Patent Publication Number: US-6708498-B2

Title: Venturiless swirl cup

Description:
This is a continuation of U.S. application Ser. No. 08/993,861; filed Dec. 18, 1997, now U.S. Pat. No. 6,550,251. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to gas turbine engines, and, more specifically, to combustors therein. 
     In a gas turbine engine, air is compressed in a compressor and mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through one or more turbine stages which extract energy therefrom. Performance of the combustor affects engine efficiency and exhaust emissions. Mixing of fuel and air in turn affects combustor performance, and the prior art is crowded with combustor designs having varying degrees of effectiveness since many tradeoffs are typically required in combustor design. 
     Undesirable exhaust emissions include unburned hydrocarbons, carbon monoxide (CO), and nitrogen oxides (NOx). These exhaust emissions are affected by uniformity of the fuel and air mixture and amount of vaporization of the fuel prior to undergoing combustion. A typical gas turbine engine carburetor which mixes the fuel and air includes a fuel injection nozzle mounted in a swirl cup attached to the upstream, dome end of the combustor. The swirl cup typically includes two rows of swirl vanes which operate either in co-rotation or counter-rotation for swirling air around the injected fuel for forming a suitable fuel and air mixture which is discharged into the combustor for combustion. 
     Gas turbine engine carburetors vary in configuration significantly depending upon the specific engine design, and whether the engine is configured for aircraft propulsion or for marine and industrial (M&amp;I) applications. NOx emissions are typically reduced by operating the combustor with a lean fuel and air mixture. However, lean mixtures typically result in poor low power performance of the combustor, increased CO and HC emissions, and are susceptible to lean flame blowout (LBO), autoignition, and flashback. 
     NOx emissions may also be reduced by configuring the combustor with a multiple dome, such as a double dome having two radially spaced apart rows of carburetors operated in stages. For example, the radially outer carburetors are sized and configured for pilot performance and operate continuously during all modes of engine operation from idle to maximum power. The radially inner carburetors are sized and configured for main operation and are fueled only above idle for higher power operation of the engine. 
     Accordingly, the required amount of fuel for operating the combustor over the different power settings may be selectively split between the outer and inner carburetors for obtaining suitable combustor performance with reduced exhaust emissions. 
     Performance of the combustor is also evaluated by conventional profile factor and pattern factor which indicate relative uniformity of radial and circumferential temperature distribution from the combustion gases at the exit of the combustor which affect efficiency and life of the high pressure turbine which firstly receives the combustion gases from the combustor. 
     A typical swirl cup used in both the outer and inner carburetors includes a tubular member in the form of a venturi disposed between the two rows of swirl vanes. The venturi has two primary purposes including a throat of minimum flow area sized for accelerating the injected fuel and swirl air from a primary row of swirl vanes to a suitably high velocity to reduce carbon formation on the face of the fuel injection nozzle and to prevent the flame front in the combustor from travelling forwardly into the swirl cup toward the fuel nozzle. The venturi also has an inner surface along which the fuel from the nozzle may form a film which may be airblast atomized by the swirl air flowing through the swirl cup. 
     In view of these many related components affecting combustion performance, it is desired to further improve combustor performance due to improved swirl cup design. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A swirl cup for a gas turbine engine combustor includes a tubular body having an inlet at one end for receiving a fuel injection nozzle, an outlet at an opposite end for discharging the fuel, and an annular septum therebetween. A row of first swirl vanes is attached to the septum adjacent the body inlet, and a row of second swirl vanes is attached to the septum adjacent the first swirl vanes and spaced upstream from the body outlet. Air from the first and second swirl vanes is swirled directly around the injected fuel without a flow barrier or venturi therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic axial sectional view through a portion of an exemplary gas turbine engine including a combustor in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is an enlarged elevational, partly sectional view of the dome end of the combustor illustrated in FIG. 1 showing a pair of swirl cups and cooperating fuel injector in accordance with an exemplary embodiment of the present invention. 
     FIG. 3 is an aft-facing-forward view of the swirl cups illustrated in FIG.  2  and taken along line  3 — 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated schematically in FIG. 1 is a portion of an exemplary gas turbine engine  10  which is axisymmetrical about a longitudinal or axial centerline axis  12 . The engine  10  includes a compressor  14  which may take any conventional form for providing compressed air  16  into an annular combustor  18 . The combustor  18  is conventionally configured with a radially outer liner  18   a , a radially inner liner  18   b , and an annular dome  18   c  joined to the upstream ends thereof to define an annular combustor chamber  18   d.    
     In the preferred embodiment, the combustor dome  18  is a double-dome in which are conventionally mounted a row of radially outer or pilot swirl cups  20 , and a row of radially inner or main swirl cups  22  configured in accordance with an exemplary embodiment of the present invention. A common fuel injector  24  includes a pair of radially outer and inner fuel injection nozzles  24   a, b  disposed in respective ones of the outer and inner swirl cups  20 , 22  for injecting fuel  26  therein in a conventional manner. 
