The Research Of Foil Optimization And Flow Visualization Of Counter-rotating Tidal Turbine

Tidal current is a new type of renewable and clean energy,which has been playing an increasingly important role in modern civilization in order to cope with the increasingly serious environmental and resource issues and promot sustainable development of social economy.The horizontal axis tidal turbine is one of the most effective methods of tidal current energy extraction,and has attracted widespread attention of scholars all over the globe.In this thesis,a brand-new counter-rotating type horizontal-axis tidal turbine was presented as the research object,and the design and optimization process was discussed from aspects of airfoil,blade element and impeller.Then the turbine working performance and flow characteristics of the near wake region were investigated by wind tunnel experiment,water tunnel experiment and flow visualization experiment.The research can be divided into the following aspects:1.A new strategy of airfoil parametric modeling and multi-objective optimization was proposed.The modeling strategy was constructed based on Bezier curve method and realized by coupling interface initialization and single objective optimization,in order to motivate the fitting curve to rapidly approach the original airfoil.The fitting error is less than 0.02%and the efficiency and accuracy are both guaranteed.The NSGAII algorithm was coupled with XFOIL program to optimize the airfoil performance under all operating conditions.The variance of lift drag ratio was introduced into the objective function to realize the differentiation and control of the optimization direction.The whole process of modeling and optimization was demonstrated in terms of airfoil E387,and the results were evaluated in detail.2.Based on the airfoil study,the basic principles and strategies of impeller design were discussed from one-dimensional momentum theory to the blade element momentum theory.The structural characteristics and performance advantages of the counter-rotating type tidal turbine were then introduced and the design method was instructed.The design process from airfoil to blade element and impeller was completed,which serves as a foundation for subsequent experimental research.3.The unique test platform for counter-rotating impellers was proposed and introduced in detail,and the performance of and traditional single impeller was compared with the counter-rotating ones.The effect of blade setting angle and axial distance on the working performance(peak value of power coefficient,high efficiency etc.)was also investigated.The experimental results show that the counter-rotating impeller posseses better performance than the traditional one.The existence of rear impeller can effectively enhance the efficiency of energy acquisition and improve the working performance.The peak value of performance and the high efficiency range can be influenced by the adjustment of the blade setting angles of front and rear impeller.The effect of axial distance is significant in the interval of middle and high blade tip speed ratio,but the effect becomes quite faint in the interval of low tip speed ratio.The comparison between the results from water tunnel and wind tunnel experiment indicate that the performance of the counter-rotating impellers have similar characteristics in those two kinds of experimental mediums.4.The 3-D flow visualization system was construced on the basis of performance test platform for counter-rotating type impellers,and the axial,radial and tangential velocities in the near wake region of both traditional single impeller and counter-rotating type impellers were measured by PIV phase-free technique.The phenomenons of near wake velocity loss and wake rotation absorption of the rear impeller were verified from the distribution analysis of axial and tangential velocity in certain criticle positions.The rotational velocity in the near wake region of rear impeller is eliminated not only under the optimal operating condition but also under conditions of low power coefficient.The structure of the tip vortex tube was successfully captured by PIV phase-locked technique,and its shape,strength and development were studied in great detail.For the counter-rotating type impellers,the radial motion of the vortex tube from the front impeller is enhanced by the existence of the rear impeller,which means the hydraulic diameter of the vortex tube is increased.The hydraulic pitch of the vortex tube is shortened with the increase of tip speed ratio.Proper increase of tip speed ratio and axial distance is beneficial to vortex strength restraining.The research contents of this thesis can serve as references for impeller design and optimization for counter-rotating horizontal axis tidal turbine.