Unsteady Numerical Simulation of Flow in Draft Tube of a Hydroturbine Operating Under Various Conditions Using a Partially Averaged Navier–Stokes Model

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Experimental Data ◽  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Pressure Recovery ◽  
Navier Stokes ◽  
Mean Velocity ◽  
Vortex Rope ◽  
Pressure Recovery Factor

The variable energy demand requires a great flexibility in operating a hydroturbine, which forces the machine to be operated far from its design point. One of the main components of a hydroturbine where undesirable flow phenomena occur under off-design conditions is the draft tube. Using computational fluid dynamics (CFD), the present paper studies the flow in the draft tube of a Francis turbine operating under various conditions. Specifically, four operating points with the same head and different flow rates corresponding to 70%, 91%, 99%, and 110% of the flow rate at the best efficiency point (BEP) are considered. Unsteady numerical simulations are performed using a recently developed partially averaged Navier–Stokes (PANS) turbulence model, and the results are compared to the available experimental data, as well as the numerical results of the traditionally used Reynolds-Averaged Navier–Stokes (RANS) models. Several parameters including the pressure recovery coefficient, mean velocity, and time-averaged and fluctuating wall pressure are investigated. It is shown that RANS and PANS both can predict the flow behavior close to the BEP operating condition. However, RANS results deviate considerably from the experimental data as the operating condition moves away from the BEP. The pressure recovery factor predicted by the RANS model shows more than 13% and 58% overprediction when the flow rate decreases to 91% and 70% of the flow rate at BEP, respectively. Predictions can be improved significantly using the present unsteady PANS simulations. Specifically, the pressure recovery factor is predicted by less than 4% and 6% deviation for these two operating conditions. A similar conclusion is reached from the analysis of the mean velocity and wall pressure data. Using unsteady PANS simulations, several transient features of the draft tube flow including the vortex rope and associated pressure fluctuations are successfully modeled. The formation of the vortex rope in partial load conditions results in severe pressure fluctuations exerting oscillatory forces on the draft tube. These pressure fluctuations are studied for several locations in the draft tube and the critical region with highest fluctuation amplitude is found to be the inner side of the elbow.

scholarly journals Effect of Air Injection on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Processes ◽  
2021 ◽  
Vol 9 (7) ◽  
pp. 1182
Author(s):  
Seung-Jun Kim ◽  
Yong Cho ◽  
Jin-Hyuk Kim
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Air Injection ◽  
Low Flow ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Vortex Rope

Under low flow-rate conditions, a Francis turbine exhibits precession of a vortex rope with pressure fluctuations in the draft tube. These undesirable flow phenomena can lead to deterioration of the turbine performance as manifested by torque and power output fluctuations. In order to suppress the rope with precession and a swirl component in the tube, the use of anti-swirl fins was investigated in a previous study. However, vortex rope generation still occurred near the cone of the tube. In this study, unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted with a scale-adaptive simulation shear stress transport turbulence model. This model was used to observe the effects of the injection in the draft tube on the unsteady internal flow and pressure phenomena considering both active and passive suppression methods. The air injection affected the generation and suppression of the vortex rope and swirl component depending on the flow rate of the air. In addition, an injection level of 0.5%Q led to a reduction in the maximum unsteady pressure characteristics.

Computational Study of Pump Turbine: Partial Load Operations

2020 ◽  
Author(s):  
Muhannad Altimemy ◽  
Justin Caspar ◽  
Alparslan Oztekin
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Computational Study ◽  
Pressure Fluctuations ◽  
System Stability ◽  
Pump Turbine ◽  
Vortex Rope ◽  
Signal Velocity ◽  
Partial Load ◽  
Les Model

