Investigation On The Flow Mechanism And Performance Optimization Of Savonius Hydrokinetic Rotor

The Savonius hydrokinetic rotor is a typical drag-type rotor and can effectively absorb the kinetic energy of the water flowing at low velocities.In recent years,the drag-type hydrokinetic rotor has gained keen interests in application occasions such as river,ocean,and pipe system due to its simple structure,low maintenance cost,good startup ability and little impact on the ecological environment.However,the efficiency of the Savonius hydrokinetic rotor is low,which therefore hampers its wide application.Hitherto,a considerable amount of literature has been devoted to the performance optimization of the Savonius hydrokinetic rotor.Nevertheless,due to lack of the knowledge about the flow characteristics and operational mechanism of the drag-type rotor,the design method of the rotor still needs improvement.Moreover,few generalizable conclusions have been obtained from the study on the Savonius hydrokinetic rotor.In this context,a systematic investigation on the flow and operational characteristics of the Savonius hydrokinetic rotor is of significance.The flow characteristics of the rotor directly reflect the interaction between the incident flow and the rotor.A good understanding of the flow characteristics serves as the basis of the optimization of the rotor.Meanwhile,the operational performance of the rotor depends on the structure and parameters of the rotor,which are closely related to the near-rotor flow patterns as well.In the present study,the Savonius hydrokinetic rotor was studied,advanced non-invasive optical flow measurement and numerical techniques were used to plumb flow mechanisms of the rotor and to describe characteristic flow structures and unsteady flows.The operational performance of the Savonius hydrokinetic rotor was measured using a developed test bench.The relationship between typical flow structures and the performance of the rotor was to be established.An attempt was made to reveal energy conversion mechanisms of the rotor.Furthermore,an auxiliary device was installed upstream of rotor and corresponding operational performance and flow characteristics of the rotor were investigated.The effects of operational parameters on the performance of the rotor were analyzed.The study was expected to lend a support to the design and optimization of the rotor.Main works and conclusions of this dissertation are as follows:(1)Based on a circulation loop,the flow near the Savonius rotor was measured using time-resolved particle image velocimetry(TR-PIV).The rotor was fixed at different azimuthal angles and characteristic flow structures were described.The influence of upstream velocity on flow characteristics of the rotor was studied.The results indicate that the Coanda-like flow at the convex side of the advancing blade,the position and shape of large-scale vortices in the wake flow,as well as the width of the wake flow,vary considerably with the azimuthal angle and upstream velocity.(2)The effects of the blade shape on the performance of the rotor and flow characteristics were investigated using computational fluid dynamics(CFD)method.Three rotors with different blade shapes were proposed and their dynamic performance and corresponding flows were obtained numerically.Instantaneous flow structures near the rotor were analyzed and the mechanism of the influence of blade shape on the performance of the rotor was revealed.The results show that instantaneous torque coefficient of the rotor depends largely on the area and positions of high-pressure elements upstream of the rotor.Large-scale vortices in the wake of the rotor exhibit distinct unsteady characteristics,and their shapes and positions significantly influence instantaneous torque coefficient.As the blade shape factor increases,the concave side of the advancing blade shows improved capability of collecting the incident water and the rotor performance is improved as well.The rotor with the blade shape factor of 0.81 has the maximum power coefficient of 0.164,which is 58% higher than the rotor with the blade shape factor of 0.46.(3)For the in-line configuration of two rotors,the effect of downstream rotor on the upstream rotor was studied using both experimental and numerical methods.The flow patterns and the performance of the upstream rotor were investigated.The effects of the azimuthal angle of the downstream rotor,inter-rotor distance,and upstream flow velocity were considered,which enabled a comprehensive comparison between the results obtained under various operating conditions.The results indicate that large-scale vortices downstream of the upstream rotor vary distinctly with the azimuthal angle of the downstream rotor and the inter-rotor distance;the performance of the rotor varies as well.With increasing azimuthal angle of the downstream rotor,the influence of the downstream rotor on the wake flow and large-scale vortices is enhanced firstly and then weakened gradually.Meanwhile,the influence of the downstream rotor on the upstream rotor decays gradually as upstream velocity and inter-rotor distance increase.For the in-line configuration of two rotors,the inter-rotor distance larger than five times the rotor diameter is suggested.The maximum power coefficient is higher than that associated with an inter-rotor distance of three times the rotor diameter by about 8%.(4)To make up for the shortcomings of existing performance test bench for the drag-type hydrokinetic rotors,a high precision performance test device was designed,which can measure the rotational speed,instantaneous torque,and rotational angle of the rotor with high time resolution.Furthermore,an assistant deflector plate was proposed.The effects of the deflector plate on the performance and flow characteristics of the rotor were investigated experimentally.The instantaneous rotor performance and the flow near the rotor were measured synchronously.The effects of the setting angle of the deflector plate and upstream velocity on the performance of the rotor and the flow characteristics were analyzed.The results indicate that large setting angles of the deflector plate causes high rotor performance.The maximum power coefficient of the rotor reaches 0.463 at the setting angle of 60°,increasing by 148.9% relative to that at the setting angle of 30°.Meanwhile,the maximum power coefficient is higher than those documented in literature.The deflector plate not only prevents the impingement of the upstream flow on the returning blade,but also guides the upstream flow towards the concave side of the advancing blade.As the setting angle of the deflector plate increases from 30° to 60°,the guiding effect is enhanced.Under the influence of deflector plate,the power coefficient of the rotor increases with the upstream velocity.As the upstream velocity increases from 0.63 m/s to 0.73 m/s,the maximum power coefficient of the rotor increases by about 22.8%.