Airflow distributions in small diffuser augmented wind turbines

Diffuser Augmented Wind Turbines (DAWTs) offer higher power outputs than a bare wind turbine of the same size. Most DAWTs have the rotors at the diffuser throat but Bagheri-Sadeghi et al. suggested that optimal power may be obtained with the rotor located on the verge of the flow separation point and for a diffuser angle higher than normal. This proposition was tested by 3D computational fluid dynamics (CFD) simulations on nine small DAWTs with diffuser angles over a standard range. The test results indicate that higher power output can be obtained from DAWTs with the rotor placed aft of the diffuser through the throat and with longer blade span than those with rotor at the throat. Some DAWTs had low power output suffer from severe airflow leakages through the blade plane, both azimuthally and radially. An azimuthally non-uniform flow separation was induced when the rotor was placed close to the diffuser exit. A blade tip vortex was detected in all tested geometries.