between flows  82  and  92 . 
         [0046]    As shown in  FIG. 5 , the flow of unburned F/A of flow  82  is bounded at its inner radius by the hot inner recirculation flow or zone  86  and at its outer radius by a secondary hot trapped vortex flow or zone  92 . Both boundaries or zones are serving to ignite the unburned F/A flow  82 , so that the unburned F/A is ignited from two sources, and infuse chemical radicals from two shear layers to further increase the reactivity of flow  82 . This method of combustion therefore can postpone flame extinguishment relative to a traditional premixed combustor that uses a vortex-breakdown structure to establish an inner recirculation zone alone. 
         [0047]    With continued reference to  FIGS. 5 and 8 , the trapped vortex chamber  90  is formed of an imperforate wall  94 . Neither fuel nor air is injected into the trapped vortex chamber  90 . Rather, the flow of fuel and air into the trapped vortex chamber  90  is provided by premixed F/A from within the combustor  52 , i.e., flow  82  from the prechamber  64 . Injection of fuel, air or a fuel/air mixture into the trapped vortex chamber  90  could interfere with the swirling, rotating flow pattern established within the trapped vortex chamber  90 , reducing g-loads and residence times within the trapped vortex chamber  90 . Injection of fuel, air or a fuel/air mixture into the trapped vortex chamber  90  would result in variations in the fuel/air composition of the trapped vortex chamber  90  depending on whether the engine was being operated at full power or low power. Flame extinction within the trapped vortex chamber  90  could occur due to the purging effect of the air injected into the trapped vortex chamber  90 , reducing the exchange of chemical radicals with the vortex flow  82  and ignition of the prechamber vortex flow  82 . 
         [0048]    Injection of air alone into the trapped vortex chamber  90  could result in a leaning of the F/A ratio in the trapped vortex chamber  90  and LBO within the trapped vortex chamber  90 . Injection of fuel alone into the trapped vortex chamber  90  could richen the F/A ratio in the trapped vortex chamber  90 , resulting in overheating of the trapped vortex chamber  90  and high levels of NOx production. Increased heat production within the trapped vortex chamber  90  would require additional costly fuel injection manifolds and cooling features. Direct injection of both fuel and air into the trapped vortex chamber  90  could create a diffusion flame, which can result in locally richer combustion spots producing higher levels of NOx. Because fuel and air within the trapped vortex chamber  90  is provided solely through the flow of premixed fuel and air from the prechamber  64 , the F/A ratio within the trapped vortex chamber  90  is equivalent to that of the combustion chamber  66  and excess NOx production is avoided. 
         [0049]    Combustion in the trapped vortex chamber  90  will cause heat to be transferred into the combustor liner walls, including the trapped vortex chamber wall  94 . A cooling system is provided for maintaining wall temperatures at acceptable levels to achieve long component life. 
         [0050]    To cool the inner liner or wall  94  around the trapped vortex chamber  90 , wherein combustion reactions are occurring, passive backside convection cooling of the liner  94  can be provided.  FIGS. 8 and 9  illustrate construction of the trapped vortex chamber  90  according to an embodiment of the invention. An outer liner  96  is coupled between a flange  98  at an upstream end of the prechamber  64  and a downstream end of the prechamber  64 . In the illustrated embodiment, the outer liner  96  includes first and second portions  96   a ,  96   b  coupled to one another and to the flange  98  with fasteners (not shown) and spacers  99 . The outer liner  96  is spaced apart from the liner  94 , creating a gap  100  therebetween. 
         [0051]    Cool air from the recuperator outlet manifold  58  enters the gap  100  at U, flows in the gap  100  between the liners  94 ,  96  over an outer or backside of the liner  94  and exhausts into the combustion chamber  66  downstream of the vortex flow  82 . In the embodiment illustrated in  FIG. 8 , air is sequentially flowed from U to V to W to X to Y by a means of holes, slots, and openings. The outer liner  96  includes apertures which convey jets (flow Z) of air onto the backside of the liner  94 . These jets also cool the liner  94 . The air from flows U and Z is conveyed in the gap  100  between the two liners  94 ,  96  and is exhausted into the main combustion chamber  66  with flow Y. 
         [0052]    Because the liner  94  is imperforate, as previously discussed, at no point is the backside cooling air (i.e., flows U, V, W, X, Y, Z) permitted to enter or interfere with the reacting flow  92  within the trapped vortex chamber  90 . Also of note, flow Y enters combustion chamber  66  at a location downstream of the prechamber  64  so as not to cool the inner recirculation zone  86  or otherwise interfere with the flame holding flow features, including, for example, vortex flow  82 . 
         [0053]    One skilled in the art of combustion design could apply other methods of backside cooling of trap liner  94 . For instance, bumps or ribs could be situated on liner  94  to enhance the cooling convection. In addition, or alternately, a plasma-sprayed thermal barrier coating, such as a partially stabilized zirconia, can be applied to the hot (inner) surface of the liner  94 . 
         [0054]    In the illustrated embodiment, the air at flow U is recuperator air, which can have a temperature of about 1100 F. While this is a high temperature, the temperatures within the trapped vortex chamber  90  can be in excess of about 2400 F. In other embodiments, cooling air flow U can be from other sources, including, for example, compressor discharge air. 
         [0055]    In the illustrated embodiment, the trapped vortex chamber  90  is provided with a radial slip feature in the form of ring  101 . The ring  101  includes slots and/or apertures for permitting the flow of cooling air therethrough from flow V to flow W. The outer liner portions  96   a ,  96   b  are secured together with fasteners, such as screws, that fit through the spacers  99 , to the flange  98 . The inner liner  94  is coupled to the ring  101  by, for example, welding or adhering, at their inner diameters, but not to the flange  98 . When the liner  94 /ring  101  assembly is pressed to the flange  98  with the fasteners through outer liner portions  96   a ,  96   b , the inner surface between the flange  98  and the ring  101  is not connected to one another other than a clamping force holding them together. This configuration permits the liner  94 /ring  101  assembly to expand in a radial direction when the trapped vortex chamber  90  heats up during combustion without creating thermal stresses. This can reduce creep distortion and fatigue cracking of the components of the trapped vortex chamber  90 , including the liner  94 . Furthermore, if the liner  94  and the ring  101  are permanently secured to one another, this assembly can easily be removed and replaced because it is only clamped into place. 
         [0056]      FIG. 10  illustrates a trapped vortex chamber  190  according to another embodiment of the invention. Trapped vortex chamber  190  is generally similar in form and function to trapped vortex chamber  90 , with the exception that an inner profile of the wall  194  is approximately square or rectangular rather than semi-circular as illustrated in  FIG. 8 . 
         [0057]    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. 
         [0058]    The present invention addresses the issue of LP and LPP combustion (premixed combustion) by increasing the fuel/air range where a combustor will not produce high levels of NOx and will not extinguish (LBO). This permits a gas turbine to operate over a wider range of power without the need for a diffusion pilot feature. This method of lean combustion uses a highly swirled mixture of fuel/air in combination with a trapped vortex cavity to increase flame stability. Two flame-holding features (central recirculation zone, trapped vortex combustion) enhance the stability of the LP system. 
         [0059]    Thus, the invention provides, among other things, a method and apparatus to combust lean mixtures of fuel and air stably in a gas turbine engine for power generating equipment including microturbines. Various features and advantages of the invention are set forth in the following claims.