Turbine blade tip cooling construction

Abstract

A gas turbine blade (1) with a tip squealer (7) comprises cooling passages (12, 13) that provide cooling fluid for the cooling of the tip squealer (7). A first portion of the cooling passages (12) extends from an internal hollow space (11) through an end cap to a tip pocket. A second portion (13) of the cooling passage extends from the end cap in part through the rails (8) of the tip squealer (7) to the tip crown (9). The second portion (13) is partially bounded by the rail (8) and partially open to the tip pocket (14). The cooling passages according to the invention provide an improved cooling of the tip squealer and have the advantage that cooling fluid can cool the tip squealer (7) even in the event of clogging of the second portions (13) of the passages.

Description

FIELD OF INVENTION

[0001] This invention pertains to gas turbine blades and in particular to a cooling construction for their tip portion.

BACKGROUND ART

[0002] Gas turbine components are exposed to the very high temperatures of the gas flow driving the turbine. In order to maintain the metal temperatures of the components within acceptable limits for structural integrity and longevity they are actively cooled by means of a cooling fluid. Typically, cooling air bled from the compressor of the gas turbine is used as a cooling fluid.

[0003] The gas turbine blades, which operate directly downstream of the combustion process, pose a particular technical challenge for cooling. Especially the edges and tips of these blades are difficult to cool as access for cooling is restricted. The tip region is often subjected to the highest heat loads as the hottest gases, which flow through the center of the hot gas flow path, are pulled to the tip by secondary flows generated on the pressure side of the airfoil. These secondary flows are usually driven by leakage of hot gas from the concave or high-pressure side of the airfoil to the convex or low-pressure side of the airfoil through the tip gap. The tip gap is the clearance gap between the blade tip and stationary heat shield and serves to prevent interference between the rotating blade tips and stationary heat shields.

[0004] Gas turbine blades typically incorporate a so-called tip squealer, which consists of a recessed pocket at the blade tip surrounded by a raised wall, or rails, which extend along the tip of the pressure side and suction side of the airfoil and radially outward to the tip crown. The squealer provides a dual orifice at the tip gap that increases the resistance against hot gas flow across the tip. It also provides rub tolerance in the event that the tip clearance is diminished during turbine operation and the blade tip rubs against the stationary heat shield. The tip squealer further increases the challenge of cooling the tip because access to the rails of the squealer is restricted.

[0005] A cooling construction for the tip of a gas turbine blade is disclosed in U.S. Pat. No. 5,183,385. It comprises several cooling holes, which extend from a cooling passage within the blade through the end cap of the blade tip portion to the tip pocket. The exit ports of the cooling holes are positioned adjacent to the inner wall of the tip squealer that is adjacent to the inner wall of the rail. The cooling hole comprises two sections, of which the first section has a cylindrical shape and the second section is diffused forming a rectangular trapezoid. In this type of cooling construction there is a potential that the cooling flow detaches from the surface of the squealer rail such that the cooling effectiveness in the region of the rails is much reduced.

[0006] A further cooling construction for the tip portion of a gas turbine blade is disclosed in U.S. Pat. No. 5,660,523. It comprises several cooling holes leading from internal cooling passages of the blade through the rails to the outer surface, or crown, of the tip squealer rails. The outer surface of the rails has a groove, which contains the exit ports of the cooling holes. The cooling construction provides more direct cooling of the rails of the squealer. However, as the exit ports are located at the outermost surface of the rails, there is a high risk that the cooling holes get plugged by material rubbed off the heat shield or blade itself in the event of contact between the blade and the stationary components near it. The groove on the outer surface provides protection against such plugging to a certain degree. However, a robust tip cooling configuration would require such rub tolerance all the way to the bottom of the tip pocket.

SUMMARY OF INVENTION

[0007] It is the object of the invention to provide a cooling construction for the tip squealer of a gas turbine blade that provides improved cooling of the rails or walls of the tip squealer as well as improved protection from plugging of the cooling holes by rubbed-off material.

[0008] According to the invention a gas turbine blade comprises a pressure sidewall and suction sidewall extending from a root to a tip and from a leading edge to a trailing edge. The tip of the blade comprises a tip squealer with rails or raised walls extending radially from an end cap of the blade to a tip crown and along the radial outer end of the suction and pressure sidewalls of the blade. The rails furthermore surround a tip pocket. Between the inner surfaces of the pressure and suction sidewalls of the blade a hollow space is arranged, through which a cooling fluid can flow and cool the blade from within.

