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

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.

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.

Improvements of Flow Control With Fluid Injection for the Suppression of Flow Instabilities in Pump-Turbines

2021 ◽  
Author(s):  
Sabri Deniz ◽  
Fabio Asaro
Keyword(s):  
Flow Control ◽  
Flow Injection ◽  
Water Injection ◽  
Operating Mode ◽  
Flow Instabilities ◽  
Fluid Injection ◽  
Cfd Simulations ◽  
Pump Turbine ◽  
Flow Features ◽  
Load Conditions

Abstract A stable and reliable pump-turbine operation under continuously expanding operating range requirement often imposes challenges on the hydraulic design of the pump-turbines and requires new developments. During a previous study carried out at the HSLU (Lucerne University of Applied Sciences, Switzerland), the flow instabilities of a low specific speed (i.e., nq = 25) pump-turbine were analyzed while a CFD methodology was developed through taking different numerical approaches and applying several turbulence models. The goal was to predict the turbine-mode characteristics of the reversible pump-turbines in the S-shaped region (at speed no load conditions) accurately as well as analyzing the flow features especially at off-design conditions. This CFD model was validated against the experimental data at different guide vane openings in turbine operating mode. 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 were explored. Furthermore, a flow control technology that entailed injecting air and water in the vaneless space of a model pump-turbine was implemented for suppressing the flow instabilities and thus extending the operating range of the pump-turbine. Both air- and water-injection were applied by using an external energy source (compressor and pump) and discrete nozzles that are distributed in the vaneless space circumferentially. The S-shaped pump-turbine characteristics in turbine operating mode were modified so that the slope at speed no load conditions was no more positive indicating an improvement in the stability behavior. 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 positive effect of fluid injection on the pump-turbine characteristics was also demonstrated in the CFD calculations. CFD was able to predict the pump-turbine dimensionless discharge, Kcm1, - speed, Ku1, characteristic curve with water injection correctly. After the CFD tool is validated for the prediction of the pump-turbine characteristics with fluid injection, further CFD simulations were carried out in order to improve the effectiveness of flow control and if possible, using less amount of injected fluid in the vaneless space. The goal was to optimize the fluid injection so that the instabilities can be suppressed with the lowest possible water/energy consumption. Parameters such as number of injection nozzles, nozzle position, nozzle diameter, and injection direction are varied. Several configurations of water injection system i.e., changing the number, location, and distribution of injection nozzles circumferentially and radially, direction of flow injection with respect to the main flow in the vaneless space, symmetrical and asymmetrical circumferential distribution of the nozzles in the vaneless space were analyzed using the CFD simulations. In addition to the flow injection in the vaneless space from the hub wall, fluid injection through the guide vanes was also investigated. The results of the fluid injection modifications were compared with the results of the baseline flow injection case. Using a parameter study, optimal nozzle configurations were found, that resulted in stable pump-turbine behavior in turbine operating mode with fewer injection nozzles and lower injected flow rate in comparison to the baseline case.

scholarly journals On the Rotor Stator Interaction Effects of Low Specific Speed Francis Turbines

2019 ◽  
Vol 2019 ◽  
pp. 1-11 ◽  
Author(s):  
Einar Agnalt ◽  
Igor Iliev ◽  
Bjørn W. Solemslie ◽  
Ole G. Dahlhaug
Keyword(s):  
Guide Vane ◽  
Pressure Fluctuations ◽  
Angular Position ◽  
Pressure Sensors ◽  
Pump Turbine ◽  
Blade Loading ◽  
Loading Case ◽  
Rotor Stator Interaction ◽  
Low Specific Speed ◽  
Specific Speed

The rotor stator interaction in a low specific speed Francis model turbine and a pump-turbine is analyzed utilizing pressure sensors in the vaneless space and in the guide vane cascade. The measurements are analyzed relative to the runner angular position by utilizing an absolute encoder mounted on the shaft end. From the literature, the pressure in the analyzed area is known to be a combination of two effects: the rotating runner pressure and the throttling of the guide vane channels. The measured pressure is fitted to a mathematical pressure model to separate the two effects for two different runners. One turbine with 15+15 splitter blades and full-length blades and one pump-turbine with six blades are investigated. The blade loading on the two runners is different, giving different input for the pressure model. The main findings show that the pressure fluctuations in the guide vane cascade are mainly controlled by throttling for the low blade loading case and the rotating runner pressure for the higher blade loading case.

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 Energy Balance and Local Unsteady Loss Analysis of Flows in a Low Specific Speed Model Pump-Turbine in the Positive Slope Region on the Pump Performance Curve

Energies ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1829 ◽  
Author(s):  
Guocheng Lu ◽  
Zhigang Zuo ◽  
Demin Liu ◽  
Shuhong Liu
Keyword(s):  
Discharge Coefficient ◽  
Guide Vane ◽  
Flow Patterns ◽  
Rotating Stall ◽  
Performance Curve ◽  
Positive Slope ◽  
Pump Turbine ◽  
Pump Performance ◽  
Loss Analysis ◽  
Low Specific Speed

The positive slope on the pump performance curve of pump-turbines suggests potential operational instabilities in pump mode. Previous research has indicated that the increase of the hydraulic loss caused by sudden changes of flow patterns in pump-turbines is responsible for the positive slope, however its detailed flow mechanism is still unclear. A low specific speed model pump-turbine was numerically investigated against experiments in the present study, by applying unsteady RANS (Reynolds-Averaged Navier–Stokes equations) simulations with a v2-f turbulence model. The mechanism of occurrence of the positive slope on the pump performance curve was discussed regarding the energy balance, as this region appears when the value of ∂ P u ∂ Q is larger than the critical value P u Q . An unsteady local loss analysis, derived from the energy equation, was conducted to illustrate the contribution of local flow patterns to the loss in corresponding hydraulic components. The variation of the kinetic energy of the mean flow was taken into account for the first time so that this method can be applied to highly time dependent flow patterns, e.g., a rotating stall in the present study. The investigations on the flow patterns revealed that some guide vane channels stalled with a larger discharge coefficient than the positive slope region. Several guide vane channels near the stalled channels were stalling with minor decrease of the discharge coefficient, leading to sudden increases of the input power and the loss. When the discharge coefficient slightly decreased in further, the pump-turbine operated into the positive slope region, and the rotating stall with 3 stall cells appeared, proven by the FFT (Fast Fourier Transform) and cross-phase analysis on the pressure fluctuations.

scholarly journals Experiments and numerical simulations of a flow instability in a low-specific-speed pump-turbine

2019 ◽  
Vol 240 ◽  
pp. 072042
Author(s):  
Koichiro Matsumura ◽  
Kazuhiko Yokota ◽  
Wakana Tsuru ◽  
Donghyuk Kang ◽  
Hayate Sugiyama
Keyword(s):  
Numerical Simulations ◽  
Flow Instability ◽  
Pump Turbine ◽  
Low Specific Speed ◽  
Specific Speed

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.

Part Load Instability and Rotating Stall in a Multistage Low Specific Speed Pump

2021 ◽  
Author(s):  
Erik Vermunt ◽  
Martijn Van Der Schoot ◽  
Bruurs Bruurs ◽  
Bart Van Esch
Keyword(s):  
Rotating Stall ◽  
Part Load ◽  
Low Specific Speed ◽  
Specific Speed
Sign in / Sign up

Export Citation Format

Share Document