Effects of blade section on performance of butterfly wind turbines as double-blade VAWTs

A butterfly wind turbine (BWT) is a kind of vertical axis wind turbine (VAWT) with closed-loop blades. These blades form a double-blade structure, which is expected to improve self-starting properties and reduce energy costs because of their simple construction. Two models of micro BWTs (diameter: 0.4 m; height: 0.3 m) were built and subjected to wind tunnel testing. One of the models had a symmetrical blade section and the other had a cambered blade section with a mean line that followed a curved path in a flow curvilinear relative to the blade. Experimental results showed that the cambered blade rotor was superior to the symmetrical blade rotor in terms of torque and power coefficients at higher tip speed ratios (TSR). However, at low TSRs, the performance of the symmetrical blade rotor tended to be higher than that of the cambered blade rotor. To investigate the effects of blade section on the performance and flow field of the double-blade rotor, two-dimensional computational fluid dynamics (2D-CFD) analysis was carried out for two double-blade rotors with symmetrical and cambered blades. Although 2D-CFD analysis is not suitable for the quantitative performance analysis of the three-dimensional BWT, the CFD results showed the same tendency of the torque and power performance as the experimental results. If the outer blades alone are considered, the cambered blades generate larger torque (or power) than the symmetrical blades at all TSR values, in the case of a large chord-to-radius ratio as with the present rotors. On the other hand, the inner symmetrical blades generate more torque (or power) than the inner cambered blades at TSRs less than 1.5. A TSR of 0.75, at which the symmetrical blade rotor showed the highest torque coefficient, was intensively analyzed in terms of the aerodynamic forces and torques calculated by the 2D-CFD. Under this condition, the inner blade of the symmetrical blade rotor generated positive torque at a wider range of azimuth angles than the cambered blade rotor.