In the figure below, the blades are listed in eight positions from 0 degrees to 315 degrees. The wind enters from the left. The light blue vector v is the wind speed, and the green vector u is the linear velocity inverse of the blade's circular motion. direction (that is, the airflow speed felt by the blade when there is no wind), the blue vector w is the resultant airflow speed felt by the blade (that is, the relative wind speed), and the purple vector L is the lift force experienced by the blade.
Let’s analyze the stress on the blades at these eight angles. At the positions of 90 degrees and 270 degrees, the relative wind speed does not produce lift. At the other six positions, the lift received by the blades can all move. The direction produces a torque force, which is why the Darieu wind turbine can rotate under the force of the wind.
In fact, the situation is much more complicated. The previous analysis diagram is an ideal state, which is the state when the ideal tip speed ratio and no resistance from the blades are present. The torque force used by the blades to push the rotor to rotate is the component of the combined force of lift and drag in the forward direction of the blades. Let’s take the situation at 315 degrees to analyze the situation with resistance. The black vector D in the figure is the resistance experienced by the blades. The brown vector F is the combined force of lift L and resistance D. The component force M of this force in the forward direction of the blades is M. This is the actual torque force. Obviously, the torque force at this time is significantly less than the ideal condition.
And at angles near 180 degrees and 270 degrees, the combined force of lift and drag produces a reverse torque force.
The Darieux wind turbine has a greater output force only when the blades are near 360 degrees and 180 degrees. Even so, it can only operate when the tip speed ratio is above 3.5. This can be illustrated by the following figure.
In the figure on the left, the blade is affected by the relative wind speed W to generate lift L and drag. D. The angle between the relative wind speed W and the blade chord line, that is, the angle of attack α of the blade, is about 14 degrees. The relative wind speed W is composed of the wind speed V and the blade movement speed u. At this time, the blade movement speed is about 4 times the wind speed, that is The tip speed ratio is 4. The resultant force of lift L and resistance D is F, and the moment force exerted by this force on the wind wheel is M, which is the force that drives the wind wheel to rotate. When the tip speed ratio is 4, the blade can generate a moment to push the rotor when running on the windward or leeward side. Only near the two sides (90 degrees and 180 degrees), the lift force is very small and there will be a small negative impact. directional moment.
The wind speed on the right side of the figure has doubled, but the speed of the blade movement has not changed. The tip speed ratio is about 2, and the angle of attack α of the blade is about 27 degrees. At this time, the blade is working in a stall state. , the lift L generated by the blades decreases significantly, but the resistance D increases greatly. The moment force M generated by the wind wheel is negative, which prevents the rotation of the wind wheel. At this time, the blades generate negative moments in most positions. For most common airfoils, when the tip speed ratio is less than 3.5, the blades basically generate no force to push the wind wheel to rotate.
It is difficult for the Darrieux wind turbine to operate under low wind speeds. Only under higher wind speeds can the wind turbine rotate at a tip speed ratio of 3.5 or above to operate normally
, higher power output can be obtained when the tip speed ratio is 4-6. In order to reduce drag and increase lift, the cross-sectional shape of the wind turbine blades isShape (airfoil) selection and surface finish requirements are relatively high. Since the Darieu wind turbine cannot rely on lift when the tip speed ratio is below 3.5, can it operate on drag? Since each fin is evenly fixed on the circumference of the wind wheel, the resistance moment generated by each fin due to the wind is not large and the total moment combined by each fin is very small. Even if a certain moment can be generated at a certain angle, it may not be generated at another angle. Reverse torque is generated, so the Darieu wind turbine cannot start automatically by wind power alone. It must be started by external force to make the blade tip speed ratio reach more than 3.5 before it can operate with lift. The typical Darieu wind turbine wing is not straight, but curved, and the two wings form a φ shape. The picture below shows a Darieux wind turbine model.
Today’s Darieu wind turbines mostly use straight blades, which some people call H-shaped wind turbines. The number of blades of H-type wind turbines is generally 2 to 6.
The blades of Darieu wind turbines are fixed on the rotating shaft through both ends or the middle, which is beneficial to increasing the mechanical strength and can be made very lightweight; The Darieux wind turbine is not top-heavy, has lower requirements on the tower, is suitable for fixing with cables, is easy to install, and is convenient for maintenance. These are its advantages. Regarding the problem that the Darieu wind turbine cannot start automatically, the general method is to use a generator as a motor to drive the wind turbine to rotate during starting, so that the blade tip speed ratio reaches more than 3.5. Due to the strict requirements on wind speed changes and load changes, it is difficult to operate smoothly and efficiently, and due to the disadvantages of not being able to start automatically, the development of Darieu wind turbines was slow. It was not until recent years that after technical improvements, it began to develop significantly. .
To analyze the aerodynamic performance of the wind wheel, it is necessary to understand the flow field at the wind wheel in order to analyze the generated aerodynamic force, torque and power. For this purpose, an aerodynamic model of the lift-type wind wheel must be established.
What are the differences in the performance of wind turbines operating under constant tip speed ratio and constant speed conditions?
Blade tip speed ratio (λ) = blade tip speed/ Wind speed = blade rotation number (ω) * rotation radius (R) / wind speed at the blade root (V)
Cp = actual wind energy obtained / theoretical maximum wind energy = P / (1/2 * ρ * A * V^3), ρ is the air density, A is the blade sweep area, and V is the wind speed.
Answer: Tip speed ratio λ = RΩ/?, where Ω is the angular speed, ? is the wind speed, and R is the radius of the fan. When the wind turbine operates at a constant blade tip speed ratio, that is, λ is always constant. When the wind speed of the wind turbine changes, the speed of the wind turbine needs to change accordingly to ensure that the ratio between the two is constant. This is generally applicable when the wind speed is less than the rated wind speed.
When the wind turbine runs at a constant speed, no matter how the wind speed changes, the speed of the wind turbine must remain constant until it reaches the rated power. When the wind speed is greater than the rated wind speed, the wind rotor is limited to absorb wind energy from the wind through active stall or pitch adjustment, so that the generator maintains constant output power.