High Head Pump Turbine Research Articles

Investigation into the Energy Loss Mechanism of Pump Turbine During the Runaway Process Based on Entropy Production Theory

Abstract The power-trip of the pump under certain conditions leads to the runaway of the unit, causing dangerous transitional process in pumped storage power stations. This process is characterized by transient hydraulic features like flow separation and vortex structures, which increase hydraulic losses significantly and severely impact unit efficiency and stability. This paper aims to elucidate the mechanism of energy loss due to unstable flow during pump turbine runaway. It focuses on a high-head model pump turbine, examining the transient flow process from pump conditions to runaway conditions. It conducts quantitative analysis of energy loss using entropy production theory, besides, further clarifies the location and causes of hydraulic losses by integrating with internal flow analysis. The results show that a significant high-entropy production zone is captured in the pump braking condition, indicating extremely chaotic internal flow in this area during the runaway transition process. Additionally, throughout the transition process, turbulent entropy production initially increases, then decreases. Turbulent entropy production accounts for more than 80% of total entropy production, indicating that turbulent entropy production dominates throughout the entire transition process. The energy of the water is primarily dissipated within the runner during the runaway process. The maximum proportion is 87%, which is in the pump braking condition. Correlation analysis between flow and hydraulic losses reveals that vortex structure induced by flow separation under off-design conditions is the primary contributor to increased hydraulic losses.

Research on the influence of labyrinth ring configuration on runaway characteristics of pump-turbines

The labyrinth ring can reduce the volume loss, mitigate the influence of force characteristics when the unit is operating, and ensure the efficient, safe, and stable operation of the unit. It is usually installed in the upper crown and lower ring of the pump-turbines runner. When the pump-turbines unit is running in runaway condition, the internal flow field structure is more complicated due to the different forms of labyrinth ring, so a new method needs to be adopted to analyze it from a new perspective. In this paper, a high head pump-turbines was selected as the research object. Combined with the experimental and numerical simulation results, the flow field structure corresponding to different upper labyrinth ring forms under runaway condition was analyzed and visualized in many aspects. The results show that the mixed type labyrinth ring can improve the undesirable flow regime in the runner and draft tube. The guide vanes under the serrated type labyrinth ring are more likely to be damaged due to greater force. The influence of the mixed type labyrinth ring on the radial force of the runner is only half that of the ladder type, which has a great advantage in maintaining the radial stability of the runner. The ladder and serrated type labyrinth ring can make the unit obtain less axial water thrust. In addition, based on the Finite-Time Lyapunov Exponent (FTLE) method, the flow field structure in the vaneless region was analyzed, and it was found that the labyrinth ring may affect the flow in the flow path of the guide vane and the vaneless region, thus causing the change of axial water thrust. This study reveals the internal flow and force characteristics of pump-turbines with different types of labyrinth rings under runaway condition and provides a technical reference for further understanding and studying the influence of labyrinth ring forms on pump-turbines performance and force characteristics.

Transient Flow-Induced Stress Investigation on a Prototype Reversible Pump–Turbine Runner

Pump–turbine units with high heads are subjected to strong pressure pulsations from the unsteady transient flow in fluid channels, which can produce severe vibrations and high stresses on the pump–turbine structural components. Therefore, reducing transient flow-induced stresses on prototype reversible pump–turbine units is an important measure for ensuring their safe and efficient operation. A high-head prototype reversible pump–turbine with a rated head of 440 m was used to investigate the transient flow characteristics and the flow-induced-stresses in this study. First, the flow passages of the pump–turbine unit and the structure of the reversible pump–turbine runner were constructed with CAD tools. Next, CFD simulations at the full load were performed to investigate the pressure pulsation characteristics of the pump turbine in both the time domain and the frequency domain. After this, the pressure files calculated by the CFD were exported and applied to a finite element model of the pump–turbine runner to calculate the transient flow-induced dynamic stresses. The results show that the pressure pulsations in the flow passage are closely related to the rotational speed, the guide vane number, and the runner blade number of the pump–turbine unit. The maximum flow-induced stresses on the pump–turbine runner at the full load were below 2 MPa and lower than the allowable value, which reveals that the designs of the pump–turbine runner and the flow passage are acceptable. The conclusions can be used as a reference to evaluate the design of high-head pump–turbines units. The approaches used to carry out the transient flow-induced stress calculations can be applied not only to pump–turbines units but also to other types of fluid turbomachinery such as pumps, turbines, fans, compressors, turbochargers, etc.

