Patent Publication Number: US-8985956-B2

Title: Compressive stress system for a gas turbine engine

Description:
FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under Contract No. DE-FC26-05NT42643, awarded by the U.S. Department of Energy (DOE). The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present application and the resultant patent relate generally to gas turbine engines and more particularly relate to systems and methods for imparting compressive stress to composite airfoils so as to minimize interlaminar tensile stress about the shanks thereof. 
     BACKGROUND OF THE INVENTION 
     Airfoils used in gas turbine engines generally have been made from high temperature superalloys given the high temperature operating environment and the various stresses created during operation. Various types of composite materials also have been used given the lightweight nature and the high temperature capabilities of such composite materials. One drawback with such composite materials, however, includes relatively poor interlaminar properties. Moreover, the overall turbine bucket generally may be subject to nonuniform stress patterns under normal operating conditions. As such, the bucket may experience varying degrees of localized stress at different times and at different locations. Turbine buckets therefore may be designed with more composite material at locations such as the shank and the minimum neck areas so as to accommodate high localized tensile stresses. 
     There is thus a desire for an improved composite materials turbine bucket design. Preferably such an improved turbine bucket design should accommodate increased interlaminar stresses with the use of less material. Such reduced stresses should increase component life while reducing the amount of material also should result in reduced component costs. 
     SUMMARY OF THE INVENTION 
     The present application and the resultant patent provide a compressive stress system for a gas turbine engine. The compressive stress system may include a first bucket attached to a rotor, a second bucket attached to the rotor, the first and the second buckets defining a shank pocket therebetween, and a compressive stress spring positioned within the shank pocket. The compressive stress spring asserts a force on the buckets so as to reduce the interlaminar stresses therein. 
     The present application and the resultant patent further provide a method of reducing interlaminar stresses in a composite material bucket. The method may include the steps of positioning a compressive stress spring in a shank pocket between adjacent buckets, releasing a pair of arms of the compressive stress spring into contact with each of the adjacent buckets, and asserting a compressive force on each of the adjacent buckets by the pair of arms so as to reduce the interlaminar stresses in each of the adjacent buckets. 
     The present application and the resultant patent further provide a compressive stress system for a gas turbine engine. The compressive stress system may include a first bucket and a second bucket attached to the rotor. The first bucket and the second bucket may include a composite material and may define a shank pocket therebetween. A compressive stress spring may be positioned within the shank pocket so as to assert a force on the first bucket and the second bucket. 
     These and other features and improvements of the present application and the resultant patent will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a gas turbine engine with a compressor, a combustor, and a turbine. 
         FIG. 2  is a side plan view of a compressive stress system for a turbine bucket as may be described herein showing a compressive stress spring positioned between adjacent buckets. 
         FIG. 3  is a side plan view of an alternative embodiment of a compressive stress system as may be described herein. 
         FIG. 4  is a side plan view of an alternative embodiment of a compressive stress system as may be described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows a schematic view of gas turbine engine  10  as may be used herein. The gas turbine engine  10  may include a compressor  15 . The compressor  15  compresses an incoming flow of air  20 . The compressor  15  delivers the compressed flow of air  20  to a combustor  25 . The combustor  25  mixes the compressed flow of air  20  with a compressed flow of fuel  30  and ignites the mixture to create a flow of combustion gases  35 . Although only a single combustor  25  is shown, the gas turbine engine  10  may include any number of combustors  25 . The flow of combustion gases  35  is in turn delivered to a turbine  40 . The flow of combustion gases  35  drives the turbine  40  so as to produce mechanical work. The mechanical work produced in the turbine  40  drives the compressor  15  via a shaft  45  and an external load  50  such as an electrical generator and the like. 
     The gas turbine engine  10  may use natural gas, various types of syngas, and/or other types of fuels. The gas turbine engine  10  may be any one of a number of different gas turbine engines offered by General Electric Company of Schenectady, N.Y. including, but not limited to, those such as a 7 or a 9 series heavy duty gas turbine engine and the like. The gas turbine engine  10  may have different configurations and may use other types of components. Other types of gas turbine engines also may be used herein. Multiple gas turbine engines, other types of turbines, and other types of power generation equipment also may be used herein together. 
