Perforated Turbine Bucket Tip Cover

Abstract

An airfoil with a tip cover surface treatment for reducing leakage flows, minimizing tip vortex size and penetration into main flow that will improve turbine efficiency. The surface treatment for the airfoil tip covers includes a series of concave shapes, such as grooves or holes. These grooves and holes will cause the leakage flow into separate flow paths within the cavities and generate more resistance to leakage flows through the airfoil tip clearance, thereby reducing the leakage mass flow and weakening tip vortex and its interaction with turbine main flows. The material removed from the tip cover provides the additional benefit of reducing the weight of the tip cover, thereby enhancing blade reliability.

Description

BACKGROUND OF THE INVENTION

[0001] The invention relates generally to turbine blades for a turbine engine and more specifically to a tip shroud for turbine blades.

[0002] Turbine blades are rotating airfoil-shaped components in series of stages designed to convert thermal energy from a working fluid, such as gas or steam, into mechanical work of turning a rotor. Performance of a turbine can be enhanced by sealing the outer edge of the blade tip to prevent the working gas from escaping the working flowpath into the gaps between a blade tip and an outer casing of turbine. A common manner for sealing the gap between the turbine blade tips and the turbine casing is through blade tip shrouds. Not only do shrouds enhance turbine performance, but also serve as a vibration damper, especially for large, radial-length turbine blades. The shroud acts as a mechanism to raise the blade natural frequency and in turn minimizes failures due to extended resonance time of the blade at a natural frequency.

[0003] A portion of a typical turbine blade with a shroud (also referred to as turbine bucket cover or tip cover) is shown in FIG. 1. The turbine blade 10 includes an airfoil section 11 and shroud 12. The shroud 12 may be manufactured integral to the airfoil 11. The airfoil further contains a leading edge 15 and trailing edge 16 that run generally perpendicular to shroud 12. The shroud 12 has a thickness and has sidewalls 17, which may be cut to create an interlocking configuration when adjacent turbine blades are present. The interlocking mechanism occurs along two bearing faces 13, where adjacent turbine blades (not shown) contact at shroud 12. It is the interlocking of the turbine blade shrouds 12 at bearing faces 13, that creates means for damping out vibrations, as well as for sealing the working fluid within the turbine gas-path. An additional feature of a typical turbine blade shroud may be a knife-edge tip seal (tooth seal) 14. Depending upon the size of the blade shroud, one or more tip seals may be utilized. These seals run parallel to each other, typically perpendicular to the engine axis, and extend outward from shroud 12. The purpose of these seals is to engage the shroud blocks of the turbine casing (not shown) to further minimize leakage around the blade tip and reduce mechanical impact in case the bucket tip rubs the casing. Tip leakage diverts working fluid that would otherwise flow in a main flow path and perform work on the turbine blades. Tip leakage may further result in elevated tip votex size and intensity, which may penetrate the main steam flow path downstream from the blade, raising backpressure and thereby lowering the efficiency of the stage. However, a clearance between the tip seal and the casing shroud needs to be provided to account for thermal expansion and asymmetry of rotation. A fully covered turbine blade tip has better aerodynamics performance over uncovered bucket tip because of reduced tip vortex size, intensity, and tip leakage.

[0004] While the purpose of the shroud is to seal the working fluid within the flow path as well as to provide a means to dampen vibrations, the shroud has its disadvantages as well. A drawback to the shroud concept is the weight the shroud adds to the turbine blade. During operation, the turbine blades spin on a disk, about the engine axis 18. A typical industrial application includes disk speeds up to 3600 revolutions per minute. The blades are held in the disk by an interlocking cut-out between the blade root and the disk. As the turbine blade spins, the centrifugal forces cause the blade to load outward on the turbine disk at this attachment point. The amount of loading on the disk and hence the blade root, which holds the blade in the disk, is a function of the blade weight. That is, the heavier the blade, the more load and stresses are found on the interface between the blade root and disk, for a given revolutions per minute. Excessive loading on the blade root and disk can reduce the overall life of each component. Another drawback to shrouds is creep curling of the blade shrouds. Depending on the thickness of the shroud, the shroud edges can "curl" up at their ends and introduce severe bending stresses in the fillets between the shroud and blade tip. Shrouds curl due to the bending load on the edges of the shroud from gas pressure loads as well as centrifugal loads. The curling of a shroud is analogous to the bending of a cantilevered beam due to a load at the free end of the beam. An industry known fix to this curling phenomenon is to increase the section thickness of the shroud uniformly which will result in a stiffer shroud and more resistance to curling. The downside to simply increasing the shroud thickness uniformly is the additional weight that is added to the shroud by this additional material.

