Experimental and Numerical Investigations on the Origins of Rotating Stall in a Propeller Turbine Runner Operating in No-Load Conditions

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Sébastien Houde ◽  
Guy Dumas ◽  
Claire Deschênes
Keyword(s):  
Pressure Fluctuations ◽  
Low Frequency ◽  
High Amplitude ◽  
Rotating Stall ◽  
Spinning Reserve ◽  
Electrical Grids ◽  
Large Pressure ◽  
Turbine Runner ◽  
Synchronous Rotation ◽  
Hydraulic Turbines

Hydraulic turbines are more frequently used for power regulation and thus spend more time providing spinning reserve for electrical grids. Spinning reserve requires the turbine to operate at its synchronous rotation speed, ready to be linked to the grid in what is termed the speed-no-load (SNL) condition. The turbine's runner flow in SNL is characterized by low discharge and high swirl leading to low-frequency high amplitude pressure fluctuations potentially leading to blade damage and more maintenance downtime. For low-head hydraulic turbines operating at SNL, the large pressure fluctuations in the runner are sometimes attributed to rotating stall. Using embedded pressure transducer measurements, mounted on runner blades of a model propeller turbine, and numerical flow simulations, this paper provides an insight into the inception mechanism associated with rotating stall in SNL conditions. The results offer evidence that the rotating stall is in fact associated with an unstable vorticity distribution not associated with the runner blades themselves.

THE PHYSICAL ORIGIN OF SEVERE LOW-FREQUENCY PRESSURE FLUCTUATIONS IN GIANT FRANCIS TURBINES

2005 ◽  
Vol 19 (28n29) ◽  
pp. 1527-1530 ◽  
Author(s):  
R.-K. ZHANG ◽  
Q.-D. CAI ◽  
J.-Z. WU ◽  
Y.-L. WU ◽  
S.-H. LIU ◽  
...  
Keyword(s):  
Swirling Flow ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Physical Origin ◽  
Navier Stokes Equation ◽  
Power Stations ◽  
Hydraulic Turbines ◽  
Hydrodynamic Stability Theory

The physical origin of severe low-frequency pressure fluctuation frequently observed in Francis hydraulic turbines under off-design conditions, which greatly damages the structural stability of turbines and even power stations, is analyzed based on the hydrodynamic stability theory and our Reynolds-averaged Navier-Stokes equation simulation (RANS) of the flow in the entire passage of a Francis turbine. We find that spontaneous unsteady vortex ropes, which induce severe pressure fluctuations, are formed due to the absolute instability of the swirling flow at the conical inlet of the turbine's draft tube.

Investigation on Complex Pressure Fluctuations in a Gas-Solids Circulating Fluidized Bed Rieser

2012 ◽  
Vol 538-541 ◽  
pp. 610-615
Author(s):  
Jun Xu ◽  
Xing Xing Chen ◽  
Gui Lei Wang ◽  
Yao Dong Wei
Keyword(s):  
Fluidized Bed ◽  
Pressure Fluctuation ◽  
Circulating Fluidized Bed ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Axial Direction ◽  
High Amplitude ◽  
Solid Mass ◽  
Two Phase ◽  
Mass Fluxes

The experiment is carried out in a 13-meter-high circulating fluidized bed(CFB) to investigate gas-solid two-phase flow by pressure sensor. The axial pressure and pressure fluctuation are measured in different solid mass fluxes. With the solid mass flux increasing, pressure gradually increases, and pressure gradually decreases along the riser upwards. The characteristic of pressure fluctuation in the riser is analyzed, which indicates that pressure fluctuation in the riser originates from the inlet. The intensity of the pressure fluctuation decreases along the riser upwards. This pressure fluctuation is composed of two types: one is of low frequency and high amplitude, which is resulted from unstable feeding to the riser and keeps coherent along the axial direction. And the other is of high frequency and low amplitude, which is the result of a variety of factors, such as cluster movement, gas-solid interaction and gas velocity fluctuation.

