WO1992001156A1

WO1992001156A1 - A wing for a windmill

Info

Publication number: WO1992001156A1
Application number: PCT/DK1991/000201
Authority: WO
Inventors: Henrik Stiesdal
Original assignee: Danregn Vindkraft A/S
Priority date: 1990-07-11
Filing date: 1991-07-11
Publication date: 1992-01-23
Also published as: AU8234691A; DK164925B; DK167690A; DK167690D0

Abstract

In order to accommodate the sharpened demands regarding the generation of noise by windmills, the windmill wing (1) according to the invention is configured as a traditional wing (1) in the area from the root and up to a distance from the end (2), i.e. with optimum angle of incidence (α) under normal conditions of operation, while the outer part of the wing, the wing tip (2), is configured with an angle (α) which results in the lift being as close to zero as possible under normal conditions of operation. In practice, this means that the tip (2) appears as if 'twisted back a little' in relation to the remaining part of the wing (1), so that the difference in pressure between the pressure side (5) and the suction side (6) of the wing tip (2) is reduced, whereby the tip-whirl is reduced and the noise from the wing (1) is reduced to a considerable degree.

Description

A WING FOR A WINDMILL

The invention relates to a wing for a windmill, where the angle between the rotor plane and the chord in a given section of the wing decreases in a substantially uniform manner with the distance of the cross-section from the rotor axis, or is constant.

Two or three of such wings are disposed at a uniform angular distance on a hub for the formation of a wind rotor which is made to rotate by the wind around a rotor axis, so that the wings rotate in one plane, the rotation plane or the rotor plane.

It will be known that during operation, the wings of a windmill emit considerable noise which to a substantial de¬ gree stems from the extreme radial end of the wing, which in the following is called the wing tip, and by the so- called tip-whirl generated by the wing tip.

The cross-section of the wings for windmills is twisted in relation to the rotor plane in such a manner that the chord in a given cross-section of the wing forms an angle with said plane. This angle is calculated on the basis of considerations concerning how the optimum angle of incidence can be achieved. For a windmill with constant revolutions, the wing is optimal at a certain wind velocity, and it is with this wind velocity that the calculation is carried out.

The load in the vicinity of the wing tip will thus be relatively high, and the pressure difference between the pressure and the suction side of the wing is corresponding¬ ly great. Unfortunately, there is that disadvantage that the wing and especially the wing tip emits a corresponding¬ ly high level of noise. It is the object of the invention to present a configuration for a windmill wing which reduces the level of noise emitted.

This object is achieved with a windmill wing of the kind presented in the preamble, in that said wing according to the invention is characteristic in that the angle between the rotor plane and the chord in a given cross-section of the wing at its extremity is set in such a way that the lift on the wing's extremity is limited or moves towards 0 at the tip.

With a windmill wing which is configured in the manner pre¬ sented, it is achieved that the loads in the vicinity of the wing tip become approximately 0, and that the pressure difference between pressure and suction sides becomes very small, whereby the energy which is lost in the wing's tip- whirl is reduced. The result of this is that the tip noise will be very limited.

This is achieved advantageously by increasing the said angle to a value of approx. 5° at the tip.

The extremity of the wing comprises with advantage the radially outer 15-20% of the wing's total length.

The windmill wing according to the invention will now be described in more detail with reference to the drawing, where:

figs, la, b and c respectively show a windmill wing seen at right angles to the wing's plane of rotation, tangentially in this plane and at right angles to the wing's longitudinal axis and radially from the outside inwards in said plane, and fig. 2 shows a random cross-section of the wing depicted in fig. 1.

In fig. 1 is shown a windmill wing 1, of which the radially outer end 2 in the position of use of the windmill wing 1 constitutes the so-called wing tip.

In relation to its direction of rotation, the windmill wing 1 has a substantially straight front edge 3 and an opposite rear edge 4 which extends in a substantially inclined manner forwardly and outwardly towards the wing tip 2, so that the breadth of the wing seen at right angles in to¬ wards the rotor plane decreases outwardly. As will be seen in fig. lb, the wing 1 has a relatively flat side 5, the so-called wind or pressure side, and a relatively curved side 6 which forms the wing's suction side.

In fig. 2, the reference figure 7 indicates the chord in a random cross-section of the wing 1, and the reference figure 8 indicates the rotor plane for the wing or for the wind rotor. The chord 7 forms an angle α with the rotor plane 8, and with the known wing 1 the size of this angle diminishes with increasing distance from the rotor axis, so that the angle α at the wing tip 2 is 0.

In fig. la, the dot-and-dash lines indicate nine different cross-sections in the wing 1, in that the radial distance of the cross-sections from the rotor axis will be seen in the following tables 1 and 2, which respectively show some related conditions for the individual sections of a known wing which is marketed under the designation Aerostar 9 m, and for a wing according to the present invention.

In the tables, c indicates the breadth of the wing 1 for each of the given radii, t indicates the thickness of the wing in relation to the chord 7 at the relevant radius, and α indicates the angle which the chord in the relevant cross-section forms with the wind rotor's plane of rotation.

It will be seen in table 1 that the angle α between the chord 7 and the rotor plane 8 in a given cross-section in the known wing diminishes in a substantially uniform manner with increasing distance of the cross-section from the rotor axis.

With this configuration of the wing 1, a relatively large difference in pressure will exist between the wing's pressure side 5 and suction side 6, which has hitherto been considered to be an advantage, since it was thought that a better efficiency could hereby be achieved for the wing 1.

On the basis of later measurements, it would appear that this traditional configuration of windmill wings does not contribute so much towards the windmill's production as has earlier been thought to be the case, apparently because the tip-whirl disturbs the optimum flow across the extremity of the wing 1.

It will be seen in table 2 that the angle a between the chord and the rotor plane in a given cross-section diminishes in a substantially uniform manner with increasing distance of the cross-section from the rotor axis in the first sections, but that this angle then increases again in such a way that the wing tip is twisted back. Table 1

Radius 9.74 , number of sections 9.

Table 2

Radius 9.74 m, number of sections 9.

Claims

C A I M S

1. Wing for a windmill, where the angle (a) between the rotor plane (8) and the chord (7) in a given cross-section of the wing (1) diminishes in a substantially uniform manner with the distance of the cross-section from the rotor axis, or is constant, c h a r a c t e r i z e d in that the angle (a) between the rotor plane (8) and the chord (7) in a given cross-section of the wing (1) at its extremity (2) is set in such a manner that the lift of the extremity (2) of the wing (1) is limited or moves towards 0 at the tip (2).

2. Wing according to claim 1, c h a r a c t e r i z e d in that the angle (a) at the extremity (2) of the wing (1) in¬ creases to a value of approx. 5° at the tip (2).

3. Wing according to claim 1 or 2, c h a r a c t e r i z e d in that the extremity (2) of the wing (1) comprises the radially outer 15-20% of the total length of the wing (1).