Guide Vane Opening Research Articles

Experimental Investigation and Performance Characteristics of Francis Turbine with Different Guide Vane Openings in Hydro Distributed Generation Power Plants

This article presents a study on the performance characteristics of a Francis turbine operating with various guide vane openings to determine the best operating point based on unit quantities. The guide vane openings were specified based on the width between the vanes at their exit, i.e., 10 mm, 13 mm, 16 mm, and 19 mm. The performance characteristic curves of the Francis turbine—head versus speed, torque versus speed, discharge versus speed, and efficiency versus speed—were obtained at various input power and guide vane openings. From these data, unit curves were plotted and the corresponding best efficiency points were obtained. The highest efficiency of 50.25% was obtained at a guide vane opening of 19 mm. The values of head, discharge, speed, and output power at BEP were 7.84 m, 13.55 lps, 1250 rpm, and 524 W, respectively.

Investigation of flow behavior causing extraordinary stress on blade during start-up of the Francis turbine

This paper describes flow behavior which causes extraordinary transient stress on runner blade that occurs during start-up of a Francis turbine. Firstly, transient computational fluid dynamics (CFD) was conducted to evaluate hydraulic similarity for the condition of extremely low rotation speed and small guide vane opening. CFD results show that the pressure is below saturation vapor pressure all over the inside of the runner at extremely low rotation speed conditions. Secondly, model test was carried out to measure and evaluate pressure and strain on the blade. The test results revealed that there is a tendency that the phenomenon becomes more severe when rotation speed is lower, when guide vane opening is larger, and when suction pressure is lower. It is concluded that the extraordinary phenomena are not ‘extraordinary’ but commonly arisen as the transient hydraulic behavior and the impact load causing extraordinary stress on runner blades is related to the instantaneous condensation of vapor bubbles such as steam hammer accompanied by the explosive pressure rise.

Flow-induced vibration analysis of a prototype pump-turbine runner during turbine start-stop transient process

Recently more start-stops per day of the pump-turbine units are required by the power grid for load regulation. During the turbine start-up and shut-down transient processes, the flow and pressure of the prototype pump-turbine change dramatically at different guide vane opening angles. The extremely unsteady pressure fluctuation in the flow passages can induce large deformation, stress concentration, and strong vibration of the pump-turbine runner. Therefore, it is significantly important to study the unsteady flow characteristics and the corresponding flow-Induced vibration of the runner during turbine start-up and shut-down transient process. In this investigation, a 3D model of the pump-turbine unit including the flow passages of the spiral casing, stay vane, guide vane, runner, and draft tube, crown chamber, band chamber, labyrinth seals, and balance pipes are constructed. The flow characteristics of the unit during the turbine start-up and shut-down process were analysed by coupling the 1D pipeline calculation and 3D turbine flow domain calculation. By mapping the pressure distribution of the fluid domain to the runner structure domain, the flow-induced dynamic behavior of the pump-turbine runner is performed, and the large deformation and stress concentration of the runner are investigated in detail. The flow-induced vibration results achieved are able to provide meaningful suggestions for safe operation and for improving the pump-turbine runner design.

Characterization of the hydrodynamic instabilities in a pump-turbine operating at part load in turbine mode

Pump-turbines (RPT) nowadays represents the most common mechanical equipment adopted in the new generation of storage hydro plant. In order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources, RPT are forced to rapidly switch between the pumping and generating mode also extending their operation under off-design conditions in unstable operating areas. Because of the design criterion adopted for the development of a RPT, an unstable behavior represented by a typical S-shaped profile with a positive slope in the machine’s characteristic can occur near to the runaway condition.With the purpose of evaluating the evolution of the fluid field near to the no-load condition, an in-depth CFD analysis of the RPT model test of the Norwegian Hydropower Center is performed by retracing the machine’s characteristic curve running through the flow-speed characteristic curve up to the turbine brake region for fixed guide vanes opening. To validate the numerical results, a comparison with the experimental results in terms of characteristic curves and pressure signals is performed.The results allow to capture the 3D characteristics of the unsteady phenomena, progressively evolving in an organized rotating stall, highlighting also the influence of the flow rate change from partial loads to the turbine brake operation on their development. In order to characterize the pulsating nature of the instability phenomena developing in the runner and in the rotor-stator interaction, a time-frequency analysis is performed on the numeric pressure and torque signals. The combination between fluid-dynamic and time-frequency analysis makes it possible to identify and characterize three evolution phases: inception, growth and consolidation.

