Design And Characteristics Study On Hydrodynamic Performances Of Horizontal Axis Tidal Turbine's Blade

Considering great value of development and utilization of tidal energy,among various types of design technology of ocean tidal conversion device in foreign countries,as the representative of the key part of horizontal axis turbine blade,the efficient design technology of all kinds of turbine blades have become undisclosed core technology.Design of horizontal axis tidal turbine's(HATT)blade is not a single technology which is closely related to the overall structure form,operating strategy,rotor's pitch mechanism,diffuser device,carrier form,and especially the marine environment.Therefore,only depended on the single parameter Cp(energy efficiency),the Glauert design method for blade design has been unable to meet the needs of engineering design.In order to solve this problem,a new method is developed to design on the blade of the horizontal axis tidal current turbine based on BEM.The method focuses on how to satisfy the extra design constraints while maintaining high Cp.In addition,this paper used CFD(Computational Fluid Dynamics)to analyze the flow field,which made the blade has a strong ability to adapt to the environment and improved the the applicability of the new design method.First,in order to obtain the precise shape line of HATT's blade,the accuracy of prediction model of rotor's performance and load is obviously very important.This paper expounded the blade element momentum theory(BEM)and its correction method.The load characteristics of the blade was analyzed in detail by using the theory of BEM at different TSR and positon along the radius.The effect of the grid quality and quantity,turbulence model,boundary conditions and reference system was discussed.According to the comparition with the model test,the results showed that SST turbulence model could obtain the accurate load state when the blade attack angle is large.Under high quality grid conditions and the wall parameter y~+<2,it would meet the grid independence condition when the grid number of blade region and whole region was controlled respectively in 1 million and 2 millions.The steady state had a close result to the transient state.So it was proposed to used the steady state simulation for its much less time cost.Secondly,targeting the designation and optimization of HATT's blade,a contour-map method is developed to design on the blade of the horizontal axis tidal current turbine based on BEM.The method could ignore the assumptions in Glauert method by the different given initial parameters and the iterative sequence.Although the equation is more complex,depending on calculation code,the numerical solution of equations can be obtained more quickly.Through a preliminary calculation example of the N3L55 blade,the result was compared with the original Glauert design method.The results showed that the contour-map method has a more accurate shape line.Based on the contour-map method,the additional optimizing conditions were considerd using its own algorithm,and it successfully achieved the goal of optimizing.In addition,this paper has used the contour-map method to design blades N2L55.The CFD method was used to analyze the N2L55's hydrodynamic performance compared with the N3L55 blade.The results showed the N2L55's Cp reduced 3%and has an equivalent Ct to the N3L55's.Furthermore,through the compariton of model test and CFD method,a detailed analysis of Cp,thust coefficient Ct and self-start velocity at different blade pitch angle of blade was carried out.It showed that more the pitch angle of blade were,the top value of Cp and Ct smaller.Also the self-start velocity was reduced.At present,one of the main problems in the design of HATT's blade is the contradiction between the unidirectional rotor and the direction switching mode.This paper proposed a simplified blade profile aims on adapting bidirectional tidal power which is unneccesary to have any direction switching mechanism primarily depends on the symmetrical shape line.The design results of SN2L5 was presented in this paper using the courtor-map method and validated by the CFD simulations.The results showed that Cp's(power coefficient)top value is up to32.4%.Finally,aimed at the influence of the operating environment to design parameters,based on CFD simulation technology,the hydrodynamic characteristics of the rotor in diffuser,the scale effect,and the performance under the cavitation was analyzed,which is difficult to be calculated,measured and observed with the BEM theory and model test.The performance of a diffuser turbine is numerically investigated by CFD.This approach was validated by comparing the simulation results with experimental data.A good qualitative match between the experimental and simulation results was observed,indicating that the CFD approach is feasible and reasonable.Based on the velocity and pressure distribution analysis,two main reasons of the power increase of turbine with diffuser were proposed.According to the similarity theory,a series of comparition were carried out to investigate the effects of Reynolds number which contains the model with the same scale under different flow and others with different scale under the same flow.The comparition results indicated that:1)changes in the Reynold numbers no matter whether the flow or blade scale,it would have an impact on the dynamic characteristics of water turbine,and the influence law is consistent;2)As Reynolds numbers increased,the torque and thrust of the blade would increase too,so that the Cp can be improved.With the increase of Reynolds number,the trend decreased;3)as Reynold numbers became larger,the Cp peak TSR(Tip speed ratio)decreased,but as the Reynolds number increased,the offset of TSR decreased gradually.Therefore,the effect of Reynolds number should be fully considered to the prototype performance prediction when based on the model test.At the same time,the blade design,the proper airfoil performance data should be choosed at operating Reynolds number in order to ensure the accuracy of blade design.A research on the cavitation which probablly occurs on the blade surface when the Cp had a sharp decline at the high flow velocity was carried in detail.How the performance of the rotor would vary when cavitation occurred on the blade surface was investigate using the CFD technology.Through the principle of cavitation of HATT's blade,the conditions of avoiding the occurrence of cavitation is derived,and effective suggestions that the relationship among TSR,flow velocity and the immersion depth should be particularly considered were given to the design of the blade.