Fluid reaction surfaces (i.e. – impellers) – Specific blade structure – Concave surface
Reexamination Certificate
2002-05-08
2004-03-16
Nguyen, Ninh H. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Specific blade structure
Concave surface
C416SDIG002, C416SDIG005
Reexamination Certificate
active
06705838
ABSTRACT:
The present invention relates in a broad aspect to a method for design and modification of airfoils useful for wind turbine applications, which airfoils possess smooth and stable characteristics in stall. These characteristics comprise: (1) No or very little tendency to double stall, (2) Insensitivity or little sensitivity of maximum lift to leading edge roughness, (3) High lift-drag ratio just before maximumlift, (4) Small variations of the aerodynamic loads in stall and (5) Sufficient aerodynamic damping to suppress blade vibrations in stall. The invention further relates to blades and/or airfoil sections in general which posses smooth and stabile characteristics in stall. Also, it relates to a method of implementing the desired shape on an airfoil or a wind turbine blade.
BACKGROUND FOR THE INVENTION AND INTRODUCTION TO THE INVENTION
An increasing problem at least within the wind turbine industry is poor power quality, high fatigue loads and unreliability of power and loads for wind turbines operating at high wind speeds where the airfoil sections or blades are just before or in stall.
The airfoil sections on the blades operate at angles of attack ranging from low angles of attack, where the lift is low on the linear part of the lift versus angle of attack curve, to high angles of attack where the airfoil section is just before stall or in stall. This is in contrast to other traditional aviation applications such as aeroplanes and helicopters. Especially operation in stall is unique for some wind turbines, which use stall as a way of controlling the peak power which is also called the rated power.
The following parameters influence the operation in stall:
DOUBLE STALL
Double stall denotes the situation wherein the flow past an airfoil section or blade having an angle of attack relative to the free stream flow shows at least two levels of lift and drag to the same angle of attack.
The problem becomes even larger when a wind turbine is operating in stall regulated mode, i.e. the power of the turbine is limited by stall on the blades and the lift and drag produced by the blade or airfoil section may jump between at least two levels showing at least two levels in generated power and/or thrust on the rotor. When the power jumps from on level to another the aerodynamic forces acting on the blade or airfoil section dramatically changes. This may cause uncertainties of the power level and thereby uncertainties in the prediction of the energy production. Furthermore, large vibrations induced in the turbine may be the result which at the end may cause break down of the turbine.
LEADING EDGE ROUGHNESS
Leading edge roughness around maximum lift both at stall and in stall can appear when the leading edge part of the airfoil section or blade is accumulating bugs, dust, ice or other kind of material changing the roughness of the existing airfoil section or blade and the effective flow pattern around the airfoil section or blade.
Leading edge roughness around maximum lift both just before stall and in stall is often observed on wind turbines. It causes uncertainties of the power level and thereby uncertainties in the prediction of the energy production. Furthermore, the loads on the structure will change, which might result in undesirable structural dynamics.
LIFT-DRAG RATIO
The lift-drag ratio is a measure of the efficiency of the airfoil section or blade. The higher ratio the better efficiency. Thus, the lift-drag ratio can be improved by increasing the lift and/or decreasing the drag.
It is desirable that the lift-drag ratio just before stall is high. This is because maximum power of the wind turbine this way will be obtained at lower wind speeds and the annual energy production will increase.
VARIATION OF AERODYNAMIC LOADS
The variation of the aerodynamic loads can be measured as the standard deviation of the aerodynamic lift and drag. A high standard deviation of lift and drag means that the variation of the aerodynamic loads is high. Especially in stall the standard deviation can be high, indicating that the flow is not smooth and stable.
When operating in stall it is important that the variation of the aerodynamic loads is as small as possible. This is because the structure will respond on the aerodynamic loads. Thus, large variations in the loads will cause vibrations in the structure resulting in higher fatigue loads and more noise. Furthermore, the quality of the produced power may become poorer with larger variation in the aerodynamic loads. Very big variations in the loads are observed when double stall appears.
AERODYNAMIC DAMPING
The total damping of a wind turbine blade is the sum of the structural damping and the aerodynamic damping. Aerodynamic damping is a measure for how well the blade structure is damped when influenced by aerodynamic loads. Especially when operating under stalled conditions there is a risk for negative total damping. If the positive structural damping is less than the negative aerodynamic damping severe vibrations of the structure will appear possibly resulting in break down of the structure.
For wind turbines operating in high wind speeds vibrations of the blades both in the rotor plane and out of the rotor plane can occur. This is a very undesirable situation and can be avoided by changing the stall characteristics of the airfoils used for the blades.
Therefore a technical problem in connection with the above mentioned parameters influencing stall is to provide airfoil sections or blades wherein the risk of presence of double stall, significant influence of leading edge roughness, low lift-drag ratio just before stall and in stall, big variations of the aerodynamic loads and/or negative aerodynamic damping is minimised and in some cases avoided.
As many turbines today are utilising airfoil sections or blades producing double stall, big influence of leading edge roughness, low lift-drag ratio just before stall and in stall, big variations of the aerodynamic loads and/or negative aerodynamic damping a further problem is to modify these turbines, that is modify the blades of existing wind turbines or modify the existing moulds for airfoils and/or blades.
BRIEF DESCRIPTION OF THE INVENTION
These problems have been solved by means of the present invention, which provides two- or three-dimensional cross sectional airfoil data, such as airfoil sections or blades, useful for aerodynamic applications such as for a wind turbine, wherein the airfoil section contour or parts thereof or wherein the contour of the blade or parts thereof have been modified to avoid one or more of the problems listed below:
Generation of double stall, that is the flow past the airfoil section or blade shows at least two different lift and/or drag levels to the same angle of attack. According to several aspects of the present invention, aerodynamic airfoil sections or blades are designed which when exposed to a stream of fluid they do not have the tendency of generating a burst of a leading edge separation bubble as such a leading edge separation may be the source of double stall phenomena.
Sensitivity to leading edge roughness, that is where the airfoil shows a decrease in the lift and an increase in the drag just before stall and in stalled conditions when bugs, dust, ice or other pollutants accumulate at the leading edge part of the airfoil. According to several aspects of the present invention, aerodynamic airfoil sections or blades are designed which when exposed to a stream of fluid they show less sensitivity to leading edge roughness in stalled conditions.
Low lift-drag ratio at angles of attacks just before maximum lift, that is where the airfoil section or blade just before maximum lift shows a certain efficiency. According to several aspects of the present invention, aerodynamic airfoil sections or blades are designed which when exposed to a stream of fluid they show an increased lift-drag ratio at angles of attacks from just before maximum stall and until lift and thereby an increased efficiency in this angle of attack interval.
Big variations in the aerodynamic loads, th
Bak Dan Christian
Fuglsang Peter
Forskningscenter Riso
Harness & Dickey & Pierce P.L.C.
Nguyen Ninh H.
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