Patent Publication Number: US-6339923-B1

Title: Fuel air mixer for a radial dome in a gas turbine engine combustor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to provisional applications having Ser. No. 60/103,652 filed Oct. 9, 1998 and Ser. No. 60/103,649, filed Oct. 10, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The U.S. Government may have certain rights in this invention pursuant to contract number NAS3-27235, NAS3-26617, and/or NAS3-25951. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to combustors in gas turbine engines and, in particular, to a fuel air mixer configured for use in a dome of a gas turbine engine combustor oriented substantially perpendicular to a longitudinal axis through the combustor. 
     It will be appreciated that emissions are a primary concern in the operation of gas turbine engines, particularly with respect to the impact on the ozone layer by nitrous oxides (NOx), carbon monoxide (CO), and hydrocarbons. In the case of supersonic commercial transport aircraft flying at high altitudes, current subsonic aircraft technology is not applicable given the detrimental effects on the stratospheric ozone. Accordingly, new fuel injection and mixing techniques have been and continue to be developed in order to provide ultra-low NOx at all engine operating conditions. 
     In response to such emissions concerns, a new combustor has been developed and is discussed in a patent application entitled “Multi-Stage Radial Axial Gas Turbine Engine Combustor,”which is filed concurrently herewith by the assignee of the present invention, has Ser. No. 09/898,557, and is hereby incorporated by reference. A key component found to provide extremely low levels of NOx at moderate to high power conditions for such aircraft engine was the use of a series of simple mixing tubes as the main fuel injection source. It was found, however, that flame stability and emissions characteristics of a combustor incorporating only such mixing tubes was less capable at low power. Thus, it was determined that an independent pilot fuel injector system would be beneficial for such combustor to improve low power flame stability and meet landing-takeoff (LTO) and idle cycle emissions requirements. 
     The use of combustion staging has been in practice within the gas turbine engine art for many years to expand the operational range of combustion systems, as well as to provide a broad range of gas turbine power output and applicability. This has typically been accomplished by staging the fuel in a plurality of fuel air mixing devices or modulating the mixing devices independently. In addition, air staging has been performed by having separate and/or isolated annular or cannular combustion zones that can be controlled independently to provide low emissions and a broad range of operation. To date, however, such staging by pilot and main combustion zones has been within substantially the same annular plane. 
     In light of the foregoing, it would be desirable for a fuel air mixer to be developed which is configured for use in a dome oriented substantially perpendicular to a longitudinal axis through the combustor. It would also be desirable for such fuel a air mixer to be constructed so as to employ a cooling scheme which also improves fuel/air mixing and assists in lowering the fuel-air ratio of the premixture provided to the combustion region of such dome. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a fuel air mixer for a gas turbine engine combustor having a longitudinal axis therethrough is disclosed, wherein the fuel air mixer is configured for use in a dome oriented substantially radial to the longitudinal axis. The fuel air mixer includes a fuel injection assembly having a first end, a second end, a fuel passage extending therethrough, and a flange portion having a plurality of spaced openings formed therein which extends from the first end. The fuel air mixer also includes a first end, a second end, a cavity formed in a central portion thereof, and a flange portion having a plurality of spaced openings formed therein which extends from the first end. The mixer assembly is configured to receive the fuel injection assembly in the cavity so that the fuel injection assembly and the mixer assembly are able to be connected to an outer casing of the combustor by means of the respective flange portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic longitudinal cross-sectional view of a gas turbine engine combustor including a fuel air mixer in accordance with the present invention; 
     FIG. 2 is a detailed longitudinal cross-sectional view of the multi-stage radial axial combustor depicted in FIG. 1 including a fuel air mixer positioned in the radial dome in accordance with the present invention; 
     FIG. 3 is an enlarged, cross-sectional view of the radial dome and fuel air mixer depicted in FIGS. 1 and 2; 
     FIG. 4 is an exploded view of the fuel air mixer depicted in FIGS. 2 and 3; and, 
     FIG. 5 is a top view of the fuel air mixer depicted in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings in detail, wherein identical numerals indicate the same elements throughout the figures, FIGS. 1 and 2 depict a gas turbine engine combustor identified generally by reference numeral  10 . As seen therein, combustor  10  has a longitudinal axis  12  extending therethrough and includes an outer liner  14 , an inner liner  16 , a first or pilot dome  18  positioned immediately upstream of outer liner  14  to form a first combustion zone  20  radially oriented to longitudinal axis  12 , and a dome plate  22  which is connected to first dome  18  at an outer portion and to inner liner  16  at an inner portion. In this way, a second or main combustion zone  24  is defined by dome plate  22 , outer liner  14  and inner liner  16  which is located substantially perpendicular to first combustion zone  20 . This combustor design is known as a multi-stage radial axial (MRA) and is discussed in greater detail in the 557 patent application entitled “Multi-Stage Radial Axial Gas Turbine Engine Combustor,” incorporated hereinabove by reference. 
