Loss mechanisms caused by rotor-stator interaction and performance improvement in bidirectional impulse turbines for wave energy conversion

Abstract

This study presents a high-fidelity computational fluid dynamics (CFD) analysis of the detailed flow characteristics and loss-generation mechanisms in a bidirectional impulse turbine used for oscillating water column (OWC) wave energy conversion. Bidirectional impulse turbines inherently experience efficiency degradation compared with unidirectional turbines, because the geometric symmetry required for reciprocating flow prevents optimal incidence control. Understanding the associated loss mechanisms is therefore essential for improving turbine performance. The CFD model was validated against existing experimental data, showing good agreement in torque coefficient, pressure-drop coefficient, and efficiency. Following this validation, the study examined pressure drop, swirl velocity, and total pressure loss in the upstream guide vane, rotor, and downstream guide vane. The results indicate that major losses originate from flow separation at the rotor leading edge and at the downstream guide vane leading edge. Additionally, an analysis of energy-flux evolution within the rotor passage quantitatively clarified the distribution of energy transfer and dissipation near the peak-efficiency operating condition. These findings highlight that reducing losses in the rotor and downstream guide vane is crucial for enhancing turbine performance. Overall, the insights obtained in this study contribute to establishing design guidelines for improving the efficiency of bidirectional impulse turbines for wave energy applications.