From Rotating Stall to Surge: A Shock Tube Mechanism

2013 ◽  
Author(s):  
Paul Xiubao Huang ◽  
JianAn Yin
Keyword(s):  
Shock Tube ◽  
Gas Flow ◽  
Pressure Ratio ◽  
Low Frequency ◽  
Physical Nature ◽  
High Amplitude ◽  
Rotating Stall ◽  
Jet Engine ◽  
Dynamic Type ◽  
Compressor Surge

Compressor surge is a complete breakdown in compression resulting in an abrupt momentary reversal of gas flow and the violent pressure fluctuation with relatively low frequency and high amplitude. It commonly exists in dynamic type turbo compressors, particularly axial compressor and jet engine, or turbo charger for reciprocating engines. It is generally accepted that surge is preceded by a rotating stall, a situation of a few stalled blades rotating around compressor annulus (cascade) with much higher frequency. In jet engine, violent surge event typically produces a frightening loud bang, lots of vibrations and could cause catastrophic structural failures if not timely managed. Naturally, as important matters as rotating stall and surge, there have been tremendous R/D efforts from academia, government and industry devoted to this area, especially since jet engines became the prime powerhouses for modern airplanes. Despite of all the efforts, there still seems to be a more urgent need to understand the physical characteristics of the transition from a rotating stall to surge that has mystified researchers due to its transient nature. Fundamental questions remain unanswered even today, such as: What exactly triggers the surge to take place from a rotating stall? What is the physical nature of a compressor system or a local incipient surge: is it a movement of wave or fluid particles or both? How to estimate the quantitative destructive forces of a severe surge, that is, the maximum possible surge strength? This paper attempts to answer these questions by applying the classical Shock Tube Theory to the transient process from rotating stall to surge. The Shock Tube analogy is established with the hypothesis (implied from experimental observations) that an instant zero through flow condition exists inside a stalled cascade cell or dynamic compressor that triggers surge. It is revealed that surge event consists of a pair of non-linear compression and expansion waves (CW & EW) that instantly reverse gas flow (IRFF) by the pushing force of upstream propagating CW and the pulling force from downstream travelling EW. The surge strength is shown to be proportional to the square root of the pressure ratio of the involved cascade or compressor. Surge Rules are deduced to predict the location of surge initiation, the minimum and maximum surge strengths, travelling directions and speed. Moreover, a pro-active control strategy called SEWI (Surge Early Warning Initiative) is proposed using the unique characteristics of CW-IRFF-EW formation of a cascade cell induced surge as precursors for subsequent warning and controls before the destructive compressor surge takes place.

Compressible Modal Instability Onset in an Aerodynamically Mistuned Transonic Fan

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Christoph Brandstetter ◽  
Benoit Paoletti ◽  
Xavier Ottavy
Keyword(s):  
Pressure Fluctuations ◽  
Low Frequency ◽  
Stability Margin ◽  
Acoustic Resonance ◽  
Feedback Mechanism ◽  
Rotating Stall ◽  
Rotor Blades ◽  
The Stability ◽  
Transonic Fan ◽  
Aeromechanical Stability

This paper describes observed modal oscillations arising from a feedback mechanism between an acoustic resonance in the exit flow channel and aerodynamic and aeroelastic disturbances in a transonic fan stage. During tests, the fan suffered from rotating stall and surge which were preceded by low frequency pressure fluctuations. Through a range of aerodynamic and aeromechanical instrumentations, it was possible to determine a clear chain of cause and effect, whereby geometrical asymmetries trigger local instabilities and modal oscillations through an interaction with the system acoustics. To the authors knowledge, this is the first time that modal oscillations occurring before stall are attributed to multiphysical interactions, showing that acoustic characteristics of the system can influence the aerodynamic as well as the aeromechanical stability of fans. This bears implications for the stability assessment of fans and compressors because first, the stability margin may be affected by standing waves generated in bypass ducts or combustion chambers, and second, geometrical variations of the rotor blades which are believed to be beneficial for aeromechanical stability may lead to complex coupling phenomena.

Unstable Flow Characteristics in S-Shaped Region of Pump-Turbine Runners With Large Blade Lean

2017 ◽  
Author(s):  
Zhe Ma ◽  
Baoshan Zhu ◽  
Cong Rao ◽  
Lei Tan
Keyword(s):  
Guide Vane ◽  
Pressure Fluctuations ◽  
Vortex Formation ◽  
Flow Characteristics ◽  
Low Frequency ◽  
Leading Edge ◽  
Rotating Stall ◽  
Scale Model ◽  
Unstable Flow ◽  
Discharge Point