Abstract The performance of a pump-turbine under partial flow rates, 85%, 75%, and 65%, is studied using the LES model. The power signal, velocity, vorticity, and pressure field is presented over the blades and throughout the draft tube. Pressure fluctuations are probed at various locations over the wall of the draft tube. Examining the flow field in the blade region can provide further insights into the system performance. Flow-induced pressure fluctuations can disrupt system stability. For this turbine, a strong swirling region is observed around the draft tube walls, causing pressure fluctuations. The size and intensity of this region decrease with the flow rate. A vortex rope is present in all cases. At the design point, the strength is constant throughout the draft tube. However, at partial load, the rope is weakened along the draft tube. Between the region dominated by the vortex rope and the wall, there is a swirling shear layer, which moves closer to the wall as the flow rate decreases. Both the magnitude of pressure fluctuations at the wall and the pressure difference over the blade decrease with the flow rate. The decreased pressure differences over the blade represent less power produced, and the decline in fluctuation magnitude at the wall represents more system stability. For this turbine, there appears to be a trade-off between power and strength of pressure fluctuations.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part II: Control of the Vortex Rope

2014 ◽  
Vol 81 (6) ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Companion Paper ◽  
Francis Turbine ◽  
Control Technique ◽  
Outer Flow ◽  
Vortex Rope ◽  
Stagnant Region ◽  
Partial Load

Numerical simulations and investigation of a method for controlling the vortex rope formation in draft tubes are carried out in this paper, which is the second part of a two-paper series. As shown in the companion paper, formation of the vortex rope is associated with a large stagnant region at the center of the draft tube. Therefore, it is concluded that a successful control technique should focus on the elimination of this region. In practice, this can be performed by axially injecting a small fraction (a few percent of the total flow rate) of water into the draft tube. Water jet is supplied from the high-pressure flow upstream of the turbine spiral case by a bypass line; thus, no extra pump is needed in this method. It is shown that this method is very effective in elimination of the stagnant region in a simplified draft tube operating at two part-load conditions, i.e., at 91% and 70% of the best efficiency point (BEP) flow rate. This results in improvement of the draft tube performance and reduction of hydraulic losses. The loss coefficient is reduced by as much as 50% for the case with 91% of BEP flow rate and 14% for the case with 70% of BEP flow rate. Unsteady, three-dimensional simulations show that the jet increases the axial momentum of flow at the center of the draft tube and decreases the wake of the crown cone and thereby decreases the shear at the interface of the stagnant region and high velocity outer flow, which ultimately results in elimination of the vortex rope. Furthermore, reduction (by about 1/3 in the case with 70% of BEP flow rate) of strong pressure fluctuations leads to reliable operation of the turbine.

scholarly journals Effect of Fins on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Energies ◽  
2020 ◽  
Vol 13 (11) ◽  
pp. 2806 ◽  
Author(s):  
Seung-Jun Kim ◽  
Young-Seok Choi ◽  
Yong Cho ◽  
Jong-Woong Choi ◽  
Jung-Jae Hyun ◽  
...  
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Low Flow ◽  
Francis Turbine ◽  
Vortex Rope ◽  
Turbine Model ◽  
Low Flow Rate

Undesirable flow phenomena in Francis turbines are caused by pressure fluctuations induced under conditions of low flow rate; the resulting vortex ropes with precession in the draft tube (DT) can degrade performance and increase the instability of turbine operations. To suppress these DT flow instabilities, flow deflectors, grooves, or other structures are often added to the DT into which air or water is injected. This preliminary study investigates the effects of anti-cavity fins on the suppression of vortex ropes in DTs without air injection. Unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted using a scale-adaptive simulation shear stress transport turbulence model to observe the unsteady internal flow and pressure characteristics by applying anti-cavity fins in the DT of a Francis turbine model. A vortex rope with precession was observed in the DT under conditions of low flow rate, and the anti-cavity fins were confirmed to affect the mitigation of the vortex rope. Moreover, at the low flow rate conditions under which the vortex rope developed, the application of anti-cavity fins was confirmed to reduce the maximum unsteady pressure.

scholarly journals Computational and Theoretical Analyses of the Precessing Vortex Rope in a Simplified Draft Tube of a Scaled Model of a Francis Turbine

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Girish K. Rajan ◽  
John M. Cimbala
Keyword(s):  
Discharge Coefficient ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Mean Flow ◽  
Scaled Model ◽  
Vortex Rope

