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.

scholarly journals Hysteresis Characteristic in Hump Region of a Pump-Turbine Model

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.

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

Water ◽  
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.

Mechanism of Fast Transition of Pressure Pulsations in the Vaneless Space of a Model Pump-Turbine During Runaway

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
Xiaoxi Zhang ◽  
Wei Zeng ◽  
Yongguang Cheng ◽  
Zhiyan Yang ◽  
Qiuhua Chen ◽  
...  
Keyword(s):  
Transient Flow ◽  
Characteristic Curve ◽  
Three Dimensional ◽  
Low Frequency ◽  
Vortex Structures ◽  
Pressure Pulsations ◽  
Pump Turbine ◽  
Whole Process ◽  
Time Variations ◽  
Flow Vortex

The pressure pulsations in the vaneless space of pump-turbines are extremely intense and always experience rapid time variations during transient scenarios, causing structural vibrations and even more serious accidents. In this study, the mechanism behind the rapid time variations of the vaneless space pressure pulsations in a model pump-turbine during runaway was analyzed through three-dimensional (3D) numerical simulations. These results show that the high-frequency pressure pulsation components originating from rotor–stator interactions (RSI) are dominant during the whole process. These components fluctuate significantly in frequency when the working point goes through the S-shaped region of the characteristic curve, with the amplitudes increasing. Meanwhile, some low-frequency pulsations are also enhanced and become obvious. These features can be attributed to the transitions of the inter blade vortex structures (IBVSs) to the forward flow vortex structures (FFVSs) and the back flow vortex structures (BFVSs) at the impeller entrance, when the pump-turbine operates in the region with S-shaped characteristics. The FFVSs mainly cause decreases in frequency and introduce low-frequency pulsations, while the BFVSs are responsible for the unstable fluctuations. These findings contribute to the understanding of how transient flow patterns evolve and may provide new ideas about avoiding severe pressure pulsations caused by rotating stalls in the pump-turbine during transient scenarios.

scholarly journals Numerical Study of Pump-Turbine Instabilities Under Pumping Mode Off-Design Conditions

2015 ◽  
Author(s):  
Uroš Ješe ◽  
Regiane Fortes-Patella ◽  
Matevž Dular
Keyword(s):  
Power Plants ◽  
Numerical Study ◽  
Guide Vane ◽  
Characteristic Curve ◽  
Rotating Stall ◽  
Electrical Network ◽  
Pump Turbine ◽  
Wide Range ◽  
Pumping Mode ◽  
And Performance

Pumped storage power plants, using reversible pump-turbines, are a great solution to maintain the stability of an electrical network. The continuous operating area of reversible pump-turbines machines is usually delimited by cavitation or a hydraulic instability called hump phenomena at part load. If the machine operates under these off-design conditions, it might be exposed to vibrations and performance losses. The paper focuses on the numerical analysis of the pumping mode regime and pays special attention to the prediction of the hump shaped characteristic curve and associated rotating stall. The investigations were made on a high head pump-turbine design (nq=27) at model scale for four different guide vane opening angles and a wide range of flow rates. Numerical simulations were performed and analyzed in LEGI and were compared to the global experimental data, provided by Alstom Hydro.

scholarly journals Analysis of the Guide Vane Jet-Vortex Flow and the Induced Noise in a Prototype Pump-Turbine

2019 ◽  
Vol 9 (10) ◽  
pp. 1971 ◽  
Author(s):  
Ran Tao ◽  
Zhengwei Wang
Keyword(s):  
Kinetic Energy ◽  
Vortex Flow ◽  
Guide Vane ◽  
Design Flow ◽  
Turbulence Kinetic Energy ◽  
Flow Structures ◽  
Opening Angle ◽  
Acoustic Analogy ◽  
Pump Turbine ◽  
Start Up

