Abstract:
A method for operating a gas turbine engine including a core engine, a fan assembly for pressurizing air, a core stream duct, an inner bypass duct, and an outer bypass duct is provided. The method includes channeling a first portion of air discharged from the fan assembly through the core gas turbine engine, channeling a second portion of the air discharged from the fan assembly through the inner bypass duct such that the second portion of air bypasses the core gas turbine engine, mixing the core gas turbine engine exhaust air and the second portion of air, channeling the mixed air through a core engine nozzle, and channeling a third portion of the air discharged from the fan assembly through a bypass nozzle.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to gas turbine engines and more particularly, to a method and apparatus for controlling gas turbine engine bypass airflows.  
         [0002]     At least one known gas turbine engine includes, in serial flow arrangement, a forward fan assembly, a core driven fan assembly, a high-pressure compressor for compressing air flowing through the engine, a combustor for mixing fuel with the compressed air such that the mixture may be ignited, a high pressure turbine for providing power to the high pressure compressor, and a low pressure turbine for providing power to the fan assembly. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. In operation, the core engine generates combustion gases, which are discharged downstream to a low pressure turbine that extracts energy therefrom for powering the forward fan assembly.  
         [0003]     At least one known gas turbine engine has been developed for use in a supersonic transport aircraft (SSBJ). These gas turbine engines must therefore be designed to meet stringent noise, weight, and performance requirements. One such engine is a variable cycle engine (VCE) that is configurable to operate in a double bypass mode. More specifically, the flow modulation potential is increased by splitting the core bypass air into two sections, each in flow communication with a separate concentric bypass duct surrounding the core engine, one duct containing a core driven compressor/fan stage (CDFS). During operation, the bypass ratio, i.e., the ratio of the quantity of airflow bypassing the core engine to that passing through the core engine can be varied by selectively bypassing or flowing air through the CDFS. through various systems of valves and mixers.  
         [0004]     Mixing the CDFS exhaust air with the bypass duct stream may limit the controllability of the core-driven fan stage (CDFS) operating line. Accordingly, at least one known gas turbine engine includes a variable area bypass injector device to facilitate reducing the likelihood that potential gas turbine engine operability and stall problems may occur. However, the variable area bypass injector device may reduce the operational efficiency of the core-driven fan stage. For example, when the variable cycle engine is operated in a “single bypass” mode, the engine may experience a relatively substantial dump pressure loss. Moreover, in applications that require relatively stringent acoustic requirements, at least one known gas turbine engine includes an exhaust nozzle that is designed to include relatively large exhaust nozzle variations thus making the exhaust nozzle relatively heavy and complex to design.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     In one aspect, a method for operating a gas turbine engine including a core engine, a fan assembly for pressurizing air, a core stream duct, an inner bypass duct, a core driven fan assembly (CDFS) for pressurizing air, and an outer bypass duct is provided. The method includes channeling a first portion of air discharged from the fan assembly through the core gas turbine engine, channeling a second portion of the air discharged from the fan assembly through the CDFS which includes a variable inlet guide vane (VIGV), and into the inner bypass duct such that the second portion of air bypasses the core gas turbine engine, mixing the core gas turbine engine exhaust air and the second portion of air, channeling the mixed air through a core engine nozzle, and channeling a third portion of the air discharged from the fan assembly through a bypass nozzle.  
         [0006]     In another aspect, a gas turbine engine assembly is provided. The gas turbine engine assembly includes a core gas turbine engine, a fan assembly for pressurizing air, a core stream duct in flow communication with the fan assembly and configured to receive a first portion of air discharged from the fan assembly, a CDFS and inner bypass duct assembly in flow communication with the fan assembly, wherein the inner bypass duct is positioned radially outward from the core gas turbine engine and configured to receive a second portion of air discharged from the fan assembly and contains a CDFS for the purpose of providing additional pressurization to that provided by the fan assembly, and an outer bypass duct in flow communication with the fan assembly, wherein the outer bypass duct positioned radially outward from the inner bypass duct and configured to receive a third portion of air discharged from the fan assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine;  
         [0008]      FIG. 2  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a first operational configuration;  
         [0009]      FIG. 3  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a second operational configuration;  
         [0010]      FIG. 4  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a third operational configuration and  
         [0011]      FIG. 5  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a fourth operational configuration. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  is a cross-sectional view of a portion of an exemplary gas turbine engine  10  that includes an outer casing or nacelle  12 , the upstream end of which forms an inlet  14  that is sized to provide a predetermined quantity of airflow to the engine  10 . Disposed within inlet  14  is a fan  16  for receiving and compressing the airflow delivered by inlet  14 .  