     The air  16  and fuel  26  are mixed together in the separate swirl cups  20 , 22  for providing a suitable fuel and air mixture which is discharged into the combustion chamber  18   d  and conventionally ignited for generating hot combustion gases  28  which are discharged from the combustor  18  into a conventional high pressure turbine nozzle  30   a  and cooperating high pressure turbine  30   b . The turbine  30   b  includes a row of turbine blades extending radially outwardly from a rotor disk, with the disk being suitably joined to the compressor  14  for providing power thereto during operation. 
     The combustor  18  illustrated in FIG. 1 is configured with the double dome  18   c  and two rows of swirl cups  20 , 22  for reducing exhaust emissions during operation of the engine from idle to maximum power while obtaining acceptable combustor performance. The fuel injector  24  and outer swirl cups  20  may take any conventional configuration, and cooperate with the inner swirl cups  22  which are suitably modified in accordance with the present invention for further reducing exhaust emissions and further improving performance of the combustor. 
     More specifically, the improved inner swirl cup  22  cooperating with a corresponding one of the outer swirl cups  20  and common fuel injector  24  are illustrated in more particularity in FIG. 2 in accordance with an exemplary embodiment of the present invention. Each of the circumferentially spaced apart inner swirl cups  22  includes a tubular body  32  which is axisymmetric about its own longitudinal or axial centerline axis, and includes an annular inlet  32   a  at a forward or upstream end thereof for receiving the inner fuel nozzle  24   b  and the fuel  26  therefrom. The body  32  also includes an annular outlet  32   b  at an opposite downstream or aft axial end thereof disposed coaxially with the body inlet  32   a  for discharging the fuel  26  into the combustion chamber  18   d . The body  32  also includes an annular septum  32   c  in the form of a flat disk with a central aperture therethrough disposed axially between the body inlet  32   a  and outlet  32   b.    
     Referring to both FIGS. 2 and 3, each of the inner swirl cups  22  further includes means in the form of a first or primary row of circumferentially spaced apart first swirl vanes  34  fixedly attached to the forward face of the septum  32   c  adjacent to the body inlet  32   a  for channeling into the body  32  first swirl air in a first swirl direction, which is counterclockwise for example as shown in FIG. 3 circumferentially around the injected fuel  26 . Means in the form of a second or secondary row of circumferentially spaced apart second swirl vanes  36  are fixedly attached to the aft face of the septum  32   c  downstream from and adjacent to the first swirl vanes  34 , and are spaced upstream from the body outlet  32   b  for channeling into the body  32  additional, or second swirl air in a second swirl direction, also counterclockwise for example as illustrated in FIG. 3, directly around both the injected fuel  26  and the first swirl air. 
     As shown in FIG. 2, the septum  32   c  terminates in accordance with the present invention axially between the first and second swirl vanes  34 , 36  without a radial flow barrier or venturi therebetween for allowing direct and immediate contact between the air discharged from the swirl vanes  34 , 36 . But for the present invention as described in more detail hereinbelow, the inner swirl cups  22  are conventionally configured without a conventional flow barrier or venturi between the swirl vanes  34 , 36 . 
     This is more apparent by examining the cooperating outer swirl cup  20  illustrated in FIG. 2 which is similarly configured in a conventional manner, but includes a tubular venturi  32   d  integrally formed with the radially inner end of the septum  32   c  and extending axially aft therefrom. The venturi  32   d  is defined by an inner surface which converges to a throat of minimum flow area to accelerate flow, and then diverges to its outlet. The outer surface of the venturi is typically straight cylindrical. The venturi accelerates the fuel and first swirl air while radially separating the second swirl air therefrom up to its outlet. 
     In both the outer and inner swirl cups  20 , 22  the first and second swirl vanes  34 , 36  may be formed in a common casting with the main body  32  including the septum  32   c . In this exemplary embodiment, the body  32  also includes an integral forward plate  32   e  commonly cast with the forward ends of the first swirl vanes  34  to provide a conventional mount containing a conventional floating ferrule  38  in which the respective fuel nozzles  24   a,b  are slidably mounted. The bodies  32  themselves are suitably fixedly joined in complementary apertures through the combustor dome  18   c  and may be welded or brazed therein. 
     Since the outer swirl cups  20  are provided for pilot performance of the combustor during all modes of operation from idle to maximum power, they are suitably sized for mixing pilot portions of the fuel  26  with pilot portions of the air  16  through the first and second swirl vanes  34 , 36  thereof. Correspondingly, the inner swirl cups  22  are specifically sized for main performance of the combustor at power setting greater than idle and up to maximum power. Other than size and the absence of the venturi  32   d  in the inner swirl cups  22 , the outer and inner swirl cups  20 , 22  may be similarly configured in a conventional manner. 
