Numerical investigation of unsteady flows in hydraulic turbines

该项目的主要目标是对水轮机内部固有的非定常流动进行数值研究，并确定reynolds -average Navier-Stokes （RANS）能够求解相关流动结构的水平。涡轮中的流动波动会对结构产生疲劳应力，从而导致设备故障。这种波动在任何工况下都存在：静叶片和旋转叶片之间的尾迹相互作用和压力波、水力不平衡、涡旋脱落、剪切层的平流。此外，在部分负荷时，强烈的旋流以大涡的形式离开流道。由于尾水管中的逆压力梯度，该涡可能会破裂并产生振荡压力，从而危及结构。为了更好地了解不稳定运行条件下的流动现象，Hydro-Quebec （HQ）最近开始了一项雄心勃勃的项目，使用ANSYS-CFX和OpenFOAM软件模拟非稳定的三维流动。然而，在充分预测流动结构和不稳定性方面，经验提出了一些问题。这段时间内的非定常流场的精确模拟对于确定涡轮叶片的动载荷至关重要，因为在尾水管中存在着强烈的非定常流场。模拟的准确性完全取决于网格的质量、数值格式的选择和湍流模型。目前对尾水管非定常流场建模的尝试揭示了一阶闭包模型的有效性；然而，对于网格的质量要求和数值格式要求却很少提及。研究网格、湍流模型和数值方案的作用是帮助HQ朝着正确方向前进的关键和急需的知识。它将为总部的IREQ团队提供有价值的技术信息以及模拟这种流动的数值工具。所提出的研究影响将使HQ能够更准确地预测水力发电机组的剩余寿命。

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.