Research On Full Field Flow And Similarity In Bidirectional Tidal Power Turbines With Upstream And Downstream Reservoirs

Tidal energy is one of the renewable energies and an abundant energy source in the world,of which the power station has little influence on the ecological environment and huge potential.The tidal power characterized by low energy density sources relative to project costs.Therefore,it should be commercially developed by accurately and reliably predicting unit performances and other related indicators.The latest research shows that there are relatively large difference between the hydraulic performance parameters,predicted by the traditional numerical simulation method,the model test and the empirical formula for scale effects,and ones of the site test results for the prototype unit.Tidal power turbines commonly operate at ultra-low operating head with rapid water level changes in the wide range,and complex two-phase flows of the water and the air in reservoirs at turbine upstream and downstream would influence unit performances.Therefore,research in ultra-low head tidal power units are very important on predicting accurately operating performances and on establishing a suitable numerical model in ultra-low head tidal power units.The present dissertation conducted the performance analysis including free-surface flows,approach flow characteristics in reservoirs as well as cavitation in turbine runner for tidal power turbines in ultra-low head turbines.A suitable numerical model for predicting internal flow fields in turbine was proposed based on two-phase threecomponent flow characteristics(water-air-vapor),which was established with turbine upstream and downstream reservoir models.It would provide a solution for the low accuracy of predicting prototype hydraulic performances influenced by complicated inflow and outflow conditions in ultra-low head turbines.The proposed numerical model has been verified by comparison in prototype unit output powers between the simulation and side test results.The change patterns in prototype hydraulic performances including cavitation at different water levels were analyzed under the constant gross head condition.The effect of turbine inflow condition,on the internal flow characteristics and the hydraulic performances,was investigated in bidirectional tidal power turbines utilizing ultra-low head hydraulic resources.The geometrical factors of upstream reservoirs influenced air-water surface vortex flows in reservoir and turbine intake flows.The turbine inflow conditions had an effect on the non-uniform and asymmetric flow characteristics,the air contents in water and the hydraulic losses in the intake channel.It was found out that the loss coefficient was relatively large in comparison to the turbine operating head,for low-head high specific speed units,which significantly influenced turbine output powers,efficiencies and cavitation performances.It was found that the hydraulic similarity has not been well established between prototype and model units for ultra-low hydraulic turbines,in contrast with typical hydraulic turbine units.Low-head unit was influenced by scale effects on the Reynolds number and the Froude number.It is recommended that the model test equipment should be designed with upstream and downstream water tanks under the geometric similarity to reservoirs for prototype units,as much as possible,to reduce errors for predicting the intake vortex and flows in reservoir.