The Influence of Preset Pitch on the Performance of Cambered Foils within Cross-flow Turbines

Abstract

The rotational reference frame in cross-flow turbines results in variations in virtual geometry and the angle of attack as the blades move through the flow field. This dynamic interaction significantly affects the performance and efficiency of the turbine. Initial studies have looked at the influence of blade geometric camber, either enhancing or reducing the virtual camber due to rotation. Other studies have shown the importance of preset pitch for overall performance. In this study, the sensitivity of the flow curvature effects to the change in preset pitch angle and airfoil camber are examined, with the virtual camber and angle of attack estimated according to the conformal mapping method. The virtual camber and angle of attack caused by rotation are shown to be more sensitive to the preset pitch angle than to changes in geometric camber. The interaction of preset pitch angle and geometric camber across a range of tip-speed ratios was examined for a single-bladed turbine with both a symmetric foil and those with ±2% camber. It was found that the preset pitch (−6◦ to −8◦) and tip-speed ratio (3.0) that yield optimal performance for each blade were insensitive to geometric camber. However, the utilization of the +2% camber airfoil does not significantly increase the peak performance of the turbine with any combination of the preset pitch angle tested. The positive preset pitch angle and the positive camber enhance the lift upstream and have a more detrimental performance downstream, while the negative preset pitch angle and negative camber do the opposite. Often the preset pitch angle or the airfoil camber has the peak performance upstream and does not correspond to the optimal performance downstream. The turbine loading has a direct relationship to the blade power. Three camber foils yield similar average turbine loading around optimal performance and are insensitive to the preset pitch angle at a lower tip-speed ratio. The negative camber airfoil could reduce the maximum loading on the turbine by 16% with no loss in performance when compared to the zero camber airfoil.