Patent Publication Number: US-8113000-B2

Title: Flashback resistant pre-mixer assembly

Description:
This invention was made with U.S. Government support under Contract Number DE-FC26-05NT42644 awarded by the U.S. Department of Energy. The U.S. Government has certain rights to this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a fuel injector system in a gas turbine engine, and more particularly, to a fuel injector system including a flashback resistant pre-mixer assembly. 
     BACKGROUND OF THE INVENTION 
     In gas turbine engines, compressed air discharged from a compressor section and fuel introduced from an external source are mixed together and burned in a combustion section. The mixture is directed through a turbine section, where the mixture expands to provide rotation of a turbine rotor. The turbine rotor may be linked to an electric generator, wherein the rotation of the turbine rotor can be used to produce electricity in the generator. 
     Gas turbine engines are known to produce an exhaust stream containing a number of combustion products. Many of these byproducts of the combustion process are considered atmospheric pollutants, and increasingly stringent regulations have been imposed on the operation of gas turbine power plants in an effort to minimize the production of these gasses. Of particular concern is the regulation of the production of the various forms of nitrogen oxides collectively known as NO x . It is known that NO x  emissions from a gas turbine increase significantly as the combustion temperature rises. One method of limiting the production of NO x  is the use of a lean mixture of fuel and combustion air, i.e. a relatively low fuel-to-air ratio, thereby limiting the peak combustion temperature to a level below the threshold for NO x  production. However, higher combustion temperatures are desirable to obtain higher efficiency and reduced production of carbon monoxide. 
     Two-stage combustion systems have been developed that provide efficient combustion and reduced NO x  emissions. In a two-stage combustion system, the majority of the fuel and air enter the pre-mixed combustion stage to reduce NO x  emissions. In pre-mixed combustion, the air and fuel are mixed together in a pre-mixer assembly that is upstream of a main combustion chamber of the engine. A small diffusion stage is included for obtaining ignition and low load flame stability. In diffusion combustion, the air and fuel are mixed together and ignited in the combustion chamber. 
     Gas turbine engines have been designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, biomass gas, etc, and more recently gas turbines engines have been designed to combust syngas produced from integrated gasification combined cycle applications. The syngas has a much higher flame speed than natural gas and is more susceptible to flame flashback when applied in pre-mixed combustion. Flame flashback in the pre-mixer assembly of gas turbine engines is undesirable, as it can cause damage to the components in and around the pre-mixer assembly, i.e., the flame may anchor onto the components and may burn through them. 
     Specifically, flame flashback may be caused when the turbulent burning velocity of the air and fuel mixture exceeds the axial flow velocity in the pre-mixer assembly, especially in low velocity regions near the boundary layer of the pre-mixer assembly. Flame flashback can also occur in recirculation zones that are caused by abrupt changes in the area of the flow path of the air and fuel mixture, such as at an aft end of a swirler assembly of the pre-mixer assembly, which provides an exit for the air and fuel mixture from the pre-mixer assembly into the combustion chamber. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, a pre-mixer assembly associated with a fuel supply system is provided for effecting a mixing of air and fuel upstream from a main combustion zone in a gas turbine engine. The pre-mixer assembly comprises a swirler assembly and a pre-mixer transition member. The swirler assembly is disposed about a fuel injector of the fuel supply system and includes a forward end defining an air inlet and an opposed aft end. The pre-mixer transition member extends from the aft end of the swirler assembly toward the main combustion zone and includes a forward end affixed to the aft end of the swirler assembly and an opposed aft end defining an outlet of the pre-mixer assembly to the main combustion zone. The aft end of the pre-mixer transition member is spaced from a base plate such that a gap is formed between the aft end of the pre-mixer transition member and the base plate. The gap permits a flow of purge air therethrough to effect an increase in a velocity of the air and fuel mixture exiting the pre-mixer assembly. 
