Hydraulic Excitation Research Articles

Study on the Internal Flow Characteristics of Long and Short Blade Runners of a 1000 MW Francis Turbine under Different Opening Conditions

The 1000 MW Francis turbine unit at the Baihetan hydropower station is the maximum capacity unit in the world at present, and it has adopted the runner type with long and short blades. For this ultra-high output Francis turbine, especially with the breakthrough runner structure, the hydraulic excitation phenomenon caused by internal dynamic and static interference is the key factor for the stability of the unit. In this study, the 1000 MW Francis turbine unit is taken as the research object, and the rated output conditions with different guide vane openings are selected for comparative analysis. The flow field structure and the pressure pulsation characteristics inside the guide vane and runner under different openings are obtained. The distribution characteristics and evolution law of the vortex in the runner under different guide vane openings are analyzed. The results show that the dynamic and static interference between the runner and the guide vane induces the local high-speed flow to appear in the vaneless area, and the larger the guide vane opening, the smaller the dynamic and static interference between the runner and the guide vane; the vortex in the runner mainly develops and evolves from the inlet to the outlet and is mainly distributed near the blade wall surface. The pressure pulsation inside the runner is mainly due to the action of dynamic and static interference. The pressure pulsation induced by the dynamic and static interferences shows a decreasing law from the runner inlet to the runner outlet.

Characteristics and factors of mode families of axial turbine runner

Axial turbines are widely used in hydropower systems. Owing to the complex hydraulic excitation force during operation, its flow-passing components, particularly the runner, are prone to severe vibrations. To further understand the vibration of the runner under different excitation forces, this study systematically investigated the modal characteristics of an axial turbine runner through numerical simulation using the example of a prototype Kaplan runner, including a shaft. The finite element method (FEM) and acoustic-structure coupling method were used to solve the natural modes of the runner in vacuum and the added mass effect of water, respectively. It was found that the modes of cantilever-blade structures without a shaft are mainly represented as vibrations of the blades, which can be classified into a series of mode families based on the single-blade modes, corresponding to a series of frequency bands with very small widths. The composition of members in each mode family depended strictly on the number of blades with a clear rule. For axial turbines, the modal vibration of the shaft and blades is prone to coupling, which leads to abnormal changes in some modes in the mode family, resulting in the scattering of the natural frequencies and greatly improving the possibility of resonance. The law of mode coupling was revealed, and the effects of the blade opening and bearing stiffness on the mode coupling and frequency bandwidth were explored. The stiffness of the bearing close to the runner significantly affected the frequency bandwidths. The greater the stiffness, the smaller are the bandwidths. This study can provide a reference for resonance cause analysis and vibration avoidance design of axial turbines and similar structures.

Prediction method of non-stationary random vibration fatigue reliability of turbine runner blade based on transfer learning

In order to solve the problems such as lack of fault information, sample variation with time and expensive calculation in the estimation of the vibration fatigue reliability of the turbine runner blade under the non-stationary hydraulic excitation. A prediction method of non-stationary random vibration fatigue reliability of the turbine runner blade based on transfer learning is proposed in this paper. Firstly, the dynamics model of the cracked turbine runner blade under the non-stationary hydraulic excitation is established to analyze the characteristics of the non-stationary random vibration fatigue of the turbine runner blade. Secondly, the transformation matrix between the source domain and target domain in the hidden space is found by the transfer learning method of balanced distribution adaptation (BDA). The adaptation of active learning and Kriging-based system reliability method (AK-SYSi) is applied to estimate the non-stationary random vibration fatigue reliability of the turbine runner blade with multi-failure-mode. Finally, an example is analyzed, and the Monte Carlo simulation (MCS) is used to verify the correctness of the proposed method. The results show that the method proposed in this paper can effectively predict the failure probability of the non-stationary vibration fatigue of the turbine runner blade in future time.

Dynamic soil-structure interaction of a single-span railway bridge, forced vibration testing and simulation

High-speed railway is expanding drastically in Sweden, necessitating new technology, and improvements of existing structures. End-shield bridges are a common and under-tested bridge type in Sweden. Their dynamic performance is significantly impacted by their boundary conditions due to the soil–structure interaction (SSI) and their large masses cantilevering beyond the footings. A specific end-shield bridge was tested under low (5 kN) and high (20 kN) amplitude-forced hydraulic excitation for a wide range of frequencies. Several train passages for typical passenger trains, ‘X62’, were measured with the same experimental setup. The results were analysed to isolate the significant modes of the system and the natural frequencies. A full 3D numerical model was calibrated and updated in Abaqus, along with a brief sensitivity study to determine the most influential parameters. Finally, the response to passing trains and Eurocode design HSLM trains was calculated. The experimental study showed that higher loading amplitudes resulted in higher damping and lower natural frequencies. The numerical analysis showed that for this bridge type the SSI cannot be neglected and can be successfully introduced in the model.

