Study On Startup Process And Performance Optimization Of The Drag-type Hydraulic Rotor

Energy exploration and utilization are of significance for the economic development.With gradual depletion of fossil fuels and increasingly aggravated environmental issues,it is urgent to develop clean and renewable energies.More than71% of the earth’s surface is covered by water and water energy is undoubtedly a green energy with huge reserves.Savonius rotor is a typical drag-type rotor and is featured by simple structure,low fabrication cost and reliable operation.Moreover,this rotor can operate without the assistance of large dams and is therefore feasible for various occasions such as rivers,ocean currents and canals.The configuration of this verticalaxis rotor facilitates the installation and maintenance of the generator module.Meanwhile,the operation of the rotor is insensitive to the direction of the incoming flow,which avoids the employment of a direction-adjusting device.However,the startup performance of the traditional Savonius rotor is greatly affected by the initial azimuthal angle,and the rotor cannot start up at certain azimuthal angles.In tides or currents,Savonius rotor is required to frequently start up and halt.If the rotor cannot start up rapidly,it is necessary to install auxiliary equipment,which will undoubtedly increase the complexity and failure rate of the unit.The present thesis focuses on the startup performance of the drag-type rotor.An optimization scheme will be proposed to cope with the shortages of current rotors;then the influence of the improvement of the startup performance on the energy-utilization efficiency of the rotor will be evaluated.In this dissertation,the operational principle of the drag-type rotor and the forces acting on the rotor blades were analyzed,and the equations governing the rotation of the rotor were constructed.The six degree of freedom(SDOF)solver was used to model the rotation of the rotor driven by the impact of water flow.Based on a water circulation loop,the rotor performance test was performed,and the near-rotor flow was measured using time-resolved particle image velocimetry(TR-PIV).The physical validity of the numerical scheme was proved through the experimental results.The difference between the assumptions of the flow-driven rotation and constant rotational speed was investigated.It is demonstrated that the assumption of constant rotational speed leads to an overestimation of the rotor performance.The startup capability of the rotor was predicted based on the static performance of the rotor.Then the dynamic startup process of current drag-type rotor was studied.The flow patterns corresponding to the static states were analyzed.The transient startup processes at different initial azimuthal angles were inspected.The results show that current rotor has low startup performance due to a wide range of negative static torque coefficient.The ranges of the initial azimuthal angles corresponding to failed startup contain the static dead zone and the dynamic dead zone.The failed startup processes share the same static equilibrium point at which the rotor eventually stops.For the successful startup,the startup process can be divided into initial acceleration stage and transition stage,according to the variation of the angular velocity.The flow transition from the static to the dynamic state is featured by the attenuation of large-scale vortices in the low-velocity wake of the rotor.To eliminate the dead zone,the blade clearance was specifically selected,and the performance of the rotor was evaluated comprehensively.The results show that the static torque coefficient of the optimized rotor is larger and remains positive,the output torque is larger,the operational efficiency is higher,and the fluctuation of the angular velocity is lower.After optimization,the startup time of the rotor extends linearly with increasing load coefficient.The larger the load coefficient,the less apparent the transition stage during the startup process,and the lower the average rotational velocity after startup is finished,and the larger the amplitude and wavelength of the curve of the angular velocity.As the rotor rotates at low load coefficients,the pressure distribution over the backflow side of the blade is rather uniform,and the resultant force acting on the rotor depends mainly on the pressure distribution over the blade side that faces the incoming water flow.However,at high load coefficients,high-intensity vortices appear on the back of the advancing blade,and corresponding local pressure distribution over the advancing blade promotes the rotation the rotor.