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Tidal energy is one of the most mature energy sources used in all kinds of ocean energy. Axial flow impeller plays a key role in converting potential energy and kinetic energy of seawater into electric energy. In this study, aiming at the problem of three different clearance inclination in axial flow impeller, the axial flow impeller is studied by numerical simulation and experimental verification. The results show that the efficiency first decreases rapidly, then fluctuates within a certain range, and then decreases rapidly as the inclination changes from negative to positive. When the positive inclination is larger, the axial force and radial force are smaller. Meanwhile, the length of the clearance leakage vortex and the included angle with the axial direction are smaller. This paper provides a reference for improving the utilization efficiency of tidal energy. It also has guiding significance for the design of axial flow impeller.

Pump-turbine plays a significant role in the pumped storage station, and it is troubled by unreasonable hydraulic thrust. Some pressure balance devices have been adopted to ensure the safety of the pump-turbine unit, but the effect mechanism has not been numerical studied. This paper establishes the CFD model of the flow passage of a pump-turbine, wherein the crown gap, band gap, pressure balance pipe, and the discharge ostium are considered. Then, the flow characteristics of the pump-turbine at four typical operating conditions in turbine mode are calculated in transient scheme, and the numerical results agree with the experimental results. Finally, the effect of the pressure balance pipe and the discharge ostium on the pressure pulsation and the hydraulic thrust is discussed. The results show that the pressure balance pipe causes the direction of the axial hydraulic thrust to change from downward to upward, and the discharge ostium helps reducing the axial hydraulic thrust to a reasonable range. The pressure balance pipe and the discharge ostium have little effect on the radial hydraulic thrust. The pressure pulsation analysis provides a basis for revealing the mechanism of the hydraulic thrust frequency, and the rotor-stator interaction frequency exists in the axial hydraulic thrust. This paper provides a deeper knowledge of the impact mechanism of the pressure balance pipe and discharge ostium on the flow characteristics of the pump-turbine, which could contribute to future designs of the pump-turbine units, and is helpful for improving the safety of the pumped storage unit.

Cavitation is an important issue in pumped storage unit as it can compromise the safe and stable operation of the unit. The present work aims revealing the dynamic behavior of a pump-turbine runner in cavitating flow from a perspective of fluid-structure interaction. A procedure is developed to map the cavities from computation fluid dynamics simulation to finite element model. The calculated flow rate and head agree well with experimental results. A nonlinear relationship between the sonic speed in cavity and vapor volume fraction is applied in the finite element model. The effect of the sonic speed and fluid density on the dynamic behavior of the runner, and their coupled effect are investigated, respectively. Then the modal characteristics of the runner in cavitating flow are analyzed by solving the first 8000 modes, and a harmonic analysis is carried out to determine the main global mode. The results show that the sonic speed (20 m/s–100 m/s) could have a greater effect than the added mass for some modes. Finally, it is shown that the complexity and number of nodal diameter modes greatly increases in cavitating flow, with a lot of local modes which are similar to global nodal diameter mode. This work provides a method for evaluating the dynamic behavior of the pump-turbine runner in cavitating flow and could contribute to the safe operation of the pump-turbine unit.

PurposeThe purpose of this paper is to reveal the transient thermo-elasto-hydrodynamic lubrication mechanism of a bidirectional thrust bearing in a pumped-storage unit, and to propose the transient simulation method of two-way fluid-solid-thermal interaction of thrust bearing.Design/methodology/approachThe transient fluid-solid-thermal interaction method is used to simulate the three-dimensional lubrication of the thrust bearing, during the start-up and shutdown process of a pumped storage unit. A pad including an oil hole is modelled to analyze the temporal variation of lubrication characteristics, such as the film pressure, thickness and temperature, during the transient operation process.FindingsThe injection of the high-pressure oil sufficiently affects the lubrication characteristics on film, in which the hysteresis phenomena were found between the start-up and shutdown possess.Originality/valueThis paper reveals the transient lubrication mechanism of tilting pad in a thrust bearing, by means of transient fluid-solid-thermal interaction method. Lubrication characteristics are simulated without assuming the temperature relationship between the oil film inlet and the outlet and the heat transfer on the pad free surface. This paper provides a theoretical basis for the safe design and stable operation of thrust bearings.

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.

In order to analyze the energy conversion characteristics of the impeller blades in multiphase pump quantitatively, the impeller performance in design working condition was studied under water and gas-liquid two-phase flow, respectively. The results show that the capacity of impeller energy conversion gradually increases when the blade wrap angle changed from 0 to 60°; when the wrap angle is larger than 60°, the capability began to degenerate gradually, and the energy transfer performance of the pressure surface began to degrade. The enhancement of the energy transfer performance of the impeller pressure surface had a positive impact on the impeller energy conversion performance, but the enhancement of the energy transfer performance of the suction surface had an adverse effect on the impeller energy conversion performance. Under the gas-liquid two-phase case, the impeller power capability degraded, the energy conversion performance of impeller was worse than that of water alone. The research results had essential theoretical significance for the design of hydraulic optimization of the multiphase pump.

Low-head hydraulic power units typically have a short intake pipe or no that structure. For this reason, the upstream and downstream flows in reservoirs could influence the internal flow characteristics in the turbine, which sometimes increase the flow non-uniformity and the local hydraulic losses. Meanwhile, the efficiency estimation methods for prototype turbines still have a limitation for determining the unit discharge, which is generally predicted by using a Winter-Kennedy method. This paper proposes a hydraulic performance estimating method focusing on low-head turbine units, which is conducted by using CFD analysis and based on site test results. A bulb type turbine unit was adopted as a research object, which is operating in a low-head run-of-the-river power station. The flow behaviors in turbine were simulated based on two water levels under single unit operation, with two reservoir modeling. The simulation results agreed well with test ones for the turbine power as well as the relative discharge curve. The prototype efficiency in operation was estimated at the same time. In addition, this paper analyzed the relationship between the gross head and net head, and introduced the concept of plant hydraulic efficiency, as regards the flow interaction between the turbine unit, intake and tailrace.

Due to the massive entrance of new renewable energies such as wind or solar, hydraulic turbines have to work far from its designed point and withstanding multiple transients, such as starts and stops, that shorten the useful life of the machine and cause fatigue damages. The present paper reviews the complex problem of fatigue in Francis turbines particularly focused on the experimental data available for static and dynamic stresses. For this purpose, many researches, which include different Francis turbines covering a wide range of design head and power, have been considered. The experimental stresses characteristics measured with strain gauges installed on the turbine runner and obtained from previous works have been analyzed for the different operating conditions and transient states occurring in the normal life of actual Francis units. The actual computational capabilities and techniques typically used to estimate such stresses have been discussed in detail. Potential future techniques to simplify complex strain measurements on the turbine runner, computational and statistical methods to estimate turbine stresses are reviewed in this paper. Finally, the relative damage of the different operating conditions and useful life estimation of the turbine, based on past strain measurements of the runner, are addressed.

During the starting up of the pump mode in pump turbines, the axial hydraulic force acting on the runner would develop with the guide vane opening. It causes deformation and stress on the support bracket, main shaft and runner, which influence the operation security. In this case, the axial hydraulic force of the pump turbine is studied during the starting up of pump mode. Its influences on the support bracket and main shaft are investigated in detail. Based on the prediction results of axial hydraulic force, the starting-up process can be divided into “unsteady region” and “Q flat region” with obviously different features. The mechanism is also discussed by analyzing pressure distributions and streamlines. The deformation of the support bracket and main shaft are found to have a relationship with the resultant force on the crown and band. A deflection is found on the deformation of the runner with the nodal diameter as the midline in the later stages of the starting-up process. The reason is discussed according to pressure distributions. The stress concentration of the support bracket is found on the connection between thrust seating and support plates. The stress of the runner is mainly on the connection between the crown and the blade’s leading-edge. This work will provide more useful information and strong references for similar cases. It will also help in the design of pump turbine units with more stabilized systems for reducing over-loaded hydraulic force, and in the solving of problems related to structural characteristics.

The present paper aims to investigate the energy dissipation related to unsteady flow phenomena inside a three-bladed impeller of a centrifugal dredge pump under over-load operating conditions. Three-dimensional unsteady numerical simulations of the centrifugal pump are performed by adopting the SAS SST-curvature correction turbulence model with the total energy equation. The simulating results are verified by comparing the performance results and pressure fluctuation with available experimental data. The unsteady flow patterns and energy dissipation in the rotating impeller are analysed by entropy distribution and pressure fluctuation spectra. A high-entropy area appears in the impeller flow passage when the discharge increases. It is indicated in the unsteady simulation results that a vortex flow with high entropy generates and detaches periodically, which causes the hydraulic energy loss under over-load operating conditions. In numerical simulations, a frequency as 3.3 times of rotating frequency is found in the pressure spectral analysis at 1.45 Q0 operating condition, which is related to the unsteady flow structure. The secondary flow near the volute tongue is found at 1.45 Q0 operating condition due to the large angle of attack when discharge increases.

The free-surface vortex is simulated with the homogeneous mixture approach. In order to define suitable boundary conditions in the two-phase flow field, a numerical strategy is proposed on the air ...

This paper presents entire flow path simulations in a prototype four-nozzle Pelton turbine under three water heads, from pipe flow to nozzle jet water sheet flow on rotating bucket and drop from bucket brim on the air. Different type flows in the Pelton turbine are analyzed by adopting the three-dimensional transient air-water two-phase flow simulation method. Hydraulic performance of stationary parts and rotating runner are evaluated along the entire flow path. Flow analyses indicate that the pressure pulsation is very low and the water flow could be regarded as steady flow in the stationary parts. The hydraulic loss in these parts drops as the water head increases due to a reduction of the frictional loss. Afterwards, the interactions between the jets and buckets in the rotating runner are discussed. The pressure pulse on the bucket surface pulsates with the spreading of the water sheet flow and takes up 10%–25% water energy. The hydraulic performance of the bucket is highest at optimal water head and decreases as the water head varies. Unsteady flow analyses show that there is a potential for interference between the two adjacent jets if the water head or the angle between the two jets decreases.

The present paper suggests a modification of the turbulent viscosity model for predicting cavitating flows. It is to improve the two-equation turbulence model, which failed to accurately predict unsteady cloud cavitation due to the turbulent viscosity overestimated. The modification approach is that the cavitating flow depends on the turbulent viscosity in not only the mixture flow field but also the surrounding flow field in the vicinity of the vapor cavity. The turbulent viscosity in the standard RNG k-e turbulence model was modified by the correlation between the cavitation model and the Barotropic equation of state. The unsteady cavitating flow around a 3-D NACA0015 hydrofoil was simulated by using the modified model as well as the standard model. The simulation result for the modified model agreed well with the experimental one. From the numerical result, the effect of the turbulent viscosity on the cavitation simulation and the cavitating flows was discussed. The present work showed that the strong turbulence is mainly produced in the condensation process, and it delays the cavitation inception in the next cycle.

The modal characteristics are important issues of a Francis turbine runner. It is prone to cause excessive vibration even resonance of turbine if the natural frequency is close to the excitation frequency. Thus, in the runner design and optimization processes, not only hydraulic characteristics but also dynamic characteristics should be considered to ensure the safe and stable operation of the unit. The modal analysis of hydrofoil NACA0009 is calculated by means of coupled acoustic fluid-structure model to verify the feasibility of the method and the results show good agreement with the experimental results. Then the effect of gap and structure parameters are discussed. Results show that the effect of fluid in the crown gap and seal on natural frequency and added mass is small, but the effect of fluid in the band gap and seal is considerable. The mode shapes are similar under different structural parameters, but the natural frequency and added mass factor are different. The effect of root rounding radius is more obvious than that of blade thickness and band thickness. Besides, the effect mechanism of band thickness on modal characteristics is quite complex, the lower-order frequency decreases while the higher-order frequency increases with the increase of the band thickness. The results in this paper are helpful to avoid the typical resonance frequency such as rotor-stator interaction frequency in the design and optimization processes.

