Patent Application
20070292274
The airfoil includes a main body or section 34 consisting essentially of metal. In this regard, the term “metal” includes “alloy” but for the purposes of describing the invention herein is not considered to mean a “metallic foam”. In the example embodiment described herein, the main body is a monolithic metallic section, although the invention is not necessarily limited in this regard. The metallic section has a first mass density and radially extends from generally the blade root to the blade tip. The pockets or recesses 30, 32 are defined in the airfoil where the metal is omitted or removed. In this regard, the main body or metallic section 34 of the blade is forged, extruded or cast, and the pockets or recesses 30, 32 may be formed by machining such as, for example, by chemical milling, electrochemical machining, water-jet milling, electro-discharge machining or high speed machining.
If deemed necessary or desirable, the filler section 38 disposed to fill pocket 32 may have different properties, such as temperature resistance, as compared to the filler section 40 used to fill pocket 30. The utilization of different filler sections, or more specifically filler materials, permits improved temperature capability of hybrid blades at reduced cost. Each material used could be formulated for specific locations on the blade based on temperature characteristics of the filler materials and temperature capability requirements of the blades in any given stage. Using the more expensive, high temperature, materials in a limited location on the blade makes the design of hybrid blades more feasible especially for those blades that experience high windage conditions, i.e. in the region 20, 22 of the last stage(s).
Choices for bonding the filler materials to the metal surface of the airfoil portion 28 include, without limitation, self adhesion, adhesion between the filler materials and the metal surface of the airfoil portion 28, adhesive bonding (adhesive film or paste), and fusion bonding.
Hybrid bucket (or blade) design allows for several beneficial outcomes in that it creates a lighter blade which allows for longer or wider cord buckets. However, typical hybrid blade designs do not have a stiff enough composite material in the pocket to help strengthen the blade. Thus, conventionally, the amount of pocketing (depth) in a hybrid blade has been limited due to stress limitations. This limits the ability to create longer, wider or tuned buckets (blades). Overcoming this limitation of conventional hybrid blade design would be advantageous.
Using a carbon fiber material as the filler material in a hybrid blade is beneficial as it can be stiffer than the metallic blade section, thereby allowing a more aggressive pocketing of the blade while keeping the blade mechanically robust. Thus, using a stiff carbon composite can help reduce the stress level in the blade outer area as it makes up for metal that has been removed. However, the inventors have recognized that the interface between the metallic section and the carbon composite can cause galvanic corrosion that would degrade the strength and efficiency of the metallic section over time. Additionally, interfacial stresses are caused by the large mismatch in thermal expansion between the carbon fiber composite, which can be as low as 0.01 ppm/° F., and e.g. steel, which is typically 7 ppm/° F.
Thus, in example embodiments of the invention, the filler material in a hybrid blade is comprised of a carbon fiber lay-up, with resin, and a glass type fiber interface (barrier) that is provided at least between the metallic blade material and the carbon composite. In this regard, the glass composite layers provide a dual benefit of serving as a barrier between the metal of the blade and the carbon composite filler, and of reducing the interfacial stresses between these thermally mismatched components. Further in this regard, the glass composite interlayer coefficient of expansion can be tuned by controlling fiber orientation as well as fiber fraction.
Thus, the use of a carbon composite as proposed herein above is stiff enough to overcome stress limitations, so as to allow for more aggressive pocketing, and the provision of a glass fiber (fabric) barrier interface interposed between the carbon composite and the metallic main body protects the metal from galvanic corrosion and reduces interfacial residual stress.
In some situations, the carbon and steam interface may also need to be protected since carbon may not be as robust in a steam environment. In this regard, there is some evidence that steam and carbon may not always be compatible. Thus, if deemed necessary or desirable, the glass composite can also be used as an erosion shield or barrier between the carbon and the steam environment. Thus, according to yet a further, optional feature of example embodiments of the invention, the glass barrier layer can also be used as a protective cover layer for the steam path facing surface(s) of the carbon fiber composite. However, the glass composite cover does not necessarily need to be used on the steam path surface(s).
As described in greater detail below, to further reduce the weight of the blade in example embodiments of the invention the pocket or portion(s) thereof may be defined to extend entirely through the blade as one or more windows to the opposite, suction side of the blade. The pocket and/or window is then filled with a composite material to reestablish the original airfoil shape or design airfoil shape.
Thus, the embodiment of
The invention further provides a means of suppressing the aerodynamic elastic response of a blade row (continuously coupled or free-standing) by facilitating mixed-tuning of the natural frequency within the row. Mixed-tuning would comprise combining a particular segment of buckets with one frequency characteristic, with one or more other groups of another frequency. The buckets are then selectively assembled in a row so as to achieve improved mechanical damping of a system. There may be more than one group of blades depending on the desired end result.
In this regard, by varying the amount of carbon 144, 244, 344 versus glass barrier fibers 146, 148; 246, 248; 346, 348 in the pockets/windows 130; 230, 231; 330 one can predictably vary the stiffness of the blade 124, 224, 324. This can be accomplished by altering the number of layers of carbon fibers versus the glass barrier fibers; more carbon to stiffen individual buckets and less carbon to allow for more flexibility. This change in stiffness typically correlates to a change in natural frequency. Buckets of varying frequency characteristics (stiffness) can be combined to alter the natural frequency of the bucket group. Thus, a plurality of blades may be provided, each with the same aerodynamic shape and profile externally, but with different filler sections to create at least two distinct groups of blades, one group could use a high strength or stiffer material while the other group could use a lower stiffness or higher damping material. Thus, using this concept two or more populations of blades may be purposefully manufactured and logically assembled so as to utilize their inherent difference in natural frequency as a means of damping the system response to synchronize and non-synchronize vibration without adversely affecting the aerodynamic properties of the blade design.
Thus, the blades 124, 224, 324 described above may be utilized to form a row of blades on a steam turbine rotor wheel as illustrated in
It is also possible to vary the pattern of blade group distribution, again so as to achieve the desired frequency characteristics. For example, a pattern AABBAA . . . or AABAAB . . . might also be employed. The mapped configuration results in mixed tuning of the set of blades via various damping responses of the blades in each group of blades to create a more damped blade row or set. This may also shift the frequencies of each blade to take even greater advantage of the mixed tuning concept.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
