Abstract:
A turbine engine includes a first fan including a plurality of fan blades rotatable about an axis and a reverse flow core engine section including a core turbine axially forward of a combustor and compressor. The core turbine drives the compressor about the axis and a transmission system. A geared architecture is driven by the transmission system to drive the first fan at a speed less than that of the core turbine. A second fan is disposed axially aft of the first fan and forwarded of the core engine and a second turbine is disposed between the second fan and the core engine for driving the second fan when not coupled to the transmission.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/782,610 filed on Mar. 14, 2013. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This subject of this disclosure was made with government support under Contract No.: FA8650-09-D-2923/D013 awarded by the United States Air Force. The government therefore may have certain rights in the disclosed subject matter. 
    
    
     BACKGROUND 
     A gas turbine engine typically includes a fan section and a core engine including compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The energetic gas flow expands through the turbine section to drive the compressor and the fan section and finally exits through a thrusting nozzle. 
     Typically, the compressor is axially forward of the compressor and turbine sections. In some gas turbine engine configurations known as reverse flow turbine engines, the turbine section is axially forward of the combustor and compressor. Airflow is ducted aft to the compressor, than forward to the combustor and turbine where exhaust gases are exhausted forward and mixed within incoming airflow. Such reverse flow engine can provide performance advantages and efficiencies. 
     Airflow through the gas turbine engine is typically divided between a core flow path and a bypass flow path. More flow through the bypass passage as compared to the core flow path typically provides increased fuel efficiency at the expense of overall thrust. Engines for high speed aircraft include smaller bypass passages to provide greater thrusts. Fuel efficiency is therefore balanced against thrust requirements and smaller bypass flows are utilized when greater thrusts are desired. 
     A variable cycle gas turbine engine may switch between highly fuel efficient operation with increased bypass airflow and high speed operation with less bypass flow with more thrust produced by the core engine. 
     Although variable cycle gas turbine engines have improved operational efficiency, turbine engine manufactures continue to seek further improvements to engine performance including improvements to propulsive efficiency. 
     SUMMARY 
     A turbine engine according to an exemplary embodiment of this disclosure, among other possible things includes a first fan including a plurality of fan blades rotatable about an axis. A reverse flow core engine section includes a core turbine axially forward of a combustor and a compressor. The core turbine drives the compressor about the axis. A transmission system is driven by the core turbine. A geared architecture is driven by the transmission system for driving the first fan at a speed less than the core turbine. A second fan is disposed axially aft of the first fan and forwarded of the core engine. A second turbine is disposed between the second fan and the core engine. The second turbine drives the second fan. 
     In a further embodiment of the foregoing turbine engine, the transmission includes a first mode. The second fan is coupled to the transmission and driven at the speed of the core turbine. 
     In a further embodiment of any of the foregoing turbine engines, the transmission includes a second mode. The second fan is driven at a speed less than that of the core turbine and greater than that of the first fan. 
     In a further embodiment of any of the foregoing turbine engines, the second fan and second turbine are fixed to rotate at a common speed. 
     In a further embodiment of any of the foregoing turbine engines, the second turbine drives the second fan responsive to the transmission decoupling from the second fan. 
     In a further embodiment of any of the foregoing turbine engines, includes a variable vane disposed axially forward of the second turbine for controlling a speed of the second turbine and thereby the second fan. 
     In a further embodiment of any of the foregoing turbine engines, the transmission includes a first transmission path coupling the core turbine to the second fan and the geared architecture such that second fan rotates at a speed common with the core turbine. 
     In a further embodiment of any of the foregoing turbine engines, the transmission includes a second transmission path through a gear reduction to drive the second fan at a speed less than that of the core turbine and greater than the first fan. 
     A method of operating a turbine engine according to an exemplary embodiment of this disclosure, among other possible things includes defining a core gas flow path through a core engine, where the core engine includes a compressor, a combustor in communication with the compressor and a core turbine driven by high energy gas flow generated by the combustor, driving a transmission with the core turbine, driving a first fan through a geared architecture driven by the transmission, driving a second fan axially aft of the first fan and forward of the core turbine, the second fan is driven by the transmission at a speed common with the core turbine in a first mode, and driving the second fan with a second turbine disposed axially forward of the core turbine when the transmission is in a second mode. 
     In a further embodiment of the foregoing method, in the second mode the second fan is driven at a speed less than that of the core turbine and greater than that of the first fan. 
     In a further embodiment of any of the foregoing methods, the second fan and second turbine are fixed to rotate at a common speed and speed of the second turbine is varied to change the speed of the second fan when decoupled from the transmission. 
     In a further embodiment of any of the foregoing methods, the second turbine drives the second fan responsive to the transmission decoupling from the second fan. 
     In a further embodiment of any of the foregoing methods, the transmission includes a first transmission path coupling the core turbine to the second fan and the geared architecture such that second fan rotates at a speed common with the core turbine in the first mode. 
     In a further embodiment of any of the foregoing methods, the transmission includes a second transmission path through a gear reduction to drive the second fan at a speed less than that of the core turbine and greater than the first fan in the second mode. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example reverse flow turbine engine. 
         FIG. 2  is a schematic view of the example reverse flow turbine with an example transmission in a first torque transmitting condition. 
         FIG. 3  is a schematic representation of the example reverse flow turbine with the example transmission in a second torque transmitting condition. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates an example gas turbine engine  10  that includes a first fan  12  that is driven by a core engine section  14 . The core engine section  14  in this engine is known as a reverse flow gas turbine engine. The reverse flow gas turbine engine  14  includes a compressor  20 , a combustor  18  and a core turbine  16 . The core turbine  16  is disposed axially forward of the combustor  18  and the compressor section  20 . Incoming air flow  15  proceeds through the first fan  12  into a core flow path  34  where it is directed to an aft portion of the core engine  14  into the compressor  20 . In the compressor  20 , the incoming air  15  is compressed and fed to a combustor  18 . In the combustor  18 , the high pressure air is mixed with fuel and ignited to create a high energy gas flow. The high energy gas flow expands through the core turbine  16 . The high energy exhaust gasses are exhausted through an exhaust mixer  40  where the exhaust gasses  42  are mixed with bypass air flows B 1  and B 2  through a corresponding first bypass passage  36  and a second bypass passage  38 . The mixed air flow of exhaust gasses  42  and incoming air flow  15  are then exhausted out of the aft end of the engine  10 . 
     The core turbine  16  drives a shaft  44  that in turn drives a transmission  28 . The transmission  28  drives the fan  12  through a geared architecture  30 . The transmission  28  is coupled through a coupling shaft  56  to the geared architecture  30  such that the first fan  12  will rotate at a speed less than the speed of the core turbine  16  and transmission  28 . 
     The transmission  28  is further coupled to a free spool  26 . The free spool  26  includes a second fan  22  that is coupled to a second turbine  24 . The second turbine  24  is disposed within a core flow path C such that incoming air  15  drives the second or cold turbine  24 . The second turbine  24  does not include a rotating shroud for the radially outer tip within the core flow passage  34 . 
     The example gas turbine engine  10  includes the reverse flow core engine  14  that drives the first fan section  12  and includes the adaptive fan  22  that rotates at variable speeds to adapt the engine to various thrust requirements. As appreciated, a significant amount of incoming air  15  is compressed and driven through the bypass passages  36  and  38 . The more air flow  15  that is directed and generates thrust through the bypass passages  36  and  38 , the more fuel efficient the engine operates. However, in some instances it is desired to increase thrust, and thereby increase air flow through the core flow path C is desired. In these circumstances, the second fan section  22  is varied in speed to distribute air between the first bypass  36  and the core flow path  34  to provide the desired specific thrust from the example engine  10 . 
     The second fan  22  is selectively driven by the transmission  28  at a speed common with the core turbine  16  or decoupled from the core turbine  16  to rotate at a speed less than that of the core turbine  16  but greater than that of the first fan  12 . 
     Referring to  FIG. 2  with continued reference to  FIG. 1 , the example engine  10  is shown schematically and includes the transmission  28 . The transmission  28  includes a clutch  50  and a gear reduction  46 . The example gear reduction  46  is coupled to the shaft  44  and drives the free spool  26  through a clutch mechanism  48 . The clutch mechanism  50  provides for the direct transmission of torque from the shaft  44  to the free spool  26 . The gear reduction  46  drives both the first fan  12  and second fan  22  at a speed slower than the core turbine  16 . 
     The example engine  10  in  FIG. 2  is shown with the transmission  28  in a first, high speed mode where torque is transmitted along a first load path  58  through the clutch  50 . The clutch  50  includes no gear reduction or other speed reduction devices and therefore directly transmits the speed of the core turbine  16  to the free spool  26 . The speed of the free spool  26  is therefore equal to that of the core turbine  16 . 
     The first fan  12  is driven by the transmission  28  through the geared reduction  30  and therefore always rotates at speed less than that of the core turbine  16 . With the first fan  12  and the second fan  22  rotating at maximum speeds, a maximum amount of airflow is driven through the core flow path  34  and the first bypass passage  40  to produce a maximum engine thrust. 
     Referring to  FIG. 3 , with continued reference to  FIG. 1 , the transmission  28  is shown in a second, low speed mode where the example gear reduction  46  is coupled to the shaft  44  and drives the free spool  26  through the clutch mechanism  48 . In the second mode of operation, torque is transmitted through the geared reduction  46  and clutch  48  such that the second fan  22  will rotate at a speed less than that of the core turbine  16 . The gear reduction  46  further drives the geared architecture  30  such that the first fan runs at a slower speed than in with the transmission in the first mode. 
     As appreciated, because the core turbine  16  is driving the gear reduction  46  of the transmission  28 , both the fan section  12  and the free spool  26  including the second fan  22  will rotate at a speed less than that of the core turbine  16 . The second fan  22  or free spool  26  will rotate at a speed that is greater than that of the first fan  12  due to the gear reduction provided by the geared architecture  30 . The gear reduction  46  as part of the transmission  28  includes a gear reduction that provides that the free spool  26  will rotate at a speed greater than the first fan  12  but less than that of the core turbine  16 . 
     In the second mode shown in  FIG. 3 , torque is transmitted through the second load path  52  through the gear reduction  46  and the clutch  48 . In this configuration, the free spool  26  will rotate at a speed that is greater than that of the first fan  12  but less than that of the core turbine  16 . 
     Moreover, the clutch  48  may be disengaged and therefore not transmit torque to the free spool  26  such that the second fan  22  is driven entirely by the second turbine  24 . The speed of the second turbine  24  is in turn varied and controlled by way of a variable vane  32 . The variable vane  32  is moveable between a first position and a second position by an actuator  54 . As appreciated, the first and second positions are positions that direct air flow into the turbine  24  to govern the speed of the second turbine  24  and thereby of the second fan  22 . The alteration and adjustment of the speed of the second turbine  24  and air swirl of the second fan  22  changes the condition of the core flow C and bypass flow B 1 . Control of air swirl of the second fan  52  controls flow through core flow C and bypass flow B 1 . 
     Accordingly, the example gas turbine engine provides for the variation of specific thrust by varying flow through the various bypass passages and by directing torque in a variable manner between the first fan  12  and the second fan  22 . 
     Although a dual annular bypass flow gas turbine engine is indicated, the features of the disclosed invention could be utilized in an engine where only a single annular bypass passage is utilized. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure.