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.

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

2017 ◽  
Vol 61 (2) ◽  
pp. 94 ◽  
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.

Flow Behavior of a Centrifugal Compressor With Vaneless Diffuser at Design and Off Design Conditions

2012 ◽  
Author(s):  
Chuang Gao ◽  
Weiguang Huang ◽  
Haiqing Liu ◽  
Hongwu Zhang ◽  
Jundang Shi
Keyword(s):  
Centrifugal Compressor ◽  
Flow Behavior ◽  
Travelling Waves ◽  
Leading Edge ◽  
Pressure Oscillation ◽  
Rotating Stall ◽  
Vaneless Diffuser ◽  
Unstable Flow ◽  
Flow Recirculation ◽  
Static Performance

This paper concerns with the numerical and experimental aspects of both steady and unsteady flow behavior in a centrifugal compressor with vaneless diffuser and downstream collector. Specifically, the appearance of flow instabilities i.e., rotating stall and surge is investigated in great detail. As the first step, the static performance of both stage and component was analyzed and possible root cause of system surge was put forward based on the classic stability theory. Then the unsteady pressure data was utilized to find rotating stall and surge in frequency domain which could be classified as mild surge and deep surge. With the circumferentially installed transducers at impeller inlet, backward travelling waves during stall ramp could be observed. The modes of stall waves could be clearly identified which is caused by impeller leading edge flow recirculation at Mu = 0.96. However, for the unstable flow at Mu = 1.08, the system instability seems to be caused by reversal flow in vaneless diffuser where the pressure oscillation was strongest. Thus steady numerical simulation were performed and validated with the experimental performance data. With the help of numerical analysis, the conjectures are proved.

Unsteady Cavitating Flow Around an Inclined Rectangular Cylinder

Volume 3 ◽  
2004 ◽  
Author(s):  
Terukazu Ota ◽  
Isao Tsubura ◽  
Hiroyuki Yoshikawa
Keyword(s):  
Separation Bubble ◽  
Fluid Dynamic ◽  
Flow Characteristics ◽  
Low Frequency ◽  
Cavitation Number ◽  
Cavitating Flow ◽  
Dynamic Force ◽  
Unstable Flow ◽  
Rectangular Cylinder ◽  
Cavity Configuration

Unsteady characteristics of cavitating flow around an inclined rectangular cylinder with a width to height ratio of 8.0 were experimentally investigated for various angles of attack and cavitation numbers. Measurements of fluid dynamic force and surface pressure were made and the cavity configuration was observed with a camera. Especially considered are the self-oscillating unstable flow characteristics along with the time variation of cavity configuration. It is found that a severe vibration occurs at some cavitation number, in which the attached cavity is formed in the separation bubble. As the cavitation number further decreases, the low frequency fluctuation of flow occurs.

Large Eddy Simulation of Francis Turbine Flow Fields under Small Opening Condition

2013 ◽  
Vol 444-445 ◽  
pp. 281-285 ◽  
Author(s):  
Tao Guo ◽  
Jun Zhou ◽  
Xiao Nan Liu
Keyword(s):  
Kinetic Energy ◽  
Large Eddy Simulation ◽  
Turbulent Kinetic Energy ◽  
Guide Vane ◽  
Francis Turbine ◽  
Scale Model ◽  
Unstable Flow ◽  
Small Opening ◽  
Eddy Simulation ◽  
Large Eddy

The vibration intensity is strong in Francis turbine occurred under the small opening conditions, such as Lijia Gorges and Three Gorges project. In paper we use large eddy simulation (LES) method base on Vreman SubGrid-Scale model to study the generation and evolution process of turbulence flow, capturing the details of the flow structures and the dissipation of the turbulent kinetic energy. The SIMPIEC algorithm is applied to solve the coupled equation of velocity and pressure. The result shows that the small guide vane opening conditions deviate the optimal conditions most. So some unstable flow characters been induced. Such as the turbulent kinetic energy of fluid in guide vanes zone, the blade passage and the draft tube are very strong. The unstable flow phenomenon including the swirl, flow separation, interruption and vortex strip. It can be deduced that the vibration of unit is induced by these flow characteristic.

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.

scholarly journals Noise characteristic analysis and sound sources identification for rod–airfoil interaction using different subgrid-scale models

2021 ◽  
Vol 233 ◽  
pp. 04036
Author(s):  
Chenghao Yang ◽  
Heying Feng ◽  
Yehui Peng
Keyword(s):  
Energy Transport ◽  
Pressure Fluctuations ◽  
Leading Edge ◽  
Scale Model ◽  
Subgrid Scale ◽  
Characteristic Analysis ◽  
Sound Sources ◽  
Eddy Simulation ◽  
Scale Models ◽  
The Impact

Four subgrid-scale models based on large eddy simulation (LES), such as Smagorinsky–Lilly (SL), dynamic Smagorinsky–Lilly (DSL), wall-adapting local eddy-viscosity (WALE), and dynamic kinetic-energy transport (KET) were used and couple Ffowcs Williams–Hawkings equation to accurately analyze and identify the characteristics and position of the sound sources of rod–airfoil interaction. The results of four models were compared with experimental data. It was found that the DSL model was the optimal subgrid-scale model for the study of the interaction noise considering the calculation accuracy. Therefore, the DSL model was selected for analyzing and identifying the characteristics and location of the interaction noise source. During the calculation, solid and permeable data surfaces were used for acoustic integral surfaces. The results show that the impact of the quadrupole source is negligible at a low Mach number, and the dipole noise coming from the pressure fluctuations is dominant. Meanwhile, the dipole noise from the airfoil is louder than that from the rod; the leading edge of about 30% chord length of airfoil the is the main sound source of interference effect. Above results can provide guidance for research of blade-vortex interaction noise.

Effect of Runner Blade Thickness on Flow Characteristics of a Francis Turbine Model at Low Flowrates

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Seung-Jun Kim ◽  
Young-Seok Choi ◽  
Yong Cho ◽  
Jong-Woong Choi ◽  
Jin-Hyuk Kim
Keyword(s):  
Stokes Equations ◽  
Guide Vane ◽  
Vortex Formation ◽  
Flow Characteristics ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Hydroelectric Power ◽  
Runner Blade ◽  
Blade Thickness ◽  
Interblade Vortices

Abstract Francis turbines are often used for generating hydroelectric power, but their performance characteristics significantly depend on the operating conditions. In particular, interblade vortices in the passages between runner blades can occur at low flowrates, which can degrade performance, and increase vibrations and instability during operation. In a previous study, we showed that the hydraulic performance and flow characteristics depend on the flow passage area of runner blades under low-flowrate conditions. Under such operating conditions, the runner blade thickness can affect the interblade vortex characteristics, and in turn, affect the performance of the turbine. In this study, we investigated the effect of runner blade thicknesses in the presence of interblade vortices under low flowrates; steady- and unsteady-state Reynolds-averaged Navier–Stokes equations were solved using a shear stress transport as a turbulence model. The interblade vortices were described well at the near leading and trailing edges near the hub. These vortex regions showed flow separation and stagnation flow, and the interblade vortex characteristics were dependent on the high-magnitude unsteady pressures at the low-frequency region. For the same guide vane opening, at lower flowrates, higher blockage ratios reduced interblade vortex formation and unsteady pressure.

scholarly journals The Effect of Vortex Shedding on the Unsteady Pressure Distribution Around the Trailing Edge of a Turbine Blade

1996 ◽  
Author(s):  
G. Cicatelli ◽  
C. H. Sieverding
Keyword(s):  
Pressure Distribution ◽  
Turbine Blade ◽  
Large Scale ◽  
Guide Vane ◽  
Turbine Blades ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Wake Flow ◽  
Trailing Edge ◽  
Unsteady Pressure

The wakes behind turbine blade trailing edge are characterized by large scale periodic vortex patterns known as the von Karman vortex street. The failure of steady-state Navier-Stokes calculations in modeling wake flows appears to be mainly due to ignoring this type of flow instabilities. In an effort to contribute to a better understanding of the time varying wake flow characteristics behind turbine blades, VKI has performed large scale turbine cascade tests to obtain very detailed information about the steady and unsteady pressure distribution around the trailing edge of a nozzle guide vane. Tests are run at an outlet Mach number of M2,is,=0.4 and a Reynolds number of Rec = 2·106. The key to the high spatial resolution of the pressure distribution around the trailing edge is a rotatable trailing edge with an embedded miniature pressure transducer underneath the surface and a pressure slot opening of about 1.5° of the trailing edge circle. Signal processing allowed or differentiation between random and periodic pressure fluctuations. Ultra-short schlieren pictures help in understanding the physics behind the pressure distribution.

scholarly journals Investigation of the Noise Induced by Unstable Flow in a Centrifugal Pump

Energies ◽  
2020 ◽  
Vol 13 (3) ◽  
pp. 589
Author(s):  
Jiaxing Lu ◽  
Xiaobing Liu ◽  
Yongzhong Zeng ◽  
Baoshan Zhu ◽  
Bo Hu ◽  
...  
Keyword(s):  
Centrifugal Pump ◽  
Frequency Band ◽  
Acoustic Pressure ◽  
Low Frequency ◽  
Internal Flow ◽  
Rotating Stall ◽  
Operating Conditions ◽  
Flow Instabilities ◽  
Unstable Flow ◽  
Pump Inlet

In order to investigate the mechanism and the characteristics of the noise induced by unstable flow in a centrifugal pump, the internal flow characteristics in the pump were numerically researched, and the acoustic pressure fluctuations at the pump inlet and outlet were experimentally investigated. Obvious corresponding relationships between the flow instabilities, the cavitation and the noise were established. It was found that the rotating stall, the backflow, the hump, the occurrence of unstable flow and the cavitation in such a centrifugal pump were effectively detected through the noise, which could help to provide fundamental information on flow instabilities and guarantee safe and steady operating conditions for the system. The recirculation and prewhirl regions in the pump upstream pipe, which were caused by the backflow and the rotation of the impeller, presented the circumferential movement with a spiral shape, causing apparent broadband fluctuations at low frequency band of the acoustic pressure. The backflow and rotating stall could also result in broadband fluctuations of the pump outlet noise, which was distributed from 100 Hz to 150 Hz. Meanwhile, the broadband fluctuations of the pump outlet acoustic pressure distributed in the low frequency range, which was produced by the occurrence of cavitation, moved to the lower frequency band as the flow rate increased. The enhanced broadband fluctuations of the pump inlet and outlet noise distributed from 1 kHz to 6 kHz were caused by the coupling between the cavitation-induced noise and the system-produced noise. The broadband fluctuations of the pump inlet noise distributed between 6 kHz and 9 kHz were regarded as the typical frequency band of cavitation in the centrifugal pump.

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