BACKGROUND 1. Field

[0001] The present invention relates to turbine blades for gas turbine engines, and in particular to turbine blade tips.

2. Description of the Related Art

[0002] Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine blade assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures.

[0003] Typically, turbine blade is formed from a root portion at one end and an elongated portion forming a blade that extends outwardly from a platform coupled to the root portion at an opposite end of the turbine blade. The blade is ordinarily composed of a tip opposite the root section, a leading edge, and a trailing edge. The tip of a turbine blade often has a tip feature to reduce the size of the gap between ring segments and blades in the gas path of the turbine to prevent tip flow leakage, which reduces the amount of torque generated by the turbine blades. The tip features are often referred to as squealer tips and are frequently incorporated onto the tips of blades to help reduce pressure losses between turbine stages. These features are designed to minimize the leakage between the blade tip and the ring segment.

SUMMARY

[0004] Briefly, aspects of the present invention provide a turbine blade having a squealer with a vortex disruptive fence. [0005] According to a first aspect of the invention, a turbine blade is provided, which comprises an airfoil comprising an outer wall formed by a pressure sidewall and a suction sidewall joined at a leading edge and at a trailing edge. The airfoil further comprises a squealer tip at a first radial end and a root at a second radial end generally opposite the first radial end for supporting the blade and for coupling the blade to a disc, and at least one internal cavity forming an internal cooling system. The squealer tip comprises an end cap disposed over the outer wall, a pressure side tip wall extending radially from the end cap and being aligned with an outer surface of the pressure sidewall, and a suction side tip wall extending radially from the end cap and being aligned with an outer surface of the suction sidewall. A plurality of coolant openings are formed through the end cap between the pressure side tip wall and the suction side tip wall, the coolant openings being fluidically connected to the internal cavity. The squealer tip further comprises a fence structure extending radially from the end cap and positioned adjacent to one or more of the coolant openings, for shielding a coolant ejected from said one or more coolant openings from a vortex formed by a gas path fluid flow over the squealer tip.

[0006] According to a second aspect of the invention, a turbine blade is provided, which comprises an airfoil comprising an outer wall formed by a pressure sidewall and a suction sidewall joined at a leading edge and at a trailing edge. The airfoil further comprises a squealer tip at a first radial end and a root at a second radial end generally opposite the first radial end for supporting the blade and for coupling the blade to a disc, and at least one internal cavity forming an internal cooling system. The squealer tip comprises an end cap disposed over the outer wall, a pressure side tip wall extending radially from the end cap and being aligned with an outer surface of the pressure sidewall, and a suction side tip wall extending radially from the end cap and being aligned with an outer surface of the suction sidewall. A plurality of coolant openings are formed through the end cap between the pressure side tip wall and the suction side tip wall, the coolant openings being fluidically connected to the internal cavity. The squealer tip further comprises one or more fence structures, each fence structure being associated with one or more of the coolant openings for shielding a coolant ejected from said one or more coolant openings from a vortex formed by a gas path fluid flow over the squealer tip. The fence structure extends height-wise radially from the end cap and is formed of: a first rib element extending transverse to a cross-flow of the gas path fluid from the pressure side tip wall to the suction side tip wall, and a second rib element extending transverse to an axial flow of the gas path fluid generally from the leading edge to the trailing edge.

[0007] According to a third aspect of the invention, a squealer tip is provided for a turbine blade. The squealer tip comprises an end cap configured to be disposed over a radially outer end of an airfoil outer wall. The squealer tip further comprises a pressure side tip wall and a suction side tip wall. The pressure side tip wall extends radially from the end cap and is configured to be aligned with an outer surface of an airfoil pressure sidewall. The suction side tip wall extends radially from the end cap and is configured to be aligned with an outer surface of an airfoil suction sidewall. A plurality of coolant openings are formed through the end cap between the pressure side tip wall and the suction side tip wall, the coolant openings being configured to be in fluid communication with an airfoil internal cavity. The squealer tip further comprises a fence structure extending radially from the end cap and positioned adjacent to one or more of the coolant openings, for shielding a coolant ejected from said one or more coolant openings from a vortex formed by a gas path fluid flow over the squealer tip.

BRIEF DESCRIPTION OF THE DRAWINGS [0008] The invention is shown in more detail by help of figures. The figures show specific configurations and do not limit the scope of the invention.

[0009] FIG 1 is a perspective view of a turbine blade with a squealer tip;

[0010] FIG 2 is a schematic cross-sectional view along the section II-II of FIG 1;

[0011] FIG 3 is an enlarged perspective view of a squealer tip of a turbine blade according one embodiment of the present invention; [0012] FIG 4 is a schematic top view looking radially inward toward a squealer tip according to one embodiment of the present invention;

[0013] FIG 5 is a schematic cross-sectional view along the section V-V of FIG 4; and

[0014] FIG 6 is a schematic cross-sectional view along the section VI-VI of FIG 4.