Numerical Simulation of Cavitation Characteristics of a Centrifugal Pump Based on an Improved ZGB Model

We modified the Zwart–Gerber–Belamri (ZGB) cavitation and RNG turbulence models based on their rotational motion characteristics, as well as simulating the phenomenon of small fluctuations in rotational speed due to the action of hydraulic excitation force, to increase the precision of numerical simulations of the cavitation characteristics of centrifugal pumps. According to the theory behind the enhanced model, the pressure gradient in the impeller runner changes uniformly, and the cavitation bubble initially manifests itself at the front edge of the blade’s suction. With the reduction in the effective margin, changes in the impeller flow channel’s pressure gradient increased, the blade’s suction-side cavitation area expanded, and the flow field’s internal disturbance enhanced, resulting in hydraulic loss to the centrifugal pump, considerably reduced operating performance, and the blade pressure side by the cavitation-affected area becoming smaller. The blade’s suction-surface pressure–load curve can be divided into a low-pressure cavitation zone, a pressure fluctuation zone, and a pressure stabilization zone. The pressure–frequency-domain diagram of each monitoring point has a discrete distribution, and the main frequency is the blade-passing frequency ƒ = 293 Hz and its n times frequency. With the deepening of the cavitation, the amplitude of the main frequency in the low-frequency band showed a trend of increasing and then decreasing, and the amplitude of the high-frequency band showed an increasing trend. When we applied the ZGB cavitation model based on the improved rotational motion characteristics to our numerical calculation of the centrifugal pumps’ cavitation characteristics, our calculated cavitation performance curve was closer to the experimental value than the ZGB model, and when the centrifugal pump’s cavitation degree was increased, the area covered by the vacuole area was 1/4 of the suction side of the vane.

A comprehensive empirical equation for the hydrodynamic damping of vibrating blade-like structures

Blade fatigue is a widely concerning issue when a turbine transforms ocean energy into electronic power. The frequent changes in flow velocity may cause a wide range of hydraulic excitation frequencies and result in blade resonance. The hydrodynamic damping ratio is of great importance in assessing the resonance amplitude in the design stage. However, it is difficult to make accurate predictions through the basic prediction equation (BPE) because some key flow parameters are not fully taken into account. The present research simplifies the blades as hydrofoils and devotes to improving the accuracy of the BPE. Firstly, multiple correction terms (including velocity, attack angle, and tip clearance correction terms) relative to the BPE are proposed, based on previously published works. Secondly, a comprehensive empirical equation (CEE) is constructed, by coupling the BPE and multiple correction terms. Accuracies of the CEE for first bending and torsional modes are evaluated separately In particular, the average error of the CEE for the first bending mode decreases from 57.16% to 9.87% in comparison with that of BPE. And the average error decreases from 259.48% to 50.72% for the first torsional mode. CEE provides a new tool for engineering evaluation of the resonance failure of blade-like structures.

Test and Numerical Simulation of Pressure Pulsation under the Forward and Reverse Working Conditions of a Horizontal Axial Flow Pump

In daily operation, the flow state in the runner chamber of an axial flow pump is extremely complex, and the pressure pulsation caused by the vibration of the unit and the hydraulic excitation is also extremely complex. The pressure pulsation will also greatly damage the flow components of the unit. In this paper, a pressure pulsation test and research on an axial flow pump in a pumping station are conducted. The research results indicate that the pressure pulsation at the inlet of the runner and the inlet of the front guide vane are basically consistent with the numerical simulation results under the rated operating conditions, and the external characteristic curves are basically consistent. When running in the forward direction, the pressure pulsation at the inlet of the runner periodically changes, and the main frequency is the rotational frequency. In the reverse operation, the main frequency of the pressure pulsation at the outlet of the runner is the rotational frequency, and the main frequency of the pressure pulsation at the outlet of the movable guide vane is the pilot frequency; the pressure coefficient changes under different flow conditions. The distribution trends are consistent and decrease with the increase in flow rate; the velocity streamlines are evenly distributed near the design flow rate but are sparse and uneven under low flow conditions. Deviation from the design flow rate should be avoided during normal operation of the unit conditions to ensure safe and stable operation.