     In accordance with a second aspect of the present invention, a pre-mixer assembly associated with a fuel supply system is provided for effecting a mixing of air and fuel upstream from a main combustion zone in a gas turbine engine. The pre-mixer assembly comprises a swirler assembly and a pre-mixer transition member. The swirler assembly is disposed about a fuel injector of the fuel supply system and includes a forward end defining an air inlet and an opposed aft end. The pre-mixer transition member extends from the aft end of the swirler assembly toward the main combustion zone and includes a forward end affixed to the aft end of the swirler assembly and an opposed aft end defining an outlet of the pre-mixer assembly to the main combustion zone. A plurality of apertures is formed in the swirler assembly and/or the pre-mixer transition member to allow purge air to flow therethrough. The purge air effects an increase in a velocity of the air and fuel mixture as the air and fuel mixture flows through the pre-mixer assembly proximate to a boundary layer of the pre-mixer assembly. 
     In accordance with yet another aspect of the present invention, a pre-mixer assembly associated with a fuel supply system is provided for effecting a mixing of air and fuel upstream from a main combustion zone in a gas turbine engine. The pre-mixer assembly comprises a swirler assembly and a pre-mixer transition member. The swirler assembly is disposed about a fuel injector of the fuel supply system and includes a forward end defining an air inlet and an opposed aft end. The pre-mixer transition member extends from the aft end of the swirler assembly toward the main combustion zone and includes a forward end affixed to the aft end of the swirler assembly and an opposed aft end defining an outlet of the pre-mixer assembly to the main combustion zone. The aft end of the pre-mixer transition member is spaced from a base plate such that a gap is formed between the aft end of the pre-mixer transition member and the base plate. The gap permits a flow of purge air therethrough to effect an increase in a velocity of the air and fuel mixture exiting the pre-mixer assembly. A plurality of apertures is formed in at least one of the swirler assembly and the pre-mixer transition member to allow additional purge air to flow therethrough. The additional purge air effects an increase in the velocity of the air and fuel mixture as the air and fuel mixture flows through the pre-mixer assembly proximate to a boundary layer of the pre-mixer assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is a sectional view of a gas turbine engine including a plurality of combustors incorporating pre-mixer assemblies according to an embodiment of the invention; 
         FIG. 2  is a side cross sectional view of a portion of one of the combustors illustrated in  FIG. 1  incorporating a plurality of pre-mixer assemblies; 
         FIG. 3  is an enlarged side cross sectional view of a portion of one of the pre-mixer assemblies illustrated in  FIG. 2 ; 
         FIG. 4  is an end view of a portion of the pre-mixer assembly shown in  FIG. 3  illustrating a base plate according to an embodiment of the invention; and 
         FIG. 5  is a side cross sectional view of a pre-mixer assembly and a base plate according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
     Referring to  FIG. 1 , a gas turbine engine  10  is shown. The engine  10  includes a compressor section  12 , a combustion section  14  including a plurality of combustors  16 , and a turbine section  18 . The compressor section  12  inducts and pressurizes inlet air which is directed to the combustors  16  in the combustion section  14 . Upon entering the combustors  16 , the compressed air from the compressor section  12  enters a head end  19  of each of the combustors  16  and is thereafter mixed with a fuel and ignited in a main combustion zone  14 A defined in an inner volume of a liner  20  (see  FIG. 2 ) to produce a high temperature and high velocity combustion gas flowing in a turbulent manner. The combustion gas then flows from the main combustion zone  14 A through a transition  22  to the turbine section  18  where the combustion gas is expanded to provide rotation of a turbine rotor  24 . 
     Referring to  FIG. 2 , a portion of one of the combustors  16  of the combustion section  14  is shown. It is understood that the remaining combustors  16  are substantially similar to the combustor  16  as described in detail herein. The combustor  16  further comprises a pilot nozzle  32  having a pilot fuel injection port  34  disposed along central axis  36  of the combustor  16  upstream from the main combustion zone  14 A. The pilot nozzle  32  is in communication with a source of fuel (not shown) for delivering fuel to pilot fuel injection port  34 . The pilot nozzle  32  is secured to a support housing (not shown) of the combustor  16 . A pilot cone  38  having an aft end portion  40  is disposed about the pilot fuel injection port  34  of the pilot nozzle  32 . A pilot flame zone  42  is formed within the pilot cone  38  adjacent and upstream from the main combustion zone  14 A. 
