Patent Publication Number: US-8973368-B2

Title: Mixer assembly for a gas turbine engine

Description:
STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. NNC08CA92C awarded by the National Aeronautics and Space Administration (NASA). The U.S. Government has certain rights in the invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending, commonly-assigned U.S. patent application (application Ser. No. 13/014,434), entitled “MIXER ASSEMBLY FOR A GAS TURBINE ENGINE,” filed on the date of filing of the present application, and is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates generally to combustors for gas turbine engines and more particularly to mixer assemblies for gas turbine engines. 
     Gas turbine engines, such as those used to power modern aircraft, to power sea vessels, to generate electrical power, and in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor, and turbine are disposed about a central engine axis with the compressor disposed axially upstream or forward of the combustor and the turbine disposed axially downstream of the combustor. In operation of a gas turbine engine, fuel is injected into and combusted in the combustor with compressed air from the compressor thereby generating high-temperature combustion exhaust gases, which pass through the turbine and produce rotational shaft power. The shaft power is used to drive a compressor to provide air to the combustion process to generate the high energy gases. Additionally, the shaft power is used to, for example, drive a generator for producing electricity, or drive a fan to produce high momentum gases for producing thrust. 
     An exemplary combustor features an annular combustion chamber defined between a radially inboard liner and a radially outboard liner extending aft from a forward bulkhead wall. The radially outboard liner extends circumferentially about and is radially spaced from the inboard liner, with the combustion chamber extending fore to aft between the liners. A plurality of circumferentially distributed fuel injectors are mounted in the forward bulkhead wall and project into the forward end of the annular combustion chamber to supply the fuel to be combusted. Air swirlers proximate to the fuel injectors impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead wall to provide rapid mixing of the fuel and inlet air. 
     Combustion of the hydrocarbon fuel in air in gas turbine engines inevitably produces emissions, such as oxides of nitrogen (NOx), carbon dioxide (CO 2 ), carbon monoxide (CO), unburned hydrocarbons (UHC), and smoke, which are delivered into the atmosphere in the exhaust gases from the gas turbine engine. Regulations limiting these emissions have become more stringent. At the same time, the engine pressure ratio is getting higher and higher for increasing engine efficiency, lowering specific fuel consumption, and lowering carbon dioxide (CO 2 ) emissions, resulting in significant challenges to designing combustors that still produce low emissions despite increased combustor inlet pressure, temperature, and fuel/air ratio. Due to the limitation of emission reduction potential for the rich burn-quick quench-lean burn (RQL) combustor, lean burn combustors, and in particular the piloted lean premixed/partially premixed pre-vaporized combustor (PLPP), have become used more frequently for further reduction of emissions. However, one of the major challenges for the development of PLPP is the requirement to sufficiently premix the injected fuel and combustion air in the main mixer of a mixer assembly within a given mixing time, which is required to be significantly shorter than the auto-ignition delay time. 
     Mixer assemblies for existing PLPP combustors typically include a pilot mixer surrounded by a main mixer with a fuel manifold provided between the two mixers to inject fuel radially into the cavity of the main mixer through fuel injection holes. The main mixer typically employs air swirlers proximate and upstream of the fuel injection holes to impart a swirl to the air entering the main mixer and to provide rapid mixing of the air and the fuel, which is injected perpendicularly into the cross flow of the air atomizing the fuel for mixing with the air. The level of atomization and mixing in this main mixer configuration is largely dependent upon the penetration of the fuel into the air, which in turn is dependent upon the ratio of the momentum of the fuel to the momentum of the air. As a result, the degree of atomization and mixing may vary greatly for different gas turbine engine operating conditions (e.g., low power conditions where there is poor atomization and mixing may result in higher emissions than high power conditions where there is better atomization and mixing). In addition, since the fuel injection holes are typically located downstream of the point where the air swirlers produce the maximum turbulence, the degree of atomization and mixing is not maximized, increasing the amount of emissions. Furthermore, since the fuel injection holes are typically located downstream of the air swirlers, the risk of flashback, flame holding and autoignition greatly increases due to the low velocity regions associated with fuel jets and walls. A highly possible source for flashback, flame holding and autoignition in the typical main mixer is caused by a wake region that can form downstream of the fuel injection holes where injected fuel that has not sufficiently penetrated into the cross flow of the air (e.g., when air is flowing at low velocity) will gather and potentially ignite. Another possible source is related to boundary layers along the wall, which is thickened by fuel jets due to reduced velocity. 
