Abstract:
Disclosed is a premix injector for mixing air and fuel and injecting the mixture into a combustion chamber. The premix injector includes an air blast fuel nozzle, a venturi, and a premix chamber. The venturi extends from an inlet to an exit which is in fluid communication with a combustion chamber. Importantly, the venturi has a jet deflector along its interior wall near its exit and on the side of the venturi nearest the exit of the combustor.

Description:
REFERENCE TO COPENDING APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/966,393, filed Nov. 7, 1997 (U.S. Pat. No. 6,070,406) which claims benefit of 60/031,780 filed Nov. 26, 1996. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to low NOx combustion systems for a gas turbine engine. More particularly, the present invention relates to a venturi for mixing fuel and air and injecting the mixture into a combustion chamber of such a system. 
     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 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 a 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. This application discloses a combustor dilution bypass system that includes a valve and a low pressure drop combustor bypass duct. The 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 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. This combustor dilution bypass system is illustrated in FIGS. 1-11 of this application. 
     Referring to FIGS. 1 and 3 there are two premix injectors  64  on each side of the combustor spaced about 180° apart. Each premixer injector  64  injects tangentially a mixture of fuel and air into the combustion chamber  60 . FIG. 12 shows the results from testing of this combustor and reveals two spikes in carbon monoxide generation as a function of angular distance around the combustion chamber  60 . These spikes occurred just downstream of the venturis  70  and indicate areas of unburned fuel. This unburned fuel is believed to be caused by some of the mixture exiting the venturis flowing directly to the dilution zone  36  instead of being mixed and combusted in the combustion chamber. To see how this could happen, viewing FIG. 2, the fuel air mixture flowing from the premixer injectors  64  enters the combustion chamber tangentially and sprays outward upon entering the combustion chamber. Therefore some the mixture flows directly to the dilution zone  36  located at the exit of the combustor and does not mix in with the gas in the combustion chamber. 
     Accordingly, a need exists in a venturi to be used in these premixer injectors that would direct the flow of gas away from the combustor exit. 
     SUMMARY OF THE INVENTION 
     In view of the above it is the object of the present invention to provide a venturi that directs the flow of gas away from the combustor exit. 
     Another object of the present inventions is to combine this venturi in a combustor system designed for low nitrous oxide emissions for a simplified method of reintroducing excess air not used for combustion, back into the flowpath downstream of the combustion zone without the complexity and expense associated with variable area dilution ports. 
     The present invention achieves these objects by providing a valve and a low pressure drop combustor bypass duct. The 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 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. 
     The premixer includes a premix injector in which air and fuel are mixed. The premix injector includes an air blast fuel nozzle, a venturi, and a premix chamber. The venturi extends from an inlet to an exit which is in fluid communication with a combustion chamber. Importantly, the venturi has a jet deflector along its interior wall near its exit and on the side of the venturi nearest the exit of the combustor. 
     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 combustor with two dilution bypass systems of the type contemplated by the present invention. 
     FIG. 2 depicts the combustor of FIG. 1 from a different perspective. 
     FIG. 3 depicts a sectional view through the combustor and one of the dilution bypass system of FIG. 2 along line A—A. 
     FIG. 4 depicts an enlarged fragmentary sectional view of a portion of FIG.  3 . 
     FIG. 5 depicts a perspective view of the valve contemplated by the present invention. 
     FIG. 6 depicts a partial cut-away perspective view of the valve contemplated by the present invention. 
     FIG. 7 depicts another partial cut-away perspective view of the valve contemplated by the present invention. 
     FIG. 8 depicts a third partial cut-away perspective view of the valve contemplated by the present invention. 
     FIG. 9 depicts a transverse sectional view of the combustor of FIG.  1 . 
     FIG. 10 depicts a perspective view of a portion of the combustor and dilution bypass system. 
     FIG. 11 depicts a schematic view of the combustor dilution bypass system. 
     FIG. 12 depicts test data showing CO generation as a function of angular distance around the combustion chamber contemplated by the present invention. 
     FIG. 13 shows a side view of the venturi contemplated by the present invention. 
     FIG. 14 shows a front view of the venturi contemplated by the present invention. 
     FIG. 15 shows a view taken along line  15 — 15  of FIG.  14 . 
    
    
     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 a 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 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  64  that injects tangentially a mixture of fuel and air into the combustion chamber  60 . The injector  64  has an air blast fuel nozzle  66 , a venturi  70 , a premix chamber  68  and an igniter  72 . In operation, the air blast nozzle inject a fuel-air mixture into the premix chamber  68 . In the premix chamber additional air is added through premixer duct  22 . To keep the nitrous oxide as low as possible the fuel air mixture exiting the venturi is as fuel lean as possible. The igniter  72  ignites this lean mixture during engine starting creating a hot gas that flows into the combustion chamber  60 . Exit port  20  connects to the bypass duct  24 . The 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 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 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 a fifty percent power condition where the orifice plate is approximately half opened. Finally, FIG. 8 shows the shut down power condition where the orifice plate is fully opened and there is no flow to the premixer injector  64 . 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. 
     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 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  at the exit of the combustor just upstream of a turbine  82 . 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 reenter 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, valve  12  is rotated closing off the port  18  leading to the premixer injector. Although engine power is substantially reduced at the idle condition, the total airflow volume being pumped by the compressor is not. Thus at idle power, 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 valve allows for the large flow of low power excess air to reach the dilution zone  36  without having to flow through the overly restrictive dilution pods. 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. 13-15, the venturi  70  is comprised of a tube  100  extending from an inlet  102  to an exit  104 . The inlet  102  is symmetric about an axis  106  and has an outer surface  108  raised and rounded about the perimeter of the inlet  102 . The exit  104  is symmetric about an axis  110  that is at an angle relative to axis  106  so that the exit  104  can mate to the spherical surface of the combustion chamber  60 . Between the inlet  102  and exit  104  are a tapered section  112  having a boss  114  for receiving the igniter  72 , a first cylindrical portion  116  and a second cylindrical portion  118 . A rocker tab  120  is formed on the exterior surface of the cylindrical portion  118  and is used in mounting the venturi to the combustion chamber  60 . On the inside of the second cylindrical portion  116  is a jet deflector  122 . The jet deflector  122  extends inward from an interior surface  117  of the first cylindrical portion  116  and is located as close to the exit  104  as possible. The jet deflector is tapered so that one end  124  extends into the flow stream in the venturi  70  and the other end  126  is flush with the interior surface  117 . Referring to FIG. 14 the jet deflector extends circumferentially around the interior surface  117  an angular distance in the range of 60 to 100 degrees. 
     As the fuel air mixture exits the venturi  70 , that part of the flow on the side of the venturi nearest the combustor exit comes in contact with the jet deflector  122  and is redirected towards the combustor inlet where it is more completely combusted. 
     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.