Patent Application
20060110248
The present invention relates to patterns placed at the surface of metal components of gas turbine engines, radial inflow compressors and radial turbines, including micro-turbines and turbo-chargers, that are exposed to high temperature environments and, in particular, to a new type of optimized pattern applied to turbine shrouds used in gas turbine engines in order to improve the performance and efficiency of the turbine blades (also known as “buckets”).
Gas turbine engines are used in a wide variety of different applications, most notably electrical power generation. Such engines typically include a turbocompressor that compresses air to a high pressure by means of a multi-stage axial flow compressor. The compressed air passes through a combustor, which accepts air and fuel from a fuel supply and provides continuous combustion, thus raising the temperature and pressure of the working gases to a high level. The combustor delivers the high temperature gases to the turbine, which in turn extracts work from the high-pressure gas working fluid as it expands from the high pressure developed by the compressor down to atmospheric pressure.
As the gases leave the combustor, the temperature can easily exceed the acceptable temperature limitations for the materials used in construction of the nozzles and buckets in the turbine. Although the hot gases cool as they expand, the temperature of the exhaust gases normally remains well above ambient. Thus, extensive cooling of the early stages of the turbine is essential to ensure that the components have adequate life. The high temperature in early stages of the turbine creates a variety of problems relating to the integrity, metallurgy and life expectancy of components coming in contact with the hot gas, such as the rotating buckets and turbine shroud. Although high combustion temperatures normally are desirable for a more efficient engine, the high gas temperatures may require that air be taken away from the compressor to cool the turbine parts, which tends to reduce overall engine efficiency.
In order to achieve maximum engine efficiency (and corresponding maximum electrical power generation), it is important that the buckets rotate within the turbine casing or “shroud” with minimal interference and with the highest possible efficiency relative to the amount of energy available from the expanding working fluid.
During operation, the turbine casing (shroud) remains fixed relative to the rotating buckets. Typically, the highest efficiencies can be achieved by maintaining a minimum threshold clearance between the shroud and the bucket tips to thereby prevent unwanted “leakage” of a hot gas over tip of the buckets. Increased clearances will lead to leakage problem and cause significant decreases in overall efficiency of the gas turbine engine. Only a minimum amount of “leakage” of the hot gases at the outer periphery of the buckets, i.e., the small annular space between the bucket tips and turbine shroud, can be tolerated without sacrificing engine efficiency. Further, there are losses caused by the flow of hot gas over a particular portion of an interior surface of the turbine shroud when the bucket is not near the particular portion.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine rotates, centrifugal forces acting on the turbine components can cause the buckets to expand in an outward direction toward the shroud, particularly when influenced by the high operating temperatures. Additionally, the clearance between a bucket tip and the shroud may be non-uniform over the entire circumference of the shroud. Non-uniformity is caused by a number of factors including machining tolerances, stack up tolerances, and non-uniform expansion due to varying thermal mass and thermal response. Thus, it is important to establish the lowest effective running clearances between the shroud and bucket tips at the maximum anticipated operating temperatures.
A significant loss of gas turbine efficiency results from wear of the bucket tips if, for example, the shroud is distorted or the bucket tips rub against the ceramic or metallic flow surface of the shroud. If bucket tips rub against a particular location of the shroud such that the bucket tip is eroded, the erosion of the bucket tip increases clearances between bucket tip and shroud in other locations. Again, any such deterioration of the buckets at the interface with the shroud when the turbine rotates will eventually cause significant reductions in overall engine performance and efficiency.
In the past, abradable type coatings have been applied to the turbine shroud to help establish a minimum, i.e., optimum, running clearance between the shroud and bucket tips under steady-state temperature conditions. In particular, coatings have been applied to the surface of the shroud facing the buckets using a material that can be readily abraded by the tips of the buckets as they turn inside the shroud at high speed with little or no damage to the bucket tips. Initially, a clearance exists between the bucket tips and the coating when the gas turbine is stopped and the components are at ambient temperature. Later, during normal operation the clearance decreases due to the centrifugal forces and temperature changes in rotating and stationary components inevitably resulting in at least some radial extension of the bucket tips, causing them to contact the coating on the shroud and wear away a part of the coating to establish the minimum running clearance. Without abradable coatings, the cold clearances between the bucket tips and shroud must be large enough to prevent contact between the rotating bucket tips and the shroud during later high temperature operation. With abradable coatings, on the other hand, the cold clearances can be reduced with the assurance that if contact occurs, the sacrificial part is the abradable coating instead of the bucket tip.
As noted in prior art patents describing abradable coatings for use in turbocompressors and gas turbines (see e.g., U.S. Pat. No. 5,472,315), a number of design factors are considered in selecting an appropriate material for use as an abradable coating on the shroud, depending upon the coating composition, the specific end use, and the operating conditions of the turbine, particularly the highest anticipated working fluid temperature. Ideally, the cutting mechanism (e.g., the bucket blade tips) can be made sufficiently strong and the coating on the shroud will be brittle enough at high temperatures to be abraded without causing damage to the bucket tips themselves. That is, at the maximum operating temperature, the shroud coating should be preferentially abraded in lieu of any loss of metal on the bucket tips.