     BRIEF SUMMARY OF THE INVENTION 
     A mixer assembly for a gas turbine engine is provided, including a main mixer with fuel injection holes located between at least one radial swirler and at least one axial swirler, wherein the fuel injected into the main mixer is atomized and dispersed by the air flowing through the radial swirler and the axial swirler. This configuration reduces the dependence upon the ratio of the momentum of the fuel to the momentum of the air, increases the degree of atomization and mixing by injecting the fuel at a point of high turbulence, and reduces the potential for flame holding by reducing the potential for forming a wake region and lengthening the potential mixing distance. 
     According to one embodiment, a mixer assembly for a gas turbine engine is provided. The mixer assembly includes a main mixer comprising an annular inner radial wall, an annular outer radial wall surrounding at least a portion of the annular inner radial wall, wherein the annular outer radial wall incorporates a first outer radial wall swirler with a first axis oriented substantially radially to a centerline axis of the mixer assembly, a forward wall substantially perpendicular to and connecting the annular inner radial wall and the annular outer radial wall forming an annular cavity, wherein the forward wall incorporates a first forward wall swirler with a second axis oriented substantially axially to the centerline axis of the mixer assembly, and a plurality of fuel injection holes in the forward wall between the first outer radial wall swirler and the first forward wall swirler, wherein the first outer radial wall swirler is on a first side of the plurality of fuel injection holes and the first forward wall swirler is on a second side of the plurality of fuel injection holes. 
     In another embodiment, a mixer assembly for a gas turbine engine is provided. The mixer assembly includes a main mixer comprising an annular inner radial wall, an annular outer radial wall surrounding at least a portion of the annular inner radial wall, wherein the annular outer radial wall incorporates a plurality of outer radial wall swirlers with a first axis oriented substantially radially to a centerline axis of the mixer assembly, a forward wall substantially perpendicular to and connecting the annular inner radial wall and the annular outer radial wall forming an annular cavity, wherein the forward wall incorporates a first forward wall swirler with a second axis oriented substantially axially to the centerline axis of the mixer assembly, and a plurality of fuel injection holes in the forward wall between the plurality of outer radial wall swirlers and the first forward wall swirler, wherein the plurality of outer radial wall swirlers is on a first side of the plurality of fuel injection holes and the first forward wall swirler is on a second side of the plurality of fuel injection holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the disclosure, reference will be made to the following detailed description which is to be read in connection with the accompanying drawing, wherein: 
         FIG. 1  is a schematic diagram of an exemplary embodiment of a gas turbine engine. 
         FIG. 2  is a partial perspective view of an exemplary embodiment of a combustor of a gas turbine engine. 
         FIG. 3  is an enlarged partial perspective view of an exemplary embodiment of a mixer assembly for the exemplary combustor of  FIG. 2 . 
         FIG. 4  is an enlarged partial perspective view of another exemplary embodiment of a mixer assembly for the exemplary combustor of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of an exemplary embodiment of a gas turbine engine  10 . The gas turbine engine  10  is depicted as a turbofan that incorporates a fan section  20 , a compressor section  30 , a combustion section  40 , and a turbine section  50 . The combustion section  40  incorporates a combustor  100  that includes a plurality of fuel injectors  150  that are positioned annularly about a centerline  2  of the engine  10  upstream of the turbines  52 ,  54 . Throughout the application, the terms “forward” or “upstream” are used to refer to directions and positions located axially closer toward a fuel/air intake side of a combustion system than directions and positions referenced as “aft” or “downstream.” The fuel injectors  150  are inserted into and provide fuel to one or more combustion chambers for mixing and/or ignition. It is to be understood that the combustor  100  and fuel injector  150  as disclosed herein are not limited in application to the depicted embodiment of a gas turbine engine  10 , but are applicable to other types of gas turbine engines, such as those used to power modern aircraft, to power sea vessels, to generate electrical power, and in industrial applications. 
