FIELD OF THE INVENTION

The present invention relates to a wind turbine blade and a method of operating a wind turbine.

BACKGROUND TO THE INVENTION

The rotor of a modern wind turbine produces a high tip speed for the blades of the rotor. To achieve high efficiency the aerofoil profiles at various sections along the blades should be selected to achieve a high lift to drag ratio in order to produce a high power coefficient for a rotor operating at relatively high tip speeds. A high power coefficient improves the annual energy production of the wind turbine.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a wind turbine blade comprising a leading edge, a trailing edge, a pressure surface extending between the leading edge and the trailing edge, a suction surface extending between the leading edge and the trailing edge, and an aerofoil section bounded by the pressure surface and the suction surface, wherein the aerofoil section has at least one rearward facing step between the leading edge and the trailing edge.

The rearward facing step has a face, an outer edge and an inner edge and defines a cavity behind the step. The step may be considered as a cut back, or surface discontinuity, as compared with an aerofoil having continuous pressure and suction surfaces. The outer edge is where the step face meets with the pressure or suction surface, whichever of those surfaces has the cut back.

The invention is advantageous in that the rearward facing step promotes rotation of the flow off the outer edge of the step. The resultant vortex may then become trapped in the cavity behind the step as the separated flow caused by the step reattaches itself downstream of the step. The step may cause an increase in pressure in the flow field adjacent the trapped vortex due to flow attachment, or re-attachment, due to the increased energy of the vortex. When the step is provided on the pressure surface of the aerofoil, the increase in pressure adjacent the vortex may contribute to an increase in lift. When the step is provided on the suction surface of the aerofoil, the flow re attachment may contribute to an increase in lift. In both cases the step may cause a small increase in drag, but the net effect may be an increase in the lift to drag ratio of the aerofoil.

A second aspect of the invention provides a method of operating a wind turbine comprising a plurality of blades, each blade having a leading edge, a trailing edge, a pressure surface extending between the leading edge and the trailing edge, a suction surface extending between the leading edge and the trailing edge, and an aerofoil section bounded by the pressure surface and the suction surface, wherein the aerofoil section has at least one rearward facing step, and wherein the at least one rearward facing step has one or more apertures, the method comprising ejecting air from the one or more apertures into a cavity immediately behind the step.

In addition to the above mentioned advantages of the rearward facing step, ejecting air from the aperture into the cavity behind the step may further advantageously affect the flow field around the aerofoil. The ejected air may energize the flow, strengthening the vortex in the cavity. The strengthened vortex may promote flow attachment, or re attachment of separated flow, reducing drag. Reducing drag in this way may increase the lift to drag ratio of the aerofoil.

The at least one rearward facing step may be provided on the pressure surface.

The at least one rearward facing step may be provided on the suction surface.

A plurality of rearward facing steps, at least one on the pressure and at least one on the suction surface, may be provided on the same blade at different spanwise locations.

The blade may have a root end and a tip end opposite the root end in a spanwise direction of the blade, and the at least one rearward facing step may extend in the spanwise direction.

The rearward facing step may extend full span or part span in the spanwise direction. The aerofoil section may have a thickness and a chordwise direction extending between the leading edge and the trailing edge, and the rearward facing step may have a height at least 10% of the aerofoil section thickness immediately forward of the step in the chordwise direction.

The rearward facing step may have sharp edges, e.g. at the inner edge and/or the outer edge. The aerofoil section may be generally in the class known as a 'Kline-Fogleman' or KF aerofoil.

The rearward facing step may have one or more apertures for ejecting air into a cavity immediately behind the step.

The blade may further comprise one or more conduits for conveying air through the blade and to exit via the one or more apertures.

The one or more conduits may extend generally spanwise along the blade.

The blade may have a root end and a tip end, and the blade may further comprise one or more inlets adjacent the root end at a location remote from the one or more apertures. The method may further comprise ingesting air through the one or more inlets and ejecting the ingested air through the one or more apertures.

