Patent Publication Number: US-8123471-B2

Title: Variable stator vane contoured button

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to aircraft gas turbine engines and, particularly, to variable stator vane buttons. 
     2. Background Information 
     Variable stator vanes are commonly used in aircraft gas turbine engine compressors and fans and in some turbine designs. Non-rotating or stationary stator vanes typically are placed downstream or upstream of rotor blades of the fans, compressors, and turbines. These vanes reduce the tangential flow component leaving the rotors, thereby increasing the static pressure of the fluid and setting the flow angle to a level appropriate for the downstream rotor. The stator vanes carry a lift on the airfoil of the stator vane due to a higher static pressure on the pressure side of the airfoil and a lower static pressure on the suction side of the airfoil. 
     Due to the large range of operating conditions experienced by an axial flow compressor over a typical operating cycle, flow rates and rotational speeds of the compressor also vary widely. This results in large shifts in the absolute flow angle entering the stator vanes. To allow the vanes to accommodate these shifts in flow angle without encountering high loss or flow separation, circumferential rows of variable stator vanes are constructed so that the vanes can be rotated about their radial (or approximately radial) axis. 
     Generally, variable stator vanes (VSVs) have spindles through their rotational axis that penetrate the casing, allowing the vanes to be rotated using an actuation mechanism. At the flowpath, there will typically be a button of material around the spindle which rotates along with the vane. However, the size of this button is normally limited by the pitchwise spacing of the VSVs, resulting in a portion of the vane chord at the endwalls where a gap exists between the flowpath and the vane. 
     Because there is a large pressure gradient between the pressure and suction sides of the vane, leakage flow is driven across this gap, resulting in reduced fluid turning and higher loss at the endwalls. 
     This leakage flow also causes flow non-uniformities (i.e. wakes) at the adjacent rotor blades, which may excite these blades causing potentially damaging vibrations in the rotor blades. It is, thus, desirable to reduce the chordwise extent of this gap and the accompanying leakage flow. To this end, VSV buttons have been designed to cover inner and outer diameter ends of the VSV airfoil. The coverage of the ends is desirable because it minimizes endwall losses due to leakage flow at the endwall gap between the vanes and the walls of the flow passageway. 
     Conventional VSV buttons typically have diameters equal to or slightly less than the pitchwise spacing between vanes at their respective locations. This is because larger buttons would overlap with one another making it physically impossible to fit the vane assemblies together. In some cases, designers have specified flats or arched cuts on the sides of the buttons to allow the use of larger button diameters, thereby achieving greater endwall coverage. However, these configurations typically result in large cavities between buttons and often have large flowpath gaps near the vane leading edges leading to undesirable losses and large wakes. 
     Thus, it is highly desirable to provide buttons which minimize endwall leakage and operate over a wide range of vane angle settings. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A variable stator vane includes an airfoil mounted on a button centered about a rotational axis and leading and trailing edges and pressure and suction sides of the airfoil. The button has circular leading and trailing edges circumscribed about the rotational axis at a button radius and that generally correspond to the airfoil leading and trailing edges respectively. The circular leading edge is upstream of the circular trailing edge. contoured pressure and suction sides of the button extend from the circular leading edge to the circular trailing edge and are recessed inwardly from a perimeter circumscribed about the rotational axis at the button radius. The contoured pressure side has upstream and downstream pressure side portions and the suction side has upstream and downstream suction side portions. One of the upstream and downstream pressure side portions is substantially straight and another of the upstream and downstream pressure side portions is substantially convexly curved. One of the upstream and downstream suction side portions is substantially straight and another of the upstream and downstream suction side portions is substantially convexly curved. One of the upstream pressure side portion and the upstream suction side portion is substantially straight and another of the upstream pressure side portion and the upstream suction side portion is substantially convexly curved. 
     Another embodiment of the variable stator vane includes a circular second curved section of the downstream pressure side portion of the button and the circular second curved section extends from a downstream end point of the downstream pressure side portion to the trailing edge. 
