Comparison of flow characteristics and performance between horizontally oriented hybrid hydrokinetic turbine rotors

The present study aimed to reveal the operation and flow characteristics of horizontally oriented hybrid hydrokinetic turbine rotors composed of Bach and H rotors. The computational fluid dynamics (CFD) technique was used in conjunction with the six degrees of freedom (SDOF) method to solve instantaneous flow field. The results indicate that at a tip speed ratio of 1.73, the highest power coefficient of 0.328 is obtained with the H(B) rotor, characterized by a Bach rotor encircled by the blades of an H rotor. For the H-B rotor, characterized by a Bach rotor arranged side by side with an H rotor, and the B-H-B rotor, which consists of Bach and H rotors arranged in series, their startup performance is similar, and their torque coefficients are about twice larger than that of the H(B) rotor. The rotational speed during the stable operation stage increases monotonically with the upstream flow velocity. The H(B) rotor features a shortest startup time of approximately 1.23 s at an upstream velocity of 2.8 m/s. A compound wake is evidenced by the H-B and B-H-B rotors; the part of the wake corresponding to the Bach rotor inclines downward, whereas the wake of the H rotor meanders in the streamwise direction. In comparison, the wake of the H(B) rotor involves a relatively high velocity deficit.