EFFECT OF TWIST ANGLE ON THE PERFORMANCE OF DARRIEUS VERTICAL AXIS WIND TURBINES

Conventional straight-bladed Darrieus wind turbines have some drawbacks including large oscillations of coefficient of torque with negative values within a wide range of angle of rotation which decrease the self-starting abilities of the turbine and increase the mechanical vibrations. To solve these weaknesses, the current study proposes twisted-bladed Darrieus turbine instead of the conventional straight-bladed design to flatten the oscillations of torque coefficient in addition to enhancing the self-starting abilities of the Darrieus turbine. The studied angles of twisting are 45, 90, and 135 degrees in addition to the traditional untwisted Darrieus rotor. The trends of coefficient of power against ratio of rotor tip speed in addition to coefficients of dynamic and static torque over a complete cycle (360 degrees) are estimated for all studied twisted-bladed Darrieus turbines. Thus, three dimensional incompressible unsteady Reynolds-Average Navier- Stokes equations in conjunction with the k-omega shear-stress transport turbulence model is developed and numerically simulated utilizing ANSYS Fluent. The developed numerical model is validated using the available measurements from literature. The predicted characteristics of flow field around the proposed twisted-bladed Darrieus turbines are presented and compared with the traditional untwisted design. Accordingly, results revealed a significant improvement of the coefficient of power is attained by increasing the angle of twisting from 0 degrees to 90 degrees. However, with an additional increase in the angle of twisting beyond 90 degrees, the coefficient of power declined. The turbine with an angle of twisting of 90 degrees attains the largest coefficient of power of 0.318 with a percentage increase of 13.2 % in comparison with the traditional straight-bladed Darrieus turbine which has a peak coefficient of power of 0.281. The traditional untwisted turbine has negative values of coefficient of static torque within a large range of angle of rotation. However, the proposed turbines with twisted blades exhibit lower torque fluctuations and positive static torque values at all angles of rotation resulting in enhancing the self-starting abilities of the Darrieus turbine.