Optimization of Flanged Diffuser for Small-Scale Wind Power Applications

The development of renewable and clean energy has become more crucial to societies due to the increasing energy demand and fast depletion of fossil fuels. A state-of-the-art design for an augmented wind turbine has been introduced in the past years to increase the efficiency of compact horizontal axis wind turbines, exceeding the ideal Betz’s limit of the maximum energy captured from the wind. The optimization of the flanged diffuser - so-called diffuser augmented wind turbine DAWT - is investigated numerically using the multi-objective genetic algorithm “MOGA”. A 2D computational model is developed using ICEM CFD and solved by ANSYS Fluent. The Turbulence model selected is shear stress transport K-omega, with a pressure-based solver and a coupled algorithm scheme. The optimization objectives are to maximize the velocity ratio at the shroud throat and minimize shroud form dimensions. 517 design points were solved, and the design dimensions were categorized into four types: compact, small, medium, and large design. The results showed that the diffuser dimensions are the main parameters to increase velocity inside the shroud throat, where a long diffuser with a low converging angle drags more air inside the shroud, reaching in some cases more than double the upwind velocity. While the nozzle and flange are also effective in the different design types. It was found that a super long diffuser with a length ratio of 2.9 LD to throat diameter D is optimal with a diverging angle of 7.6˚, accompanied by a nozzle of ratio 1.2 LN/D and 12.6˚ converging angle and a flange length ratio of 0.6 LF/D. This optimal design increased the velocity ratio by almost 2.5 times.