Fundamental Understanding of the Physics of a Small-Scale Vertical Axis Wind Turbine with Dynamic Blade Pitching: An Experimental and Computational Approach

Abstract

This paper describes the systematic experimental and computational (CFD) studies performed to investigate the performance of a small-scale vertical axis wind turbine (VAWT) utilizing dynamic blade pitching. A VAWT prototype with a simplified blade pitch mechanism was designed, built and tested in the wind tunnel to understand the role of pitch kinematics in turbine aerodynamic efficiency. The ability of the present pitch mechanism to change the blade pitch phasing instantaneously in order to adapt to changes in wind direction is the key to maximizing power extraction in urban environments. A CFD model was developed and the model predictions correlated extremely well with test data. The validated CFD model was used to develop a fundamental understanding of the physics of power extraction of such a turbine. Both experimental and CFD studies showed that the turbine efficiency is a strong function of blade pitching amplitude, with the highest efficiency occurring around ±20◦ to ±25◦ amplitude. The optimum tip speed ratio (TSR) depends on the blade pitch kinematics, and it decreased with increasing pitch amplitude for the symmetric blade pitching case. CFD analysis showed that the blade extracts all the power in the frontal half of the circular trajectory, however, it loses power into the flow in the rear half. One key reason for this being the large virtual camber and incidence induced by the flow curvature effects, which slightly enhances the power extraction in the frontal half, but increases the power loss in the rear half. Fixed-pitch turbine investigated in the present study also showed lower efficiency compared to the variable pitch turbines owing to the massive blade stall in the rear half, caused by the large angle of attack and high reverse camber. Maximum achievable CP of the turbine increases with higher Reynolds numbers, however, the fundamental flow physics remains relatively same irrespective of the operating Reynolds number. This study clearly indicates the potential for major improvements in VAWT performance with novel blade kinematics, lower chord/radius ratio, and using cambered blades.