         [0013]     Gas turbine engine  10  also includes a core engine  40 , that is positioned downstream of fan  16 . In the exemplary embodiment, core engine  40  includes an axial flow compressor  42 , with an extended tip on the first stage to operate as the CDFS  34 , having a rotor  44 .  
         [0014]     During operation, air compressed by fan  16  is channeled through a core engine inlet duct  46 , and is further compressed by the axial flow compressor  42 . The compressed air is then discharged to a combustor  48  where fuel is burned to provide high-energy combustion gases to drive a core engine turbine  50 . Turbine  50 , in turn, drives the rotor  44  through a shaft  52  in the normal manner of a gas turbine engine. The hot gases of combustion then pass to and drive a low-pressure turbine  54 , which, in turn, drives the fan  16  through shaft  56 .  
         [0015]     In the exemplary embodiment, gas turbine engine  10  also includes two bypass ducts. More specifically, gas turbine engine  10  includes an outer bypass duct  58  that is radially inward of outer casing  12 , and an inner bypass duct  60  that is positioned radially inward of outer bypass duct  58 , to facilitate bypassing a portion of the fan airflow around core engine  40 . In the exemplary embodiment, outer bypass duct  58  and inner bypass duct  60  substantially circumscribe core gas turbine engine  10 .  
         [0016]     During operation, and in the exemplary embodiment, air is channeled from fan  16  through axial space  22  wherein the airflow is separated into a plurality of flowpaths. Specifically, a first portion of the airflow is channeled through outer bypass duct  58  and aft towards a nozzle assembly  100 . A second portion of the air is channeled through CDFS  34  and inner bypass duct  60 , that is radially outward of a splitter  70 , and aft toward a variable area bypass injector (VABI)  102 , and a third portion of the air is channeled to core gas turbine engine  40 . Accordingly, as described herein, the air supplied from fan  16  is separated into three separate flowpaths within gas turbine engine  10 .  
         [0017]     In the exemplary embodiment, the airflow channeled through inner bypass duct  60  is combined and/or mixed with the core engine combustion gases exiting low-pressure turbine  54  utilizing VABI  102 . Moreover, the airflow channeled through outer bypass duct  58  is channeled through an exhaust nozzle support strut  104  that is coupled radially aft of core gas turbine engine  10 .  
         [0018]     Accordingly, and in the exemplary embodiment, gas turbine engine  10  also includes a core nozzle assembly  110 , i.e. a core nozzle flap, that is configured to regulate the quantity of combined air that is channeled from VABI  102 , and a bypass nozzle assembly  112 , i.e. a bypass nozzle flap, that is configured to regulate the quantity of airflow that is channeled from outer bypass duct  58 .  
         [0019]     In the exemplary embodiment, core nozzle assembly  110  includes a core nozzle valve  120 , i.e. a plug, that is coupled to outer casing  12 . In one embodiment, core nozzle assembly  110  is a variable area core nozzle assembly wherein actuation is accomplished using various mechanical devices to vary the size of a throat area  122 . For example, core nozzle valve  120  may be a flap actuated using a hinge (not shown). In the exemplary embodiment, core nozzle valve  120  is translatable in an axially forward direction  124  and an axially aft direction  126 . In an alternative embodiment, core nozzle valve  120  is fixedly coupled to outer casing  12 .  
         [0020]     In use, core nozzle valve  120  controls the size of throat area  122  to facilitate regulating a quantity of air channeled through throat area  122 . More specifically, and in the exemplary embodiment, core nozzle valve  120  is translated in forward direction  124  to facilitate increasing a quantity of airflow that is channeled through throat area  122 . Alternatively, core nozzle valve  120  is translated in aft direction  126  to facilitate decreasing the quantity of airflow channeled through throat area  122 . Accordingly, core nozzle assembly  110  facilitates regulating the quantity of airflow that is channeled from VABI  102  to the exhaust.  
         [0021]     In the exemplary embodiment, bypass nozzle assembly  112  includes a bypass nozzle valve  130 , i.e. a plug, that is coupled to an engine centerbody  132  for example. In one embodiment, bypass nozzle assembly  112  is a variable area bypass nozzle wherein actuation is accomplished using various mechanical devices to vary the size of a throat area  134 . For example, bypass nozzle valve  130  may be a flap actuated using a hinge (not shown). In the exemplary embodiment, bypass nozzle valve  130  is translatable in axially forward direction  124  to and an axially aft direction  126 . In an alternative embodiment, bypass nozzle valve  130  is fixedly coupled to centerbody  132 .  