     As indicated in the 557 patent application, fuel air mixers  46  are provided within each impingement baffle opening  28  so as to be aligned along an axis  25  of each segment  19  for first dome  18 . Although other configurations of fuel air mixers may be utilized, it is preferred that fuel air mixers  46  have a design similar to the cyclone mixers disclosed in U.S. Pat. Nos. 5,540,056 and 5,444,982, which are hereby incorporated by reference. It will be understood, however, that certain improvements to the cyclone design are discussed herein, particularly with regard to its application in a radial dome configuration. 
     It will be seen from FIGS. 3 and 4 that fuel air mixer  46  preferably includes a fuel injection assembly  92 , a mixer assembly  94 , and a heat shield  96  which work in concert to provide a fuel air mixture  98  to first dome  18  while maintaining desired air flow therefrom to assist in cooling and preventing boundary conditions from forming. More specifically, fuel injection assembly  92  includes an elongated fuel stem  100  which extends along axis  25  from a first end  102  to a second end  104  and has a passage  106  therein. It will be noted that the diameter of fuel stem  100  is reduced at about a midpoint thereof to second end  104 , where an end wall  108  is provided adjacent second end  104  so as to terminate passage  106 . Further, a flange portion  110  extends radially outward from axis  25  adjacent first end  102  thereof and includes a plurality of openings  112  therein. A fuel inlet  114  is provided adjacent first end  102  of fuel stem  100  which is in flow communication with passage  106 . It will be understood from FIG. 1 that fuel inlet  114  is connected to a fuel supply  116 . A plurality of fuel injectors  118  are positioned within corresponding radial openings  119  located adjacent second end  104  of fuel stem  100 , wherein fuel injectors  118  are in flow communication with passage  106 . Accordingly, fuel enters fuel air mixer  46  at fuel inlet  114 , flows through passage  106  until it is injected radially through fuel injectors  118 , is mixed with an air flow through swirlers  42 , and provided to first dome  18  as premixture  98 . 
     Mixer assembly  94  includes an elongated mixer tube  120  which extends from a first end  122  to a second end  124  and forms a cavity  126  in conjunction with an end wall  128 . It will be appreciated that mixer tube  120  is preferably configured so that cavity  126  is able to receive a majority of fuel stem  100  therein. Further, a first plurality of openings  130  are formed in mixer tube  120  approximately midway the length thereof for receiving air flow supplied to outer annular passageway  68 . Openings  130  are in flow communication with an annular passage  132  formed by fuel stem  100  and mixer tube  120  which supplies air to the fuel injected by fuel injectors  118 . Of course, a second plurality of openings  134  are provided in mixer tube  120  adjacent second end  124  thereof, where such openings  134  are aligned with fuel injectors  118  when fuel stem  100  is positioned in mixer tube  120 . It will further be seen that a flange portion  136  extends radially out from mixer tube  120  adjacent first end  122  and is configured so that fuel stem flange portion  110  lies in substantially abutting relation therewith. A plurality of openings  137  are provided in flange portion  136  which may be aligned with openings  112  in fuel stem flange portion  110 . 
     Heat shield  96  is preferably attached to a lower portion of mixer tube  120  and includes a substantially annular wall  138  with an end wall  140  located across a bottom of annular wall  138  so as to form a cavity  142  therein. It will be seen in FIGS. 3 and 4 that a plurality of openings  144  are formed therein in a position so that they align with second openings  134  of mixer tube  120 . Heat shield  96  and mixer tube  120  are then preferably connected by means of a plurality of tubes  146  inserted through openings  134  and  144 . Tubes  146  are then brazed to heat shield openings  144 , but left to form a slip joint with mixer tube openings  134  to allow for movement of mixer tube  120 . It will be appreciated that tubes  146  are positioned so as to align with fuel injectors  118 , and although not shown, fuel injectors  118  may be positioned within tubes  146 . Air entering through openings  130  and traveling down annular passage  132  then exits through tubes  146  and mixes with the fuel provided by injectors  118 . 
