Patent Application
20080213098
This application claims priority of European Patent Office application No. 07002465.8 EP filed Feb. 5, 2007, which is incorporated by reference herein in its entirety.
The invention relates to a blade for a turbomachine, wherein the blade is formed along a radial direction and has a blade tip and also a blade profile with a pressure side and a suction side.
Free standing lowpressure blades are used in turbomachines, such as in steam turbines, primarily in the low-pressure region. For constructional reasons, such turbomachines have a comparatively large radial gap between the blade tip and the casing. So that losses, however, do not become too great, it is known to arrange abrasive coatings on the blade tip or on the casing which lies opposite the blade tip. With contact between the blade tip and the casing, only the abrasive coating is slightly worn away, as a result of which a comparatively small radial gap is created.
The leakage flow which flows through between the blade tip and the casing is also referred to as gap jet. In low pressure steam turbines, these gap jets cause pulsing flows in the diffuser, which can lead to unwanted humming of the diffuser.
Furthermore, it is known to form the blades very thin in the tip region, i.e. on the final millimeters of the blade end of the blade geometry. The tip region then corresponds to the suction side contour. By means of this measure, on one hand an aerodynamically pronounced throttling effect of the gap flow is achieved, which slightly reduces the gap loss, and on the other hand the radial clearance can be designed smaller, since in the case of rubbing of the blade tip with the casing the risk of damage can be considered to be low because the tip region is worn away, as it were, on the casing without major damage.
A further measure for reducing gap flows is achieved by the use of blades with shrouds. The shrouds act as seals, as it were, as a result of which the gap losses are reduced. However, such blades with shrouds have mechanical disadvantages because the shrouds represent comparatively large additional masses which lead to an unwanted mechanical loading of the blades. A further disadvantage which is brought about by the shrouds arises as a result of interconnecting the blades via the shrouds. Such connections can lead to additional vibrational and resonance modes of the blade ring which is connected in this way.
It is an object of the invention to disclose a turbine blade for a turbomachine, which can be used in a turbomachine and during operation leads to low gap losses.
This object is achieved by means of a blade for a turbomachine, wherein the blade is formed along a radial direction, and has a blade tip and also a blade profile with a pressure side and a suction side, wherein the blade tip has a curvature.
Therefore, the geometry of the blade is greatly altered by the invention. Like in the case of the freely flow-washed wings of an aircraft, the blade tip is formed with so-called winglets. With this, an essential feature is that the blade tip has a sharp curvature compared with the major part of the blade airfoil.
By this measure, the driving pressure difference across the blade tip is reduced. A further effect is that the radial clearance between the blade tip and the casing is reduced, and consequently the gap loss becomes less. The efficiency of such a turbomachine is ultimately improved.
Furthermore, the aerodynamic conditions in the region of the blade tip can be better controlled, since aerodynamic shocks of the flow around the profile are shielded against interaction with the casing. As a result of this, shock reflexes are reduced.
A further effect of the blade according to the invention is that the diffuser shows little inclination towards the so-called diffuser humming.
Further advantageous development are disclosed in the dependent claims.
The curvature is preferably formed as a bend. Therefore, the blade tip is formed in the manner of a winglet which is known from aircraft construction.
So, it is advantageous if the blade tip, as seen in the radial direction in the direction of the blade tip, is formed in a manner in which it is first curved towards the suction side and then curved towards the pressure side.
As a result of this, the centrifugal force which acts upon the blade tip is minimized, as it were, in an optimum manner because the centrifugal force acts on the one hand upon one side (suction side) of the extension direction of the blade and on the other hand upon the other side (pressure side) of the extension direction of the blade.
In a further advantageous development, a wear-resistant coating is applied to the blade tip surface which during operation lies opposite a casing inner wall. As a result of this, contacts which occur during operation between the blade tip and the blade inner casing can be tolerated. The coating has such strength that damage to the blade inner casing hardly occurs. An exemplary embodiment of the invention is subsequently explained in more detail with reference to drawings.
In the drawings:
In
The blade tip 4 of the turbine blade 1 has a curvature 8. The curvature 8 in this case is localized on the upper region of the blade tip 4. The curvature 8 is formed as a bend, as a result of which the blade tip is formed in the manner of a winglet, which is known from aircraft construction. The blade 1 has an overall length L which can be calculated from the surface of the blade root 9 to the blade tip 4. The curvature 8 occurs in a region of basically about 5% of the overall length L, calculated from the blade end point 10.
As shown in
In a further embodiment, the blade tip 4, as seen in the radial direction 3 in the direction of the blade tip 4, is formed in a manner in which it is first curved towards the suction side 7 and then curved towards the pressure side 6.
The blade tip 4 has a blade tip edge 12 which is formed basically parallel to a circumferential direction which is formed during operation.
The blade 1 has differently formed blade cross-sectional profiles 5 along the radial direction 3. In one embodiment, the projection of the blade tip 4, as seen in the radial direction 3, is smaller than the projection of the largest blade cross-sectional profile 5 in the radial direction 3. In other words, the blade tip edge 12 does not project beyond the largest blade cross-sectional profile 5.
The blade has a blade tip surface 13 which, during operation, lies opposite a casing inner wall 11 and which is provided with a wear-resistant coating. This wear-resistant coating, as shown in the example according to
The blade 1 can be produced from steel or from titanium. The blade cross-sectional profiles 5 reduce in the direction of the radial direction 3. As is to be seen in the examples in
The curved turbine blade tips 4 are preferably used in low-pressure last stage blades in low-pressure steam turbines. The circumferential speed of the blade tip 4 in this case can be at values of more than Mach 1. As a result of this, use in the aerodynamic range of transonic flows is possible, In this case, both the inflow and the outflow are to be understood as transonic flow.
In
In
The curvature 8 preferably occurs on the pressure side 6 of the blade 1 in order to reduce gap flow losses.
The curvature 8 is designed in such a way that the blade end point 10 does not axially project beyond the axial width of the blade cross-sectional profile 5. The curvature 8 of the blade tip 4, in order to minimize further gap losses, follows a conicity of the casing.
According to
For further improvement of the gap losses, a coating 14 can be applied to the blade tip surface. This coating 14 can be formed as an abrasive wear coating, with which a contact with the inner casing does not lead to major damage.
The curvature 8 of the turbine blade tip 4 can be produced by means of close-tolerance finish forging. The curvature 8 can also be produced by means of envelope forging with subsequent milling of the blade airfoil.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07002465.8 | Feb 2007 | EP | regional |
