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Research Papers: Multiphase Flows

Flow-Feedback Method for Mitigating the Vortex Rope in Decelerated Swirling Flows

[+] Author and Article Information
Constantin Tănasă

Scientific Researcher
Research Center for Engineering of Systems With Complex Fluids,
“Politehnica” University of Timişoara,
Boulevard Mihai Viteazu 1,
RO-300222 Timişoara, Romania
e-mail: costel@mh.mec.upt.ro

Romeo Susan-Resiga

Professor
Hydraulic Machinery Department,
“Politehnica” University of Timişoara,
Boulevard Mihai Viteazu 1,
RO-300222 Timişoara, Romania
e-mail: resiga@mh.mec.upt.ro

Sebastian Muntean

Senior Researcher
e-mail: seby@acad-tim.tm.edu.ro

Alin Ilie Bosioc

Scientific Researcher
e-mail: alin@mh.mec.upt.ro
Center for Advanced Research in Engineering Science,
Romanian Academy—Timişoara Branch,
Boulevard Mihai Viteazu 24,
RO-300223 Timişoara, Romania

1Corresponding author.

Manuscript received November 7, 2012; final manuscript received March 1, 2013; published online April 12, 2013. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 135(6), 061304 (Apr 12, 2013) (11 pages) Paper No: FE-12-1561; doi: 10.1115/1.4023946 History: Received November 07, 2012; Revised March 01, 2013

When reaction hydraulic turbines are operated far from the design operating regime, particularly at partial discharge, swirling flow instability is developed downstream of the runner, in the discharge cone, with a precessing helical vortex and its associated severe pressure fluctuations. Bosioc et al. (2012, “Unsteady Pressure Analysis of a Swirling Flow With Vortex Rope and Axial Water Injection in a Discharge Cone,” ASME J. Fluids Eng., 134(8), p. 081104) showed that this instability can be successfully mitigated by injecting a water jet along the axis. However, the jet discharge is too large to be supplied with high pressure water bypassing the runner, since this discharge is associated with the volumetric loss. In the present paper we demonstrate that the control jet injected at the inlet of the conical diffuser can actually be supplied with water collected from the discharge cone outlet, thus introducing a new concept of flow feedback. In this case, the jet is driven by the pressure difference between the cone wall, where the feedback spiral case is located, and the pressure at the jet nozzle outlet. In order to reach the required threshold value of the jet discharge, we also introduce ejector pumps to partially compensate for the hydraulic losses in the return pipes. Extensive experimental investigations show that the wall pressure fluctuations are successfully mitigated when the jet reaches 12% of the main flow discharge for a typical part load turbine operating regime. About 10% of the jet discharge is supplied by the plain flow feedback, and only 2% boost is insured by the ejector pumps. As a result, this new approach paves the way towards practical applications in real hydraulic turbines.

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Figures

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Fig. 1

Cross section through a Francis hydraulic turbine (a) vortex rope in the discharge cone and (b) flow-feedback system with jet injection into the discharge cone

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Fig. 2

Cross section through the swirling flow apparatus and detail of the swirl generator

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Fig. 3

Schematic representation of the flow-feedback system (FFM) implemented on the swirl apparatus and a cross section through the swirl generator

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Fig. 4

Collecting spiral case (a) radius of the cross section, (b) top view of the twin spiral case, (c) meridian cross-section geometry, and (d) transversal cross-section

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Fig. 5

Flow-feedback system with ejector pumps (FFM+) and a detail with the ejector pump

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Fig. 6

Pressure loss on the return pipes versus jet discharge

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Fig. 7

Dimensionless pressure coefficient measured on the conical diffuser wall for swirling flow with vortex rope and with flow-feedback control, respectively

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Fig. 8

Pressure fluctuation when using the FFM: original signal (left) and its Fourier spectrum (right)

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Fig. 9

Pressure fluctuation when using the FFM+: original signal (left) and its Fourier spectrum (right)

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Fig. 10

Dimensionless amplitude (a) Strouhal number and (b) versus the jet discharge fraction presented by Bosioc et al. [22]. FFM and FFM+ are marked on both figures.

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Fig. 11

Mitigation of the pressure fluctuations using FFM and FFM+ with respect to the vortex rope case at the four levels shown in Fig. 2

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Fig. 12

Rotating and plunging pressure fluctuation decomposition for swirling flow with vortex rope (a), FFM (b), and FFM+ (c), respectively, at the four levels shown in Fig. 2

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