     A plurality of pre-mixer assemblies  44  extend in an annular array about and are substantially parallel to the pilot nozzle  32 . The pre-mixer assemblies  44  are associated with main fuel injectors  46 , each having at least one and preferably a plurality of main fuel injection ports  48  as shown in  FIG. 2 . The main fuel injectors  46  are affixed to and extend from the support housing. The main fuel injectors  46  are in communication with a source of fuel (not shown) for delivering fuel to the pre-mixer assemblies  44 . It is understood that the pilot nozzle  32  may be in communication with the same or a different source of fuel as the main fuel injectors  46 . 
     Referring now to  FIG. 3 , one of the pre-mixer assemblies  44  will now be described. It is understood that the other pre-mixer assemblies  44  are substantially similar to the pre-mixer assembly  44  as described herein. The pre-mixer assembly  44  comprises a substantially cylindrical swirler assembly  52  having a forward end  54  (see  FIG. 2 ) and an opposed aft end  56 . Although the swirler assembly  52  illustrated herein is substantially cylindrical in shape, it is understood that the swirler assembly  52  may have any suitable shape, such as, for example, oval or polygonal. The forward end  54  defines an air inlet for receiving a flow of compressed air from the head end  19  of the combustor  16 . 
     The swirler assembly  52  includes a plurality of apertures  60  formed near the aft end  56  thereof for permitting a flow of purge air therethrough, i.e., air from the head end  19  of the combustor  16 , from the outside of the swirler assembly  52  to the inside of the swirler assembly  52 . The apertures  60  are preferably aligned in at least one annular row as shown in  FIGS. 2 and 3  and are sized to allow a desired amount of purge air to flow therethrough. In the preferred embodiment, the apertures  60  comprise openings having a diameter of about 2 mm, but may have other suitable sizes. The purge air effects an increase in the velocity of the air and fuel mixture flowing proximate to a boundary layer  61  of the pre-mixer assembly  44 , i.e., an area proximate to the inner surface of the pre-mixer assembly  44  as will be described in greater detail below. 
     The pre-mixer assembly  44  also comprises a pre-mixer transition member  70 , which, in the embodiment shown, comprises a separate piece from the swirler assembly  52  but may comprise a single or integral piece with the swirler assembly  52 . The pre-mixer transition member  70  comprises a forward end  72  that is affixed to the aft end  56  of the swirler assembly  52  and an opposed aft end  74  defining an outlet of the pre-mixer assembly  44  to the main combustion zone  14 A. As shown more, clearly in  FIG. 3 , spanning members  75  may be disposed between the swirler assembly  52  and the pre-mixer transition member  70  such that a gap  77  is formed therebetween. The gap  77  permits an additional flow of purge air therethrough i.e., from the head end  19  of the combustor  16  to the inside of the pre-mixer assembly  44 . The additional purge air effects a further increase in the velocity of the air and fuel mixture flowing through the pre-mixer assembly  44  proximate to the boundary layer  61  as will be described in greater detail below. 
     The forward end  72  of the pre-mixer transition member  70  comprises a substantially cylindrical opening having a dimension D 1  of about 2.8 inches (see  FIG. 3 ) and the aft end  74  comprises a circumferentially elongated and radially compressed opening having a circumferential dimension D 2  of about 1.6 inches (see  FIG. 4 ) and a radial dimension D 3  of about 3.7 inches. The pre-mixer transition member  70  narrows radially and expands circumferentially from the forward end  72  to the aft end  74  thereof to form the circumferentially elongated and radially compressed opening shown in  FIG. 4 , such that the aft end  74  of each pre-mixer transition member  70  comes into close proximity with the aft end  74  of the two adjacent pre-mixer transition members  70 . 
