Flow mechanisms and performance enhancement of twisted-blade S-type hydrokinetic turbines

AbstractTwisted blades improve the S-type turbine's power output and mitigating torque fluctuations; however, the mechanisms underlying performance enhancement by twisted blade remain unclear. Therefore, this study conducts three-dimensional simulations to quantify the interference between twist angle (TA) and aspect ratio (AR) on power output, torque fluctuation, and self-starting capability. Compared with the straight-bladed turbine, the turbine with AR = 2 and TA = 135° achieves a 3.1% power improvement. Flow field analyses show that twisted blade introduces a vertical-flow, which substantially intensifies the low-pressure region on the advancing blade near the lower endplate, thereby markedly increasing the positive torque. A pronounced interaction is identified for TA and AR, and an optimal TA emerges for AR≥2, corresponding to a twist increment of approximately 70°/D. Twisted blade also modulates torque unsteadiness; increasing TA reduces the fluctuation amplitude and yields a minimum when TA matches an integer multiple of the torque periodicity. Additionally, turbines with TA of 90° and 180° increase averaged static torque coefficient by 24.4% and 39.0% than the straight-bladed turbine.