Numerical study on energy-converging efficiency of the ducts of vertical axis tidal current turbine in restricted water

Abstract

Installation of the channeling device around a tidal turbine can stabilize flow field, increase flow velocity and improve energy utilization efficiency significantly. The application of channeling devices on improving the efficiency of vertical axis tidal turbines in restricted water has hardly been investigated. In the present study, two ducts with different inside wall shapes have been taken as examples to investigate the bank and free surface effects on energy-converging efficiency of the ducts around a vertical axis tidal current turbine in shallow and narrow water. The three dimensional viscous flow field around the ducts is simulated by using CFD technology. RANS equations are employed as viscous-flow solvers and the realizable two-layer k-ε turbulence model is applied to predict the energy-converging efficiency of the ducts. Investigation of grid dependence is conducted and grid independent solution is obtained by analyzing the numerical results of several sets of computational grids. For the duct around the vertical axis tidal current turbine near or piercing the free-surface of the water in some cases, Volume of Fluid (VOF) method is employed to capture the free surface. Based on the numerical results, it is known that the average energy flux density in W02 model is much larger than that in W01 model, so it can be concluded that the inside wall shape of the ducts dominates the energy-converging efficiency of the ducts in great extent and bank effect on energy-converging efficiency of the duct varies with inside wall shapes of the duct. What's more, the free surface plays an important role in the energy-converging efficiency of the ducts, and smaller clearance between free surface and the top of duct will induce higher energy-converging efficiency.