Flow analysis of shrouded small wind turbine with a simple frustum diffuser with computational fluid dynamics simulations

Abstract

Reported experimental and computational fluid dynamic (CFD) studies have demonstrated significant power augmentation of diffuser shrouded horizontal axis wind turbine compared to bare one with the same swept area of the diffuser. These studies also found the degree of the augmentation is strongly dependent on the shape and geometry of the diffuser such as the length and the expansion angle. In this paper, CFD simulations of a small commercial wind turbine have been carried out with a simple frustum diffuser shrouding. The diffuser has been modeled with different shapes with the aim to understand the effect of length and area ratio on power augmentation. The simulations provide some effective frustum diffuser geometries for the small commercial wind turbine. From the analysis, sub-atmospheric back pressure is found to be the most influential factor in power augmentation. This factor is significantly affected by diffuser area ratio. Meanwhile flow separation in the diffuser can lead to significant lowering of the pressure recovery coefficient which reduces the overall power coefficient. This phenomenon can be mitigated by adjusting the length of the diffuser. The results of this study present a method on how to determine effective frustum diffuser geometries for a small wind turbine. These results can be extended for any types of wind turbines designed for nominal wind speed.