A STRATEGY TO ACCOUNT FOR THE HUB BLOCKAGE EFFECT IN THE BLADE-ELEMENT/MOMENTUM THEORY

Although the hub blockage effect is generally disregarded for large-sized horizontal axis wind machines, it can significantly affect the performance of small-sized turbines whose ratio between the hub and rotor radii can attain values up to 25-30%. The article proposes a generalization of the Blade-Element/Momentum Theory (BE/M-T) accounting for the effects of the hub presence on the rotor performance. The new procedure relies on the quantitative evaluation of the radial distribution of the axial velocity induced by the hub all along the blade span. It is assumed that this velocity is scarcely influenced by the magnitude and type of the rotor load, and it is evaluated using a classical CFD approach applied to the bare hub. The validity and accuracy of the modified BE/M-T model are tested by comparing its results with those of a more advanced, and validated against experimental data, CFD-actuator-disk (CFD-AD) approach which naturally and duly takes into account the hub blockage, the rotor presence as well as the wake divergence and rotation. The comparison shows that the correction for the hub blockage effects in the BE/M-T model significantly reduces the differences with the results of the reference method (CFD-AD) both in terms of global (power coefficient) and local (thrust and torque per unit length) quantities.