[0005] As described above, to prevent the tip cover from rubbing turbine casing wall and to further reduce tip leakage, one or several seal teeth can be placed on the top of a tip cover. According to recent computational flow dynamic (CFD) analysis of 48'' last stage bucket of a low pressure turbine, adding a seal tooth may reduce the stage efficiency by about 0.5% since increased bucket tip volume can cause tip vortex to penetrate deeper into main flow.

[0006] Accordingly, it would be desirable to limit tip leakage for turbine blades, while at the same time providing enhanced stage efficiency.

BRIEF DESCRIPTION OF THE INVENTION

[0007] According to a first aspect of the present invention, there is provided a blade for a turbine engine including a root, a tip, a leading edge, a trailing edge, and opposed pressure and suction sidewalls extending generally along a radial axis within an inner casing of the turbine engine. The blade includes a tip cover extending between a tip-cover pressure sidewall and a tip-cover suction sidewall. A top surface on the tip cover extends between the tip-cover pressure sidewall and the tip-cover suction sidewall.

[0008] The top surface is configured to permit the blade to rotate with a limited clearance to a shroud of a casing of the turbine engine. A thickness of the tip cover extends from the top surface to a tip of the blade. A plurality of depressions are provided over essentially the full top surface of the tip. The depressions extend into the thickness of the tip cover and are adapted to restrict leakage flow between the tip cover and the shroud of the casing of the turbine engine.

[0009] According to another aspect of the present invention, a method is provided for fabricating a blade for a turbine engine. The method includes providing a blade including a tip cover with a top surface extending between a tip-cover pressure sidewall and a tip-cover suction sidewall, a first interlocking end at a trailing edge of the rotor blade and a second interlocking end at a leading edge of the rotor blade, and a thickness of the tip cover extending radially from the top surface of the tip cover to the rotor blade. The method includes removing a portion of the thickness of the tip cover forming a plurality of depressions over essentially the full top surface of the tip cover, the plurality of depressions being adapted to restrict leakage flow between the tip cover and the shroud of the casing of the turbine engine.

BRIEF DESCRIPTION OF THE DRAWING

[0010] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0011] FIG. 1 illustrates a prior art shroud for a turbine blade;

[0012] FIG. 2 illustrates turbine blade incorporating an inventive tip cover;

[0013] FIG. 3 illustrates an isometric view of a first embodiment of the inventive tip cover;

[0014] FIG. 4 illustrates a top isometric view of interlocked tip covers according to the first embodiment of the present invention;

[0015] FIG. 5 illustrates an impact of the inventive depressions on a tip leakage path for the working fluid between the tip cover for a rotor blade and the turbine casing;

[0016] FIG. 6 illustrates reduced axial velocity of tip leakage flow with the inventive surface treatment;

[0017] FIG. 7 illustrates a second embodiment of the present invention wherein the depressions are formed as holes over the top surface of the tip cover;

[0018] FIG. 8 illustrates a further embodiment of the present inventive tip cover incorporating a seal tooth; and

[0019] FIG. 9 illustrates a flowchart of an exemplary embodiment of a method for fabricating a blade with a surface treatment of depressions that may be used with a turbine engine, including a steam turbine engine or a gas turbine engine.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The following embodiments of the present invention have many advantages, including reducing bucket tip leakage flow, increasing rotor torque work, minimizing mixing losses caused by tip vortex, improving turbine performance and reducing tip cover weight.

[0021] The present invention relates to an airfoil with a tip cover surface treatment for reducing leakage flows, minimizing tip vortex size and penetration into main flow that will improve turbine efficiency. An aspect of the present invention is to treat airfoil tip covers with a series of concave shapes, such as grooves or holes. These grooves and holes will cause the leakage flow into separate flow paths within the cavities and generate more resistance to leakage flows through the airfoil tip clearance, that reduces the leakage mass flow and weakens tip vortex and its interaction with turbine main flows. The material removed from the tip cover provides the additional benefit of reducing the weight of the tip cover, thereby enhancing blade reliability. The surface treatment may also eliminate the need for using one or more tooth seals on the tip cover to reduce leakage flow. The toothless fully covered blade tip cover provides superior aerodynamic performance over any of partially covered tip shrouds.

[0022] FIG. 2 illustrates a turbine blade incorporating an inventive tip cover. The tip cover 25 may be attached to an airfoil 21 of a blade 20 for a turbine engine. The airfoil 21 may include a root 30, a tip 35, a leading edge 40, a trailing edge 41, a pressure side 45 and a suction side 50. A dovetail arrangement 32 in the root section 30 engages the airfoil to the rotor wheel of the turbine engine (not shown). The tip cover 25 may be formed integral with the airfoil 21 at the tip end 35. The tip cover 25 may include a plurality of depressions 80 in a top surface 55 adapted to reducing leakage between the top surface of the airfoil and the shroud of the turbine engine casing.

[0023] FIG. 3 illustrates an expanded isometric view of a first embodiment of the inventive tip cover. The tip cover 25 may include a top surface 55, a tip-cover pressure sidewall 45, and a tip-cover suction sidewall 50, the top surface 55 extending between the tip-cover pressure sidewall 60 and the tip-cover suction sidewall 65. The tip cover 25 may have a thickness 66 in a general inward radial direction with respect to the root 30 of the airfoil 21 (FIG. 2). The top surface 55 of the tip cover may be generally conformed to an inner surface of a stage shroud for the casing of the turbine engine (not shown), so as to provide a limited clearance there-between to limit leakage of working fluid between the stages of the turbine engine. The tip cover 25 may further include a first interlocking end 70 and a second interlocking end 71 providing bearing surfaces 75 to engage opposing bearing surfaces of tip covers of a leading turbine blade and a trailing turbine blade (see FIG. 4) of a turbine wheel when the wheel is rotating at operating speed. The tip cover 25 may include a plurality of depressions 80 adapted to reducing leakage between the top surface of the airfoil and the shroud of the turbine engine casing. In the first embodiment, the depressions 80 may include a plurality of grooves 85 in the top surface 55 Each groove 85 may be aligned parallel to each other, extending in length in a direction generally between the first interlocking 70 end and the second interlocking end 71 of the tip cover 25. The groove 85 may be formed as a U-shaped depression with groove floor 81. The walls of the grooves may be essentially normal to the face of the top surface of the tip cover 25. The groove 85 may include a width 86 and a depth 87 below the top surface 55.

[0024] On each adjacent side of the parallel-arranged grooves 85 there may be situated a wall 88 of an elevated height 87 relative to the grooves. The top 89 of the elevated wall 88 is formed by the top surface 55 of the tip cover 25. The elevated wall 88 may be aligned generally in parallel with other elevated walls 88 and in generally in parallel with the plurality of grooves 85. The walls 88 may extend in length in a direction generally between the first interlocking end 70 and the second interlocking end 71 of the tip cover 25. The elevated walls 88 are also arranged normal to the leakage flow 90 across the tip cover from the tip-cover pressure sidewall 60 to the tip-cover suction sidewall 65.

[0025] FIG. 4 illustrates a top isometric view of interlocked tip covers according to the first embodiment of the present invention. The turbine blade 20 is situated between leading turbine blade 23 and a trailing turbine blade 24. Bearing surfaces 75 of tip cover 25 for the turbine blade 20 engage the complimentary bearing surface 76 of the leading turbine blade 23 and the trailing turbine blade 24 when the rotor wheel is turning at operating speed. The grooves 85 and elevated surfaces of the tip cover 25 for the turbine blade 20 are aligned with the grooves and elevated surfaces of the tip covers for the leading turbine blade 23 and the trailing turbine blade 24, thereby presenting continuous elevated walls and continuous grooves between adjacent turbine blades when the turbine is at operating speed to oppose tip leakage flow 90 from tip-cover pressure sidewall to the tip-cover suction sidewall.

[0026] FIG. 5 illustrates an impact of the inventive depressions on a tip leakage path for the working fluid between the tip cover for a rotor blade and the turbine casing. A clearance 100 is provided between the top surface 55 of tip cover 25 and the inner surface 95 of turbine casing 96. The clearance 100 must be maintained to account for thermal expansion and rotational eccentricity. Tip seal leakage 90 is driven by a differential pressure between the pressure side 60 and the suction side 65 of the tip cover 25 for the blade 20. The grooves 85 and elevated walls 88 are seen from the side. The tip leakage flow 90 passes through the clearance 100 above the top surface 55 of the tip cover 25. Low pressure in each of grooves 85 diverts a part of the tip leakage flow 90 near the top surface 55 of the tip cover 25, thereby creating vortex circulation 110 in and about the plurality of grooves 85. The vortex circulation 110 within the plurality of grooves 85 impinges into the clearance 100, restricting the effective tip clearance 105 available to pass tip leakage flow, thereby reducing the leakage flow 90.

[0027] FIG. 6 illustrates reduced axial velocity of steam flow at a rotor outlet over an outer section of a span for a blade with a grooved surface treatment. The relative axial velocity 210 of the steam flow vs. percent turbine span 215 is plotted for a flat tip shroud without seal teeth 220 and a tip shroud with the inventive grooves 230. Axial velocity is relatively constant over a span 240 from about 75% to 97.5% of blade length. For the area between the tip shroud and the casing shroud 250 (97.5% to 100% span), a large increase in the axial velocity resulting from tip leakage relative to the bulk working steam flow path the blade section is shown for both cases. The inventive groove arrangement provides approximately a 15% velocity reduction over the case with the inclined tip shroud. The inventive grooves of the tip cover slow the steam velocity between the tip cover and the casing shroud, resulting in less leakage compared to other arrangements. The reduced tip leakage further minimizes tip vortex size and penetration into the main working gas flowpath, thereby increasing rotor torque and improved stage performance.

[0028] A second embodiment of the present invention is illustrated in FIG. 7. The blade 20 and the tip cover 25 remain as described for the first embodiment. The depressions 80 may include a plurality of holes 125 essentially covering the top surface 55 of the tip cover 25. In one configuration, the holes may be aligned in a plurality of rows 130 extending between the opposite interlocking ends 70, 71 of the tip cover 25 and a plurality of columns 135 extending between the pressure side 60 and the suction side 65 of the tip cover 25. The holes 125 of adjacent columns 135 may be staggered relative to the adjacent rows 130 as illustrated in FIG. 7. The holes 125 may include a radius 126 and a depth 127. Further, it should be understood that various other arrangements of holes on the top surface of the tip cover are possible within the scope of the present invention. Similar to the operation of the grooves of the first embodiment as depicted in FIG. 5, the holes 125 of the top surface 55 may divert a portion of the tip leakage forming vortices that extend into the tip leakage path, thereby having the effect of reducing the leakage passing through the tip clearance. The shape of the holes 135 may be generally circular, although other shapes may be considered within the scope of the present invention. The holes are usually close ended.

[0029] FIG. 8 illustrates a further embodiment of the present inventive blade incorporating a tip cover including a seal tooth. Tip cover 25 is mounted on airfoil 21. The top surface 55 of the tip cover 25 includes a plurality of grooves 85 between the tip cover and a casing shroud (not shown). The plurality of grooves 85 is adapted for reducing tip leakage, as previously described. The tip cover 55 may further include a seal tooth 150. The seal tooth 150 may be disposed along a length of the top surface 55 between bearing surfaces 75, forming a barrier to tip leakage. When the. turbine blade 20 is mounted with a leading turbine blade and a trailing turbine blade (not shown), the tip seal may align with the tip seals of these adjacent blades to form a continuous barrier limiting tip leakage.

[0030] According to another aspect of the present invention, a method is provided for fabricating a blade for a turbine engine. FIG. 9 illustrates a flowchart of an exemplary embodiment of a method 200 of fabricating a blade that may be used with a turbine engine, including a steam turbine engine or a gas turbine engine. Although the method 200 may be used to fabricate any blade, the method will be described with respect to fabricating blade (20) shown in FIG. 2. Method 200 includes providing in step 202 the blade 20 (as shown in FIG. 2) The blade being provided includes a tip cover with a top surface extending between a tip-cover pressure sidewall and a tip-cover suction sidewall, a first interlocking end at a trailing edge of the rotor blade and a second interlocking end at a leading edge of the rotor blade, and a thickness of the tip cover extending radially from the top surface of the tip cover to the rotor blade. Step 204 includes removing a portion of the thickness of the tip cover forming a plurality of depressions over essentially the full top surface of the tip cover, the plurality of depressions being adapted to adapted to restrict leakage flow between the tip cover and the shroud of the casing of the turbine engine.

[0031] Removing a portion of the thickness of the top cover may include forming a plurality of grooves extending parallel to each other between a first interlocking end of the tip cover and a second interlocking end of the tip cover. Forming a plurality of grooves may include forming grooves on the tip covers of adjacent blades so when the blades are rotating at a full operating speed, the grooves at the first interlocking end of the tip cover for each blade are aligned with the grooves of the second interlocking end of a tip cover on an adjacent blade and the grooves at the second interlocking end of the tip cover are aligned with the grooves of the first interlocking end of a tip cover on an adjacent blade. The method for fabricating a blade may further include forming at least one seal tooth on the top surface of the tip cover, the seal tooth situated generally parallel with the grooves on the top surface and the seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

[0032] Alternatively, the step of removing a portion of the thickness of the top cover may include forming a plurality of holes in the top surface of the tip cover extending into the thickness of the tip cover and arranging the holes in a plurality of rows extending between the opposite interlocking ends of the tip cover and a plurality of columns extending between the pressure side and the suction side of the tip cover. The step of arranging the closed-end holes may also include staggering the holes of adjacent columns of holes relative to the holes of the adjacent rows of holes. The method for fabricating a blade may further include forming at least one seal tooth on the top surface of the tip cover, the seal tooth situated generally parallel with the rows of holes on the top surface and the seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

[0033] The surface treatment may also provide about 0.1% improvement in stage efficiency over the inclined tip shroud and about a 0.07% improvement in stage efficiency over the flat tip shroud. Further, the surface treatment may provide about a 0.33% work extraction increase over the inclined tip shroud and about a +0.08% improvement in work extraction over a flat tip shroud without seal tooth.

[0034] While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.

Claims

1. A blade for a turbine engine including a root, a tip, a leading edge, a trailing edge, and opposed pressure and suction sidewalls extending generally along a radial axis within an inner casing of the turbine engine, the blade comprising: a tip cover extending between a tip-cover pressure sidewall and a tip-cover suction sidewall; a top surface on the tip cover extending between the tip-cover pressure sidewall and the tip-cover suction sidewall and adapted to permit the blade to rotate with a limited clearance to a shroud of a casing of the turbine engine; a thickness of the tip cover extending from the top surface to a tip of the blade; a plurality of depressions over essentially the full top surface of the tip cover, the plurality of depressions extending into the thickness of the tip cover and adapted to restrict leakage flow between the tip cover and the shroud of the casing of the turbine engine.

2. The blade according to claim 1, the plurality of depressions comprising: a plurality of grooves extending into the thickness of the tip cover.

3. The blade according to claim 2, wherein the plurality of grooves extend parallel to each other between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

4. The blade according to claim 3, wherein the plurality of grooves include a U-shaped cross-section.

5. The blade according to claim 4, wherein the U-shaped cross-section includes vertical walls roughly normal to the top surface of the tip cover and a bottom surface roughly parallel to the top surface of the tip cover.

