Numerical study on a modified impulse turbine for OWC wave energy conversion

Impulse turbine is used in recent times as a self-rectifying turbine in OWC wave energy convertor. It has been found from field test data in Indian OWC plant that the profiles of air flow velocity is not symmetric in exhalation and reverse directions. The velocity is higher during the exhalation (chamber to atmosphere) than in the reverse direction. The modification proposed in this paper is to set the rotor blade pitch asymmetrically with a non-zero value of setting angle. It is expected to give a better performance in a wave cycle under the above air flow conditions. A 3D CFD model based on Fluent is validated by corresponding experimental data and is used for the numerical simulation of the turbine performance for different setting angles under steady conditions. All the calculations were carried out under the steady conditions. Pseudo-sinusoidal velocity pattern was used to represent the realistic air flows in the pilot plant. Quasi-steady analysis was then employed for calculating the mean efficiency for a certain variation of air flow velocity with time during a wave cycle. The clear flow separations were found at the suction side near the trailing edge and the high velocity domain occurs at the suction side near the midstream flow path. The pressure distribution show similar characteristics on both pressure and suction sides. Input coefficient, torque coefficient and mean efficiency are calculated for evaluating the turbine performance under various rotor blade setting angles. For the typical value of 0.6 for the ratio of velocity amplitude at inhalation to that at exhalation, the rotor blade setting angle of 5° was found to be optimum for the modified impulse turbine to achieve the best mean efficiency in a wave cycle.