Effect of blade number on flow around straight-bladed vertical axis wind turbine

Abstract

The influence on performance during the revolution of a straight-bladed vertical axis wind turbine (VAWT) is discussed, focusing on the effect of the blade number. A data campaign has been performed for two, three, four and five-bladed rotors in wind tunnel. Firstly, rotor performances have been compared with the results of an experiment based on the use of the NACA 0021 airfoil, such as rotor torque and power. And then, for each rotor configuration, flow field characteristics are investigated at optimum values of tip speed ratio by means of Laser Doppler Velocimeter (LDV), allowing a quantification of the influence of blade number on flow geometric features and dynamic quantities. Finally, the velocity distribution around wind turbine, velocity deficits in wake region and turbulence intensity in flow field are compared for the analyzed architectures, achieving a quantification of the effect of blade number on overall rotor performance. In this research, according to the wind tunnel experiments, wind velocity field and performance (power coefficient and torque coefficient) are obtained through LDV system and torque meter, respectively. For example, a wide low wind velocity field appeared from the wind turbine internal region to downstream area. In addition, velocity vector had an asymmetry with respect to the x-axis at the downstream side of the rotor, and the reverse flow was generated at some region of downstream side for the blade numbers of 4 and 5. This result provided a theoretical guiding significance to the development of VAWT simplified load model.

Keywords

• wind turbine
• straight-bladed vertical axis wind turbine
• blade number
• flow field
• velocity deficit
• turbulence intensity