Variation in Flow Characteristics and Power Performance Due to Axial Distance Optimization in the Design of Counter-Rotating Tidal Turbines

Counter-rotating turbines, designed to enhance the performance efficiency of tidal turbines, exhibit variable operational characteristics depending on the axial distance between the front and rear blades. This study encompassed both numerical analyses and performance experiments to establish the optimal design by examining the relationship between flow field alterations and the performance of a counter-rotating tidal turbine with varied axial distances. The blades of the counter-rotating tidal turbine, based on a 10-kW single turbine, were designed utilizing the Blade Element Momentum technique. The turbine blades were assessed for changes in output performance attributed to flow separation by analyzing the velocity distribution and separation points within the flow, demonstrating a maximum power coefficient of 40.3% at a design Tip Speed Ratio of 3. At y/D = 0.3. The counter-rotating tidal turbine achieved a maximum power coefficient of 47%, with performance enhancements of the rear blades driven by the accelerated wake of the front blades. Furthermore, the pressure coefficients of the blades, influenced by their shape, inflow velocity, and angle, were detailed separately for the suction and pressure sides. The study also explored the correlation between the flow characteristics and the output performance of each blade by analyzing the distribution of pressure coefficients.