Axial momentum theory-based investigation of rotor separation effects in single-shaft horizontal-axis dual-rotor turbines

Dual-rotor tidal turbines have demonstrated performance advantages over single-rotor systems; however, their optimization remains challenging due to the increased number of design variables and interactions. Existing studies often present inconsistent findings, largely constrained to specific turbine configurations, which limits their general applicability. This study addresses the need for a more unified framework by investigating the influence of axial spacing between the two rotors based on axial momentum theory. To evaluate the applicability of the simplified theoretical approach in practical settings, a numerical analysis was conducted using ANSYS-based simulations of a counter-rotating marine current turbine model. A key assumption in previous theoretical models that the pressure at the inlet of the downstream rotor equals ambient pressure is examined to see if it is necessary and how it affects the turbine's performance. The simulation results are in full agreement with the theoretical predictions. The results show that the optimal power coefficient does not necessarily occur at large axial distances between the rotors. Instead, the results show that the power coefficient improves as the projected cross-sectional diameter of the rear rotor's flow tube at front rotor location (D1int) and axial induction ratio (e1/e2) approach 0.707D and 1/3, respectively, with peak performance observed near these values. These findings can support more efficient optimization of dual-rotor systems.