Experimental Study on the Effect of Drop Size in Rain Erosion Test and on Lifetime Prediction of Wind Turbine Blades

Abstract

The effects of drop sizes in Rain Erosion Tests (RET) are studied testing glass fiber-epoxy specimens coated with a commercial polyurethane based top coat. The tests are conducted applying a whirling arm RET at impact velocities ranging from 90 to 150 m/s and with four different rain field setups generating mean droplet diameters of 0.76 mm, 1.90 mm, 2.38 mm and 3.50 mm respectively. From the RET data VH (Velocity-Impingement) curves are presented based on the ASTM-G73 definition of accumulated impingement. The slope of the VH curves vary with drop size.We propose a drop size dependent empirical model for impingement ( H ) to damage as function of impact velocity ( v ), H ( v ) = cv − m , where the parameters  m and c are functions of the drop size.The drop size-dependent impingement model is then applied for computing the expected leading edge lifetime of a virtual 15MW IEA reference turbine at 18 different meteorological stations in Northern Europe based on 10-minute time series of rain intensity and wind speeds. The drop size dependent model predicted on average 2.35 times longer lifetime compared to models based on the standard 2.4 mm drop size.

Keywords: rain erosion test, impingement to end of incubation, drop-size dependency, wind turbine blade coating, site specific lifetime modelling, phenomenological damage model