FSI simulation of a high-head pump-turbine during load rejection process

Abstract High-head pump-turbine suffers large centrifugal and hydraulic forces due to large rotational speed and severe pressure pulsations during transient processes, and the strong vibration may increase the risk of runner failure. Load rejection transient process is crucial for pumped-storage hydropower stations, and the evolution laws of the runner dynamic stress during this process need profound research. In this study, the evolution of the runner dynamic stress of a prototype pump-turbine during a load rejection transient process was simulated using the one-dimensional and three-dimensional coupling (1D-3D) method and the fluid-structure interaction (FSI) method. The results of this dynamic transient process were compared to the results of the static working points. It is shown that the rotational speed dominates the mean value of stress while the fluctuating amplitude of stress is dominated by pressure fluctuations, which are larger in the S-shaped region of characteristics. Compared with the static working point solutions, the transient process solutions can better reflect the stress fluctuations and should be used for safety assessment.

Optimization Analysis of Head Cover Structure of High Head Pump Turbine

Abstract Head cover is one of the key components of the pump turbine. Its stiffness and axial stiffness have an important influence on the safe and stable operation of the unit. The structure of head cover is related to the design pressure. In this paper, the structure of head cover of the high head pump turbine is optimized, and the strength and axial stiffness of head cover before and after optimization was analyzed by finite element method. The results show that the optimized head cover is better than the axial stiffness of head cover before optimization.

The influence of the S shaped characteristics of high head pump-turbine on the minimum pressure of draft tube

Abstract The S shaped characteristics of pump-turbines have an important impact on the minimum pressure of draft tube. The quality of the S shaped characteristics is one of the key indicators to determine the success of model runner development, and this indicator is particularly important for ultra-high head pump-turbines. In order to delve deeper into the inherent relationship between the S-shaped characteristics of the runner and the optimization design and minimum pressure of draft tube, this article first defines a method for fixed-point quantitative analysis of the S shaped characteristic curve. Then, different experimental characteristic curves obtained from multiple optimized designs of the runner are used to calculate the hydraulic transition process. At the same time, the S shaped curve is quantitatively analyzed, and the relationship between the three is explored through continuous analysis. The results show that the bending degree of the equal opening lines in the middle at the S shape characteristics curves has the greatest influence on the minimum pressure at the draft tube inlet. The greater the ratio of unit flow difference to unit speed difference in the S shape characteristics curves of the equal opening line, the greater the runaway unit flow on the same equal opening line, the more favorable it is to increase the minimum pressure at the inlet of the draft tube. The optimal design of blade type and guide vane airfoil can improve the minimum pressure value of draft tube to a certain extent. The conclusions obtained in this paper can be used for reference in the research and development design of high head pump-turbine runner and the optimization calculation of transition.

Numerical study on the performance characteristics of a high-head pump turbine based on the GEKO model

Abstract As the most mature large-scale energy storage facility in the world, the core equipment of pumped storage power stations, the pump-turbine, is currently developing towards high head and large capacity. And the computational fluid dynamics method (CFD) is one of the most important means to research the internal flow mechanism of pump turbines and optimize the hydraulic performance. Turbulence models play an important role in the study of the flow field. Many high-performance RANS turbulence models (such as SST) cleverly combine the advantages of various models to achieve high precision, but they are not universal and their applicability and accuracy for complex flows are still limited. The Generalized k-ω (GEKO) model model may be a good choice among eddy viscosity models as users can adjust the free parameters according to their specific applications. In this study, the internal flow field of pump turbine is numerically simulated by GEKO model with various values of the separation parameter C SEP and the curvature correction parameter C CURV compared with experimental results. The calculation results show that the parameter settings of C SEP and CCURV in GEKO model have a direct impact on the numerical calculation results. The parameter modification of GEKO model has the potential to improve the adaptability and accuracy of turbulence model, and is useful for improving the engineering application of turbulence model.

Effect of crown gap thickness on axial force of high-head pump turbine

Abstract The size of the runner crown gap is one of the factors affecting the volume loss of the pump turbine, and the unreasonable gap thickness is unfavourable to the unit operation. By establishing a three-dimensional full-channel fluid domain model of a high-head pump turbine, and using the computational fluid dynamics (CFD) method to perform numerical calculations on models with different crown gap thicknesses, the effects of axial water thrust on runners with different upper crown gap thicknesses are clarified. The results of simulation indicates that the resultant force of axial water thrust on the runner is vertically upward under different gap thicknesses, which is far less than the weight of the whole rotating part. The axial water thrust is very sensitive to the change of the seal gap value, the greater the seal gap value, the greater the axial water thrust on the unit, and the axial water thrust begins to decrease after increasing to a certain value. The research results are referable for the study of the gap flow characteristics and the crown gap design.