6. The blade according to claim 5, wherein when the blade is rotating at a full operating speed the plurality of grooves at the first interlocking end of the tip cover are aligned with the plurality of grooves of the second interlocking end of a tip cover on an adjacent blade and the plurality of grooves at the second interlocking end of the tip cover are aligned with the plurality of grooves of the first interlocking end of a tip cover on an adjacent blade.

7. The blade according to claim 2, further comprising at least one seal tooth on the top surface of the tip cover, the at least one seal tooth disposed generally parallel with the plurality of grooves on the top surface and the at least one seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

8. The blade according to claim 1, wherein the plurality of depressions comprise a plurality of holes in the top surface of the tip cover extending into the thickness of the tip cover, the plurality of holes being aligned in a plurality of rows extending between the opposite interlocking ends of the tip cover and a plurality of columns extending between the pressure side and the suction side of the tip cover.

9. The blade according to claim 8, wherein the holes of adjacent columns may be staggered relative to the holes of the adjacent rows of holes.

10. The blade according to claim 8 wherein when the blade is rotating at a full operating speed, the rows of holes at the first interlocking end of the tip cover are aligned with the rows of holes of the second interlocking end of a tip cover on an adjacent blade and the rows of holes at the second interlocking end of the tip cover are aligned with the rows of holes of the first interlocking end of a tip cover on an adjacent blade.

11. The blade according to claim 2, further comprising: at least one seal tooth on the top surface of the tip cover, the at least one seal tooth situated generally parallel with the rows of holes on the top surface and the at least one seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

12. The blade according to claim 1, wherein the turbine engine is a gas turbine.

13. The blade according to claim 1, wherein the turbine engine is a steam turbine.

14. A method for fabricating a blade for a turbine engine, the method comprising: providing a blade including a tip cover with a top surface extending between a tip-cover pressure sidewall and a tip-cover suction sidewall, a first interlocking end at a trailing edge of the rotor blade and a second interlocking end at a leading edge of the rotor blade, and a thickness of the tip cover extending radially from the top surface of the tip cover to the rotor blade; and removing a portion of the thickness of the tip cover forming a plurality of depressions over essentially the full top surface of the tip cover, the plurality of depressions being adapted to adapted to restrict leakage flow between the tip cover and the shroud of the casing of the turbine engine.

15. The method for fabricating a blade according to claim 14, the step of removing a portion of the thickness of the top cover comprising: forming a plurality of grooves extending parallel to each other between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

16. The method for fabricating a blade according to claim 15, the step of forming a plurality of grooves comprising: forming grooves on the tip covers of adjacent blades so when the blades are rotating at a full operating speed, the grooves at the first interlocking end of the tip cover for each blade are aligned with the grooves of the second interlocking end of a tip cover on an adjacent blade and the grooves at the second interlocking end of the tip cover are aligned with the grooves of the first interlocking end of a tip cover on an adjacent blade.

17. The method for fabricating a blade according to claim 15, further comprising: forming at least one seal tooth on the top surface of the tip cover, the seal tooth situated generally parallel with the grooves on the top surface and the seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.

18. The method for fabricating a blade according to claim 17, the step of removing a portion of the thickness of the top cover comprising: forming a plurality of holes in the top surface of the tip cover extending into the thickness of the tip cover; and arranging the holes in a plurality of rows extending between the opposite interlocking ends of the tip cover and a plurality of columns extending between the pressure side and the suction side of the tip cover.

19. The method for fabricating a blade according to claim 14, the step of arranging the closed-end holes comprising: staggering the holes of adjacent columns of holes relative to the holes of the adjacent rows of holes.

20. The method for fabricating a blade according to claim 14, further comprising: forming at least one seal tooth on the top surface of the tip cover, the seal tooth situated generally parallel with the rows of holes on the top surface and the seal tooth extending between a first interlocking end of the tip cover and a second interlocking end of the tip cover.