Comparison of low wind speed aerodynamics of unsymmetrical blade H-Darrieus rotors-blade camber and curvature signatures for performance improvement

Abstract

Unsymmetrical blade Vertical axis wind turbine (VAWT) can be nicely adapted in the built environment because of its improved performance, self-starting ability, simple construction, easy maintenance and capability to adapt to wind direction changes. For proliferation of such VAWT installations, the challenge is to improve its efficiency in low wind speed condition (usually less than 10 m/s) of the built environment, which depends on its blade profile design. In an unsymmetrical blade, blade thickness-to-chord ratio, percentage camber, blade curvature around aerodynamic moment center are the controlling parameters for effective blade-fluid interactions for performance improvement, thus requiring a detailed investigation of the aerodynamics of unsymmetrical blade VAWT. The objective of this paper is to compare low wind speed aerodynamics of two different unsymmetrical airfoil blade profiles to obtain design information of blade camber and blade curvature around aerodynamic moment center for performance improvement of H-Darrieus VAWT, which is hardly available in the existing literature. Detailed blade-fluid interactions are analysed by Ansys Computational Fluid Dynamics (CFD) software for different low wind speeds between 4 m/s and 8 m/s. It is found that more rounded curvature of the suction side surface of advancing S815 blade around aerodynamic moment center and thicker blade profile (maximum thickness is 26.2% at 25.7% chord) has a distinct influence for performance improvement of the rotor in the power stroke. In the returning stroke, the VAWT is benefited from higher camber of EN0005 blade (maximum camber percentage of 10% at 55% chord) and lesser blade curvature of the blade suction side. Next, vorticity structures of each blade profile have also been monitored at different azimuthal positions and necessary performance insights with regards the development of static/deep stall vortices and other boundary layer features have been drawn. Further, blade polars, power performance, and blade loading have also been obtained and compared with published results. Overall better designs of unsymmetrical blade profiles have also been recommended. The present study leads to the understanding of the design blade camber and curvature signatures in both power and returning strokes for possible performance improvement of small-sized VAWT in low wind speed condition.