Analysis of the cause of the 40Hz frequency component in spiral casing of a high-head pump turbine

Abstract Vibration and noise problems affect the safe and stable operation of pump turbines. For the design of pump turbines, it is crucial to accurately identify the excitation sources and their evolution laws that cause vibration and noise. Based on numerical calculation methods, this paper studies the 40Hz frequency component that occurs in the spiral case of a prototype pump turbine during operation in a power plant. Upon analysis of the internal flow characteristics of the spiral case, the study examines the causes for the formation of pressure pulsations at this frequency component and their transmission laws.

Numerical Study on the Effect of Crown Clearance Thickness on High-Head Pump Turbines

In order to promote the development of new power systems and the consumption of clean energy, pumped storage hydropower stations tend to become increasingly larger in capacity and higher in head height. In this paper, a three-dimensional model of the full channel of a high-head pump turbine is established, and the influence of the gap thickness between the runner crown and the headcover on the internal flow field characteristics, pressure fluctuation characteristics and axial water thrust are studied by means of computational fluid dynamics (CFD). The results indicate that the area where the pressure and velocity vary greatly in the flow field of the crown clearance is at the seal of the labyrinth ring. In addition, the pressure balance pipe has a greater impact on the pressure and flow rate of the water passing through the clearance, and the crown clearance near the pressure balance pipe generates a vortex, resulting in energy loss. Decreasing the thickness of the clearance has no obvious effect on the pressure on the flow-passing components, and increasing the thickness of the clearance has a great change in the pressure values at the upper crown inlet, in front of the labyrinth ring of the crown and in front of the labyrinth ring of the band. The upper part of the vaneless area, the crown inlet and the lower ring inlet are close to the runner; hence, the interference from the rotating parts is the strongest. The effect of increasing the thickness of the crown clearance on the axial water thrust is greater than that of decreasing the thickness of the crown clearance. The pulsation frequency of the axial thrust on the crown, band and blade and the resultant force of the axial thrust increase with the increase in the crown clearance thickness.

Analysis of the Energy Loss Mechanism of Pump-Turbines with Splitter Blades under Different Characteristic Heads

The operating efficiency of high-head pump turbines is closely related to the internal hydraulic losses within the system. Conventional methods for calculating hydraulic losses based on pressure differences often lack detailed information on their distribution and specific sources. Additionally, the presence of splitter blades further complicates the hydraulic loss characteristics, necessitating further study. In this study, Reynolds-averaged Navier–Stokes (RANS) simulations were employed to analyze the performance of a pump turbine with splitter blades at three different head conditions and a guide vane opening (GVO) of 10°. The numerical simulations were validated by experimental tests using laser doppler velocimetry (LDV). Quantitative analysis of flow components and hydraulic losses was conducted using entropy production theory in combination with an examination of flow field distributions to identify the origins and features of hydraulic losses. The results indicate that higher heads are associated with lower growth rates of total hydraulic losses. In particular, the significant velocity gradients at the trailing edge of the splitter blades contribute to higher hydraulic losses. Furthermore, the hydraulic losses in the runner (RN) region are predominantly influenced by velocity gradients and not by vortices, with the flow conditions in the RN region impacting the hydraulic losses in the draft tube (DT).

Experimental and numerical investigation of vortex flows and pressure fluctuations in a high-head pump-turbine

Vortex flows and pressure fluctuations, especially in the draft tube and vaneless space, have crucial impact on stable and safety operation for a high-head pump-turbine due to be fast adjusted and frequently switched between pump and turbine modes and often down to the deep part load. To more accurately investigate the vortex flows and pressure fluctuations in a high-head pump-turbine with splitter blades runner, a reliable approach by combined experiments with numerical simulations is presented in this paper. Laser Doppler Velocimetry measurements and high-speed camera observations inside the draft tube have been conducted at all the investigated points. Velocity profiles, vortex ropes visualization, the instantaneous tangential velocity and pressure fluctuations in both measured and simulated are presented to validate the numerical simulations and classify the vortex flow patterns. Meanwhile, the relationship between formation, development and breakdown of the vortex rope, the processional frequency and swirl intensity has been linked up and experimentally validated. Variation regularity of pressure fluctuations along the flow passage at different operating conditions has been quantitatively substantiated with measurements and predictions. Validation of predicted results versus experimental data shows that the presented numerical simulations can give very good results of vortex structures and the predicted fluctuation's frequencies. The pressure fluctuations amplitude of both the measured and the predicted in turbine mode agree well in the vortex rope-free zone, but has relatively larger deviations out of the vortex rope-free zone. It also demonstrated that the runner with splitter blades can restrain development of vortex flow and decrease the amplitude of pressure fluctuations for a high-head pump-turbine. The presented approach has been applied to successfully developed the high-head pump-turbines for several Pumped Storage Power Plants in China and demonstrated very useful in hydraulic optimizations.

