Numerical investigation of unsteady flows in hydraulic turbines

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The primary objective of the project is to numerically investigate unsteady flows that are inherently present in hydraulic turbines and determine the level at which the Reynolds-averaged Navier-Stokes (RANS) is able to resolve the pertinent flow structures. Flow fluctuations in a turbine generate fatigue stresses on the structure that can lead to equipment failure. Such fluctuations are present under any operating condition: wake interactions and pressure waves between stationary and rotating blades, hydraulic unbalance, vortex shedding, advection of shear layers. Moreover, at part load, a strong swirling flow exits the runner in the form of a large vortex. Because of the adverse pressure gradient in the draft tube, this vortex may break down and give rise to an oscillating pressure that endangers the structure. To better understand the flow phenomena in unstable operation conditions, Hydro-Quebec (HQ) has recently embarked on an ambitious project to simulate unsteady three-dimensional flows using both the ANSYS-CFX and OpenFOAM softwares. However, the experience has raised a number of questions in regards to the ability to adequately predict the flow structures and instabilities. The accurate simulation of the unsteady flow during this period of time is crucial in determining the dynamic loading on the turbine blades, where strong non-stationary flow are present in the draft tube. The accuracy of the simulation is entirely dependent on the quality of grid, the choice of numerical schemes, and turbulence models. Current attempts at modelling the unsteady flow in the draft tube has shed light on the ineffectiveness of first-order closure models; however, very little is mentioned in the required quality of grid as well as the numerical scheme. The proposed project to investigate the role of the grid, turbulence model, and numerical scheme is a critical and much needed knowledge to help advance HQ in the right direction. It will provide HQ's IREQ team with the valuable technical information as well as a numerical tool to simulate such flows. The impact of the research presented will enable HQ to predict with greater accuracy the residual life of hydroelectric units.