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Research Papers: Flows in Complex Systems

A Numerical Study of Francis Turbine Operation at No-Load Condition

[+] Author and Article Information
Hossein Hosseinimanesh

École Polytechnique de Montréal,
CP 6079, succ. Centre-ville,
Montréal, QC H3C 3A7, Canada
e-mail: hossein.hosseinimanesh@polymtl.ca

Christophe Devals

École Polytechnique de Montréal,
CP 6079, succ. Centre-ville,
Montréal, QC H3C 3A7, Canada
e-mail: christophe.devals@polymtl.ca

Bernd Nennemann

CFD/Tools,
Andritz Hydro Canada, Inc.,
6100 Transcanadienne,
Pointe-Claire, QC H9R 1B9, Canada
e-mail: bernd.nennemann@andritz.com

Marcelo Reggio

Professor
Department of Mechanical Engineering,
École Polytechnique de Montréal,
CP 6079, succ. Centre-ville,
Montréal, QC H3C 3A7, Canada
e-mail: marcelo.reggio@polymtl.ca

François Guibault

Professor
Department of Computer Engineering,
École Polytechnique de Montréal,
CP 6079, succ. Centre-ville,
Montréal, QC H3C 3A7, Canada
e-mail: francois.guibault@polymtl.ca

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 2, 2015; final manuscript received August 1, 2016; published online November 2, 2016. Assoc. Editor: Bart van Esch.

J. Fluids Eng 139(1), 011104 (Nov 02, 2016) (15 pages) Paper No: FE-15-1714; doi: 10.1115/1.4034422 History: Received October 02, 2015; Revised August 01, 2016

This paper presents a numerical methodology to study Francis turbines at no-load condition, an important operating condition regarding static and dynamic stresses. The proposed methodology uses unsteady Reynolds-averaged Navier–Stokes (RANS) simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step, and friction torque. The modeling tool is the commercial software ansys-cfx 14. The research compares the simulations that were performed using transient rotor–stator (TRS) and stage interface models and validate the results through experiments over the full range of admissible guide vane angles (GVAs). Both TRS and stage interface models yielded similar trends for all turbine runner parameters during the no-load process. Results show sizable differences in the average and maximum pressure on the blades between TRS and stage simulations. Analysis of the flow behavior in TRS simulation demonstrates complex flow phenomena involving a vortex breakdown within the draft tube, and strong vortices blocking the runner inlet, which dissipate the input energy into the turbine and yield a near zero-torque at no-load condition.

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Figures

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Fig. 1

Mesh for Francis turbine and distributor (left) and computational domain of complete turbine in medium head-TRS simulation (right)

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Fig. 2

Mesh for components in medium head-stage simulation: (a) distributor passage, stay vane, and guide vane, (b) runner passage, and (c) draft tube

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Fig. 3

Mesh quality histograms: (a) element volume (log value) distribution, (b) minimum angle distribution, and (c) expansion factor distribution

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Fig. 4

Comparison of simulated and experimental discharge factor Ned at no-load condition for different mesh densities

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Fig. 5

Comparison of simulated and experimental discharge factor Qed at no-load condition for different mesh densities

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Fig. 6

Distribution of y+ at no-load on medium head Francis turbine components (GVA 16 deg): (a) stay vane and guide vane, (b) runner blade pressure side (PS) and suction side (SS), and (c) draft tube

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Fig. 7

Geometry and boundary conditions of computational domains in medium head-stage simulation

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Fig. 8

Variation of the normalized runner angular speed, flow rate, and torque in medium head-TRS and stage simulations for a GVA 16 deg: (a) normalized engineering quantities versus time, (b) TRS simulation, and (c) stage simulation

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Fig. 9

Comparison between CFD predictions and experimental measurements [33] of speed coefficients Ned at no-load conditions (E = error bar): (a) medium head Francis turbine and (b) high head Francis turbine

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Fig. 10

Comparison between CFD predictions and experimental measurements [33] of flow coefficients Qed at no-load conditions (E = error bar): (a) medium head Francis turbine and (b) high head Francis turbine

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Fig. 11

Time history of normalized pressure fluctuation at PS and SS in medium head-TRS and stage simulations for a GVA 16 deg: (a) TRS—PS, (b) TRS—SS, (c) stage—PS, and (d) stage—SS

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Fig. 12

Monitoring points on PS and SS sides of blade in medium head-TRS and stage simulations

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Fig. 13

Spectral analysis of normalized pressure fluctuations at SS and PS at no-load from medium head-TRS and stage simulations for a GVA 16 deg

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Fig. 14

Normalized time-averaged pressure distribution on the blade pressure (left) and suction (right) sides at BEP (top) and no-load (bottom) from medium head-TRS simulation for a GVA 16 deg

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Fig. 15

Three-dimensional streamlines of time-averaged velocity within runner at no-load in medium head-TRS simulation for a GVA 16 deg

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Fig. 16

Normalized time-averaged axial vorticity at 1% span (left) and 50% span (right) runner at no-load from medium head-TRS simulation for a GVA 16 deg

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Fig. 17

Normalized time-averaged velocity streamlines and vorticity magnitude at mid surface in blade channel at no-load condition from medium head-TRS simulation for a GVA 16 deg

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Fig. 18

Normalized time-averaged tangential velocity (left) and axial velocity field (right) on a plane section through the draft tube at no-load from medium head-TRS simulation for a GVA 16 deg

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Fig. 19

Comparison of the average pressure coefficient Cp at no-load from medium head-TRS and stage simulations for a GVA 16 deg at different spans: (a) 0% span, (b) 50% span, and (c) 75% span

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Fig. 20

Comparison of normalized time-averaged velocity field and two-dimensional velocity streamlines at no-load condition between medium head-TRS (left) and medium head-stage (right) simulations for a GVA 16 deg on the plane z/L = −0.4 crossing the runner

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Fig. 21

Comparison of normalized time-averaged pressure distribution on the blade suction side at no-load condition between medium head-TRS (left) and medium head-stage (right) simulations for a GVA 16 deg

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Fig. 22

Comparison of normalized time-averaged pressure distribution on the blade pressure side at no-load condition between medium head-TRS (left) and medium head-stage (right) simulations for a GVA 16 deg

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Fig. 23

Comparison of normalized time-averaged velocity field and two-dimensional velocity streamlines at no-load condition between medium head-TRS (left) and medium head-stage (right) simulations for a GVA 16 deg at a plane crossing the draft tube

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