Detection of Draft Tube Surge and Erosive Blade Cavitation in a Full-Scale Francis Turbine

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Xavier Escaler ◽  
Jarle V. Ekanger ◽  
Håkon H. Francke ◽  
Morten Kjeldsen ◽  
Torbjørn K. Nielsen
Keyword(s):  
Draft Tube ◽  
Guide Vane ◽  
Full Range ◽  
Pressure Fluctuations ◽  
Acoustic Emissions ◽  
Suction Side ◽  
Full Scale ◽  
Francis Turbine ◽  
Complete Measurement ◽  
Rotor Stator Interaction

A full-scale Francis turbine has been experimentally investigated over its full range of operation to detect draft tube swirling flows and cavitation. The unit is of interest due to the presence of severe pressure fluctuations at part load and of advanced blade suction-side cavitation erosion. Moreover, the turbine has a particular combination of guide vanes (20) to runner blades (15) that makes it prone to significant rotor-stator interaction (RSI). For that, a complete measurement system of dynamic pressures, temperatures, vibrations, and acoustic emissions has been setup with the corresponding transducers mounted at selected sensitive locations. The experiments have comprised an efficiency measurement, a signal transmissibility evaluation, and the recording of the raw signals at high sampling rates. Signal processing methods for demodulation, peak power estimation, and cross correlation have also been applied. As a result, draft tube pressure fluctuations have been detected around the Rheingans frequency for low loads and at 4% of the rotating frequency for high loads. Moreover, maximum turbine guide bearing acoustic emissions have been measured at full load with amplitude modulations at both the guide vane passing frequency and the draft tube surge frequency.

Simulations and Measurements of Pressure Oscillations Caused by Vortex Ropes

2006 ◽  
Vol 128 (4) ◽  
pp. 649-655 ◽  
Author(s):  
Zhengwei Wang ◽  
Lingjiu Zhou
Keyword(s):  
Stokes Equations ◽  
Draft Tube ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Suction Side ◽  
Rotational Frequency ◽  
Francis Turbine ◽  
Pressure Oscillations ◽  
Hydraulic Unit ◽  
Load Conditions

Pressure oscillations caused by vortex rope were measured in the draft tube of a prototype Francis turbine. The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with the RNG κ−ϵ turbulence model were solved to model the flow within the entire flow path of the prototype hydraulic unit including the guide vanes, the runner, and the draft tube. The model was able to predict the pressure fluctuations that occur when operating at 67–83% of the optimum opening. The calculated frequencies and amplitudes of the oscillation show reasonable agreement with the experiment data. However, the results at 50% opening were not satisfactory. Pressure oscillations on the runner blades were found to be related to the precession of vortex ropes which caused pressure on the blades to fluctuate with frequencies of −fn+fd (fn is the rotational frequency and fd is vortex procession frequency). The peak-to-peak amplitudes of the pressure oscillations on the blades at the lower load conditions (67% opening) were higher than at higher load conditions (83% opening). Fluctuations on the suction side tended to be stronger than on the pressure side.

Experimental Study of the Francis Turbine Pressure Fluctuations and the Pressure Fluctuations Superposition Phenomenon Inside the Runner

2017 ◽  
Vol 140 (4) ◽  
Author(s):  
Zhongxin Gao ◽  
Wenruo Zhu ◽  
Long Meng ◽  
Jianguang Zhang ◽  
Fei Zhang ◽  
...  
Keyword(s):  
Data Storage ◽  
Measuring Equipment ◽  
Draft Tube ◽  
Guide Vane ◽  
Pressure Fluctuations ◽  
Francis Turbine ◽  
Storage Device ◽  
Pressure Side ◽  
Suction Surface ◽  
Large Pressure

The pressure fluctuations in both the rotating runner and the other fixed components in a model Francis turbine under various loads were experimentally measured by means of onboard measuring equipment in the runner and data storage device on the shaft in this study. Large pressure fluctuations were observed under both small guide vane opening and large guide vane opening conditions. Flow separation at the blade suction surface led to large pressure fluctuations for small guide vane openings, the unsteady flow around the inlet on the blade pressure side led to large pressure fluctuations for large openings. The pressure fluctuations correlation between the runner and other components of the turbine, mainly the draft tube, was analyzed in detail for both small guide vane opening (12 deg) and large guide vane opening (30 deg). The results show that the pressure fluctuations in the runner space increased by the superposition of draft tube vortex rope pressure fluctuations and runner inter blade vortices pressure fluctuations, resulting in much larger pressure fluctuations in the runner space than in other components.

