Flow instability transferability characteristics within a reversible pump turbine (RPT) under large guide vane opening (GVO)

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.

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Investigation of the Flow Instabilities of a Low Specific Speed Pump-Turbine: Part 2 — Flow Control With Fluid Injection

Volume 1: Fluid Applications and Systems; Fluid Measurement and Instrumentation ◽

10.1115/fedsm2020-20275 ◽

2020 ◽

Author(s):

Sabri Deniz ◽

Martin von Burg ◽

Manuel Tiefenthaler

Keyword(s):

Flow Control ◽

Flow Instability ◽

Guide Vane ◽

Water Injection ◽

Rotating Stall ◽

Flow Instabilities ◽

Fluid Injection ◽

Pump Turbine ◽

Low Specific Speed ◽

Specific Speed

Abstract This the second part of a two-part paper focusing on the flow instabilities of low-specific pump turbines. In this part, results of the flow control application with fluid injection (using both water and air) in the vaneless space in order to suppress the flow instabilities of a low specific speed model pump-turbine in turbine mode operation at HSLU (Lucerne University of Applied Sciences) Switzerland are presented. Based on the analysis of the experimental data, flow visualization, and CFD results focusing especially on the flow features in the vaneless space and at the runner inlet, the onset and development of the flow instabilities are explored as presented in the first part of this paper. Based on these analyses, the flow control technology by injecting air and water as well as suction of the fluid in the vaneless space of the model pump-turbine is implemented for suppressing the flow instabilities and thus extending the operating range of the pump-turbine. Both air- and water-injection are applied by using an external energy source (compressor and pump) and discrete nozzles circumferentially distributed in the vaneless space. The S-shaped pump-turbine characteristics in turbine operating mode are modified so that the slope at speed no load conditions is no more positive meaning an improvement in the stability behavior. To the best of our knowledge, this is the first successful application of flow control with fluid injection in the vaneless space of pump-turbines. Fluid injection is applied at two different guide vane openings, i.e. at 6° and 18°. The analysis of the unsteady pressure data indicates the suppression of flow instability such as rotating stall with fluid injection in the vaneless space. The water injection is more effective than the air injection for modifying the slope of the pump-turbine characteristics.

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Simulation and Experimental Investigation of the Stay Vane Channel Flow in a Reversible Pump Turbine at Off-Design Conditions

Periodica Polytechnica Mechanical Engineering ◽

10.3311/ppme.9345 ◽

2017 ◽

Vol 61 (2) ◽

pp. 94 ◽

Cited By ~ 1

Author(s):

Sandro Erne ◽

Gernot Edinger ◽

Anton Maly ◽

Christian Bauer

Keyword(s):

Transient Flow ◽

Guide Vane ◽

Low Frequency ◽

Rotating Stall ◽

Design Flow ◽

Flow State ◽

Mean Flow ◽

Load Range ◽

Pump Turbine ◽

Stall Inception

This work presents the assessment of the mean flow field and low frequency disturbances in the stay vane channel of a model pump turbine using transient numerical simulations and LDV-based measurements. The focus is laid on transient CFD simulations of characteristic flow states in the stay vane channel when operating at off-design conditions in pump mode. Experimental and numerical investigations obtained a shifting velocity distribution between the shroud and hub of the distributor when continuously increasing the discharge in the part-load range. Simulations captured the occurrence of this changing flow state in the stay vane channel reasonably well. A further increase of the discharge showed a uniformly redistributed mean flow of both hub and shroud side. Monitoring points and integral quantities from measurements and transient simulations were used to interpret the development of transient flow patterns in the stay vane channel at the operating point of strongest asymmetrical flow. During simulation and measurement, a dominant rotating stall inception was observed near the design flow of the pump turbine. At this point where the stall becomes severe, a high level of correlation between the signals of the upper and lower stalled flow in the stay vane channel was calculated. Further simulations for different guide vane positions predicted a strong influence of the guide vane position on the structure of rotating stall.

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Hysteresis Characteristic in Hump Region of a Pump-Turbine Model

10.20944/preprints201608.0007.v1 ◽

2016 ◽

Author(s):

Deyou Li ◽

Hongjie Wang ◽

Jinxia Chen ◽

Torbjørn K. Nielsen ◽

Daqing Qin ◽

...