As the reversible pump-turbines operate in the S-shaped region, instability problems including backflow, vortex formation and rotating stall may appear. Previous researches studied instabilities at different guide vane opening (GVO) on their inception and evolution but few studies explored the effect of the blade lean at the leading edge. In present work, two runners tested by experiments, the runner A with a negative and the runner B with a positive blade lean at leading edge, were studied in CFD mode with a reduced scale model. Six operating points, namely, best efficiency point (OP#1), two points in the normal operating region (OP#2, OP#3), two points near runaway line (OP#4, OP#5) and a low discharge point in turbine brake (OP#6) were calculated for both runners. As the discharge reduces, the flow in the runners loses its symmetry and the efficiency becomes lower and lower. The flow of OP#1, OP#2 and OP#3 is healthy but slight separations locate near the inlet of the passages. At OP#4, obvious vortexes occupy the passages and the visible vortexes prevent the flow from entering the channels. The blockage generates strong backflow near the inlet of the runner. Moreover, the main backflow area locates near the hub for runner A while for runner B it is near the shroud. Unsteady vortex formation and rotating stall respectively exist at the near runaway points (OP#4 and OP#5) and low discharge point (OP#6). At these three points, the pressure fluctuations in the vaneless gap between the runner and guide vanes are very high and the amplitude shows a small difference between the two runners. Dramatic distinction appears on the frequency of the fluctuation. For both of the two runners, a peak corresponding to 70% fn, where fn is the runner rotating frequency, rises in the spectra of OP#4 and OP#5. This peak appears at all the monitors in the vaneless space at the same time standing for the unsteady vortex formation, which does not rotate with the blades. In addition, at OP#6, 40% and 50% fn are detected as the dominant frequencies for runner A and runner B respectively. In addition, the propagation of such two low frequency signal along the annulus in the vaneless space proves the existence of the rotating stalls.

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.

Performance of Kaplan Turbine Operating at Design Condition

2021 ◽  
Author(s):  
Muhannad Altimemy ◽  
Saif Watheq ◽  
Justin Caspar ◽  
Alparslan Oztekin
Keyword(s):  
Draft Tube ◽  
Tube Wall ◽  
Negative Impact ◽  
Pressure Fluctuations ◽  
Average Power ◽  
Low Frequency ◽  
High Amplitude ◽  
Tip Vortex ◽  
Design And Optimization ◽  
Kaplan Turbine

Abstract Design and optimization using computational fluid dynamics to enhance the hydro turbine’s performance are becoming gradually more common because of its flexibility, minor detailed flow description, and cost-effectiveness. These features are not easily achievable in model testing. k–ω simulations conducted in OpenFOAM 7 characterize the flow structure inside an industrial-sized Kaplan turbine module operating at the peak design flowrate. The power signal, velocity, vorticity, and pressure field are presented over the blades and throughout the draft tube. Additionally, pressure fluctuations were probed along the draft tube wall. The simulation shows a tip vortex rope in the narrow gap between the blade tip and turbine casing. The strong influence of the swirl leaving the runner had a negative impact on the flow pressure fluctuation. Also, high vortical activity was presented near the draft tube wall, leading to turbine instability. It was demonstrated that the turbine generates 14.923 MW of average power. The power signal showed minor fluctuations induced by the vortical activity close to the runner region and the corresponding pressure fluctuations. The Fast Fourier Transform showed the system is dominated by low frequency, high amplitude fluctuations.

scholarly journals Effect of Stall Cells on Pressure Fluctuations Characteristics in a Centrifugal Pump

Symmetry ◽  
2019 ◽  
Vol 11 (9) ◽  
pp. 1116 ◽  
Author(s):  
Peijian Zhou ◽  
Jiacheng Dai ◽  
Chaoshou Yan ◽  
Shuihua Zheng ◽  
Changliang Ye ◽  
...  
Keyword(s):  
Centrifugal Pump ◽  
Flow Rate ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Rotating Stall ◽  
Design Flow ◽  
Eddy Simulation ◽  
Leading Role ◽  
Stall Cells ◽  
Design Flow Rate

Rotating stall is an unsteady flow phenomenon, which always leads to instability and efficiency degradation. In order to reveal pressure fluctuations in the impeller of centrifugal pump induced by stall cells, the flow structures in a volute-type centrifugal pump were calculated using Large Eddy Simulation (LES) method. The predicted results of the numerical model were compared with experimental flow-head curve. The simulation results were in good agreement with the experimental results. The stall phenomenon occurred when the flow rate dropped to 70% of design flow rate. Three stall cells located at the entrance of passages could be observed, which remained stationary relative to the rotating impeller. With the decrease of flow rate, the area occupied by stall cells gradually increased. The peak value of pressure fluctuation at 25% of design flow rate is obviously larger than that at 50% of design flow rate. For the unstalled or stalled passage, the impeller-volute interaction played a leading role in the pressure fluctuations of the impeller. For the stalled passage, the amplitude of the low frequency induced by stall cell is relatively insignificant.

APPLICATION OF THE EMPIRICAL MODE DECOMPOSITION METHOD TO GROUND-ROLL NOISE ATTENUATION IN SEISMIC DATA

2013 ◽  
Vol 31 (4) ◽  
pp. 619 ◽  
Author(s):  
Luiz Eduardo Soares Ferreira ◽  
Milton José Porsani ◽  
Michelângelo G. Da Silva ◽  
Giovani Lopes Vasconcelos
Keyword(s):  
Empirical Mode Decomposition ◽  
Seismic Data ◽  
Low Frequency ◽  
High Amplitude ◽  
Seismic Line ◽  
Seismic Processing ◽  
Mode Decomposition ◽  
Ground Roll ◽  
Fortran 90 ◽  
Emd Method

ABSTRACT. Seismic processing aims to provide an adequate image of the subsurface geology. During seismic processing, the filtering of signals considered noise is of utmost importance. Among these signals is the surface rolling noise, better known as ground-roll. Ground-roll occurs mainly in land seismic data, masking reflections, and this roll has the following main features: high amplitude, low frequency and low speed. The attenuation of this noise is generally performed through so-called conventional methods using 1-D or 2-D frequency filters in the fk domain. This study uses the empirical mode decomposition (EMD) method for ground-roll attenuation. The EMD method was implemented in the programming language FORTRAN 90 and applied in the time and frequency domains. The application of this method to the processing of land seismic line 204-RL-247 in Tacutu Basin resulted in stacked seismic sections that were of similar or sometimes better quality compared with those obtained using the fk and high-pass filtering methods.Keywords: seismic processing, empirical mode decomposition, seismic data filtering, ground-roll. RESUMO. O processamento sísmico tem como principal objetivo fornecer uma imagem adequada da geologia da subsuperfície. Nas etapas do processamento sísmico a filtragem de sinais considerados como ruídos é de fundamental importância. Dentre esses ruídos encontramos o ruído de rolamento superficial, mais conhecido como ground-roll . O ground-roll ocorre principalmente em dados sísmicos terrestres, mascarando as reflexões e possui como principais características: alta amplitude, baixa frequência e baixa velocidade. A atenuação desse ruído é geralmente realizada através de métodos de filtragem ditos convencionais, que utilizam filtros de frequência 1D ou filtro 2D no domínio fk. Este trabalho utiliza o método de Decomposição em Modos Empíricos (DME) para a atenuação do ground-roll. O método DME foi implementado em linguagem de programação FORTRAN 90, e foi aplicado no domínio do tempo e da frequência. Sua aplicação no processamento da linha sísmica terrestre 204-RL-247 da Bacia do Tacutu gerou como resultados, seções sísmicas empilhadas de qualidade semelhante e por vezes melhor, quando comparadas as obtidas com os métodos de filtragem fk e passa-alta.Palavras-chave: processamento sísmico, decomposição em modos empíricos, filtragem dados sísmicos, atenuação do ground-roll.

Frequency Response Analysis of Piecewise Nonlinear Vibration Isolator

2005 ◽  
Author(s):  
Patrick Stahl ◽  
G. Nakhaie Jazar
Keyword(s):  
Low Frequency ◽  
High Amplitude ◽  
Relative Displacement ◽  
Response Analysis ◽  
Frequency Response Analysis ◽  
State Behavior ◽  
Vibration Isolators ◽  
Short Period ◽  
Secondary Suspension ◽  
Low Amplitude

Non-smooth piecewise functional isolators are smart passive vibration isolators that can provide effective isolation for high frequency/low amplitude excitation by introducing a soft primary suspension, and by preventing a high relative displacement in low frequency/high amplitude excitation by introducing a relatively damped secondary suspension. In this investigation a linear secondary suspension is attached to a nonlinear primary suspension. The primary is assumed to be nonlinear to model the inherent nonlinearities involved in real suspensions. However, the secondary suspension comes into action only during a short period of time, and in mall domain around resonance. Therefore, a linear assumption for the secondary suspension is reasonable. The dynamic behavior of the system subject to a harmonic base excitation has been analyzed utilizing the analytic results derived by applying the averaging method. The analytic results match very well in the transition between the two suspensions. A sensitivity analysis has shown the effect of varying dynamic parameters in the steady state behavior of the system.

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