Study On Flow Characteristics And Performance Of The Vertical-axis Hydrokinetic Rotor Under Complex Inflow Conditions

As a new energy conversion equipment,Savonius hydrokinetic rotor has gradually become the research focus in harnessing the flow energy of river,channel and ocean.Savonius hydrokinetic rotor has the advantages of simple structure,low maintenance cost,large startup torque,strong environmental adaptability and no cavitation.Thus far,a considerable amount of literature has been devoted to the drag-type rotor in wind energy utilization.In comparison,investigations of the drag-type rotor used to absorb the kinetic energy of water have seldom been reported.The application of hydrokinetic rotor is new relative to that of the wind turbine rotor;therefore,studies on fundamental flow characteristics of the rotor and the effect of upstream flow on the rotor performance are greatly necessitated.The blade structure determines the distribution of flow parameters around the rotor,and the flow in turn influences the performance of the rotor.Therefore,the analysis of such a relationship provides a reference for the optimization of the rotor.In this dissertation,Savonius hydrokinetic rotor was focused,and the advanced flow measurement and numerical simulation methods were used to investigate the influence of the rotation angle on the near-rotor flow,as well as the effect of the flow on the startup and operation performance of the rotor.Subsequently,under the condition of shear inflow,the flow characteristics and performance of the rotor were analyzed.The principle of Savonius hydrokinetic rotor is expected to be replenished and a sound reference will be provided for the optimization of the rotor.Major works and the conclusions drawn from the study are as follows:(1)Flow measurement was performed based on a circulation loop.Flow velocity distribution in the rotor wake was measured using the time-resolved particle image velocimetry(TR-PIV).The effects of the rotation angle and upstream flow velocity were considered.Flow characteristics near the rotor were described.The results show that the startup performance and energy conversion capability of the rotor are closely related to the surrounding flow.At different rotation angles,flow characteristics downstream of the rotor,especially the large-scale vortex structure,are significantly different.Irregular vortex shapes and vortex shedding promote the startup of the rotor at any initial rotation angle;however,it is thereby difficult for the rotor to maintain a relatively stable rotational speed during its operation.Based on the timeaveraged flow field near the rotor,it is inferable that the width of the rotor wake is determined by the rotor area facing the upstream flow.Similarity is observed between velocity distributions upstream and downstream of the rotor.As the upstream flow velocity increases,the vortex area and low-velocity zone downstream of the rotor shrink,and the vortex shedding frequency increases.(2)The commercial CFD software ANSYS Fluent was used to simulate threedimensional turbulent flows around the rotating rotor.The effects of uniform and shear upstream flows on the performance of the rotor were comparatively analyzed.Flow characteristics of the rotor under different flow conditions are described.The relationship between torque output and flow is analyzed.The results show that the upstream flow of the rotor inclines downward under the condition of shear upstream flow.As the rotor arrives at the vertical state,the influence of the shear upstream flow on the near-rotor flow is weak.As the rotor approaches the horizontal configuration,the vortex structure near the rotor is greatly influenced by the shear effect of the upstream flow.The variation of the torque coefficient of the rotor with the rotation angle is featured by a butterfly pattern as the rotor faces low-velocity upstream flow.As the shear coefficient increases,the variation of the torque coefficient is delayed.The rotation angles where critical torque coefficients are obtained are similar under different upstream flow conditions,and the pressure distributions near the rotor are analogous as well.(3)Under the same condition of shear upstream flow,flows around the rotor were simulated at different tip speed ratios,which were accomplished through regulating the rotational speed of the rotor.Variations of the torque performance of the rotor with the tip speed ratio were explained.Flow characteristics of the rotor at the selected tip speed ratios were described.The results indicate that vortices in the rotor wake alter apparently as the tip speed ratio varies,which imposes an immediate effect on the wake flow characteristics of the rotor.At a tip speed ratio of 0.5,variation of the torque coefficient is smooth,which enables a high average rate of energy utilization and a stable power output.At a tip speed ratio of 0.7,flow patterns near the rotor are appreciably influenced by the rotation of the rotor,and the vortex structures involved are considerably deformed.