Hybrid bucket dovetail pocket design for mechanical retainment

A method for mechanically attaching a composite or polymeric material to a bucket in a radial airfoil includes creating at least one dovetail shape pocket in the bucket having inclined interfaces with respect to the radial airfoil, and filling the pocket with the composite or polymeric material.

BACKGROUND OF THE INVENTION

This invention relates generally to steam turbines and more generally to methods and apparatus for retaining material in hybrid buckets.

Steam turbine buckets (blades) operate in an environment in which they are subject to high centrifugal loads. Additionally, they are in a steam environment with a varying angle of flow incidence to the bucket. A hybrid bucket is a steam turbine bucket that is made primarily of a metallic substance with at least one “pocket” of a non-metallic composite filler material. The filler material may further comprise a polyimide or other type of polymeric resin combined with continuous glass, carbon, KEVLAR® or other fiber reinforcement to achieve the original airfoil surface. This composite matrix is now being designed to be used in units that have high bucket temperatures during windage conditions (low flow, high speed “wind milling” of buckets). One issue with the very stiff high temperature composites is that the adhesion to the metal becomes one of the weakest links in the system.

U.S. Pat. No. 5,720,597, entitled “Multi-Component Blade for Gas Turbine,” describes gas turbine aircraft blades constructed of metal and foam are provided with a composite skin, an erosion coating, or both. Configurations are disclosed that are applicable to fan blades, and more specifically to “propulsion engines.” As such, the sizes and shapes of the pockets are significantly limited. Moreover, U.S. Pat. No. 6,139,728, entitled “Poly-Component Blade for a Steam Turbine,” discloses configurations similar to those disclosed in U.S. Pat. No. 5,720,597, but for steam turbines. Benefits described include lower weight, which allows less robust blade alignment and thereby reduces cost. Furthermore, U.S. Pat. No. 6,042,338, entitled “Detuned Fan Blade Apparatus and Method,” describes a “propulsion engine fan” and various types of blades with different pocket locations, but does not disclose blades of essentially one pocket with different rib structures. In addition, the disclosure is limited to pockets with radial location from a tip to 5%-38% span and chord wise from 15% to 35% from the leading edge and 20% to 45% from the trailing edge with similar limitations on the second or alternative pocket design.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, some configurations of the present invention provide a method for mechanically attaching a composite or polymeric material to a bucket in a radial airfoil. The method includes creating at least one dovetail shape pocket in the bucket having inclined interfaces with respect to the radial airfoil, and filling the pocket with the composite or polymeric material.

In another aspect, some configurations of the present invention provide an airfoil having a bucket that has forward and aft internal interfaces. The bucket has a plurality of inclined surfaces along forward and aft internal interfaces and a pocket filled with a filler material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a schematic illustration of an exemplary opposed-flow, low-pressure (LP) steam turbine10. Turbine10includes first and second low pressure sections12and14. As is known in the art, each turbine section12and14includes a plurality of stages of diaphragms (not shown inFIG. 1). A rotor shaft16extends through sections12and14. Each LP section12and14includes a nozzle18and20. A single outer shell or casing22is divided along a horizontal plane and axially into upper and lower half sections24and26, respectively, and spans both LP sections12and14. A central section28of shell22includes a low pressure steam inlet30. Within outer shell or casing22, LP sections12and14are arranged in a single bearing span supported by journal bearings32and34. A flow splitter40extends between first and second turbine sections12and14.

FIG. 2is a perspective view of a steam turbine bucket100that may be used with turbine10(shown inFIG. 1).FIG. 3is a perspective view of a portion of a composite material101used to fill a pocket122formed in turbine bucket100. Turbine bucket100includes a pressure side102and a suction side (not shown inFIG. 2) connected together at a leading edge104and a trailing edge106. Pressure side102is generally concave and the suction side is generally convex. Turbine bucket100includes a dovetail108, an airfoil portion110, and a root112extending therebetween. In the exemplary embodiment, airfoil portion110and root112are fabricated from one unitary piece and are coupled to dovetail108. In an alternative embodiment, airfoil portion110, root112, and dovetail108may all be fabricated as a unitary component. In the exemplary embodiment, bucket100couples to rotor shaft16via dovetail108and extends radially outward from rotor shaft16. In an alternative embodiment, bucket100may be coupled to rotor shaft16by other devices configured to couple a bucket to a rotor shaft, such as, a blisk.