     A flow passage  148  is formed by annular wall  138  of heat shield  96  and a portion of mixer tube  120 , where flow passage  148  is in flow communication with air flow provided to outer annular passageway  68  so as to provide air to cavity  142 . An impingement baffle  150  is preferably provided within cavity  142  so as to meter the air flow to end wall  140 . In this way, the air flow into cavity  142  is able to assist in cooling heat shield end wall  140 , although end wall  140  preferably includes a thermal barrier coating applied thereto as indicated by reference numeral  152 . It will also be seen that a plurality of openings  154  are formed in end wall  140  to release spent cooling air from a cavity  143  in flow communication with cavity  142 . The spent cooling air is injected into first combustion zone  20 , where it improves mixing, helps prevent flashback into throat area  60 , and further lowers the fuel-air ratio of premixture  98  entering first combustion zone  20 . Additional openings  156  may be provided within a portion of annular wall  138  (preferably below impingement baffle  150 ) so as to improve fuel/air mixing through throat area  60 . 
     In order for fuel air mixers  46  to be properly aligned with each impingement baffle opening  28 , they are preferably connected to outer casing  70  by means of a mechanical connection with flange portions  110  and  136  of fuel stem  100  and mixer tube  120 , respectively. This is accomplished by means of bolts  158  or other similar devices provided in the aforementioned plurality of openings  112  and  137  formed in flange portions  110  and  136 . In this way, fuel air mixers  46  may be removed for a maintenance purposes without teardown of combustor  10 . Because openings  112  and  137  are typically provided in symmetrical relation about their respective flange portions, an additional opening  160  and  162  is formed in flange portions  110  and  136  so as to ensure proper alignment and orientation of openings  134  and fuel injectors  118  (see FIG.  4 ). Alternatively, fuel stem  100  and mixer tube  120  may be manufactured with the same number of bolt openings as openings  134  and fuel injectors  118 , and be positioned in the same respective circumferential locations. In any event, the connection of flanges  110  and  136  (apart from combustor casing  70 ) by a mechanical connection through additional openings  160  and  162  permits fuel air mixers  46  to be removed as a whole (as opposed to fuel injection assembly  92  and mixer assembly  94  separately) from combustor  10  after bolts  158  have been removed. 
     It will also be appreciated that fuel air mixers  46  are sized with respect to a swirler assembly  36  positioned in each baffle opening  28  so as to permit a minimal gap  50  (see FIG. 3) between fuel air mixers  46  and an outer ring portion  38  thereof. Gap  50  not only accounts for thermal growth of outer ring portion  38  and fuel air mixer  46 , but movement of first dome  18  relative to outer casing  70 . Gap  50  also allows air to be injected therethrough which assists in blowing out a recirculation zone bounded by swirler assembly  36  and fuel air mixer  46 . 
     In operation, fuel air mixer  46  receives fuel through fuel inlet  114  from a pilot supply tube  254  in flow communication with fuel supply  116  (shown in FIG.  1 ), enters passage  106  in fuel stem  100 , and exits passage  106  by injection through fuel injectors  118 . The fuel is mixed with air supplied from outer annular passageway  68 , which enters annular passage  132  via first mixer tube openings  130 . A fuel air mixture  98  is then injected through tubes  146  connecting openings  134  and  144  in mixer tube  120  and heat shield  96 , respectively. Fuel air mixture  98  is swirled by air flowing through swirlers  42  and rotationally flows through throat area  60  into first combustion region  20 . Of course, fuel air mixture  98  is also influenced by air flowing through openings  154  and  156  in heat shield end wall  140  and heat shield annular wall  138 , as well as liner segment openings  74 . With respect to the former, it is seen that air is provided by means of a flow passage  148 , which is in flow communication with air flow in outer annular passageway  68  and heat shield cavities  142  and  143 . Thus, it will be appreciated that fuel air mixer  46  has a dual air flow circuit therethrough. 
     Having shown and described the preferred embodiment of the present invention, further adaptations of the fuel air mixer for a radial combustor dome can be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the invention.