Results on flows in a draft tube of a constant-head, constant-specific speed, model Francis turbine are presented based on computational fluid dynamics (CFD) simulations and theoretical analysis. A three-dimensional, unsteady, Navier–Stokes solver with the detached-eddy simulation (DES) model and the realizable k–ϵ (RKE) model is used to analyze the vortex rope formed at different discharge coefficients. The dominant amplitude of the pressure fluctuations at a fixed point in the draft tube increases by 13 times, and the length of the rope increases by 3.4 times when the operating point of the turbine shifts from a discharge coefficient of 0.37 to 0.34. A perturbation analysis based on a steady, axisymmetric, inviscid, incompressible model for the mean flow is performed to obtain a Sturm–Liouville (SL) system, the solutions of which are oscillatory if the discharge coefficient is greater than 0.3635, and nonoscillatory otherwise.

Analysis and Prevention of Vortex Rope Formation in the Draft Tube Cone of a Hydraulic Turbine

2012 ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Flow Rate ◽  
Draft Tube ◽  
Control Technique ◽  
Low Velocity ◽  
Recovery Coefficient ◽  
Outer Flow ◽  
Vortex Rope ◽  
Stagnant Region ◽  
Inner Region ◽  
Pressure Recovery Coefficient

Numerical simulations and analysis of the vortex rope formation in a draft tube cone of a Francis turbine operating at part-load conditions are carried out. Steady simulations are performed using a 2-D axisymmetric computational grid and unsteady simulations are carried out using a 3-D computational grid for a simplified axisymmetric draft tube. Two part-load operating conditions with same head and different flow rates are considered. The flow rates of these two operating points correspond to 91% of the flow rate at best efficiency point (case I) and 70% of the flow rate at best efficiency point (case II). Steady, axisymmetric simulations show the formation of a central stagnant region in the draft tube which becomes larger as flow rate decreases. This region results in flow blockage and reduction of the pressure recovery coefficient. It is shown that the pressure recovery coefficient is reduced by 46% by decreasing the flow rate from 91% of the best efficiency point (case I) to 70% of the best efficiency point (case II) while loss coefficient becomes 5 times larger. Present unsteady, three-dimensional simulations correctly predict the overall shape of the vortex rope and the calculated vortex rope frequency differs only 5% from experimental data. It is shown that the vortex rope is formed due to the roll-up of the shear layer at the interface between the low-velocity inner region (results from the crown cone wake) and highly swirling outer flow. Finally a flow control technique which uses a water jet injected from the runner crown tip along the axis is investigated. The jet accelerates the flow near the centerline (stagnant region) and decreases the relative velocity and thereby the shear between low-velocity inner region and high-velocity outer flow and hence prevents the vortex rope formation. The jet discharge is optimized for minimum overall losses. The optimized jet decreases total losses by 13% for case I and the vortex rope is eliminated. The fraction of water used for the optimized jet is less than 0.5% of the turbine discharge. As shown in the present study, this control technique can suppress severe pressure fluctuations resulting from the vortex rope formation.

Flow in the Simplified Draft Tube of a Francis Turbine Operating at Partial Load—Part I: Simulation of the Vortex Rope

2014 ◽  
Vol 81 (6) ◽  
Author(s):  
Hosein Foroutan ◽  
Savas Yavuzkurt
Keyword(s):  
Experimental Data ◽  
Axial Velocity ◽  
Draft Tube ◽  
Three Dimensional ◽  
Operating Conditions ◽  
Turbulence Closure ◽  
Francis Turbine ◽  
Outer Flow ◽  
Vortex Rope ◽  
Partial Load