     The pre-mixer transition member  70  includes a plurality of apertures  78  formed therein for permitting an additional flow of purge air therethrough i.e., from the head end  19  of the combustor  16  to the inside of the pre-mixer transition member  70 , as shown in  FIGS. 2 and 3 . The apertures  78  are preferably aligned in at least one annular row as shown in  FIGS. 2 and 3  and are sized to allow a desired amount of additional purge air to flow therethrough. In the embodiment shown, the apertures  78  comprise openings having a diameter of about 1 mm, and in a preferred embodiment are smaller than the apertures  60  formed in the swirler assembly  52 . Preferably, the apertures  78  formed in the pre-mixer transition member  70  are circumferentially staggered from the apertures  60  formed in the swirler assembly  52 . The additional purge air effects a further increase in the velocity of the air and fuel mixture flowing through the pre-mixer assembly  44  proximate to the boundary layer  61  as will be described in greater detail below. 
     As shown in  FIG. 2 , pins  80 A 1 ,  80 A 2 ,  80 B 1 ,  80 B 2  secure the pre-mixer assemblies  44  to a radially inner surface  82  of a liner head  83  that is affixed to the liner  20 , such as, for example, by welding. The pins  80 A 1 ,  80 A 2 ,  80 B 1 ,  80 B 2  may comprise, for example, hourglass shaped or straight members that are welded or otherwise secured at one end to the radially inner surface  82  of the liner head  83  and at the other end to the corresponding pre-mixer assembly  44 . Any suitable number of pins  80 A 1 ,  80 A 2 ,  80 B 1 ,  80 B 2  may be used for attachment of the pre-mixer assemblies  44  to the liner  20 . 
     Referring now to  FIGS. 2-4 , a base plate  90  of the combustor  16  is shown. It is understood that each of the combustors  16  includes a base plate  90  that is substantially similar to the base plate  90  described in detail herein. The base plate  90  comprises a radially inner wall  89  that surrounds the pilot cone  38  and a radially outer wall  91  proximate to the liner  20 . In the embodiment shown in  FIGS. 2 and 3 , a forward end  92  of the radially outer wall  91  of the base plate  90  is attached to the radially inner surface  82  of the liner head  83  downstream from the pins  80 A,  80 B, such as by welding, for example. A radial wall  93  of the base plate  90  extends between the radially inner wall  89  and the radially outer wall  91  and defines an aft end of the base plate  90 . The aft end  94  of the base plate  90  extends to an axial location slightly upstream from the axial location of the aft end  74  of the pre-mixer transition member  70 , although it is understood that the aft end  94  of the base plate  90  could extend to an axial location slightly downstream from or substantially the same as the axial location of the aft end  74  of the pre-mixer transition member  70 . 
     As shown in  FIG. 4 , the radial wall  93  of the base plate  90  includes a plurality of apertures  96  formed therein, each aperture  96  having a size slightly larger than the dimensions D 2 , D 3  of the aft end  74  of the pre-mixer transition member  70 , i.e., such that a gap  98  is formed between the aft end  74  of the pre-mixer transition member  70  and the radial wall  93  of the base plate  90 . Each of the apertures  96  is associated with a respective one of the pre-mixer assemblies  44  of the engine  10  and permits the air and fuel mixture in each associated pre-mixer assembly  44  to flow out of the pre-mixer assembly  44  and into the main combustion zone  14 A. 
     In the embodiment shown, the gaps  98  comprise dimensions in the radial and circumferential directions such as, for example, 1.0 mm around the circumference of the pre-mixer transition member  70  between the pre-mixer transition member  70  and the aft end  94  of the base plate  90 . The gaps  98  are maintained by a plurality of first protuberances  95  formed in the base plate  90  that extend toward the pre-mixer transition members  70 , as shown in  FIG. 3 . It is noted that since the pre-mixer transition member  70  and the aft end  94  of the base plate  90  extend to slightly different axial locations, the gaps  98  also comprise a dimension in the axial direction. It is also noted that the gaps  98  may comprise any combination of dimensions in the radial, circumferential, and/or axial directions depending on the locations of the pre-mixer transition member  70  and the aft end  94  of the base plate  90 . The gaps  98  permit a flow of purge air therethrough i.e., from the head end  19  of the combustor  16  to effect an increase in the velocity of the air and fuel mixture as it exits the pre-mixer assemblies  44 . The gaps  98  are maintained by a plurality of first protuberances  95  formed in the base plate  90  that extend toward the pre-mixer transition member  70 , as shown in  FIG. 3 . 
