Blockage corrections for the experimental results obtained for a small-scale wind turbine
in a wind tunnel are required in order to estimate how the same turbine would perform in real
conditions. The tunnel blockage is defined as the ratio of the wind turbine swept area to the wind
tunnel cross-section area. Experimental measurements of the power coefficient were performed
on a horizontal-axis wind turbine with two rotors of diameter equal to 2 m and different numbers
of blades, namely three and five. Measurements were carried out for different tip speed ratios in
the closed circuit open test section wind tunnel of the University of Perugia (Italy). The obtained
experimental results were compared with the numerical ones carried out in free conditions by using a
CFD approach based on the steady-RANS method with the SST k-w turbulence model, adopting the
multiple reference frame (MRF) strategy to reduce the computational effort. The comparison showed
that the maximum value of blockage, which is reached in the asymptotic limit at very large tip speed
ratio (TSR) values, does not depend appreciably on the number of blades. A higher number of blades,
however, makes the occurrence of the maximum blockage come earlier at lower TSRs. |
