A method of evaluating the vortex rope strength in draft tube of Francis turbine. (June 2020)

• Record Type: Journal Article
• Authors: Cheng, Huan; Zhou, Lingjiu; Liang, Quanwei; Guan, Ziwu; Liu, Demin; Wang, Zhaoning; Kang, Wenzhe
• Is Part Of: Renewable energy. Volume 152(2020)
• Page Start: 770
• Page End: 780
• Publication Date: 2020-06
• Subjects: Vortex rope -- Pressure fluctuation -- Velocity gradient -- Model test -- LDV -- Part-load condition; Renewable energy sources -- Periodicals; Power resources -- Periodicals; Énergies renouvelables -- Périodiques; Ressources énergétiques -- Périodiques; 333.794
• DOI: 10.1016/j.renene.2020.01.020
• Languages: English
• ISSNs: 0960-1481

Abstract

Abstract: A precessing vortex core in the draft tube of Francis turbine is considered as unstable flow conditions and the cause of many undesirable system instabilities for hydraulic power stations. The feature of vortex rope, pressure fluctuations and runner outlet velocity at part-load conditions are investigated experimentally and numerically in this paper. An obvious vortex rope could be observed at 0.88 Q 11BEP point and produces the most intensive periodic pressure fluctuation at the vortex rotation frequency f v along the whole passage of Francis turbine. At other conditions away from this point, the vortex rope becomes unobvious or even invisible, and the associated pressure fluctuation amplitude also reduces. This trend could be well represented by the new parameters G and V s, which could be used to determine the strength of helical vortex field based on the gradient of time-averaged velocity on horizontal section of the draft tube cone. Transient simulations using VLES turbulence model could obtain satisfying results by comparing with experimental data, especially in terms of the time-averaged axial velocity profile resolution. Therefore, this new method enables the designers to evaluate vortex strength in engineering applications. Highlights: Experimental and numerical investigations of vortex rope in draft tube are performed for model hydraulic turbine. The size and length of vortex rope have peak values with the discharge close to BEP. The relative amplitude of the vortex precession frequency is affected by the vortex rope strength. A new method based on the profile of axial velocity gradient is proposed to evaluate the strength of helical vortex rope. It is validated that this method is useful for another runner to determine the characteristics of vortex rope.