Effect Analysis of Step Input Smoothing Optimization on Hydraulic Excitation Suppression of Position Servo System

Based on step input ramping optimization, a further optimization method is proposed by superimposing a sine signal to smooth the mutation of the step signal. This sine smoothing method has zero signal change rate and acceleration at the beginning and the end of the signal mutation. That is, the optimization has good smoothing characteristics. With the position servo system as the research object, the simulation model is established and the effect analysis of parameters such as rising time, friction, and external load is carried out. It can be concluded based on the analysis that the ramp and the sine smoothing optimizations are very effective in reducing hydraulic excitation, especially in suppressing the pressure pulsation of the system; meanwhile, it would cause a system response millisecond delay. Different optimization methods should be selected according to the specific application scenarios.

Vibration characteristics analysis of shaft system for bulb hydroelectric generating unit based on magnetorheological fluid damper

In order to reveal the influence law of magnetorheological fluid damper on shaft system vibration for bulb hydroelectric generating unit, based on the modified LuGre friction model, the rub-impact dynamic model of magnetorheological fluid damper-rotor-runner-bearing system is established considering multi-effect of unbalanced magnetic pull, oil film force, and unbalanced hydraulic excitation. The suppression pattern of passive control for the magnetorheological fluid damper on vibration from rotor as well as runner under different rotating speed and mass eccentricity is investigated by using numerical method, and the stability of periodic solution for system is analyzed combined with Floquet theory. Models with and without magnetorheological fluid damper are studied, and the results show that the nonlinear dynamic characteristics of bulb hydroelectric generating unit, can be significantly improved by valuable passive control from magnetorheological fluid damper damping. The system response becomes orderly due to the existence of magnetorheological fluid damper, which can attenuate vibration generated during the parameters variation including rotating speed and mass eccentricity, with trend towards synchronous periodic motion form. Magnetorheological fluid damping passive control has good applicability to bulb hydroelectric generating unit, which can provide a conducive reference for shaft system vibration reduction of unit.

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.

Review of the Hydraulic and Structural Design of High-Speed Centrifugal Pumps

In recent years, much attention has been paid to the application of high-speed centrifugal pumps; still, the development of this pump faces several challenges. In order to obtain a more comprehensive understanding of the high-speed centrifugal pump, this paper reviewed the engineering application, technical challenges, and feasible solutions of this pump from the aspects of hydraulic design, including cavitation, hydraulic excitation, efficiency issue at an ultra-low specific speed, and the solution to these problems. The current state of the structural design of the high-speed centrifugal pump was briefly described in addition. For the faults in the existing high-speed centrifugal pump, some research studies on pump monitoring were presented. Finally, the status and shortcomings of the design of the pump were simply analyzed and summarized. It is hoped that this study can provide some references for the design and practical usage of high-speed centrifugal pumps.

Research on the Flow-Induced Stress Characteristics of Head-Cover Bolts of a Pump-Turbine during Turbine Start-Up

Nowadays, pump-turbine units have to experience multiple start-stops every day to balance the power production and consumption on the grid. During the transient process of turbine start-up, the hydraulic forces applied to the head-cover would change dramatically and induce high-level stresses on the head-cover bolts. As key components of large hydraulic turbine units, the head-cover bolts are subjected to tens of thousands of tonnes of hydraulic excitation force during operation. Special attention should be paid to the design of the head-cover bolts of large hydraulic pump-turbine units because these units have high water heads and high hydraulic excitation forces. Therefore, the safe design of the head-cover bolts is extremely important to maintain the operational safety of the whole unit. This paper investigates the flow-induced stress characteristics of the head-cover bolts during turbine start-up in a large prototype pump-turbine unit. A complete 3D fluid model and a corresponding 3D structural model, including the head-cover bolts of the pump-turbine unit, were created. The fluid–structure coupling method was used to calculate the structural stresses caused by fluid flow during turbine start-up. The pressure files during turbine start-up calculated by the CFD tool were transferred and mapped to the finite element model of the structural components of the pump-turbine unit. Subsequently, the flow-induced stress characteristics of the head-cover bolts were numerically simulated. The simulation results showed that the hydraulic excitation force on the head-cover bolts increased significantly during turbine start-up, and the displacement and the stress distributions of different head-cover bolts were not uniform. The calculation methods and conclusions in this paper can also be applied to evaluate the flow-induced stress characteristics of head-cover bolts for similar hydraulic pump-turbine units.