     The radially inner wall  89  of the base plate  90  also includes a plurality of second protuberances  97  formed therein that extend outwardly toward the pilot cone  38 , as shown in  FIG. 3 . The second protuberances  97  create a passageway  99  between the pilot cone  38  and the base plate  90  that allows the flow of cooling air therethrough that can be used to provide cooling for the pilot cone  38 . Air from the head end  19  of the combustor  16  is permitted to flow into the passageway  99  through apertures  99 A formed in the base plate  90  proximate to a forward end  38 A of the pilot cone  38  as shown in  FIG. 3 . It is noted that this air flows out of the passageway  99  adjacent the aft end  94  of the pre-mixer transition member  70  and may be used to further increase the velocity of the air and fuel mixture exiting the pre-mixer assemblies  44  and/or prevent flame holding in this region. 
     As shown in  FIG. 4 , the base plate  90  also includes a plurality of small holes  100  having a diameter of about 1.0 mm formed in the radial wall  93  thereof for permitting additional purge air to flow therethrough i.e., from the head end  19  of the combustor  16  to prevent flame holding on the base plate  90  by further increasing the velocity of the air and fuel mixture as it exits the pre-mixer assemblies  44 . It is understood that an amount of fuel, i.e., supplied from an upstream fuel injector (not shown), such as, for example, a C-stage fuel injector, may be mixed with the air flowing through the holes  100  during different operating conditions of the engine. It is understood that the fuel flow is preferably provided such that the fuel air mixture is always below the flammability limit of the mixture. 
     During operation of the engine  101  the compressed air from compressor section  12  flows through a compressor section exit diffuser  101  (see  FIG. 1 ) into a combustor plenum  102  (see  FIG. 1 ). The compressed air then flows into the head end  19  of each of the combustors  16  and into the pilot flame zone  42  where it is mixed with fuel from the pilot fuel injection port  34 . The mixture of air and fuel from the pilot fuel injection port  34  then exits the pilot flame zone  42  and enters the main combustion zone  14 A. 
     Compressed air from compressor section  12  also flows from the head ends  19  of the combustors  16  into each of the pre-mixer assemblies  44  through the forward ends  54  of the pre-mixer assembly swirler assemblies  52 . The air is mixed with fuel from the main fuel injectors  46  and the air and fuel mixture flows through the swirler assemblies  52  and into the pre-mixer transition members  70 . 
     The purge air flowing through the apertures  60  in the swirler assemblies  52  increases the velocity of the air and fuel mixture proximate to the boundary layer  61  to assist in preventing flame flashback from occurring in the pre-mixer assemblies  44 , i.e., by assisting in keeping the velocity of the air and fuel mixture proximate to the boundary layer  61  above the turbulent burning velocity of the air and fuel mixture. In addition to increasing the velocity of the air and fuel mixture, this air also lowers the fuel air ratio in this region. Further, the close proximity of the forward ends  72  of the pre-mixer transition members  70  to the aft ends  56  of the swirler assemblies  52  provide for a smooth transition for the mixture of air and fuel through the pre-mixer assembly  44 , thus preventing air and fuel mixture recirculation zones that could otherwise be caused by abrupt transitions in the flow path between components in the combustion section  14 . Additionally, the extended structure provided by the pre-mixer transition members  70  provides a smooth flow path for the air and fuel mixture flowing out of the pre-mixer assemblies  44  to further prevent air and fuel mixture recirculation zones. Moreover, the additional purge air that flows through the gaps  77  between the swirler assemblies  52  and the pre-mixer transition members  70 , and the additional purge air that flows through the apertures  78  in the pre-mixer transition members  70  provide for additional increases in the velocity of the air and fuel mixture flowing through the pre-mixer assemblies  44  proximate to the boundary layer  61  to further assist in preventing flame flashback from occurring in the pre-mixer assemblies  44 . 
