WO2017200549A1

WO2017200549A1 - Tip shroud with a fence feature for discouraging pitch-wise over-tip leakage flow

Info

Publication number: WO2017200549A1
Application number: PCT/US2016/033471
Authority: WO
Inventors: Li Shing Wong; Kok-Mun Tham; Ching-Pang Lee
Original assignee: Siemens Aktiengesellschaft
Priority date: 2016-05-20
Filing date: 2016-05-20
Publication date: 2017-11-23

Abstract

A turbine blade tip shroud (22) includes a shroud base (20) extending in a circumferential direction of a turbine engine (64) and further extending in an axial direction of the turbine engine (64) from an upstream edge (56) to a downstream edge (58). A circumferential fin (50) extends radially outward from the shroud base (20) to run a tight gap with a stator component (51), to provide a sealing against a stream-wise over-tip leakage flow (FS) of gas path fluid in a direction generally from an airfoil leading edge (34) to an airfoil trailing edge (36). The shroud (22) includes a fence feature (70) extending generally radially outward with respect to an airfoil tip (24), and extending transverse to the circumferential fin (50), to discourage a pitch-wise over-tip leakage flow (FP) of gas path fluid in a direction from an airfoil pressure side (38) to an airfoil suction side (40).

Description

TIP SHROUD WITH A FENCE FEATURE FOR DISCOURAGING PITCH-WISE

OVER-TIP LEAKAGE FLOW

BACKGROUND 1. Field [0001] This invention is directed generally to turbine engines, and more particularly to a turbine blade having a tip shroud.

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] A turbine blade is formed from a root portion at one end and an elongated portion forming an airfoil that extends outwardly from a platform coupled to the root portion. 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. [0004] Some turbine blades include tip shrouds, attached to the tips of the turbine blades. Tip leakage loss, is essentially lost opportunity for work extraction and also contributes towards aerodynamic secondary loss. To reduce over-tip leakage, shrouded blades may include a circumferential fin, typically embodied as a knife edge seal, for running tight tip gaps with a stator component, typically a honeycomb structure. The turbine tip shrouds are also used for the purpose of blade damping. SUMMARY

[0005] Briefly, aspects of the present invention provide a turbine blade having a tip shroud with a fence feature for discouraging pitch-wise over-tip leakage flow.

[0006] According to an aspect of the present invention, a turbine blade is provided. The turbine blade comprises a generally elongated airfoil having a leading edge, a trailing edge, a pressure side, a suction side on a side opposite to the pressure side, a tip at a radially outer end of the airfoil, a root coupled to a radially inner end of the airfoil for coupling the airfoil to a disc. The turbine blade further comprises a shroud coupled to the tip of the airfoil. The shroud comprises a shroud base extending in a circumferential direction of a turbine engine and further extending in an axial direction of the turbine engine from an upstream edge to a downstream edge. The shroud also comprises a circumferential fin extending along the circumferential direction of the turbine engine. The circumferential fin extends radially outward from the shroud base to run a tight gap with a stator component of the turbine engine, to provide a sealing against a stream- wise over-tip leakage flow of gas path fluid in a direction generally from the leading edge to the trailing edge. The shroud further comprises a fence feature extending generally radially outward with respect to the tip of the airfoil, and extending transverse to the circumferential fin, to discourage a pitch-wise over-tip leakage flow of gas path fluid in a direction from the pressure side to the suction side.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is shown in more detail by help of figures. The figures show preferred configurations and do not limit the scope of the invention.

[0008] FIG 1 is a perspective view of a typical tip shroud for a turbine blade; [0009] FIG 2 is a perspective view of a gas turbine engine with shrouded turbine airfoils with at least one fence feature according to embodiments of the present invention;

[0010] FIG 3 is a perspective view of a turbine blade with a tip shroud comprising a transverse fin, according to a first embodiment of the present invention;

[0011] FIG 4 is a radial top view of the tip shroud shown in FIG 3; [0012] FIG 5 is a perspective view of a turbine blade with a tip shroud in which a pressure side portion of the shroud base is curved radially outward, according to a second embodiment of the present invention;

[0013] FIG 6 is a cross-sectional view along the section VI-VI of FIG 5; and

[0014] FIG 7 is a perspective view of the embodiment shown in FIG 5, looking against the direction of stream-wise flow of gas path fluid.

DETAILED DESCRIPTION

[0015] In the following detailed description, across different embodiments, like reference characters have been used to designate like or corresponding elements for the sake of simplicity. [0016] In this description, various specific details are set forth in order to provide a thorough understanding of such embodiments. However, those skilled in the art will understand that disclosed embodiments may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, methods, procedures, and components, which would be well-understood by one skilled in the art have not been described in detail to avoid unnecessary and burdensome explanation.

[0017] Furthermore, usage of the phrase "in one embodiment" does not necessarily refer to the same embodiment, although it may. It is noted that disclosed embodiments need not be construed as mutually exclusive embodiments, since aspects of such disclosed embodiments may be appropriately combined by one skilled in the art depending on the needs of a given application.

[0018] The terms "comprising", "including", "having", and the like, as used in the present application, are intended to be synonymous unless otherwise indicated. Also, unless otherwise specified, the connector "or", as used herein, implies an inclusive "or", which is to say that the phrase "A or B" implies: A; or B; or both A and B. Lastly, as used herein, the phrases "configured to" or "arranged to" embrace the concept that the feature preceding the phrases "configured to" or "arranged to" is intentionally and specifically designed or made to act or function in a specific way and should not be construed to mean that the feature just has a capability or suitability to act or function in the specified way, unless so indicated. [0019] In a turbine engine, a tip shroud, may be used, particularly for long blades in aft turbine stages, primarily because they offer increased damping, and hence design robustness against frequency problems. Referring to FIG 1, to reduce over-tip leakage, a tip shroud 1 may include one or more circumferential fins 2, typically embodied as knife edge seals, for running tight tip gaps with a stator component, typically a honeycomb structure (not shown in FIG 1). A full coverage shroud design largely prevents gas path fluid flow from leaking in a pitch-wise direction, i.e., in a direction from the pressure side to the suction side of the airfoil. The one or more knife edge seals act to provide a sealing against a stream-wise over-tip leakage flow of gas path fluid. The stream-wise over-tip leakage flow, designated in the drawings as FS, takes place in a direction generally from the leading edge to the trailing edge of the airfoil.

[0020] The drive to improve engine performance, i.e., power and efficiency, has resulted in longer blades and higher gas path temperatures. This leads to increased stresses on the blade and disk and the designer had sought to reduce the weight carried by the blade. One measure to reduce the overall weight of the blade is to scallop or cut-off material from the shroud around the blade, resulting in reduced shroud coverage in the inter-blade passages in a blade row. The material removed from the shroud 1 is shown by the shaded region 3 in FIG 1. The present inventors have recognized that such a scalloping of the shroud may compromise the capability of the shroud to prevent a pitch- wise leakage flow of gas path fluid. The pitch-wise over-tip leakage flow, designated in the drawings as FP, takes place in a direction generally from the pressure side to the suction side of the airfoil. The trailing edge of one airfoil typically forms a throat of minimum flow area along its perpendicular intersection near the mid-chord of the suction of the adjacent airfoil. It has been seen that the pitch-wise leakage is typically most pronounced at the region just upstream of the throat of the inter-blade passage, where the pressure gradient is maximum and the shroud coverage is reduced by scalloping. Embodiments of the present invention illustrated in FIGS 2-7 provide a turbine blade having a tip shroud with a fence feature for discouraging pitch-wise over-tip leakage flow, thereby improving turbine aerodynamic efficiency and power. The illustrated embodiments may make it possible to recover aerodynamic performance, particularly of a heavily-scalloped light-weight shroud, back to that of an equivalent free-standing blade of the same clearance.

[0021] Referring to FIG 2, a turbine engine 64 is illustrated, that comprises a turbine blade 10 wherein embodiments of the present invention may be incorporated. The turbine blade 10 is formed from a generally elongated airfoil 32 extending in a generally radial direction in the turbine engine 64 from a rotor disc. The airfoil 32 includes an outer wall formed of a generally concave pressure side 38 and generally convex suction side 40, extending span-wise along a radial direction of the turbine engine 64. The pressure side 38 and the suction side 40 are joined at a leading edge 34 and at a trailing edge 36. The airfoil 32 includes a tip 24 at a first radially outer end 44 of the airfoil 32 and a root 46 coupled to the airfoil 32 at a second radially inner end 48 of the airfoil 32 for supporting the airfoil 32 and for coupling the airfoil 32 to the rotor disc. The turbine blade 10 further includes a shroud 22, also referred to as a tip shroud, coupled to the tip 24 of the generally elongated airfoil 32. The shroud 22 comprises a shroud base 20 coupled to the tip 24 of the generally elongated airfoil 32 and a circumferential fin 50, which may be embodied, for example, as a knife edge seal. The knife edge seal 50 extends radially outward from the shroud base 20 and also extends in a circumferential direction of the turbine engine 64, running tight tip gaps against a honeycomb structure 51 on the stator of the turbine engine 64 to reduce stream-wise over-tip leakage of a gas path fluid comprising hot gases.

[0022] FIGS 3-4 illustrate a first example embodiment of the present invention. As shown, the shroud 22 includes a shroud base 20 with a knife edge seal 50 extending in the circumferential direction C of the turbine engine. The shroud base 20 further extends in an axial direction of the turbine engine from an upstream edge 56 to a downstream edge 58. The knife edge seal 50 is configured to provide sealing against a stream-wise over-tip leakage flow FS of gas path fluid. In relation to the stream-wise direction, the shroud base 20 comprises a forward section 52 located upstream of the knife edge seal 50 and terminating at the upstream edge 56, and an aft section 54 located downstream of the knife edge seal 50 and terminating at the downstream edge 58. In this example, the shroud base 20 is scalloped such that the axial distance between the upstream edge 56 and the downstream edge 58 varies in an undulating manner in the circumferential direction C. In other embodiments, a full coverage shroud may be used, in which the upstream and downstream edges 56, 58 run essentially parallel to each other in the circumferential direction C. It is to be noted that the benefits of the exemplary embodiments are particularly pronounced in case of a scalloped shroud. As stated above, such a scalloping may expose the throat area of the inter-blade passage to over-tip leakage flow FP in the pitch-wise direction, i.e., in a direction from the pressure side 38 to the suction side 40. In accordance with the first example embodiment, the shroud 22 is provided with a fence feature 70 in the form of a transverse fin 70a, to discourage the pitch- wise leakage flow. [0023] As shown in FIG 3, the transverse fin 70a extends generally radially outward with respect to the airfoil tip 24. In particular, the transverse fin 70a may be positioned on the shroud base 20 directly over the airfoil tip 24, extending radially outward therefrom. Further, as shown in FIG 4, the transverse fin 70a extends transverse to the knife edge seal 50. The transverse fin 70a may extend along the entire chord -wise axial length of the airfoil tip 24 from the leading edge 34 to the trailing edge 36, or only a portion thereof. In the shown embodiment, to minimize additional mass on the blade, the transverse fin 70a is provided only at the aft section 54 of the shroud base 20. That is to say, the transverse fin 70a extends for only a portion of the chord-wise axial length, from a mid-chord region to the trailing edge 36 at the airfoil tip 24. The above embodiment is based on the finding that in many blade designs, maximum pressure gradient exists in the region from the mid- chord to the trailing edge of the blade, whereby the aft section 54 of the shroud base 20 downstream of the knife edge seal 50 is most susceptible to pitch-wise leakage. The precise positioning of the transverse fin 70a may however be a function of the specific blade configuration. The mid-chord region may span, for example between 30-60 % of the chord-wise axial length from the leading edge 34 to the trailing edge 36 at the airfoil tip 24. As shown in FIG 4, the transverse fin 70a may extend generally parallel to a contour of the tip 24 of the airfoil 32. For example, the transverse fin 70a may closely follow the contour of the pressure side 38, or the suction side 40 or a mean camber line at the airfoil tip 24.

[0024] Referring back to FIG 3, the transverse fin 70a has a radially outer edge 72 which is configured to sealingly interface with the honeycomb structure 51 (see FIG 2), for preventing a leakage of gas path fluid from the pressure side 38 to the suction side 40. As the transverse fin 70a is not meant to be stiffener for the shroud 22, the transverse fin 70a does not have to be particularly thick. To minimize the mass of the blade, the transverse fin 70a may, for example, have the minimum thickness to ensure survivability when interacting with the honeycomb. In a further embodiment, to provide more effective blocking of pitch-wise leakage flow, the transverse fin 70a may be inclined at an angle to the radial direction. In particular, the transverse fin 70a may be angled with respect to the radial direction such that the transverse fin 70a extends radially outward against the direction of pitch-wise flow FP of gas path fluid.

[0025] Referring now to FIGS 5-7, a second embodiment of the present invention is illustrated, in which the fence feature 70 is formed by extending a portion of the shroud base 20 radially outward. As shown in FIG 6, in a pitch-wise cross-section, the shroud base 20 may comprise a mid portion 60 disposed directly over the airfoil tip 24, a pressure side portion 62 extending from the mid portion 60 to a pressure side edge 66 of the shroud base 20, and a suction side portion 64 extending from the mid portion 60 to a suction side edge 68 of the shroud base 20. In the shown embodiment, a fence feature 70b is formed by curving or angling the pressure side portion 62 of the shroud base 20 radially outward. In this case, the pressure side edge 66 forms a radially outermost edge 72 of the fence feature 70b. In an alternate embodiment, the fence structure 70b may be likewise realized by curving or angling the suction side portion 64 radially outward. The fence feature 70b thus formed, extends generally radially outward with respect to the airfoil tip 24 (see FIG 5-7) and extends transverse to the knife edge seal 50, to discourage pitch-wise over-tip leakage flow FP (see FIGS 5 and 7). The fence feature 70b may extend along the entire chord- wise axial length from the leading edge 34 to the trailing edge 36, or only a portion thereof. In the shown embodiment, the fence feature 70b is provided only at the aft section 54 of the shroud 22, for example, in view of the higher pressure gradient in this region as stated above. In particular, as shown in FIGS 5 and 7, the fence feature 70b may extend from the knife edge seal 50 to the downstream edge 58 of the shroud base 20.

[0026] In one embodiment, the fence feature 70b may be formed by modifying an existing shroud design by curving the pressure side portion 62 or the suction side portion 64 radially outward, such that there is no net mass addition to the existing shroud design. Furthermore, as this configuration does not rely on the shroud base 20 for the mid-to-aft section of the airfoil to control pitch-wise leakages, further more aggressive scalloping (shown by shaded region 80 in FIG 5) is possible. Thus, it may be possible to actually achieve a net reduction in shroud mass.

[0027] The radially outer edge 72 of the fence feature 70b is configured to sealingly interface with the honeycomb structure 51 (see FIG 2), for preventing a leakage of gas path fluid from the pressure side 38 to the suction side 40. To provide more effective blocking of pitch-wise leakage flow, the fence feature 70b may be inclined at an angle to the radial direction. In the illustrated embodiment, as shown in FIGS 6 and 7, the fence feature 70b is angled with respect to the radial direction such that the fence feature 70b extends radially outward against the direction of pitch-wise flow FP of gas path fluid.

[0028] 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.

Claims

1. A turbine blade (10) comprising:

a generally elongated airfoil (32) having a leading edge (34), a trailing edge (36), a pressure side (38), a suction side (40) on a side opposite to the pressure side (38), a tip (24) at a radially outer end (44) of the airfoil (32), a root (46) coupled to a radially inner end (48) of the airfoil (32) for coupling the airfoil (32) to a disc;

a shroud (22) coupled to the tip (24) of the airfoil (32) and comprising:

a shroud base (20) extending in a circumferential direction of a turbine engine (64) and further extending in an axial direction of the turbine engine (64) from an upstream edge (56) to a downstream edge (58) of the shroud base (20), and

a circumferential fin (50) extending along the circumferential direction of the turbine engine (64), and extending radially outward from the shroud base (20) to run a tight gap with a stator component (51) of the turbine engine (64), to provide a sealing against a stream-wise over-tip leakage flow (FS) of gas path fluid in a direction generally from the leading edge (34) to the trailing edge (36),

wherein the shroud (22) comprises a fence feature (70) extending generally radially outward with respect to the tip (24) of the airfoil (32), and extending transverse to the circumferential fin (50), to discourage a pitch-wise over-tip leakage flow (FP) of gas path fluid in a direction from the pressure side (38) to the suction side (40).

2. The turbine blade (10) according to claim 1, wherein the fence feature (70) comprises a transverse fin (70a) positioned directly over the tip (24) of the airfoil (32) and extending radially outward therefrom, the transverse fin (70a) extending along a chord-wise axial length of the tip (24) of the airfoil (32).

3. The turbine blade (10) according to claim 2, wherein the transverse fin (70a) extends from a mid-chord region to the trailing edge (36) at the tip (24) of the airfoil (32).

4. The turbine blade (10) according to claim 2, wherein the transverse fin (70a) extends from the leading edge (34) to the trailing edge (36) at the tip (24) of the airfoil (32).

5. The turbine blade (10) according to claim 2, wherein the transverse fin

(70a) extends generally parallel to a contour of the tip (24) of the airfoil (32).

6. The turbine blade (10) according to claim 1, wherein the fence feature (70, 70b) is formed by extending a pressure side portion (62) or a suction side (64) portion of the shroud base (20) radially outward.

7. The turbine blade (10) according to claim 6, wherein the fence feature (70, 70b) is located in a stream-wise aft section (54) of the shroud base (20).

8. The turbine blade (10) according to claim 7, wherein the fence feature (70,

70b) extends from the circumferential fin (50) to the downstream edge (58) of the shroud base (20).

9. The turbine blade (10) according to claim 6, wherein the fence feature (70, 70b) is formed by curving the pressure side portion (62) or the suction side portion (64) radially outward, such that a pressure side edge (66) or a suction side edge (68) respectively of the shroud base (20) forms a radially outermost edge (72) of the fence feature (70, 70b).

10. The turbine blade (10) according to claim 1, wherein the fence feature (70) is angled with respect to the radial direction such that the fence feature (70) extends radially outward against a direction of pitch-wise flow (FP) of gas path fluid.

11. The turbine blade (10) according to claim 1, wherein a radially outermost edge (72) of the fence feature (70) sealingly interfaces with the stator component (51) of the turbine engine (64).

12. The turbine blade (10) according to claim 1, wherein the upstream edge (56) and/or the downstream edge (58) of the shroud base (20) are scalloped along the circumferential direction to define a region of reduced mass of the shroud base (20) between adjacent turbine blades (10) of a blade ring in the turbine engine (64).