       FIG. 2  is a partial perspective view of an exemplary embodiment of a combustor  100  of a gas turbine engine  10 . The combustor  100  is positioned between the compressor section  30  and the turbine section  50  of a gas turbine engine  10 . The exemplary combustor  100  includes an annular combustion chamber  130  bounded by an inner (inboard) wall  132  and an outer (outboard) wall  134  and a forward bulkhead wall  136  spanning between the walls  132 ,  134  at the forward end of the combustor  100 . The bulkhead wall  136  of the combustor  100  carries a plurality of mixer assemblies  200 , including the fuel nozzle  152  of a fuel injector  150 , a main mixer  220 , and a pilot mixer  210 . It will be understood that, although only a single mixer assembly  200  is shown in  FIG. 2  for illustrative purposes, the combustor  100  may include a plurality of mixer assemblies  200  circumferentially distributed and mounted at the forward end of the combustor  100 . A number of sparkplugs (not shown) are positioned with their working ends along a forward portion of the combustion chamber  130  to initiate combustion of the fuel and air mixture. The combusting mixture is driven downstream within the combustor  100  along a principal flowpath  170  toward the turbine section  50  of the engine  10 . The fuel and air provided to the pilot mixer  210  produce a primary combustion zone  110  within a central portion of the combustion chamber  130 . The fuel and air provided to the main mixer  220  produce a secondary combustion zone  120  in the combustion chamber  130  that is radially outwardly spaced from and concentrically surrounds the primary combustion zone  110 . 
       FIG. 3  is an enlarged partial perspective view of an exemplary embodiment of the mixer assembly  200  for the exemplary combustor  100  of  FIG. 2 . The exemplary mixer assembly  200  includes a main mixer  220  and a pilot mixer  210 . The pilot mixer  210  and the main mixer  220  are concentrically arranged with the pilot mixer  210  located in the center of the main mixer  220 , which surrounds a portion of the pilot mixer  210 . The mixer assembly  200  has a centerline axis  218 . The pilot mixer  210  includes an annular pilot mixer housing  212  separating and sheltering the pilot mixer  210  from the main mixer  220 . The main mixer  220  further includes an annular main mixer outer radial wall  222  radially surrounding a portion of the annular pilot mixer housing  212 , the outer surface of which forms an annular main mixer inner radial wall  219 , and a main mixer forward wall  224  substantially perpendicular to and connecting the annular main mixer outer radial wall  222  and the annular main mixer inner radial wall  219 , forming a main mixer annular cavity  228 . The annular main mixer outer radial wall  222  further incorporates a first outer radial wall swirler  240 , while the main mixer forward wall  224  further incorporates a first forward wall swirler  230  and a plurality of fuel injection holes  226  circumferentially distributed between the first outer radial wall swirler  240  and the first forward wall swirler  230  around the main mixer forward wall  224 . Although shown proximate to the first outer radial wall swirler  240  in the main mixer forward wall  224 , the fuel injection holes  226  can be located proximate the first forward wall swirler  230  in the main mixer forward wall  224  as well. The fuel injection holes  226  are in flow communication with a fuel manifold (not shown), which in turn is in flow communication with a fuel supply. Although described with respect to liquid fuel, the exemplary embodiments of mixer assemblies  200  can also be used with gaseous fuel or partially vaporized fuel. As can be seen in  FIG. 3 , the first outer radial wall swirler  240  is positioned on a first side of the fuel injection holes  226 , while the first forward wall swirler  230  is positioned on a second side of the fuel injection holes  226 . In one embodiment, the first side is substantially opposite of the second side. 
     The first outer radial wall swirler  240  is incorporated into the annular main mixer outer radial wall  222  and has an axis  248  oriented substantially radially to the centerline axis  218  of the mixer assembly  200 . The first forward wall swirler  230  is incorporated into the main mixer forward wall  224  and is oriented substantially parallel or axially to the centerline axis  218  of the mixer assembly  200 . The swirlers  230 ,  240  each have a plurality of vanes for swirling air traveling through the swirlers to mix the air and the fuel dispensed by the fuel injection holes  226 . The first outer radial wall swirler  240  includes a first plurality of vanes  242  forming a first plurality of air passages  244  between the vanes  242 . The vanes  242  are oriented at an angle with respect to axis  248  to cause the air to rotate in the main mixer annular cavity  228  in a first direction (e.g., clockwise). The first forward wall swirler  230  includes a second plurality of vanes  232  forming a second plurality of air passages  234  between the vanes  232 . The vanes  232  are oriented at an angle with respect to the centerline axis  218  to cause the air to rotate in the main mixer annular cavity  228  in a second direction (e.g., counterclockwise). 
