| Vertical axis tidal current generator is one of the important forms of developing ocean energy,which has been widely studied and applied in tidal current energy development in recent years.The Darrieus vertical axis tidal current energy turbine studied in this paper depends on its unique geometric structure and blade airfoil,and the blades rotate with the lift generated by different blade angles of attack at different phase angles,so it is also called lift turbine.In this paper,the hydrodynamic performance of vertical axis tidal current hydraulic turbine is studied by numerical calculation,and its energy conversion process and flow characteristics are discussed,the influence of low inflow velocity on the energy performance and flow characteristics of hydraulic turbine is analyzed,which provides guidance for the optimal design and application of Darrieus hydraulic turbine.Firstly,the theoretical basis and numerical calculation method of vertical axis tidal current energy turbine are introduced,and the calculation physical model of vertical axis tidal current energy turbine is established and the flow area grid is divided.The experimental results verify the correctness of the calculation method.The energy and flow characteristics of vertical axis tidal current turbine are studied and analyzed.It is found that the energy performance of the turbine has strong time series fluctuation,and the power output coefficient is close to sinusoidal variation at high tip speed ratio,while the power output coefficient of a single blade is mainly divided into two stages,a peak stage in the upstream region of the blade and a fluctuation stage in the downstream region.The distribution of internal flow field is discussed,and the vorticity transport equation is introduced to analyze the coherent structure of vortex.It is found that vortex is mainly driven by relative stretching term It is found that the radial force of the hydraulic turbine is not completely symmetrical,and there is an offset at 210°,which is due to the radial force caused by a certain tensile force caused by the vortex shedding on the blade surface.The existence of blade surface vortex and tip vortex has certain influence on blade surface pressure,and the shedding of blade surface vortex causes instability of turbine wake flow,and this phenomenon has a strong correlation with impeller tip speed ratio,which is caused by stall characteristics of blade airfoil.Secondly,the energy performance and flow characteristics of hydraulic turbine at low inflow velocity are analyzed.It is found that the decrease of inflow velocity mainly affects the peak value of instantaneous power coefficient of single blade of hydraulic turbine,and the maximum peak value of blade will also shift to low phase angle.At the same time,it is found that the decrease of inflow velocity will reduce the vorticity intensity in the turbine and make the flow field in the turbine become more stable.Finally,the influence of turbine blade parameters on energy performance and flow state is discussed,and the main factors affecting turbine operation are determined by orthogonal experiment.Based on the theory of vorticity,the optimization and redesign are carried out,and the influence of different geometric structures on the performance and flow field of the turbine is analyzed and discussed.The vortex suppression structure at the tip of the blade is put forward to further improve the performance of the turbine,improve the internal flow state of the turbine,reduce the secondary effect and make it have higher energy conversion efficiency. |
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