This study presents an experimental and numerical investigation of the effects of tunnel blockage, defined as the ratio of the free portion of a wind tunnel cross-section with and without the rotor of a small-size (diameter 2 m) horizontal-axis turbine (HAWT). Experimental measurements were performed on three rotors with different number of blades, namely three, five and six, and different tip speed ratios, in the closed-loop open-test section wind tunnel of the University of Perugia. Numerical wind tunnel simulations were performed at the University of Genoa through a steady-RANS method with SST k-w turbulence model by means of the CFD solver OpenFOAM. The wind turbine was represented numerically by the actuator disc method. Simulations have been compared to experimental results for validation.