With the development of large-scale tidal current turbines and the increase of tidal current velocity, the possibility of cavitation increases. Furthermore, unsteady cavitation is a complicated multiphase flow that causes the power degradation of tidal current turbine blade. There has been no comprehensive investigation of it so far. In this study, the blade captured power is obtained at different cavitation numbers using the Schnerr-Sauer cavitation model. The numerical uncertainty for the mesh and the time step is calculated by the GCI method. It has been shown that, when the cavitation number is 5 and 2, cavitation has no effect on the blade power. With the decrease of the cavitation number, the rise in cavitation intensity occurs when the vapor distribution area stretches from the blade tip to the blade root and from the leading edge to the trailing edge, respectively. With a fall in cavitation number to 1.3, the vapor volume fraction rises, and the viscosity of the mixed phase reduces, resulting in a reduction in viscous power. When the cavitation number is 0.8, there exists a larger region in which an absolute value of minimum pressure coefficient is less than the cavitation number, a smaller blade load is present, and the pressure difference power is substantially decreased. Because of the huge inverse pressure gradient created by cavitation, the negative pressure difference power is generated, resulting in a decline of the blade power coefficient to 14%, when the cavitation number is 0.5.
The rivers connecting oceans generally carry sediment due to water and soil losses in China. Additionally, the maximum sediment concentration is 300 g/L, which is much higher than that of other countries. It is unknown whether seawater with sand particles will affect the power of tidal current turbine blades. It is therefore necessary to study the capture power of tidal current turbines in the water-sediment environment. In this study, the blade was divided into a number of transversal airfoil elements based on the blade element theory. The CFD-DPM model was employed to study the lift and drag coefficients of airfoil under multiphase flow, and the fluid–particle interaction was considered. The accuracy of this presented model was assessed using the experimental data of a 120 kW tidal current turbine in a water-sediment environment. Good agreement between the predictions and experimental data was observed. The effect of particle properties on the lift coefficient and the drag coefficient of airfoil were investigated in detail. Furthermore, the 120 kW tidal current turbine power was calculated based on the Blade Element Momentum theory under different particle concentrations. The results show that small diameter particles can improve the tidal current turbine power and the large diameter particle can reduce the power.
The blade erosion of tidal current turbine can lead to significant energy losses and affect stall behavior. To maintain good performance and prevent turbine malfunction, it is important to determine the location and rate of blade erosion caused by particle impact. In this study, a computational fluid dynamics and discrete phase model (CFD-DPM) method is employed to study the erosion characteristics in blades subjected to multiphase flow. The fluid-particle interactions and the influence of turbulence on the particle trajectories are considered in the CFD-DPM model. The maximum erosion location and the average erosion rate are investigated under different particle diameters, particle concentrations, particle shape factors and airfoil parameters. The fluid velocity, particle velocity and particle trajectory are further analyzed to reveal the erosion mechanism under different influencing factors. The results show that while both the maximum erosion location and the average erosion rate depend upon particle independence (the greater the degree to which particles deviate from the fluid streamline, the better the particle independence), the latter is also related to the drag force exerted by the particle. For tidal current turbine, erosion occurs first at the tip and leading edge of the blade, and the most severe erosion area is the blade tip. The erosion laws obtained in this work can provide guidance for erosion prediction, tidal current turbine field site selection and blade optimization.
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