The power output of a wind turbine can be increased by adding a duct around it. The ducts, which tend to use high-lift airfoil shapes for their cross-section with the suction side inside the duct, cause flow acceleration and a greater velocity at the rotor. These ducted wind turbines (DWTs), which are also called shrouded wind turbines or diffuser augmented wind turbines, can be advantageous to their bare turbine counterparts as they have lower cut-in speeds, produce more power per unit device area, and have the potential to reduce noise and protect against broken blades or bird collisions. Hence, there has been a growing interest in the study and optimization of DWTs. However, there have been few studies on the application of these devices in arrays, which requires the study of the wake behind the DWTs.

An axisymmetric Reynolds-Averaged Navier-Stokes (RANS) CFD model using a *k*-ω SST turbulence model and an actuator disc model for the rotor was used to study the wake of DWTs and compare them with bare wind turbines. The actuator disc RANS simulations showed a strong dependence of the wake recovery behavior on the values of the ambient turbulence variables. Results agreed with experimental data when the turbulence intensity at the rotor was significantly greater than the recommended values for the k-ω SST model and an integral length scale equal to the diameter of the rotor was used. Therefore, simulations were done with freestream conditions specified using a turbulence intensity of 10% and an integral length scale of turbulence equal to the total diameter of the power producing unit (i.e. the rotor diameter for open rotors or the exit diameter for DWT simulations). As the RANS simulations at elevated turbulence intensities resulted in values of thrust and power coefficients which were about 10% greater than actuator disc theory, the studies were done at a slightly lower than the optimal rotor thrust coefficient.  The  studies suggest that variations in thrust coefficient do not significantly change the far wake behavior. 

 

In order to make a reasonable comparison between a ducted and open wind turbine, the ducted wind turbine chosen for this study had a total power coefficient (based on the total or exit area of the duct) slightly greater than Betz’s limit and was compared to an open rotor operating close to Betz’s limit. Therefore, both devices produced almost the same amount of power per unit area.

 

The simulations showed that the Gaussian shape of velocity profile in the far wake observed for an open rotor is still observed for a DWT wake. However, the near wake of the DWT was pushed further downstream and the velocity deficit was significantly greater. Although the change in flux of streamwise momentum for a DWT should be close to the open rotor, there is a significant additional drag force on the duct which leads to a larger momentum deficit downstream of the DWT. These results suggest that DWTs will require larger spacings than open wind turbines if they are utilized in arrays.


Wake Comparison of Open and Ducted Wind Turbines Using Actuator Disc Simulations

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.