DETAILED DESCRIPTION

[0015] In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

[0016] Referring to the drawings wherein identical reference characters denote the same elements throughout the various views, FIG 1 illustrates an exemplary turbine blade 1. The turbine blade 1 includes a conventional dovetail 2, which may have any suitable form including tangs that engage with complementary tangs of a dovetail slot in a rotor disc (not shown) for radially retaining the blade 1 to the rotor disc as it rotates during operation of the turbine engine. A blade shank 4 extends radially outwardly from the dovetail 2 and terminates in a platform 6 that projects laterally outwardly from and surrounds the shank 4. A generally hollow airfoil 10 extends radially outwardly from the platform 6 and into a stream of a hot gas path fluid. The airfoil 10 comprises an outer wall 12 which is formed of a generally concave pressure sidewall 14 and a generally convex suction sidewall 16 joined together at a leading edge 18 and at a trailing edge 20. The airfoil 10 extends from a root 26 at a radially inner first end to a tip 30 at a radially outer second end, and may take any configuration suitable for extracting energy from the hot gas stream and causing rotation of the rotor disc. As shown in FIG 2, the interior of the hollow airfoil 10 comprises at least one internal cavity 28 defined between an inner surface 14b of the pressure sidewall 14 and an inner surface 16b of the suction sidewall 16, to form an internal cooling system for the turbine blade 1. The internal cooling system may receive a coolant, such as air diverted from a compressor section (not shown), which may enter the internal cavity 28 via coolant supply passages typically provided in the blade root 26. Within the internal cavity 28, the coolant may flow in a generally radial direction, absorbing heat from the inner surfaces 14b, 16b of the outer wall 12, before being discharged via external orifices 17, 19, 37, 38 into the hot gas path.

[0017] Particularly in high pressure turbine stages, the tip 30 may be formed as a so- called "squealer tip". Referring jointly to FIGS 1-2, the squealer tip 30 may be formed of an end cap 32, a pressure side tip wall 34 and a suction side tip wall 36. The end cap 32 is disposed over the outer wall 12 at the radially outer end of the outer wall 12. The pressure and suction side tip walls 34 and 36 extend radially outwardly from the end cap 32 and may extend substantially or entirely along the perimeter of the end cap 32 to define a tip cavity 35 between an inner surface 34b of the pressure side tip wall 34 and an inner surface 36b of the suction side tip wall 36. An outer surface 34a of the pressure side tip wall 34 may be aligned flush with an outer surface 14a of the pressure sidewall 14, while an outer surface 36a of the suction side tip wall 36 may be aligned flush with an outer surface 16a of the suction sidewall 16.

[0018] The squealer tip 30 further includes a plurality of cooling holes 37, 38 that fluidically connect the internal cavity 28 with an external surface of the squealer tip 30 exposed to the hot gas path fluid. In the shown example, the cooling holes 37, referred to as pressure side tip cooling holes, are formed through the pressure side tip wall 34 and are arranged in an array along at least a portion of the perimeter of the pressure side tip wall 34 from the leading edge 18 to the trailing edge 20. The cooling holes 38, referred to as suction side tip cooling holes, are formed through the end cap 32 and open into the tip cavity 35. The suction side tip cooling holes 38 are arranged in an array generally parallel to and in close proximity to the suction side tip wall 36, i.e., offset toward the suction side tip wall 36 in relation to an airfoil centerline 29 extending centrally between the pressure and suction sidewalls 14 and 16.

[0019] The present inventor has recognized that particularly in case of the suction side tip cooling holes 38, the coolant ejected from the holes 38 into the tip cavity 35 tends to be entrained in a vortex FV formed by a gas path fluid flow over the squealer tip 30. The vortex is understood to be formed by an interaction of a cross-flow FX of the gas path fluid with an axial flow FA of the gas path fluid over the tip cavity 35. The cross-flow FX takes place from the pressure side tip wall 34 to the suction side tip wall 36, while the axial flow FA takes place along an engine axis generally in a chordal direction from the leading edge 18 toward the trailing edge 20. The vortex FV tends to pull the coolant rejected from the cooling holes 38 away from the surface to be cooled, leading to insufficient cooling of the squealer tip 30. Consequently, regions adjacent to the suction side tip cooling holes 38 tend to be overheated by the gas path fluid, leading to the formation of hotspots. Such an overheating is however typically less pronounced in regions adjacent to the pressure side tip cooling holes 37. It is an endeavor of the present inventor to address one or more of the above technical problems.

