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Research Papers

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part II: Control of the Vortex Rope

[+] Author and Article Information
Hosein Foroutan

Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
338C Reber Building,
University Park, PA 16802
e-mail: hosein@psu.edu

Savas Yavuzkurt

Department of Mechanical and
Nuclear Engineering,
The Pennsylvania State University,
327 Reber Building,
University Park, PA 16802
e-mail: sqy@psu.edu

1Corresponding author.

Manuscript received October 6, 2013; final manuscript received February 7, 2014; accepted manuscript posted February 12, 2014; published online March 6, 2014. Assoc. Editor: Kenji Takizawa.

J. Appl. Mech 81(6), 061011 (Mar 06, 2014) (7 pages) Paper No: JAM-13-1424; doi: 10.1115/1.4026818 History: Received October 06, 2013; Revised February 07, 2014; Accepted February 12, 2014

Numerical simulations and investigation of a method for controlling the vortex rope formation in draft tubes are carried out in this paper, which is the second part of a two-paper series. As shown in the companion paper, formation of the vortex rope is associated with a large stagnant region at the center of the draft tube. Therefore, it is concluded that a successful control technique should focus on the elimination of this region. In practice, this can be performed by axially injecting a small fraction (a few percent of the total flow rate) of water into the draft tube. Water jet is supplied from the high-pressure flow upstream of the turbine spiral case by a bypass line; thus, no extra pump is needed in this method. It is shown that this method is very effective in elimination of the stagnant region in a simplified draft tube operating at two part-load conditions, i.e., at 91% and 70% of the best efficiency point (BEP) flow rate. This results in improvement of the draft tube performance and reduction of hydraulic losses. The loss coefficient is reduced by as much as 50% for the case with 91% of BEP flow rate and 14% for the case with 70% of BEP flow rate. Unsteady, three-dimensional simulations show that the jet increases the axial momentum of flow at the center of the draft tube and decreases the wake of the crown cone and thereby decreases the shear at the interface of the stagnant region and high velocity outer flow, which ultimately results in elimination of the vortex rope. Furthermore, reduction (by about 1/3 in the case with 70% of BEP flow rate) of strong pressure fluctuations leads to reliable operation of the turbine.

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Figures

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Fig. 1

Computational domain for the simplified draft tube investigated in this study

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Fig. 2

Water jet injection from the runner crown cone into the draft tube

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Fig. 3

Axial velocity profiles at the inlet section to the draft tube for (a) case I and (b) case II, effect of jet injection

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Fig. 4

Streamline patterns for the steady axisymmetric simulation of flow in the draft tube, effect of water jet injection (a) case I (91% of BEP flow rate) and (b) case II (70% of BEP flow rate)

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Fig. 5

Losses in the draft tube for (a) case I and (b) case II as a function of the jet radius

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Fig. 6

Contours of instantaneous axial velocity showing the stagnant region and vortex rope for (a) case I (91% of BEP flow rate) and (b) case II (70% of BEP flow rate) with and without water jet injection

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Fig. 7

Time evolution of the controlled draft tube flow by water jet injection showing the reduction and elimination of the vortex rope (top left to bottom right)

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Fig. 8

Unsteady pressure on the draft tube wall for case II, effect of water jet injection

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