Patent Grant
7467924
This application relates generally to a turbine blade for a gas turbine engine wherein a tab structure under the platform is modified.
Conventional gas turbine engines include a compressor, a combustor and a turbine assembly that has a plurality of adjacent turbine blades disposed about a circumference of a turbine rotor. Each turbine blade typically includes a root that attaches to the turbine rotor, a platform, and a blade that extends radially outwardly from the turbine rotor.
The compressor receives intake air. The intake air is compressed by the compressor and delivered primarily to the combustor where the compressed air and fuel are mixed and burned in a constant pressure process. A portion of the compressed air is bled from the compressor and fed to the turbine to cool the turbine blades.
The turbine blades are used to provide power in turbo machines by exerting a torque on a shaft that is rotating at a high speed. As such, the turbine blades are subjected to a myriad of mechanical stress factors. In addition, the turbine blades are typically cooled using relatively cool air bled from the compressor resulting in temperature gradients being formed, which can lead to additional elements of thermal-mechanical stress within the turbine blades.
Further, because the turbine blades are located downstream of the combustor where fuel and air are mixed and burned in a constant pressure process, they are required to operate in an extremely harsh environment. Traditionally, a chromium-based coating is applied to the entire turbine blade to resist the corrosive effects associated with this harsh environment. The traditional coating protects primarily against stress corrosion in areas of low stress concentration, however, the traditional coating does not provide adequate protection against stress corrosion in areas of high stress concentration, for example, under the platform.
As such, it is desirable to provide a turbine blade that is optimized to reduce the effects of the mechanical and environmental stress factors.
The present invention provides a turbine blade having a revised under-platform structure, including a novel coating process and a configuration that reduces mechanical and environmental stress factors within the turbine blade.
The turbine blade includes a platform with an airfoil extending upwardly from the platform and a root portion extending downwardly from the platform. The turbine blade has a pressure side and a suction side. Two suction side tabs extend a first distance outwardly from the suction side of the root portion below the platform. Two pressure side tabs extend outwardly from the pressure side of the root portion below the platform. One of the two pressure side tabs extends outwardly a distance similar to the first distance, however, the other of the two pressure side tabs extends outwardly a second distance that is significantly less than the first distance. The shorter of the two pressure tabs regionally decreases mechanical stress factors within the turbine blade.
In addition, a plurality of coatings are systematically placed and layered to reduce mechanical and environmental stress factors. A first coating is applied to substantially cover the turbine blade on both sides of the platform. The first coating protects against corrosion in areas of low stress concentration. However, the area under the platform of the turbine blade at the root portion is subjected to much higher stress concentrations than other areas of the turbine blade. Therefore, a second coating is applied over the first coating only under the platform. The second coating is added to resist corrosion cracking in areas of high stress concentration. The second coating is applied using a line-of-sight coating process through an access area that is created as a result of the shortened pressure side tab. The second coating is applied underneath the platform by spraying the coating directly at the shorter of the two pressure side tabs. Additional coatings are applied to the turbine blade to further reduce the effects of stress.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
The second set of tabs 43 extends outwardly from the root 36 on the suction side 40 in a first direction that is substantially parallel to the platform 32. The first set of tabs 42 extends outwardly from the root 36 on the pressure side 38 in a second direction, substantially opposite the first direction. The second direction is also substantially parallel to the platform 32.
The first tab 72 and the second tab 76 extend outwardly from the pressure side 68 of the root 66 in a direction substantially parallel to the platform 64. The second tab 76 is significantly shorter than the first tab 72. A third tab and a fourth tab are positioned on the suction side 70 of the root 66, similar to the prior art, and have lengths that are similar to the first tab 72. The tabs are used to position the damper as shown in
The first tab 72, the third tab and the fourth tab respectively include a base portion 72A and a post portion 72B. The second tab 76 includes only a base portion 76A. By only using the base portion 76A in this region, an amount of mechanical stress imposed on the turbine blade 60 in this region is reduced. While the inventive turbine blade 60 is disclosed for use in a first stage turbine assembly, the inventive turbine blade 60 may be used in any stage.
To further reduce the effects of stress on the turbine blade 60, a plurality of coatings are applied to specified portions of the turbine blade 60. A first coating, which in this example is a chromium-based coating, is applied to substantially cover the turbine blade 60 for corrosion protection. The first coating is applied to resist stress corrosion in areas of low stress concentration. Any type of chromium-based coating may be used.
A second coating is applied over the first coating to address high stress areas on the turbine blade 60. One high stress area is an area under the platform 64, more specifically a region surrounding the base portion 72A of the first tab 72 and including the first tab 72. This area is subjected to much higher stress concentrations than the remainder of the turbine blade 60. Further, the area under the platform 64 is susceptible to a different type of corrosion, that is, corrosion that occurs as a result of the high stress concentration. As such, the second coating, which is also chromium-based, is applied only under the platform 64 to resist stress corrosion is areas of high stress concentrations. This second coating is applied using a line-of-sight application process in which a sprayer, shown schematically at 200 in
A third coating is applied over the first coating only on the airfoil 62. In this example, the third coating is a metallic-bond coating which assists in adherence of a fourth coating applied over the third coating only on the airfoil 62. This improves adhesion of a fourth coating, which in this example is a ceramic coating. The combination of coatings used on the airfoil 62 may include a heat treat process to ensure adhesion. Further, the combination of coatings reduces the effects of the harsh environment on the turbine blade 60.
Finally, a fifth coating is applied over the fourth coating only to a tip 80 of the turbine blade 60 to facilitate blade cutting. The fifth coating is a cubic boron nitride (CBN) coating. To ensure the tight clearances required by the turbine engine, the tips of the turbine blades are required to cut-in to the case surrounding the turbine engine. As such, the fifth coating is sacrificial, maintaining its integrity only long enough to ensure adequate run-in.
The types of coatings discussed above are examples of each coating and other types of coatings could also be used to provide the desired characteristics.
A comparison of the geometries of the tabs of the prior art and the present invention is more clearly illustrated in
The second tab 76 includes only a base portion 76A. This base portion 76A extends outwardly from the pressure side 68 along a third distance D3, which is approximately equal to D1. The overall length L of the first tab 72 is D1+D2, which is significantly greater than D3.
Because the second tab 76 only includes the base portion 76A, the mechanical stress in the region surrounding the base portion 76A under the platform 64 is reduced. That is, because the second tab 76 of the present invention is shorter than the prior art tab 47, it does not extend into the cavity created between two adjacent turbine blades 30A and 30B to support the damper 44. As such, the mechanical stress, more specifically, the torsional stress induced by the damper 44 into the region under the platform 64 through the length of the prior art tab 47 no longer exists in the present invention.
Further, as discussed above, because the second tab 76 only includes the base portion 76A, the shorter second tab 76 provides an access area for coating application. This access provides an unimpeded line-of-sight for application of the second coating under the platform 64, which ensures complete coverage of the area of highest stress concentration including the first tab 72.
While the present invention is illustrated in a turbine blade, it should be understood that the invention would also be beneficial in a static structure such as a stator or a vane.
Although preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
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| Number | Date | Country | |
|---|---|---|---|
| 20070041838 A1 | Feb 2007 | US |