Bucket dovetail108has a length114that facilitates securing bucket100to rotor shaft16. As rotor shaft16may vary in size, length114may also vary to facilitate providing optimal performance of bucket100and, more specifically, turbine10. Root112extends radially outward from dovetail108and has a length that is approximately equal to dovetail length114. Airfoil portion110extends radially outward from root112and also has an initial length that is approximately equal to dovetail length114. Notably, in the exemplary embodiment, root112and airfoil portion110are fabricated unitarily together such that there are no seams or inconsistencies in bucket100where root112transitions to airfoil portion110.

Airfoil portion110extends radially outward from root112and increases in length to a tip116of bucket100. In the exemplary embodiment, tip116has a length118that is longer than length114. Airfoil portion110also has a width (not shown) sized to facilitate locking a snub cover (not shown). As such, tip length118and the tip width may vary depending on the application of bucket100and, more specifically, turbine10. Bucket100has a radial length120measured from dovetail108to tip116. Length120is selected to facilitate optimizing performance of bucket100. As such, bucket length120may also vary depending on the application of bucket100and, more specifically, turbine10.

In some configurations of the present invention, and referring toFIG. 2andFIG. 3, a directional fiber136orientation is used in a hybrid bucket configuration. Bucket100can be fabricated of a metallic base metal and include a pocket or pockets122that can be filled with a polymer composite material101.

Composite material101can be a polyimide based composite material or any other suitable material that enables bucket100to function as described herein. Composite material101includes fibers136, such as, but not limited to, glass, carbon, Kevlar or other fibers, which are bonded together, for example, in a resin matrix138. Fibers136may be contained in a single layer133, in a plurality of layers133, in one or more layers of fabric, or dispersed throughout matrix138.

In the exemplary embodiment, bucket100also includes a pocket122defined within airfoil portion110. Alternatively, airfoil portion110may include more than one pocket122. Pocket122is formed with a bottom surface124that is recessed from pressure side102of airfoil portion110. Alternatively, pocket122may be formed with a bottom surface124that is recessed from the suction side (not shown inFIG. 2). In the exemplary embodiment, pocket122is substantially rectangular and has a width126and a length128. Alternatively, as is known in the art, pocket122may be formed with any cross-sectional shape that enables bucket100to function as described herein. Width126and length128are selected to ensure that pocket122is circumscribed by pressure side102. In other embodiments, although pocket122may be shaped differently, in each configuration, pocket122is circumscribed by pressure side102. The shape of pocket122is selected to facilitate optimizing performance of bucket100.

In some configurations of the present invention, a method is provided for providing mechanical attachment of a composite or polymeric material101to a bucket100in a radial airfoil102. This method advantageously assists in reducing shear stress in an adhesive layer between metal of bucket100and composite material101as well as to provide a positive mechanical lock of composite material101to bucket100. Some configurations of the present invention use a composite material matrix101that comprises one or several different layers of fiber material136and/or fiber material136in different weave orientations. Also, some configurations of the present invention utilize a “dovetail” shaped pocket190in bucket100that has “dovetail” shaped forward and aft edges (i.e., interfaces)168that help to distribute composite material101load into metallic bucket100during centrifugal loading. Some configurations of the present invention can use either a soft (low temperature) composite material101matrix or a stiff (high temperature) composite material101matrix, in configurations in which the pocket has a backwall.

In other configurations of the present invention, a method for tuning a row of continuously coupled or freestanding turbine buckets100is provided that facilitates reducing the amplitude of vibration and/or damping characteristics. The method includes using a directional fiber orientation in a hybrid bucket100configuration. Bucket100can be made of a metallic base metal with a pocket or pockets122that can be filled with a polymer composite. Composite material101can be a polyimide based composite or another suitable material type, and the material101may include fibers, such as glass, carbon, Kevlar® or other fibers, which are bonded, for example, in a resin matrix. The fibers may be contained in a single layer, in a plurality of layers, in one or more layers of fabric, or throughout matrix18. The orientation of fibers is selected to facilitate tuning bucket100in a particular fashion and/or may be used to “mixed tune” the set. In other words, the fiber orientation is determined in accordance with a pre-selected tuning of bucket100. The frequency characteristic is controlled in some configurations of the present invention by tailoring the fiber orientation during composite lay up and cure. By fine tuning the fiber orientation and/or the weave of a fabric, some configurations of the present invention facilitate controlling strengths and elastic modulus in different directions in fabric constructed from these fibers.

