Analysis Of The Hydrodynamic Performance Of Shaftless Turbines Based On S-Wing Type

Tidal power generation devices generate electricity by capturing the kinetic energy of seawater through the impeller of a hydraulic turbine in a cyclical reciprocal manner.The current mainstream power generation device is a horizontal shaft turbine with a guide tube,but due to the gap pressure flow between the guide tube and the impeller blade tip,the rotating shaft hub has no effective working area,which greatly affects the efficiency of hydrokinetic energy capture under low flow conditions.In order to improve the energy capture efficiency of the turbine,this thesis innovatively proposes a shaftless impeller solution with the blade root placed in the slide groove of the guide tube wall,the blade tip pointing to the axis and no hub,which structurally expands the effective working area of the blade and circumvents the back pressure turbulence resistance generated by the interstitial pressure flow.To match the shaftless impeller,the design and optimisation of the blade is based on the relationship between lift,drag and kinetic energy capture efficiency of the airfoil at low flow conditions.The NACA4415 was selected as the base airfoil for the shaftless impeller by combining the basic theoretical calculations of fluid dynamics with an existing commercial airfoil library.A multi-objective genetic algorithm is used to optimise the airfoil lift and drag parameters by setting coordinate control points on the airfoil surface.The velocity and pressure of the airfoil in the same flow field with different angles of attack are simulated using the SST turbulence model and the"parametric scan"method.The results show that the optimized airfoil improves the lift-to-drag ratio by 4.5%,10.8%and 22%for the angles of attack=1°、4°、13°respectively.In order to meet the operating conditions of reciprocal impact of seawater currents,a centrosymmetric structure of S-airfoil is proposed on the basis of the initial optimised airfoil to achieve bi-directional hydrodynamic energy capture and a reasonable layout of the blade pressure centre,and the optimised design of S-airfoil with 12.7°angle of attack is obtained by using numerical simulation and vector equation graphical solution.The shaftless impeller uses an odd number of blades to ensure the rotational dynamic balance performance.According to the design requirements and working conditions,a 5-bladed S-wing shaftless impeller is modelled in 3D and a control simulation study is conducted on a shafted impeller of the same diameter.The test results show that the same size shaftless impeller has a higher energy gain coefficient and lower axial thrust at the same tip speed ratio;and the energy gain coefficient increases and the thrust coefficient decreases as the shaft diameter ratio increases.The impeller flow field velocity results show that the shaftless impeller eliminates the downstream low-speed rotating basin compared to the shafted impeller,which plays a role in recovering the rotating wake energy and further improves the hydrokinetic energy capture efficiency.The S-wing optimization research scheme based on the shaftless impeller proposed in this paper provides a new solution to the energy conversion efficiency and energy conversion stability of tidal power generation devices,provides new research ideas for subsequent engineering experiments on the shaftless structure to solve problems such as the gap pressure flow of tidal energy turbines,has a certain reference value for the new structure of tidal energy generation devices and blade design methods,and provides a research basis for promoting the practicalization of marine energy generation.