Load on a wind turbine blade and its stress condition
Abstract
The article provides an analysis of the stress and deformation conditions of NACA blade at wind speed of 3-12 m/s and in a parking position (over 20 m/s). The wind speed of 9 m/s and Poisson's glass-epoxy laminate with a value of 0.43 initiates yaw of the blade tip from the axis of rotation by 651 mm. Von Mises stress reaches a value of 75.58 MPa. Visualization of airflow around the blade demonstrates that flow separation occurs at the point of blade mounting in the hub and the speed is higher than airflow speed around the blade tip.
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Introduction
Wind power installations are used to convert kinetic energy of wind into electricity. This conversion can be performed in various ways, however, so-called Betz limit with a power coefficient of 0.593 cannot be surpassed. When the value of the coefficient is greater than 0.5, aerodynamic efficiency rises up to 85%. Precisely manufactured blades, e.g. NACA type blades, must be utilised in order to achieve such efficiency. This type of blades represents the most effective method how to generate the pressure difference required in a wind turbine rotor. Blades of this type can also reach a considerable proportion of the aerodynamic lift cy to aerodynamic drag coefficient cx. The values of both coefficients are affected by the angle of attack α, which determines pressure distribution on the blade as well as its stress state.
Conclusion
The examined stress and load conditions are on the turbine blade of a wind power plant to be located in the area with lower wind-energy potential. The maximum assumed wind speeds 30m above ground range between 5 - 9 m/s.
The airflow around the root attachment is shown in Fig. 5 and the blade tip in Fig. 6. A wake occurs on the upwind side of the root section and the flow separates from the surface of the root attachment. This is due to the fact that the root attachment has a cylindrical shape, which is, in terms of airflow, the least favourable shape. The wind speed on the upwind side reaches up to 37.6 m/s. The wake is weaker on the blade tip due to a more suitable airfoil profile. The maximum speed is about 24 m/s.