Insight into stall delay and computation of 3D sectional aerofoil characteristics of NREL phase VI wind turbine using inverse BEM and improvement in BEM analysis accounting for stall delay effect

In this paper, analysis of stall delay phenomenon for the NREL Phase VI wind turbine is done in order to improve Blade Element Momentum (BEM) method. The current study includes different proposals for extrapolation of 2D aerofoil characteristics for S809 aerofoil at higher angles of attack and their implementation in BEM analysis and comparison with experiment results. Also four existing stall delay corrections models are examined. In general BEM with no stall delay models did not account for stall delay and BEM with existing stall delay models result in over-prediction of lift coefficients at inboard sections of the blade at higher wind speed. Improved inverse BEM method is developed here to compute 3D aerofoil characteristics. In addition to five radial locations as described in the NREL/NASA Ames test analysis, thirteen additional radial locations are considered for a better understanding of stall delay. A new BEM model with the local radius effect for aerofoil characteristics (other than as a function of Reynolds number and angle of attack only) is proposed. Implementation of the new model showed a good agreement with aerofoil characteristics distribution along the blade span. It further revealed a good matching for overall thrust comparison and promising comparison in power computation.