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Research Papers: Flows in Complex Systems

Experimental Evidence of Rotating Stall in a Pump-Turbine at Off-Design Conditions in Generating Mode

[+] Author and Article Information
Vlad Hasmatuchi1

 École Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Hydraulic Machines, Avenue de Cour 33bis, CH-1007 Lausanne, Switzerlandvlad.hasmatuchi@epfl.ch

Mohamed Farhat

 École Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Hydraulic Machines, Avenue de Cour 33bis, CH-1007 Lausanne, Switzerlandmohamed.farhat@epfl.ch

Steven Roth

 École Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Hydraulic Machines, Avenue de Cour 33bis, CH-1007 Lausanne, Switzerlandsteven.roth@epfl.ch

Francisco Botero

 École Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Hydraulic Machines, Avenue de Cour 33bis, CH-1007 Lausanne, Switzerlandfrancisco.botero@epfl.ch

François Avellan

 École Polytechnique Fédérale de Lausanne (EPFL), Laboratory for Hydraulic Machines, Avenue de Cour 33bis, CH-1007 Lausanne, Switzerlandfrancois.avellan@epfl.ch

1

Corresponding author.

J. Fluids Eng 133(5), 051104 (Jun 07, 2011) (8 pages) doi:10.1115/1.4004088 History: Received December 10, 2010; Revised April 20, 2011; Published June 07, 2011; Online June 07, 2011

An experimental investigation of the rotating stall in reduced scale model of a low specific speed radial pump-turbine at runaway and turbine brake conditions in generating mode is achieved. Measurements of wall pressure in the stator are performed along with high-speed flow visualizations in the vaneless gap with the help of air bubbles injection. When starting from the best efficiency point (BEP) and increasing the impeller speed, a significant increase of the pressure fluctuations is observed mainly in the wicket gates channels. The spectral analysis shows a rise of a low frequency component (about 70% of the impeller rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Analysis of the instantaneous pressure peripheral distribution in the vaneless gap reveals one stall cell rotating with the impeller at sub-synchronous speed. High-speed movies reveal a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas at runaway the flow is highly disturbed by the rotating stall passage. The situation is even more critical at very low positive discharge, where backflow and vortices in the guide vanes channels develop during the stall cell passage. A specific image processing technique is applied to reconstruct the rotating stall evolution in the entire guide vanes circumference for a low positive discharge operating point. The findings of this study suggest that one stall cell rotates with the impeller at sub-synchronous velocity in the vaneless gap between the impeller and the guide vanes. It is the result of rotating flow separations developed in several consecutive impeller channels which lead to their blockage.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

“S-shaped” characteristic curve of a pump-turbine in generating mode

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Figure 2

HYDRODYNA reduced scale model

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Figure 3

Experimental setup for pressure measurements and high-speed flow visualization

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Figure 4

Pressure sensors location in the model

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Figure 5

Resulting “S-shaped” curves for the discharge and torque factors in generating mode

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Figure 6

Pressure fluctuations standard deviation (STD)

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Figure 7

Investigated operating points

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Figure 8

Pressure fluctuations amplitude spectra in the spiral casing, sensor location Sc1

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Figure 9

Pressure fluctuations amplitude spectra in the guide vanes channel, sensor location GV1

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Figure 10

Time history of the pressure fluctuation in the vaneless gap for 3 operating conditions

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Figure 11

Time history of the pressure fluctuation in the whole vaneless gap at low discharge condition (OP. #4)

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Figure 12

Instantaneous air bubbles flow visualizations in one guide vanes channel

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Figure 13

Superposition of air bubbles visualizations in one guide vanes channel during 3 impeller revolutions

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Figure 14

Instantaneous synthetic flow visualization of the rotating stall evolution in the full guide vanes ring and vaneless gap

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Figure 15

Theoretical mean velocity triangles at the impeller inlet for BEP and off-design operating points

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Figure 16

Schematic representation of the impeller inlet flow pattern at BEP and off-design operation

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