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

2011 ◽  
Vol 133 (5) ◽  
Author(s):  
Vlad Hasmatuchi ◽  
Mohamed Farhat ◽  
Steven Roth ◽  
Francisco Botero ◽  
François Avellan
Keyword(s):  
High Speed ◽  
Pressure Fluctuations ◽  
Frequency Component ◽  
Rotating Stall ◽  
Image Processing Technique ◽  
Scale Model ◽  
Pump Turbine ◽  
Guide Vanes ◽  
Radial Pump ◽  
Generating Mode

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.

Hydrodynamics of a Pump-Turbine at Off-Design Operating Conditions: Numerical Simulation

2011 ◽  
Author(s):  
Vlad Hasmatuchi ◽  
Steven Roth ◽  
Francisco Botero ◽  
Mohamed Farhat ◽  
Franc¸ois Avellan
Keyword(s):  
Numerical Simulation ◽  
Stokes Equations ◽  
Operating Conditions ◽  
Scale Model ◽  
Navier Stokes ◽  
Computational Domain ◽  
Pump Turbine ◽  
Navier Stokes Equations ◽  
Guide Vanes ◽  
Radial Pump

Flow numerical simulations in a low specific speed radial pump-turbine scale model are performed to investigate off-design operating conditions in generating mode. The Best Efficiency Point (BEP) and the runaway operating conditions at 10° guide vanes opening are addressed. The computational domain includes the full reduced scale model water passage from the spiral casing inlet to the draft tube outlet. The numerical simulation is performed using the Ansys CFX code, solving the incompressible unsteady Reynolds-Averaged Navier-Stokes equations. Wall pressure measurements in the stator are used to validate the numerical results. Then, detailed analysis is focused on the onset of the flow instabilities when the machine is brought from BEP to runaway. In these severe operating conditions, one single stall cell is found to rotate with the impeller at subsynchronous speed in the vaneless gap between the impeller and the guide vanes. It is found to be the effect of flow separation developed at the inlet of several consecutive impeller channels which lead to their blockage.

scholarly journals A review of rotating stall in reversible pump turbine

2016 ◽  
Vol 231 (7) ◽  
pp. 1181-1204 ◽  
Author(s):  
Yuning Zhang ◽  
Yuning Zhang ◽  
Yulin Wu
Keyword(s):  
Energy Storage ◽  
Pressure Fluctuations ◽  
Rotational Frequency ◽  
Rotating Stall ◽  
Discharge Condition ◽  
Pump Turbine ◽  
Large Pressure ◽  
Channel Blockage ◽  
Generating Mode ◽  
Future Work

Currently, operations of reversible pump turbines in pumped hydro energy storage plant suffer great instability problems in the well-known S-shaped characteristic regions, leading to the failure of start, significant pressure fluctuations, and severe vibrations of the whole system. One of the physical origins of the above instability of reversible pump turbines is the rotating stall phenomenon generated at off-design conditions in generating mode. In this review, recent studies on the rotating stall of reversible pump turbines are critically reviewed with a focus on the generating mode. In reversible pump turbine, the rotating stall initiates at runaway and is fully developed at low discharge condition with characteristic rotating frequency being 50–70% of the impeller rotational frequency. Notorious effects induced by rotating stall include generation of large pressure fluctuation, channel blockage, and strong backflow, all of which contribute significantly to the instability of reversible pump turbine. Methods for identification of rotating stall are also introduced with plenty of examples. Finally, several suggestions on the future work are given and discussed.

scholarly journals Distribution of Pressure Fluctuations in a Prototype Pump Turbine at Pump Mode

2014 ◽  
Vol 6 ◽  
pp. 923937 ◽  
Author(s):  
Yuekun Sun ◽  
Zhigang Zuo ◽  
Shuhong Liu ◽  
Jintao Liu ◽  
Yulin Wu
Keyword(s):  
Mass Flow ◽  
Pressure Fluctuations ◽  
Dominant Frequency ◽  
Pump Mode ◽  
Pump Turbine ◽  
Guide Vanes ◽  
Path Direction ◽  
Mass Flow Rates ◽  
Operating Points ◽  
Spiral Casing

Pressure fluctuations are very important characteristics in pump turbine's operation. Many researches have focused on the characteristics (amplitude and frequencies) of pressure fluctuations at specific locations, but little researches mentioned the distribution of pressure fluctuations in a pump turbine. In this paper, 3D numerical simulations using SSTk − ω turbulence model were carried out to predict the pressure fluctuations distribution in a prototype pump turbine at pump mode. Three operating points with different mass flow rates and different guide vanes’ openings were simulated. The numerical results show how pressure fluctuations at blade passing frequency (BPF) and its harmonics vary along the whole flow path direction, as well as along the circumferential direction. BPF is the first dominant frequency in vaneless space. Pressure fluctuation component at this frequency rapidly decays towards upstream (to draft tube) and downstream (to spiral casing). In contrast, pressure fluctuations component at 3BPF spreads to upstream and downstream with almost constant amplitude. Amplitude and frequencies of pressure fluctuations also vary along different circumferential locations in vaneless space. When the mass flow and guide vanes’ opening are different, the distribution of pressure fluctuations along the two directions is different basically.

scholarly journals Experimental Characterization of a Pump–Turbine in Pump Mode at Hump Instability Region

