Abstract:
A gas turbine engine component has an airfoil and a squealer tip. The airfoil has a pressure side and a suction side. The squealer tip is located at one end of the airfoil to engage with an adjacent surface and thereby form a seal. The squealer tip terminates in a squealer tip apex with an arched cross-sectional profile in a plane extending from the pressure side to the suction side of the airfoil. A method for producing an airfoil seal for the gas turbine engine component is also provided.

Description:
BACKGROUND 
     The present invention relates generally to an airfoil seal arrangement, and more particularly to an arrangement of a gas turbine engine having airfoils with squealer tips. 
     A gas turbine engine comprises a compressor that pressurizes air, a combustor that mixes pressurized air from the compressor with fuel and ignites the resulting fuel-air mixture, and a turbine that extracts energy from the ignited mixture downstream of the combustor. Both the compressor and turbine includes a plurality of airfoil elements, often in multiple stages. These airfoil elements comprise rotor blades and stator vanes located in airflow passages generally defined by gas turbine engine casings, rotors, and shrouds. Rotor blades rotate relative to stator vanes that generally remain stationary with respect to the body of the gas turbine engine. Airflow leakage around the tips of blades and vanes at respective outer and inner airflow diameters of airflow passages reduces gas turbine engine efficiency. To avoid this, a compressor is conventionally constructed with a minimal gap between blade or vane tips and adjacent stationary or rotating surfaces, respectively. Blades and vanes need not form perfect air seals with these adjacent surfaces, but are designed to reduce gas bleed. To this end, squealer tips of blades and vanes are commonly manufactured with labyrinth or knife-edge seals. Some blades or vanes with knife-edge seals use thin or tapered “squealer” tips. During a break-in cycle of the gas turbine engine, these squealer tips are abraded by contact with adjacent engine components. Stator vane squealer tips, for instance, make contact with an adjacent inner airflow diameter shroud or rotor land surfaces within the gas turbine engine. Frictional contact between the shroud or rotor land and the stator vane squealer tip abrades the squealer tip until only a uniform minimum gap remains between the stator vane and the rotor. This abrasion process can melt blade or vane squealer tips, and sometimes liberates abraded debris from the stator vane, rotor surface, or both. Liberated debris can reduce component lifetimes within the gas turbine engine. 
     SUMMARY 
     The present invention relates to a gas turbine engine component and a method of forming a seal with the gas turbine engine component. The gas turbine engine component has an airfoil and a squealer tip. The airfoil has a pressure side and a suction side. The squealer tip is located at one end of the airfoil to engage with an adjacent surface and thereby form a seal. The squealer tip terminates in a squealer tip apex with an arched cross-sectional profile in a plane extending from the pressure side to the suction side of the airfoil. A method for producing an airfoil seal for the gas turbine engine component is also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified cross-sectional view of a gas turbine engine comprising a compressor, a combustor, and a turbine. 
         FIG. 2  is a cross-sectional view of the compressor of  FIG. 1 . 
         FIG. 3   a  is a perspective view of a stator section of the compressor of  FIG. 2 . 
         FIG. 3   b  is a cross-sectional view of the stator section of  FIG. 3   a.    
         FIG. 4  is a close-up cross-sectional view of a squealer tip of a stator vane from the stator section of  FIGS. 3   a  and  3   b.    
         FIG. 5  is close-up cross-sectional view of a machining step for forming the squealer tip of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified cross-sectional view of gas turbine engine  10 , comprising compressor  12 , combustor  14 , and turbine  16 . Compressor  12  has stator vanes  20  and rotor  17  with rotor blades  18 . Turbine  16  drives rotor  17  of compressor  12 , and may also drive an electrical generator (not shown). In some embodiments, compressor  12  and turbine  14  may have a plurality of stages. Air flows along indicated airflow path AF through gas turbine engine  10 . Compressor  12  receives and pressurizes atmospheric gas or air by rotational movement of rotor blades  18  relative to stator vanes  20  and about rotational axis A. Rotor blades  18  and stator vanes  20  are rigid airfoil elements with pressure and suction sides that pressurize and decelerate gas, respectively. Fuel is injected into combustor  14 , where it mixes with pressurized gas from combustor  12 . Combustor  14  ignites the resulting fuel-air mixture, increasing the temperature of the gas. Turbine  16  extracts mechanical energy from hot, high-pressure gas downstream of combustor  14 . 
     Gas leakage along airflow path AF around inner or outer radial extents of rotor blades  18  or stator vanes  20  results in diminished compression efficiency. To reduce such leakage, stator vane  20  is formed with a narrow squealer tip that minimizes a gap distance between stator vane  20  and an adjacent surface, such as a shroud or a rotor surface, as described below with respect to squealer tips  28  of  FIGS. 2 ,  3   a , and  3   b.    