Identification on Vortex Rope in Francis Turbine Draft Tube Based on Convkurgram

Francis turbine is a core energy conversion part and low frequency vibrations caused by unsteady vortex rope in the draft tube may damage the unit through resonance. Therefore, a more comprehensive assessment and monitoring of the status of Francis turbine is of great significance for unit stability. In this hydraulic machinery, mechanical impulses and hydraulic excitation are co-existing. The former manifests as a narrow low frequency band with high kurtosis, while the latter represents a wide high frequency band with energy associated with the vortex rope. Based on that, a novel method coined Convkurgram is proposed to suppress the extreme high kurtosis mechanical components and extract the hydraulic carrier wave efficiently. Besides, it takes multiple information frequency bands into consideration in case of contamination waves exist. Finally, the method is evaluated by means of actual vibration signals collected from the hydroelectric generating unit under normal and cavitation conditions. The results show the proposed method recognizes the characteristic frequency bands of mechanical components and hydraulic carriers as well as extracts the shaft frequency and blade passing frequency successfully.

Analysis of Centroid Trajectory Characteristics of Axial-Flow Pump Impeller Under Hydraulic Excitation

The hydraulic excitation characteristics of axial flow pump unit are studied through theoretical analysis, numerical simulation and field test in this paper. The correlation between impeller hydraulic and radial vibration displacement of impeller centroid is obtained through theoretical analysis. Through the 1-way fluid-solid-interaction (FSI) numerical simulation, the distributions of water pressure and displacement on the impeller surface are obtained, and the time-domain and frequency-domain characteristics of transient hydraulic and radial displacement are revealed. Through the field test, the external characteristics of axial flow pump unit and the time-frequency characteristics of the pressure pulsation at the measuring points beside the inlet of the impeller are obtained. The comparisons between simulation results and experimental results show that the former is accurate and reliable.

Influence of blade leaning on hydraulic excitation and structural response of a reversible pump turbine

Blade leaning is commonly seen in the runner design of reversible pump turbines which operate under varying conditions. However, there is no certain law in determine the leaning mode and level. Considering performance, hydraulic excitation and structural response, five runners with strong rotational (RL+), rotational (RL), strong counter-rotational (CL+), counter-rotational (CL) and without (NL) blade leaning are compared under high-efficiency condition in pump mode and turbine mode. The head, efficiency, internal flow pressure pulsation and runner stress are comparatively studied. Among the five runners, CL+ runner is found has the highest efficiency as pump when RL+ runner has the highest efficiency as turbine. Pressure pulsation results show that the rotor-stator interaction region is the strongest pulsation source especially for runner and blade frequencies. In pump mode, pressure pulsation intensity decreases when blade leaning mode gradually changes from rotational to counter-rotational. In turbine mode, the NL runner has the strongest pressure pulsation intensity in runner and guide vane. Both rotational and counter-rotational leaning will reduce pressure pulsation. Velocity contours indicate that blade leaning will affect the velocity uniformity especially along rotational direction and cause stronger or weaker local hydraulic excitation. Under hydraulic excitation, RL+ runner suffers the highest equivalent stress as pump while CL runner suffers the highest equivalent stress as turbine. From rotational to counter-rotational blade leaning, the maximum stress moves on the crown from low pressure side to high pressure side. Considering hydraulic excitation and structural response, the strong counter-rotational leaning blade is found better in reversible runner design.

Effect of the Diameter of Pressure-Balance Pipe on Axial Hydraulic Thrust

The axial hydraulic thrust has great influence on the safety and stability of a pump turbine. A common way to balance hydraulic thrust is to install a pressure-balance pipe, and the change in pipe diameter is one of the important factors affecting axial hydraulic thrust. In this paper, the influence of the diameter changes in a pressure-balance pipe on axial hydraulic thrust of a pump turbine, plus the seal clearance flow, is studied and analyzed under three work conditions, i.e., 100%, 75%, and 50% loads. It is found that under 100% and 75% load conditions, the axial hydraulic thrust increases vertically with the increase in pipe diameter; whereas, under 50% load condition, the axial hydraulic thrust increases first and then decreases with the increase in pipe diameter. The results aim to give guidelines for the choice of pressure-balance pipe diameters and to control the axial hydraulic thrust of a pumped-storage power station, so that the hydraulic excitation force can be better matched with the hydraulic mechanism.

