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Research Papers: Flows in Complex Systems

Analysis of the Unstable Behavior of a Pump-Turbine in Turbine Mode: Fluid-Dynamical and Spectral Characterization of the S-shape Characteristic

[+] Author and Article Information
Giovanna Cavazzini

Department of Industrial Engineering,
University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: giovanna.cavazzini@unipd.it

Alberto Covi

Department of Industrial Engineering,
University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: alberto.covi@unipd.it

Giorgio Pavesi

Department of Industrial Engineering,
University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: giorgio.pavesi@unipd.it

Guido Ardizzon

Department of Industrial Engineering,
University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: guido.ardizzon@unipd.it

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 2, 2015; final manuscript received August 14, 2015; published online September 10, 2015. Assoc. Editor: Frank C. Visser.

J. Fluids Eng 138(2), 021105 (Sep 10, 2015) (12 pages) Paper No: FE-15-1085; doi: 10.1115/1.4031368 History: Received February 02, 2015; Revised August 14, 2015

The most common mechanical equipment adopted in the new generation of pumped-hydro power plants is represented by reversible pump-turbines (RPT), required to rapidly switch between pumping and generating modes in order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources. As a consequence, pump-turbines are required to extend their operation under off-design conditions in unstable operating areas. The paper presents a numerical analysis of the unstable behavior of a pump-turbine operating in turbine mode near the no-load condition. To study in depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, a load rejection scenario at constant and large guide vane opening (GVO) was numerically analyzed by running through the flow-speed characteristic up to the turbine brake region. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating stall (RS) (65.1% of the runner rotation frequency) during the turbine brake operation. These phenomena were characterized by frequency and time–frequency analyses of several numerical signals (static pressure, blade torque, mass flow rate in blade passages). The influence of the development of these unsteady phenomena on the pump-turbine performance in a turbine operation was also analyzed, and the potential causes that generated the S-shaped characteristic curve were also investigated.

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References

Figures

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Fig. 1

S-shaped characteristic curve of a pump-turbine in generating mode: discharge–speed curve at a constant GVO

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Fig. 2

Scheme of the tested configuration

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Fig. 3

Sketch of the runner and distributor with references of the main diameters

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Fig. 9

Percentage errors in performance between experimental and numerical results

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Fig. 8

Comparison between numerical and experimental performance curves

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Fig. 7

Scheme of the experimental setup

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Fig. 6

Detail of the coarse mesh of the numerical model: inlet duct, return channel, guide vanes, runner, draft tube, and part of the meshed leakage system

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Fig. 5

Meridional view of the numerical model. Regions filled in gray refer to the blades; region filled in black refers to the leakage system.

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Fig. 4

Guide vanes reference position (λ = 0 deg)

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Fig. 10

Starting and ending points of the numerical simulation of the turbine brake (at constant GVO and constant speed)

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Fig. 11

Flow pattern in the unstable branch of the characteristic (Q = 21.8%Qbep) inside the runner (relative streamlines) and inside the guide vanes (absolute streamlines)

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Fig. 12

Instantaneous velocity flow field inside runner and guide vanes at midspan at three different instants: (a) t1 − Q = 45.1%Qbep; (b) t2 − Q = 37.8%Qbep; and (c) t3 − Q = 21.8%Qbep

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Fig. 13

Evolution of the instantaneous flow rate passing through each channel of the runner (a) and of the guide vanes (b) during the turbine brake. εi=−1 means blocked channel; εi<−1 means back flow in the channel

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Fig. 17

Scheme of the positions of the acquired numerical signals

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Fig. 18

Power-spectra of the mass flow rates through runner channels (a) and of the torques acting on the runner blades (b)

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Fig. 19

Power-spectra of the mass flow rates through the distributor channels (a) and of the torques (b) acting on seven guide vanes blades

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Fig. 20

Power-spectra of the static pressure signals acquired at the guide vanes inlet (a) and outlet (b)

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Fig. 14

Pressure profiles on a runner blade (blade 2) at midspan at four different instants: (a) Q = 45.1%Qbep; (b) Q = 37.8%Qbep; and (c) Q = 21.8%Qbep

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Fig. 27

Performance curve of the pump-turbine: (a) head and (b) mechanical power

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Fig. 28

Contributions in head of the runner and guide vanes

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Fig. 29

Numerical dimensionless discharge–speed curve

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Fig. 15

Evolution of the blade torque during the turbine brake: (a) runner (blades 2, 4, and 6) and (b) guide vanes (blades 1, 7, 13, and 19)

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Fig. 16

Flow field inside runner and guide vanes for Q = 20.3%Qbep: (a) flow pattern by means of absolute (guide vanes) and relative (runner) streamlines; (b) instantaneous velocity flow field at midspan

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Fig. 21

Cross-spectra of the mass flow rates (a) and torque signals (b) between different runner blades

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Fig. 22

Cross-spectra of the mass flow rates (a) and torque signals (b) between different guide vanes

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Fig. 23

Cross-spectra of the static pressure at (a) guide vanes outlet and (b) guide vanes inlet

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Fig. 24

Wavelets of (a) the mass flow rate in runner channel 2 and (b) of the torque acting on runner blade 2

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Fig. 25

Wavelets of (a) the mass flow rate in guide vanes channel 1 and (b) of the torque acting on guide vanes blade 1

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Fig. 26

Wavelets of static pressure data series at (a) p3,1 and (b) p4,1

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