Fluid reaction surfaces (i.e. – impellers) – With weight-balancing means – Self-shifting or selectively adjustable mass
Reexamination Certificate
1999-02-23
2001-10-09
Verdier, Christopher (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With weight-balancing means
Self-shifting or selectively adjustable mass
C416S500000, C415S119000, C415S010000, C415S014000
Reexamination Certificate
active
06299410
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and apparatus for damping vibration in turbomachine components and more particularly to a method and apparatus that employs a mechanical-to-electromagnetic energy transformer for such damping.
BACKGROUND
Under operating conditions, a turbomachine component, for example, a gas turbine airfoil, is subjected to a variety of forces. Some forces are dependent on rotor speed, e.g. centrifugal force, resulting in a steady state or slowly varying strain (change in dimension, e.g., stretching or shortening) of the airfoil. Others result in a more dynamically varying strain, i.e., commonly referred to as vibratory strain, and airfoil vibration, e.g., forced vibration (resonance or buffeting) and aero elastic instability (flutter). The magnitudes of the forces and resulting strains depend on the engine operating conditions and the aircraft structural and aerodynamic properties.
To prevent damage to the airfoil, the magnitudes of the steady state and vibratory strains must not exceed the structural capabilities (limits) of the airfoil. In order to keep the vibratory strain of the airfoil within limits, the engine is often operated at lower than optimum conditions, resulting in a reduced engine operating efficiency.
Various approaches exist for reducing airfoil vibration. Some of these approaches involve stiffening the structure of the airfoil. The effect of stiffening is to adjust the resonant frequency of the airfoil to a value that is different from that of the vibratory force. Increased stiffness helps to prevent flutter-type vibratory strain. For example, a more rigidly constructed airfoil results in less vibration. However, a more rigid airfoil is often heavier (with associated disadvantages) and the optimum degree of rigidity is often not precisely known at the time that the airfoil is initially designed. Another approach makes use of a shroud, disposed at a midspan point on the airfoil. A midspan shroud has the effect of stiffening the airfoil. In addition, the shrouds interact with one another to reduce vibration of multiple adjacent blades. However, a midspan shroud tends to obstruct the airflow and thereby reduce turbomachine efficiency.
Passive vibration damping is another approach for reducing the magnitude of airfoil vibration. Passive vibration damping is a form of structural damping that involves the dissipation of energy. One approach for passive damping employs sliding friction devices, such as those employed under blade platforms. This approach relies on friction to dampen vibratory motion. However, most blade vibratory motion occurs above the platform, for which under-platform devices have limited effectiveness.
An active vibration control scheme has been proposed by Acton et al. in U.S. Pat. No. 4,967,550. The scheme uses a control system with actuators to counter blade vibration. Acton et al. disclose that two categories of actuators involving direct contact with the blades: “(i) electromagnetically actuated shakers attached to the blades for introducing forces in the blades, and (ii) piezoelectric or magnetostrictive means internal of the blades to deform them by changing the relevant characteristics of such, for examples embedded piezoelectric crystals which could distort the blade and thereby affect the local structural properties of the blades, e.g. by increasing the structural damping.” Piezoelectric materials convert electrical energy to mechanical energy, and visa versa. Unlike passive methods, an active control system, sometimes referred to as a feedback system, is complex, requiring sensors, signal processing circuits, actuators, and a power supply. Embedding piezoelectric crystals in the blade requires a complex fabrication process. The combination of an active control system and embedded piezoelectric crystals is not practical in terms of cost and complexity.
SUMMARY OF THE INVENTION
An object of one aspect of the present invention is to provide a method and apparatus for passive damping of vibratory strain in a turbomachine component that experiences high steady state strain using mechanical-to-electromagnetic energy conversion without an active control system.
Another object of another aspect of the present invention is to provide a method and apparatus for airfoil vibration damping using mechanical-to-electromagnetic energy conversion without embedded actuators.
The present invention reduces the magnitude of vibratory strain in a turbomachine component that experiences high steady state strain by, in the case of a passive embodiment, coupling a mechanical-to-electromagnetic energy converter to an interior and/or exterior surface of the component, and/or embedding the energy converter within the component, and by, in the case of an active embodiment, coupling a mechanical-to-electromagnetic energy converter to an interior and/or exterior surface of the component. Thus, the present invention eliminates the need for an active embodiment with embedded piezoelectric crystals.
Damping as referred to herein is defined to mean reducing vibratory strain in a component, whether accomplished by dissipation or by stiffening.
Although passive vibration dampers that employ mechanical-to-electromagnetic energy conversion are known, until now, they have not been employed to dampen vibration of turbomachine components that experience high steady state stress. For example, with respect to rotating airfoils, the materials commonly used for passive damping, e.g., piezoelectric material, were not considered capable of providing sufficient damping, i.e., dissipation of energy, to be of practical value for passive damping vibration in. However, in accordance with the present invention, it has been determined that in regard to some types of turbomachine airfoil vibration, e.g., flutter, high frequency vibration modes, only a small amount of damping is needed. It has further been determined that passive vibration damping using mechanical-to-electrical energy conversion can often provide sufficient damping to be effective. In some embodiments, for example, such damping provides sufficient reduction in the magnitude of the strain on the airfoil that it enables the engine to be operated at closer to optimum conditions, and thereby achieve greater turbomachine efficiency.
Although systems that employ mechanical-to-electromagnetic energy conversion are known, until now, the mechanical-to-electrical energy converters in such systems have not been affixed to a surface of a turbomachine component that experiences high steady state stress. For example, with respect to a rotating airfoil, the mechanical-to-electromagnetic energy converters suggested for such systems have consisted of piezoelectric and magnetostrictive means internal of the blades, e.g., embedded piezoelectric crystals. There are many reasons for not affixing the mechanical-to-electrical energy converter to the surface of the blade. For example, a blade experiences very high steady state tensile strain during engine operation. Piezoelectric materials typically comprise a ceramic type of material and are thus susceptible to damage, i.e., cracking (breaking), or detaching (flying off) from the blade, in environments of such high steady state tensile strain. Furthermore, the magnitudes of the vibratory strains on an exterior surface of the blade is considered extreme for piezoelectrics. Another reason is to keep the mechanical-to-electrical energy converters out of the airflow, so as not to upset the aerodynamic shape of the blade. Embedded crystals may also have been considered necessary to achieve effective damping. However, it has been determined that the such materials may be suitably positioned on and attached to turbomachine airfoils to achieve effective damping without embedding it, in the form of crystals, within the structure of a blade. Moreover, it has been determined that because the vibratory stress is greater on the exterior of the blade, more effective vibration damping is often possible if the mechanical-to-electrical energy converters are coupled to an exter
Crawley Edward F.
Hilbert Gary R.
Pearson David D.
United Technologies Corporation
Verdier Christopher
Woo Richard
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