The start-up process of a pump-turbine in pump mode is found with obvious noise, especially at the small guide vane opening angle. The turbulent-flow-induced noise is an important part and must be reduced by flow control. Therefore, the computational fluid dynamics (CFD) method is used in this study to predict the internal flow in a high head prototype pump-turbine (the specific speed nq is 31.5) under an extremely off-design condition (Cφ = 0.015 and Cα = 0.096). The acoustic analogy method is also used to predict the near-field noise based on the turbulence field. Special undesirable flow structures including the flow ring between the runner trailing-edge and the guide vane, guide vane jet, twin-vortexes adjacent to guide vane jet, inter stay vane vortex, stay vane jet, and volute vortex-ring are found in a pump-turbine. These complex jet-vortex flow structures induce local high turbulence kinetic energy and an eddy dissipation rate, which is the reason why noise is generated at small guide vane opening angle. Three dominating frequencies are found on the turbulence kinetic energy pulsation. They are the runner blade frequency fb = 64.5 Hz, the dominate frequency in the guide vane and the stay vane fgsv = 9.6 Hz, and the dominate frequency in volute fvl = 3.2 Hz. The flow pulsation tracing topology gives a good visualization of frequency propagation. The dominating regions of the three specific frequencies are clearly visualized. Results show that different flow structures may induce different frequencies, and the induced specific frequencies will propagate to adjacent sites. This study helps us to understand the off-design flow regime in this prototype pump-turbine and provides guidance when encountering the noise and stability problems during pump mode’s start-up.

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.

Validation of Numerical Simulation for Rotating Stall in a Transonic Fan

2011 ◽  
Author(s):  
Minsuk Choi ◽  
Nigel H. S. Smith ◽  
Mehdi Vahdati
Keyword(s):  
Mass Flow ◽  
Transient Flow ◽  
Rotating Stall ◽  
Implicit Time ◽  
Stall Inception ◽  
Rotating Speed ◽  
Design Speed ◽  
Fan Speed ◽  
Loading Parameter ◽  
Transonic Fan

This paper addresses a comparison of numerical stall simulations with experimental data at 60% (subsonic) and 95% (supersonic) of the design speed in a modern transonic fan rig. The unsteady static pressures were obtained with high frequency Kulite transducers mounted on the casing upstream and downstream of the fan. The casing pressure variation was clearly visible in the measurements when a stall cell passed below the transducers. Numerical stall simulations were conducted using an implicit, time-accurate 3D compressible RANS solver. The comparisons between the experiment and simulation mainly cover performance curves and time-domain pressure traces of Kulites during rotating stall. At two different fan speeds, the stall characteristics such as the number and rotating speed of the stall cells were well-matched to the experimental values. The mass flow rate and the loading parameter under the fully-developed rotating stall also showed good agreement with the experiment. In both numerical and experimental results, a large stall cell was eventually formed after stall inception regardless of the fan speed. Based on the validation, the detailed flow has been evaluated to understand rotating stall in a transonic fan. In addition, it was found that the mass flow measurement using casing static pressure might be wrong during transient flow if the Kulites were mounted too close to the fan blade.

Aeromechanical Control of High-Speed Axial Compressor Stall and Engine Performance— Part I: Control-Theoretic Models

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
O. G. McGee ◽  
K. L. Coleman
Keyword(s):  
High Speed ◽  
Axial Compressor ◽  
Engine Performance ◽  
Maximum Flow ◽  
Rotating Stall ◽  
Design Flow ◽  
Systematic Evaluation ◽  
Stable Operation ◽  
Stall Inception ◽  
Axial Compressors

General methodologies are proposed in this two-part paper that further phenomenological understanding of compressible stall inception and aeromechanical control of high-speed axial compressors and engine performance. Developed in Part I are strategies for passive stabilization of compressible rotating stall, using tailored structural design and aeromechanical feedback control, implemented in certain classes of high-speed axial compressors used in research laboratories and by industry. Fundamentals of the stability of various dynamically-compensated, high-speed compressors was set down from linearized, compressible structural-hydrodynamic equations of modal stall inception extended further in this study from previous work. A dimensionless framework for performance-based design of aeromechanically-controlled compression system stall mitigation and engine performance is established, linking specified design flow and work-transfer (pressure) operability to model stages or local blade components, velocity triangle environment, optimum efficiency, extended stall margin and operability loci, and aeromechanical detailed design. A systematic evaluation was made in Part II (Coleman and McGee, 2013, “Aeromechanical Control of High-Speed Axial Compressor Stall and Engine Performance—Part II: Assessments of Methodology,” ASME J. Fluids Eng. (to be published)) on the performance of ten aeromechanical feedback controller schemes to increase the predicted range of stable operation of two laboratory compressor characteristics assumed, using static pressure sensing and local structural actuation to rudimentary postpone high-speed modal stall inception. The maximum flow operating range for each of the ten dynamically-compensated, high-speed compression systems was determined using optimized or “tailored” structural controllers, and the results described in Part II of the companion paper are compared to maximum operating ranges achieved in corresponding low-speed compression systems.

