Patent Publication Number: US-7594794-B2

Title: Leaned high pressure compressor inlet guide vane

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to the field of variable geometry guide vanes for gas turbine engines. More specifically, the invention relates to variable geometry guide vane assemblies that reduce stress placed on downstream compressor blades. 
     A gas turbine engine compressor typically includes inlet guide vanes followed by a row, or stage of compressor rotor blades. A fan (military style) or high pressure compressor will only have one row of inlet guide vanes. There may be other rows of variable vanes, but they may differ in their principle of operation. During operation, air is sequentially compressed by the compressor stages. The compressed air is channeled to a combustor and mixed with fuel and ignited. The hot combustion gases generated power the engine. 
     Axial compressors rely on spinning blades that have airfoil sections similar to airplane wings. As with airplane wings, in some conditions the blades can stall or surge. If this occurs, the airflow around the stalled compressor can reverse direction violently. Many compressors are fitted with anti-stall systems such as bleed bands or variable geometry guide vanes to decrease the likelihood of surge. 
     To ensure compressor stability over a wide range of mass flow rates and operating speeds, variable guide vanes are employed. Guide vanes are usually cast structures having an airfoil and a platform. The aerodynamic vanes turn the airstreams through an angle to meet the blades of a following compressor stage and reduce the effective inlet area of the stage. 
     Variable guide vane assemblies use blades that can be individually rotated around their axis, as opposed to the power axis of the engine. For startup they are rotated to open, reducing compression, and then are rotated back into the airflow as operating conditions require. Closing the guide vanes progressively as compressor speed falls reduces the slope of the surge (or stall) line, improving the surge margin of the engine. 
     Vane movement is accomplished by coupling a corresponding vane arm to the outer ends of each vane and joining the vane arms to a common actuation or unison ring for providing uniform adjustment of the individual vanes. Each vane must be identically angled relative to the other vanes in the ring to maximize efficiency and prevent undesirable aerodynamic distortion from a misaligned vane. 
     Current variable geometry inlet guide vanes are positioned radially around the longitudinal engine axis. A typical variable inlet guide vane assembly is shown in  FIG. 1 . A problem experienced with current variable geometry guide vane designs is a stress that manifests itself at the root, or inner radial ends of the downstream compressor blades. The high stress experienced is due to unsteady air formed at their outer radial ends. The unsteady air pushes and pulls on the blades, stressing where they couple to an inner concentric engine structure. 
     Radial inlet guide vanes do not direct a uniform velocity of air across the downstream compressor blades as their geometry changes in response to engine demands. As a result, the compressor blades experience an unbalanced loading of air velocities with slower moving, separated air concentrated near the outer radial end regions. 
     What is desired is a variable geometry guide vane assembly that reduces unwanted compressor blade or fan blade stresses. The invention provides a solution to this problem. 
     SUMMARY OF THE INVENTION 
     The inventors have discovered that it would be desirable to have variable geometry guide vane assemblies that reduce stress placed on downstream compressor blades in gas turbine engines. 
     The invention circumferentially leans the guide vanes away from the pressure side at the outer radial diameter, effectively pushing the engine core air flow radially, towards the outer diameter and reducing airflow separations on the guide vane near the outer radial diameter. This allows for aerodynamic stresses on the downstream rotor blades to be reduced. 
     One aspect of the invention provides a variable geometry guide vane assembly for a gas turbine engine. Variable geometry guide vane assemblies according to this aspect comprise a plurality of vanes having a leading section and a trailing section pivotally mounted about an axis defined through a lower trunnion and an upper trunnion, the plurality of vanes extend between an inner concentric structure and an outer engine casing, where the lower trunnion is located at the inner concentric structure and the upper trunnion is located at the outer engine casing, and the axes for the plurality of vanes are not radial from the inner concentric structure. 
     Another aspect of the invention is where each vane axis is radially offset by an angular difference in a range of from greater than 0° to 30°. 
     Yet another aspect of the invention is a vane arm for a variable geometry guide vane assembly. Vane arms according to this aspect comprise a mounting end, a spherical bearing end having located therein a spherical-type bearing, and a hinge coupling the mounting end with the spherical bearing end. 
     Another aspect of the vane arm is the bearing end further comprises an end plane, a hinge plane, and a line of intersection wherein the line of intersection is defined where both planes meet. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial front axial view of a variable geometry radial guide vane assembly. 
         FIG. 2  is a partial front axial view of a variable geometry leaned guide vane assembly according to the invention. 
         FIG. 3  is a partial front sectional, axial view of an exemplary variable geometry leaned guide vane mounted according to the invention. 
         FIG. 4  is a partial top sectional view through an exemplary mounting portion of the leaned guide vane shown in  FIG. 3  taken along line  4 - 4 . 
         FIG. 5  is an exemplary exploded view of the variable geometry leaned guide vane shown in  FIG. 3  with a vane arm. 
         FIG. 6  is a partial perspective axial view of an exemplary variable geometry leaned guide vane assembly according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Further, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     The invention is a variable geometry leaned inlet guide vane assembly as shown in  FIGS. 2 ,  3  and  6 . The invention “leans” each guide vane away from the pressure side (direction of rotation) at the outer radial end. The lean for each vane may be set at one angular position. The vane axis is offset from a radius r by an angular difference θ in a range of 0°&lt;θ≦30°. 
       FIG. 2  shows a plurality of leaned guide vanes spaced apart equidistantly around the intake annulus of a gas turbine engine. Surrounding the intake annulus is an engine casing structure. The plurality of leaned guide vanes extends in a skewed, non-radial direction between an inner concentric structure and an outer engine casing. 
     The moveable vanes are mounted for selective rotation about an axis which passes through two trunnions. The angular rotation required of the movable vanes may be up to a maximum deflection of approximately 70°. Over the range of movement, the arc swept by the radially outer edges of the vanes has potential for interference with the annular shape of the inner surface of the engine casing. In order to accommodate this range of vane movement and to avoid gaps between the vane radially outer edge and the casing surface, these both conform to a part spherical surface configuration. Therefore a constant and minimal gap between the edge and surface may be maintained over the whole range of vane movement. 
     A vane actuating mechanism is provided on the radially outer side of the annular engine casing (not shown). This comprises a circumferentially movable unison ring to which the outer trunnion of each vane is connected by means of a vane arm. 
     Shown in  FIG. 3  is a portion of an annular stator casing  301  of an exemplary axial compressor for a gas turbine engine to which is mounted a plurality of circumferentially spaced apart variable geometry leaned guide vanes  303 . Each vane includes an airfoil  305  comprising leading and trailing edges, and high and low pressure sides. 
     Each vane  303  may be a cast structure and may be formed using any suitable casting technique known in the art. While the vanes  303  are preferably cast structures, they may also be machined if desired. 
     Each vane  303  further includes a radially outer trunnion  307  extending coaxially and integrally outwardly from the top of the airfoil  305  for pivotally mounting the airfoil  305  in a corresponding bushing  309  in the casing  301 . The vane  303  also includes a radially inner trunnion  311  mounted in a sealing ring  313 . Other variants of the invention may use other means to pivotally mount the airfoil  305  to the engine casing  301  and inner concentric structure  302 . 
     In order to selectively rotate the airfoil  305  during operation, the airfoil  305  includes a keyed, D-shaped seat  401  as shown in  FIG. 4  which extends radially outward from the trunnion  307  as shown in  FIG. 5 . A threaded stem  403  extends radially outward from the seat  401 . 
     The threaded stem  403  is cylindrical with a substantially constant outer diameter, whereas the seat  401  is unidirectional in an exemplary D-shaped configuration below the stem  403  to provide a self alignment feature for mounting a vane arm  405  atop the airfoil  305  for selective rotation during operation. The vane arm  405  is secured to the airfoil  305  by a threaded retaining nut  315 . Other variants of the invention may use other means such as keyed splines, crenulated surfaces in matching correspondence, or others to secure a vane arm  405  to a vane  303 . 
     Each vane arm  405  has a spherical bearing (Heim-type bearing)  503  end which cooperates with a pin  317  located on an annular actuation, or unison ring  319  for simultaneously rotating in unison each of the airfoils  305  in an individual leaned guide vane assembly. Actuating a leaned vane is difficult since a non-articulating, planar vane arm  405  motion is not tangential with respect to the unison ring  319 . 
     To compensate for the non-tangential travel the vane arm  405  experiences with respect to a unison ring  319  (radially offset 0°&lt;θ≦30°), the vane arm  405  includes a hinge  505 . The hinge  505  divides the vane arm  405  into a spherical bearing  503  end and a mounting end  509 . The hinge allows for rotational freedom in the range of about ±30° from a mounting end plane  509 . For guide vanes having approximately a 14° lean, a hinge rotation of ±9° should be sufficient. For guide vanes having approximately a 30° lean, a hinge rotation of ±20° should be sufficient. 
     The spherical bearing  503  end comprises two planes, an end plane  507  and a hinge plane  508  that form a line of intersection  511 . The intersection  511  is at an angle α with respect to a vane arm  405  longitude. The angle α may be placed on either side of the longitudinal reference depending on the embodiment desired. 
     The end plane  507  is angled at a dihedral from the hinge plane  508  at an angle of β. The angle β may be placed on either side of the hinge plane  508  depending on the embodiment desired. The range of motion offered by the hinge  505  in conjunction with the dihedral of the end  507  and hinge  508  planes allow for a non-binding freedom of movement as the unison ring  319  rotates to selectively pivot the airfoils  305 . 
     The function of the end plane  507  and hinge plane  508  is to position the end plane  507  tangent to the unison ring  319  when the guide vanes  303  are at the midpoint of rotation. Most applications may have a in a range of 90°≦α≦150° and β in a range of 0°&lt;β≦45°. 
     In a preferred embodiment, the mounting hole  407  is generally a D-shaped configuration in matching correspondence with the seat  401  around which it is seated. The seat  401  preferably includes a pair of opposite, parallel side flats  409  which define a width A of the seat  401 . The seat  401  also has an arcuate front  411  and a flat back  413  which define a length B of the seat  401 . The seat  401  may be narrower in width A than in length B. The mounting hole  407  includes a pair of opposite, parallel side walls  501  spaced apart at a width C. The mounting hole  407  also includes a generally arcuate front and a flat back which are spaced apart over a length D. The hole width C may be less than the hole length D to correspond with the configuration of the seat  401  and allow for precise alignment. As described above, other configurations for coupling a vane arm  405  to a guide vane  303  are possible. 
     The invention reduces stress placed on compressor blades which use upstream guide vanes, and fan blades which use upstream guide vanes in turbofan engines. The invention leans the guide vanes circumferentially, pushing engine core air flow towards the downstream blades. This allows the stresses on the downstream blades to be significantly reduced. 
     The invention overcomes the difference in articulation between a unison ring  319  and vane arm  405 . The hinged vane arm  405  of the invention couples with a unison ring  319  using a spherical joint  503 . The hinge  505  dividing the vane arm  405  permits the end plane  507  to follow the path of the unison ring  319 . This arrangement allows a leaned guide vane assembly to be actuated by a conventional unison ring. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.