     In the exemplary embodiment of the main mixer  220  shown in  FIG. 3 , the air flowing through the first outer radial wall swirler  240  will be swirled in a first direction and the air flowing through the first forward wall swirler  230  will be swirled in a direction substantially opposite of the first direction. Also, in the exemplary embodiment of the main mixer  220  shown in  FIG. 3 , the air flowing through the first outer radial wall swirler  240  has an axis  248  oriented substantially radially to the centerline axis  218  of the mixer assembly  200 , while the air flowing through the first forward wall swirler  230  has an axis oriented substantially axially to the centerline axis  218  of the mixer assembly  200 . In this configuration, the fuel is injected through the fuel injection holes  226  between the radial first outer radial wall swirler  240  and the axial first forward wall swirler  230 . In one embodiment, the fuel is injected through the fuel injection holes  226  that are oriented substantially perpendicularly to axis  248  and the flow of air from the radial first outer radial wall swirler  240 , which atomizes and disperses the fuel. The fuel then is atomized and dispersed again by the flow of air from the axial first forward wall swirler  230 , thus atomizing the fuel by airflow from two sides. Although shown proximate to the first outer radial wall swirler  240  in the main mixer forward wall  224 , the fuel injection holes  226  can be located proximate the first forward wall swirler  230  in the main mixer forward wall  224  and be oriented substantially perpendicularly to the axis of the first forward wall swirler  230  and the flow of air from the radial first forward wall swirler  230 , which atomizes and disperses the fuel. The fuel then is atomized and dispersed again by the flow of air from the axial first outer radial wall swirler  240 , thus atomizing the fuel by airflow from two sides. In either configuration, an intense mixing region  229  of fuel and air is created within annular main mixer cavity  228  axially adjacent to the fuel injection holes  226 , allowing the majority of fuel and air to be mixed before entering the downstream end of the annular main mixer cavity  228 . This configuration reduces the dependence upon the ratio of the momentum of the fuel to the momentum of the air, increases the degree of atomization and mixing by injecting the fuel at a point of high turbulence, and reduces the potential for flame holding by reducing the potential for forming a wake region and lengthening the potential mixing distance. The configuration of the vanes in the swirlers may be altered to vary the swirl direction of air flowing and are not limited to the exemplary swirl directions indicated. Furthermore, the number of radial and axial swirlers can be modified (e.g., the first outer radial wall swirler  240  can be replaced by a plurality of radial swirlers and the first forward wall swirler  230  can be replaced by a plurality of axial swirlers). 
       FIG. 4  is an enlarged partial perspective view of another exemplary embodiment of the mixer assembly  200  for the exemplary combustor  100  of  FIG. 2 . As in  FIG. 3 , the exemplary mixer assembly  200  includes a main mixer  220  and a pilot mixer  210 . The pilot mixer  210  includes an annular pilot mixer housing  212  separating and sheltering the pilot mixer  210  from the main mixer  220 . The main mixer  220  further includes an annular main mixer outer radial wall  222  radially surrounding a portion of the annular pilot mixer housing  212 , the outer surface of which forms an annular main mixer inner radial wall  219 , and a main mixer forward wall  224  substantially perpendicular to and connecting the annular main mixer outer radial wall  222  and the annular main mixer inner radial wall  219 , forming a main mixer annular cavity  228 . The annular main mixer outer radial wall  222  further incorporates a plurality of outer radial wall swirlers, including a first outer radial wall swirler  270 , a second outer radial wall swirler  280 , and a third outer radial wall swirler  290 , while the main mixer forward wall  224  further incorporates a plurality of forward wall swirlers, including a first forward wall swirler  250 , a second forward wall swirler  260 , and a plurality of fuel injection holes  226  circumferentially distributed between the second forward wall swirler  260  and the first outer radial wall swirler  270  around the main mixer forward wall  224 . Although shown proximate to the first outer radial wall swirler  270  in the main mixer forward wall  224 , the fuel injection holes  226  can be located proximate the second forward wall swirler  260  in the main mixer forward wall  224  as well. The fuel injection holes  226  are in flow communication with a fuel manifold (not shown), which in turn is in flow communication with a fuel supply. Although described with respect to liquid fuel, the exemplary embodiments of mixer assemblies  200  can also be used with gaseous fuel or partially vaporized fuel. As can be seen in  FIG. 4 , the first, second, and third outer radial wall swirlers  270 ,  280 ,  290  are positioned on a first side of the fuel injection holes  226 , while the first and second forward wall swirlers  250 ,  260  are positioned on the second side of the fuel injection holes  226 . In one embodiment, the first side is substantially opposite of the second side. 
     The first, second, and third outer radial wall swirlers  270 ,  280 ,  290  are incorporated into the annular main mixer outer radial wall  222  and each have an axis  248  oriented substantially radially to the centerline axis  218  of the mixer assembly  200 . The first and second forward wall swirlers  250 ,  260  are incorporated into the main mixer forward wall  224  and are oriented substantially parallel or axially to the centerline axis  218  of the mixer assembly  200 . Swirlers  250 ,  260 ,  270 ,  280 ,  290  each have a plurality of vanes for swirling air traveling through the swirlers to mix the air and the fuel dispensed by the fuel injection holes  226 . 