Prediction on the axial water thrust characteristics of pump turbine runner under partial load

For the high head pump turbine, the unit instability during its operation mainly dues to the hydraulic vibration inside the machinery, which will cause the mechanical fatigue failure under certain circumstances. In this paper, the flow pattern and the runner axial water thrust in turbine model under partial load were analyzed by numerical simulation. The internal flow field instability and vortex load characteristics of the runner were analyzed to obtain the reasons for the different axial water thrust characteristics of the runner under two different loads. The results showed that due to the reduction of the guide vane opening, the flow rate is smaller at 50% load, and the free flow vortex structure on the high-pressure side of the runner blade inlet strengthens the impact of the water flow on the runner. Moreover, when the pressure difference between the runner blades decreases and the axial water thrust of the blades decreases, the total axial water thrust of the runner decreases and fluctuates frequently with time.

Pressure fluctuations during the pump power-trip of a low-head pump-turbine with the co-closing of guide vane and ball valve

The pressure fluctuations (PFs) were significantly different for different head pump turbines (PTs). To study the evolutionary mechanisms of PFs inside a low-head PT, the co-closing of the guide vane (GV) and ball valve after a pump power trip was simulated by adopting the one- and three-dimensional coupled approach and dynamic mesh technology. Additionally, the simulation results were validated through comparison with the experimental data. The results suggest that the transient performance and pressure fluctuate severely before the PT passes through the reverse maximum flowrate condition, which causes severe fluctuations in the impeller radial hydraulic thrust (FR). Moreover, the study found that PFs during the transition of low-head PTs also include two new frequency components, fDV caused by the Dean vortices in the spiral casing and the 38–40-fold impeller-rated rotation frequency caused by the periodical interactions between the rotational impeller and the regular high-pressure (HP) regions in the stay/GV, in addition to the frequency components fRSI-S and fRSI-R caused by rotor-stator interactions and fBFV caused by local backflow vortexes close to the impeller inlet. The two new fluctuation frequency components cannot be captured for the middle-head and high-head PTs and primarily influence the axial hydraulic thrust of the impeller (FZ). This finding can provide an important theoretical guide for the design and optimization of low-head PTs.

Flow-Induced Vibration of Non-Rotating Structures of a High-Head Pump-Turbine during Start-Up in Turbine Mode

Pumped storage-power plants play an extremely important role in the modern smart grid due to their irreplaceable advantages in load peak-valley regulation, frequency modulation, and phase modulation. The number of start-stops per day of pump-turbine units is therefore also increasing. During the start-up transient process in turbine mode, the complex flow in runner passage, crown and band chambers, and seal labyrinth is able to induce severe vibration of non-rotating structures such as head cover, stay-ring, and pose a threat to the safe operation of the pump-turbine unit. In this article, the flow-induced vibration of the structures of a pump-turbine unit during its start-up process in turbine mode is studied. In the first place, this investigation establishes a three-dimensional model of the full flow passage and carries out a full three-dimensional CFD calculation based on one-dimensional pipeline calculation results for the start-up transient process. In the next place, by applying the fluid–structure interaction calculation method, the finite element analysis of non-rotating components of the pump-turbine unit is carried out. The flow-induced stresses and deformations of head cover, stay-ring, etc., are obtained and analyzed. The results reveal that the maximum deformation of the non-rotating structures is located at the inner edge of the head cover while the maximum stress appears at the trailing edge fillet of a stay vane. In summary, the dynamic stress of the non-rotating structures changes largely during the start-up process. The stress is strongly related to the axial thrust caused by the fluid flow. The achieved results can provide guidance for further fatigue life assessment of non-rotating structures and contribute to the structural safety design of pump-turbine units.

