The Effects of Blockage Ratio and Distance from a Free Surface on the Performance of a Hydrokinetic Turbine

Blockage ratio and distance below a free surface are two factors that potentially affect hydrokinetic turbine performance. The extent of the effects of these parameters is considered by determining the coefficient of performance of a simple turbine blade rotating at 250 RPM in a 3.5-m/s flow of water using FLUENT Computational Fluid Dynamics software. The effects of blockage ratio are tested in a circular duct of varying diameter (0.5588 to 10.668 meters (22 to 420 inches)), while the effects of distance below a free surface are tested in a river bed, where the depth of the turbine is varied (0.3175 to 4.1148 meters (12.5 to 162 inches)). The coefficient of performance is shown to vary only slightly for values of blockage ratio below about 0.4. As the blockage ratio increases beyond 0.4, the coefficient of performance increases greatly and graphically tends toward a vertical asymptote as the blockage ratio approaches 1. Turbine depth is shown to have little or no effect on a turbine's coefficient of performance, except when the turbine is very close to the surface, at which point the coefficient of performance increases; however, the model or mesh employed to obtain these results may not be sufficient. It is suggested that a finer mesh or a two-phase model including air above the river be used in the further study of this topic.