EP1925780A1

EP1925780A1 - Blade for an axial-flow turbine

Info

Publication number: EP1925780A1
Application number: EP06024322A
Authority: EP
Inventors: Andrey Shukin; Sergey Shukin
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-11-23
Filing date: 2006-11-23
Publication date: 2008-05-28

Abstract

A blade for an axial-flow turbine comprises a cooling passage (6, 7 , 19, 21) formed inside the blade (1) to direct a cooling medium along and beneath at least one of the suction side surface (4) of the blade (1) and the pressure side surface (3) of the blade (1) such that a core (8) of the blade (1) defined by being jacketed by the cooling passage (6, 7, 19, 21) is thermally isolated towards the blade surface (2,4).

Description

Field of the invention

The invention relates to a blade for an axial-flow turbine, in particular a blade for an axial-flow steam turbine.

Background of the invention

Blades, in particular rotor blades and stator vanes, in axial-flow turbines, in particular in steam turbines, are subjected to mechanical and thermal loads during operation of the turbines. The thermal and mechanical loads are caused by hot gas flow, for example super heated steam in case of a steam turbine, heating up the blades and applying gas forces to the blades. In case of rotor blades the rotational speed of the rotor additionally strains the blades due to centrifugal forces.
Therefore, high demands are made to design and construction of blades having sufficient mechanical integrity in order to withstand applied loads during operation.
Further, an excessive length of operation time affects the life endurance of the blades. In particular, when a blade is subjected to a high temperature in combination with a high strain an excessive long time, creeping of the blade can occur resulting in cracks in the blade material and finally in mechanical failure.
The strength of the blade material is dependent on stresses applied during operation, operation temperature and operation time. In order to improve the mechanical integrity and life endurance of the blade it is a common remedy to reduce temperature of the gas flow.
However, in general a reduction of the temperature level of the gas flow in the turbines leads to a loss in thermodynamic efficiency of the turbines.
It is an object of the invention to provide a blade for an axial-flow turbine with an increased strength and, nevertheless, an increased thermodynamic efficiency.

