Extensive design and aerodynamic performance investigation of diffuser augmented wind turbine (DAWT) guided by generalized actuator disc theory

Abstract

Shrouding a turbine boosts power, lowers cut-in speed, but raises installation costs, and limits adaptability to wind shifts. A compact, wide-angle DAWT with competitive capacity is crucial for practical installations. Small turbines, often in fluctuating wind sites, necessitate thorough off-design performance analysis. A relatively short, wide-angle GOE431 diffuser is optimized by an efficient response surface method. Inverse blade element method, combined with actuator disc DAWT CFD considering wake swirl, shapes 90 cm-diameter rotor blade with minimal iterations. Implications of Generalized Actuator Disc Theory on DAWT design and performance are addressed with three-dimensional CFD for the first time. This approach unveiled substantial room for improving DAWT efficiency and uncovered key factors causing deviations from ideal performance. Tip leakage and diffuser losses constituted 9.5% of overall energy losses, with wake rotation, blade efficiency, and blade drag at 9.2%. Tip-hub losses, finite blade number, rotor-diffuser interaction, suboptimal rotor, and turbulence contributed to 12.1% in three-bladed DAWT, reaching CP,max = 0.746, with tip losses about one-third of bare turbine. Six-bladed DAWT raised CP by 93%, from 0.417 to 0.805, achieving 75% of ideal DAWT. Finite blade number led to reduced attack angles, higher tip losses, and limited flow expansion, contributing significantly to energy losses. As blade number and design tip-speed-ratio increased, blade Reynolds number decreased, suggesting an optimal combination to minimize energy losses. At off-design, a strong connection existed between thrust coefficient, diffuser efficiency, and Cp increase in wide-angle diffuser DAWTs. Maintaining CT near CT,opt (0.786) at high tip-speedratio led to significant Cp rise.