Procedure to minimize rotor vibrations from flow-induced excitations in Kaplan turbines

The flow pattern in the draft tube is commonly considered when investigating rotor vibration problems, especially for Francis turbines. However, phenomenon in the vaneless space for Kaplan turbines have been shown to induce rotor vibrations when operating in non-standard conditions such as speed no load (SNL). These flow disturbances develop in the vaneless space at small guide vane openings and high swirls. Depending on the number of flow disturbances and runner blades they can excite transversal, axial or torsional vibrations. In the case of transversal excitation they can in some cases cause resonance problems if the frequency is close to a natural frequency of the rotating structure. Recently it was observed that the excitation causes severe resonance in the rotating structure on a refurbished turbine when operated at SNL.The present paper presents frequency analysis of the flow-induced excitations coupled to the shaft vibrations on an old and a refurbished Kaplan turbine. To investigate the causes of the resonance problem, measurements have been performed on both the model and prototype turbine and on a twin unit with the old runner design. The measurements on the prototypes consist of pressure, shaft bending moment, and vibrations. Pressure and radial forces were measured on the model.The result shows the dependence of the runner blade opening on the transversal excitation and the frequency of the flow disturbances. The rotordynamic analysis showed the same phenomena with forward and backward precession. The results agree well between the model and prototype when measured data are normalized by the runner rotational frequency. It is proposed, when refurbishing a unit, to use measured pressures, shaft bending moment and vibrations on the prototype before the refurbishment and the new model turbine to identify critical excitation frequencies for the new prototype turbine, i.e., use the information from the tests to set rotordynamic requirements for the new prototype unit.

Degradation Trend Prediction of Hydropower Units Based on a Comprehensive Deterioration Index and LSTM

Deterioration trend prediction of hydropower units helps to detect abnormal conditions of hydropower units and can prevent early failures. The reliability and accuracy of the prediction results are crucial to ensure the safe operation of the units and promote the stable operation of the power system. In this paper, the long short-term neural network (LSTM) is introduced, a comprehensive deterioration index (CDI) trend prediction model based on the time–frequency domain is proposed, and the prediction accuracy of the situation trend of hydropower units is improved. Firstly, the time–domain health model (THM) is constructed with back-propagation neural network (BPNN) and condition parameters of active power, guide vane opening and blade opening and the time–domain indicators. Subsequently, a frequency-domain health model (FHM) is established based on ensemble empirical mode decomposition (EEMD), approximate entropy (ApEn), and k-means clustering algorithm. Later, the time–domain degradation index (TDI) is developed according to THM, the frequency-domain degradation index (FDI) is constructed according to FHM, and the CDI is calculated as a weighted sum by TDI and FDI. Finally, the prediction model of LSTM is proposed based on the CDI to achieve degradation trend prediction. In order to validate the effectiveness of the CDI and the accuracy of the prediction model, the vibration waveform dataset of a hydropower plant in China is taken as a case study and compared with four different prediction models. The results demonstrate that the proposed model outperforms other comparison models in terms of predicting accuracy and stability.

Analysis of the Flow Energy Loss and Q-H Stability in Reversible Pump Turbine as Pump with Different Guide Vane Opening Angles

The internal flow problem of a reversible pump turbine restricts its safe and stable operation. Among them, the influence of the guide vane on the internal flow field is very crucial. The flow–head relationship is of great significance in the performance stability of the unit. In this study, the performance and flow field characteristics under different flow rates were analyzed for different guide vane opening angles. By comparing the results of the model test and computational fluid dynamics simulation, it was found that the simulation can well predict the energy characteristics and flow field distribution. There is an optimal efficiency range under each guide vane opening angle. The increase or decrease in flow will reduce the efficiency. For the head, it will decrease significantly with a decrease in the flow rate, especially when it deviates seriously from the optimal efficiency region. From the contour of the flow energy loss and the vector of velocity, it can be seen that the head drop is closely related to the flow blockage caused by the difference between the runner incoming flow direction and the installation direction of the guide vane. This study deeply revealed the valley and peak of head variation under different guide vane opening conditions. It can provide technical support for improving the wide range operation stability of a pump turbine.

