GB2216959A

GB2216959A - Air turbine or propellor blade

Info

Publication number: GB2216959A
Application number: GB8906639A
Authority: GB
Inventors: William Scott Bannister
Original assignee: MARLEC ENGINEERING CO Ltd
Current assignee: MARLEC ENGINEERING CO Ltd
Priority date: 1988-03-22
Filing date: 1989-03-22
Publication date: 1989-10-18
Anticipated expiration: 2009-03-22
Also published as: GB2216959B; GB8906639D0; GB8806739D0

Abstract

A turbine or propellor blade 1 has brake means to avoid overspeed in high wind conditions. The brake means comprise 9 disposed within the blade to interconnect a first opening 11 and a second opening 10 in the blade, and further comprise valve means 13 for controlling fluid flow through the passage 9, in which a pressure differential between the first and the second openings as a result of rotation of the rotor can effect fluid flow. The first opening 11 may be radially inward of the second 10 or may be disposed in the nose cone 3 of the rotor, or on the high-pressure side of an aerofoil cross-section of the blade, the second opening 10 then being on the low pressure side. A further inlet may be provided by a tube 12. <IMAGE>

Description

TITLE: Air Turbine The invention relates to rotational speed control of air turbines or propellers generally. It is particularly, but not exclusively, concerned with means for controlling the speed of a multi-blade wind turbine.

Wind turbines driven by an abundant though variable supply of motive power are commonly used at remote locations for pumping water or generating electricity. The output of such a turbine is directly related to the rotational speed of its blades which in turn is generally a function of the incident wind speed.

To control output and prevent 'overspeed' in high wind conditions, it is necessary to govern the rotational speed of the turbine blades. This is usually accomplished by employing brake means acting directly on the turbine rotor, or means which alter the effective attitude of the turbine blades (thereby reducing the power extracted therefrom).

The latter can be achieved by rotating the whole turbine, by means of a rear rudder, to move the blades 'out of' the wind. Alternatively, the pitch of each or sometimes the tip of each blade relative to the rotor hub may be changed.

Unfortunately, the control means hitherto described all complicate the basic mechanical structure of the turbine, thus providing additional parts and possibly complex mechanisms that must be maintained.

It is an object of the invention to provide a mechanically simple brake means for controlling the rotational speed of a turbine which overcomes at least some of the attendant disadvantages of prior known constructions.

According to one aspect of the invention a blade of a rotor of a turbine or propeller has brake means comprising an open-ended passage disposed within the blade which interconnects a first opening and a second opening in the blade, and further comprising valve means for controlling fluid flow through the passage, in which a pressure differential between the first and the second openings as a result of rotation of the rotor can effect said fluid flow.

The blade may have an aerofoil cross-section and the first opening is disposed in the high pressure side of the aerofoil and the second opening is disposed on the low pressure side of the aerofoil.

Alternatively the rotor may have a nose cone and the first opening may be disposed therein.

As a further alternative, the first opening may be disposed in a leading edge of the blade.

At least the major portion of the passage may be co-axial with the longitudinal axis of the blade.

The valve means may comprise a control valve.

The control valve may be normally closed and controlled to open at a pre-determined rotor speed or may be normally open and held closed until a pre-determined rotor speed is reached.

From a further aspect, the invention provides a rotor of a turbine or propeller including a plurality of blades as set out above.

It will be appreciated that a rotationally balanced arrangement is employed with said blade being one of a plurality of blades of the rotor. Preferably each blade of a multi-blade rotor is provided with brake means in accordance with the invention.

The invention will now be further described with reference to the accompanying drawing which illustrates, by way of example only, a preferred embodiment of the invention.

In the drawing: Fig. 1 is a partial diagrammatic front elevation of the rotor of a wind turbine comprising brake means in accordance with the preferred embodiment and showing one of the rotor blades in a static position.

A wind turbine rotor comprises two or more blades, one of which is illustrated in Fig. 1. This blade 1 extends radially from and is rigidly attached to a hub of the rotor, usually fitted with a nose cone 2, and the rotor is mounted concentrically on a turbine shaft (not shown). The shaft typically drives an electrical generator and is rotatable about a horizontal axis 3.

The blade 1 has an aerofoil cross-section along substantially all its length. This is formed by a front surface 4 confronting the oncoming wind and an opposite rear surface 5 which join on one side at a leading edge 6 and on the other at a trailing edge 7. At the radially outer end of the blade 1, the surfaces 4 and 5 meet a blade tip 8.

Within the body of the blade 1, bounded by the surfaces 4 and 5, a longitudinal passage 9 is disposed which extends longitudinally within the blade 1. This passage connects a perforated outlet spoiler 10 at its radially outer end adjacent the blade tip 8, and a perforated inlet 11 at its radially inner end adjacent the hub. Alternatively an opening may be provided on the axis 3 of the nose cone 2.

The spoiler 10 and inlet 11 are disposed on opposite faces of the blade 1, extending respectively through the front surface 4 and rear surface 5. Also, a second inlet to the passage may be provided by an inlet tube 12 which extends perpendicular to the leading edge 6, both externally and internally of the blade 1. Finally, to complete the construction of the brake means associated with the blade 1, a control valve 13 is disposed in the passage 9 intermediate the spoiler 10 and inlet 11. Alternatively, the valve 13 could be situated in the nose cone 2. This valve 13 when open allows air to flow along the passage 9, and when closed prevents such flow.

