Numerical Simulation Of Unsteady Turbulent Flow In A Francis Hydro-turbine

As a core component of hydroelectric system, hydraulic turbine will be confronted with serious questions such as unsteady pressure pulsation, cavitation, silt abrasion and flow-induced vibration, which will influence hydraulic turbine work. The study of such problems has practical significance as well as theoretical values. Numerical simulations are an important tool to study the internal flow features for a Francis hydro-turbine. The dissertation mainly focuses on several technical problems which are affecting the high efficiency and the stable operation of hydraulic turbine by using the state-of-the-art numerical methods in modern computational fluid dynamics. The main work and results are stated as following.(1) Numerical simulation of three dimensional transient turbulent flow in the whole flow passage of a Francis hydro turbine based upon the Reynolds averaged Navier-Stokes equations was conducted with the large eddy simulation (LES) technique on Smargorinsky-Lilly model and sliding mesh technology. The steady flows simulated with the standard k-ε model were used as the initial conditions of the flow in the unsteady simulation. The large scale structures evolving in spatially and temporally were visualized by using unstructured hybrid-grid and PISO algorithm. The evolution details of the large scale structures such as eddy from generate to shedding in vane cascades and blade passages are well captured at special case. The results show that LES can simulate well transient turbulent flow and its evolution in a Francis hydro turbine with complex geometry. The computational method provides some reference for exploring mechanism of eddy formation in a complex turbulent of hydraulic machinery.(2) Finely numerical simulation of the three-dimension unsteady turbulent flows in whole flow passage of a Francis hydro-turbine based on the uncompressible viscous Navier-Stokes equations was conducted. The steady turbulent flow was firstly simulated with the standard k-ε model in different opening cases of the moving guide vanes and then used as the initial flow of the unsteady flow to be simulated. The detached eddy simulation (DES) and the sliding mesh technique were applied to the simulation of the unsteady turbulent flow. The flow structures in the turbine in the different opening cases were well captured, and the evolving of the large scale structures was finely shown spatially and temporally. The results show that DES is able to well simulate the unsteady turbulent flow and to obtain the finely dynamic eddy structures in a Francis hydro-turbine. The research relies on the understandings of the vibrating of a hydro-turbine due to the turbulent flow.(3) The numerical simulation of three dimensional cavitation turbulent flow in Francis hydro-turbine passage was conducted based on the mixture model for homogeneous multiphase flow hypothesis in the Euler-Euler approach and the Schnerr and Sauer cavitation models. The standard k-s model, SIMPLEC algorithm and multiple reference frame model were used. The distributions of the water-vapor volume fraction in blade passage and draft tube were obtained in special case. The position and degree of cavition in turbine passage were analyzed. The results show that the method can be used to simulate the3D cavitation turbulent flow and true multiphase flow with cavitation in Francis turbine than one-phase model. The calculation provides significant reference for revelation the feature of gas-liquid phase flow of hydraulic machinery and optimal design for the runner. The study is valuable for improving the cavitation of performance in Francis turbine.(4) The numerical simulation of three dimensional two phases turbulent flow in Francis hydro-turbine passage was conducted based on the mixture model for an algebraic slip formulation in the Euler-Euler approach. The standard k-s model, SIMPLEC algorithm and multiple reference frame model were used. The distributions of the silt volume fraction in blade passage and draft tube were obtained in special case. The position and degree of silt abrasion in turbine passage were analyzed. The results show that the method can be used to simulate the3D turbulent flow and true multiphase flow with silt abrasion in Francis turbine. The calculation provides significant reference for revelation the feature of solid-liquid phase flow of hydraulic machinery and improved hydraulic performance. The study is valuable for improving the abrasion of performance in Francis turbine.(5) The numerical simulation of two dimensional transient turbulent flow in a guide vane passage of wicket gate in a benchmark hydro-turbine was conducted based on the ALE(Arbitrary Lagrangian-Eulerian) and dynamic mesh technology. The dynamical changes of the pressure, velocity and turbulence characteristic are simulated, using unstructured-triangle grid, standard k-ε turbulence model and PISO algorithm as the guide vane is moving at linear and polyline closure. The results show that the evolution of the flow field is unsteady with decrease of the vane opening. Based on these researches, the eddy structures evolving spatially and temporally were visualized. The calculation provides some reference for vortex-induced vibration in a complex turbulent flow of hydraulic machinery. The study is valuable for simulating the turbulence induced by the guide vane adjustment.(6) The fine numerical simulation of three dimensional dynamic turbulent flow around one moving guide vane in a channel, linear and nonlinear oscillating cascade of the wicket gate of a benchmark hydro turbine unit was conducted based on the uncompressible viscous Navier-Stokes equations flow and immersed boundary method (IBM) for moving boundary. The dynamical subgrid stresses (SGS) on Smagorinsky-Lilly model was used in large eddy simulation (LES). The distributions of the pressure and the topological structures of wake were well captured as closure of the vane. The production and evolution of the wake vortices passing the kinetic guide vane was clearly shown, and the physics of the structures were analyzed. The results show that the flow around a moving guide vane will be produced numerous complex transient structure whose induce water hammer of upstream duct system and transient behavior of downstream runner passage. The numerical results show that the numerical method of LES-IBM with dynamical Smagorinsky-Lilly SGS model is able to well simulate the interaction between high Reynolds number turbulent flow with motion of the vane closure and reveal the mechanism evolution properties of the wake to the downstream in the hydro turbine adjustment process.