This invention is related generally to turbine airfoils, and more particularly to hollow turbine airfoils such as blades and vanes with internal cooling channels for passing fluids such as air to cool the airfoils.
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 and vane assembly for producing power. Combustors operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose the turbine vane and blade assemblies to these high temperatures. Turbine vanes and blades must be made of materials capable of withstanding such temperatures. Turbine vanes and blades often contain cooling systems for prolonging their life and reducing the likelihood of failure as a result of excessive temperatures.
A turbine blade is a rotating airfoil attached to a disk on the turbine rotor by a platform and blade shank. A turbine vane is a stationary airfoil that is radially oriented with respect to a rotation axis of the turbine rotor. The vanes direct the combustion gas flow optimally against the blades. One or each end of a vane airfoil is coupled to a platform, also known as an endwall. A radially outer vane platform is connected to a retention ring on the engine casing. An inner vane platform, if present, is supported by the vane.
Blades and vanes often contain cooling circuits forming a cooling system. The cooling circuits receive a cooling fluid such as air bled from the compressor of the turbine engine via a plenum and supply port in one or each platform. The cooling circuits often include multiple flow paths inside the airfoil designed to maintain all portions of the airfoil at a relatively uniform temperature. At least some of the air passing through these cooling circuits may be exhausted through film cooling holes in the leading edge, trailing edge, suction side, and pressure side of the airfoil.
Some turbine airfoils have a dual wall structure formed of inner and outer walls. This is called a 4-wall airfoil construction, since the pressure and suction sides of the airfoil each have two walls. The outer wall is exposed to hotter temperatures, so it is subject to greater thermal expansion, and stress develops at the connection between the inner and outer walls.
It is known that high cooling efficiency can be achieved by near-wall cooling in which cooling air flows in channels between the inner and outer walls of a 4-wall airfoil. However, differential thermal expansion between the hot outer walls and the cooler inner walls can cause Low Cycle Fatigue (LCF) limitations for reasons later described.
The invention is explained in the following description in view of the drawings that show:
The invention reduces and relocates stress on a 4-wall turbine airfoil by controlling the thermal expansion mismatch between the relatively hotter outer walls and the relatively cooler inner walls to reduce low cycle fatigue (LCF) in the airfoil.
The airfoil 20A of
Many different 4-wall airfoil constructions have been evaluated in the past. One hurdle has been manufacturability. However, with advances in metal investment casting and ceramic core processing, this limitation can be overcome. Another problem has been differential thermal growth stress between the hot outer walls 26, 32 and cooler inner walls 28, 34. Previous 4-wall airfoils as in
Attempts have been made to solve this by either: 1) overcooling the outer wall, or 2) using better wall materials and fabrication technology such as advanced single-crystal casting. These solutions improve the airfoil life by changing the fabrication and additional cooling, but they do not address the design geometry. In contrast, the present invention reduces thermal stress via an airfoil sectional geometry combined with a particular cooling flow pattern, which together control macro deflections in the airfoil due to thermal expansion in a way not previous known in the art.
The near-wall channels are designated as forward pressure-side channels 31F, aft pressure-side channels 31A, forward suction-side channels 33F, and aft suction-side channels 33A. One or more forward passages 38 may transfer cooling air 50H from the forward central plenum 37 to the leading edge cooling channel 44. Film-cooling holes 39 may be provided anywhere on the exterior surface of the airfoil 20B, including ones such as shown passing from the leading edge cooling channel 44 to provide film cooling flows 51 and coolant exhaust. One or more aft coolant passages 41 may communicate from the central aft plenum 40 through the trailing edge 24 as shown.
The difference in temperature of the cooling air is used to relieve thermal stress in the airfoil by creating an inverse temperature gradient across the pressure side inner wall 28B. In prior art designs, this wall is normally hotter toward the pressure side outer wall 26 and colder toward the central cooling plenums 37, 40. However, in the present flow paths the cooling air 50C is coldest in the pressure side near-wall channels 31F, 31A, and is hotter 50H in the central plenums 37, 40. As a result, the pressure side inner wall 28B is colder toward the pressure side outer wall 26 and hotter toward the central plenums 37, 40, reversing the normal gradient (i.e. inverse gradient). The resulting differential thermal expansion across this wall causes its curvature to increase. A thermal gradient of only about 20° C. (for example 435 to 455° C.) is enough to control the strain state of the airfoil in one embodiment.
The pressure side outer wall 26 also tends to grow and tries to reduce its concavity in the dual-wall geometry. However, the curling of the thicker pressure side inner wall 28B dominates, increasing the concavity of the pressure side outer wall 26. The pressure side outer wall 26 and the suction side inner wall 34 oppose curling 70 of the pressure side inner wall 28B. These opposing walls 26, 34 are made thin enough not to negate the curling effect of the pressure side inner wall and to have some compliance. The pressure side inner wall 28B may be at least as thick as the combined thicknesses of the pressure side outer wall 26 and the suction side inner wall 34B as previously described. Stress states and predicted thermal growth geometries in various airfoil embodiments of the present invention can be calculated with commonly available design tools.
The net effect is that thermal strain is off-loaded from the suction side outer wall 32 onto the pressure side outer wall 26 and the suction side inner wall 34. This is a net advantage for the following reasons:
In
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. For example, the invention has been described as a gas turbine engine airfoil including thermal strain state control arrangement effective to allow the suction side outer wall to increase its curl during operation of the gas turbine engine so that a region of peak strain in the airfoil during operation of the gas turbine engine is located remote from the suction side outer wall. The airfoil may have a thermal expansion control mechanism causing its camber to increase under differential thermal expansion of the airfoil during operational heating in order to improve its LCF life. Herein, camber means the degree of curvature of a line halfway between the pressure side and the suction side of an airfoil section. In one embodiment, the airfoil sectional geometry and an internal cooling flow pattern cause the airfoil camber to increase by controlling a temperature gradient on an internal wall structure of the airfoil. In the embodiments described above, it was the relatively thicker pressure side inner wall that curled and controlled thermal strain to off-load one of the outer walls, but in other embodiments it may be the suction side inner wall that is sized to control thermal strain and to off-load an outer wall. Other embodiments may utilize a temperature difference between the average metal temperature of the pressure side and suction side of the airfoil. This may be accomplished with a difference in the cooling air temperature between the pressure and suction sides of the airfoil. This could also be accomplished by using thermal barrier coatings having different insulating abilities on opposed sides of the airfoil. Alternatively, active heating of the backside of the strain-controlling wall may be used instead of the passive cooling scheme described above. Alternatively, bi-material may be used to achieve a desired thermal curl, for example by spraying a low or high coefficient of thermal expansion (CTE) alloy on only one side of the strain-controlling wall.
Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention.
Number | Name | Date | Kind |
---|---|---|---|
3698834 | Meginnis | Oct 1972 | A |
4768700 | Chen | Sep 1988 | A |
5392515 | Auxier et al. | Feb 1995 | A |
5405242 | Auxier et al. | Apr 1995 | A |
5702232 | Moore | Dec 1997 | A |
5931638 | Krause et al. | Aug 1999 | A |
6183192 | Tressler et al. | Feb 2001 | B1 |
6264428 | Dailey et al. | Jul 2001 | B1 |
6582194 | Birkner et al. | Jun 2003 | B1 |
6705836 | Bourriaud et al. | Mar 2004 | B2 |
6955523 | McClelland | Oct 2005 | B2 |
6974308 | Halfmann et al. | Dec 2005 | B2 |
7303376 | Liang | Dec 2007 | B2 |
7377746 | Brassfield et al. | May 2008 | B2 |
7488156 | Liang | Feb 2009 | B2 |
7527475 | Liang | May 2009 | B1 |
7563072 | Liang | Jul 2009 | B1 |
7568887 | Liang | Aug 2009 | B1 |
7819629 | Liang | Oct 2010 | B2 |
7866948 | Liang | Jan 2011 | B1 |
8047790 | Liang | Nov 2011 | B1 |
20050025623 | Botrel et al. | Feb 2005 | A1 |
Number | Date | Country | |
---|---|---|---|
20110236221 A1 | Sep 2011 | US |