Summary of the invention

According to the invention, this object is achieved by a blade for an axial-flow turbine, comprising a cooling passage formed inside the blade to direct a cooling medium along and beneath at least one of the suction side surface of the blade and the pressure side surface of the blade such that a core of the blade defined by being jacketed by the cooling passage is thermally isolated towards the blade surface.
The fact that the blade core is thermally isolated from the blade surface prevents a heat input from outside the blade through the blade surface, in particular from a gas flow surrounding the blade, to the blade core. Therefore, even when the blade surface is heated up to a high temperature, the blade core material remains at a moderate temperature level. Hence, the mechanical integrity of the blade core is high, i.e. creeping of the blade core material is low and the life endurance of the blade core is high.
Construction-conditioned the blade core bears the main mechanical load affecting upon the blade during operation. As a result of this the overall mechanical strength of the blade is generally determined by the mechanical strength of the blade core. Therefore, an increase of the mechanical integrity of the blade core, in particular by thermally isolating the blade core from the blade surface, results in an increased overall mechanical strength of the blade. This is achieved even so when the gas flow temperature is high. Therefore, the blade can be operated with hot gas flow and thereby with high thermal efficiency without suffering from deficiencies in mechanical strength.
Preferred for the use of the cooling medium is compressed air.
For example, the cooling passage is formed into a helical tube shaped channel encircling the blade core. However, it is preferred that the cooling passage defines at the pressure side blade surface a pressure side blade surface wall, at the suction side blade surface a suction side blade surface wall, and the cooling passage comprises a plurality of webs extending therethrough thereby connecting the blade core to the respective blade surface wall.
Therefore, the cooling medium flowing through the cooling passage contacts the pressure and suction side blade surface via the pressure side blade surface wall and the suction side blade surface wall, respectively. This results in a large-area cooling of the pressure and suction side blade surface leading to an increased heat transfer from the blade surface to the cooling medium, wherein the temperature gradients within the blade material are smooth. Therefore, the cooling of the blade surface is effective, nevertheless the thermal stresses in the blade are reduced.
Further, the provision of the plurality of webs extending through the cooling passage increases the mechanical stability of the blade.
Generally, the pressure side blade surface wall and the suction side blade surface wall provide aerodynamically effective surfaces, whereas the blade core is provided for carrying mechanical loads. Therefore, the thickness of said walls can be minimized leading to an increase of cooling efficiency. Further, the thickness of the blade core is increased leading to an increase in mechanical strength of the blade core.
Preferably, the plurality of webs is comprised by a plurality of pressure side rips extending along the pressure side of the blade, and/or a plurality of suction side rips extending along the suction side of the blade.
Between the individual rips cooling channels are defined guiding the cooling medium along the pressure and suction sides of the blade, respectively. Therefore, the cooling medium flow is controlled by the rips, in particular with respect to velocity and flow rate. Hence, by means of the rips in particular cooling parameters, e.g. blade surface temperature and heat transfer rate, can be adjusted in order to achieve an efficient cooling performance for the pressure side as well as for the suction side of the blade. Even said cooling parameters can be adjusted for each blade side individually thereby achieving an optimized cooling for each blade side. Therefore, the required flow rate of the cooling medium can be reduced in order to obtain a desired cooling performance.
Further, construction-conditioned the rips are elongated and slim in shape. Therefore, the mechanical coupling between said walls and the blade core is soft, i.e. a movement of the blade core relative to said walls can be compensated by deforming the rips. This relative movement can occur in particular when said walls and the blade core are reaching different temperature levels. For example, in case the gas flow is hot, the surface of the blade and said walls are consequently heated up, whereas due to the cooling effect of the cooling medium the blade core remains at a moderate temperature level. Hence, said walls are thermally expanding resulting in a dilatation toward the blade core. Owing to the soft character of coupling between said walls and the blade core only low thermal stresses, if any, occur in the blade material. This leads to a high mechanical integrity of the blade.
In particular this soft connection between said walls and the blade core is desired when the temperature difference between said walls and the blade core is high, i.e. the cooling performance of the cooling medium is high.
Since the rips are slim and elongated formed, their cross sections areas are small. Therefore, the heat transfer from the blade surface to the blade core via the rips is low. Hence, the cooling efficiency within the blade is high.
The rips may extend parallel to turbine axis direction. Preferably, the rips are formed to be straight and are arranged to extend parallel to each other, wherein the pressure side rips are inclined from the root of the blade to the tip of the blade by a first predetermined angle, and/or the suction side rips are inclined from the blade tip to the blade root by a second predetermined angle.
Due to the fact that the rips are provided inclined toward turbine axis direction, the cooling medium is advantageously guided diagonally along the pressure and suction side blade surface, respectively. Therefore, the time of contact of the cooling medium with the blade surface is high resulting in an increased cooling performance.
Further, since the pressure side rips are inclined from the root of the blade to the tip of the blade and the suction side rips are inclined from the blade tip to the blade root, at the pressure side the cooling medium is directed from the blade root to the blade tip by the pressure side rips, whereas at the suction side the cooling medium is directed from the blade tip to the blade root by the suction side rips. Thereby, it is advantageously achieved that in particular at inlet and discharge areas of the cooling medium the flow distribution is balanced.
For example, the first predetermined angle equals the second predetermined angle. However, it is preferred that the first predetermined angle and/or the second predetermined angle is in a range between 30° and 70°.
It is preferred that the blade core comprises a rear end being arranged to be retracted from the trailing edge of the blade such that in the cooling passage between the blade core rear end and the blade trailing edge a cooling medium mixing zone is defined in which the pressure side rips are connected to the suction side rips thereby forming cross-over points.
Thereby, it is advantageously achieved that in the mixing zone the pressure side rips together with the suction side rips form a stable and reliable structure. Therefore, during operation of the blade the mechanical load of the blade walls in particular at the tailing edge of the blade is reduced.
According to an alternative embodiment of the invention, it is preferred that each the plurality of pressure side rips and/or the plurality of suction side rips are formed by a rip matrix comprised by a plurality of outer rips and a plurality of inner rips, wherein the outer and inner rips are formed to be straight and are arranged to extend parallel to each other, wherein the outer rips are inclined from the root of the blade to the tip of the blade by a third predetermined angle and the inner rips are inclined from the blade tip to the blade root by a forth predetermined angle such that the outer rips are connected to the inner rips thereby forming cross-over points.
The cross-over points included in the rip matrix have small cross sectional areas. Therefore, in general the heat flux between the blade walls and the blade core is reduced leading to a higher temperature difference between the blade walls and the blade core. Hence, the thermal isolation of the blade core toward the blade walls is more effective thereby increasing the mechanical integrity of the blade core and of the blade.
Further, the mechanical coupling of the blade walls to the blade core through the cross-over points is soft. Due to this, the risk of thermal stresses in the blade walls, the blade core, and in particular in the inner and outer rips is reduced.
For example, the third predetermined angle equals the forth predetermined angle. However, it is preferred that the third predetermined angle as well as the forth predetermined angle is in a range between 30° and 70°.
Further, according to an alternative embodiment of the invention, it is preferred that the plurality of webs is comprised by a plurality of pin-fins. It is also preferred to combine the rip structure, the rip matrix and/or the pin-fins. For example, the rip structure can be provided at the pressure and suction side of the blade, whereas at the trailing edge of the blade the pin-fins are provided.
Preferably the blade root is integrally provided at the blade core. Since the blade core is provided in particular for carrying mechanical loads, the integrally provision of the blade root at the blade core increases the overall stability of the blade.
It is preferred that the blade core comprises a front end being arranged to be retracted from the leading edge of the blade such that in the cooling passage between the blade core front end and the blade leading edge a cooling medium entry channel is defined adapted to feed the cooling medium to the downstream portion of the cooling passage.
By providing the entry channel for feeding the cooling medium, a steady supply of the cooling medium to the pressure and suction side surface of the blade is achieved. Thereby, the risk of cooling discontinuity is reduced and the reliability of the cooling system is increased.
Preferably, the trailing edge of the blade comprises blade trailing edge apertures connecting the cooling passage toward the outside and being adapted to discharge the cooling medium.
By providing the blade trailing edge apertures for discharging the cooling medium, a steady discharge flow of the cooling medium along the trailing edge is achieved. Since through the blade trailing edge apertures heated up cooling medium is mixed to the main gas flow downstream the blade, a return of the cooling medium has not to be provided. Therefore, the cooling system is simple and cost saving in operation.
The blade can be integrally formed. However, it is preferred that the pressure side blade surface wall, the suction side surface wall, and the blade core are formed from an individual material, respectively.
This advantageously offers the possibility to manufacture the blade walls in particular from a high oxidation resistance material or a material with high LCF prosperities, and the blade core from a high corrosion resistance material or a material with high strength and plasticity at a low temperature level. This effect is desirable in particular when manufacturing thick profiled blades subjected to high aerodynamic loads and high gas velocities.
It is preferred hollowly forming the blade core, especially in order to reduce the weight of the blade.
The blade is preferably manufactured using investment casting, wherein the blade core is hollowly formed in the centre. Preferably, the blade core comprising the rips can be manufactured at first, and then the blade walls are added for example by diffusion.
Preferably, the blade is a rotor blade or a stator vane. It is also preferred that the blade is adapted to be used in a steam turbine.