A method for the experimental estimation of direct and cross-coupled dynamic coefficients of tilting-pad journal bearings of vertical hydro-generators

This article presents a method to experimentally estimate the direct and cross-coupled dynamic coefficients of tilting-pad journal bearings of vertical hydro-generators and other similar rotating machinery for damage detection purposes. Based on a simplified second-order model of a journal bearing in the state-space, the method employs only the usually monitored vibrations, the shaft radial relative, and the bearing radial absolute vibrations originated by the hydro-generator residual unbalance or by hydraulic excitations in the turbine rotor. This article shows that the method was successfully tested using the shaft and bearing vibration signals synthesized by a mathematical model of a 700 MW hydro-generator, even when these signals are contaminated with random noise. This article also shows the method’s performance when applied to real vibration signals acquired from the modeled hydro-generator. Finally, it discusses the possible measures to improve the method’s efficiency.

Increasing the operating range and energy production in Francis turbines by an early detection of the overload instability

With the increasing entrance of wind and solar power for the generation of electricity, more flexibility is demanded to hydropower plants. More flexibility means that hydro turbines have to increase the operating range between minimum and maximum power. In Francis turbines the maximum power is limited by the appearance of a strong hydraulic excitation called overload instability. When the turbine operates at loads higher than design, the cavitating vortex rope that is generated in the draft tube may become unstable, producing huge pressure fluctuations, vibrations and power swing. Turbines are not allowed to operate under these conditions in order to avoid the destruction of the unit. The overload instability emerges abruptly, even when the machine is operating in a smooth condition. No visible transition can be detected by the monitoring system, so turbine operators have no margin to react. To avoid this phenomenon, operators limit the maximum power much before reaching this condition. By doing that, the maximum power is limited as well as the regulation capacity of the unit. In this paper, the feasibility of detecting the onset of this phenomenon is analyzed. Data-driven methods and artificial intelligence techniques, including principal component analysis, self-organizing map and artificial neural networks, are applied to the data available from experimental tests in a Francis turbine. The signals of vibration, pressure fluctuations and other parameters are combined and studied. The possibilities of a premature detection of the instability before it occurs are discussed. The method could be implemented in the monitoring system of the unit so that the operating range could be safely increased.

Experimental and Numerical Investigations of Vibration Characteristics Induced by Pressure Fluctuations in a Parallel Operating Pumping System

AbstractParallel pumping systems may operate with excessive vibrations as a result of complex hydraulic excitation sources. To remove excessive vibrations, it is essential to understand the relatio...

Fuel rod nonlinear vibrations to detect and characterize Pellet-Cladding Interaction

Pellet-Cladding Interaction (PCI) in a Light Water Reactor is one of the major concerns to guarantee clad integrity while attempting at increasing the flexibility of PWR nuclear reactor operations to follow power grid demand. In order to forbid operations leading to clad failure, modeling capability to simulate the mechanism has improved through the years. Code development needs detailed and precise experimental data. Those data result from dedicated irradiation programs, named “power ramp tests”, carried out in experimental devices in Material Testing Reactors (e.g. in France ISABELLE in OSIRIS in the past (Alberman et al., 1997), the ADELINE loop in the Jules Horowitz Reactor JHR in the next future (Cheymol et al., 2011)). Those irradiation devices are highly instrumented to collect the most relevant information with the highest possible accuracy. In parallel with information gained thanks to post-irradiation examination programs, research is working on innovative methodologies to detect and characterize PCI kinetics during the tests. In this frame, we investigate the technological feasibility to measure the effects of PCI, on the vibrations of a nuclear fuel rod externally submitted to the turbulent axial flow rate excitation. We designed and realized the out-of-pile IMPIGRITIA set-up to reproduce, in controlled laboratory conditions, the mechanical interaction originating in the nuclear reactor. A single PWR rod test section, reproducing the main relevant geometrical and material characteristics of ADELINE experimental system, is submitted to the hydraulic excitation representative of the real case. The local closure of the gap is obtained by means of a remotely controlled expansion system. Measurements of the transverse displacement of the sample rod are collected by Laser Doppler Vibrometry. In this paper we introduce the design of the mock-up and the associated measurement method, then we present the two experimental phases and their results: the first one in air, to show the feasibility of the measurement in air in controlled conditions; the second one under turbulent flow rate to state on the feasibility of the passive detection. At the end, conclusions and perspectives for the work are discussed.