Multiparametric optimization of the Middle Vanes of a counter-rotating axial impulse turbine for Oscillating Water Column devices

Oscillating Water Column devices have been historically one of the most studied Wave Energy Converters. Despite the existence of other alternatives, bidirectional turbines are the most widespread solution to take advantage of the characteristic bidirectional air flows in these devices. This category mainly includes Wells and impulse turbines. M. E. McCormick introduced the counter rotating impulse turbines in 1978. Unlike the aforementioned, these specific impulse turbines mount two counter-rotating rotors. Thus, the downstream rotor is able to harness the kinetic energy at the output of the upstream rotor. These turbines traditionally achieved poor efficiencies at low flow rates due to the misalignment of the flow between rotors. More recently, the introduction of a row of Middle Vanes between the two rotors was proposed in order to overcome this drawback. In this work, a CFD-based optimization of the Middle Vanes of a counter-rotating axial impulse turbine is presented. The Mixing Plane technique was used to simulate the relative movement of the blades. Three geometric parameters used to define the shape of the Middle Vanes, as well as the number of them, were considered as Input Parameters for the optimization. Meanwhile, the total-to-static efficiency reached by the turbine at a flow coefficient of 1 was set as the only Output Parameter to maximize in this multiparametric and single-objective optimization, this leading to a 1.2% improvement in terms of peak efficiency.