Complete Francis turbine flow simulation for the whole range of discharges

CFD (Computational Fluid Dynamic) is today standard procedure for simulating and analysing fluid flow through hydraulic machines. This is a process where fluid flow domain is divided into many small volumes where governing equations are converted into algebraic equations, which can be solved numerically. Computational result strongly depends on methods used for converting governing to algebraic equations and on the choice of the fluid flow domain. Chosen technique depends on available computer resources and on possibility to simplify domain of interest. Mean techniques used for simulating fluid flow in hydraulic machines will be shortly described. Simulating fluid flow through different turbine components separately is today standard procedure. The main idea is to assume steady state uniform conditions in circumferential direction at the spiral-casing outflow. Accordingly only one geometrically periodic part of tandem cascade and runner blades has to be considered. This simulation results in severe saving of computer resources but with less accurate results. The obtained results are still sufficiently accurate for the routine everyday design, but are inaccurate for optimisation. Fluid flow simulation of the entire Francis turbine is desirable when spiral casing has nonuniform distribution of the incoming water or when strong interaction between turbine components exists [3]. Obtained results are more accurate because calculation domain contains all cascade channels and no averaging is applied in circumferential direction. Above methods assumes that the flow field is steady and such methods are sometimes called “frozen rotor” approach. For more accurate results, unsteady fluid flow simulation of entire turbine is required where interaction of turbine components is taken into account [6]. Resulting computational grids are huge and computationally expensive. Such grids can be handled by parallel computing using domain decomposition.