Investigation of flow field and torque coefficient of Darrieus wind turbine with respect to pitch angle variations and tip speed ratios

The Darrieus lift-based VAWT has been considered by many scientists due to the simplicity of design and independence to the wind direction. Due to increasing angle of attack at low tip speed ratios, the straight-bladed Darrieus wind turbines have the inherent problem of self-starting inability and have less power coefficient as compared to the horizontal axis wind turbines. In this study, has been showed that use of the variable pitch angle turbines is a suitable solution to overcome the self-starting problem and increasing the output power of Darrieus wind turbine. So, the effect of variable blade pitch angle mechanism with different amplitudes on output torque, flow field around the rotor and self-starting of Darrieus wind turbine has been investigated. Also, unsteady two-dimensional simulation is conducted using computational fluid dynamics with SST kω turbulence model and moving mesh technique is used for simulation of the rotating rotor. The numerical investigation shows that variable pitch mechanism decreases the angle of attack and makes the turbine have a chance to produce more torque at different azimuthal angles. Investigation of the flow around the blade also shows that the use of variable pitch blades increases the pressure difference between the low and high pressure of around the blade and delays stall. So, the Darrieus turbine with variable pitch angle in comparison with zero fixed pitch angle has the ability to produce more power at the middle and lower tip speeds ratios.