Effect of Diffuser Inlet Angles on the Power and Torque Coefficients of a Hydrokinetic Turbine

Abstract Ocean current energy is a promising ocean renewable energy source. The Savonius hydrokinetic turbine (HKT) is known for its simplicity and adaptability in low-velocity ocean currents, but its efficiency is generally lower than that of other turbine types. This study investigates the optimization of diffuser inlet opening angles and their effect on the power output of a Savonius HKT using flow-3d simulations with the unsteady Reynolds-averaged Navier–Stokes equations and the RNG k-ε turbulence model. Five inlet angles (θi = 15 deg, 20 deg, 25 deg, 30 deg, and 35 deg) are tested to evaluate their impact on flow velocity, power coefficient (Cp), and torque coefficient (Ct). Refinement level 7 (RL7) is used for all analyses, ensuring consistent mesh resolution. Six domain sizes are tested to assess boundary effects, confirming that D4 balances accuracy and computational efficiency. The time-step analysis identifies TS5 as the optimal time-step. Results show that the 20 deg inlet angle achieves the highest Cp of 0.447 and Ct of 0.628 compared other angles. It also enhances Cp by 38% and Ct by 28% compared to a turbine without a diffuser. This validates the effectiveness of the 20 deg configuration. This study demonstrates that optimizing the diffuser inlet opening angle, particularly θi = 20 deg, can enhance the HKT efficiency.