A Computational Fluid Dynamics Investigation on the Axial Induction Factor of a Small Horizontal Axis Wind Turbine

Abstract

The evolution of wind and hydrokinetic turbines stimulated the development of several tools to evaluate and to predict horizontal axis rotor behavior. From this perspective, the blade element momentum methods stand out as one of the most common approaches due to its reliability and computing speed. In the classical blade element momentum, the axial induction factor is a crucial variable to compute correctly the turbine parameters. Usually, the axial induction is determined by an interactive process that balances the forces at blade sections with momentum equations. The forces are computed based on the airfoil polars evaluated at each blade section with local inlet velocity. This procedure assumes that the swirl terms are linearized, where the lateral pressure forces is neglected. In order to evaluate these tri-dimensional effects on the blade element momentum method, the present work introduces a different methodology to determine the axial induction factor employing computational fluid dynamics simulations. The method was applied for a full-scale horizontal axis rotor with three blades and 1 m of diameter, with wind tunnel experiments for validation. The axial induction factor obtained with the new technique was compared to the classical blade element momentum method. The results show axial induction factor variations along the radial and axial coordinates. An analogy with Glauert power coefficient limit was made, finding a specific limit curve for the tested turbine, and, moreover, a correlation between turbine firing speed and induction factor.