     The downstream suction side portion of the button may generally coincide with the suction side of the airfoil. 
     A more particular embodiment of the variable stator vane includes the airfoil disposed between spaced apart outer and inner buttons centered about a rotational axis. An outer spindle may extend outwardly from the outer button and an inner spindle may extend inwardly from the inner button. 
     The variable stator vane design may be incorporated in a gas turbine engine variable vane assembly having at least one circular row of variable stator vanes wherein each of the variable stator vanes includes an airfoil disposed between spaced apart outer and inner buttons centered about a rotational axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustration of a portion of a gas turbine engine high pressure compressor variable stator vanes and contoured buttons. 
         FIG. 2  is a perspective view illustration of several of the compressor variable stator vanes and contoured buttons illustrated in  FIG. 1 . 
         FIG. 3  is an enlarged perspective view illustration of one of the compressor variable stator vanes and its contoured buttons illustrated in  FIG. 2 . 
         FIG. 4  is another enlarged perspective view illustration looking radially outwardly of one of the compressor variable stator vanes illustrated in  FIG. 3 . 
         FIG. 5  is a perspective view illustration looking radially inwardly of three adjacent compressor variable stator vanes illustrated in  FIG. 3 . 
         FIG. 6  is a diagrammatic illustration of an airfoil cross-section superimposed on a contoured button of one of the vanes illustrated in  FIG. 3 . 
         FIG. 7  is a diagrammatic illustration of an exemplary method used to contour the buttons illustrated in  FIG. 3 . 
         FIG. 8  is a diagrammatic illustration of results from the exemplary method illustrated in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in  FIG. 1  is a portion of an exemplary turbofan gas turbine engine high pressure compressor  10  axisymmetrical about a longitudinal or axial centerline axis  12 . Circular first and second rows  11 ,  13  of variable stator vanes  15  (VSVs) are disposed in the compressor  10  and used to optimize the direction at which gases flowing through the compressor  10  enter first and second rows  17 ,  18  of rotatable blades  16 . Though the exemplary embodiment of the VSVs disclosed herein is for a high pressure compressor, the VSV&#39;s may be used in other compressor sections and in fan and turbine sections of a gas turbine engine as well. A compressor casing  61  supports variable stator vane assemblies  56  which include the variable stator vanes  15 . 
     Referring to  FIGS. 2-4 , each variable stator vane assembly  56  includes a plurality of variable stator vanes  15 . Each variable stator vane  15  is pivotable or rotatable about a rotational axis  20 . Each variable stator vane  15  has an airfoil  31  disposed between spaced apart outer and inner buttons  32 ,  33 . An outer spindle  34  extends outwardly from the outer button  32  and an inner spindle  35  extends inwardly from the inner button  33 . The outer and inner spindles  34 ,  35  are rotatably supported in outer and inner trunnions  36 ,  37  respectively as illustrated in  FIG. 1 . 
     Referring to  FIG. 1 , the outer spindle  34  is rotatably disposed through the outer trunnion  36  which, in turn, is mounted in an outer opening  78  in the casing  61 . The inner spindle  35  is rotatably disposed through the inner trunnion  37  which, in turn, is mounted in an inner opening  79  in an inner ring  81  which is spaced radially inwardly of the casing  61 . A lever arm  80  extends from the outer spindle  34  and is linked to an actuation ring  82  for rotating or pivoting and setting the flow angle of the variable stator vanes  15 . 
