EP2412927A1

EP2412927A1 - Turbine blade

Info

Publication number: EP2412927A1
Application number: EP10171273A
Authority: EP
Inventors: Alexander Bone; Dimitri Vinogradov
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2010-07-29
Filing date: 2010-07-29
Publication date: 2012-02-01

Abstract

The invention relates to a turbine blade (1) for a gas turbine, comprising a blade tip (2) and a tip crown (3), which extends generally radially outward from the blade tip (2), the tip crown has a radially outer surface (5) for forming a seal between the blade and a shroud (4) of a gas turbine. The radially outer surface of the tip crown is inclined relative to an opposing sealing surface of the shroud (4).

Description

Field of technology

The present invention relates to a turbine blade for a gas turbine engine, in particular to a turbine blade tip configuration.

Prior art

In multi-stage expansion gas turbines, gas is expanded though the turbine stages converting energy into mechanical work by driving the turbine shaft.
One method of improving the performance of a gas turbine is to reduce the leakage flow between rotor blade tips of the gas turbine and the outer stationary shroud that forms a seal with the tips. The shroud can be in the form of a heat shield. In order to optimize turbine performance, the clearances must be as small as possible, but safe operation must be given at any time. After assembly of the gas turbine, the clearance between the blade tip and the shroud should therefore be within specified tolerances.
During operation of the gas turbine the clearance can however be affected by a number of factors, for example by ovalization of the turbine vane carrier or deformation during special operation conditions, for example during a hot restart. Reconditioning the gas turbine parts such as the blades, the stator heat shields of the turbine vane carrier can also lead to the original tolerances not being met. As a result, when the gas turbine cycles through temperatures, the tip clearance varies and sometimes, the tip touches against the shroud, causing rubbing to occur. Rubbing can damage the blade tips. Providing a larger tip clearance will reduce the chance of rubbing, but will also allow for more hot gas flow to leak across the gap reducing the turbine efficiency. Other methods to prevent rubbing include adjusting the clearances by axial shifting of the rotor during operation or changing the dimensions of the turbine vane carrier by thermal expansion i.e. by adjustable cooling of the turbine vane carrier. These methods are however complex and costly.
A simpler alternative is to provide the blade tip with a crown, which extends from the blade top to form the seal with the shroud. The crown is relatively thin compared with the thickness of the blade and therefore does not provide as much surface area against the shroud when rubbing occurs. Therefore the effect of rubbing is reduced. An example of such a crown can be seen in US-Patent US7494319B1 . Here the blade tip is provided with a crown or tip rail which provides an outer surface parallel with the shroud surface.
A problem with this design is that when rubbing occurs, the whole of the outer crown surface of the blade tip is in contact with the shroud and the resulting frictional forces on the blade are very high. Another problem of this design is that the rubbing of the surface heats the tip of the blade so that cooling of the blade tip is necessary.
It is therefore an object of the present invention to provide an improved blade design to reduce the rubbing of the blade tip against the stationary shroud, thereby reducing the forces exerted on the blade and increasing the life of the blade and shroud. A further object of the present invention is to reduce or eliminate the need for cooling of the blade tip.

