Transient flow characteristics and performance of horizontal-axis hybrid hydrokinetic turbine rotors during startup process

The startup performance of the hybrid hydrokinetic turbine rotor has seldom been reported. The relationship between time-varying rotor performance and instantaneous flow field during the startup process remains uninterpreted. The present study combined the H-type lift rotor with Bach rotor(s) through three different manners of configuration and attempted to clarify the effect of the drag rotor on the flow and startup characteristics of the lift rotor. The proposed hybrid rotors were horizontally installed. The method incorporating the computational fluid dynamics technique and six degrees-of-freedom model was applied. The results show that when Bach rotor is enclosed by the lift blades, the highest rotational speed and shortest startup time are achieved. At a tip speed ratio of 1.26, the maximum power coefficient of 0.257 is obtained. When the drag and lift rotors are installed side by side, the torque coefficients are relatively large, but the startup performance degrades. The wake flows significantly differ with the rotor scheme. A mixed wake is evidenced downstream of the side-by-side rotor, while for the inner–outer rotor structure, the wake of the drag rotor is immediately disintegrated by the lift blades, and the wake is explicitly dominated by the lift rotor. The obtained conclusions serve as a sound reference to effective utilization of the kinetic energy of water through the hybrid hydrokinetic turbine rotor.