Numerical Simulation And Performance Prediction Of Three-Dimensional Turbulent Flow In Francis Turbine

With the development of large-sized hydraulic turbine in recent years, high energy performance good cavitation performance and stability of turbine are more important then ever. Considering the achievement aboard on the design of high performance hydraulic turbine, CFD is an important design approach to obtain accurate and detailed data of inner flow. Therefore, it is significant to totally understand the inner flow of all passage components to predict the performance of hydraulic turbine and improve its design.The inner flow of turbine is 3D viscid, incompressible and unsteady turbulent flow. Based on the 3D time-average Navier-Stokes equations, standard k-e model and RNG k-ε model are used to simulate the 3D turbulent flow field of whole flow passage in the Francis turbine including guid wickets, runner and draft tube.The energe performation and cavitation performation are calculated from the simulation results. The reasonable pressure and flow velocity are drawn from the results of combined calculation of the spiral case, stay ring and guide vane. The value of angular moment distributing in the whole spiral case is good coherent with the value of the design theory, VuR = Const. The calculation results also show that the assumption of the velocity is acceptable which radial velocity is uniform along the circumference direction at the outlet of the spiral case. The flow direction at the outlet of the stay ring provides the suggustion for the hydraulic design of stay ring. The flow direction at the outlet of guid vanes is used for 3D turbulence calculation of the runner.The hydraulic loss in guid wickets is also calculated and analyzed in this paper. The rusult of the simulation shows that computation precision increases with the grid density, especially with the grid desity around the leading edge of the blade. Secondly, the inlet boundary condition is very important.The hydraulic losses equal about the experience data when pressure outlet boundary condition is applied under the acceptable grid density, local grid refine and proper compute parameter. Thirdly, the outlet position of computational region has low influence on the hydraulic loss in the guid wickets.Comparing the calculation results of 3D steady turbulent flow in single flow passage of the runner model with that in the combination computation of runner and draft tube, both of computation methods can draw the detail flow field, but the predicted effiency of hydraulic turbine matches with the experiment value in single flow passage analysis. Under the limited computing resource, a method that combines the whole passage computation with single passage computation for a model runner is suggested. This method predicts well the efficiecy of hydraulic turbine. The accuracy could be 0.9%. In addition of this, this method is feasible. It provides the valuable reference for the prediction of the energe performation in the Francis turbine.According to the results of 3D turbulent simulation in the single flow passage, the initial cavitation coefficient is calculated. The computing results agree with the experiment critical cavitation coefficient.The result of this study has significant effect tu direct simulation of 3D flow in hydrnulic turbine and to predict the hydraulic performation of turbines.