Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part I: Simulation of the Vortex Rope
Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part I: Simulation of the Vortex Rope
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DOI：
10.1115/1.4026817
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发表时间：
2014-06
期刊：
影响因子：
--
通讯作者：
H. Foroutan;S. Yavuzkurt
中科院分区：
文献类型：
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作者：
H. Foroutan;S. Yavuzkurt
本文对模型弗朗西斯涡轮机尾水管内涡带的形成进行了数值模拟和分析。这一部分的重点是采用不同的湍流闭合模型对流动进行模拟和研究。两个部分负载的操作条件下，相同的头和不同的流量（91%和70%的最佳效率点（BEP）流量）被认为是。在简化的轴对称几何条件下，对轴对称和三维网格进行了定常和非定常模拟，并将结果与实验数据进行了比较。可以看出，稳态模拟与雷诺平均Navier-Stokes（RANS）模型不能解决涡绳和轴对称和三维流动的几何形状给出相同的对称结果。这些RANS模拟低估了尾水管中心附近的轴向速度（至少14%）和湍流动能（至少40%），甚至非常接近设计条件。远离设计点，模型无法预测尾水管中轴向速度的正确水平。非定常模拟采用非定常RANS（URANS）和分离涡模拟（DES）湍流闭合方法。URANS模型不能捕捉涡带的自诱导非定常性并给出定常解，而DES模型给出了足够的非定常解。使用适当的不稳定模型，即，DES，涡绳的整体形状是正确的预测和计算的涡绳频率与实验数据相差只有6%。证实了涡带的形成是由于在冠锥尾流产生的低速内区与高旋流外流之间的界面处剪切层的卷起。
Numerical simulations and analysis of the vortex rope formation in a simplified draft tube of a model Francis turbine are carried out in this paper, which is the first part of a two-paper series. The emphasis of this part is on the simulation and investigation of flow using different turbulence closure models. Two part-load operating conditions with same head and different flow rates (91% and 70% of the best efficiency point (BEP) flow rate) are considered. Steady and unsteady simulations are carried out for axisymmetric and three-dimensional grid in a simplified axisymmetric geometry, and results are compared with experimental data. It is seen that steady simulations with Reynolds-averaged Navier–Stokes (RANS) models cannot resolve the vortex rope and give identical symmetric results for both the axisymmetric and three-dimensional flow geometries. These RANS simulations underpredict the axial velocity (by at least 14%) and turbulent kinetic energy (by at least 40%) near the center of the draft tube, even quite close to the design condition. Moving farther from the design point, models fail in predicting the correct levels of the axial velocity in the draft tube. Unsteady simulations are performed using unsteady RANS (URANS) and detached eddy simulation (DES) turbulence closure approaches. URANS models cannot capture the self-induced unsteadiness of the vortex rope and give steady solutions while DES model gives sufficient unsteady results. Using the proper unsteady model, i.e., DES, the overall shape of the vortex rope is correctly predicted and the calculated vortex rope frequency differs only 6% from experimental data. It is confirmed that the vortex rope is formed due to the roll-up of the shear layer at the interface between the low-velocity inner region created by the wake of the crown cone and highly swirling outer flow.