Summary of the invention

The present invention addresses these problems.
According to the invention these problems are solved by providing a turbine blade for a gas turbine having the features of claim 1. Preferred embodiments of the turbine blade according to the invention can be found in the dependent claims.
According to the invention a turbine blade comprises a tip crown having a radial outer surface for forming a seal between the blade and a shroud of a gas turbine, the radial outer surface of the tip crown being inclined relative to an opposing sealing surface of the shroud. In the case of rubbing of the tip crown with a shroud or heat shield of the gas turbine the surface area of the tip crown in contact with the shroud is smaller. This results in lower forces on the blade and increases the life of the blade. Furthermore the clearance between the tip crown and the shroud can be reduced in normal operation as the damage associated with rubbing is reduced. If the blades are cooled, then there is also less cooling leakages past the tips of the blades.
In a further preferred embodiment of the invention the tip crown extends substantially between the trailing edge and the leading edge of the turbine blade. This limits secondary leakage flow past the tips of the blade.
In a further preferred embodiment of the invention the tip crown is made from a different material to the blade. In this way the properties of tip crown can therefore be tailored to its specific function.
A further aspect of the invention provides a gas turbine comprising a turbine blade according to claim 1. The gas turbine comprises a heat shield circumferentially surrounding the turbine rotor, whereby the heat shield comprises a porous ceramic material on a radially inner surface thereof. The porous ceramic material contributes to reducing the forces on the blade by providing less resistance to the blade movement.
The above and other objects, features and advantages of the invention will become more apparent from the following description of certain preferred embodiments thereof, when taken in conjunction with the accompanying drawings.

Short description of the drawings

The invention is described referring to an embodiment depicted schematically in the drawings, and will be described with reference to the drawings in more details in the following.
The drawings show schematically in:

Figure 1 an arrangement of a turbine blade according to the invention,
Figure 2 the turbine blade of claim 1,
Figure 3 a cross section along Q-Q of the tip of the turbine blade in figure 1.

Detailed description of preferred embodiments

Referring to figure 1, a turbine blade 1 for a gas turbine is shown. The turbine blade 1 forms part of a turbine rotor (not shown) comprising a plurality of such blades, which rotate around a central axis (x) of the gas turbine. The turbine rotor may comprise several rows of blades and may have a generally conical or cylindrical form. The angle α describes the inclination of the blade tips 2 with respect to the axial direction.
The turbine blade 1 extends generally in the radial direction (z). The turbine blade 1 comprises a blade tip 2 and a tip crown 3, which extends generally radially outward from the blade tip 2. Surrounding the turbine blades 1 in a radial direction is a turbine shroud 4 of a gas turbine. The turbine shroud 4 can be in the form of a heat shield or a plurality of heat shields and is stationary relative to the rotating blades.
The symbol 8 describes the clearance between the turbine blade tip crown 3 and the shroud 4. This clearance can be affected during operation of the turbine by a number of factors for example by deformation of the turbine rotor or of the shroud 4 during a hot restart. This can result in rubbing of the tip crown 3 against the shroud 4.
Figure 2 shows the turbine blade 1 of claim 1 without the stationary turbine shroud 4 for clarity. Figure 3 shows a cross section through the tip 2 of the blade in figure 2 along the line Q-Q.
The tip crown 3 of the blade has a radial outer surface 5 for forming a seal between the blade and the turbine shroud 4. The angle # describes the inclination angle of the surface 5 of the tip of the crown relative to an opposing sealing surface 6 of the shroud 4. The angle β is preferably greater than 3 degrees. The radial outer surface 5 of the tip crown 3 may be curved or planar. By providing non-parallel surfaces 5, 6 between the radial outer surface 5 of the tip crown 3 and the opposing sealing surface 6 of the shroud 4, when rubbing occurs, the surface area of the tip crown 3 in contact with the shroud 4 is smaller. The smaller contact area reduces the forces on the blade 1, decreasing the risk of damage to the blade 1 when rubbing occurs. In the case of heavy rubbing of the surface 5 of the tip crown 3 with the shroud 4, the tip crown 3 may be made of an appropriate material relative to the material of the shroud 4 such that the radial outer surface 5 of the tip crown 3 is ground down, in which case the radial outer surface 5 of the tip crown 3 will be parallel with the shroud surface 6 and further rubbing of the tip crown 3 with the shroud 4 will increase the forces on the blade.
The tip crown 3 extends preferably substantially the full length of the blade tip 2 between the leading 7 and trailing edges 8 of the blade. The tip crown 3 is preferably formed as a continuation of the suction surface 9 of the blade in the radial direction (z). The tip crown 3 can however also be a continuation of the pressure surface 10 in the radial direction (z). In another embodiment (not shown) the tip crown 3 is formed along the periphery of the blade tip 2 whereby cooling channels are provided in a region of the blade tip 2 surrounded by the tip crown 3.
The tip crown 3 may be formed of a material different to that of the blade. This material is preferably more wear resistant than the blade material. The tip crown 3 can advantageously be formed separately from the blade tip 2 and attached to the blade tip 2. The tip crown 3 can alternatively be provided by a coating to the blade tip 2.
The thickness Δ of the tip crown 3 and the height (h) of the tip crown 3 are free parameters and depend on the geometry of the shroud 4 and of the blade to which the invention is applied. These dimensions are chosen to ensure the required mechanical integrity of the tip crown 3.
In a gas turbine comprising a blade according to the invention and a heat shield, the heat shield may be advantageously made of a porous ceramic material. Alternatively a porous ceramic material may be applied as a coating on a radially inner surface 6 of the heat shield. The porous ceramic material is softer than the material of the blade tip 2 and the tip crown 3 and reduces the rubbing forces. The tip crown 3 according to the invention acts as a knife-edge cutting through the porous ceramic material.
The invention relates particularly to uncooled and unshrouded turbine blades 1. The invention may however also be applied to cooled turbine blades and turbine blades the tips of which are connected by a shroud.
The preceding description of the embodiments according to the present invention serves only an illustrative purpose and should not be considered to limit the scope of the invention.
Particularly, in view of the preferred embodiments, the man skilled in the art different changes and modifications in the form and details can be made without departing from the scope of the invention. Accordingly the disclosure of the current invention should not be limiting. The disclosure of the current invention should instead serve to clarify the scope of the invention which is set forth in the following claims.

