Research And Experiments On The Hydrodynamic Performance Of Vertical Axis Tidal Current Turbine

The international community is actively seeking alternative energy sources due to concerns about pollution and cliamate change caused by fossil fuel consumption.Compared with other renewable energy resources,tidal current energy has the comparative advantages of predictable,low visual impact and low environmental impact.Tidal current turbine is the device extracting kinetic energy from tidal currents.According to their structural form,tidal current turbines can be categorized as vertical axis or horizontal axis.Compared with horizontal axis turbines,vertical axis turbines have some unique advantages such as working independently of current direction,more accessible maintenance,little effect on the turbine's flow field from generator,low noise and little cavitation.However there are still some problems that hinder their application for example,poor self-starting performance,relatively low energy efficiency and torque pulsation.According to the vertical axis turbine's disadvantages of relatively low energy efficiency,poor self-starting performance and torque pulsation,some research have been made on the tip vortex of turbine blades,the arrangement of turbine blades and the diffuser for vertical axis turbines.Some research results were adopted in the 15 kW vertical axis tidal current energy conversion device and good results were obtained from the field test.In this paper,the main content is as follows:The mechanism and influencing factors of the turbine blade's tip vortex were studied by three-dimensional airfoil theory.The method of adding end-plates to the tip of blade were proposed to block the tip vortex.The hydrodynamic characteristics of three-blade turbine were studied by numerical(Ansys Fluent software)and experimental methods.According to the influence area of tip flow,the shape of end-plate was designed and the effectiveness of adding this end-plate was verified.To further enhance the output of the turbine,the shape of end-plate was cut half on the basis of the original end-plate to reduce the drag and regulate the pressure distribution between the two sides of blade.The performance of the turbine with inward-half end-plate and outward-half end-plate was studied by numerical method.The output characteristic and mechanism of the blade with inward-half and outward-half end-plates were analyzed.The change of single-layer-blade turbine's(three-blade,four-blade and six-blade turbine)energy efficiency(Cp)with tip speed ratio(TSR)was studied by experimental method.By calculating the starting torque of the turbine in different stopping positions,the self-starting performance of the turbine was analyzed.The output characteristic of double-layer six-blade turbine with different shift angles and inner radius was studied by the coupled numerical method,and their self-starting performances and load characteristic were also analyzed.The self-starting performance,energy efficiency and output characteristic of the coaxial dual rotor turbine with different shift angles were studied,and were compared with the three-blade turbine of same span.According to the performance of different forms of turbine,their scopes of application were analyzed.A series of diffusers of simple structure,easily building and being available in reciprocating flow were studied in flume by experimental and numerical methods.The two-dimensional and three-dimensional numerical simulations of the diffliser in open water were also conducted to obtain the general result of its flow augmented effect.Based on the three-dimensional results in open water,the influence of the diffuser plate's length and angle on the diffuser's flow augmented effect in different flow directions were analyzed.The axial force on diffuser was also studied.The change of the three-blade turbine's tip speed ratio range,energy efficiency,load characteristic and output characteristic were studied when the turbine was with and without diffuser by the coupled numerical method.The influence of the diffuser plate's length and angle on the turbine's performance was also analyzed.A field test was carried out for a 15 kW vertical axis tidal current energy conversion device with coaxial dual rotor turbine of 60° shift angle.The range of tip speed tatio,the optimum tip speed tatio,the average power efficiency,generated energy and capacity factor of one flood tide and ebb tide,the distuibution of self-starting velocity and the change of system efficiency with current velocity were analyzed according to the data of field test.In the later part of the field test a diffuser was installed on one side along the turbine's span.The change of the device's performanc after installing the diffuser was analyzed.The influence of side walls and piles on the turbine's performance was studied by two-dimensional numerical method and the velocimeter's diviation caused by the turine's induced flow was also studied.The wave action and adhesion of marine organism can be the threat to the energy conversion device.The pulsation of current velocity and the failure of load regulation on turbine may cause the stop of the device.These should be noted in the future design and operation of tidal current power generation project.