In an oscillating water column (OWC) based wave energy device, a water column that oscillates due to the sea wave motion generates a bi-directional airflow in an air chamber, and finally, air turbine driven by the bi-directional airflow converts the pneumatic energy into the mechanical energy. A counter-rotating impulse turbine for bi-directional airflow has been proposed by M. E. McCormick of the United States Naval Academy in 1978. In previous studies by using CFD analysis, the authors investigated the effect of the turbine geometry on the performance of the counter-rotating impulse turbine for bi-directional airflow, and it was clarified that the efficiency of the turbine is higher than that of an impulse turbine with a single rotor for bi-directional airflow in a range of high flow coefficient.
On the other hand, this impulse turbine has a disadvantage that the efficiency in a range of low flow coefficient is remarkably low due to the deterioration of the flow between the two rotors. In a previous study, the authors proposed to installed middle vanes between the two rotors in order to overcome the above drawback. And the effect of the middle vanes the turbine performance was investigated by CFD analysis. As a result, it was found that the middle vanes installed to the turbine rectify the flow between the rotors in a range of low flow coefficient and it drastically increases the efficiency.
In this study, the middle vanes was actually installed in the counter-rotating impulse turbine for bi-directional airflow, and its performance was elucidated by wind tunnel tests which has the turbine with a inner diameter of 240 mm and a centrifugal fan. It was found from the experiment result that the turbine performance at low flow coefficient is improved and the peak efficiency increases by 1.14 times, by means of the installation of middle vanes to the turbine.