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.

Performance of Francis Turbine Operating at Excess Load Condition

2020 ◽  
Author(s):  
Muhannad Altimemy ◽  
Justin Caspar ◽  
Alparslan Oztekin
Keyword(s):  
Flow Field ◽  
Draft Tube ◽  
Turbine Blades ◽  
Pressure Fluctuations ◽  
Load Condition ◽  
Operating Conditions ◽  
Francis Turbine ◽  
Partial Load ◽  
Dynamics Simulations ◽  
Excess Load

Abstract Computational fluid dynamics simulations are conducted to characterize the spatial and temporal characteristics of the flow field inside a Francis turbine operating in the excess load regime. A high-fidelity Large Eddy Simulation (LES) turbulence model is applied to investigate the flow-induced pressure fluctuations in the draft tube of a Francis Turbine. Probes placed alongside the wall and in the center of the draft tube measure the pressure signal in the draft tube, the pressure over the turbine blades, and the power generated to compare against previous studies featuring design point and partial load operating conditions. The excess load is seen during Francis turbines in order to satisfy a spike in the electrical demand. By characterizing the flow field during these conditions, we can find potential problems with running the turbine at excess load and inspire future studies regarding mitigation methods. Our studies found a robust low-pressure region on the edges of turbine blades, which could cause cavitation in the runner region, which would extend through the draft tube, and high magnitude of pressure fluctuations were observed in the center of the draft tube.

scholarly journals Investigation of Pressure Fluctuation and Pulsating Hydraulic Axial Thrust in Francis Turbines

Energies ◽  
2020 ◽  
Vol 13 (7) ◽  
pp. 1734
Author(s):  
Xing Zhou ◽  
Changzheng Shi ◽  
Kazuyoshi Miyagawa ◽  
Hegao Wu ◽  
Jinhong Yu ◽  
...  
Keyword(s):  
Draft Tube ◽  
Renewable Energy Sources ◽  
Pressure Fluctuations ◽  
Dominant Frequency ◽  
Rotational Frequency ◽  
Rapid Expansion ◽  
Francis Turbine ◽  
Axial Thrust ◽  
Partial Load ◽  
Hydropower Stations

Under the circumstances of rapid expansion of diverse forms of volatile and intermittent renewable energy sources, hydropower stations have become increasingly indispensable for improving the quality of energy conversion processes. As a consequence, Francis turbines, one of the most popular options, need to operate under off-design conditions, particularly for partial load operation. In this paper, a prototype Francis turbine was used to investigate the pressure fluctuations and hydraulic axial thrust pulsation under four partial load conditions. The analyses of pressure fluctuations in the vaneless space, runner, and draft tube are discussed in detail. The observed precession frequency of the vortex rope is 0.24 times that of the runner rotational frequency, which is able to travel upstream (from the draft tube to the vaneless space). Frequencies of both 24.0 and 15.0 times that of the runner rotational frequency are detected in the recording points of the runner surface, while the main dominant frequency recorded in the vaneless zone is 15.0 times that of the runner rotational frequency. Apart from unsteady pressure fluctuations, the pulsating property of hydraulic axial thrust is discussed in depth. In conclusion, the pulsation of hydraulic axial thrust is derived from the pressure fluctuations of the runner surface and is more complicated than the pressure fluctuations.

