The influence of the lobed ejector on the horizontal axial tidal turbine and flow characteristic analysis

The power coefficient of the horizontal axis tidal turbine (HATT) is limited to 0.593 by the Betz limitation, however, the turbine's efficiency can be improved by incorporating a diffuser or lobe ejector. In this study. In this paper, water tunnel experiments were conducted to obtain the power and thrust coefficients of the original turbine and the turbine with a lobe ejector at different tip speed ratios (TSR). The accuracy of the numerical simulation was then verified by comparing it with the experimental results. The results reveal that the power coefficient of the turbine with a lobe ejector is up to 623% higher than that of the original turbine; however, it also leads to a negative thrust coefficient higher than the original turbine by 94.64%. To determine the optimal position of the lobe ejector, a numerical simulation of the turbine with a diffuser, which consists of a single inner lobe channel, was conducted under the same conditions. The findings demonstrate that the power coefficient of the diffuser is 123% higher than that of the original turbine, and the negative thrust is higher at 26.64%, significantly less than that of the lobe ejector. This indicates that the effect of the lobe ejector largely depends on the ejector effect of the outflow channel. Subsequently, to investigate the ejection effect of the lobe ejector, various aspects, including the vortex structure of the flow field, pressure coefficient of the blade section, velocity vector field of the rotor's center plane, and wake velocity curve, were analyzed. Moreover, the changes in the position of flow separation points along different sections of the blade from the blade root to the blade tip, the variation of the spiral structure of the blade root vortex, the evolution of the velocity vector field in the low-velocity region of the turbine's wake flow field and the alteration of the velocity curve of the wake flow field were further examined.