For tidal power stations, turbines operate in a wide head range and sometimes near the free surface due to tide changes, which commonly consists of short intake channel. When they operate under low water levels, free surface flows would be a major concern for hydraulic stability. The present paper establishes a simulation method for predicting all flow fields in tidal power stations. Two-phase simulation model is introduced with the acceleration of gravity and the free surface, which consists of a turbine unit and two reservoir models (sea side and reservoir side). Hydraulic performance of a prototype turbine was predicted by using the proposed model, based on water levels of both two sides. The predicted unit performance agreed well with the test result. The flow fields were compared with ones by two other simulation methods. The proposed method could predict non-uniform intake flow inducing head losses by the sudden contraction and the gravity force, which sufficiently influenced turbine internal flows. Consequently, head drops in each part of the turbine were distributed differently, under the same net head. The present model provides a foundation for predicting prototype turbine performances under not only tide changes but also air-water interactions.

Purpose The purpose of this paper is to study the pressure fluctuation characteristics in the sidewall gaps of a centrifugal dredging pump in detail and discover the excitation sources. Design/methodology/approach An unsteady numerical simulation with shear–stress transport–scale-adaptive simulation (SAS-SST) model was conducted for a centrifugal pump considering the sidewall gaps. The numerical codes were validated by a model test carried out in China Water Resources Beifang Investigation, Design and Research Co., Ltd. Fast Fourier transform was used to obtain the frequency components of the pressure fluctuation. Findings Pressure fluctuation characteristics inside the pump were analyzed for a condition near the design point. In the sidewall gaps, the circumferential, radial and axial distribution of the pressure fluctuation amplitude follow different laws. The non-axisymmetrical distribution of pressure fluctuation in the sidewall gaps shows that the unsteady flow in the volute casing which has a non-axisymmetrical geometry imposes an evident effect on the flow field in the sidewall gaps and the interaction between the main flow and the clearance flow cannot be neglected. There are several frequency components appearing as the dominant frequencies at different locations in the sidewall gaps, but the relatively stronger pressure fluctuations are all dominated by the rotating frequency. It indicates that the rotating impeller, which originally makes the shrouds rotate, is the primarily excitation source of the pressure fluctuations in the sidewall gaps. Originality/value The pressure fluctuation characteristics in the sidewall gaps of centrifugal pumps were first comprehensively analyzed. Unsteady flows in the sidewall gaps should be considered during the design and operation of centrifugal pumps.

Purpose The purpose of this paper is to analyze lubrication characteristics of a tilting pad thrust bearing considering the effect of the thermal elastic deformation of the pad and collar. Design/methodology/approach This study used the fluid–solid interaction (FSI) technique to investigate the lubrication characteristics of a tilting pad thrust bearing for several typical operating conditions. The influences of the rotational speed, the thrust load and the oil supply temperature on the lubrication characteristics were analyzed. Findings The three-dimensional (3D) film model clearly shows that there is no pressure gradient but large temperature gradients across the film thickness. The wall heat transfer coefficients on the pad surfaces distribute in a very complex way and change within a large range. The rotational speed, the thrust load and the oil supply temperature have great but different influences on the lubrication characteristics. Originality/value This paper has preliminarily revealed the lubrication mechanism of the tilting-pad thrust bearings. The 3D FSI method is suggested to evaluate the thermal-elastic-hydrodynamic deformations of thrust bearings instead of the conventional method which iteratively solves the Reynolds equation, the energy equation, the heat conduction equation and the elastic equilibrium equation. Using FSI method, the heat transfer coefficients on the pad surfaces can be evaluated better.

