Hydrodynamic performance optimization of horizontal Pinwheel and vertical Savonius tidal turbines using CFD

• Main Author: Mittal, Shivansh
• Other Authors: Ng Yin Kwee
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Mechanical engineering::Alternative, renewable energy sources
• Online Access: https://hdl.handle.net/10356/168013
• Institution: Nanyang Technological University

Drag-dominant turbines play a vital role in urban windfarms and multi-flow direction tidal arrays due to their low cut-in speed and omnidirectional characteristics. This research focuses on verifying the Moment Balancing method proposed by Mr. Yixiao Zhang, a PhD researcher, to determine the maximum power coefficient of drag-dominated turbines using Computational Fluid Dynamics (CFD) analysis of two different models. The proposed method overcomes the limitations of the Disk actuator model, which includes the varying sweeping area of the rotating virtual disc of the vertical axis turbine, the unsteady effect of the unaccounted bypass flow interaction on the downstream flow boundary of the horizontal axis turbine, and parasitic force acting on the rotor or support walls for both models. The algorithm assesses the net moment exerted on the turbine as an alternative to the cyclic force calculation technique employed by the actuator disk method. The Pinwheel turbine's computational setup was previously designed by Mr. Yixiao Zhang, and it was analyzed in this study. The Savonius turbine's setup was fully developed and analyzed in this research project. The two CFD models were made comparable by conducting a parametric study to ensure equal blockage area of 12%. After conducting a thorough analysis, the steady k-ε turbulence model was chosen, and grid independence tests were conducted for both turbines. The TSR range for the simulations was 0.64 - 5.0 for Pinwheel and 0.3 -1.0 for Savonius. This study establishes algebraic relationships and equations between newly proposed parameters of thrust and idle moment with the net moment of Pinwheel and Savonius turbines through linear regression analysis. The Pinwheel's rotational speed was found to be an excellent predictor for idle moment, while the inlet velocity was an excellent predictor for thrust moment in both models with R2 value approximately 1. Rotational speed proved to be a very weak predictor in the case of Savonius, with an R2 value approximately 0.22. Lastly, it is found that the power coefficient is maximum or zero when idle and thrust moment offset each other at the neutral point. The optimal TSRs are found for Pinwheel at 2.37 and Savonius at 0.63 with 15.6% and 10% error rate respectively for experimental validation. The study is successful in supporting Mr. Zhang's proposed method, which is expected to improve and simplify an engineer’s understanding of the turbine’s optimal TSR by adjusting the rotor speed to suit the inlet flow case and reduce computational costs for simulations, thus enhancing the affordability and robustness of tidal energy development.