TURBULENCE MODELS COMPARATION IN A FRANCIS TURBINE FLOW SIMULATION: AN APPLIED CASE ON THE ITAIPU TURBINES
João M. Marra1; Liliana M. Gramani2; Luiz F. Zubeldia3; Eloy Kaviski2
1 Itaipu Binacional; 2 UFPR; 3 Unioeste
doi:10.20906/CPS/CILAMCE2015-0350
Resumo
In the Brazilian electric power matrix and the potential to be installed in Brazil and in the world the Francis turbines represent significant contribution in generating. Given the versatility of the hydraulic turbines in meeting of the demand of an interconnected power system and of unfavorable hydrological conditions, these turbines are subjected to operating outside of its optimum conditions of its flow, damaging its hydraulic behavior, increasing the risk of resonance and instability phenomena in the hydraulic system, as well as the occurrence of power swing and fluid structure interactions, that can also increase the vibration and noise levels in the equipment and associated civil structure. Despite of some rotation (torsion) induced in the fluid flow by set formed by the elements of the spiral case, stay vanes and guide vanes the flow which leaves the runner in the best efficiency point is basically axial. However, outside of that ideal condition design of a Francis turbine, a rotating flow component induced by the tangential flow component in the runner outlet cannot be avoided and this gives rise to a vortex core in the draft tube. The vortex core is a source of excitation for the whole hydraulic circuit, which can operate as forced excitation or as self-excitation in hydraulic instability process. To better understand the complex flow in a Francis turbine and evaluate the disturbances that occur throughout the whole hydraulic circuit due the pressure pulsations, the Computational Fluid Dynamics (CFD) can be used. However the knowledge of the inherent peculiarities application of this technique is extremely important for obtaining satisfactory results. In this work were performed studies with variation of range system (domain), spatial discretization (mesh) and turbulence models (SST, k-ε e k-ω) in the solution of governing equations of three-dimensional turbulent flow, aiming to evaluate the numerical solution to reproduce the hydraulic system behavior as to pressure pulsation and frequencies
Palavras-chave: Francis turbine; Turbulence models; Numerical simulation; CFD