Abstract:
An augmentor system minimizes acoustic pressure fluctuations on the heat release process within the augmentor by staggering the vanes therein. The alternating axial vane stagger pattern arranges the downstream set of vanes as baffles which prevent the propagation of tangential acoustic waves between the vanes to protect the upstream set of vanes from the effects of transverse acoustic velocity fluctuations which minimizes screech without substantially affecting augmentor performance.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a gas turbine engine augmentor, and more particularly to an augmentor segment which reduces screech. 
   Augmentors, or “afterburners” increase the thrust of a gas turbine engine. Additional thrust is produced within an augmentor when oxygen contained within the core gas flow of the engine is mixed with fuel and burned. In some instances, additional thrust is produced by mixing and burning fuel with cooling or bypass air entering the augmentor through the inner liner of the augmentor shell as well. 
   One type of augmentor includes radially oriented vanes, which are circumferentially disposed around a central tail cone. The vanes include a plurality of fuel distribution apertures positioned on both sides of a line of high-pressure air apertures. The fuel distribution apertures provide fuel distribution and the line of high-pressure air apertures collectively provide pneumatic bluff bodies analogous to prior art mechanical flame holders. 
   Screech is a term known in the art as high-frequency pressure oscillations induced by intense combustion, which, under certain conditions, are generated in the augmentor. Uncontrolled screech reduces the high-cycle fatigue life of the augmentor due to primarily three modes of screech-induced vibration including radial, circumferential, axial, and combinations thereof. 
   Gas turbine engine augmentors typically include cooling liners which provide for shielding the structural augmentor casing from hot augmentor combustion gases, for providing cooling air to an exhaust nozzle disposed at the downstream end of the augmentor and for providing screech suppression. Augmentor combustion efficiency is directly proportional to the amount of available discharge gases utilized in the combustion process. Any quantity of engine discharge gas that is utilized for cooling and screech suppression and not used in the augmentor combustion process decreases augmentor thrust capability and efficiency. 
   Augmentors are relatively long structures when compared to overall engine size and must accommodate relatively high combustion gas temperatures. Conversely, as engine packaging constraints are reduced to minimize thermal and radar signatures, less space is available for the augmentor cooling and screech suppression systems. 
   Accordingly, it is desirable to provide a gas turbine engine augmentor that minimizes screech without substantially affecting augmentor performance. 
   SUMMARY OF THE INVENTION 
   The augmentor section according to the present invention minimizes acoustic pressure fluctuations on the heat release process within the augmentor by staggering the vanes therein. 
   Combustion that occurs downstream of the vanes is affected by the resonant acoustic pressure fluctuations within the augmentor. The acoustic pressure fluctuation typically relate to a “tangential” mode and cause fluctuation in the combustion heat release. When the heat release fluctuations are in-phase with the acoustic pressure fluctuations, the process is unstable and the pressure fluctuations are amplified. 
   Screech is a phenomenon in which the combustion that occurs downstream of the vanes is affected by resonant acoustic pressure fluctuations in the augmentor. These acoustic pressure fluctuations usually relate to a “tangential” acoustic mode and cause fluctuations in the combustion heat release. When these heat release fluctuations are in-phase with the acoustic pressure fluctuations, the process is unstable and the pressure fluctuations are amplified. Large pressure fluctuations result in excessive noise and vibration and decreased component durability. 
   The alternating axial vane stagger pattern arranges the downstream set of vanes as “baffles” to prevent and/or dampen the propagation of tangential acoustic waves between the vanes thereby protecting the flame stabilization location on the upstream set of vanes from the effects of transverse acoustic velocity fluctuations to minimize screech without substantially affecting augmentor performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  is a general diagrammatic sectional view of a gas turbine engine; 
       FIG. 2  is a an expanded diagrammatic view of an augmentor shown from the rear of the engine; 
       FIG. 3  is a “unwrapped” radial view of an augmentor illustrating the axial vane arrangement; 
       FIG. 4  is a schematic view of fuel disbursement from vanes within the augmentor of the present invention; and 
       FIG. 5  is another “unwrapped” radial view of an augmentor designed according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a general schematic view of a gas turbine engine  10 . The gas turbine engine  10  is defined about an engine centerline axis A about which the various engine sections rotate. The engine  10  typically includes a fan section  11 , a compressor section  12 , a combustor  14 , a turbine section  16 , and an augmentor section  18  defined along the engine axis A. It should be understood that although a particular arrangement is disclosed in the illustrated embodiment, other arrangements will benefit from the instant invention. 
