Job Sheet 1 Blade Aerodynamics

Transcription

1 Job Sheet 1 Blade Aerodynamics The rotor is the most important part of a wind turbine. It is through the rotor that the energy of the wind is converted into mechanical energy, which turns the main shaft and subsequent electrical generator. The rotor is made up of two basic components: the blades, which catch the wind, and the hub, which secures the blades to the main shaft and encloses the blade wind turbines. Some alternatives to the three-blade rotor designs are the two- and single- blades, including additional blade assembly time and maintenance, an increase in drag, and an increase in the complexity of rotor lifting at the job site during wind turbine assembly or to lift into place, the blades must be larger to capture the same amount of energy (Figure 1-1). The longer blade length increases the complexity of blade construction and transportation to of a failure. Blade Basics To begin this discussion, a few questions must be answered regarding the shape of the rotor blade, and the forces that act on the blades and cause them to rotate. These questions relate to the science of aerodynamics. Aerodynamics is the study of the motion of air, particularly when it interacts with a moving object. trailing edges, and the short blade edges, which are made up of the blade tip at the end farthest from the hub and the blade root, which bolts the blade to the hub. Festo Didactic Inc

2 Blade Root Trailing Edge Hub T L Blade Cross Section Leading Edge Blade Tip Figure 1-2. Blade Description. affect the blade s performance at different wind speeds. Wind turbine manufacturers have Another name for a turbine blade is an airfoil. Airfoils produce lift or suction by forcing the of the air along this path thins the air and produces a lower air pressure than on the concave or pressure side. The imbalance of air pressure means air wants to move from the high pressure side to the low pressure side, which produces suction to move the air, or a resulting lifting force on the airfoil (Figure 1-3). Air Flow Lift Airfoil Lift Air Flow Figure 1-3. Airfoil Air Flow Producing Lift. the hub if the blade is positioned properly. 2 Festo Didactic Inc

3 Blade Pressure and Force can be calculated. Figure 1-4. Force Acting on a Blade. about 9 miles per hour (mph). The wind-exposed blade area is approximately 16 square meters (m 2 ), which is roughly 172 square feet (ft 2 ). 2 ) is: ) NOTE: 3. obtained by dividing the number of meters in 1 mile, 1609, by the number of seconds in 1 hour, One square meter = square foot. Multiply the square foot value by to convert the value into square meters. Example: x 172 ft 2 = m 2 Festo Didactic Inc

4 Sphere = 0.47 Half-sphere = 0.42 Cone = 0.5 Corner of a cube = 0.8 Long cylinder = 0.82 Short cylinder = 1.15 Streamlined body = 0.04 Streamlined half-body = 0.09 is assumed. We can now plug the values into the equation and calculate an answer: Calculate the total force (F) acting on the blade: Force = Pressure x Area The blade area is 16 m 2 2 x16 m 2 Until now, we assumed the blade is at rest and the wind is blowing at a right angle to the direction of blade rotation. How does this change if the blade is moving? Direction of Rotation When a blade is rotating (Figure 1-5), it encounters a head wind (U). The strength of the head wind is dependent on two things: the speed of rotation and the distance from the center of the 4 Festo Didactic Inc

5 rpm, a blade length of 19 m (62.3 ft), and a diameter of 44 m (144.4 ft). The natural wind speed Which is the rotational speed in revolutions per minute times the swept circumference, divided by 60 seconds. The circumference of a circle is pi times the diameter or C = x D = Therefore: As the blade turns the Resulting wind (W r C 2 = (A 2 + B 2 ) C = square root of (A 2 + B 2 ) W r 2 + U 2 ) 0.5 W r = ( ) 0.5 This resulting wind (W r wind. It is split into a component in the direction of rotation (Fd) and a second component at against the tower, while the force (Fd) points in the direction of rotation and provides the driving torque to the main shaft. the blade as a result of its own movement is of great importance to the wind turbine blade, and its structural design must cope with the large forces and stress of operation. It is of great importance that the force on the blade is almost at a right angle to the resulting While a race car might feel wind resistance as a burden that must be overcome with extra Festo Didactic Inc

6 engine power and fuel, a wind turbine blade uses this extra wind resistance to its advantage. experiences both lift and drag. While high blade velocity near the blade tip can result in large increases in wind and drag, the blade performance near the root of the hub is less affected by these conditions. The head resulting wind has a greater angle in relation to the plane of rotation. Therefore, the blade angle Blade Pitch and Angle of Attack between the apparent wind direction (current wind direction adjusted for the wind forces created by the rotating blades) and a hypothetical blade centerline running from a blade s leading edge to its trailing edge. The blade pitch is the blade angle from the same hypothetical blade changes in wind direction and the speed of rotation. Pitch Angle Angle of Attack Hub Figure 1-6. Pitch Angle and Angle of Attack. and either stop or control the speed of the turbine. Turbines have a pitch control system to vary 6 Festo Didactic Inc

7 mechanical slewing drive is necessary. This drive precisely angles the blade while withstanding high torque loads. Many modern turbines use hydraulic pitch drive systems. These systems are often spring loaded. If hydraulic power fails, the blades automatically furl or turn into the wind. Some turbines use an electric servomotor for every rotor blade. They have a small battery design, and employ no electric or hydraulic controls. Wind Exposed Surface Force Vectors Figure 1-7. Variations in Pitch Angle. Speed control of the wind turbine can be accomplished using a pitch control system. As the increases (Figure 1-7). The force on a turbine blade is the result of the wind pressure multiplied effect of reducing the lifting force and force angle slowing the turbine. A rapid reduction in angle the blades turns perpendicular to the rotational head wind increases their drag. NOTE: This effect is similar to holding a canoe paddle in the current of a rapidly moving Festo Didactic Inc

