Abstract:
The present invention provides a premix fuel injector for use in gas turbine engines and in combustion systems having controllable pressure drops. The premix fuel injector comprises a premix chamber having an inlet for receiving a flow of pressurized air and having an exit. A venturi is coupled to the exit of the premix chamber and an inlet of a combustion chamber. Gaseous fuel is flowed into the premix chamber by a plurality of circumferentially disposed tubes extending into the premix chamber with each of said tubes having at least one hole for flowing a stream of the gaseous fuel.

Description:
TECHNICAL FIELD 
     This invention relates generally to low emission combustion systems used in gas turbine engines and in particular to fuel injectors for use in such systems. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines of the type used for industrial applications may employ combustor systems designed to minimize nitrogen oxide emissions. One such combustor system, disclosed in U.S. Pat. No. 5,481,866, entitled Single Stage Premixed Constant Fuel/Air Ratio Combustor, issued to Mowill on Jan. 9, 1996, is incorporated herein by reference to the extent necessary for a full understanding of such a combustor. The &#39;866 patent discloses a combustor having an externally cooled non-perforated combustor liner that receives all combustion air from a venturi shaped premixer. Excess air that does not enter the combustor through the premixer is ducted so as to externally cool the combustor liner, and eventually re-enters the flowpath downstream of the combustion region through dilution ports. An air valve is used to directly control the amount of air supplied to the premixer so as to minimize nitrous oxide emissions at all power settings. The air valve has the effect of indirectly controlling the amount of air routed to the dilution ports. 
     A problem occurs when combustors of the type disclosed in the &#39;866 patent are used in conjunction with an engine having a compressor with a relatively high compression ratio. At low engine power settings, the air valve used to control air to the premixer is mostly closed forcing most of the compressed air through the dilution ports. Although engine power is reduced, the total volume of air being pumped by the compressor at low power or idle settings remains high, resulting in a substantial increase in dilution airflow at reduced power. However, the dilution ports are necessarily sized to provide adequate backflow margin at the lower flow, higher power settings. Thus at reduced power, higher dilution flow conditions, the dilution ports overly restrict the dilution airflow causing a larger than desired pressure drop across the combustor and a loss of engine efficiency. 
     One solution has been to provide a separate apparatus for varying the flow area of the dilution ports at different power settings in addition to an air valve for controlling air to the premixer. A disadvantage is that such apparatus are typically very complex, adding significantly to the total cost of the combustor system. 
     Another solution is disclosed in the copending U.S. patent application Ser. No. 08/966,393, filed Nov. 7, 1997 which is assigned to the assignee of this application. The &#39;393 application discloses a combustor dilution bypass system that includes an air valve and a low pressure drop combustor bypass duct. The air valve simultaneously controls both the supply of air to the premixer, and the amount of air directed into a large bypass duct. Air entering the bypass duct is reintroduced into the gas flowpath as dilution air downstream of the primary combustion zone. At low power settings the air valve directs most of the air to the bypass duct, in effect adding dilution flow to that provided through the fixed area dilution ports, whereby the pressure drop across the combustor may be controlled at an optimal level. 
     Notwithstanding the amount of air being bypassed, to achieve low emission there is a need to have the fuel and air thoroughly mixed in the premix injector prior to the mixture entering the combustion chamber. Failure to mix the fuel and air results in fuel rich and/or fuel lean concentrations in the combustion chamber. These concentrations lead to local flame temperatures that depart from the optimum for the minimum production of carbon monoxide and nitrogen oxides. The eventual burning of these rich concentrations results in the generation of hot regions which produce nitrogen oxides and can damage turbine components downstream of the combustor. The lean concentrations promote incomplete combustion and production of carbon monoxide and unburned hydrocarbons. This is especially a concern where the fuel is a gas as opposed to a liquid. Because a gaseous fuel will have very low momentum when injected, the compressed air with which it needs to mix can in affect trap the gas and prevent it from mixing. 
     Accordingly, a need exists in a low emissions combustor for a premix fuel injector that thoroughly mixes gaseous fuel and air before injecting the mixture into the combustion chamber. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a premix fuel injector that mixes gaseous fuel and air before injecting the mixture into the combustion chamber. 
     Another object of the present invention is to provide a gas turbine engine having such a premix fuel injector. 
     Yet another object of the present is to provide a premix fuel injector for use in a combustion system having controllable pressure drops. 
     The present invention achieves these objects by providing a premix fuel injector with a premix chamber having an inlet for receiving a flow of pressurized air and having an exit. A venturi is coupled to the exit of the premix chamber and an inlet of a combustion chamber. Gaseous fuel is flowed into the premix chamber by a plurality of circumferentially disposed tubes extending into the premix chamber with each of said tubes having at least one hole for flowing a stream of the gaseous fuel. 
     These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a perspective view of a low emissions combustion system with two dilution bypass systems as contemplated by the present invention. 
     FIG. 2 depicts the combustion system of FIG. 1 from a different perspective. 
     FIG. 3 depicts a sectional view one of the dilution bypass systems. 