Experimental and Numerical Investigation of Rotor–Stator Interaction in a Large Prototype Pump–Turbine in Turbine Mode

In recent years, large-capacity, high-head pump–turbine units have been developed for pumped storage power plants to effectively utilise water energy and store large amounts of electricity. Compared with the traditional Francis turbine unit, the radial distance between the trailing edge of the guide vanes and the leading edge of runner blades of high-head pump–turbine unit is smaller, so the rotor–stator interaction and the corresponding pressure fluctuations in the vaneless space of pumped storage units are more intense. The pressure fluctuations with high amplitudes and high frequencies induced by rotor–stator interaction (RSI) become the main hydraulic excitation source for the structures of the unit and may cause violent vibration and fatigue damage to structural components, and seriously affect the safe operation of the units. In this paper, the RSI of a high-head pump–turbine in turbine mode of operation is studied in detail by means of site measurement and full three-dimensional unsteady simulations. The results of RSI-induced pressure fluctuations in turbine mode are analysed experimentally and numerically. The accuracy of the numerical calculations is verified by comparing with the measured results, and the variation law of RSI is deeply analysed. The results show that the pressure fluctuations in the vaneless space are affected by the wake of the guide vane, the rotating excitation of the runner, the low-frequency excitation of the draft tube, and the asymmetric characteristics of the incoming flow of the spiral case, and shows significant differences in spatial position. The findings of the investigation are an important and valuable reference for the design and safe operation of the pumped storage power station. It is recommended to design the runner with inclined inlets to reduce the amplitudes of RSI-induced pressure fluctuations and to avoid operating the pump–turbine units under partial load for long periods of time to reduce the risk of pressure fluctuation induced severe vibration on the structures.

A new start-up law of pump turbines with misaligned guide vane devices

The frequency of high head pump turbines easily fluctuates under the low head no-load condition, which makes it difficult to achieve the synchronous grid connection. In this study, a new start-up strategy of linkage between misaligned guide vane (MGV) devices and conventional guide vanes is proposed, and the optimal arrangement scheme of MGV devices is selected through experiments. Based on the characteristic method, improved Suter transform and the characteristics of MGV devices, the calculation model of transition process is established. The simulated rotational speed of emergency shutdown process fits well with the field measured data. Under start-up conditions, the newly proposed control law can effectively reduce the fluctuation amplitude of frequency, draft tube inlet pressure and volute inlet pressure, and shorten the synchronization time. Additionally, the newly proposed law can significantly improve the no-load stability of the pump turbine under different head conditions, especially low head conditions.

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.

Stability Analysis of Vaneless Space in High-Head Pump-Turbine under Turbine Mode: Computational Fluid Dynamics Simulation and Particle Imaging Velocimetry Measurement

When the Francis-type reversible pump-turbine runs under partial load, the pressure pulsation amplitude and frequency in vaneless space are high, posing a serious threat to the stability of unit operation. Water presents weak compressibility in a high-head pump-turbine, thereby affecting the amplitude–frequency characteristics of pressure pulsation. This study used numerical simulations in a model and prototype pump-turbine and particle image velocimetry (PIV) in a model pump-turbine to examine the internal flow field and pressure pulsation characteristics and determine the effect of the flow in the vaneless space on the amplitude–frequency characteristics of the pressure pulsation. The pressure pulsation amplitude–frequency characteristics were verified through prototype tests. The effects of the weak compressibility of the water on the propagation law of pressure pulsation throughout the flow passage of the prototype and model pump-turbine were roughly similar but exhibited certain differences. Considering the weak compressibility of water, the pressure pulsation fluctuations in each flow passage of the prototype and model pump-turbine exhibit varying degrees of improvement, which is more obvious at the prototype scale. Therefore, the pressure wave disturbance caused by the weak compressibility of the water has different effects on the prototype scale and model scale of the high-head Francis pump-turbine.

Investigation on runner resonance and fatigue life of a high–head pump–turbine

The structural stability and fatigue of the runner of a high-head pump-turbine was explored by conducting a numerical simulation of the entire flow passage under different working conditions. A two-way fluid-structure interaction calculation method was used, which captured the values for 5.75 fn, 9 fn, 13 fn, 18 fn, and 20 fn ( fn is rotational frequency) in pressure fluctuation. The resonance of the runner was compared with the results of the modal analysis. The rain flow and damage accumulation methods were used to predict the fatigue life. The study found that the 13 fn pressure fluctuation of the fluid in the flow passage of the pump-turbine is the primary cause of the resonance of the runner blade. The main frequency of vibration is the third natural frequency of the runner. The vibration form is up and down along the Z–axis, and there are some high-order resonances. The most vulnerable part of the entire runner is located at the T-shaped connection between the blade inlet edge and the runner crown and bottom ring. The excitation force under the small flow condition is more complicated than that under the large flow condition, which is more likely to cause fatigue damage to the runner. In addition, the damage extent of the high-stress amplitude to the runner is obviously greater than the influence of the number of stress cycles, accounting for approximately 60% of the total damage. The research results can provide a reference for the operational stability and life prediction of a high-head pump-turbine runner.