Numerical simulations and analysis of the vortex rope formation in a simplified draft tube of a model Francis turbine are carried out in this paper, which is the first part of a two-paper series. The emphasis of this part is on the simulation and investigation of flow using different turbulence closure models. Two part-load operating conditions with same head and different flow rates (91% and 70% of the best efficiency point (BEP) flow rate) are considered. Steady and unsteady simulations are carried out for axisymmetric and three-dimensional grid in a simplified axisymmetric geometry, and results are compared with experimental data. It is seen that steady simulations with Reynolds-averaged Navier–Stokes (RANS) models cannot resolve the vortex rope and give identical symmetric results for both the axisymmetric and three-dimensional flow geometries. These RANS simulations underpredict the axial velocity (by at least 14%) and turbulent kinetic energy (by at least 40%) near the center of the draft tube, even quite close to the design condition. Moving farther from the design point, models fail in predicting the correct levels of the axial velocity in the draft tube. Unsteady simulations are performed using unsteady RANS (URANS) and detached eddy simulation (DES) turbulence closure approaches. URANS models cannot capture the self-induced unsteadiness of the vortex rope and give steady solutions while DES model gives sufficient unsteady results. Using the proper unsteady model, i.e., DES, the overall shape of the vortex rope is correctly predicted and the calculated vortex rope frequency differs only 6% from experimental data. It is confirmed that the vortex rope is formed due to the roll-up of the shear layer at the interface between the low-velocity inner region created by the wake of the crown cone and highly swirling outer flow.

Simulation of Wall Pressure Fluctuations on Simplified Automobile Shapes Using a Lattice Based Method

2005 ◽  
Author(s):  
Sivapalan Senthooran ◽  
Bernd Crouse ◽  
Ganapathi Balasubramanian ◽  
David Freed ◽  
Swen Noelting ◽  
...  
Keyword(s):  
Experimental Data ◽  
Numerical Scheme ◽  
Cfd Simulation ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Complex Flow ◽  
Wall Pressure Fluctuations ◽  
Flow Phenomena ◽  
High Reynolds ◽  
Rng Turbulence Model

A comparison of experimental data and CFD simulation results of wall pressure fluctuations on simplified geometries that generate flow structures similar to an automobile are presented. The numerical results have been obtained using the commercial software PowerFLOW 3.4p4a. The simulation kernel of this software is based on the numerical scheme known as the Lattice Boltzmann Method (LBM), combined with an RNG turbulence model. This scheme accurately captures time-dependent aerodynamic behavior of high Reynolds number flows over complex geometries, together with the acoustics. The geometries considered for this study represent the green house and the side mirror of a car. Spectral analysis is performed on the simulation data and the results are compared to the experimental data. This comparison provides good correlation between the simulation and experiment, and demonstrates the capability of this numerical scheme in predicting turbulent fluctuations due to complex flow phenomena.

Generation of Twin Vortex Rope in the Draft-Tube Elbow of a Francis Turbine During Deep Part-Load Operation

2021 ◽  
Author(s):  
Mohammad Hossein Khozaei ◽  
Arthur Favrel ◽  
Toshitake Masuko ◽  
Naoki Yamaguchi ◽  
Kazuyoshi Miyagawa
Keyword(s):  
Linear Correlation ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Model Tests ◽  
Francis Turbine ◽  
Deep Part ◽  
Swirl Number ◽  
Vortex Rope ◽  
Part Load

Abstract This paper focuses on the generation of twin vortex rope in the draft-tube elbow of a Francis turbine at deep part-load operation through analyzing the results of model tests along with numerical simulations. Model tests, including pressure fluctuations measurements, are conducted over 10 speed factors. By considering the frequency of the pressure fluctuations with respect to the swirl intensity at the runner outlet, the part-load operating range is divided into three regimes, with two clear transitions between each occurring at swirl numbers 0.4 and 1.7. For operating conditions with a swirl number S>0.4, a linear correlation between the frequency of the precessing vortex core and the swirl number is established. During deep part-load regime (S>1.7), low-frequency pressure fluctuations appear. Their frequency feature another linear correlation with the swirl number. Unsteady CFD simulation of the full domain is performed to elucidate the generation mechanisms of the low-frequency fluctuations. By tracking the center of the vortical structures along the draft-tube, generation of three vortices in the elbow responsible for the pressure fluctuations at the lowest frequency is highlighted: the main PVC hits the draft-tube wall in the elbow resulting in its break down into three vortices rotating with half the rotational speed of the PVC. Two of the vortices rotate with opposite angular position, constituting a structure of twin vortices. The periodic rotation of these three vortices in the elbow induces the low-frequency pressure fluctuations.

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