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Experimental study of mitigation of a spiral vortex breakdown at high Reynolds number under an adverse pressure gradient
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
Department of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. Water Power Laboratory, Department of Energy and Process Engineering, Norwegian University of Science and Technology.ORCID iD: 0000-0001-7599-0895
2017 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 9, 104104Article in journal (Refereed) Published
Abstract [en]

The flow in the off-design operation of a Francis turbine may lead to the formation of spiral vortex breakdowns in the draft tube, a diffuser installed after the runner. The spiral vortex breakdown, also named a vortex rope, may induce several low-frequency fluctuations leading to structural vibrations and a reduction in the overall efficiency of the turbine. In the present study, synchronized particle image velocimetry, pressure, and turbine flow parameter (Q, H, α, and T) measurements have been carried out in the draft tube cone of a high head model Francis turbine. The transient operating condition from the part load to the best efficiency point was selected to investigate the mitigation of the vortex rope in the draft tube cone. The experiments were performed 20 times to assess the significance of the results. A precession frequency of 1.61 Hz [i.e., 0.29 times the runner rotational frequency (Rheingans frequency)] is observed in the draft tube cone. The frequency is captured in both pressure and velocity data with its harmonics. The accelerating flow condition at the center of the cone with a guide vane opening is observed to diminish the spiral form of the vortex breakdown in the quasi-stagnant region. This further mitigates the stagnant part of the cone with a highly dominated axial flow condition of the turbine at the best efficiency point. The disappearance of the stagnant region is the most important state in the present case, which mitigates the spiral vortex breakdown of the cone at high Reynolds numbers. In contrast to a typical transition, a new type of transition from wake to jet is observed during the mitigation of the breakdown. The obtained 2D instantaneous velocity fields demonstrate the disappearance region of shear layers and stagnation in the cone. The results also demonstrate the existence of high axial velocity gradients in an elbow draft tube cone.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017. Vol. 9, 104104
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-66373DOI: 10.1063/1.4999123ISI: 000414227600037Scopus ID: 2-s2.0-85032731423OAI: oai:DiVA.org:ltu-66373DiVA: diva2:1154622
Note

Validerad;2017;Nivå 2;2017-11-03 (andbra)

Available from: 2017-11-03 Created: 2017-11-03 Last updated: 2017-11-24Bibliographically approved

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