Numerical investigation of unsteady flows in hydraulic turbines

该项目的主要目标是对水轮机中固有存在的非定常流动进行数值研究，并确定雷诺平均纳维斯托克斯(RANS)能够解决相关流动结构的水平。涡轮机中的流量波动会在结构上产生疲劳应力，从而可能导致设备故障。这种波动在任何运行条件下都存在：静止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，旋涡脱落，剪切层的平流。此外，在部分负荷下，一股强烈的旋流以大漩涡的形式离开转轮。由于尾水管内的逆压差，这种旋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，魁北克水电(HQ)最近开始了一个雄心勃勃的项目，使用Ansys-CFX和OpenFOAM软件模拟非恒定三维流动。然而，这一经验对充分预测流动结构和不稳定性的能力提出了一些问题。对这段时间内非定常流动的准确模拟对于确定涡轮叶片上的动态载荷至关重要，因为在尾水管中存在强烈的非定常流动。模拟的精度完全取决于网格的质量、数值格式的选择和湍流模型。目前在尾水管中模拟非定常流动的尝试已经揭示了一阶闭合模型的无效；然而，在所需的网格质量以及数值格式中提到的很少。研究网格、湍流模型和数值方案的作用的拟议项目是帮助总部朝着正确方向发展的关键和迫切需要的知识。它将为总部的IREQ团队提供有价值的技术信息以及模拟此类流动的数值工具。所提出的研究的影响将使总部能够更准确地预测水电机组的剩余寿命。

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.