Numerical investigation of the impact of the diameter and rotational motion of a multi-blade rotor on the hydrokinetic combined turbine performance

The vertical axis combined hydrokinetic turbine (VACHT) presents promising applications in marine environments due to its high self-starting performance and efficiency. However, traditional VACHTs suffer from significant performance losses in the outer rotor when the diameter ratio is large and the startup dead zone issue of the inner Savonius rotor. To address these challenges, this study proposes a VACHT composed of an H-type rotor and a multi-blade (M-type) rotor, categorized into two structures based on counter-rotating and co-rotating operations. Numerical simulations reveal that the diameter ratio (f) has a greater impact on VACHT performance than the tip speed ratio. Counter-rotating allows the VACHTs to accommodate larger diameter ratios (f > 0.6) than co-rotation. The diameter ratio primarily influences the interference between the inner and outer rotors. The M-type rotor enhances the self-starting performance of both VACHTs, providing enhancements exceeding 200 %. Moreover, cluster arrangements of VACHTs benefit from increased power output due to enhanced blockage effects, with co-rotating VACHTs exhibiting higher gains (13.65 %) compared to counter-rotating VACHTs (9.84 %). The lengths of both VACHTs’ wake are similar to H-type turbines, indicating that the compactness of the VACHT cluster remains unaffected.