Comparison of performance between two compound hydrokinetic rotors equipped separately with Bach and helical-bladed drag rotors

Abstract

For the compound hydrokinetic rotor, although fundamental characteristics have been recognized, a comprehensive interpretation of the power coefficient, flow characteristics, and startup performance has rarely been reported. The present study aims to reveal operating and flow characteristics of the compound hydrokinetic rotor characterized by a helical-bladed lift rotor. A method combining computational fluid dynamics and the six degrees-of-freedom model was used to solve instantaneous flow field. The results showed that the combination of the lift and Bach rotor yields the largest static torque coefficient. Under the no-load condition, the average rotational speed of the compound rotors in the stable operation stage is lower than that of the single lift rotor. Both the maximum power coefficient and optimum tip speed ratio decrease with the addition of the drag rotor. For the compound rotor comprising the helical drag rotor, at a blockage ratio of 0.41, the energy-conversion ability is degraded. The effect of the lift rotor on the wake flow is characterized by dispersedly distributed low-velocity elements in axial direction. Apart from clarifying instantaneous flow characteristics during the startup process of the compound rotor, the study sheds light on the interaction between the drag and lift rotors in the compound rotor and provides a sound reference to further improvement of rotor geometry. © 2025 Elsevier Ltd