Brief Description of the Drawings

In the following the invention is explained on the basis of preferred embodiments with reference to the drawings. In the drawings:

Figure 1 shows a cross-sectional view taken along the pressure side of a first embodiment of a blade according to the invention,
figure 2 shows a cross-sectional view taken along the suction side of the first embodiment,
figure 3 shows a cross-sectional view taken along the line A-A in figure 1,
figure 4 shows a cross-sectional view taken along the pressure side of a second embodiment of a blade according to the invention, and
figure 5 shows a cross-sectional view taken along the line A-A in figure 4.

Detailed Description of preferred Embodiments of the Invention

Referring to figures 1 to 3, according to a first embodiment of the invention a rotor blade 1 for a steam turbine comprises a pressure side blade surface 2, a suction side blade surface 4, a leading edge 13, a trailing edge 14, a blade root 11, and a blade tip 12.
The rotational axis of the steam turbine is denoted with x, the circumferential direction of the blade rotation is denoted with y, and the radial axis along the blade height is denoted with z.
The blade 1 comprises at its pressure side a pressure side blade surface wall 3 defining the pressure side blade surface 2, and at its suction side a suction side blade surface wall 5 defining the pressure side blade surface 4. Further, the blade 1 comprises a blade core 8 being integrally formed with the blade root 11.
Within the blade 1 a cooling passage is formed separating the blade core 8 from the pressure side surface blade wall 3 as well as from suction side blade surface wall 5.
The cooling passage comprises an entry channel 19 with a U-shaped cross section and being arranged in the vicinity of the leading edge 13 and extending in parallel thereto from the blade root 11 to the blade tip 11. Within the blade root 11 the entry channel 11 is provided with an entry chamber 20 having an enlarged cross section with respect to the entry channel 19.
Further, the cooling passage comprises a pressure side cooling passage 6 and a suction side cooling passage 7. The pressure and suction side cooling passages 6, 7 are formed into a substantially equally spaced gap defining the pressure side blade surface wall 3 and the suction side blade surface wall 5 as a boundary wall with constant thickness.
Cooling air is flown through the cooling passage for thermally isolating the blade core 8 from the pressure and suction side blade surfaces 2, 3 by cooling down the pressure and suction side blade surface walls 3, 4. The inlet flow of the cooling air through the entry channel 19 is shown in figures 1 and 2 with arrows 23.
The entry channel 19 runs into the pressure side cooling passage 6 as well as into the suction side cooling passage 7.
A front end 9 of the blade core 8 forms the inner boundary of the entry channel 19 such that the inlet flow of the cooling air 23 is separated into a first branch guided to the pressure side cooling passage 6 and a second branch guided to the.suction side cooling passage 7.
A rear end 10 of the blade core 8 is arranged retracted from and parallel to the trailing edge 14, wherein between the trailing edge 14 and the blade core rear end 10 both the pressure and suction side cooling passages 6, 7 merge together thereby forming a mixing zone 21. In the mixing zone 21 cooling air discharging from the pressure and suction side cooling passages 6, 7 is mixed together in the mixing zone 21.
The trailing edge 14 comprises trailing edge apertures 22 such that cooling air having passed the entry chamber 20, the entry channel 19, the pressure side cooling passage 6 as well as the suction side cooling passage 7, and the mixing zone 21 is discharged through the trailing edge apertures 22 towards the outside. The outlet flow of the cooling air through the trailing edge apertures 22 is shown in figures 1 and 2 with arrows 24.
The blade 1 is provided with fourteen pressure side rips 15 extending through the pressure side cooling passage 6, and with fourteen suction side rips 16 extending through the suction side cooling passage 7. The rips 15, 16 are formed slim and straight and define between each other channels to be passed through by cooling air.
The pressure side rips 15 are inclined from the blade root 11 to the blade tip 12 seen from the leading edge 13 to the trailing edge 13. The inclination is denoted by α and has a value of 60°. The suction side rips 16 are inclined from the blade tip 12 to the blade root 11 seen from the leading edge 13 to the trailing edge 13. The inclination is denoted by β and has a value of 60°.
In the mixing zone 21 the pressure side rips 15 and the suction side rips 16 meet and connect each other thereby forming a grid like rip matrix comprising cross-over points 18.
Referring to figures 4 and 5, a second embodiment of the invention is shown.
Differences of the second embodiment vis-à-vis the first embodiment are described.
According to the second embodiment, the blade 1 is provided with a rip matrix 17 extending through the pressure side cooling passage 6 and through the suction side cooling passage 7. The rip matrix 17 is comprised by fourteen outer rips 17a facing the blade surface and fourteen inner rips 17b facing the blade core 8.
The outer rips 17a are inclined from the blade root 11 to the blade tip 12 seen from the leading edge 13 to the trailing edge 13. The inclination is denoted by α' and has a value of 60°. The inner rips 17b are inclined from the blade tip 12 to the blade root 11 seen from the leading edge 13 to the trailing edge 13. The inclination is denoted by β' and has a value of 60°.
The rips 17a, 17b are formed slim and straight and define between each other channels to be passed through by cooling air. Further, the outer rips 17a and the inner rips 17b meet and connect each other thereby forming the rip matrix 17 comprising cross-over points 18. Since the channels formed by the outer rips 17a and the channels formed by the inner rips 17b are arranged to one another and are connected to each other, cooling air passing through the cooling passages 6, 7 is swirled.
Cooling air having passed the pressure side cooling passage 6 and the suction side cooling passage 7 is directly flown through the trailing edge apertures 22 through outside.

