Performance Of Vertical Axis Tidal Turbine With Variable Pitching Blades

A vertical-axis tidal turbine gets the name for its shaft is perpendicular to the direction offlow.Its unique superiority made it occupied an extremely important position in tidal energy conversion devices.However,there are several shortcomings in practical engineering application,such as low efficiency,self-starting difficulty and poor reliability.To overcome the disadvantages of a vertical-axis turbine,this thesis is inspired by the variable pitching blade strategy comes from vertical-axis wind turbine,and applies it to the tidal turbine.By comparing with the performance of fixed blade turbine and variable pitch turbine,the optimized blade pitching control strategy is proposed finally.This thesis is mainly about optimizing the blade pitching control strategy by CFD numerical simulations.It is found that the hydrodynamic performance of the vertical-axis turbine with sinusoidal variation blades is obviously improved than that of fixed blade turbine.At first,the flow domain around vertical-axis turbine is divided into some sub-domains,ICEM software is used to discretize the domain.Around the blade,O-C blocks is adopted to generate 2d grids,which can effectively optimize the grids quality in the sharp area at the trailing edge of the blade.Sliding mesh movement is adopted between the grid areas.Considering the actual start-up process of the turbine,the UDF function is used at the entrance of the turbine flow field which the flow velocity increased from zero to the given constant value.A commercial CFD software Fluent is used,the shear pressure transfer(SST)k-ω turbulence model combined with Navier-Stokes equation is used to solve the problem.Then,according to the hydrodynamic performance of the vertical-axis turbine under different load coefficients,the optimal tip speed ratio of the maximum power generation efficiency is around 2.0.The results reveal that the hydrodynamic performance is improved by variable pitching because it postpone the occurrence of stall in the upstream half of the rotor.The maximum power generation efficiency of the variable pitching turbine is improved by 30% over the fixed blades turbine,and the fluctuation of the output power and rotational speed is also decreased.In this paper,the optimal amplitude of pitch range is 5°-10°。Based on the optimal variable pitch parameters obtained from numerical simulations,experiments are conducted in a circulating flow flume.The experiment models are fixed blades turbine,variable pitching blades turbine.The results show that the flow velocity of self-starting for variable blades is lower than the fixed blades turbine.The efficiency and torque coefficient of variable pitch blades turbine are higher than fixed blades turbine.