The inlets at the root end may be located just downstream of the expected flow separation line. Ingesting the separated air flow through the inlets may reduce the amount of separated air flow at the root end and may reduce the amount of turbulent flow that can feed into the spanwise flow near the trailing edge of the blade outboard of the root end, so reducing drag and increasing the lift to drag ratio of the blade. The one or more inlets may be in the pressure surface of the blade adjacent the root end, and/or in the suction surface of the blade adjacent the root end, and/or in a hub connection interface adjacent the root end for conveying air into the blade from a hub. The one or more inlets in either the pressure surface or the suction surface of the blade may be at a location remote from the one or more apertures.

The method may further comprise ingesting air through the one or more inlets and ejecting the ingested air through the one or more apertures.

The blade may further comprise an aperture controller for actively controlling one or more of the apertures to vary the flow rate or direction of the air ejected into the cavity.

A further aspect of the invention provides a wind turbine comprising a plurality of blades according to the first aspect.

The wind turbine may further comprise a hub to which the plurality of blades are attached. The hub may have one or more inlets.

The method may further comprise ingesting air through the one or more hub inlets and ejecting the ingested air through the one or more apertures.

The method may further comprise passively conveying air from the inlet(s) to the aperture(s) by centrifugal force generated from rotation of the blades.

The method may further comprise actively controlling one or more of the apertures to vary the flow rate or direction of the air ejected into the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates a wind turbine having blades mounted to a hub;

Figure 2 illustrates a perspective view of one of the blades;

Figure 3 illustrates a section view along A-A in Figure 2;

Figure 4 illustrates a schematic plan view of the blade in Figure 2;

Figure 5 illustrates a portion of a blade having apertures in the rearward facing step; Figure 6 illustrates schematically the air flow through the apertures and over the blade potion;

Figure 7 illustrates inlets in the root end of the blade;

Figure 8 illustrates schematically the air flow through the inlets and around the root end of the blade;

Figure 9 illustrates a schematic plan view of a blade having inlets at the root end and apertures in the rearward facing step; and

Figures 10 to 13 illustrate a variety of stepped aerofoil sections for the blade.

DETAILED DESCRIPTION OF EMBODIMENT(S)

In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness and chord are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction.

A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when in use, has a higher pressure than a suction surface of the blade.

A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when in use. The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

Figure 1 shows a wind turbine 1 including a nacelle 2 supported on a tower 3 that is mounted on a foundation 4. The wind turbine 1 depicted here is an onshore wind turbine such that the foundation 4 is embedded in the ground, but the wind turbine 1 could be an offshore installation in which case the foundation 4 would be provided by a suitable marine platform, such as a monopile or jacket.

The nacelle 2 supports a rotor 5 comprising a hub 6 to which three blades 7 are attached. It will be noted that the wind turbine 1 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 5 is mounted at the nacelle 2 to rotate about a substantially horizontal axis defined at the centre at the hub 6. As is known, the blades 7 are acted on by the wind which causes the rotor 5 to rotate about its axis thereby operating generating equipment through a gearbox (not shown) that is housed in the nacelle 2. The generating equipment is not shown in figure 1 since it is not central to the examples of the invention.

Each of the blades 7 has a root end 8 proximal to the hub 6 and a tip end 9 distal from the hub 6. A leading edge 10 and a trailing edge 1 1 extend between the root end 8 and tip end 9, and each of the blades 7 has a respective aerodynamic high pressure surface (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) surface extending between the leading 10 and trailing edges 1 1 of the blade 7.

Figure 2 shows a perspective view of one of the blades 7, and figure 3 shows a section view along A-A in figure 2 in an outboard region of the blade 7 towards the tip end 9.