In operation the wind turbine may be turned about a vertical swivel axis so that the blades are facing 'into' the oncoming wind. Typically, this is facilitated by means of a fixed rear vane. The generally attached flow of air over the aerofoil cross-section of the blade 1 results in a pressure differential between a low pressure region on the confronting surface 4 (by virtue of the higher wind speed thereover) and a high pressure region on the opposite rear surface 5. It will be understood that the linear speed of the blade 1 increased radially outwardly from the hub 2 for any given rotational speed, and thus the pressure differential across the aerofoil will also vary along the blade length.The resultant 'aerodynamic lift' has a component tangential to the hub axis 3 applying an 'aerodynamic' or input torque thereto, which opposes the aerodynamic drag on the blade 1 and produces rotation of the hub 2 in the direction A.

Clearly the output of the turbine rotor is a function of the aerodynamic torque on the blade 1 less the aerodynamic drag thereon, thus to maximise the output it is desirable to minimise the drag.

Conventionally aerodynamic brake means function by changing the effective attitude of the blades to the wind direction to reduce the aerodynamic torque thereon, consequently the turbine speed reduces. In the illustrated embodiment of the invention the valve 13 is operative not only to reduce the aerodynamic torque but also to increase the aerodynamic drag on the blade 1.

With the valve 13 closed the blade 1 is free to rotate at a speed determined by the incident wind speed and the aerodynamics of its external surfaces. Desirably the spoiler 10 and the inlet 11 are shaped so that with the valve 13 closed, the airflow thereover is attached. At high rotational speeds of the turbine, the valve 13 is opened and air enters the radially inner end of the passage 9 by way of the tube 12 (when provided) acting as a 'ram' and through the inlet 11. Air is drawn along the passage 9 and exhausted through the spoiler 10 under the influence of centripetal acceleration and by virtue of the pressure differential between the high pressure region adjacent the inlet 11 and the low pressure region adjacent the spoiler 10.

Such movement of air through the passage 9 acts to brake the rotation of the turbine blades by decreasing the aerodynamic torque, increasing the aerodynamic drag and reducing the usable output of the turbine. The aerodynamic torque is decreased because the flow of air into the inlet 11 and out of the spoiler 10 causes a reduction in the pressure differential across the aerofoil at those regions.

Therefore, the lift applied to the blade 1 and consequent torque applied to the shaft is also reduced. The passage of air through the inlet 11 and spoiler 10 also breaks up the attached flow thereabouts and greatly increases the turbulence adjacent the blade tip 8. The resultant turbulence effects a consequent increase in the drag on the blade 1. The final braking effect is due to the centripetal acceleration (or centrifugal pumping) of air along the passage 9 by the walls thereof, which reduces the usable turbine output.

It will be apparent that the braking effect due to the passage 9 is variable and determined by the volume of air passing therethrough and the wind speed over the blade 1.

These factors may be controlled by selective operation and restriction of the passage 9 by the valve 13 to provide variable braking of the blade 1 at any given speed.

Typically, the valve 13 is normally closed and controlled to open at a pre-determined rotor speed, although the valve 13 may alternatively be normally open and held closed until such speed is reached to provide a fail-safe system.

It will be appreciated that the type of valve 13 employed, its positioning and its manner of actuation or operation is only limited by the requirement that the passage 9 is disposed within outer surfaces of the blade 1 and that the valve controls the airflow therethrough. Also, the blade 1 may have any suitable aerofoil form and be manufactured in any suitable manner so long as an air passage is provided therealong.

Claims

1. A blade of a rotor of a turbine or propeller having brake means comprising an open-ended passage disposed within the blade to interconnect a first opening and a second opening in the blade, and further comprising valve means for controlling fluid flow through the passage, in which a pressure differential between the first and the second openings as a result of rotation of the rotor can effect said fluid flow.

2. A blade according to claim 1 wherein the first opening is a radially inner opening and the second opening is a radially outer opening in the blade.

3. A blade according to claim 1 or claim 2 wherein the blade has an aerofoil cross-section and the first opening is disposed in the high pressure side of the aerofoil and the second opening is disposed on the low pressure side of the aerofoil.

4. A blade according to claim 1 or claim 2 wherein the rotor has a nose cone and the first opening is disposed therein.

5. A blade according to claim 1 or claim 2 wherein the first opening is disposed in a leading edge of the blade.

6. A blade according to any preceding claim wherein at least the major portion of the passage is co-axial with the longitudinal axis of the blade.

7. A blade according to any preceding claim wherein the valve means comprise a control valve.

8. A blade according to claim 7 wherein the control valve is normally closed and is controlled to open at a pre-determined rotor speed.

9. A blade according to claim 7 wherein the control valve is normally open and is held closed until a pre-determined rotor speed is reached.

10. A rotor of a turbine or propeller including a plurality of blades according to any preceding claim.

11. A blade substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.