[0020] FIGS 3-6 illustrate exemplary embodiments of the present invention, wherein like reference characters are retained for like elements in relation to the configuration shown in FIGS 1-2. As shown in FIGS 3-4, a squealer tip 30 according to one embodiment may comprise one or more fence structures 40 extending radially outwardly from the end cap 32, for shielding the coolant ejected from one or more of the tip cooling holes 38 from the above-described vortex formed by gas path fluid flow over the squealer tip 30. In the illustrated embodiment, a plurality of fence structures 40 are arranged in series generally parallel to the suction side tip wall 36. The fence structures 40 are positioned adjacent to the suction side tip cooling holes 38, with each fence structure 40 being associated with one or more of the suction side tip cooling holes 38. In the shown example, each fence structure 40 is positioned to shield two suction side tip cooling holes 38a, 38b. However, the above example is not meant to be limiting as the number of cooling holes 38 associated with each fence structure 40 may be a matter of design choice. It is to be noted in the views of FIGS 3 and 5, the cooling holes 38a-b are actually obstructed by the fence structure 40 and therefore not visible, but are shown nevertheless for illustration purposes only. [0021] Referring to FIGS 3-5, in the illustrated embodiment, each fence structure 40 comprises a first rib element 42, namely a cross-flow blocking rib 42. The cross-flow blocking rib 42 extends height-wise (H42) radially from the end cap 32 and further extends length-wise (L42) transverse to the cross-flow FX of the gas path fluid from the pressure side tip wall 34 to the suction side tip wall 36. In the illustrated embodiment, as shown in FIG 4, the lengthwise (L42) extension of the cross-flow blocking rib 42 may be approximately perpendicular to a direction of the cross-flow FX of the gas path fluid. The geometric configuration of the cross-flow blocking rib 42 shields the coolant K ejected from the respective associated cooling holes 38a-b from the cross-flow FX. To this end, the cross-flow blocking rib 42 may be positioned such that the associated cooling holes 38a-b are located between the cross-flow blocking rib 42 and the suction side tip wall 36. In this example embodiment, the cross-flow blocking rib 42 extends length-wise (L42) generally parallel to the suction side tip wall 36.

[0022] Referring to FIGS 3, 4 and 6, the illustrated fence structure 40 may further comprise a second rib element 44, namely an axial flow blocking rib 44. The axial flow blocking rib extends height-wise (H44) radially from the end cap 32 and further extends lengthwise (L44) transverse to an axial flow FA of the gas path fluid (which is directed into the plane of FIG 6). In the illustrated embodiment, as shown in FIG 4, the lengthwise (L44) extension of the axial flow blocking rib 44 may be approximately perpendicular to a direction the axial flow FA of the gas path fluid. The geometric configuration of the axial flow blocking rib 44 shields the coolant K ejected from the respective associated cooling holes 38a-b from the axial flow FA of the gas path fluid. In this example embodiment, the axial flow blocking rib 44 extends lengthwise (L44) transverse to the cross-flow blocking rib 42 and the suction side tip wall 36, connecting the cross-flow blocking rib 42 to the suction side tip wall 36.

[0023] The first rib element 42 and the second rib element 44, individually and more so in combination, serve to disrupt the vortex formation resulting from the interaction of the cross-flow FX and the axial flow FA of the gas path fluid in the tip cavity 35. The vortex disruption by the fence structure 40 prevents the coolant flow K in the tip cavity 35 from being pulled out of the tip cavity 35, but instead to be guided within the tip cavity 35 to provide proper cooling to the regions adjoining the suction side tip cooling holes 38a-b. The inventive design thus eliminates overheating and formation of hot spots in the squealer tip 30, resulting in a cooler blade tip with less risk of damage and longer life. As an added benefit, on account of the improved cooling, it may be possible to reduce the number of cooling holes 37 on the pressure side tip wall 34, thereby reducing coolant flow requirement and increasing turbine efficiency. Yet another technical effect that may result from the inventive configuration is that the coolant flow induced by the geometry of the fence structure 40 may block the leakage of gas path fluid from the pressure side to the suction side reducing the tip leakage and increasing aerodynamic performance.

[0024] In the illustrated embodiment, each fence structure 40 comprises a generally reshaped profile defined by the first and second rib elements 42 and 44. It should however be noted that the shapes of the fence structures 40 may be optimized to maximize cooling while reducing the aerodynamic losses of the tip leakage. For example, in alternate embodiments, one or more of the fence structures 40 may include an arc- shaped, U- shaped or V-shaped profile, among others. It is also to be noted that the first and second rib elements 42, 44 may be seamlessly connected or may be discrete from each other. The radial heights H42 and H44 respectively of the cross-flow blocking rib 42 and the axial flow blocking rib 44 may be configured based on aerodynamic and/or heat transfer considerations. However, in order to keep the mass of the squealer tip 30 as low as possible, the width W42 of the cross-flow blocking rib 42 may be preferably much smaller than the width W36 of the suction side tip wall 36. Likewise, for at least the same reason, the width W44 of the axial flow blocking rib 44 may be preferably much smaller than the width W36 of the suction side tip wall 36. In still other embodiments, the fence structure 40 may consist of only one of the cross-flow blocking rib 42 or the axial flow blocking rib 44.

[0025] In one embodiment, the inventive turbine blade may be formed as a one-piece casting of a suitable superalloy, such as a nickel-based superalloy, which has acceptable strength at the elevated temperatures of operation in a gas turbine engine. In an alternate embodiment, the turbine blade with the squealer tip may be monolithically formed by additive manufacturing techniques or 3-D printing, for example, using selective laser melting (SLM). In yet another embodiment, aspects of the present invention may be implemented as a low cost modification of an existing turbine blade wherein an inventive squealer tip may be separately formed and retrofitted to an existing turbine blade, for example as a service upgrade.

[0026] While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.