Abstract:
A gas turbine engine includes: a front fan having at least one rotor carrying fan blades disposed within a fan duct, and drivingly connected to a low pressure turbine, the front fan including at least one row of outer fan blades disposed outwardly of and connected to the fan blades, the outer fan blades extending across an outer bypass duct which circumscribes the fan duct; a core engine located between the front fan and the low pressure turbine; an aft fan disposed downstream of the low pressure turbine, having at least one rotor carrying aft turbine blades disposed within an aft turbine duct, the aft turbine further including at least one row of aft fan blades disposed outwardly of and connected to the aft turbine blades, and extending across an aft fan duct that surrounds the core engine; and apparatus operable to selectively throttle flow through the outer bypass duct.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0001]    The U.S. Government may have certain rights in this invention pursuant to contract number F33615-03-D-2352 awarded by the Department of the Air Force. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to gas turbine engines and more particularly to and adaptive cycle gas turbine engine including an aft fan. 
         [0003]    A gas turbine engine includes a turbomachinery core that is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure spool with a turbine driven by the core exhaust gases that in turn drives a fan through a shaft to generate a bypass flow of propulsive gas. A turbofan engine may be characterized as “low bypass” or “high bypass” based on the ratio of bypass flow to core flow. 
         [0004]    Low-bypass turbofan engines are commonly used in military aircraft. Increasingly, emphasis is being applied to low-observable and integrated powerplant technology. These considerations drive the use of embedded installations in which an engine is “buried” deep within an airframe (as opposed to being mounted in a pod or nacelle) and may have indirect and long inlet and exhaust ducts. 
         [0005]    Advanced military aircraft concepts will require improved performance moderate- to high-bypass turbine engines in embedded installations. Performance requirements are demanding engine cycles with increasingly higher overall pressure ratios, but with low pressure ratio fans. Prior art engine architectures require a large number of stages in the low pressure spool in order to achieve a high overall pressure ratio while maintaining relatively low rotational speeds required by the fan. Fans in such engines are large diameter and therefore create difficulties in the sharp turns required in the embedded inlet due to the short distance from the front of the vehicle to the fan face. Other vehicle requirements such as large air offtakes for high lift devices, large mechanical and/or electrical power extraction for aircraft mission systems and high thermal loads are difficult to meet with a fixed cycle turbine engine. 
         [0006]    Accordingly, there is a need for an adaptive cycle, high performance turbine engine suitable for use in embedded installations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    These and other shortcomings of the prior art are addressed by the present invention, which provides a variable-cycle engine having a front fan with a FLADE stage and an aft fan. 
         [0008]    According to one aspect of the invention, a gas turbine engine includes: a front fan having at least one rotor carrying fan blades disposed within a fan duct, and drivingly connected to a low pressure turbine, the front fan including at least one row of outer fan blades disposed outwardly of and connected to the fan blades, the outer fan blades extending across an outer bypass duct which circumscribes the fan duct; a core engine located between the front fan and the low pressure turbine; an aft fan disposed downstream of the low pressure turbine, having at least one rotor carrying aft turbine blades disposed within an aft turbine duct, the aft turbine further including at least one row of aft fan blades disposed outwardly of and connected to the aft turbine blades, and extending across an aft fan duct that surrounds the core engine; and apparatus operable to selectively throttle flow through the outer bypass duct. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine constructed according to an aspect of the present invention; 
           [0011]      FIG. 2  is a schematic cross-sectional view of alternative gas turbine engine constructed according to an aspect of the present invention; and 
           [0012]      FIG. 3  is a schematic cross-sectional view of another alternative gas turbine engine constructed according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates an aft fan adaptive cycle gas turbine engine constructed in accordance with an aspect of the present invention, generally designated  10 . The engine  10  has a longitudinal center line or axis A. Its major components, in sequential flow order, are a front fan  12 , a core engine  14  (also referred to as a gas generator), and an aft fan  16 . 
         [0014]    The front fan  12  rotates in a single direction and includes three fan stages, each having an annular array of stationary, airfoil-shaped guide vanes  20  and a downstream array of rotating, airfoil-shaped fan blades  22 . A fan duct  24  surrounds the fan stages and terminates at an annular splitter  26  that communicates with the core engine  14  and with an inner bypass duct  28  that circumscribes the core engine  14 . The flow capacity of the front fan  12  relative to the flow capacity of the core engine  14  determines the corresponding mass flow ratio at the splitter  26 , commonly referred to as the “bypass ratio”. The illustrated engine  10  has a low-to-moderate bypass ratio, for example about 0.3 to about 1. The guide vanes  20  of the first front fan stage are positioned at a fan inlet  30  (these are commonly referred to as “inlet guide vanes”). Their angle of incidence may be varied to throttle flow through the front fan  12 , for example using actuators  32  of a known type (shown schematically). 
         [0015]    The front fan  12  is sized (e.g. through selection of airfoil sections and dimensions, duct dimensions, and intended rotational speed) to operate at a moderate pressure ratio (i.e. the ratio of fan discharge pressure to fan inlet pressure). As used herein, the term “moderate” refers to a ratio of about 2 to about 5. 
         [0016]    The front fan  12  incorporates a “FLADE” stage (FLADE being an acronym for “fan on blade”). Each blade of the FLADE stage includes an arcuate platform segment  34  disposed at the tip of one of the fan blades  22 , and an outer fan blade  36  extending from the platform segment  34 . The outer fan blades  36  are disposed within an outer bypass duct  38  that circumscribes the fan duct  24 . 
         [0017]    An array of stationary, airfoil-shaped outer guide vanes  40  are positioned in the outer bypass duct upstream of the outer fan blades  36 . Their angle of incidence may be varied to throttle flow through the outer bypass duct  38 , for example using actuators  42  of a known type (shown schematically). The actuators  42  (and other actuators described herein) may be operated under the control of a FADEC, PMC, manual control, or other known type of engine control (not shown). 
         [0018]    The outer bypass duct  38  terminates in an annular manifold  44  that in turn connects to one or more offtake ducts  46 . The offtake ducts  46  may be used to supply flow from the outer fan blades  36  (referred to herein as “FLADE flow”) to an aircraft system (not shown) such as a powered high-lift system, a cooling system, thrust vectoring nozzles, etc. 
         [0019]    The core engine  14  includes, in sequential flow order, a compressor  48 , a combustor  50 , and a high-pressure turbine  52 . The high-pressure turbine  52  drives the compressor  48  through an outer shaft  54 . A low-pressure turbine  56  is disposed downstream of the high-pressure turbine  52  and drives the front fan  12  through an inner shaft  58 . A forward mixer  60  is disposed aft of the low-pressure turbine  56  and communicates with both the inner bypass duct  28  and the exit of the core engine  14 . 
         [0020]    The aft fan  16  is positioned downstream of the mixer  60 . It includes two fan stages, each having a rotor  61  carrying an annular array of compound blades. Each of the compound blades includes an aft turbine blade  62 , an arcuate platform segment  64 , and an aft fan blade  66 . The aft turbine blades  62  lie within an aft turbine duct  68  that receives the mixed flow from the front fan  12  and the core engine  14 . The aft fan blades  66  lie within an aft fan duct  70  that surrounds the core engine  14  and the inner bypass duct  28 . The camber of the aft turbine blades  62  and aft fan blades  66  of the two stages are opposite to each other, such that in operation the two stages rotate in opposite directions (counter-rotation). 
         [0021]    In operation, air is pressurized in the compressor  48  and subsequently mixed with fuel and burned in the combustor  50  to generate combustion gases. The high pressure turbine  52  extracts energy from the combustion gases to drive the compressor  48 . The low pressure turbine  56  extracts energy from the mixture of the combustion gases and the inner bypass flow to drive the front fan  12 . The aft turbine blades  62  extract energy from the combustion gases to drive the aft fan  16 , which generates propulsive thrust. In the illustrated example, an aft mixer  72  is positioned downstream of the aft fan  16 , and the flow streams from the aft turbine blades  62 , and the aft fan  16  mix therein and are discharged into a common exhaust duct  74 . Alternatively, the aft mixer  72  could be eliminated, and the flow stream from the aft fan  16  may be discharged through a separate duct  76  (indicated schematically by the dashed lines in  FIG. 1 ) while the flow stream from the aft turbine blades  62  is discharged through an inner duct  78 . 
         [0022]    During operation, the outer fan blades  36  provide a constant-pressure FLADE flow to the manifold  44  and offtake ducts  46 . The FLADE flow may be throttled as necessary by the variable-position outer guide vanes  40 . The flow of the front fan  12  may be throttled as necessary by the variable position inlet guide vanes  20 . By proper selection of the outer guide vane  40  position and the inner guide vane  20  position, constant (or nearly constant) flow and pressure of the FLADE stream can be maintained over a thrust range from maximum to under 40% of maximum (e.g. at typical approach-to-landing conditions). 
         [0023]      FIG. 2  illustrates an alternative aft fan adaptive cycle engine  110  constructed according to another aspect of the invention. It is similar in overall construction to the engine  10  described above and includes a front fan  112 , a core engine or gas generator  114 , and an aft fan  116 . The engine  110  differs from the engine  10  in the configuration of the aft fan  116 . 
         [0024]    The aft fan  116  includes one or more rotors  161 , all rotating in a single direction and coupled together to form an aft spool. Each rotor  161  carries an annular array of aft turbine blades  162 . An annular array of airfoil-shaped stationary aft turbine vanes  163  is disposed upstream of each rotor  161 . One of the rotors  161  carries compound blades having an aft turbine blade  162 , an arcuate platform segment  164  disposed at the tip of the aft turbine blade  162 , and an aft fan blade  166  extending outward therefrom. The aft turbine blades  162  lie within an aft turbine duct  168  that receives the mixed flow from the front fan  112  and the core engine  114 . The aft fan blades  166  lie within an aft fan duct  170  that surrounds the core engine  114  and the inner bypass duct  128 . The flow energy absorbed by the aft turbine blades  162  is transferred through the rotors  161  to the single row of aft fan blades  166 . Operation of the engine  110  is substantially the same as the engine  10  described above, the primary difference being a lower pressure ratio in the single-stage aft fan  116  as compared to the two-stage aft fan  16 . If desired, the aft fan  116  may be provided with a second fan stage by providing another aft spool (with rotors  161 , aft turbine blades  162 , aft turbine blades  163 , and aft fan blades  166  as described above) positioned aft of the aft fan spool as shown in  FIG. 2 . The two aft fan spools may be co-rotating or counter-rotating relative to each other. If the spools are co-rotating, an annular array of airfoil-shaped vanes would be provided in the aft fan duct  170  between the rows of aft fan blades  166 , to redirect the flow exiting the upstream spool into the downstream spool. 
         [0025]      FIG. 3  illustrates yet another alternative aft fan adaptive cycle engine  210  constructed according to another aspect of the invention. Like the engines  10  and  110  described above, its major components, in sequential flow order, are a front fan  212 , a core engine  214 , and an aft fan  216 . 
         [0026]    The front fan  212  operates as the front fan  12  described above and includes variable inlet guide vanes  220 , and a FLADE stage including outer fan blades  236  and variable-incidence outer guide vanes  240 . 
         [0027]    The outer fan blades  236  are disposed within an outer bypass duct  238  that circumscribes the fan duct  224 . In this configuration, the outer bypass duct  238  extends the full length of the core engine  214  and communicates with the interior of a streamlined centerbody  278  that encloses the rotors  261  of the aft fan  216 . FLADE flow produced by the outer fan blades  236  may be used to provide cooling of the centerbody  278  by being discharged through cooling holes (shown by the arrows marked “C”) and/or additional thrust, if directed axially aft (shown by the arrows marked “T”). 
         [0028]    As shown, the aft fan  216  is counter-rotating and includes two rows of aft fan blades  266 . A single-stage configuration similar to that shown in  FIG. 2  is possible as well. 
         [0029]    The engine  210  incorporates a core drive fan stage (“CDFS”) illustrated generally at  280 . This includes a duct  282  at the axially upstream end of the compressor  248 , having an inlet  284  and an outlet  286  which both communicate with the inner bypass duct  228 . Core drive fan blades  288  extend from the tips of compressor blades of a stage of the compressor  248  and are positioned within the duct  282 . An array of stationary, airfoil-shaped guide vanes  290  are positioned in the duct  282  upstream of the core drive fan blades  288 . Their angle of incidence may be varied to throttle flow through the duct  282 , for example using actuators  292  (shown schematically). When the guide vanes  290  are open, the CDFS is effective to increase the pressure of the flow in the inner bypass duct  228  and thus the overall effective fan pressure ratio of the engine  210 . 
         [0030]    The engine  210  may also include a generator  294  configured for significant electrical power extraction. The illustrated generator  294  is coupled to the inner shaft  258 , but it could also be driven through an accessory gearbox (not shown). The FLADE flow may be modulated to manage the power extracted from the low-pressure spool, for example by closing down the outer guide vanes  240  as the electrical load increases. 
         [0031]    The engine configurations described above all provide a FLADE air stream which may be used in various ways to meet particular vehicle system requirements, while still maintaining a desired flow rate, bypass ratio, and mass flow rate from the aft fan. 
         [0032]    Initial cycle studies have shown the potential to maintain a constant flow and pressure to the vehicle from a FLADE stream over a thrust range from maximum to under 40% of maximum at typical approach conditions. Overall gas generator pressure ratios of 60 or higher are achievable with a well balanced split of pressure ratio between the front fan and the high pressure compressor. 
         [0033]    The foregoing has described an adaptive cycle, aft fan gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.