Reference signs

1:: rotor blade
2:: pressure side blade surface
3:: pressure side blade surface wall
4:: suction side blade surface
5:: suction side blade surface wall
6:: pressure side cooling passage
7:: suction side cooling passage
8:: blade core
9:: blade core front end
10:: blade core rear end
11:: blade root
12:: blade tip
13:: leading edge
14:: trailing edge
15:: pressure side rips
16:: suction side rips
17:: rip matrix
17a:: outer rips
17b:: inner rips
18:: cross-over point
19:: entry channel
20:: entry chamber
21:: mixing zone
22:: trailing edge aperture
23:: inlet flow of cooling air
24:: discharge flow of cooling air

α:: inclination angle of pressure side rips toward x-axis
β:: inclination angle of suction side rips toward x-axis

α' :: inclination angle of outer rips toward x-axis
β' :: inclination angle of inner rips toward x-axis

x:: rotational axis
y:: circumferential direction
z:: radial axis along blade height

Claims

Blade for an axial-flow turbine, comprising a cooling passage (6, 7, 19, 21) formed inside the blade (1) to direct a cooling medium along and beneath at least one of the suction side surface (4) of the blade (1) and the pressure side surface (3) of the blade (1) such that a core (8) of the blade (1) defined by being jacketed by the cooling passage (6, 7, 19, 21) is thermally isolated towards the blade surface (2, 4).
Blade according to claim 1, wherein the cooling passage (6, 7) defines at the pressure side blade surface (2) a pressure side blade surface wall (3), at the suction side blade surface (4) a suction side blade surface wall (5), and the cooling passage (6, 7) comprises a plurality of webs extending therethrough thereby connecting the blade core (8) to the respective blade surface wall (3, 5).
Blade according to claim 2, wherein the plurality of webs is comprised by a plurality of pressure side rips (15, 17a, 17b) extending along the pressure side of the blade (1), and/or a plurality of suction side rips (16, 17a, 17b) extending along the suction side of the blade (1).
Blade according to claim 3, wherein the rips (15, 16) are formed to be straight and are arranged to extend parallel to each other, wherein the pressure side rips (15) are inclined from the root (11) of the blade (1) to the tip (12) of the blade (1) by a first predetermined angle (α), and/or the suction side rips (16) are inclined from the blade tip (12) to the blade root (11) by a second predetermined angle (β).
Blade according to claim 4, wherein the first predetermined angle (α) and for the second predetermined angle (β) is in a range between 30° and 70°.
Blade according to claims 4 or 5, wherein the blade core (8) comprises a rear end (10) being arranged to be retracted from the trailing edge (14) of the blade (1) such that in the cooling passage between the blade core rear end (10) and the blade trailing edge (14) a cooling medium mixing zone (21) is defined in which the pressure side rips (15) are connected to the suction side rips (16) thereby forming cross-over points (18).
Blade according to claim 3, wherein each the plurality of pressure side rips and/or the plurality of suction side rips are formed by a rip matrix (17) comprised by a plurality of outer rips (17a) and a plurality of inner rips (17b), wherein the outer and inner rips (17a, 17b) are formed to be straight and are arranged to extend parallel to each other, wherein the outer rips (17a) are inclined from the root (11) of the blade (1) to the tip (12) of the blade (1) by a third predetermined angle (α') and the inner rips (17b) are inclined from the blade tip (12) to the blade root (11) by a forth predetermined angle (β') such that the outer rips (17a) are connected to the inner rips (17b) thereby forming cross-over points (18).
Blade according to claim 7, wherein the third predetermined angle (α') as well as the forth predetermined angle (β') is in a range between 30° and 70°.
Blade according to claim 2, wherein the plurality of webs is comprised by a plurality of pin-fins.
Blade according to any of claims 1 to 9, wherein the root (11) of the blade (1) is integrally provided at the blade core (8).
Blade according to any of claims 1 to 10, wherein the blade core (8) comprises a front end (9) being arranged to be retracted from the leading edge (13) of the blade (1) such that in the cooling passage between the blade core front end (9) and the blade leading edge (13) a cooling medium entry channel (20) is defined adapted to feed the cooling medium to the downstream portion of the cooling passage (6, 7).
Blade according to any of claims 1 to 11, wherein the trailing edge (14) of the blade (1) comprises blade trailing edge apertures (22) connecting the cooling passage (6, 7, 19, 21) toward the outside and being adapted to discharge the cooling medium.
Blade according to any of claims 2 to 12, wherein the pressure side blade surface wall (3), the suction side surface wall (5), and the blade core (8) are formed from an individual material, respectively.
Blade according to any of claims 1 to 13, wherein the blade core (8) is hollowly formed.
Blade according to any of claims 1 to 14, wherein the blade (1) is a rotor blade or a stator vane.
Blade according to any of claims 1 to 15, wherein the blade (1) is adapted to be used in a steam turbine.