The power failure process of pumped storage unit is one of the most dangerous transient processes. The unit enters the pumping braking mode and then generation mode after power failure. In this process, the rotating speed changes from the negative direction to the positive direction, and the hydraulic torque of the blade also changes rapidly, resulting in the great change of runner stress. In this paper, the amplitude of dynamic stress and the location of stress concentration of runner were calculated and analysed to evaluate the safety of runner in the process of power failure in pump mode. The coupling method of one-dimensional pressure wave calculation and three-dimensional flow simulation is used to predict the change of hydraulic loads and the fluid-structure-interaction method is used to calculate the dynamic characteristics of runner. It is found that the position of stress concentration changes in the process of rapid decrease of rotating speed to zero.

During the start-up, the unit enters into speed-no-load from static state. The runner is full of complex vortex, which hinders the water flow into the runner, and even leads to the failure of starting up. It is necessary to study the vortex and flow state to evaluate the stability of start-up. In this paper, the coupling method of one-dimensional pipeline system and three-dimensional flow inside the unit is used to simulate the startup process and analyse the three-dimensional flow state in runner. The flow instability during the start-up process is deeply studied by analysing the pressure distribution, streamline and turbulent kinetic energy. The results show that the turbulence intensity in the runner decreases with the increase of discharge. During start-up, the vortices in the runner occupy the whole blade passage, and then disappear. With the decrease of discharge, the turbulent kinetic energy at the runner inlet rises and small vortex appears at runner inlet.

In the structure of water pump turbine unit, the top cover and the seat ring structure are the key structural components, and the high-strength bolts distributed in the circumferential direction are usually used to connect. With the increase of the unit capacity and size, the number of connecting bolts is increasing gradually, so as to ensure the effective connection of key structural components. Taking a mixed flow pump turbine of a pumped storage power station as an example, the finite element calculation of the joint screw pair structure of the head cover and the seat ring is carried out under different working conditions. The stress distribution law of the screw pair is studied in this paper, which provides important theoretical basis and design reference for the practical engineering application of the head cover ring connection of the pump turbine.

The stresses of head cover bolts in a pump turbine were analyzed based on Fluid-Structure-Interaction (FSI). A nonlinear finite element model including head cover, bolts and stay ring was set up by means of contact elements, two head cover models with different flanges were set up to evaluate the effect of head cover to bolts. Results showed that both the uneven pressure distribution and the uneven head cover structure along the circumference have great influence on the stresses of the bolts. In this study, for the head cover with double flange, the single bolt has larger axial stresses amplitude along the outside circle, and the average values of the axial stresses for the 96 bolts are larger too, as a result, the bolts have higher average relative stiffness.

Pumped storage power station is regarded as a key component in modern social power system. It usually requires pump-turbine start and stop frequently. The hydraulic axial thrust is one of the most essential effects on the stable operation during start and stop. The hydraulic axial thrusts of pump-turbine pump mode’s stop with power down based on 1D transient flow simulation and 3D CFD simulation were carried out in this paper. The hydraulic axial thrusts of two operation conditions show different trends in early stage. In later stage, results show similar characteristics with load-rejection process of turbine mode. The components of Fz including the crown, band and blade reach an obvious peak value with unit operation status change. This study provides the reference for the safety and stable operation of pump storage power stations.

Thrust bearing is a key component of hydroelectric generator and bears the axial load of the unit. During transient processes, such as start and stop of the unit, thrust bearing usually occurs wear phenomenon at pad surface, which affects the stable operation of the unit and reduces service life of the bearing. Based on two-way fluid-solid-thermal interaction, this paper did transient thermal-elastic-hydrodynamic (TEHD) analysis of a tilting pad thrust bearing under the pump mode. For the fluid region, the Reynolds equation and the viscosity-temperature equation are solved. For the solid region, transient heat conduction equation and basic thermal-elasticity equation are considered. The data transferred in the fluid-solid interface satisfy the fluid-solid-thermal interaction equation. The variation of the lubricant oil temperature, the oil film force, the pad surface inclination angle and axial velocity of pad and mirror plate are analysed, which provide theoretical basis for the design of bearing and the safe operation of the unit.