Keyword(s):

Guide Vane ◽

Pressure Sensors ◽

Low Frequency ◽

Rotational Frequency ◽

Hysteresis Phenomenon ◽

Pump Turbine ◽

Hysteresis Characteristic ◽

Hydraulic Losses ◽

Measurement Direction ◽

Turbine Model

The hump characteristic is one of the major instabilities in pump-turbines. When pump-turbines operate in the hump region, strong noise and serious fluctuations could be observed, which are harmful to the safe and stable operations and even destroy the whole unit as well as water conveyance system. In this paper, a low specific speed (nq = 36.1 min−1) pump-turbine model was experimentally investigated. Firstly, the hump characteristic was obtained under 19 mm guide vane opening. More interestingly, when the hump characteristic was measured in two directions (increasing and decreasing the discharge), the hysteresis characteristic was found during the hump region. The analysis of performance characteristics reveals that the hump instability is resultant of Euler momentum and hydraulic losses, and different Euler momentum and hydraulic losses in the two development processes lead to hysteresis phenomenon. Then, 12 pressure sensors were mounted in the different parts of the pump-turbine model to obtain the time and frequency characteristics. The analysis of fast Fourier transform confirms that the hump characteristic is related to the low-frequency (0.04–0.15 times rotational frequency) vortices. The occurrence and cease of vortices depend on the operating condition and measurement direction, which contribute to the hysteresis characteristic. Finally, the type of the low-frequency vortices was analyzed through the cross power spectrum.

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Evolutions of guide vane moment of a pump-turbine during runaway transient process after pump trip

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/240/7/072033 ◽

2019 ◽

Vol 240 ◽

pp. 072033 ◽

Cited By ~ 1

Author(s):

Z Y Yang ◽

Y G Cheng ◽

L S Xia ◽

W W Meng ◽

L W Gao ◽

...

Keyword(s):

Transient Process ◽

Guide Vane ◽

Pump Turbine

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Pressure fluctuation and flow instability in S-shaped region of a reversible pump-turbine

Renewable Energy ◽

10.1016/j.renene.2020.03.069 ◽

2020 ◽

Vol 154 ◽

pp. 826-840 ◽

Cited By ~ 1

Author(s):

Wenwu Zhang ◽

Zhenmu Chen ◽

Baoshan Zhu ◽

Fei Zhang

Keyword(s):

Pressure Fluctuation ◽

Flow Instability ◽

Pump Turbine

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Experimental Research on the Rotating Stall of a Pump Turbine in Pump Mode

Water ◽

10.3390/w11112426 ◽

2019 ◽

Vol 11 (11) ◽

pp. 2426

Author(s):

Xue ◽

Liu ◽

Lu ◽

Gao ◽

Meng

Keyword(s):

Flow Field ◽

Experimental Research ◽

Flow Rate ◽

Guide Vane ◽

Rotating Stall ◽

Dynamics Simulation ◽

Positive Slope ◽

Pump Mode ◽

Pump Turbine ◽

Pump Performance

The rotating stall is an unstable flow phenomenon of pump turbines in pump mode, which is of increasing concern to scientists and engineers working on pump turbines. However, at present, various studies are carried out based on CFD (computational fluid dynamics) simulation, while directly measured data and experimental research on flow fields are seldom reported. By utilizing PIV (particle image velocimetry) measuring equipment, the flow field within the guide vane zone of a low specific speed pump turbine in pump mode was measured. By measuring and analyzing the transient flow field, the evolutionary process of the rotating stall within the guide vane passages was determined. We found that for all three tested guide vane openings, regardless of whether the positive slope appeared or not, a pre-stall operating point was found for each opening in the process of decreasing the flow rate. The analysis of the loss within the flow field indicated that the dissipation-induced loss increased greatly after the rotating stall appeared. The pump performance curves at the three guide vane openings showed an inflection at the pre-stall point. When the flow rate is larger than that of the pre-stall point, the head of the pump turbine dramatically increases as the flow rate decreases. However, when the flow rate is smaller than the pre-stall point, such increases noticeably slows down.The research results showed that whether the positive slope on the pump performance curve occurred or not, instability caused by the rotating stall should be of great concern.

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