Research Papers: Flows in Complex Systems

Unstable Characteristics and Rotating Stall in Turbine Brake Operation of Pump-Turbines

[+] Author and Article Information
Christian Widmer

 Hochschule Luzern, Technik & Architektur, Technikumstrasse 21, CH-6048 Horw, Switzerlandchristian.widmer@andritz.com

Thomas Staubli

 Hochschule Luzern, Technik & Architektur, Technikumstrasse 21, CH-6048 Horw, Switzerlandthomas.staubli@alumni.ethz.ch

Nathan Ledergerber

 Hochschule Luzern, Technik & Architektur, Technikumstrasse 21, CH-6048 Horw, Switzerlandnathan.ledergerber@hslu.ch

J. Fluids Eng 133(4), 041101 (May 11, 2011) (9 pages) doi:10.1115/1.4003874 History: Received September 20, 2010; Revised March 17, 2011; Published May 11, 2011; Online May 11, 2011

Reversible pump-turbines are versatile in the electricity market since they can be switched between pump and turbine operation within a few minutes. The emphasis on the design of the more sensitive pump flow however often leads to stability problems in no load or turbine brake operation. Unstable characteristics can be responsible for hydraulic system oscillations in these operating points. The cause of the unstable characteristics can be found in the blocking effect of either stationary vortex formation or rotating stall. The so-called unstable characteristic in turbine brake operation is defined by the change of sign of the slope of the head curve. This change of sign or “S-shape” can be traced back to flow recirculation and vortex formation within the runner and the vaneless space between runner and guide vanes. When approaching part load from sound turbine flow the vortices initially develop and collapse again. This unsteady vortex formation induces periodical pressure fluctuations. In the turbine brake operation at small guide vane openings the vortices increase in intensity, stabilize and circumferentially block the flow passages. This stationary vortex formation is associated with a total pressure rise over the machine and leads to the slope change of the characteristic. Rotating stall is a flow instability which extends from the runner, the vaneless space to the guide and the stay vane channels at large guide vane openings. A certain number of channels is blocked (rotating stall cell) while the other channels comprise sound flow. Due to a momentum exchange between rotor and stator at the front and the rear cell boundary, the cell is rotating with subsynchronous frequency of about 60 percent of the rotational speed for the investigated pump-turbine (nq  = 45). The enforced rotating pressure distributions in the vaneless space lead to large dynamic radial forces on the runner. The mechanisms leading to stationary vortex formation and rotating stall were analyzed with a pump-turbine model by the means of numerical simulations and test rig measurements. It was found that stationary vortex formation and rotating stall have initially the same physical cause, but it depends on the mean convective acceleration within the guide vane channels, whether the vortex formations will rotate or not. Both phenomena lead to an unstable characteristic.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Outline of the investigated pump-turbine with numbered GV channels (left) including the meridian view (middle) and sensor arrangement in the guide vane channels (right)

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Figure 2

Grid for numerical computation of the complete domain (left) and zoomed to the rotor and stator (right)

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Figure 3

Varying boundary conditions during simulation 1 (left) and simulation 2 (right), specifying the regions of rotating stall as well as unsteady and stationary vortex formation

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Figure 4

Left: flow pattern in the unstable branch of the characteristic at 6 degree GVO in runner and vaneless space (relative streamlines) and in the stator (absolute streamlines); right: meridional view of the vortices in the vaneless space between runner (left edge) and guide vanes (right edge)

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Figure 5

Pressure signals GV1…GV5 in OP1 of the simulation (left) and the measurement (right) showing unsteady vortex formation

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Figure 6

Pressure signals GV1…GV5 in OP3 of the simulation (left) and the measurement (right) showing rotating stall

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Figure 7

Rotating stall cell with high pressure (red) at the front boundary and low pressure (blue) at the rear boundary

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Figure 8

Turbine quadrant with measured characteristics for different guide vane angles and the results of the transient simulation with varying flow rate (sim 1) and with varying GVO and flow rate (sim 2), (left) overview, (right) zoom on turbine brake range

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Figure 9

Spectrogram of the pressure signal GV1 processed by a JTFA during simulation 1 (left) and simulation 2 (right)

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Figure 10

Mass flow through the guide vane channels in terms of revolution for simulation 1 (left) and simulation 2 (right) with detailed view on onset of rotating stall in simulation 2

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Figure 11

Streamlines in rotor and stator at 30 degree GVO (sim 1): small rotating stall cell at 56 kg/s (left) and large cell at 16 kg/s (right)

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Figure 12

Pressure signal of GV1 and GV5 during simulation 2 with increasing GVO and corresponding phase shift between the two signals

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Figure 13

Extension of stationary vortex formation (left) and rotating stall (right)



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