       FIG. 2  is a simplified cross-sectional view of a section of compressor  12  of gas turbine engine  10 . Compressor  12  comprises rotor  17 , rotor blades  18 , stator vanes  20 , casing  22 , rotor land  24 , and abrasive layer  26 . Each stator vane  20  has squealer tip  28 , a sacrificial section at the innermost radial extent of stator vane  20 . In the depicted embodiment, stator vane  20  is mounted on casing  22  of compressor  12 , and projects generally radially inward from outer diameter OD to squealer tip  28  of vane  20  near rotor land  24  carried by rotor  17 , generally at inner diameter ID. In some embodiments, compressor  12  may further include shrouds located at inner diameter ID or outer diameter OD. Rotor land  24  is a smooth portion of rotor  15  that includes a region radially adjacent to stator vane  20 . In some embodiments, rotor blades  18 , stator vanes  20  (including squealer tip  28 ), and rotor land  24  may be formed of a precipitation strengthened high Ni-based alloy, such as austenitic nickel-chromium-based superalloys IN100 or Inconel 718. 
     Operation of gas turbine engine  10  produces large amounts of heat, causing components to thermally expand. Different components heat and expand at different rates, causing gaps between some components—most significantly between rotating and non-rotating components—to vary over the course of each operational cycle of gas turbine engine  10 . 
     To minimize gas leakage between squealer tip  28  and rotor land  24 , squealer tip  28  is constructed to impinge slightly on rotor land  24  during a portion of an initial break-in cycle of gas turbine engine  10 , because of thermal expansion. During this break-in cycle, squealer tip  28  contacts and rubs against rotor land  24 , and is abraded or worn down such that all squealer tips  28  terminate at a uniform radius that minimizes any gap or clearance from rotor land  24 , and that exhibits minimal eccentricity. In some embodiments, rotor land  24  may be coated with abrasive layer  26 . Abrasive layer  26  is a thin coating of abrasive material that helps to mill or grind squealer tip  28  during the break-in cycle. Abrasive layer  26  may be formed as an ablative layer of sacrificial material deposited on rotor land  24 , such as aluminum oxide or zirconium oxide. In such embodiments, both abrasive layer  26  and squealer tip  28  are abradable. During the break-in cycle, contact between squealer tip  28  and abrasive layer  26  on rotor land  24  grinds both squealer tip  28  and abrasive layer  26 , thereby forming a final stator structure with little eccentricity and minimum separation between rotor land  24  and stator vane  20 . 
       FIG. 3   a  is a perspective view of stator section  30  of compressor  12 .  FIG. 3   b  is a cross-sectional view of stator section  30  through section plane  3   b - 3   b  of  FIG. 3   a . Section plane  3   b - 3   b  extends through pressure and suction sides of stator vanes  20 . Stator section  30  forms one angular segment of a stage of stator vanes  20  of compressor  12 . Stator section  30  comprises a plurality of stator vanes  20  having a common stator root  32  anchored in casing  22  (see  FIG. 2 ), or in a compressor shroud (not shown). Stator vanes  20  each have squealer tips  28  with squealer tip edges  34 . In the depicted embodiment, squealer tips  28  are elongated, tapered tips with a squealer tip thickness t st  considerably narrower than the bodies of stator vanes  20 , and squealer tip length 1 st &gt;2t st . Such narrow, elongated squealer tips are widely used in the art to reduce the amount of contact between stator vanes  20  and rotor land  24 , there reducing grinding and frictional heating of stator vanes  20 . Squealer tips  28  may, for instance, be tapered, cast faired squealer tips at an obtuse angle Θ to direction of rotation D rot  of adjacent rotor land  24 . Squealer tips  28  may be cast-in during the formation of stator section  30 , for instance to a squealer tip thickness t st  as low as approximately 0.02 inches (˜0.5 mm). Alternatively, squealer tips  28  may be ground or otherwise machined to form narrow, tapered tips. 
     Each squealer tip  28  has squealer tip apex  34 . Squealer tip apex  34  has an arched profile which further reduces contact area between squealer tip  28  and rotor land  24 . Squealer tip apex  34  may, for instance, have a circular or elliptical profile. Squealer tip  28 , and in particular squealer tip apex  34 , provides a narrow point of contact between stator vane  20  and rotor land  24  (see  FIG. 2 ). Contact width W contact  on squealer tip apex  34  increases as stator vane  20  rubs in to rotor land  24 , up to a maximum of approximately the thickness of squealer tip  28 , as depicted in  FIG. 4  and described below. 
       FIG. 4  is a close-up cross-sectional view of squealer tip  28  with squealer tip apex  34 .  FIG. 4  indicates grind distance d g , squealer tip thickness t st , and contact width W contact  between squealer tip  28  and adjacent rotor land  24  (not shown). During a break-in cycle, squealer tip  28  and rotor land  24  abrade one another, grinding away at least a portion of squealer tip  28  such that squealer tip  28  is shortened by grind distance d g . For instance, where squealer tip  28  is a narrow, tapered tip with squealer tip thickness t st =0.02 in. (˜0.5 mm), and squealer tip apex  34  has circular profile with corresponding radius 0.01 in. (˜0.25 mm), stator vane  20  may have grind distance d g  up to 0.001 in. (˜0.25 mm). As discussed above, rotor land  24  may also be abraded during the break-in cycle. 
