Blockage corrections for wind tunnel tests conducted on a Darrieus wind turbine

In this study, the influence of the blockage ratio in three wind tunnel tests conducted on a small vertical-axis Darrieus wind turbine was quantitatively investigated. The vertical-axis Darrieus wind turbine was installed in three wind tunnels with different test-section sizes corresponding to blockage ratios of 3.5, 13.4, and 24.7%. The rotor torques, drag force, and upstream wind speeds were measured for the different blockage ratios. It is recommended from the tests that the reference wind speed of the wind turbine should be measured upstream at a distance of 3.5 times or more than the turbine diameter. The power coefficient of the wind turbine was severely distorted based on the blockage ratio. The power coefficients at blockage ratios of 13.4 and 24.7% were respectively 1.3 and 2 times higher than that at a blockage ratio of 3.5%. The optimal tip speed ratio also significantly shifted to a higher range. Based on the method proposed by Maskell (1963), a new correction coefficient is proposed for the Darrieus turbine from the measured drag coefficients obtained at three different blockage ratios. The validity of the proposed correction coefficient was confirmed by comparing it with the corrected power coefficients obtained using other correction methods.