Simulation validation of moment balancing method for drag-dominant tidal turbines

• Main Authors: Zhang, Yixiao, Mittal, Shivansh, Ng, Eddie Yin Kwee
• Other Authors: Interdisciplinary Graduate School (IGS)
• Format: Conference or Workshop Item
• Language: English
• Published: 2023
• Subjects: Engineering::Mechanical engineering; Drag-Dominant Tidal Turbine; Performance Optimization; Savonius; Pinwheel; Steady-State
• Online Access: https://hdl.handle.net/10356/169199; https://www.iage-net.org/igec2023
• Institution: Nanyang Technological University

Drag-dominated turbines play a key role in the application of urban windfarm and multi-flow direction tidal arrays because of their low cut-in speed and omnidirectional characteristics. A performance analysis study of Pinwheel and Savonius tidal turbines has been carried out using Computational Fluid Dynamics (CFD) software to define the optimal power coefficient (Cp) and Tip-Speed-Ratios (TSR). The classic Disk Actuator model assumes a fixed virtual disc with or without porous holes perpendicular to the inflow direction. This is unsuitable for drag-dominant turbine because of the rotating virtual disc of the rotor plate of a vertical-axis turbine, the unaccounted bypass flow interaction on the downstream flow boundary for a horizontal-axis turbine, and parasitic force acting on the rotor/support walls for both. Therefore, a more applicable model is required for the tidal turbine realm. The focus of this study is to propose a novel method to find the optimal TSR of a drag-dominant turbine with a cost-effective and user-friendly Moment Balancing algorithm. The CFD models were inspired and scaled from experimental findings in the literature review. Both models were made comparable using a parametric study to equalize the blockage area at 12%. After careful analysis of different solver settings, steady k-epsilon model was selected, and grid independence tests were conducted. V-shaped TSR matrix was developed with varying turbine rotational speeds and fluid inlet velocity, unlike previous works simulated at a fixed velocity. For Pinwheel and Savonius, the TSR range for simulations is 0.64-5.0 and 0.3-1.0 respectively. Thrust Moment (Acting) is calculated when the turbine is stationary, but the fluid motion exerts load and rotates it. Idle Moment (Resisting) is calculated when the turbine is rotating at a given speed and the water is stationary hence, a load is exerted on the turbine. Linear regression analysis was performed and coefficients for thrust and idle moment were calculated, thus, formulating an equation for the net moment of Pinwheel and Savonius. 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 TSR are found for Pinwheel at 2.37 and Savonius at 0.63 with 15.6% and 11.1% error rate respectively for experimental validation. Based on the findings, thrust and idle moment have a positive and negative quadratic relationship respectively with the inlet velocity. A hill-shaped curve is observed between power coefficient and TSR. The optimal TSR for Pinwheel is higher than Savonius, thereby a higher rotational and lower inlet speed should be adjusted accordingly and vice versa. The proposed algorithm 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. The computational cost is greatly reduced through replacing net moment simulations by combining thrust and idle moment simulations. Upon commercial launch of the algorithm, the tidal energy development will become robust and more affordable.