AN INVESTIGATION OF THE INFLUENCES OF DIAMETER RATIO AND ATTACHING ANGLE ON THE PERFORMANCE OF A HELICAL-BLADED COMBINED DARRIEUS-SAVONIUS WIND TURBINE

Although Darrieus wind turbines are capable of producing high power coefficients, they also exhibit a low self-starting capability. Savonius wind turbines on the other hand are capable of providing relatively high self-starting capabilities but have a lower power coefficient. The objective of the present study is thus to develop a modified design of a combined Darrieus-Savonius wind turbine in order to improve its self-starting capability and power coefficient. The modified combined design consists of helical-bladed Darrieus and Savonius rotors with helical angles of 90 and 45 degrees, respectively. The influences of various diameter ratios in addition to various attaching angles between Darrieus and Savonius rotors in the proposed design are numerically investigated. The Reynolds-Average Navier-Stokes equations in three dimensions along with the turbulence model of k-omega (SST) are developed and numerically simulated. The predicted results are validated using the available measurements. Thus, numerical predictions of the variations of the coefficient of power versus tip speed ratio as well as the coefficient of torque over a whole cycle can be made. As part of the evaluation of the starting characteristics of the proposed configurations, the static torque coefficient during a complete revolution is also calculated. Based on the current study, some significant findings can be derived. The coefficient of power remarkably increases with rising the diameter ratio up to an optimal value of 0.45, then a significant reduction in coefficient of power is observed with further increment of diameter ratio beyond 0.45 due to increasing the drag losses by Savonius rotor. In comparison to a solo Darrieus rotor at the same dimensions, the combined turbine with the best design parameters achieved a peak coefficient of power of 0.386 with a performance gain of about 19 %. Furthermore, the combined design significantly reduces the fluctuations of coefficient of static torque in addition to achieving a higher average coefficient of static torque with positive values at all azimuth angles compared to solo Darrieus rotor.