Investigation of the Flow Instabilities of a Low Specific Speed Pump-Turbine: Part 1 — Experimental and Numerical Analysis

2020 ◽  
Author(s):  
Sabri Deniz ◽  
Armando Del Rio ◽  
Martin von Burg ◽  
Manuel Tiefenthaler
Keyword(s):  
Experimental Data ◽  
Guide Vane ◽  
Turbulence Models ◽  
Operating Mode ◽  
Rotating Stall ◽  
Flow Instabilities ◽  
Pump Turbine ◽  
Flow Features ◽  
Low Specific Speed ◽  
Specific Speed

Abstract This is the first part of a two-part paper focusing on the flow instabilities of low-specific pump turbines. In this part, results of the CFD simulations and experiments of the research carried out on a low specific speed model pump-turbine at HSLU (Lucerne University of Applied Sciences) Switzerland are presented. The requirements of a stable and reliable pump-turbine operation under continuously expanding operating ranges, challenges the hydraulic design and requires new developments. Previous research at the HSLU [1] analyzed the instabilities of a medium specific speed (i.e. nq = 45) pump turbine. This paper presents the results of experimental (model pump-turbine at the test rig) and numerical (CFD) investigations of the pump-turbine instabilities of a low specific speed (nq = 25) pump-turbine in the turbine operating mode in the region of S-shaped characteristics (that is where the pump-turbine is synchronized and oscillations may occur during load rejection). The four-quadrant characteristics of a low specific speed model pump-turbine with two similar runners differentiating in the size (diameter) are measured. Testing of both runners with the same guide vane system provided information about the effects of the increased vaneless space (the distance between the guide vanes and runner) on the pump-turbine performance and stability both in turbine- and pump operating modes. A CFD methodology by using different numerical approaches and applying several turbulence models is developed in order to accurately predicting the characteristics of the reversible pump-turbines in the S-shaped region (speed no load conditions) as well as analyzing the flow features especially at off-design conditions. This CFD model is validated against the experimental data at 6° and 18° guide vane openings in turbine operating mode. With the measured data of the unsteady pressure measurements and detailed investigation of unstable ranges on the pump-turbine characteristics, flow instabilities in the low-specific speed model pump-turbine are analyzed. Relevant frequencies such as rotating stall, steady and unsteady vortex formations are determined. Based on the analysis of the experimental data 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.

Validation of Numerical Simulation for Rotating Stall in a Transonic Fan

2012 ◽  
Vol 135 (2) ◽  
Author(s):  
Minsuk Choi ◽  
Nigel H. S. Smith ◽  
Mehdi Vahdati
Keyword(s):  
Mass Flow ◽  
Transient Flow ◽  
Rotating Stall ◽  
Navier Stokes ◽  
Implicit Time ◽  
Stall Inception ◽  
Rotating Speed ◽  
Design Speed ◽  
Loading Parameter ◽  
Transonic Fan

This paper addresses a comparison of numerical stall simulations with experimental data at 60% (subsonic) and 95% (supersonic) of the design speed in a modern transonic fan rig. The unsteady static pressures were obtained with high frequency Kulite transducers mounted on the casing upstream and downstream of the fan. The casing pressure variation was clearly visible in the measurements when a stall cell passed below the transducers. Numerical stall simulations were conducted using an implicit, time-accurate, 3D compressible Reynolds-averaged Navier-Stokes (RANS) solver. The comparisons between the experiment and simulation mainly cover performance curves and time-domain pressure traces of Kulites during rotating stall. At two different fan speeds, the stall characteristics such as the number and rotating speed of the stall cells were well-matched to the experimental values. The mass flow rate and the loading parameter under the fully-developed rotating stall also showed good agreement with the experiment. In both the numerical and experimental results, a large stall cell was eventually formed after stall inception regardless of the fan speed. Based on the validation, the detailed flow has been evaluated to understand rotating stall in a transonic fan. In addition, it was found that the mass flow measurement using casing static pressure might be wrong during transient flow if the Kulites were mounted too close to the fan blade.

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