Experimental and computational investigation of a small-scale vertical axis wind turbine with dynamic blade pitching
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2015 by the American Helicopter Society International, Inc. This paper describes the systematic performance measurements and computational (CFD) studies conducted to investigate the performance of a small-scale dynamic-pitch vertical axis wind turbine (VAWT). The VAWT prototype was built and tested in a wind tunnel to understand the role of blade-pitch kinematics and flow curvature effects on turbine aerodynamic efficiency. The three parameters investigated in the experimental study were blade pitching amplitude (symmetric pitching), asymmetry in pitch kinematics between frontal and rear halves, and blade chord (or chord/radius ratio). Even though the optimal pitch amplitude is dependent on the tip speed ratio (TSR), moderate pitch amplitudes ( 20) had the highest overall efficiency for the symmetric pitch cases. The tip speed ratio corresponding to the maximum Cp decreased with increasing pitch amplitudes. The TSR corresponding to maximum Cp for 20 pitch amplitude was around 1.4, while the optimal TSR for the 40 case was around 0.7. Because of the differences in the flow velocities in the front and rear halves, for maximizing power extraction, the pitch angles required in the front is significantly higher than that in the rear. The optimal performance of the turbine occurred at a phasing of 0. However, the performance was observed to be forgiving for small changes in phasing (<10) in the positive direction (phase-lead), however, not in the negative direction. Increasing the chord/radius from 0.19 to 0.25 caused significant improvements in turbine efficiency especially at higher pitch amplitudes because of the flow curvature effects. A CFD model was developed and extensively validated with the present experimental data. The validated CFD model was used to understand the effect of the different parameters on turbine performance by analyzing the blade aerodynamics at various azimuthal locations. CFD analysis showed that the blade extracts most of the power in the frontal half of its circular trajectory and in some cases even lose power in the rear half. This study clearly indicates the potential for major improvements in VAWT performance with novel blade kinematics, optimal chord/radius ratio, and using cambered blades.