     The air and fuel mixture then flows out of the aft end  74  of the pre-mixer transition members  70  and through the apertures  96  in the base plate  90  into the main combustion zone  14 A. The purge air that flows through the gaps  98  between the pre-mixer transition members  70  and the base plate  90  and the holes  100  in the base plate  90 , in addition to the air flowing out of the passageways  99  between the pilot cone  38  and the base plate  90 , assists in preventing flame flashback from occurring at the aft ends  74  of the pre-mixer transition members  70 , i.e., by assisting in keeping the velocity of the air and fuel mixture above the turbulent burning velocity of the air and fuel mixture at the exits of the pre-mixer assemblies  44  to the main combustion zone  14 A, which, in prior art configurations, are locations that are prone to the formation of flame recirculation zones and flame flashback. 
     It is noted that the location of the aft end  94  of the base plate  90  proximate to the aft ends  74  of the pre-mixer transition members  70  is advantageous since the sizes of the gaps  98  between the radial wall  93  of the base plate  90  and the aft ends  74  of the pre-mixer transition members  70  can be controlled for providing a desired amount of purge air therethrough to prevent flame recirculation zones from occurring at the exits of the pre-mixer assemblies  44 . 
     It is also noted that the base plate  90  eliminates the need for an additional structure to provide cooling for the pilot cone  38 . Specifically, prior art systems typically employ an outer cone that surrounds the pilot cone  38  and creates a passageway between the outer cone and the pilot cone  38  that allows the flow of cooling air therethrough, which is used to provide cooling for the pilot cone  38 . 
     Referring now to  FIG. 5 , a combustor  116  according another embodiment of the invention is shown, wherein elements corresponding to elements of the first described embodiment of the combustor  16  ( FIGS. 1-4 ) are identified by the same reference numeral increased by 100. A base plate  190  according to this embodiment of the invention includes a radially outer wall  191  having a forward end  192  that is attached to a radially inner surface  120 A of a liner  120  at a location that is axially downstream from where the forward end  92  of the base plate  90  is mounted in the embodiment described above for  FIGS. 1-4 . An aft end  194  of the base plate  190  extends to substantially the same axial location as an aft end  174  of a pre-mixer transition member  170 , although it is understood that the aft end  194  of the base plate  190  could extend to an axial location slightly in front of or behind the aft end  174  of the pre-mixer transition member  170 . It is noted that the radially outer wall  191  of the base plate  190  according to this embodiment is axially shorter in length than the length of the base plate  90  described above for  FIGS. 1-4 , which accounts for the difference in mounting locations between the base plates  90 ,  190  while maintaining the same axial location of the aft ends  94 ,  194  of the respective base plates  90 ,  190 . 
     As with the base plate  90  described above for  FIGS. 1-4 , gaps  198  are formed between the aft ends  174  of the pre-mixer transition members  170  and the aft end  194  of the base plate  190 . In the embodiment shown in  FIG. 5 , the gaps  198  comprise dimensions in the radial and circumferential directions such as, for example, 1.0 mm around the circumferences of the pre-mixer transition members  170  between the pre-mixer transition members  170  and the aft end  194  of the base plate  190 . However, it is understood that the gaps  198  could comprise a dimension in the axial direction, instead of or in addition to the dimensions in the radial and circumferential directions depending on the locations of the aft ends  174  of the pre-mixer transition members  170  and the aft end  194  of the base plate  190 . The gaps  198  permit a flow of purge air therethrough to effect an increase in a velocity of the air and fuel mixture exiting the pre-mixer assemblies  144 . 
     The remaining structure of the combustor  116  and use thereof is substantially the same as for the combustor  16  described above for  FIGS. 1-4 . 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.