Over the recent decades, many different ducted turbines have been designed to augment efficiency of wind turbines. The design and number of blades are the most important parameters to optimize efficiency of wind turbines. In this paper, effects of design, number and attack angles of blades on rotational speed are experimentally studied in a duct which increases wind velocity up to 2.46 times numerically and 232 times experimentally as great as far-field flow. In order to realize this, 3 different types of aerodynamic blades were designed and then, 2-bladed, 3-bladed and 4-bladed impellers were created by these blades; finally, 9 impellers were built on aggregate. The rotational speed of each impeller was recorded at 7 attack angles in two operating states with and without the duct at the same conditions in a wind tunnel. Results demonstrate that firstly, the rotational speed is reduced by adding more blades; secondly, the impellers with wide top blades have lower rotational speeds; thirdly, the rotational speed is augmented by increasing attack angle from 0 to 75; therefore, the rotational speed increase ratio is dependent on these 3 parameters and the wind velocity increase ratio at the throat as well. (C) 2016 Elsevier Ltd. All rights reserved.