     Referring to  FIGS. 1 and 2 , the outer and inner buttons  32 ,  33  are rotatably disposed in outer and inner circular recesses  42 ,  43  in the casing  61  and the inner ring  81  respectively. Each airfoil  31  has an airfoil leading edge LE upstream U of an airfoil trailing edges TE and pressure and suction sides PS, SS. Referring to  FIGS. 2-6 , the outer and inner buttons  32 ,  33  each have circular leading and trailing edges  52 ,  53  generally corresponding to the airfoil leading and trailing edges LE, TE and the circular leading edge  52  is upstream of the circular trailing edge  53 . The circular leading and trailing edges  52 ,  53  are circumscribed about the rotational axis  20  at a button radius R. The outer and inner buttons  32 ,  33  each have contoured pressure and suction sides  58 ,  59  extending downstream D from the circular leading edge  52  to the circular trailing edge  53 . The contoured pressure and suction sides  58 ,  59  generally correspond to and face in the same circumferential directions as the airfoil pressure and suction sides PS, SS respectively. 
     Illustrated in  FIG. 6 , is an exemplary button  54  representative of the outer and inner buttons  32 ,  33 . The button  54  includes the circular leading and trailing edges  52 ,  53  which define a circular perimeter  22  within which the button  54  rotates about the rotational axis  20 . The circular perimeter  22  is circumscribed about the rotational axis  20  at the button radius R from the rotational axis  20 . The contoured pressure and suction sides  58 ,  59  are cut out or recessed in from the perimeter  22 . The contoured pressure side  58  has upstream and downstream pressure side portions  24 ,  26 . The contoured suction side  59  has upstream and downstream suction side portions  28 ,  30 . The side portions are either substantially straight (linear) or substantially convexly curved (curvilinear). Side portions, in diagonally opposite quadrants of the button, are similarly shaped and are either substantially straight or convexly curved. Upstream pressure and suction side portions have opposite shapes, one being substantially straight and the other being substantially convexly curved. Note that the convexly curved side portions, in diagonally opposite quadrants of the button, are similarly shaped but most likely do not have the same curved shape. 
     Another way of describing this is as follows: one of the upstream and downstream pressure side portions  24 ,  26  is substantially straight and another of the upstream and downstream pressure side portions  24 ,  26  is substantially convexly curved; one of the upstream and downstream suction side portions  28 ,  30  is substantially straight and another of the upstream and downstream suction side portions  28 ,  30  is substantially convexly curved; and one of the upstream pressure side portion  24  and the upstream suction side portion  28  is substantially straight and another of the upstream pressure side portion  24  and the upstream suction side portion  28  is substantially convexly curved. 
     The button  54  illustrated herein has a linear upstream pressure side portion  24  and a linear downstream suction side portion  30 . Thus, the button  54  illustrated herein also has a convexly curved upstream suction side portion  28  and a convexly curved downstream pressure side portion  26 . Alternatively, the upstream pressure side portion  24  and the downstream suction side portion  30  may be convexly curved and the upstream suction side portion  28  and the downstream pressure side portion  26  may be straight. The combinations are designed to maximize the area A of the button  54  while accommodating a large turning angle (not shown) of the variable stator vanes  15 . In order to further maximize the area A of the button  54 , the downstream suction side portion  30  of the button  54  generally coincides with the suction side SS of the airfoil  31  in the exemplary embodiment of the button  54  illustrated in  FIG. 6 . 
     The contoured pressure and suction sides  58 ,  59  are cut out or recessed in from the perimeter  22  and shaped to accommodate button diameters  44  of the buttons that are greater than pitchwise spacing SP between adjacent ones of the airfoils  31  as measured from rotational axes  20  of the airfoils  31  of adjacent ones of the variable stator vanes  15  as illustrated in  FIGS. 6 and 7 . Buttons having button diameters greater than pitchwise spacing would otherwise overlap with one another, making it physically impossible to fit the vane assemblies together. This button geometry allows increased VSV endwall coverage while simultaneously limiting the size of the exposed cavities in the outer and inner circular recesses  42 ,  43  as illustrated in  FIG. 1  as well as in inner and outer endwall regions  19  and  21  at critical operating conditions. 
       FIG. 7  illustrates a method for sizing and shaping the buttons  54  illustrated in  FIG. 6  using adjacent first and second button templates  60 ,  62  each of which includes an airfoil template  66  mounted thereon. The button diameter  44  of the first and second button template  60 ,  62  is set to a maximum reasonable size giving a combination of high VSV endwall coverage and acceptable overlap. The exemplary embodiment of the method illustrated herein uses 80-100% coverage of the airfoil endwall, which is represented by the airfoil template  66 , or 10-40% button overlap which is overlap of adjacent button perimeters  22 . An exemplary method of drawing profiles for contoured pressure and suction sides  58 ,  59  illustrated herein includes the following steps. 
     Step  1 , the first and second button templates  60 ,  62  are rotated so the airfoil templates  66  are positioned at their maximum closed position as illustrated by the narrowest allowable opening  94  between the leading edge LE and the suction side SS of adjacent airfoil endwalls or airfoil template  66 . A first point P 1  is located on the perimeter  22  of the second button template  62  substantially nearest the leading edge LE of the airfoil template  66  of the second button template  62 . Point P 1  is generally located within 50%-200% of an airfoil max thickness TM of the leading edge LE. 
     A second point P 2  is located substantially near an intersection of the perimeter  22  of the first button template  60  and the suction side SS of the airfoil template  66  on the adjacent first button template  60 . Point P 2  is generally located within 50% of airfoil max thickness TM of the airfoil suction side SS. A first straight line  90  between the first and second points P 1 , P 2  defines the upstream pressure side portion  24  of the contoured pressure side  58  and the downstream suction side portion  30  of the contoured suction side  59  of the button  54 . The first point P 1  also defines the intersection of the circular leading edge  52  and the upstream pressure side portion  24  of the contoured pressure side  58  of the button  54 . 
     The airfoil templates  66  are then rotated incrementally open until the airfoil templates  66  are positioned at their maximum open position as illustrated by the widest allowable opening  95  between the leading edge LE and the suction side SS of adjacent airfoil endwalls or airfoil template  66 . By rotating the respective button templates  62  third and fourth points P 3  and P 4  are defined on the buttons to clear the corners (the first and second points P 1 , P 2 ) of the adjacent buttons. This process is repeated to define or locate fifth through tenth points P 5 -P 10  until the corners clear the adjacent button. Then, the points are connected to create first and second smooth curve  126 ,  127  and combined with the first and a second straight lines  90 ,  91  respectively, as illustrated in  FIG. 8 , to define the contoured pressure and suction sides  58 ,  59  of the buttons  54 . 
     If the vanes are rotated to their full open position, and the second point P 2  on the first button template  60  has not cleared the trailing edge  53  of the second button template  62 , then a second curved section  133  of the downstream pressure side portion  26  of the button  54  is needed. The second curved section  133  is defined by a circular curve between the tenth point P 10 , or last point, of the first smooth curve  126  and the trailing edge  53  of the second button template  62  and is concentric with the trailing edge  53  of the first button template  60 . The above process describes how to generate first and second nominal button cutouts  158 ,  159  for the first and second button templates  60 ,  62  used to define the contoured pressure and suction sides  58 ,  59  of the buttons  54 . The nominal cutouts will be offset closer to each other by a small amount, typically 0-0.02″, to allow actual parts to be assembled with normal manufacturers variation, internal corners between adjacent surfaces of the upstream and downstream suction side portions  28 ,  30 ; upstream and downstream pressure side portions  24 ,  26 ; and the second curved section  133  will be blended, typically, with a fillet radius in a range of about 0.03-0.10 inches, for manufacturability and mechanical robustness. 
     The preferred embodiment provides a minimum overall gap between the buttons, although not necessarily the minimum pocket at the nominal design angle, and provides another potential benefit in that, in the event of a broken lever arm  80  (which sets the angle of the VSV), the affected vane will actually be guided to follow the adjacent vanes (without broken arms), rather than simply be subject to aero loads or lock in place due to friction, which can cause excessive aero distortion and induce damaging vibration to the rotor blades. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.