Turbine specific fatigue life prediction model for wind turbine blade coatings subjected to rain erosion

Abstract

The leading edge of a wind turbine blade is subjected to repeated impacts, by dust particles and rain droplets, leading to erosion. Hence, the surface of leading edge is painted or coated with protective coating materials to mitigate the erosion. However, even the coatings tend to fail and erode in fatigue under repetitive impact loading caused by these particles and water droplets. The numerical models available in the literature assume uniform tip velocity of the blade for fatigue life prediction of coatings. However, magnitude of rain drop impact induced erosion is dependent on the tip velocity, surface roughness and the rain intensities over the operating lifespan. The tip velocities of a turbine blade vary with the wind speed and the power characteristics of the turbine model. Thus, the wind velocity and tip velocity variation must be incorporated along with surface roughness for realistic estimation of erosion in wind turbine blade coatings. This paper presents a turbine specific computational framework to predict the fatigue life of the wind turbine blade coating against rain erosion using turbine power characteristics, annual wind velocity variation, blade profile and rain drop size distribution using finite element modelling. The coating surface is modelled for both smooth and rough surfaces to present the effect of surface roughness on the estimated fatigue life of the coating. It was found that the raindrop impact has no significant influence on the leading-edge erosion of a smooth coating surface. The obtained results show that the assumption of uniform tip velocity significantly overestimates the fatigue damage due to rain drop erosion. The surface roughness of the coating was found to be the most critical parameter in the rain induced erosion of the turbine coatings. Simulation results indicate a reduction in fatigue life with increased surface roughness and are qualitatively similar to experimental findings from literature. Results from the current study estimate the fatigue life of a coating with a surface roughness of 50 µm to be 1.36 years by considering an uniform drop diameter of 2.5 mm. This finding is in a good agreement with the on-site studies which report observations of erosion in turbine coatings between 1 and 3 years. The framework presented in this paper can be used for conducting realistic numerical and experimental investigations on the leading-edge erosion of the different turbine models and coatings. Furthermore, the numerical results presented can provide useful insights to wind turbine and turbine coating manufacturers in mitigating the leading-edge erosion.