Enhanced stability and failure avoidance of hydropower plant in contingent island operation by model predictive frequency control

The challenges of contingent island operation of hydropower plants are addressed by proposing an enhanced frequency control with advanced control algorithms. The study is based on a real power plant and considers the relevant power system in its entirety along with the load and transmission lines. Integrated transfer functions between the guide vane opening of a hydro turbine and the frequency of the associated generator were determined by in-situ identification. A model predictive controller, a fractional order PID controller, and a PID controller utilizing integral absolute error and integral time absolute error criteria were designed and developed. The performance of the proposed algorithms with the identified plant model was tested on a NI-cRIO 9049 FPGA platform. Improved frequency control in terms of setpoint change was achieved with the MPC and FOPID controllers. The MPC controller also features betters disturbance rejection compared to conventional PID controllers, which are tuned according to integral error criteria. The considered algorithms have been simulated in different operating scenarios, such as variations of active power load and transmission length. The proposed approach enhances stability and consequently avoids operational failures of hydropower plants in contingent islanding mode.

Experimental and numerical simulation research on flow characteristics of model pump-turbine in four-quadrant operating quadrants

Owing to the need for power grid peak regulation and frequency regulation, the pumped storage unit has to undergo frequent working condition conversion, and the pump turbine runs across the four quadrants of the full characteristics during the transition. This study used the full flow numerical simulation and particle imaging velocimetry test to examine the internal flow of three guide vane openings. Consequently, the evolution law of the internal flow characteristics of the pump turbine was obtained. According to our results, in the high-efficiency areas of the turbine and the pump, the runner and guide vane areas flowed steadily, whereas, under other unsteady conditions, there was a vortex structure between the runner area and the vane area. Except for the pump-braking mode, all the most unstable flow areas appeared in the vaneless space. The flow in the runner channel was most unstable in pump-braking mode. Finally, we obtained test images of different operating points in the four-quadrant operating area as well as vortex motion trends.

Experimental and Computational Fluid Dynamics Study of the Flow Field of a Model Pump Turbine

The pumped storage unit has a hump area and S characteristics during operation. The operating conditions of the unit are complex, and it is necessary to switch between working conditions. The unit frequently crosses the hump area and the S characteristic area, thus hindering the unit to connect to the grid. Severe vibration affects the safe and stable operation of a power station. With the joint efforts of the industry, the impact of the hump and S-shaped problems on project commissioning has been fundamentally resolved, but the mechanism of inducing hump and S-shaped problems has not yet been well established. Therefore, in this study, the particle image velocity measurement method based on a full-characteristic test was adopted. According to the operating characteristics of the unit, 32 operating points with four guide-vane opening degrees were selected to test the flow pattern in the vaneless space. After subtracting the average flow velocity, the vortex image of the leafless area was obtained, which provided a reference for the design of the runner of the water pump turbine.

Research on the Internal Flow Characteristics of Pump Turbines for Smoothing the Output Fluctuation of the Wind–Photovoltaic Complementary System

Wind and photovoltaic (PV) power generation and other distributed energy sources are developing rapidly. But due to the influence of the environment and climate, the output is very unstable, which affects the power quality and power system stability. Pumped hydroelectric energy storage (PHES) systems are suitable as peaking power sources for wind and photovoltaic (Wind–PV) complementary systems because of their fast start–stop and long life. The mathematical models and operational characteristics of the three subsystems in the wind–PV–PHES complementary system are analyzed to improve the generation efficiency and access capacity of wind and PV power. The peaking characteristics of the PHES system are used to balance the maximum benefit and minimum output fluctuation of the wind–PV complementary system. The stable operation of the pump turbine is an important guarantee for the smooth output of the wind–PV complementary system. Three operating points are selected from the net load curve and converted to the pump turbine model parameters. The internal flow characteristics and laws of the pump turbine under different guide vane opening conditions are summarized through the analysis of the computational fluid dynamics (CFD) numerical simulation post-processing results. The study shows that the output of wind and PV power generation varies with the changes in wind speed and solar radiation, respectively. The output of the wind–PV complementary system still has large fluctuations, and the PHES system can effectively suppress the power fluctuation of the wind–PV complementary system and reduce the abandoned wind and light rate. CFD technology can accurately and efficiently characterize the internal flow characteristics of the pump turbine, which provides a basis for the design, optimization, and transformation of the pump turbine.

Study on the characteristics of vortex motion and pressure pulsation in vaneless zone of Kaplan turbine

Kaplan turbine with adjustable blade has wide operating efficiency and is widely used in medium and low head hydropower stations. Pressure pulsation has always been a key problem affecting the operating efficiency and stability of hydraulic turbines. The numerical simulation of the hydraulic characteristics of Kaplan turbine is carried out to analyze the pressure pulsation and flow distribution characteristic in the vaneless zone under different guide vane openings and blade installation angles. Results show that: At the small flow condition, vortices appear in the vaneless zone, which leads to the increase of pressure pulsation in the vaneless zone and the expansion of the fluctuation range of water moment on blade. At the small flow condition with different unit speed, the dominant frequency are observed to be between 4-5 times rotation frequency in the vaneless zone far from the runner inlet. The frequency source is that the rotating vortex flow and the rotating runner blade will form the effect similar to rotor-stator interference.The pressure pulsation observed at the monitoring point near the runner inlet mainly comes from the disturbance of the runner blade to the flow and the pressure pulsation frequency is 6fn .

Experimental study of Crossflow turbine under different operating conditions

In the present study, experiments were conducted to investigate the performance of Cross-flow turbines (CFT). A CFT runner with 16 degrees of angle of attack and a ratio of 0.67 inner to the outer diameter is selected for the experiment. The efficiency of the turbine was studied using a 3D printed runner with 30 blades at a constant head of 10m and varying rpm from 350 to 750 with an increment of 100 rpm and guide vane opening ranging from 10% to 100% with an increment of 10%. The results of the study indicate that CFT can be used effectively as it contains a flat curve that works well over wide flow conditions. CFT efficiency increases with increasing rpm to a certain point, after which it begins to decrease with increasing rpm.

Head-Cover vibration investigation of a Prototype Reversible Pump-Turbine Unit during Start-up in Pump mode. Part I: Fluid dynamic analysis

Reversible pump-turbine units of pumped storage power stations can generate power in turbine mode and store electric energy in pump mode for load balancing of the electric power grid. During the start-up transient process in pump mode of a reversible pump-turbine, the flow pattern and the pressure distribution in the flow channel change dramatically with the increasing opening angle of guide vanes. The unsteady pressure of the flow passing channel can cause severe vibration on the head-cover, bottom ring, and other stationary structures around the impeller. In this paper, a coupled 1D/3D co-simulation approach is developed to investigate the pressure distribution characteristics of the flow field during the start-up transient process in pump mode for a large prototype reversible pump-turbine unit. The entire model of flow channel from spiral casing to draft tube is constructed and the flow characteristics of key fluid fields such as spiral casing, stay vanes, guide vanes, impeller, and draft tube are analyzed. The simulation results show that under the condition of minimum guide vane opening the flow is insufficient and vortices occur on the impeller blades. The pressure distribution results of the flow channel from 3D fluid dynamic analysis during the start-up transient process in pump mode are exported as boundary conditions for the subsequent fluid-structure coupling analysis of the head-cover vibration investigation.

Numerical study on the effect of guide vane opening and draft tube outlet pressure on cavitation in Kaplan turbine

variations in electricity demand and site conditions like mass flow rate and net head, force the turbines to operate under off-design conditions. This increases cyclic loading in stationary and rotating components and severe cavitation in low head turbines like Kaplan turbine. The present analysis aims to examine the influence of various operating conditions on hydraulic efficiency and cavitation characteristics using ANSYS CFX. The stages of cavitation development in the runner are extensively studied for different operating conditions. The key point of present work is the critical analysis of various flow parameters for cavitating and non-cavitating conditions. The value of critical cavitation factor is found to increase from 0.34 to 0.61 with an increase in guide vane opening from 35º to 60º. The blade loading and flow parameters are found to be affected due to changes in guide vane opening and cavitation. The computed results are validated with the experimental model testing results and found to bear close comparison.

Startup Process Optimization of Hydropower Unit Based on Stress Measurement of Francis Turbine Runner

The energy efficiency characteristics of the turbine, cavitation performance of the runner, and operation stability of the unit are the three major concerns of a hydropower field, the latter two factors leading to the units’ cavitation and stability problems under some operation conditions and fatigue failure of the runner blades, which are the main causes of structural and mechanical failure for the hydropower units. In this article, three conventional unit startup processes are introduced, and combined with the optimal impulse control strategy, an optimized unit startup method is proposed. As the optimal guide vane opening adjustment setting value is obtained by solving the planned startup control strategy, 3D modeling and CFD simulation are carried out, then the equivalent stress characteristics are obtained by finite element calculation when the unit is running from startup to no load at a rated speed on three different startup modes. At last, the dynamic stress tests of the runner blade are performed at the hydropower station by using wireless acquisition and transmission technology, and the peak value and characteristics of the dynamic and static stresses of the blade under different startup processes are obtained. In this, the validity of the model analysis is proved by the comparison between the simulation calculation and the measured results, and the effectiveness of the startup control strategy is proved by combining the runner blade dynamic stress simulation, calculation, and the field dynamic stress test results. The optimization method of the startup process proposed in the article can alleviate the abnormal vibration of the unit and provide a basis for long-term stable operation of the hydropower unit.

Distribution Features of Flow Patterns and Pressure Pulsations of Pump-Turbine in Five Operating Modes on the Four-Quadrant Plane

To meet the demand of the power grid for flexible regulation, pump turbines have to frequently and fleetly stop, start, and transition between different modes, causing many instability problems. To solve the problems, understanding the complex flow and pressure pulsation features in different working conditions is the prerequisite. In this study, the flow patterns, pressure pulsations, cavitation characteristics, and runner forces of a prototype pump-turbine working under the five operating modes were displayed and analyzed based on computational fluid dynamics simulations. It is shown that when working points are near the pump and turbine design working points, the flow is smooth, with small pressure pulsation and without cavitation; however, when working points deviate from the optimal ones, flow patterns become worse and cavitation appears. In the pump-brake mode, the flow hits the runner blades violently, causing obvious pressure and runner force fluctuations. In the hump region, the flow has a strong impact on the guide vanes, and the rotating stall vortices are apparent in the vane area. In the S-shaped region, pressure pulsations are the highest with prominent rotor–stator interaction and rotating stall. Under similar working conditions, a larger guide-vane opening corresponds to a larger pressure pulsation amplitude. On the spatial distribution, the largest pressure pulsations happen in the vaneless space.

Influence of End Wall Clearance on Guide Vane Self-Excited Vibrations at Small Openings during Pump Mode’s Starting Up Process of a Reversible Pump Turbine

Unstable guide vane torsional mode self-excited vibrations that occur at small guide vane openings during the transient operations with pump flow, such as the starting and closing of the pump mode, are considered to have potentially severe consequences, such as guide vane slippage or damage to the link and lever mechanism. Related site tests have indicated that the end wall clearance of a guide vane may have important influences on torsional mode self-excited vibrations. In this paper, numerical investigations, which were based on computational fluid dynamics (CFD) with a single degree of freedom (1DOF) mass-spring oscillator, were carried out on a prototype high-head reversible pump turbine. The results showed that the guide vane self-excited vibrations are unstable under steady-state conditions and during the pump mode’s starting up process for cases with small end wall clearances. In addition, the critical conditions of self-excitation instability under steady-state conditions have larger safety margins than those during the pump mode’s starting up process. After further discussion, it was concluded that increasing the end wall clearance to suppress unstable guide vane self-excited vibration is unreliable due to the complexity and randomness of the initial vibration excitations.

Measurement of dynamic stress and optimized start-up method for prototype Francis turbine

Abstract This paper describes the measurement method and results of dynamic stress for a prototype high head Francis turbine runner, blades of which had fatigue cracks near the root of the blade outlet band side within one-year operation. In order to specify the cause of the cracks, first of all, dynamic stress of the runner blade was measured throughout complete one cycle operation including start-up, no load operation, increase/decrease of load and stopping. Excessive variable strain at the portion near the crack occurred at very low rotational speed during start-up. The large strains occurring during start-up are not due to Rotor-Stator Interaction, other definite cyclic forced vibration or resonance, but the stochastic vibration. Consequently, start-up mode survey was conducted with dynamic stress measurement. As a conclusion, maintaining a small guide vane opening below a certain rotational speed is an effective indicator to reduce the strain during start-up and to minimize the damage caused by this phenomenon.

Experimental and Numerical Simulation Study on the Flow Characteristics of the Draft Tube in Francis Turbine

The flow characteristics of the draft tube of a Francis turbine have a significant influence on turbine stability. Numerical simulations were performed for a Francis turbine under three different output conditions of 20%, 100%, and 120% at the rated and maximum heads. Laser Doppler velocimetry (LDV) tests were conducted to test the flow characteristics of the draft tube of the Francis turbine model. The flow characteristics in the draft tube, the mechanism of the flow characteristics change, and the effect of the opening on the vortex rope were analyzed. The results showed that the large and invisible vortex in the conical cross-section at the inlet of the draft tube gradually changed to a tangible vortex rope as the guide vane opening (GVO) increased. The pressure and velocity are significantly influenced by the GVO, and the flow characteristics in the draft tube improve as the GVO increases. Simultaneously, the influence range of the vortex rope increased as the head increased.