As shown in figure 3 the blade 7 has an aerofoil section bounded by a pressure surface 12 and a suction surface 13. The suction surface 13 is generally continuous and extends from the leading edge 10 to the trailing edge 1 1 with a smooth curvature. The pressure surface 12 extends generally rearwardly from the leading edge 10 to a surface discontinuity or step 14. The step 14 has a face 15, an outer edge 16 and an inner edge 17. The outer edge 16 is where the face 15 meets the pressure surface 12. The inner edge 17 is where the face 15 meets a base 18, or floor, of a cavity 19 formed behind the face 15 of the step 14. The base 18 extends rearwardly towards the trailing 1 1 of the aerofoil section. The step 14 is located between the leading edge 10 and the trailing edge 1 1. The step 14 is thus a discontinuity in the pressure surface 12 and is located some distance forward of the trailing edge 1 1.

The outer edge 16 and the inner edge 17 are relatively sharp corners in the example illustrated in figure 3. The outer edge 16 is a convex corner and the inner edge 17 is a concave corner. A small radius at the outer edge 16 promotes flow separation and the formation of a vortex within the cavity 19. A small radius at the inner edge 17 may further promote the creation of a smaller secondary vortex adjacent to the inner edge 17. In alternative examples the inner edge 17 may have a larger radius. Depending on manufacturing constraints the outer edge 16 may be provided by a separate component carrying the outer edge of small radius which is attached to a main load bearing structure of the blade 7, where the main load bearing structure has a larger radius at the attachment region.

The step 14 may have a height at least 10% of the aerofoil section thickness immediately forward of the step 14 in the chordwise direction. In the illustrated example the step 14 has a height approximately 50% of the aerofoil section thickness immediately forward of the step in the chordwise direction. It will be appreciated that the height of the step may be varied and tailored to optimise the aerodynamic objective of the aerofoil section.

Figure 4 illustrates a plan view of the blade 7 and showing in broken line the projection of the step 14 cut back in the pressure surface 12 of the blade. As can be seen in figure 4, the step 14 extends from a mid-span region towards the tip end 9 of the blade 7. The step 14 may extend part-span or full-span in the spanwise direction. The face 15 of the step 14 is generally planar extending substantially linearly when viewed in plan view as shown in figure 4. However in alternative examples the face 15 may be curved in either the thickness or spanwise directions. In operation of the wind turbine 1 having the blades 7 with the step 14 as described above, the step 14 promotes rotation of the flow off the outer edge 16 of the step 14. The resultant vortex may then become trapped in the cavity 19 behind the step 14 as the separated flow caused by the step reattaches itself to the base 18 of the cavity 19 downstream of the step. The step may cause an increase in pressure in the flow field adjacent to the trapped vortex due to flow reattachment and the increased energy of the vortex. Figure 5 shows another example of a wind turbine blade 7 having a step 14 in the pressure surface 12 similar to the blade shown and described previously. In the portion of the blade 7 shown in figure 5 the face 15 of the step 14 includes a plurality of apertures 20. The apertures 20 are spaced in the spanwise direction across the face

15 of the step 14. The apertures 20 may be provided along only a portion of the face 15 or may be provided along the entire face 15. The apertures 20 may be regularly spaced or irregularly spaced. The apertures 20 may be provided as slits or slots or may take any other suitable form.

Figure 6 illustrates the portion of the blade 7 shown in figure 5 and graphically illustrating the effect of ejecting air through the apertures 20. Ejecting air through the apertures 20 into the cavity 19 energises the vortex which rolls up off the outer edge

16 of the step 14. The energised flow in the vortex encourages flow over the pressure surface 12 to remain attached in the downstream direction towards the trailing edge 1 1 .

Figure 7 shows a plurality of inlets 21 provided in the suction surface 13 of the blade 7 adjacent to the root end 8. As shown in figure 7 the blade 7 has a generally circular cross section at the root end 8. The cross section of the blade transitions from a circular section to an aerofoil section in the outboard spanwise direction of the blade. A shoulder 22 of the blade is located at the spanwise location of maximum chord length. The inlets 21 are located between the root end 8 and the shoulder 22. Aerodynamic bodies of circular cross section inevitably cause separated flow. In the transition region of the blade, between the circular root end section and the shoulder 22, management of the separated/separating flow can reduce drag. The air inlets 21 near the root end 8 can ingest separated or separating flow back through the suction surface 13, thus reducing flow separation at the root end and reducing the amount of turbulent flow that can feed into the spanwise flow near the trailing edge 1 1 . By reducing the amount of turbulent flow that can feed spanwise along the blade by the Coriolis effect, the amount of separated flow in the region between the root end 8 and the location near the shoulder 22 can be reduced.