     Grinding during the break-in cycle produces a uniform inner rotor diameter ID (see  FIG. 2 ). Over the course of the break-in cycle, the contact area between each squealer tip apex  34  and adjacent rotor land  24  increases, as squealer tip  28  is abraded. Because grind takes place primarily at depths substantially less than the radius of curvature of squealer tip edge  28  (i.e. d g &lt;½t st ), the contact area between stator vane  20  and rotor land  24  remains less than the thickness of squealer tip  28  during the majority of the break-in cycle. Where squealer tip apex  34  has a circular profile, for instance:
 
 W   contact ≈2√{square root over ( t   st   d   g   −d   g   2 )}  [Equation 1]
 
     (where W contact  is the width of the contact area at a particular grind distance d g ). 
     The circular or elliptical profile of squealer tip apex  34  thus reduces initial contact area between stator vane  20  and rotor land  24  during a break-in cycle of compressor  12 . Although squealer tip  28  has been described as a narrow, tapered tip, a worker skilled in the art will recognize that providing squealer tip apex  34  with a circular or elliptical cross-sectional profile will reduce contact area between stator vane  20  and rotor land  24 , even where squealer tip  28  does not narrow near squealer tip apex  34 . 
     Reduced contact area between rotor land  24  and stator vanes  20  results in decreased frictional heating of rotor land  24  and stator vanes  20  while stator vanes  20  rub in against rotor land  24  at pinch point or points of the aforementioned break-in cycle. At high temperatures, squealer tip apex  34  can melt, rather than grind. Squealer tip apex  34  reduces melting by minimizing contact area between stator vanes  20  and rotor land  24 , thereby reducing frictional heating. Additionally, the narrow cross-section of squealer tips  28  results in a low total volume of material ablated from stator vanes  20  and rotor land  24  (or abrasive layer  26  on rotor land  24 ), and thus a decrease in liberated debris. Although the preceding discussion has focused on a squealer tip structure that reduces contact area between stator vanes  20  and rotor land  24  (or abrasive layer  26  thereon), a worker skilled in the art will recognize that some compressor rotor blades  18  may also benefit from squealer tips with arched profiles at their radially outermost extents, which reduce contact area between rotor blades  18  and radially adjacent shroud or casing sections. Similarly, although the preceding discussion has focused on air seals for compressor  12 , squealer tips with arched profiles may also be provided for rotor blades or stator vanes of turbine  16 . 
       FIG. 5  is a close-up cross-sectional view of a machining step for stator vane  20 . In particular,  FIG. 5  depicts squealer tip apex  34  of squealer tip  28  being shaped by brush wheel  100 . At least one brush wheel  100  is used to shape the rounded cross-section of squealer tip apex  34 , characterized above. In one embodiment, squealer tip edges  34  are machined in-case with stator vanes  20  in an assembled state to provide a close match between stator vanes  20  and rotor land  24 , and a uniform inner diameter ID. In this embodiment, stator sections  30  are assembled in casing  22  (see  FIG. 2 ), while at least one rotary brush wheel  100  is inserted in the place of rotor  17  to grind or shape squealer tip edges  34 . 
     In one embodiment a conventional rotary grinder is used to grind squealer tip edges  34  to a uniform inner diameter ID (see  FIG. 2 ) close to the eventual location of rotor land  24 . This rotary grinder is then removed, and replaced with brush wheel  100 . This brush wheel may, for instance, be a ring of nylon bristles impregnated with abrasive material such as aluminum oxide or silicon carbide. Rotation of brush wheel  100  relative to squealer tip apex  34  removes burrs left from previous machining steps, and rounds squealer tip apex  34  to produce the circular or elliptical profile previously discussed. The rotation speed of brush wheel  100  and the dwell time of the machining process are adjusted to optimize inner diameter ID and the cross-section of squealer tip edges  34 . In some embodiments, stator sections  30  are also rotated about the axis of compressor  12  during these machining steps. In such embodiments, the rotation speed of stator sections  30  can also be adjusted to optimize inner diameter ID and the cross-section of squealer tip edges  34 . Once squealer tip edges  34  have been machined to a desired cross-sectional profile, stator sections  30  are reassembled with other components of gas turbine engine  10 . 
     The circular or elliptical cross-section of squealer tip apex  34  provides reduced contact area between stator vane  20  and rotor land  24 . Because d g &lt;t st , This reduced contact area results in less melting and less debris liberation during break-in cycles of compressor  12 . Squealer tip apex  34  can be inexpensively and quickly produced using brush wheel  100 . 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.