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Numerical Calculation of Air Flow Around and at Wake of the Darrieus Turbine

Ebrahimi Saryazdi, Mohammad | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46402 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Boroushaki, Mehrdad; Rajabi, Abbas
  7. Abstract:
  8. Recently, a lot of attention has been devoted to the use of Darrieus wind turbines because of high power density and environment aspect in urban areas. Turbine airflows don’t effect on the power coefficient of Darrieus turbine. This turbine has an acceptable power coefficient depend on the other wind turbine in the small scale turbine. Aerodynamic performance of a Darrieus turbine is very complex due to phenomena such as dynamic stall and changing forces on the turbine caused by changing horizontal angles. So study airflows around turbine must be preformed. In this thesis the aerodynamics of an H-rotor vertical axis wind turbine (VAWT) has been studied using computational fluid dynamics in two different turbulence models. It should be noted that previous research focused primarily on one blade and using the simple turbulence model (DMSM or RANS model). In this study, CFD was used to evaluate the aerodynamic performance of H-Darrieus. SST k-ω turbulence model was used to simulate a 2D and 3D unsteady model of the H - Darrieus turbine. In order to complete this simulation sensitivity analysis on effective parameters of turbine such as solidity factor, airfoil shape, wind velocity and shaft diameter were done. Finally, 3D simulation of the turbine was done till 3D effective parameter study. Simulation of Darrieus turbine with URANS method and sensitivity analysis of important and effective parameters were done in this study for the first time.
    To simulate the flow through the turbine a 2D and 3D simplified computational domain was generated. Then fine mesh for each case consist of different turbulence model and different dimensions were generated. Each mesh in this simulation dependent from effective parameters consist of domain size, mesh quality, time step and total revolution. The sliding mesh method was applied to evaluate the unsteady interaction between the stationary and rotating components. In modeling of turbine calculation, when the simulation has converged. The Y+ parameter is calculated as follow to show the quality results and also the mesh generated. The SST k-ω simulation results closely match the experimental data and provide an efficient and reliable way to study wind turbine aerodynamics. In this simulation power coefficient curve in low speed ratio predict well. Although this value have overpredict at high tip speed ratios. The RNG k-ε simulation results can not predict current flow near airfoil and dynamic stall correctly. This results have overpredict at each tip speed ratios.
    The solidity factor sensitivity analysis, low, medium and high solidity factor were used. The results of this sensitivity analysis reveal medium solidity has a better performance. Sensitivity analysis of airfoil shape consisted with three NACA airfoil of NACA0012, NACA0015 and NACA0021. The results of this analysis show NACA0021 have a better power coefficient curve. The sensitivity analysis of wind speed was done on 3, 6 and 9 m/s. According to the results of turbine at 9m/s, this situation shows the best performance. Although turbine at 3m/s show better results at low tip speed ratios than high wind speeds. Eventually to evaluate the effect of turbine structure, three turbines consist of a turbine without main shaft, turbine with 3 cm main shaft and turbine with 5 cm main shaft. However, by increasing the diameter of the shaft turbine performance drop, but yet the effect of this parameter is not significant compared to the other parameters evaluated in this project.
    Finally, 3D modeling of the turbine were modify evaluate 3D effect. This simulation has a good agreement with experimental data results. The results of this modeling indicate that the vortices in the airfoil tip more power and then this vortex converged at the wake and drop turbine power coefficient.
  9. Keywords:
  10. Darrieus Turbine ; Computational Fluid Dynamics (CFD) ; Low Reynolds Number ; Vertical Axis Wind Turbine ; Dynamic Stall

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