   Air entering the fan section  11  is divided between core gas flow  20  and bypass air flow  22 . The core gas flow  20  generally follows a path essentially parallel to the axis A of the engine  10 , through the compressor section  12 , combustor  14 , turbine section  16 , and augmentor section  18 . Bypass air  22  also follows a path parallel to the axis  26  of the engine  10 , passing through an annulus  28  along the periphery of the engine  10 . 
   Core gas flow  20  follows a path initially passing through the compressor section  12  and subsequently through the combustor  14  and turbine section  16 . The core gas flow  20  passes through the augmentor section  12 , where fuel is selectively added, mixed with the flow  20  and burned to impart more energy to the flow  20  and consequently more thrust exiting the nozzle  24  of the engine  10 . 
   Referring to  FIG. 2 , an end view of the augmentor section  18  is illustrated as viewed from the rear of the engine  10 . The augmenter section  18  includes a central cone  30 , a case  32  having an inner lining  34 , an outer wall  36 , and a plurality of circumferentially disposed vanes  38  extending radially outward from the cone  30  to the inner lining  34 . 
   One or more fuel distributors  40  are attached to the outer wall  36  of the case  32 . Fuel feed lines  42  extending from a fuel supply are coupled to the fuel distributors  40  to distribute fuel into each vane  38  and out of a multiple of fuel injection orifices  44 . 
   In operation, when the augmentor section  18  is actuated, fuel is admitted into a fuel distribution system within each of the vanes  38  and exits the multiple of fuel injection orifices  44  to extend out a distance into the core gas flow  20 . After distribution from the vanes  38 , the fuel mixes with the core gas flow  20  and the bypass air  22  introduced in the core gas flow  20 . This mixture is combusted and proceeds downstream to increase the thrust of the engine  10  ( FIG. 1 ). For further understanding of other aspects of the augmentor operation, attention is directed to U.S. Pat. No. 5,685,140 METHOD FOR DISTRIBUTING FUEL WITHIN AN AUGMENTOR which is assigned to the assignee of the instant invention and which is hereby incorporated herein in its entirety. 
   Referring to  FIG. 3 , an “unwrapped” radial view of the vanes  38  is illustrated. The vanes  38  are preferably arranged in an axial displaced arrangement in which a first set of vanes  38   a  are located axially upstream of a second set of vane  38   b . That is, the second set of vanes  38   b  are setback from the first set of vanes  38   a . Preferably, the vanes are arranged in a 2-vane alternating axial stagger pattern. 
   The alternating axial stagger pattern provides numerous benefits. The downstream set of vanes  38   b  operate as “baffles” which prevent the propagation of tangential acoustic waves between the vanes  38  thereby “protecting” the flame anchoring locations on the upstream set of vanes  38   a  from the effects of transverse acoustic velocity fluctuations. The “setback” distance between the trailing edges of the two sets of vanes is preferably arranged such that the heat release responses of the flame systems from the sets of vanes  38   a ,  38   b  are out of phase with each other when subjected to longitudinal velocity fluctuation. It should be understood that the actual distance relationship of the setback is well within the knowledge of one of ordinary skill in the art of augmentor design. 
   Referring to  FIG. 4 , the upstream set of vanes  38   a  include fuel injection orifices  44   a  which provide low fuel jet penetration into the air stream. That is, the fuel injection orifices  44   a  in the first set of vanes exits discharges fuel Fa to extend out a distance from the vane  38   a  less than the distance fuel Fb exits from fuel orifices  40   b  defined in the second set of vanes  38   b . This pattern enriches fuel in the wake behind the first set of vanes  38   a  and provides enhanced flame anchoring. The downstream set of vanes  38   b  preferably include fuel injection orifices  44   b  which provide fuel jet penetration into the air stream equal to or greater than the first set of vanes  38   a . The actual penetration of the fuel jets from the sets of vanes is tailored depending on radial position for optimal augmentor efficiency. This arrangement increases fuel/air mixing in the augmentor section  18 , leading to higher combustion efficiency. The flame anchoring characteristics of the downstream set of vanes  38   b  also provide increased heat input from the flames attached to the upstream vanes  38   a.    
   Referring to  FIG. 5 , another vane pattern is illustrated. The vane pattern differs from the  FIG. 3 , pattern in that every third vane is part of the first set of vanes  38   a ′ which are located axially forward of a second set of vane  38   b ′. It should be understood that any pattern and/or spacing arrangement of axially displaced vane sets will benefit from the present invention, including as few as just a single downstream vane to baffle the tangential waves. 
   It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting. 
   Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention. 
   The foregoing description is,exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.