Fluid reaction surfaces (i.e. – impellers) – Oscillatory reaction motion
Reexamination Certificate
2001-07-18
2003-09-30
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Oscillatory reaction motion
Reexamination Certificate
active
06626642
ABSTRACT:
The present invention relates to wind turbine blades with oscillation damping means provided in the blade for damping oscillations of the blade, in particular edgewise oscillations of the blade in the rotational plane of a rotor on which the blade is mounted.
The invention relates in particular to damping means comprising a U-shaped cavity in which a liquid, preferably a saturated solution of sodium chloride in water, may oscillate in counter-phase with the oscillations of the blade. The upper end of the arms of the U-shaped cavity are in an advantageous embodiment connected with a passage way so that an O-shaped cavity is provided. The cavity may be tuned to dampen oscillations of the first natural edgewise frequency or the second natural edgewise frequency of the blade or the damping means may comprise both types of dampers.
Furthermore, the invention relates to blades comprising dampers having a cylindrical element that may roll in a box in the blade and to dampers having a resiliently suspended pendulum.
BACKGROUND
Undesired oscillations may occur in wind turbines for power generation. These oscillations may include the entire turbine, i.e. several parts are oscillating in a combined mode, or the oscillations may occur locally in single part. Of particular severity are oscillations in the rotor blades either edgewise (in the rotor plane), flapwise (perpendicular to the rotor plane), or in a combined edge-and flapwise mode. Whether these oscillations do occur is dependent on the wind turbine design and the meteorological conditions.
The oscillation phenomena may cause dangerously high loads on the blade and other parts of the wind turbine, which may lead to a sudden collapse or alternatively may be the cause of fatigue damage and lifetime reduction, as cracks in the components slowly grow ultimately leading to failure. The occurrence of oscillations adds an uncertainty factor to predictions of lifetime loads on the various parts of a wind turbine, making it necessary to make the design stronger and heavier than would otherwise be the case.
The concept of damping of oscillations in blade arranged on a rotor has been known for many years from turbine blades as described in U.S. Pat. No. 2,292,072, U.S. Pat. No. 2,349,187 and U.S. Pat. No. 2,999,669.
The general principle of dampen oscillations of wind turbine blades by building a damping device into the blades is described in WO-A-95/21327, and a number of examples of such devices are given, such as devices having a movable mass, a spring and a damper combined in different configurations, an electromagnetic device comprising a suspended aluminium disk in which eddy currents are induced during movement of the disk to provide resistance to movement and a device comprising a container partly filled with a liquid and having a perforated damper plate, a Tuned Liquid Damper (TLD).
A damping device comprising a number of cavities, mainly spherical cavities, filled with granulate or a liquid, is disclosed in DK 95 00222 U3, and a damping device comprising a number of containers partly filled with liquid, a Tuned Liquid Damper system (TLD), is disclosed in WO 99/32789.
A general problem is that it is difficult to produce an efficient damper of a feasible design that is sufficiently compact and flat in order to satisfy the severe spatial restrictions within the blade.
A wind turbine blade having a mechanical resonator comprising two coupled pendulums is disclosed in EP 0 853 197 A1. A compact design of the damper may be provided but the coupling mechanisms are mechanically rather complex.
DESCRIPTION OF THE INVENTION
Damping of oscillations in wind turbine blades, in particular edgewise oscillations, is important for preventing fatigue damages and failure of the blades, and the issue is becoming increasingly important with the development of larger wind turbines and a more light construction of the blades.
It is an object of the present invention to provide a wind turbine blade having an efficient damper of a feasible design that is sufficiently compact and flat in order to satisfy the severe spatial restrictions within the blade.
It is a further object of the present invention to provide a blade having a damper that is of a simple construction in order to reduce the costs of the damper and at the same time provide a damper that requires no or very little maintenance during its expected lifetime.
As the design of the construction of wind turbine blades becomes more light and the blades are made longer, an increasing demand arises for damping of oscillations of the second natural frequency of the blade, in particular oscillations of the second natural frequency of the blade in the rotational plane of the rotor, i.e. second order edgewise oscillations. It is a still further object of the present invention to provide a blade having a damper for damping oscillations of the second natural frequency of the blade, in particular edgewise oscillations.
It is a yet still further object of the present invention to provide a blade having means for damping oscillations of the second natural frequency as well as means for damping oscillations of the first natural frequency.
Thus, the present invention relates to a wind turbine blade for being arranged on a rotor of a wind turbine, the blade comprising oscillation damping means defining a U-shaped cavity within the blade, the cavity being partly filled with a liquid, of which cavity the bottom of the U-shape is arranged towards a tip end of the blade and the arms of the U-shape are oriented substantially toward a hub end of the blade.
The cavity may typically be defined by means of a tube or a shell, but the cavity may also be an integrated part of the construction elements of the blade. The hub end of the blade is understood to be the end connected to the hub of the rotor and the tip end is understood to be the free end of the blade. The U-shape should be understood in a broad sense as a cavity having a middle or bottom part and two arms, that preferably are arranged symmetrically with respect to the bottom part so that oscillations are dampened symmetrically. The bottom part may be quite short as compared to the liquid filled part of the arms so that the shape is more like a V which may be advantageous because the natural frequency of a damper of such design varies less at high amplitudes of the liquid column inside the cavity than for a damper having a longer bottom part. However, a feasible design has shown to be more U-shaped than V-shaped and a design in which the length of the liquid column contained within the bottom part is approximately equal to twice the length of the liquid column of each of the arms provides satisfactory damping properties of the damper as well as relatively small amplitudes of the liquid column.
The arms of the cavity are in a particular embodiment arranged in a rotational plane of the rotor and the oscillation damping means is tuned to dampen oscillations of the first natural frequency of the blade in the rotational plane so that edgewise oscillations of the blade are dampened. These oscillations are mainly within the range of 1-5 Hz to which frequency range the oscillation damping means preferably is tuned.
In order to provide a proper damping of the blade, the oscillation damping means should be designed to dampen the oscillations of the first natural frequency of the blade in the rotational plane with a magnitude equivalent to a logarithmic decrement of the amplitude of at least 2%.
The logarithmic decrement &dgr; of the amplitude is defined as
δ
=
1
n
ln
(
a
1
a
n
)
where n is the number of oscillations
a
1
is the amplitude of the first oscillation, and
a
n
is the amplitude of the nth oscillation.
The structure of the blade has in itself a damping effect in the oscillations, and the oscillations of the first natural frequency of the blade in the rotational plane should totally dampen with the combined effect of the structure and the damper with a magnitude equivalent to a logarithmic decrement of the amplitude of at least 5% and pref
Birch & Stewart Kolasch & Birch, LLP
Kershteyn Igor
Look Edward K.
NEG Micon A/S
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