Successive load rejection is one of the most crucial transient processes for calculating the maximum pressure in spiral case and maximum rotating speed of runner. One-dimensional mathematical models were established for a pump-storage power station and characteristics method was used to study the successive load rejection. The condition in which the maximum pressure occurs was chosen for three-dimensional unsteady calculation. The boundary conditions at spiral-casing inlet and draft tube outlet were determined by using the one-dimensional result. Based on numerical simulation, the axial hydraulic thrust and transient flow characteristic during the successive load rejection process were analysed. The axial hydraulic thrust during the successive load rejection process was mainly caused by the complex flow in the runner. The complex flow was affected both by hydraulic disturbance and the successive load rejection process.

During load rejection process, the rotation rate, head and discharge change a lot and the internal flow of the runner is complex, leading to fatigue cyclic loads on the runner. In some cases, fatigue cracks appear at the stress concentration location of the runner. This paper studied the dynamic stresses of a pump-turbine runner during load rejection process in turbine mode using finite element method. The hydraulic loads were obtained from CFD analysis, and the clearance flow was taken into consideration in the CFD simulation. The variation of performance characteristics was calculated through the one-dimensional method of characteristic (1D-MOC). The results show that the pressure on outer surfaces of runner changed the stress of the runner, and the rotation rate had a great influence on the dynamic stresses of the runner.

The startup process occurs frequently for pumped storage units. During this process, the rotating rate that changes rapidly and unsteady flow in runner cause the complex dynamic response of runner, sometimes even resonance. The sharp rise of stress and the large-amplitude dynamic stresses of runner will greatly shorten the fatigue life. Thus, the study of start-up process in turbine mode is critical to the safety operation. This paper introduced a method of coupling one dimensional (1D) pipeline calculation and three-dimensional computational dynamics (3D CFD) simulation to analyze transient unsteady flow in units and to obtain more accurate and reliable dynamic stresses results during start up process. According to the results, stress of the ring near fixed support increased quickly as rotating rate rose and became larger than at fillets of leading edge and band in the later stages of start-up. In addition, it was found that dynamic response can be caused by rotor stator interaction (RSI), but also could even be generated by the severe pressure fluctuation in clearance, which can also be a leading factor of dynamic stresses. This study will facilitate further estimation of dynamic stresses in complex flow and changing rotating rate cases, as well as fatigue analysis of runner during transient operation.

For tidal power stations, the turbine operates under low-head near the free surface. The water levels change both the reservoir and sea sides, furthermore, the intake channel length is short. In this paper, the objective is to investigate the effect of surface vortices on operating conditions for tidal power units. Two-phase flows were simulated with a simplified reservoir model. The numerical method was verified with experimental data. In the reservoir model, the predicted surface vortex agreed well with various analytical models. From the numerical results, two-phase flows in the reservoir includes surface vortices sufficiently influenced the intake flows. It induces the non-uniformity of the pipe flow, which is by the strong vorticity and the air content in the water. The pipe entrance loss also directly influences operating conditions. Surface vortices influenced the net head and the flow rate passing through a pipe, and these effects increase exponentially as the water level decreases or the water level difference increases. Consequently, predicting the net head and the flow rate of the prototype could be complicated for site tests, and it would be also difficult to accurately predict prototype performances with the typical model test and numerical simulation methods for tidal power units.

The present paper aims to find out the influence of operating head on the rotating bucket free surface sheet flow and hydrodynamics performance for a Pelton turbine. Three-dimensional unsteady air-water two-phase flow simulations in the rotating buckets were performed by adopting the shear stress transport curvature correction turbulence model and homogenous model. The sensitivities of the unsteady simulation results to moving mesh resolution and computational fluid dynamics solver time-step have been evaluated to discuss the effect of Courant–Friedrichs–Lewy conditions on the two-phase flow simulation. The accuracy of the numerical predicted flow pattern and the hydrodynamic performance results are reasonable when compared with the experimental data. The simulation results indicate that the remaining kinetic energy carried by the outflow under the nondesigned water head is the main reason for the efficiency loss in the Pelton turbine. Under low water head conditions, jet distortion caused by the Coanda effect and flow interference will lead to more severe efficiency deterioration.