     FIG. 4 depicts an enlarged fragmentary sectional view of a portion of FIG.  3 . 
     FIG. 5 depicts a perspective view of an air valve. 
     FIG. 6 depicts a partial cut-away perspective view of the air valve of FIG.  5 . 
     FIG. 7 depicts another partial cut-away perspective view of the air valve of FIG.  5 . 
     FIG. 8 depicts a third partial cut-away perspective view of the air valve of FIG.  5 . 
     FIG. 9 depicts a transverse sectional view of the combustion system of FIG.  1 . 
     FIG. 10 depicts a perspective view of a portion of the combustion system and dilution bypass system of FIG.  1 . 
     FIG. 11 depicts a schematic view of the dilution bypass systems of FIG.  1 . 
     FIG. 12 is a cross section of the premix injector as contemplated by the present invention. 
     FIG. 13 is a perspective view of a portion of the premix injector of FIG.  12 . 
     FIG. 14 is a cross section of an alternative embodiment of the premix injector contemplated by the present invention. 
     FIG. 15 is a view taken along line  15 — 15  of FIG.  14 . 
     FIG. 16 is a cross section of yet another alternative embodiment of the premix injector contemplated by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 the bypass system of the subject invention is indicated generally by the numeral  10 . The bypass system  10  includes an air valve  12  connected to a combustor bypass  13 . In the preferred embodiment, two bypass systems  10  are used, one on each side of the combustor and spaced about 180 degrees apart. A different number or arrangement of bypass systems than what is shown here may be preferable depending on the particular engine and application. 
     Referring to FIGS. 2 through 4, the air valve  12  comprises a cylindrical housing  14  defining an inlet port  16 , and two exit ports  18  and  20 . Inlet port  16  is connected to an inlet duct  17  for receiving compressed air from the combustor plenum  19  that circumscribes the combustion chamber  60  which is defined by a combustor wall  61 . Exit port  18  connects to the premixer duct  22  which leads to the premixer injector  100  that injects tangentially a mixture of fuel and air into the combustion chamber  60 . Exit port  20  connects to the bypass duct  24 . The air valve  12  includes a crescent shaped rotatable valve rotor  26  for selectively controlling the relative proportions of airflow to premixer duct  22  and bypass duct  24 . 
     This flow distributing or dividing function of the air valve can be best visualized by referring to FIGS. 3 and 4. As shown in FIG. 4, when valve rotor  26  is in the idle position, (broken line), most of the airflow is directed to bypass duct  24 , and very little is directed to the premixer duct  22 . Conversely, at maximum power condition, (solid line), most of the airflow is directed to the premixer duct  22 , and very little to the bypass duct  24 . FIG. 3 depicts an intermediate power setting wherein the air valve plate  26  is positioned to evenly divide the flow between the premixer duct and bypass duct. As evident from the drawings, the crescent shape of the rotatable valve rotor  26  provides for a smooth and efficient air flowpath from inlet port  16  to either of the exit ports  18  or  20 , particularly at idle and max power conditions. 
     Referring now to FIGS. 5-8, valve  12  further comprises an exchangeable bypass orifice plate  30  replaceably mounted in the exit port  20 . To maintain a constant pressure drop across the combustor and to assure that the right amount of air flows to the premixer injector  64  requires controlling or scheduling the ratio of air supplied to the premixer duct  22  and to the bypass duct  24 . The bypass orifice plate  30  includes a variable width orifice  32  for this purpose. By shaping the orifice  32 , the ratio of the flow areas of the bypass port to the premixer port can be controlled, and thereby control the ratio of air supplied to each. FIGS. 6 through 8 show valve rotor  26  exposing orifice plate  30  to varying degrees for three power settings. FIG. 6 shows the maximum power condition where the orifice plate is covered. FIG. 7 shows an intermediate percent power condition where the orifice plate is approximately half opened. Finally, FIG. 8 shows the low power condition where the orifice plate is fully opened and the flow to the premixer injector  64  is reduced. The shape and dimensions of the orifice plate  32  are selected, in a manner familiar to those skilled in the art, for the particular engine design or installation, or desired pressure drop changes at low power conditions. It should be appreciated that the orifice plate is not essential the present invention. 
     Referring to FIG. 9, compressed air from compressor  70  enters the combustor plenum  19 . As previously described a portion of this air flows from the plenum  19  through the bypass  13 . The bypass  13  further includes an annular bypass manifold  28  which receives air from bypass ducts  24 . A plurality of tubes  34  extend from and connect bypass manifold  28  to the dilution zone  36  of combustor chamber  60 . Together, the air valve  12 , bypass ducts  24 , bypass manifold  28 , and tubes  34  provide a clear flowpath with minimal pressure drop for routing compressed air directly from the compressor exit to the dilution zone  36  in generally the same location has the dilution ports  40  just upstream of a turbine  72 . Independent of the bypassed air, the dilution ports  40  also receive air from plenum  19 . 
     FIG. 11 shows schematically how the two bypass systems  10  operate. At maximum power condition, the path to the bypass  13  is closed off, forcing most of the air to the premixer injector  64  and through the combustor chamber  60 . Any excess air is then indirectly caused to re-enter the gas flowpath through the dilution ports  40  surrounding the dilution zone  36 . Dilution ports  40  are sized for providing efficient flow at this maximum power setting, and so as to produce the desired pressure drop across the combustor. In this condition, the bypass is essentially not utilized. 
     As power is decreased from maximum, air valve  12  is rotated closing off the port  18  leading to the premixer injector. Although fuel flow is substantially reduced at low power conditions, the total airflow volume being pumped by the compressor is not reduced in the same proportion. Thus at low power, to maintain the correct fuel to air ratio in the premixers, the volume of excess air, i.e. air not going to the premixer injector increases dramatically. Were it not for the bypass  13 , all of the excess air would be directed through the dilution ports  40  resulting in a larger than desired pressure drop across the combustor. However by simultaneously opening the alternate path through the bypass duct, the three way air valves allow for the large flow of low power excess air to reach the dilution zone  36  without having to flow through the overly restrictive dilution ports. Rather, the flow is divided, with an appropriate amount flowing through dilution ports  40 , and the majority of the excess air flowing through the bypass. Through use of the bypass orifice plate  30 , the proper distribution of bypass air, to air through ports  40  can be achieved such that the combustor pressure drop is maintained constant for all operating conditions or can be adjusted as desired at low power settings. 
     Referring to FIGS. 12 and 13, the premix injector  100  includes a gaseous fuel injector  104  with a body  106  having flange  108  that is bolted to the premix injector casing  102 . The fuel injector  104  has gas fuel inlet port  112 . The fuel injector also has a commercially available air blast nozzle  116  that injects liquid fuel into the premix chamber  118  along an axial centerline  120  of the premix injector  100 . The fuel injector has an air inlet port  110  which communicates with a plenum  19 , (not shown). This provides air to assist the atomization of the liquid fuel. Mounted to the body  106  and extending into the premix chamber  118  are a plurality of circumferentially disposed fuel injector tubes  122 . Each tube is generally cylindrical and closed at the end disposed in the premix chamber  118 . Each tube  122  also has a plurality of holes  124  which are also referred to as fuel injection ports. The ports  124  are disposed along the length of each of the tubes  122 . Some of the holes are directed towards the centerline while others can be angled away from the centerline  120  in the tangential direction. In the preferred embodiment, there are six tubes  122  equally spaced apart and circumscribing the nozzle  116 . The number and spacing of the tubes as well as the number of holes  124  and their angular position will of course vary from application to application. In a manner familiar to those skilled in the art, the body  106  has internal passages, not shown, for delivering a gaseous fuel from inlet port  112  to each of the tubes  122  and other passages for delivering air and liquid fuel to the air blast nozzle  116 . 
     The premix injector also includes a venturi  126  downstream of the premix chamber  118 . The venturi  126  is a tube that tapers outward as it extends from an inlet to an exit and is symmetric about the centerline  120 . The inlet of the venturi is in fluid communication with the premix chamber  118  and its exit is in fluid communication with the combustion chamber  60 . The venturi has a boss for receiving an igniter  128 , shown in FIG.  3 . 
     In operation, gaseous fuel enters the premix chamber  118  through the tubes  122 . At the same time air enters the premix chamber  118  from the premixer duct  22 . The fuel and air mixing process is completed in the venturi  126  to form a premixed gas that enters the combustion chamber  60 . Because the gaseous fuel entering through the tubes  122  is not concentrated around the centerline  120 , the air entering from duct  22  cannot trap the gas and as result there is improved mixing of the fuel and air. 
     To further enhance the mixing of the fuel and air, a mixing screen  133  can be disposed between the duct  22  and the premix chamber  118 . If the screen  118  is used, the tubes  122  should extend through the screen  118  so that all the holes  124  are downstream of the screen. 
     FIGS. 14 and 15 show an alternative embodiment  130  of the tubes  122 . The tubes  130  are cylindrical but have an angled end  132  disposed in the premix chamber  118 . The angle of the ends  132  is about 33 degrees from the centerline  120 . Each of the ends  132  has a first radial facing hole  134  and two holes  136  angled an equal amount from the radial direction about 20 degrees. The holes  136  are coplanar with each other but not with the hole  134 . 
     FIG. 16 shows another embodiment of the present invention where swirling vanes  140  are mounted to each of the tubes  122  and extend inward therefrom. The gaseous fuel mixes with the air in the passages between the vanes  140  and then flows to the venturi  126 , Besides enhancing fuel-air mixing, the vanes also inhibit flashback of the flame into the premix chamber  118  as a result of improved air feed to the venturi inlet and by the promotion of positive, forward flowing mixture velocities along the venturi wall as a result of the swirl. The vanes  140  can also be used with the embodiment shown in FIGS. 14 and 15. 
     Various modifications and alterations of the above described sealing apparatus will be apparent to those skilled in the art. Accordingly, the foregoing detailed description of the preferred embodiment of the invention should be considered exemplary in nature and not as limiting to the scope and spirit of the invention.