     Although some form of the venturi  32   d  or other radial flow barrier between the first and second swirl vanes  34 , 36  is used in conventional combustors, it has been discovered in accordance with the present invention that improved fuel and air mixing with a correspondingly longer premixer residence time in the inner swirl cups  22  may be obtained by eliminating the venturi  32   d  therein. In this way, the air from the second swirl vanes  36  directly and immediately contacts the air from the first swirl vanes  34  and injected fuel  26  therein without the barrier or delay as in the outer swirl cups  20 . Improved fuel atomization and vaporization are obtained in the inner swirl cups  22 , along with improved uniformity of the fuel and air mixture discharged therefrom into the combustion chamber  18   d.    
     The venturiless inner swirl cups  22  illustrated in FIGS. 2 and 3 allow an improved method of operation of the combustor  18  by firstly injecting the fuel  26  into the upstream end of the inner swirl cup  22 . This is followed in turn by firstly swirling a portion of the air  16  in a first swirl direction into the inner swirl cup  22  coaxially around the injected fuel  26 , followed in turn by secondly swirling another portion of the air  16  in a second swirl direction into the inner swirl cup  22  coaxially around both the injected fuel  26  and the firstly swirled air without a radial flow barrier or venturi therebetween. This improves the premixing of the fuel and air inside the inner swirl cups  22 , which mixture is then discharged into the combustion chamber  18   d  for being ignited and undergoing combustion to form the combustion gases  28 . 
     As illustrated in FIGS. 2 and 3, the first and second swirl vanes  34 , 36  are preferably inclined radially inwardly to swirl the air  16  radially inwardly and circumferentially around the injected fuel  26 . This is in contrast to conventional axial swirl vanes which are inclined in the circumferential direction for axially swirling airflow in a manner related to but different than the radial swirling effected by the radial swirl vanes  34 , 36 . However, the invention may be extended to axial swirl vanes if desired. 
     In the preferred embodiment illustrated in FIG. 3, the first and second swirl vanes  34 , 36  are similarly inclined, or co-inclined, for effecting equal first and second swirl directions which are counterclockwise in the FIG. 3 example. In this way, the first and second swirl vanes  34 , 36  swirl the respective air portions radially around the injected fuel  26  in co-rotation. 
     This is in contrast with the orientation of the first and second swirl vanes  34 , 36  of the outer swirl cups  20  as illustrated in FIGS. 2 and 3. In the outer swirl cups  20 , the first and second swirl vanes  34 , 36  are oppositely inclined radially inwardly for effecting counter-rotation of the respective air portions therefrom with opposite first and second swirl directions, with clockwise rotation being illustrated for the first swirl vanes  34  and counterclockwise rotation being illustrated for the second swirl vanes  36  in this exemplary embodiment. 
     Although both counter-rotation and co-rotation swirl vanes are conventional in the art, tests have shown the advantage of co-rotation due to the first and second swirl vanes  34 , 36  of the inner swirl cup  22  in the preferred embodiment. For example, a significant reduction in carbon monoxide (CO) emissions have been confirmed over a significant range of swirler equivalency ratio, or fuel/air ratio, when comparing the inner swirl cups  22  to a baseline or similar design using a conventional venturi like that illustrated for the outer swirl cups  20 . 
     In order to offset the loss of the flow accelerating effect by the missing venturi in the inner swirl cup  22 , the body outlets  32   b  may be suitably reduced in flow area for accelerating the flow therethrough. The body outlets  32   b  are otherwise conventionally configured and include an integral splashplate in a conventional manner. 
     An additional and unexpected advantage of the venturiless swirl cup  22  according to the present invention is attributable to the double dome design illustrated in the Figures. As indicated above, combustor performance is also evaluated on the conventionally known profile factor which is an indication of the radial uniformity of temperature of the combustion gases  28  discharged from the outlet of the combustor  18 . 
     During engine idle, injection of the fuel  26  from the inner nozzles  24   b  into the inner swirl cups  22  is stopped, while the respective air portions through the first and second swirl vanes  34 , 36  in the inner swirl cups  22  continues to flow and simply mixes together without fuel inside the inner swirl cups  22  and without the flow barrier venturi therebetween. During idle, the fuel  26  is injected solely from the outer nozzles  24   a  into the corresponding outer swirl cups  20 , with the fuel and air mixture being ignited for sustaining the combustion process. However, the swirled air from the inner swirl cups  22  continues to mix with the combustion gases  28  during travel through the combustor  18  and improves the profile factor as confirmed by tests. 
     The venturi  32   d  is kept in the outer swirl cups  20  for its conventional benefits including flame stability and lean flame blowout margin. This is particularly important for idle operation since the inner swirl cups  20  are venturiless. 
     As indicated above, combustor performance is evaluated using various evaluation criteria, and tradeoffs in performance are typically required in view of specific combustion and fuel injection designs. The present invention introduces yet another variable in combustor design in eliminating the venturi  32   d  in the inner swirl cups  22  for providing enhanced performance of the combustor including reduction in exhaust emissions such as carbon monoxide, and an improved profile factor in the double-dome configuration disclosed. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.