A hub blockage model for small and micro horizontal axis wind turbines

The classical Blade-Element/Momentum-Theory for horizontal-axis wind turbines neglects hub-blockage effects, which can be significant for small and micro turbines with moderate or high dimensionless hub radii. This paper presents an innovative strategy to introduce a hub blockage factor in the Blade-Element/Momentum-Theory without altering its classical solution algorithm. The model assumes that the axial velocity at the disk can be computed as the superposition of its value in the hubless case and the bare-hub induced-velocity. Hence, the approach implicitly assumes that the hub induced-velocity is independent of operating conditions, and, therefore, it can be pre-determined and provided as input to the solution procedure. The validity of this assumption is thoroughly and successfully verified. The model accuracy is tested by comparing its results against experimental data and an advanced Computational-Fluid-Dynamics-Actuator-Disk model, which inherently considers the hub blockage. Detailed analyses are conducted for turbines with high (25%), moderate (15%) and low (7%) values of the dimensionless hub radius. In all cases, the enhanced Blade-Element/Momentum-Theory demonstrates significantly improved accuracy in predicting both global (power and thrust coefficient curves) and local flow quantities (blade forces, angle of attack, mean axial induction factor).