FIG. 4is a perspective view of a plurality of buckets100that may, in some configurations, be used with steam turbine10(shown inFIG. 1).FIG. 5is an enlarged view of an exemplary uniaxial fiber orientation.FIG. 6is an enlarged view of an exemplary biaxial fiber orientation.FIG. 7is an enlarged view of an exemplary quasi-isotropic fiber orientation.

It should be noted that configurations of the present invention can be used with other steam or gas turbine buckets or blades where permitted by the environment (e.g., gas turbine forward stage compressor blades).

Some configurations of the present invention provide a method for mechanically attaching composite or polymeric material101into a bucket100in a radial airfoil102. To hold composite or polymeric material101in bucket100and referring toFIGS. 2,8, and9, a shallow pocket122is created in bucket100and filled with composite or polymeric material101. Adhesion between metallic bucket100and composite or polymeric material101is thereby increased, and sheer stress at the adhesion layer is reduced using mechanical methods. Pocket or pockets108have a gradual incline up to an interface with a flowpath surface168of bucket100. The embodiment illustrated inFIG. 8has a convex interface180, while the embodiment illustrated inFIG. 9has a concave interface182.

In some configurations of the present invention and referring toFIGS. 10,11,12, and13, one or more dovetail-shaped hybrid buckets are provided. Inclined interfaces184,186with respect to radial airfoil102help retain the composite or polymeric101material and help reduce adhesion shear stress between composite or polymeric material101and metallic bucket100for either through window190or shallow pocket configurations122(the latter shown inFIGS. 8 and 9).FIG. 10is an illustration of a full through window190having a convex interface184, whileFIG. 11is an illustration of a full through window with a concave interface186. To produce through wall window190, a high stiffness composite material101is used. Prior art configurations with hybrid buckets used a polymer that could tolerate only low temperatures and had little stiffness, so going through a bucket wall was not possible.

In some configurations of the present invention and referring toFIGS. 12 and 13(in which dotted lines represent edges hidden from view and solid lines represent visible edges), a hybrid bucket100is provided that comprises a plurality of inclined surfaces184,186along a forward and an aft bucket interface. In the configuration illustrated inFIG. 12, convex interfaces184are used, whereas in the configuration illustrated inFIG. 13, concave interfaces186are used. Convex interfaces184and concave interfaces186provide a radial compression feature to retain a composite or polymer filler (not shown inFIG. 12or13) in bucket100. Such “dovetail” configurations can be used to replace prior art shallow “pocket” configurations or be used in conjunction with the prior art configuration. (The prior art pocket configuration is a shallow pocket that does not go through the airfoil. The pocket is filled with a filler material to achieve the original airfoil shape). The dovetail surface in some configurations is either concave or convex around the edge, as shown inFIGS. 12 and 13, respectively, in accordance with that which proves most beneficial during the composite lamination process and/or that which proves to have the best retainment characteristics. The determination is made empirically in some configurations.

Also in some configurations, through pocket window190is configured to minimize or at least reduce stress concentrations on a larger pocket or bucket. The dovetail interface can have any of a variety of geometric shapes in accordance with a finite element analysis of the bucket.

Composite or polymeric material101in some configurations comprises a fabric material136(by way of example without excluding others, glass, carbon, or KEVLAR®) situated in layers using a resin binder or filler. Referring toFIGS. 6,7, and8, This composite is made in some configurations using pre-impregnated unidirectional175, quasi-isotropic176, quasi-isotropic177, or woven fabric tape lay-up, or in other configurations, resin is injected over the fibers during a casting process. In some of these configurations, the material base is a high temperature polyimide base, but configurations use different polymers with high temperature capabilities.

Referring toFIGS. 8,9,10, and11, some configurations of the present invention provide “caul sheets”170on either or both sides of an airfoil102during a composite cure in a pocket122or190. Caul sheet170is used to make the airfoil shape where a pocket122or190is machined away. In some configurations of the present invention, resin fillers are used to create an airfoil shape that existed prior to pocketing.

In addition to single-stage turbine configurations, multi-stage configurations are possible when the temperature is sufficiently low and buckets can be made sufficiently large.

Aside from single through wall dovetail configurations of the present invention, some configurations are used with a shallow pocket122. In the latter case, a dovetail interface pocket can be smaller than a main pocket. The dovetail interface pocket also assists in reducing shear stress at a composite to metal adhesion layer.