Any coating material that is removed (abraded) from the shroud, however, should not affect downstream engine components. Ideally, the abradable coating material remains bonded to the shroud for the entire operational life of the gas turbine and does not significantly degrade over time. In other words, the abradable material is securely bonded to the turbine shroud and remains bonded while portions of the coating are removed by the bucket blades during startup, shutdown or a hot-restart. Preferably, the coating should also remain secured to the shroud during a large number of operational cycles, that is, despite repeated thermal cycling of the gas turbine engine during startup and shutdown, or periodic off-loading of power.
Thus, the need exists for an improved pattern that will allow for the use of bucket tips at elevated temperatures without requiring any tip treatment (such as the application of aluminum oxide and/or abrasive grits such as cubic boron nitride). A need also exists for an improved abradable coating system that can be used if necessary in conjunction with reinforced bucket tips in order to provide even longer-term reliability and improved operating efficiency.
Exemplary embodiments of the invention include a pattern for improving aerodynamic performance of a turbine including a material disposed at a base surface disposed at an interior surface of a turbine shroud such that the material is capable of abradable contact with a tip portion of a turbine bucket. The material is disposed in a pattern including a first plurality of ridges disposed at the base surface such that at least a first portion of the first plurality of ridges corresponding to at least a back portion of the turbine bucket is oriented at a first angle with respect to an axis of rotation of the turbine bucket. Each ridge of the first plurality of ridges has a first sidewall and a second sidewall. The first and second sidewalls each have a first end and an opposite second end. The first end of the first and second sidewalls extends from the base surface. The first and second sidewalls slope toward each other until meeting at the second ends of respective first and second sidewalls defining a centerline and a top portion of the ridge. The first and second sidewalls are inclined with substantially equal but opposite slopes with respect to the base surface.
Further exemplary embodiments of the invention include a pattern for improving aerodynamic performance of a turbine including a first plurality of ridges disposed at a base surface disposed at an interior surface of a turbine shroud such that at least a first portion of the first plurality of ridges corresponding to at least a back portion of a turbine bucket is oriented at a first angle with respect to an axis of rotation of the turbine bucket. Each ridge of the first plurality of ridges has a first sidewall and a second sidewall. The first and second sidewalls each have a first end and an opposite second end. The first end of the first and second sidewalls extends from the base surface. The first and second sidewalls are disposed substantially perpendicular to the base surface. The second ends of respective first and second sidewalls are connected by a top portion of the ridge.
Further exemplary embodiments of the invention include a turbine casing including a turbine shroud and a material disposed at a base surface disposed at an interior surface of a turbine shroud such that the material is capable of abradable contact with a tip portion of a turbine bucket. The material is disposed in a pattern including a first plurality of ridges disposed at the base surface such that at least a first portion of the first plurality of ridges corresponding to at least a back portion of the turbine bucket is oriented at a first angle with respect to an axis of rotation of the turbine bucket. Each ridge of the first plurality of ridges has a first sidewall and a second sidewall. The first and second sidewalls each have a first end and an opposite second end. The first end of the first and second sidewalls extends from the base surface. The first and second sidewalls slope toward each other until meeting at the second ends of respective first and second sidewalls defining a centerline and a top portion of the ridge. The first and second sidewalls are inclined with substantially equal but opposite slopes with respect to the base surface.
Further exemplary embodiments of the invention include a turbine including a rotatable shaft, a turbine shroud, and a material disposed at a base surface disposed at an interior surface of a turbine shroud such that the material is capable of abradable contact with a tip portion of a turbine bucket. The material is disposed in a pattern including a first plurality of ridges disposed at the base surface such that at least a first portion of the first plurality of ridges corresponding to at least a back portion of the turbine bucket is oriented at a first angle with respect to an axis of rotation of the turbine bucket. Each ridge of the first plurality of ridges has a first sidewall and a second sidewall. The first and second sidewalls each have a first end and an opposite second end. The first end of the first and second sidewalls extends from the base surface. The first and second sidewalls slope toward each other until meeting at the second ends of respective first and second sidewalls defining a centerline and a top portion of the ridge. The first and second sidewalls are inclined with substantially equal but opposite slopes with respect to the base surface.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
Exemplary embodiments of the present invention include an abradable coating defining a pattern that improves abradability of an abradable material and improves the aerodynamic performance of a turbine by improving a seal around a turbine bucket tip. Another exemplary embodiment includes the pattern formed in an interior surface of a turbine shroud. Generally, the pattern is formed from a plurality of ridges. Exemplary embodiments of the pattern improve aerodynamic performance of the turbine by decreasing a space between the turbine bucket tip and a turbine shroud, thereby improving the seal around the turbine bucket tip. An additional aerodynamic performance improvement is realized due to the pattern reducing aerodynamic losses between each turbine bucket tip of a plurality of turbine bucket tips. A patterned surface on the interior surface of the turbine shroud provides a direction to the mainstream flow on the outer wall. Thus, even if the seal were not improved, the patterned surface reduces aerodynamic losses.
Abradable pattern 12 is defined by a first plurality of ridges 16 disposed on a base surface 20. Each ridge 16 of the plurality of ridges 16 is substantially parallel with each other ridge 16. Each ridge 16 of the plurality of ridges 16 is also substantially equidistant from each other ridge 16.
In addition, while the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