List of reference numerals

1: Turbine blade
2: Blade tip
3: Tip crown
4: Turbine shroud
5: Tip crown radial outer surface
6: Shroud radial inner surface
7: Blade leading edge
8: Blade trailing edge
9: Suction surface
10: Pressure surface

Claims

A turbine blade (1) for a gas turbine, comprising a blade tip (2) and a tip crown (3), which extends generally radially outward from the blade tip (2), the tip crown (3) having a radial outer surface for forming a seal between the blade and a shroud (4) of a gas turbine, characterized in that the radial outer surface of the tip crown (3) is inclined relative to an opposing sealing surface (6) of the shroud (4).
A turbine blade (1) according to claim 1, characterized in that the radial outer surface of the tip crown (3) is inclined relative to an opposing sealing surface (6) of the shroud (4) at an angle greater than 3 degrees.
A turbine blade (1) according to claim 1, characterized in that the tip crown (3) extends substantially between the trailing edge (8) and the leading edge (7) of the turbine blade.
A turbine blade (1) according to claim 1, characterized in that the tip crown (3) is an extension of the suction surface (9) and/or of the pressure surface (10) of the blade.
A turbine blade (1) according to claim 1, characterized in that the tip crown (3) extends substantially around the periphery of the blade tip (2).
A turbine blade (1) according to claim 1, characterized in that the tip crown (3) is formed from a different material to the blade tip (2).
A turbine blade (1) according to claim 1, characterized in that the tip crown (3) is formed as a coating or as an element, which is fixed to the blade, tip (2).
A turbine blade (1) according to any one of the preceding claims, characterized in that in a cross section of the blade perpendicular to a chord line of the blade the tip crown (3) is inclined relative to the thickness direction of the blade (1).
A turbine blade (1) according to any one of the preceding claims, characterized in that the blade tip (2) is uncooled.
A turbine blade (1) according to any one of the preceding claims, characterized in that the blade tip (2) is unshrouded.
A gas turbine comprising a turbine blade (1) according to any one of the preceding claims
A gas turbine according to claim 11, characterized in that the turbine blade (1) is provided on a turbine rotor and the gas turbine comprises a heat shield (4) circumferentially surrounding the turbine rotor, whereby the heat shield (4) comprises a porous ceramic material on a radially inner surface thereof.