         [0022]     In use, and in the exemplary embodiment, bypass nozzle valve  130  is movable to facilitate regulating and/or varying a quantity of airflow channeled through throat area  134 . More specifically, and in the exemplary embodiment, bypass nozzle valve  130  is translated in forward direction  124  to facilitate increasing a quantity of airflow that is channeled through throat area  134 . Alternatively, bypass nozzle valve  130  is translated in aft direction  126  to facilitate decreasing the quantity of airflow channeled through throat area  134 . Accordingly, variable area nozzle assembly  120  facilitates regulating the quantity of airflow that is channeled from outer bypass duct  58  to the exhaust without mixing with the gas turbine exhaust.  
         [0023]      FIG. 2  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a first operational configuration. In the exemplary embodiment, VABI  102  and core nozzle assembly  110  are maintained in a fixed position, whereas bypass nozzle assembly  112  is movable to facilitate varying the size of throat area  134 .  
         [0024]      FIG. 3  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a second operational configuration. In the exemplary embodiment, VABI  102 , core nozzle assembly  110 , and bypass nozzle assembly  112  are all movable to facilitate varying the size of mixer inlet area  160 , core throat area  122 , and bypass throat area  134  respectively.  
         [0025]      FIG. 4  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a third operational configuration. In the exemplary embodiment, VABI  102  is maintained in a fixed position, whereas core nozzle assembly  110  and bypass nozzle assembly  112  are movable to facilitate varying the size of throat area  122  and throat area  134  respectively.  
         [0026]      FIG. 5  is schematic illustration of the gas turbine engine shown in  FIG. 1  in a fourth operational configuration. In the exemplary embodiment, core nozzle assembly  110  is maintained in a fixed position, whereas VABI  102  and bypass nozzle assembly  112  are movable to facilitate varying the size of mixer inlet area  160  and throat area  134  respectively.  
         [0027]     Each of these operational configurations exercises a different combination of variable geometry features. In general, the bypass nozzle assembly  112  is used to control the fan assembly  16  operating pressure ratio, the core nozzle assembly  110  is used to control the CDFS assembly  34  operating pressure ratio, and the VABI  102  is used to control the gas energy extraction of the core assembly  40 . The necessity to exercise these features is dependent on the application of the invention. For example, in the supersonic business jet application, where high temperatures limit the flow capacity of the core  40 , the fan discharge flow is distributed to the outer bypass duct  58  and the variable bypass nozzle assembly  110  throat area is increased to accept the increased flow without a need to increase core nozzle throat area.  
         [0028]     The gas turbine engine assembly described herein facilitates dividing the air produced by the fan assembly into three separate airstreams, i.e. core, inner bypass, outer bypass. The fan tip flow, i.e. outer bypass air is channeled into a dedicated duct and exits through a variable area nozzle, where as air generated by the fan hub and pitch flows are channeled through and around the core gas turbine engine and then mixed utilizing the VABI. More specifically, the hub flow is channeled into the core gas turbine engine and the pitch flow is channeled through the CDFS stage, including variable inlet guide vane. The CDFS flow is then mixed in with the core exhaust flow at the turbine exit. The mixed core flow is then channeled through a separate exhaust nozzle. In the exemplary embodiment, the variable area bypass nozzle is an inverted flow nozzle that facilitates maintaining a relatively low pressure and jet velocity radially inside of the bypass nozzle and a relatively higher jet velocity radially outward of the bypass nozzle therefore decreasing an acoustic signature of the gas turbine engine.  
         [0029]     Accordingly, the gas turbine engine described herein facilitates providing the ability to independently specify the fan and CDFS operating lines at the same time, thus allowing for increased thrust per unit airflow at performance levels comparable to standard mixed flow turbofan cycles. In addition, the relatively small amount of flow channeled through the CDFS facilitates reducing the requirement for a variable area mixer and variable core exhaust nozzle, and under some circumstances eliminating them. Moreover, utilizing a separate nozzle for the fan tip flow incorporates many of the benefits associated with a fladed cycle engine, while also decreasing the overall engine weight, thus increasing engine thrust per unit weight over fladed Adaptive Cycle Engines and/or VCE&#39;s.  
         [0030]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.