[0009] Cooling passages providing cooling fluid to the tip squealer have a first portion that leads from the internal space through the end cap to the tip pocket and a second portion that extends in part through the rails of the tip squealer to the tip crown. The second portion of the cooling passages is in one part bounded by a sidewall in the rail and is in another part open to the tip pocket.

[0010] By a cooling construction according to the invention the cooling fluid flow is bounded in one part, which provides a more predictable air flow compared to the cooling construction disclosed in U.S. Pat. No. 5,183,385. The high velocity jet exiting the cooling passages directly influences the heat transfer on the bounding wall. In particular, the cooling fluid can flow unimpeded through the cooling passage to the outer surface or tip crown of the squealer rail. Even in the event of rubbed-off material present due to a contact between the tip squealer and the stationary components cooling fluid can reach the tip crown unimpeded and cool the tip squealer effectively.

[0011] The cooling of the squealer pocket by means of the cooling construction according to the invention occurs by convection as well as by film cooling. The convective heat transfer between the ejected cooling flow and the surfaces of the squealer is greater the larger the surface of the rails. Hence, the convective heat transfer between the cooling flow and the surfaces with the slots according to the invention is increased over the heat transfer between a cooling flow and a rail with a flat surface.

[0012] Film cooling is accomplished by reducing the driving temperatures of the hot gases. The mere presence of the ejected cooling flow will dilute the hot gases to some degree. The film cooling effect is also increased if the cooling flow is kept close to the rail wall. This is accomplished with the help of the slots in the rail for the ejected flow.

[0013] In a first particular embodiment of the invention the cooling passages extend radially from the internal hollow space for the cooling fluid within the blade to the tip crown. The passages then extend in parallel to the pressure and suction sidewalls and the radially extending rails.

[0014] In a further particular embodiment of the invention the cooling passages extend at an angle with respect to the radial direction either in the plane perpendicular to the blade walls or in the plane of the blade sidewalls and rails or at a compound angle in both of these planes. The cooling passages extending at an angle in the plane of the blade sidewalls and rails can be oriented either toward the leading edge or toward the trailing edge of the turbine blade.

[0015] Cooling passages with such orientations have a longer path to the tip crown and the wetted surface or effective area for convective cooling is enlarged compared to the embodiment with radially extending cooling passages.

[0016] In a further particular embodiment the first portion of the cooling passages has a cylindrical shape beginning from the internal hollow space between the inner surfaces of the suction and pressure sidewall through the end cap of the blade to the tip pocket. The second portion of the cooling passage has the shape of a partial cylinder extending through the rails.

[0017] In a further particular embodiment of the invention the first and second portion of the cooling passages are shaped or elongated in the direction along the crown of the tip squealer.

[0018] This elongated or oval shape provides a further increase of the wetted surface exposed to the cooling fluid.

[0019] In a further particular embodiment of the invention the first portion of the cooling passages has parallel extending sidewalls over a first part of their length and a diffused shape over a second part of their length. The first part having a cylindrical or elongated shape serves as a metering section streamlining the cooling fluid. The second portion of the cooling passage has sidewalls that diffuse at an angle with respect to the longitudinal axis of the passage. This allows a reduction of the cooling fluid flow velocity as it flows along the rails and a spreading of the flow over a larger surface, which increases the film cooling effectiveness. Also, in this case, a larger area is available for convective heat transfer.

BRIEF DESCRIPTION OF THE FIGURES

[0020] FIG. 1 shows a perspective view of a gas turbine blade with a tip squealer and cooling passages at the tip portion according to the invention.

[0021] FIG. 2 shows a cross-section of the blade of FIG. 1 along the line II-II and the cooling passages extending radially through the end cap and through the rails up to the tip crowns.

[0022] FIG. 3 shows in a further cross-sectional view of a gas turbine blade a variant of cooling passages that extend at an angle with respect to the radial direction and in the plane perpendicular to the blade walls.

[0023] FIG. 4 shows a perspective view of a gas turbine blade with a further variant of the cooling passages according to the invention that extend at an angle with respect to the radial direction and in the plane of the blade walls and rails.

[0024] FIG. 5 shows a perspective view of a gas turbine blade with cooling passages with an elongated shape.

[0025] FIG. 6a shows a cross-sectional view of a gas turbine blade with cooling passages having diffused sidewalls over a portion of their length and

[0026] FIG. 6b shows a further cross-section of the same cooling passages along the line B-B.

DETAILED DESCRIPTION OF THE INVENTION

[0027] FIG. 1 shows a gas turbine blade 1 according to the invention comprising a pressure sidewall 2 and a suction sidewall 3, which extend from the blade's leading edge 4 to its trailing edge 5 and from a root section 6 to a tip with a tip squealer 7. The tip squealer 7 comprises a rail 8 that extends radially away from the pressure and suction sidewalls 2 and 3 ending at the tip crown 9. The tip squealer 7 furthermore comprises a tip pocket that is bounded by the tip cap or end cap and on the sides by the rails 8.

[0028] Between the pressure and suction sidewall 2 and 3 the blade comprises a hollow space 11 indicated by the broken lines. The hollow space comprises for example several passages for a cooling fluid to flow through and convectively cool the blade to a temperature at which the blade's material takes no damage. From the internal space 11 several cooling passages 12 lead radially outward toward the tip cap to the tip pocket. The same passages 12 each have an extension 13 through the rails 8 and to the tip crown 9. The passage extensions 13 are bounded on one side by a sidewall in the rail 8. On their other side they are partially open to the tip pocket. The cooling passages shown in FIG. 1 are a first variant of the invention. The passages extend radially outward and have a straightforward cylindrical shape in their first portion and the shape of a partial cylinder in their second portion or extension 13. The cooling fluid, typically air bled from a compressor, flows from the hollow space 11 through the passages 12 to the tip pocket while cooling the tip cap from within. From the tip pocket it follows the passage extension 13 to the tip crown 9 cooling the rails by convection and film cooling.

[0029] In the event of an accidental or intentional contact of the blade with stationary components of the gas turbine material can rub off and smear the material of the radially outmost portions of the tip squealer. The extensions 13 of the cooling passages can then be filled with material. However, in such a case the cooling fluid can still reach the tip crown unimpeded by flowing around the rubbed off material.

[0030] The cross-sectional view in FIG. 2 shows a first variant of the cooling passages according to the invention. They comprise a first portion 12 leading from the hollow space 11 between the pressure and suction sidewall 2 and 3 through the tip cap 10 to the tip pocket 14. From there the cooling passages extend through the rail 8 to the tip crown 9 in a second portion or extension 13 with a sidewall 15 in the rail 8 and being partially open to the tip pocket 14.

[0031] A vortex will typically form in the tip pocket 14 as a result of the flow driven through the tip cap 10. The vortex flow is in the same direction as the ejected cooling flow on the pressure side rail 2 and opposed to the ejected cooling flow on the suction side rail 3. On the pressure rail 2 the vortex flow acts to hold the ejected flow in the slot formed in the rail, which benefits the film cooling in that region. On the suction side rail 3 the opposed flow of the vortex will tend to pull the cooling jet off the rail wall. However, the presence of the slots 13 helps to retain that flow along the rail wall, which again benefits the film cooling of the rail.

[0032] FIG. 3 shows a second variant of the cooling passages according to the invention. They are shown in a similar cross-section as in FIG. 2. The longitudinal axis 16 of the passages extend from the hollow space 11 at an angle &thgr; with respect to the inner surface of the pressure or suction sidewall 2 and 3 of the blade and in a plane perpendicular to the blade sidewalls. Again, the first portion 12 of the passages has a cylindrical shape while the second portion 13 has a partial cylindrical shape. The angle &thgr; is for example 5-15° and preferably optimized for specific applications to maximize cooling within established criteria for manufacturing.

[0033] FIG. 4 shows a further variant of the cooling passages 12, 13 according to the invention. Their longitudinal axes 16 are oriented at an angle &phgr; with respect to the radial direction and in the plane of the rail 8. This orientation yields a greater surface over which the cooling fluid can cool the tip squealer. In this example, the passages are oriented toward the trailing edge.

[0034] The angle &phgr; between the radial direction and the longitudinal axis of the passages is preferably about 45°.

[0035] Furthermore, the longitudinal axis of the cooling passages can also be oriented at a compound angle with respect to the radial direction where this compound angle is in the plane perpendicular to the plane of angle &thgr; as well as in the plane angle &phgr;.

[0036] FIG. 5 shows another variant of the cooling passages. In this example, the cooling passages are oriented radially outward. They have an elongated shape in the direction along the rail. In this variant the surface over which the cooling fluid can cool the tip squealer is yet enlarged compared to the variants in FIGS. 3 or 4.

[0037] FIGS. 6a and b show a further variant of the passages according to the invention. FIG. 6a shows a similar cross-section of the blade as in FIGS. 2 and 3. FIG. 6b shows a cross-section along line B-B. The passages for the cooling fluid are cylindrically shaped over a first part 12′ within the tip cap 10 and have a diffused shape over a second part extending from the first part to the tip crown. The sidewalls of the diffused part extend at an angle &agr; with respect to the longitudinal axis of the cooling passages and in the plane perpendicular to the rail 8. The sidewalls are diffused at a further angle &bgr; in the plane tangent to the rail according to FIG. 6b. The angles &agr; and &bgr; are each preferably in the range from 5 to 10°.

[0038] In further variants of the invention the features described in the figures may be combined.

Terms Used in the Figures

[0039] 1 gas turbine blade

[0040] 2 pressure sidewall

[0041] 3 suction sidewall

[0042] 4 leading edge

[0043] 5 trailing edge

[0044] 6 root portion

[0045] 7 tip portion, tip squealer

[0046] 8 rail

[0047] 9 tip crown

[0048] 10 end cap or tip cap

[0049] 11 hollow space within blade

[0050] 12 first portion of cooling passage

[0051] 12′ first part of first portion, metering section

[0052] 13 second, partially bounded, partially open portion of cooling passage

[0053] 14 tip pocket

[0054] 15 sidewall of second portion of cooling passage

[0055] 16 longitudinal axis of cooling passage

[0056] &agr; angle of diffusion of sidewall with respect to longitudinal axis in direction of blade sidewall

[0057] &bgr; angle of diffusion of sidewall with respect to longitudinal axis in direction along rail

[0058] &thgr; angle of orientation of longitudinal axis with respect to radial direction in plane perpendicular to rails 8

[0059] &phgr; angle of orientation of longitudinal axis with respect to radial direction in plane of rail 8.

Claims

1. Gas turbine blade (1) comprising a pressure sidewall (2) and suction sidewall (3) extending from a leading edge (4) to a trailing edge (5) and from a root (6) to a tip with a tip having an end cap (10) and a tip squealer (7) with rails (8) that extend radially from the end cap (10) to a tip crown (9) and along the suction sidewall (3) and pressure sidewall (2) and surrounding a tip pocket (14), and

furthermore comprising an internal hollow space (11) between the inner surfaces of the pressure and suction sidewall (2,3), through which a cooling fluid can flow and

cooling passages for directing the cooling fluid to the tip squealer (7) characterized in that

the cooling passages have a first portion (12) leading from the internal hollow space (11) through the end cap (10) to the tip pocket (14) and a second portion that extends in part through the rails (8) to the tip crown (9) of the tip squealer (7), where the second portion (13) is in one part bounded by a sidewall (15) in the rail and in another open to the tip pocket (14).

2. Gas turbine blade (1) according to claim 1 characterized in that

the first and second portion (12, 13) of the cooling passages extend radially from the internal hollow space (11) within the blade to the tip crown (9).

3. Gas turbine blade (1) according to claim 1 characterized in that

the longitudinal axis of the first and second portion (12, 13) of the cooling passages is oriented at an angle (&thgr;) with respect to the radial direction either in the plane perpendicular to the blade sidewalls (2,3) or at an angle (&phgr;) with respect to the radial direction in the plane of the blade sidewalls (2, 3) or at a compound angle with respect to the radial direction in each of these planes.

4. Gas turbine blade (1) according to claim 3 characterized in that

the angle of orientation (&thgr;) of the longitudinal axis of the first and second portion (12, 13) of the cooling passages in the plane perpendicular to the blade sidewalls (2,3) is in the range from 0 to 45°.

5. Gas turbine blade (1) according to claim 3 characterized in that

the angle of orientation (&phgr;) of the longitudinal axis of the first and second portion (12, 13) of the cooling passages in the plane of the blade sidewalls (2,3) is in the range from 0 to 60°

6. Gas turbine blade (1) according to one of the foregoing claims 1 through 3 characterized in that

the first portion of the cooling passages (12) has a cylindrical shape leading from the internal hollow space (11) between the inner surfaces of the suction and pressure sidewall (2, 3) through the end cap (10) to the tip pocket (14) and the second portion of the cooling passage has the shape of a partial cylinder within the rails (8) of the tip squealer (7).

7. Gas turbine blade (1) according to one of the foregoing claims 1 through 3 characterized in that

the first and second portions of the cooling passages (12, 13) have an elongated or oval shape in the direction along the rails (8) of the tip squealer (7).

8. Gas turbine blade (1) according to one of the foregoing claims 1 through 3 characterized in that

the cooling passages have parallel extending sidewalls over a first part (12′) of the first portion (12) within the tip cap (10) and a diffused shape over a second part of its first portion (12) within the tip cap (10) and over its second portion or extension (13).

9. Gas turbine blade (1) according to claim 8 characterized in that

the diffused shape of the cooling passages is defined by angles (&agr;,&bgr;) between the passage sidewalls and the longitudinal axis of the cooling passages that are in the range from 5 to 10°.