Improved performance of a slotted blade using a novel slot design

Abstract

Efficient capture of wind energy requires optimal dynamic performance of turbine blades, desirably including delaying stall to higher angles of attack (AoAs). Stall is associated with flow separation close to the leading edge on the suction side of a blade so that delaying flow separation mitigates this condition. Merits of passive slots in wings for increasing their lift are well known. Typically, however, the increase in lift is accompanied by a corresponding increase in drag so that the overall aerodynamic efficiency is at most marginally improved. Here, we discuss application of a custom optimized-design internal slot on a NACA 634-021 airfoil blade to allow ventilation of flow through the slot from the pressure side to the suction side of the blade, in support of delaying flow separation, and stall. Delaying flow separation, results in increase in lift and decrease in drag at significantly high angles of attack. Slot width and inclined angle were varied to determine the optimum configuration. Dynamic performance of the blade was investigated both numerically and experimentally, with angles of attack in the range 0o≤AoA≤30o at a Reynolds number of 105. Lift and drag coefficients of the slotted airfoils predicted from the computational fluid dynamics (CFD) study correlate well with the corresponding coefficients determined via experiments. Experimental results show that a 58% increase in maximum lift coefficient and an 14% increase in maximum lift-to-drag ratio could be achieved with the optimum design case. Flow details obtained from the CFD study provide better insight into the underlying control mechanism of the internal slot. The findings could have significant implications for aeronautics and for improving capture of renewable energy from wind.