     The first outer radial wall swirler  270  includes a first plurality of vanes  272  forming a first plurality of air passages  274  between the vanes  272 . The vanes  272  are oriented at an angle with respect to axis  248  to cause the air to rotate in the main mixer annular cavity  228  in a first direction (e.g., clockwise). The second outer radial wall swirler  280  includes a second plurality of vanes  282  forming a second plurality of air passages  284  between the vanes  282 . The vanes  282  are oriented at an angle with respect to axis  248  to cause the air to rotate in the main mixer annular cavity  228  in a second direction (e.g., counterclockwise). The third outer radial wall swirler  290  includes a third plurality of vanes  292  forming a third plurality of air passages  294  between the vanes  292 . The vanes  292  are oriented at an angle with respect to axis  248  to cause the air to rotate in the main mixer annular cavity  228  in a third direction. In one embodiment, the third direction can be substantially the same as the first direction which are substantially opposite of the second direction. 
     The first forward wall swirler  250  includes a fourth plurality of vanes  252  forming a fourth plurality of air passages  254  between the vanes  252 . The vanes  252  are oriented at an angle with respect to the centerline axis  218  to cause the air to rotate in the main mixer annular cavity  228  in a fourth direction (e.g., counterclockwise). The second forward wall swirler  260  includes a fifth plurality of vanes  262  forming a fifth plurality of air passages  264  between the vanes  262 . The vanes  262  are oriented at an angle with respect to the centerline axis  218  to cause the air to rotate in the main mixer annular cavity  228  in a fifth direction (e.g., clockwise). In one embodiment, the fourth direction is substantially opposite of the fifth direction. 
     In the exemplary embodiment of the main mixer  220  shown in  FIG. 4 , the clockwise air passing through the first outer radial wall swirler  270  and the third outer radial wall swirler  290  counter-rates against the counterclockwise air passing through the second outer radial wall swirler  280 , increasing the turbulence, which improves mixing. Also, the counterclockwise air passing through the first forward wall swirler  250  counter-rates against the clockwise air passing through the second forward wall swirler  260 , increasing the turbulence, which improves mixing. In addition, the air flowing through the first, second, and third outer radial wall swirlers  270 ,  280 ,  290  has an axis  248  oriented substantially radially to the centerline axis  218  of the mixer assembly  200 , while the air flowing through the first and second forward wall swirlers  250 ,  260  has an axis oriented substantially axially to the centerline axis  218  of the mixer assembly  200 . In this configuration, the fuel is injected through the fuel injection holes  226  between the radial first, second, and third outer radial wall swirlers  270 ,  280 ,  290  and the axial first and second forward wall swirlers  250 ,  260 . 
     In one embodiment, the fuel is injected through the fuel injection holes  226  that are oriented substantially perpendicularly to axis  248  and the flow of air from the plurality of outer radial wall swirlers (first, second, and third outer radial wall swirlers  270 ,  280 ,  290 ), which atomizes and disperses the fuel. The fuel then is atomized and dispersed again by the flow of air from the plurality of forward wall swirlers (first and second forward wall swirlers  240 ,  250 ), thus atomizing the fuel by airflow from two sides. Although shown proximate to the plurality of outer radial wall swirlers  270 ,  280 ,  290  in the main mixer forward wall  224 , the fuel injection holes  226  can be located proximate the plurality of forward wall swirlers  250 ,  260  in the main mixer forward wall  224  and be oriented substantially perpendicularly to the axis and the flow of air from the plurality of forward wall swirlers  250 ,  260 , which atomizes and disperses the fuel. The fuel then is atomized and dispersed again by the flow of air from the plurality of outer radial wall swirlers  270 ,  280 ,  290 , thus atomizing the fuel by airflow from two sides. In either configuration, an intense mixing region  229  of fuel and air is created within annular main mixer cavity  228  axially adjacent to the fuel injection holes  226 , allowing the majority of fuel and air to be mixed before entering the downstream end of the annular main mixer cavity  228 . The number of axial swirlers, the number of radial swirlers, and the configuration of the vanes in the swirlers may be altered to vary the swirl direction of air flowing and are not limited to the exemplary swirl directions indicated. 
     The terminology used herein is for the purpose of description, not limitation. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as basis for teaching one skilled in the art to employ the present invention. While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. Those skilled in the art will also recognize the equivalents that may be substituted for elements described with reference to the exemplary embodiments disclosed herein without departing from the scope of the present invention. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims.