Abstract:
A turbofan engine has a fan portion in fluid communication with a core stream and a bypass stream of air separated by splitters disposed both upstream and downstream of the fan portion. A fluid passage is defined between the splitters. The turbofan engine has a plurality of vortex generators, each of the vortex generators positioned on the leading edge of a respective fan blade proximate the upstream splitter and the core stream restricting the migration of the core stream into the bypass stream through the fluid passage.

Description:
RELATED APPLICATIONS 
       [0001]    This application is related to concurrently filed and co-pending applications U.S. patent application Ser. No. ______ entitled “Splayed Inlet Guide Vanes”; U.S. patent application Ser. No. ______ entitled “Morphing Vane”; U.S. patent application Ser. No. ______ entitled “Propulsive Force Vectoring”; U.S. patent application Ser. No. ______ entitled “A System and Method for a Fluidic Barrier on the Low Pressure Side of a Fan Blade”; U.S. patent application Ser. No. ______ entitled “Integrated Aircraft Propulsion System”; U.S. patent application Ser. No. ______ entitled “A System and Method for a Fluidic Barrier from the Upstream Splitter”; U.S. patent application Ser. No. ______ entitled “Gas Turbine Engine Having Radially-Split Inlet Guide Vanes”; U.S. patent application Ser. No. ______ entitled “A System and Method for a Fluidic Barrier with Vortices from the Upstream Splitter”; U.S. patent application Ser. No. ______ entitled “Methods of Creating Fluidic Barriers in a Turbine Engine.” The entirety of these applications are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Fluid propulsion devices achieve thrust by imparting momentum to a fluid called the propellant. An air-breathing engine, as the name implies, uses the atmosphere for most of its propellant. The gas turbine produces high-temperature gas which may be used either to generate power for a propeller, fan, generator or other mechanical apparatus or to develop thrust directly by expansion and acceleration of the hot gas in a nozzle. In any case, an air breathing engine continuously draws air from the atmosphere, compresses it, adds energy in the form of heat, and then expands it in order to convert the added energy to shaft work or jet kinetic energy. Thus, in addition to acting as propellant, the air acts as the working fluid in a thermodynamic process in which a fraction of the energy is made available for propulsive purposes or work. 
         [0003]    Typically turbofan engines include at least two air streams. All air utilized by the engine initially passes through a fan, and then it is split into the two air streams. The inner air stream is referred to as core air and passes into the compressor portion of the engine, where it is compressed. This air then is fed to the combustor portion of the engine where it is mixed with fuel and the fuel is combusted. The combustion gases are then expanded through the turbine portion of the engine, which extracts energy from the hot combustion gases, the extracted energy being used to run the compressor, the fan and other accessory systems. The remaining hot gases then flow into the exhaust portion of the engine, which may be used to produce thrust for forward motion to the aircraft. 
         [0004]    The outer air flow stream bypasses the engine core and is pressurized by the fan. Typically, no other work is done on the outer air flow stream which continues axially down the engine but outside the core. The bypass air flow stream also can be used to accomplish aircraft cooling by the introduction of heat exchangers in the fan stream. Downstream of the turbine, the outer air flow stream is used to cool engine hardware in the exhaust system. When additional thrust is required (demanded), some of the fans bypass air flow stream may be redirected to the augmenter (afterburner) where it is mixed with core flow and fuel to provide the additional thrust to move the aircraft. 
         [0005]    Many current and most future aircrafts need efficient installed propulsion system performance capabilities at diverse flight conditions and over widely varying power settings for a variety of missions. Current turbofan engines are limited in their capabilities to supply this type of mission adaptive performance, in great part due to the fundamental operating characteristics of their core systems which have limited flexibility in load shifting between shaft and fan loading. 
         [0006]    When defining a conventional engine cycle and configuration for a mixed mission application, compromises have to be made in the selection of fan pressure ratio, bypass ratio, and overall pressure ratio to allow a reasonably sized engine to operate effectively. In particular, the fan pressure ratio and related bypass ratio selection needed to obtain a reasonably sized engine capable of developing the thrusts needed for combat maneuvers are non-optimum for efficient low power flight where a significant portion of the engine output is transmitted to the shaft. Engine performance may suffer due to the bypass/core pressure leakage that may occur at reduced fan power/load settings. 
         [0007]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1 a    shows a general orientation of a turbofan engine in a cut away view. In the turbofan engine shown, the flow of the air is generally axial. The engine direction along the axis is generally defined using the terms “upstream” and “downstream” generally which refer to a position in a jet engine in relation to the ambient air inlet and the engine exhaust at the back of the engine. For example, the inlet fan is upstream of the combustion chamber. Likewise, the terms “fore” and “aft” generally refer to a position in relation to the ambient air inlet and the engine exhaust nozzle. Additionally, outward/outboard and inward/inboard refer to the radial direction. For example the bypass duct is outboard the core duct. The ducts are generally circular and co-axial with each other. 
         [0008]    As ambient inlet airflow  12  enters inlet fan duct  14  of turbofan engine  10 , through the guide vanes  15 , passes by fan spinner  16  and through fan rotor (fan blade)  42 . The airflow  12  is split into primary (core) flow stream  28  and bypass flow stream  30  by upstream splitter  24  and downstream splitter  25 . In  FIG. 2 , the bypass flow stream  30  along with the core/primary flow stream  28  is shown, the bypass stream  30  being outboard of the core stream  28 . The inward portion of the bypass steam  30  and the outward portion of the core streams are partially defined by the splitters upstream of the compressor  26 . The fan  42  has a plurality of fan blades. 
         [0009]    As shown in  FIGS. 1 a  and 1 b    the fan blade  42  shown is rotating about the engine axis into the page, therefor the low pressure side of the blade  42  is shown, the high pressure side being on the opposite side. The Primary flow stream  28  flows through compressor  26  that compresses the air to a higher pressure. The compressed air typically passes through an outlet guide vane to straighten the airflow and eliminate swirling motion or turbulence, a diffuser where air spreads out, and a compressor manifold to distribute the air in a smooth flow. The core flow stream  28  is then mixed with fuel in combustion chamber  36  and the mixture is ignited and burned. The resultant combustion products flow through turbines  38  that extract energy from the combustion gases to turn fan rotor  42 , compressor  26  and any shaft work by way of turbine shaft  40 . The gases, passing exhaust cone, expand through an exhaust nozzle  43  to produce thrust. Primary flow stream  28  leaves the engine at a higher velocity than when it entered. Bypass flow stream  30  flows through fan rotor  42 , flows by bypass duct outer wall  27 , an annular duct concentric with the core engine flows through fan discharge outlet and is expanded through an exhaust nozzle to produce additional thrust. Turbofan engine  10  has a generally longitudinally extending centerline represented by engine axis  46 . 
         [0010]    Current conventionally bladed core engines cannot maintain constant or near constant operating pressure ratios as bypass flow is reduced. Current conventionally bladed fan rotors do not have the flexibility in efficiently reducing fan pressure ratio while maintaining core pressure. 
         [0011]    With reduced or no flow in the Bypass stream  30 , the core stream  28  relative pressure is greater than that in the Bypass stream  30 . In the area of the fan shown as  50  in  FIG. 1   b,  higher pressure air may leak across the region  50  from the core stream  28  into the bypass stream  30  thus reducing the core pressure which has a deleterious effect on the operation of the core and un-necessarily loading the turbine to recover the lost pressure. 
         [0012]    A fluid barrier separating the core and bypass streams as described herein, can limit the pressure loss in the core and the subsequent degradation in output of the core engine. High pressure jets along with vortices may be arranged proximate to the fan at the interface between the bypass and core streams. The jets and vortices are imparted with significant momentum to resist passage of the higher pressure core stream into the bypass stream, where the use of splitters are not practical due to the positioning and location of the fan. 
         [0013]    These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIGS. 1 a  and 1 b    are illustrations representing conventional turbofan engines. 
           [0015]      FIG. 2  is an illustration of the Bypass and primary stream flow paths. 
           [0016]      FIG. 3  is an illustration of a turbofan engine with high pressure jets projecting from the low pressure side of Fan blades according to an embodiment of the disclosed subject matter. 
           [0017]      FIG. 4  is an illustration of a turbofan engine with high pressure jets projection from the trailing edge of an upstream splitter according to an embodiment of the disclosed subject matter. 
           [0018]      FIG. 5  is an illustration of a third splitter with multiple fan stages according to embodiments of the disclosed subject matter. 
           [0019]      FIG. 6  illustrates a turbofan with vortices origination from the splitter through the fan region according to an embodiment of the disclosed subject matter. 
           [0020]      FIGS. 7 a -7 b    shows the generation of vortices from ramps according to an embodiment of the disclosed subject matter 
           [0021]      FIGS. 8 a -8 d    are different surface interruptions for the generation of vortices as described for embodiments of the disclosed subject matter. 
           [0022]      FIGS. 9 a  and 9 b    are illustrations of vortex generators on the leading edge of the fan according to embodiments of the present subject matter. 
           [0023]      FIG. 10  is a flow chart of a method of preventing pressure leakage. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 3  illustrates a Bypass flow duct  31  lying radially outward from the core flow duct  29 . The fan blade  42  is positioned upstream from the splitter  25  that separates air flow between the ducts. The upstream splitter  24  is positioned upstream from the fan blade  42  at the bottom of the Inlet guide vane  15 . As the inlet guide vane angle is changed, the bypass flow can be inhibited and pressure within the bypass flow duct  31  can differ from the pressure present in the core flow duct  29 . Air can cross between the two ducts in the vicinity of the fan blade in region  50  as shown in  FIG. 1 b    thus causing detrimental engine performance in the core as described previously. 
         [0025]    A plurality of fluidic jets  60  that inject high pressure compressor air form the fan blade  42  into the region  50  between the upstream  24  and downstream splitter  25  form a fluid barrier  51 . The high velocity jets  60  of compressed air contain enough momentum to inhibit flow leakage between the core  28  and the bypass streams  30 . The jets  60  have inertia that the low pressure air flowing in the ducts cannot overcome, thereby acting as a fluid barrier  51  to limit cross flow and pressure leakage in the region  50  between the ducts. 
         [0026]    The fluid jets  60  may advantageously have a directional component in a substantial opposite direction of the local velocity or rotation of the fan proximate to the splitters and may also have a radial component towards the axis to prevent pressure leakage across the fluid jets into the bypass stream. 
         [0027]    A valve  62  in the system modulates the high pressure air such that flow can be turned on and off depending on the predicted or actual cross flow between ducts and the detrimental effects upon the engine. 
         [0028]    As noted previously, the control of air flow through the duct may be throttled to a point where it can be minimized to the point where it is almost non-existent through the use of a small and inexpensive actuator. 
         [0029]    The high pressure gas for the jets may be provided by the compressor  26  though passages  61  to the jets. An accumulator  63  may also be provided prior to the actuator/valve in order to provide an immediate source of pressure unstrained by downstream frictional losses in the passages  61 . Alternatively, another source may be used to provide the high pressure air to the jets  60 . 
         [0030]    The high pressure fluid jets  60  originate from orifices on the low pressure side of the blades  42  wherein the plurality of orifices are radially proximate the upstream and downstream splitters in the region  50 . The plurality of orifices extend between the trailing edge of the upstream splitter  24  and the leading edge of the downstream splitter  25 . The fluid jets may advantageously having radial component directed into the core flow  28  as well as an axial component pointing downstream in the core flow  28 . It is envisioned that the compressed air drawn from the compressor  26  would represent 2-3% of the total compressor output and thus would not be a significant source of loss. 
         [0031]      FIG. 4  illustrates another embodiment of a turbofan engine  10 . As shown the fan blade  42  is positioned upstream from a splitter  24  that separates air flow between the ducts. An upstream splitter  24  is positioned upstream from the fan blade  42  at the bottom of the inlet guide vane  15 . As the inlet guide vane angle is changed, pressure within the bypass flow duct  31  can differ from the pressure present in the core flow duct  29 . Air can cross between the two ducts in the vicinity of the fan blade  42  in region  50  thus causing detrimental engine performance. 
         [0032]    As shown in  FIG. 4 , fluid jets  60  that inject high pressure compressor air from the trailing edge of the upstream splitter  24  into the region  50  between the upstream  24  and downstream splitters  25 . As described previously these jets  50  have enough momentum or inertia such that the low pressure air flowing in the ducts cannot overcome it and thus the jets  50  acts as a fluid barrier  51  to limit cross flow between the ducts. As also discussed previously, a valve/actuator  66  in the system may regulate the high pressure air such that flow can be turned on and off depending on the predicted or actual cross flow between ducts, and the corresponding detrimental effects on engine performance, an accumulator  63  may also be added. The high pressure jets  60  are preferably distributed proximate to the trailing edge of the upstream splitter  24 . The high pressure jets may be also be position on either the core  28  or bypass side  30  of the upstream splitter  24  since the jets  60  have an energy independent of the flow within the ducts. The reliance on free stream flow is discussed below with respect to creation of vortices. The high pressure gas for the jets  60  is supplied by the compressor  26  via passages  61 . The jets  60  and passages  61  are distributed circumferentially along the trailing edge. 
         [0033]      FIG. 6  illustrates a turbofan engine with concentric core and bypass flow paths and variable inlet guide vanes in the bypass duct. As shown, pressure differences between the core duct and the bypass duct can cause cross flow between the ducts in the area of the fan blade. In  FIG. 6 , the Bypass flow duct lies radially outward from the Core flow duct. A fan blade is positioned upstream from a downstream splitter that separates air flow between the ducts. An upstream splitter is positioned upstream from the fan blade at the bottom of the inlet guide vane. 
         [0034]    As the inlet guide vane angle is changed, pressure within the bypass flow duct can differ from the pressure present in the core flow duct. The working fluid in this example air can cross between the two ducts in the vicinity of the fan blade and cause detrimental engine performance as explained previously. 
         [0035]      FIGS. 7 a -7 b    illustrate the upstream splitter  25  with a plurality of vortex generators  70 . The vortex generator  70  results in counter rotating vortices  72  that are paired and circumferentially positioned around the exit plane (trailing edge) of the upstream splitter  24 . The vortices  72  are generated or tripped by several mechanisms as described further below, these mechanisms involve interruptions in the surface which disrupt and trip the bypass flow  30  or core flow  28 . In  FIGS. 7 a   - 7   b,  the vortex pairs are tripped using intermittent subtle ramps or wedges that initiate a vortex  72 . It is a localized pressure differential in the flow which initiates the vortices  72 . The vortices  72  have momentum that tends to maintain its flow position in the region  50  between the ducts that inhibits flow and pressure loss between the ducts. The vortex  72  has momentum that the relative low pressure air from flowing in the ducts cannot overcome thereby causing the series of adjacent vortices to act as a fluidic barrier  51  to limit cross flow between the ducts. 
         [0036]    While for ease of illustration, the surface interruptions are shown on the top side or outside surface of the upstream splitter  24 . In  FIGS. 7 a   - 7   b,  the surface interruptions are preferably on the inner surface of the upstream splitter interrupting the core flow. This arrangement becomes more advantageous as the bypass flow/pressure is substantially decreased by the by the closing inlet vane guides  15  in the bypass flow path  30 . 
         [0037]      FIGS. 8 a -8 d    illustrate several examples of surface interruptions envisioned for creating the vortices.  FIG. 8 a    shows a plurality of ridges  77  extending into the core stream  28 . The ridges  77  are oblique to the flow in order to initiate the vortices  72 .  FIG. 8 b    shows a plurality of blades  73  also oriented oblique to the air flow. The blades  73  may also be rotated as to change their orientation. For example, where the pressure differential between the bypass  30  and core paths  28  proximate the fan in region  50  is small, the need for a fluid barrier  51  is diminished and thus the blades  73  may be oriented with the flow in a first position  74  and only orient oblique to the flow when the pressure differential becomes significant in a second position  75 .  FIG. 8 c    shows the plurality of flaps  76  extending into the core stream  28 . Similarly as described with respect to the blades, the flaps  76  may be in a flush first position  74  when a fluid barrier  51  is not required and may be extended to a second position  75  to initiate the vortices  72  when desired.  FIG. 8 d    shows a plurality of grooves  78  recessed into the upstream splitter  24  in order to trip the flow and generate the vortices  72  as the fluid barrier  51 . The grooves  78  may extend to the end of the upstream splitter  24  or terminate proximate but before the trailing edge. 
         [0038]    As noted previously, the interruptions may be arranged to create complimentary pairs of vortices as shown in  FIGS. 7 a   - 7   b,  one rotating clockwise and the other rotating counter clockwise. Alternatively, the interruption may be arranged to create vortices that each rotate the same direction, or alternating between different directions as shown in  FIGS. 7 a -7 b    and  FIG. 8   a.    
         [0039]      FIG. 9 a    illustrates the generation of vortices  72  from the leading edge of the fan. As shown, a vane  82  extends from the leading edge  81  of the fan proximate the upstream splitter  24 . In  FIG. 9 a    or  9   b , the vane  82  is shown in the core stream  28 , however while less preferable, the vane  82  may be in the bypass flow  30  as well. The vane  82 , in  FIG. 9 a    acts as a low aspect ratio wing, and thus spills air from the high pressure side of the blade  42  to the low pressure side, thus generating vortices  72  that extend along the border region  50  between the bypass  30  and core stream  28 . As shown in  FIG. 9 b   , the vane  82  may be an extension of the fan blade  42  upstream, in which a significant gap  83  between the vane  82  and the upstream splitter  24  allows high pressure air to escape to the low pressure side which also results in the creation of vortices  72  as a fluid barrier  51 . In addition the vane  82  may be stepped in order to produce a series of vortices on each blade and creating a radial gradient of vortices. 
         [0040]    An embodiment of the vane may be triangular with a root and vane leading edge. The root extending upstream of a trailing edge of the upstream splitter and the vane leading edge extending from an upstream portion of the root into the core stream and terminating on the leading edge of one of the plurality of fan blades. The vane  82  may also be of many other known wing shapes that facilitate spillage to create vortices. 
         [0041]    Alternatively other surface disruptions may be utilized on the leading edge  81  of the fan  42  to create the vortices which act as fluid barriers between the core and bypass streams. For example, groves or protrusions similar to those described in  FIGS. 8 a -8 d    can be added to the fan blades to generate the vortices. 
         [0042]      FIG. 5  is an illustration of a third splitter with multiple fan stages according to embodiments of the disclosed subject matter. The fans  42  may be nested with a midstream or third splitter  19  between them. In such case, the third splitter  19  would advantageously also be provided with similar surface interruptions or jets to provide a fluid barrier  51  between the third splitter  19  and downstream splitter  25 . 
         [0043]      FIG. 10  shows a flow chart of a method of reducing the work performed on the bypass stream  30 , while preventing a pressure drop in the core stream  28 . The ambient air stream is divided into a bypass stream and a core stream as shown in block  101 . It is not uncommon for the ambient air stream to be divided into multiple bypass or core streams, and the method is equally applicable in those instances, and thus is not so limited to the examples shown. In block  103 , the flow in the bypass stream in restricted. Typically this will be through the use of inlet guide vanes  15  as shown in  FIG. 1B  and described above. The flow may also be restricted by completely or partially closing off the bypass duct or ducts. The step of restricting the bypass flow may be accomplished prior to, contemporaneously or subsequent to the step of dividing the streams. 
         [0044]    A fluid barrier  51  is then created between the upstream and downstream splitters proximate the fan to prevent leakage and pressure loss from the core duct to the lower pressure bypass duct as shown in block  105 . As discussed above, the fluid barrier  51  may be established through jets  60  on the low pressure side of the blade as shown in  FIG. 3 , jets  60  originating from the upstream splitter  25  as shown in  FIG. 4 . The fluid barrier  51  may also be established through the use of vortices, from the splitter  24  as shown in  FIGS. 7 a   - 8   d,  or vortices created from vane  82  or gap  83  by the fan  42  as shown in  FIGS. 9 a   - 9   b.  The core stream is compressed by the fan, without the leakage into the bypass duct as shown in block  107 , and work on the bypass field by the fan is thus reduced by minimizing pressure leakage and restricting the amount of mass flow in the bypass stream. 
         [0045]    While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence. Many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.