| The depletion of fossil energy and the resulting environmental pollution problems motivate the pursuit of new environmentally friendly alternative energy sources.Ocean energy is an important alternative clean energy source in addition to wind energy,solar energy and biomass energy.Research on wave energy and tidal energy has been the main form of ocean energy development.Tidal energy has great development potential due to its regular occurrence cycle and large overall reserves across the globe.Many countries have built experimental tidal energy power stations and proved their feasibility for development.Some countries,such as UK and Korea,have also formulated future usage goals and policies to support the marketization of tidal energy development.The horizontal axis tidal turbine is the main form for developing tidal energy.In this paper,3 blades for counter-rotating horizontal-axis tidal turbines(CRHATT)are designed based on blade element momentum theory.The steady numerical simulation method is used to study the parameters that can affect the energy harvesting performance and energy output performance of the CRHATT.Firstly,the flow field characteristics of the CRHATT and the single-rotor horizontal axis tidal turbine are compared.The results show that the wake vortex region of the CRHATT is shorter,and the fluid in the wake vortex region is more ordered than that of the single-rotor horizontal axis tidal energy turbines.The optimal distance between the dual rotors that can maximize power coefficient(C_p)of the CRHATT is found under various blade chord combination of the dual rotors.Under various distance between the dual rotors,the feasibility of the method for optimizing the C_p difference between the dual rotors by optimizing blade chord combination to change solidity ratio between the dual rotors was verified.The optimal blade chord for the dual rotors is found.The change rule of the C_pfor the CRHATT within the effective tip speed ratio range is also determined.The steady numerical simulation results of the CRHATT provide technical reference and support for the engineering application and design of the horizontal axis tidal turbine.Then based on the measured tidal data of the marine near the southern part of Zhoushan Island,the working characteristics of the CRHATT is analyzed by time-domain characteristic numerical simulation.The existence of the optimal distance between the dual rotors and the optimal blade chord combination between the dual rotors is also verified.The results show the optimal distance and optimal blade chord combination is consistent with the steady numerical simulation results.Through the analysis of the degree of dispersion of the power coefficient of the CRHATT in 5 cycles,the stability of the turbine under conditions close to the real marine conditions is verified by the analysis of the dispersion of the C_p in 5 times domain characteristic periods.The study on the working characteristics of the CRHATT under the time domain characteristics provides a reference for the future marine experiment.Meanwhile,a new research process is proposed for the research of the horizontal-axis tidal turbine.A mathematical model for predicting the output power of the horizontal-axis tidal turbine generator working in real situation is proposed and verified.The mathematical model for predicting output power is mainly based on flow filed parameters,the blade parameters of the turbine and the C_pobtained by CFD.The implement of the verification via the flexible direct-drive horizontal axis tidal turbine(FHATT),which is designed by the research group before.Firstly,a numerical simulation is carried out based on the measured parameters of the drag pool of the FHATT.Then the calculated output power can be obtained by putting the numerical simulation results,blade parameters and flow field parameters,the energy loss of the transmission structure and the output efficiency of the generator into the mathematical model.The results show that the average deviation between the calculated output power and the measured real output power is 17.47%.The average deviation can be introduced into the mathematical model as a correction factor,so that to improve the correction of the mathematical model.Then the sliced blade of the rotor of FHATT is tested by numerical simulation to analyze the torque coefficient axial distribution along the incoming flow direction.The reliability of the blade designed for working under two-way flow conditions can be verified by the test of the sliced blade.The proposed mathematical model for estimating the calculated output power by flow field parameters and blade parameters can facilitate the work prediction of tidal turbines and provides a reference for the engineering design of tidal turbines. |
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