Figure 8 shows graphically the effect on the flow due to ingesting air through the inlets 21 . Advantageously, the inlets 21 may be located just downstream of the expected separation line which tracks along a curved line generally towards the trailing edge 1 1 in the outboard spanwise direction.

The air ingested by the inlets 21 may be used to provide a source of air to be ejected from the apertures 20. In other words, the blown step shown in figures 5 and 6 may be driven by the air ingested by the inlets 21 shown in figures 7 and 8.

Figure 9 shows schematically a system for passively driving the blown stepped aerofoil of figures 5 and 6. As shown in figure 9, the blade 7 has the step 14 with a plurality of apertures 20. The apertures 20 are coupled via a generally spanwise extending conduit 23 which passes through the blade 7 and is fluidly coupled to the inlets 21 near the root end 8. A valve 24 on the conduit 23 may be opened and closed and used to selectively allow a flow of air through the conduit 23. The system shown in figure 9 takes advantage of the centrifugal force generated as the blade 7 rotates during operation of the turbine 1 . The centrifugal force forces the air to move radially outwardly along the blade 7 in the outboard spanwise direction through the conduit 23 when the valve 24 is open. In this way, air is ingested through the inlets 21 , passes along the conduit 23 and is ejected from the apertures 20. The movement of the air through the conduit 23 is purely passively driven by rotation of the blade 7 about the horizontal axis of the rotor 5 of the wind turbine 1 . A controller (not shown) may be used to control the opening and closing of the valve 24 so that the ejection of air from the apertures 20 and intake of air through the inlets 21 may be selectively controlled.

Of course, the source of air for ejection from the apertures 20 may be derived from sources other than inlets near the blade root end 8. For example, inlets may be provided in the hub 6 for ingestion of air which may then pass through conduits into the blades 7. In another example the flow of air through the conduit 23 may not be purely passive and active means for driving the air flow, such as an electrically driven pump may be used.

Figures 10-13 illustrate various alternative aerofoil sections for the blade 7. In each of the illustrated examples 1 or more steps 14 is provided in either the pressure surface 12 or the suction surface 13 of the blade 7. The steps 14 may be a blown step similar to that illustrated with reference to figures 5 and 6 for which the source of air may be as described above. Alternatively the steps 14 may have no apertures.

In figure 10 a plurality of steps 14 are provided in the pressure surface 12. In figure 1 1 a step 14 is provided in the suction surface 13. In figure 12 a plurality of steps 14 are provided in the suction surface 13. In figure 13 a step 14 is provided in the pressure surface and a step 14 is provided in the suction surface 13. Where a plurality of steps are provided the steps may number 2, 3, 4 or more. Where steps are provided in the pressure surface 12 and the suction surface 13 a plurality of steps may be provided in either one or both of the pressure surface 12 and the suction surface 13.

Any combination of the aerofoils sections previously described may be used in combination on a single blade 7. For example, a step 14 may be provided in the pressure surface 12 in a tip region of the blade, and a step 14 may be provided in the suction surface 12 in a mid-span or root region of the same blade 7. Broadly, the tip region is nearest the tip end 9, the root region is nearest the root end 8 and the mid span region is between the tip region and the root region.

Although in the illustrated examples the apertures 20 are fixed, passive apertures, it will be appreciated that the apertures 20 could be replaced by active apertures. The active apertures may be used to vary the flow rate or direction of the air ejected into the cavity 19. An aperture controller connected to and associated with the active aperture may be used for controlling the aperture size and direction.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.