Long-Period Pressure Pulsation Estimated in Numerical Simulations for Excessive Flow Rate Condition of Francis Turbine

2014 ◽  
Vol 136 (7) ◽  
Author(s):  
Kenji Shingai ◽  
Nobuaki Okamoto ◽  
Yuta Tamura ◽  
Kiyohito Tani
Keyword(s):  
Numerical Simulations ◽  
Flow Rate ◽  
Draft Tube ◽  
Homogeneous Model ◽  
Francis Turbine ◽  
Computational Time ◽  
Condensation Process ◽  
Rate Condition ◽  
Long Period ◽  
Rotor Stator Interaction

A series of numerical simulations for a Francis turbine were carried out to estimate the unsteady motion of the cavity in the draft tube of the turbine under a much larger flow rate condition than the swirl-free flow rate. The evaporation and condensation process was described by using a simplified Rayleigh–Plesset equation. A two-phase homogeneous model was adopted to calculate the mixture of gas and liquid phases. Instantaneous pressure monitored at a point on the draft tube formed long-period pulsations. Detailed analysis of the simulation results clarified the occurrence of a uniquely shaped cavity and the corresponding flow pattern in every period of the pressure pulsations. The existence of a uniquely shaped cavity was verified with an experimental approach. A simulation without rotor-stator interaction also obtained long-period pulsations after an extremely long computational time. This result shows that the rotor-stator interaction does not contribute to the excitation of long-period pulsations.

Hydraulic Stability Analysis of a Large Prototype Francis Turbine Based on Field Test

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Weiyu Wang ◽  
Qijuan Chen ◽  
Donglin Yan
Keyword(s):  
Full Range ◽  
Pressure Fluctuations ◽  
Field Tests ◽  
Francis Turbine ◽  
Operation Conditions ◽  
Amplitude Increase ◽  
Long Time ◽  
The Time Domain ◽  
Frequency Band Energy ◽  
Hydraulic Stability

Long time field tests of a 200 MW prototype Francis turbine over its full range of operation were conducted. From the experimental data, the time domain and frequency domain characteristics of the pressure fluctuations in the Francis turbine at different operation conditions were analyzed. Furthermore, the reason for the amplitude increase of pressure fluctuations and the correlation between the vibration and the pressure fluctuation was studied by using a multidimensional frequency band energy ratio analysis method. Based on the above analysis, some hydraulic stability characteristics of the large prototype Francis turbine are found, and other results are also obtained.

scholarly journals Investigating the Performance of a Super High-head Francis Turbine under Variable Discharge Conditions Using Numerical and Experimental Approach

Energies ◽  
2020 ◽  
Vol 13 (15) ◽  
pp. 3868 ◽  
Author(s):  
Zheming Tong ◽  
Hao Liu ◽  
Jianfeng Ma ◽  
Shuiguang Tong ◽  
Ye Zhou ◽  
...  
Keyword(s):  
Draft Tube ◽  
Numerical Models ◽  
Guide Vane ◽  
Maximum Velocity ◽  
Francis Turbine ◽  
Inlet Pipe ◽  
Guide Vanes ◽  
Flow Acceleration ◽  
High Head ◽  
Reduced Scale

A super high-head Francis turbine with a gross head of nearly 700 m was designed with computational fluid dynamics (CFD) simulation and laboratory tests. Reduced-scale (1:3.7) physical and numerical models of the real-scale prototype were created to investigate the hydraulic performance. According to the CFD analysis, a strong rotor–stator interaction (RSI) between guide vanes and runner blades is observed as a result of the high-speed tangential flow towards runner created by the super high water head as well as the small gaps between the radial blades. At the designed best efficiency point (BEP), there is no significant flow recirculation inside the flow passage and minor loss occurs at the trailing edge of the stay vanes and guide vanes. Maximum velocity is observed at runner inlets due to flow acceleration through the narrow passages between the guide vanes. The elbow-shaped draft tube gradually decreases the flow velocity to keep the kinetic energy loss at a minimum. The laboratory test was conducted on a reduced-scale physical model to investigate the pressure pulsations and guide vane torque (GVT) under variable-discharge configurations, which are key concerns in the design of a high head turbine. Pressure sensor networks were installed at the inlet pipe, vaneless space and draft tube, respectively. The most intense pressure variation occurs at the inlet pipe and elbow at 0.04–0.2 GVOBEP and 1.5–1.8 GVOBEP with a low frequency about 0.3 times of the runner frequency, while the vibration in vaneless zone performs stable with the blade passing frequency caused by RSI. The GVT shows a declining trend and then keeps stable as GVOs increases at synchronized condition. For the misaligned conditions, the torque of adjacent guide vanes differs a lot except at the synchronous angle and maximum absolute value at least doubles than the synchronized condition.

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.

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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