8 reduces the rotation force (Fd) to near zero and reduces the wind exposed surface to a the blades during system idol such as during mechanical servicing or during unstable weather can prevent unexpected wind gusts from overspeeding the hub and damaging the turbine. Stall discourse to the direction in which the air molecules move (Figure 1-8). Laminar Turbulent Figure 1-8. Laminar and Turbulent Air Flow. stalling (Figure 1-9). Figure 1-9. Positive Stall. 8 Festo Didactic Inc

9 is disrupted. This can adversely affect the performance and operation of the wind turbine. Some stall effects might include increased drag or vibration, which can damage blades, gears, or bearing systems. aerodynamically designed to ensure that, the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade that does not face the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. A turbine with this design is called a stall-controlled wind turbine. The rotor is designed to ensure that the blade stalls gradually rather than abruptly when the wind speed reaches its critical value. This reduces adverse vibration. Fixed blades are the basic advantage of stall control because they avoid moving parts in the rotor. They also have a complex control system, which saves on construction and maintenance costs. Blade Inspection and Maintenance Modern day wind turbines have blades that can reach lengths of 45 meters (148 ft) and beyond. Although blades are designed to last 20 years, many manufacturers only offer 2-year warranties. Blades are most often made from composites of epoxy resins, foam or balsa, and therefore, it is important that turbine owners and operators ensure blade quality. Failure to inspections are often done by independent services, post-warranty inspection and maintenance is often the job of the operator. into three main categories: on-site arrival inspection, installation inspection, and routine maintenance inspections. On-site arrival inspections start with an exterior inspection to determine if any transportation damage has occurred. It is best to have blade repair personnel on site at this stage to rapidly assess damages and repair blades to minimize construction delays and costs. If multiple wind turbines are being assembled, blades may also require proper matching to their balanced sets to ensure good performance and reduced operational vibration. A sample internal inspection of several blades should be performed to compare the construction process and ensure that their more economical, less time-consuming, simpler, and safer on the ground. All inspections and repairs should be documented with reports and photos, and archived for the life of the rotor. Festo Didactic Inc

10 Installation inspections begin with a review of the lifting equipment and installation procedure. hardware. Two additional inspections must then be performed: dynamic balance and aerodynamic alignment, which focuses on cone angle, partition angle, blade contour and twist, and pitch angel. Figure Cone Angle. Cone angle refers to the angle of the blades in reference to the plane of rotation (Figure 1-10). angle can be determined by setting the tilt angle, or angle of rotation relative to the tower, to optical equipment to determine misalignments (Figure 1-11). Figure Optical Blade Measurement. Partition angle refers to the space of blade angles on the rotor hub. The blade contour and twist refers to the shape of the blade and the changes in blade pitch along its length. Pitch angle refers to the angle between the blade chordline (Figure 1-12) and the rotor s plane of rotation. 10 Festo Didactic Inc

11 Leading Edge Plane of Rotation Mean Camber Line Pitch Angle Camber Chordline Trailing Edge P Figure Blade Chordline and Pitch Angle. The last element of a proper wind turbine blade inspection and maintenance program are the routine or maintenance inspections. Because the rotor blades must endure harsh weather conditions, constant vibration, and wind and rotational-induced stress, it is important that a period inspection schedule be developed to ensure a long operational life. A general buildup compromise the integrity and performance of the blades. Examples of common maintenance procedures include: Inspect root-securing bolts for corrosion. Bolts are re-tightened to proper torque and ice. Cosmetic repairs can also be completed. damage are performed. Leading edge tape can be applied to protect and prolong the life of the blades by reducing edge erosion. Festo Didactic Inc

12 More sophisticated, non-destructive testing methods are also utilized. This include tests such as ultrasonic, tap, or infrared thermography. Automated testing methods have an advantage on visual inspections because they identify the suspected damaged area and provide measured data points. This is useful when determining and maintaining a permanent record of any damage. An ultrasonic inspection uses a sound source and receiver to graphically display a sound echo non-destructive composite inspection method in the industry. Full Tk Blade Wall Full Tk Full Tk Void Figure Ultrasonic Tester. The advantage of ultrasound scanning versus a general visual inspection is that it enables lamination. A tap test can sometimes be used to verify the results of an ultrasonic test, and it is another good method of discovering irregularities in the blade structure. The method is based on the and the main spar. There are three types of tap testing equipment: a manual tapping hammer, 12 Festo Didactic Inc

13 The last method mentioned is Infrared thermography. Large blades made of joined sections have adhesive joints that are critical points in the blade structure, and therefore must be inspected with care. Infrared (IR) scanners are used to examine a blade throughout its image, any point can be highlighted and analyzed later using electronic image processing. If Festo Didactic Inc

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15 OBJECTIVE In this job, you will become familiar with the basic principles and terminology of blade aerodynamics by performing calculations associated with blade pressure and force. PROCEDURE Use the information provided in the Information Job Sheet to respond to these questions. Assuming a wind turbine is at sea level and the turbine blades have a What is the circumference of a turbine with a total diameter of 17 meters? Round your answer to two decimal places. m What is the value of the head wind (U) near the tip of a 14 m blade on a hub rotating at 22 rpm? What is the value of the resulting wind (W r ) velocity from the natural decimal places. What is 200 square feet in square meters? m 2 Festo Didactic Inc

16 Review 1. Describe how blades stall. What positive and negative effects can occur as a result of a stall? 2. their basic functions. 3. What term is used to describe blades that are pitched out of the wind? When is this procedure necessary? 4. What is the purpose of a tap test? 16 Festo Didactic Inc

17 5. What are the three main categories of a proper wind turbine blade inspection and maintenance program? Name: Date: Instructor approval: Festo Didactic Inc