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
J. Yang ◽  
G. Pavesi ◽  
S. Yuan ◽  
G. Cavazzini ◽  
G. Ardizzon
Keyword(s):  
High Speed ◽  
Dynamic Pressure ◽  
Load Condition ◽  
Rotating Stall ◽  
Pump Mode ◽  
Pump Turbine ◽  
Part Load ◽  
Time Frequency ◽  
Speed Flow ◽  
Dynamic Pressure Measurements

The unsteady phenomena of a low specific speed pump–turbine operating in pump mode were characterized by dynamic pressure measurements and high-speed flow visualization of injected air bubbles. Analyses were carried out on the pressure signals both in frequency and time–frequency domains and by bispectral protocol. The results obtained by high-speed camera were used to reveal the flow pattern in the diffuser and return vanes channels The unsteady structure identified in the return vane channel appeared both at full and part load condition. Furthermore, a rotating stall structure was found and characterized in the diffuser when the pump operated at part load. The characteristics of these two unsteady structures are described in the paper.

Stall Precursor Identification in High Speed Compressor Stages Using Chaotic Time Series Analysis Methods

1996 ◽  
Author(s):  
Michelle M. Bright ◽  
Helen K. Qammar ◽  
Harald J. Weigl ◽  
James D. Paduano
Keyword(s):  
Wave Energy ◽  
Energy Method ◽  
Traveling Wave ◽  
High Speed ◽  
Pressure Fluctuations ◽  
Correlation Method ◽  
Rotating Stall ◽  
Front Face ◽  
Stable Operation ◽  
Correlation Integral

This paper presents a new technique for precursor identification in high speed compressors. The technique is a pseudo-correlation integral method referred to as the correlation method. To provide a basis for comparison, the traveling wave energy technique, which has been used extensively to study pre-stall data, is also briefly presented and applied. The correlation method has a potential advantage over the traveling wave energy method because it uses a single sensor for detection. It also requires no predisposition about the expected behavior of the data to detect “changes” in the behavior of the compressor. Both methods are used in this study to identify stall procursive events in the pressure fluctuations measured from circumferential pressure transducers located at the front face of the compressor rig. The correlation method successfully identified stall formation or changes in the compressor dynamics from data captured from four different configurations of a NASA Lewis single stage high speed compressor while it was transitioned from stable operation into stall. This paper includes an exposition on the use of nonlinear methods to identify stall precursors, a description of the methodologies used for the study, information on the NASA high speed compressor rig and experimental data acquisition, and results from the four compressor configurations. The experimental results indicate that the correlation method provides ample warning of the onset of rotating stall at high speed, in some tests on the order of 2000 rotor revolutions. Complementary features of the correlation method and the traveling wave energy method are discussed, and suggestions for future developments are made.

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

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Christian Widmer ◽  
Thomas Staubli ◽  
Nathan Ledergerber
Keyword(s):  
Electricity Market ◽  
Flow Instability ◽  
Guide Vane ◽  
Pressure Fluctuations ◽  
Vortex Formation ◽  
Pressure Rise ◽  
Rotating Stall ◽  
Momentum Exchange ◽  
Pump Turbine ◽  
Stationary Vortex

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.

scholarly journals Fluid-structure coupling in the guide vanes cascade of a pump-turbine scale model

2010 ◽  
Vol 12 ◽  
pp. 012074 ◽  
Author(s):  
S Roth ◽  
V Hasmatuchi ◽  
F Botero ◽  
M Farhat ◽  
F Avellan
Keyword(s):  
Scale Model ◽  
Pump Turbine ◽  
Fluid Structure ◽  
Guide Vanes

Large Eddy Simulation of the Rotating Stall in a Pump-Turbine Operated in Pumping Mode at a Part-Load Condition

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Olivier Pacot ◽  
Chisachi Kato ◽  
Yang Guo ◽  
Yoshinobu Yamade ◽  
François Avellan
Keyword(s):  
Reynolds Number ◽  
Large Eddy Simulation ◽  
Rotating Stall ◽  
Scale Model ◽  
Pump Turbine ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Pumping Mode ◽  
Stall Cells ◽  
Reduced Scale

The investigation of the rotating stall phenomenon appearing in the HYDRODYNA pump-turbine reduced scale model is carried out by performing a large-scale large eddy simulation (LES) computation using a mesh featuring approximately 85 × 106 elements. The internal flow is computed for the pump-turbine operated at 76% of the best efficiency point (BEP) in pumping mode, for which previous experimental research evidenced four rotating stall cells. To achieve an adequate resolution near the wall, the Reynolds number is decreased by a factor of 25 than that of the experiment, by assuming that the flow of our interest is not strongly affected by the Reynolds number. The computations are performed on the supercomputer PRIMEHPC FX10 of the University of Tokyo using the overset finite-element open source code FrontFlow/blue with the dynamic Smagorinsky turbulence model. It is shown that the rotating stall phenomenon is accurately simulated using the LES approach. The results show an excellent agreement with available experimental data from the reduced scale model tested at the EPFL Laboratory for hydraulic machines. The number of stall cells as well as the propagation speed agree well with the experiment. Detailed investigations on the computed flow fields have clarified the propagation mechanism of the stall cells.

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