The performance of a vertical axis wind turbine with and without a diffuser was studied using direct force measurement technique applied to a scaled model of the rotor in a water tunnel. The experiment was conducted at different tip-speed ratios. The maximum power coefficient for the turbine was found to be equal to 0.35 for the rotor with diffuser and to 0.26 for the rotor without diffuser. Therefore, the maximum power coefficient was increased by 35% when the diffuser was used in the configuration. In the second part of this work, the flow patterns downstream of the turbine were studied by the particle image velocimetry (PIV) technique. Six different tip-speed ratios were considered for each configuration (with and without a diffuser). The vorticity and the streamline plots provide insight into the flow physics in each configuration. In addition, the swept area of a full-scale rotor was calculated for both a diffuser-augmented and a bare turbine for a range of power outputs. 德國(guó)LaVision PIV/PLIF粒子成像測(cè)速場(chǎng)儀