       FIG. 2  shows an example of a turbine bucket compressive stress system  100  as may be described herein. The turbine bucket compressive stress system  100  includes a number of turbine buckets  110 . Although the turbine bucket compressive stress system  100  herein will be described in the context of a first turbine bucket  120  and a second turbine bucket  130 , any number of turbine buckets  110  may be used herein. The turbine buckets  110  may be made out of a composite material. For example, a number of different ceramic matrix composites and the like may be used herein as well as other types of composites. 
     Generally described and by way of example only, each turbine bucket  110  may include a dovetail  140 , a shank  150 , and a platform  160 . An airfoil  170  may extend from the platform  160 . Each turbine bucket  110  may be positioned within a rotor  180  for rotation therewith. The rotor  180  may include a number of rotor slots  190  separated by rotor posts  200 . The rotor slots  190  may be sized and shaped to mate with the dovetails  140  of each turbine bucket  110 . The shank  150  may extend from a minimum neck width region  155  to the platform  160 . A shank pocket  205  may be defined between the shanks  150  of the adjacent turbine buckets  120 ,  130  and the rotor post  200 . Other components and other configurations may be used herein. 
     The turbine bucket compressive stress system  100  further may include a compressive stress spring  210 . The compressive stress spring  210  may be in the form of a substantially U-shaped clip  220  with a first arm  230  and a second arm  240 . The compressive stress spring  210  may be made from any high temperature metallic or composite material with sufficient restoring strength. The compressive stress spring  210  may have any desired size, shape, or configuration. The compressive stress spring  210  also may include a spring dovetail  250 . The spring dovetail  250  may be positioned within a spring slot  260  on the rotor  180 . 
     In use, the compressive stress spring  210  may be positioned within the shank pocket  205 . The arms  230 ,  240  of the U-shaped clip  220  may be compressed and then placed in contact with the shanks  150  of the adjacent buckets  120 ,  130  about the minimum neck width region  155  towards the platform  160 . When released, the arms  230 ,  240  of the U-shaped clip  220  impart a force and therefore compressive stress about the shanks  150 . This compressive stress helps to minimize the interlaminar tensile stress generally present in this region of the buckets  120 ,  130 . The compressive stress spring  210  may be retained by the rotor  180  via the spring dovetail  250  so as to minimize any radial load increase on the buckets  120 ,  130 . 
     The force of the arms  230 ,  240  returning to their non-deformed shape thus contacts the shanks  150  so as to impart this compressive force. This force generates compressive stress that counteracts the interlaminar tensile stress therein. High interlaminar tensile stress about the shank  150  and the minimum neck region  150  generally dictate how thick the shank  150  must be in order to carry the load of the airfoil  170 . The interlaminar tensile stress also impact on the overall life span of the component. By reducing the interlaminar tensile stresses in the shank  150  and the minimum neck region  155 , a wider range of design choices may be possible. Moreover, less material may used to reduce the overall costs while lower stresses should improve overall component lifetime. 
       FIG. 3  shows a further embodiment of a turbine bucket compressive stress system  300  as may be described herein. In this example, an array  310  of buckets is shown. Specifically, a first bucket  320 , a second bucket  330 , and a third bucket  340  are shown. Any number of buckets, however, may be used herein. A compressive stress spring may be positioned between each pair of buckets. In this example, a first compressive stress spring  350  and a second compressive string  360  are shown. Any number of compressive stress springs may be used herein. In this example, each compressive stress spring  350 ,  360  may have a variation of a U-shaped clip  370 . In this example, the U-shaped clip  370  also includes a pair of inward curls. Specifically, a first inward curl  380  on a first arm  390  and a second inward curl  400  on a second arm  410 . Other variations on the U-shaped clip  370  and the inward curls  380 ,  400  may be used herein. 
       FIG. 4  shows a further example of a turbine bucket compressive stress system  500  as may be described herein. The turbine bucket compressive stress system  500  may include an array  510  of buckets. Specifically, a first bucket  520 , a second bucket  530 , and a third bucket  540  are shown. Any number of buckets may be used herein. Likewise, a compressive stress spring may be positioned between each pair of buckets. In this example, a first compressive stress spring  550  and a second compressive stress spring  560  are shown. Any number of compressive stress springs may be used herein. In this example, the compressive stress springs take the form of a U-shaped clip  570 . In this example, the U-shaped clip  570  includes a first outward curl  570  on a first arm  590  and a second outward curl  600  on a second arm  610 . Other types of U-shaped clips  570  and the outward curls  580 ,  600  may be used herein. 
     It should be apparent that the foregoing relates only to certain embodiments of the present application and the resultant patent. Numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof,