Runner Blades Research Articles

Low-head high specific speed Kaplan turbine for small hydropower – design, CFD loss analysis and basic, cavitation and runaway investigations: A case study

In an era of growing energy demand, it is necessary to reach for lower water potentials to be used by a hydro unit. Due to low damming, a hydro unit must be designed to allow the flow of as much water as possible with maximum efficiency. This ensures optimal use of hydropower and minimization of turbine size. Therefore, this paper presents the construction of a model Kaplan turbine (runner diameter of 300 mm) with a high specific speed, designed to work under ultra-low and low heads up to 3 m.Adjustable guide vanes and runner blades were designed by means of an inverse problem method using a two-dimensional axisymmetric model. In the case of the adjustable guide vane, two shapes have been designed. The first has a classical shape and the second has a non-classical shape. The influence of both guide vanes on the turbine operation was analysed using the CFD technique. As a result, the turbine with a non-classical shape blade has a higher hydraulic efficiency and the efficiency curve is flatter. As part of these calculations, many variants of turbine settings were analysed (192 cases), namely 4 angles of classical guide vanes, 4 angles of non-classical guide vanes, and 4 angles of runner as well as 2 angles of a conical draft tube and 3 rotational speeds. The energy losses occurring in the flow elements of the turbine and their mutual contribution are presented in this paper.Model experimental tests of the turbine were carried out on the test stand with a wide range of settings of the guide vanes (the tests used a guide vane with a non-classical shape), the runner and its rotational speed. The research is aimed at determining the basic parameters of efficiency, the effect of cavitation and the runaway speed. Cavitation and runaway characteristics as well as the results of the resistance torque (friction losses) and its share in the total moment are presented.As a result of design, analytical, and experimental works the low-head high specific speed model Kaplan turbine was obtained and diagnosed. The realization of large flows, combined with the high efficiency of the machine is always a difficult task. The paper briefly describes the shapes of the designed guide vanes and runner blade obtained by the inverse problem method. However, more emphasis has been focused on the presentation of model tests that are rare for this type of machine. Therefore, the measurement uncertainty, measuring devices and the course of the conducted basic performance, cavitation and runaway tests are described in detail. Additionally, for the selected cases, a visualization of the flow with forced cavitation in the runner during cavitation tests is also presented.

Numerical analysis of cavitation hydrodynamics at different Kaplan runner solidity

The power developed by turbine is mainly affected by the runner design (including blade profile and runner solidity) and its rotational speed. However, the solidity of runner greatly affects the hydraulic efficiency as well as the cavitation performance of the turbine. Thus, the most crucial aspect of hydraulic turbine design is to ensure efficient and cavitation-free operation. The objective of proposed work is to analyze the effect of runner solidity on cavitation characteristics and hydrodynamic parameters to obtain the optimal number of runner blades for Kaplan turbine. The computational efficiency and value of critical cavitation factor at the rated operating condition are also compared with experimental results for validation and found to bear the close comparison. The critical analysis of various flow parameters for cavitating and non-cavitating analysis is also presented. The values of critical cavitation factor are found to be 0.58, 0.39, 0.26 and 0.18 for 3, 4, 5 and 6 number of runner blades, respectively. It is found that the critical cavitation factor decreases as the runner solidity increases. In addition to this, the effect of runner solidity on the location of cavitation is also investigated. The proposed work is very realistic and suited for the actual site conditions.

Digital Hydraulic Design for Low-Specific-Speed Propeller Runners with Fixed Blades

The operating point of a propeller hydropower station will deviate from the effective workspace while the discharge reduces excessively during dry seasons. It usually leads to a decrease in efficiency and even to being unable to work. To solve the above problem, a scheme named decreasing capacity to increase efficiency was presented in this article. A low-specific-speed propeller runner with fixed blades that has the same installing dimensions as the original one was redesigned and equipped in dry seasons. A positive circulation at the outlet of the blades bigger than in conventional runners is allowed. Some key technologies about hydraulic design for runner blades were researched, which include distribution of velocity circulation at the inlet and outlet of the runner, thickening of the epiphyseal line of an aerofoil, unfolding aerofoil being converted to a cylindrical section, etc. In the section on digital modeling for runner blades, aerofoils on the cylindrical sections at the rim and at the hub were constructed employing the trend extrapolation method. Moreover, a blade digital model was built at one time according to the aerofoils on all cylindrical sections by means of a successful redevelopment to UniGraphics, and it has perfect symmetry. A case presented indicates that the method of decreasing capacity to increase efficiency is feasible. Using the method, the turbine efficiency increased from less than 28.6% to 83.4% while the discharge decreased from 3.20 m3s−1 to 1.00 m3s−1, and then the hydropower unit was able to work properly.

System and method for enhancing a net head of a reaction turbine

The most useful form of renewable energy is Hydropower. This holds 16–20% share of total energy generation. Reaction type hydro turbines are predominantly used due to its advantages on the operation under high flow and low/medium head and high discharge. This paper proposes a system and method for enhancing a net head of a turbine by the way of proper design consideration of tail race channel. The system includes a reaction turbine including scroll casing, guide vanes, and a draft tube. Further, the system also includes a first flow regulating means which reduces a water level in the tailrace to a minimum threshold level from a maximum threshold level when the reaction turbine is operating at a load power corresponding to a high load and maintains the water level in the tailrace to the minimum threshold level to avoid cavitation during the flow of water in the tailrace, thereby enhancing the net head of the reaction turbine. The guide vanes and multiple runner blades reduce a flow rate of the water discharged from the draft tube upon operating on an arrangement of the guide vanes and the multiple runner blades, upon enhancing the net head of the reaction turbine.

Effects of added mass and moments of inertia on hydroelectric turbines for dynamic applications using structural acoustic simulation

This paper proposes and evaluate coupled structural acoustic simulation, SAS, as the preferred method to predict the added mass and moments of inertia of turbine runners in hydroelectric applications. The method is briefly described and focus is primarily on highlighting the effects and benefits of SAS in comparison with previously published work (Jasper, 1956; Benkö and Holmén, 1966; Hofstad, 2004). These traditional methods scale the mass and moments of inertia of the runner using rule-of-thumb guidelines with different scaling factors. SAS in comparison can directly be applied on the CAD geometry of the runner and calculate the mass and moments of inertia dependent on its direction of motion (or rotation). Added mass is highly dependent on the geometry of the runner and the enclosing waterway. This is apparent from the SAS calculations which shows that the added mass of a Kaplan runner changes due to runner geometry, runner blade angle, and shape of the enclosing waterway. All of which the traditional methods are incapable of. In addition to describe SAS and its benefits the numerically derived added mass and moments of inertia is included in a rotor dynamic analysis of two examples and the results are compared to those obtained using traditional methods. Parameters important for rotor dynamic analysis, such as the polar moment of inertia and the radial added mass, is in the present example increased with about 300% and 60% respectively, when a Kaplan runner is operated between speed-no-load to full load.This paper highlights the differences between SAS and the traditional empirical methods. It is shown that SAS gives more detailed results than the traditional methods while still being computationally effective. Consequently, SAS is the preferred method of choice for deriving the added properties.

Entropy Production Evaluation within a Prototype Pump-Turbine Operated in Pump Mode for a Wide Range of Flow Conditions

Inside the pump-turbine, energy is irreversibly lost due to turbulent pulsations in the high Reynolds number zone and actions of viscous forces close to the wall. The conventional differential pressure method cannot obtain specific details of the hydraulic loss within the machine’s flow passages; on the other hand, the entropy production method can provide accurate information on the location of irreversible losses and the spatial distribution of energy dissipation. Therefore, based on the entropy production theory, this study investigates the composition and distribution of hydraulic losses under different flow conditions for a prototype pump-turbine in pump mode. Study results indicated that total hydraulic losses significantly decreased, then slowly increased with an increase in flow rate. The entropy production rate caused by turbulence dissipation (EPTD), direct dissipation (EPDD), and wall shear stress (EPWS) displayed the same variation patterns as that of total hydraulic losses, with EPTD and EPDD being the most dominating. The location of hydraulic loss within the pump-turbine’s flow domain strongly depended on flow conditions. High hydraulic losses primarily occurred in the guide vanes (GV) and draft tube under low flow rates. Under high flow conditions, however, high hydraulic losses were mostly concentrated in the stay vanes (SV), spiral casing, and GV. Hydraulic losses at low flow rates were primarily caused by flow separation within the GV flow channels, vortices in the vaneless region, and inlet flow impacts on the runner blade’s leading edge. On the other hand, large vortices within the GV and SV flow channels, GV wake flow, and unsteady flow at the spiral casing were the main contributors to hydraulic loss under high flow conditions. EPDD was mainly caused by strain rate, so it was closer to the main vortex regions, whereas EPTD was affected by turbulence intensity and had a wider distribution range in the unsteady flow.

Application of an adaptive controller for controlling the rotational velocity of hydrogenerators taking into account the technological state of a Kaplan turbine

The impact of oil leakages in a turbine on the main power system parameters is investigated in reactive hydraulic turbines with adjustable-blade runners (Kaplan turbines) installed at Maynskaya, Nizhne-Bureiskaya and Vilyuyskaya hydroelectric power plants. The main theoretical relations and conclusions were obtained by the methods of mathematical simulation and integral calculations in the MATLAB software environment. A method is proposed for monitoring leakages in the housing of an adjustable-blade runner and smoothing the fluctuations of various parameters (amplitude, rotational velocity, phase angle, active power and generator current) of the power system in the case of oil leak-ages. The control is performed by means of a sensor installed in the runner, a fiber-optic cable and an optical-electrical converter located along the shaft from the runner blades to the oil receiver of the corresponding hydraulic unit. The performed analysis of the obtained mathematical model (frequency response and Nyquist plots were built) relative to the basic parameters of the electrical energy generated by a hydraulic unit showed that the proposed method of monitoring oil leakages contributes to an increase in the operational stability of a hydraulic unit. When constructing the model, the following parameters of the hydraulic unit were taken into account: rotational angle of runner blades and opening angle of hydraulic turbine guide vanes. The developed block diagram can be used to compare variations in the parameters both without oil leakage control and taking into account the automated control system proposed by the authors. It is shown that the obtained logarithmic Nyquist plot can be used to monitor variations in the amplitude, as well as its smoothening, both under normal conditions and taking into account oil leakages in the hydraulic turbine housing.

Effect of Cavitation with Vibration on the Powerhouse Structure of Bulb Turbines Installed in Hydropower Stations

Hydro energy is one of the world’s most abundant and valuable renewable electricity sources. Hydropower is an important source since it is a clean, sustainable, and cost-effective source of energy. The most perilous characteristic that affects the performance of the hydraulic turbine and its allied parts is the cavitation phenomenon, which is clear as the increase of vapor bubbles in the liquid through any hydraulic turbine. Cavitation causes vibration, which is very harmful to turbine guide bearings and their supporting structures. Sometimes heavy vibration causes cracks in the civil structure of the powerhouse where the bulb turbines are installed. The performance of the bulb turbine and the stability of the powerhouse structure are studied with the effect of cavitation, vibration, and deformation of the turbine casing. Experimental measurements are used to determine at what force the shape of cavitation is very destructive and crucial when the local pressure is less than the vapor pressure of the flowing water, at which static pressure cavities begin to breed and ruin. This paper focuses on small hydropower stations with bulb turbine installations, which emphasized the performance improvement of these turbines by avoiding cavitation on the runner blades. Allowing some cavitation on these machines is also recommended, which is within repairable condition, and the cavitation pitting can be repaired during annual maintenance.

Study on the Effect of the Guide Vane Opening on the Band Clearance Sediment Erosion in a Francis Turbine

Sediment erosion is a negative phenomenon for the water turbine. The purpose of this paper is to study the effect of the guide vane opening on the particle motion and sediment erosion in the band chamber using the Euler–Lagrangian approach. The software Ansys CFX and Tabakoff erosion model are used to simulate the sediment laden flow in the full flow passage of the hydraulic turbine. The results are in good agreement with the actual erosion character on site. Results show that the guide vane opening has a positive correlation with the flow in the non-clearance channel. The increase of the guide vane opening will increase the erosion of the runner blade head, but the friction wear on the outlet side of the blade surface will decrease. The rotating action of the runner makes the sediment particles in the band chamber rotate rapidly around the center of the runner and constantly collide with the band chamber wall. Under the small opening, the smaller the opening, the easier the leakage of the band clearance occurs. The unsteady flow in the band chamber will disturb the motion trajectory of particles, change the impact angle of particles, and affect the wear in the band chamber. Under different openings, the change law of the erosion in the band clearance is closely related to the change law of clearance leakage.

RANCANG BANGUN PROTOTYPE TURBIN VORTEX UNTUK PEMBANGKIT LISTRIK TENAGA MIKROHIDRO (PLTMH)

MHPP is a run of river hydroelectric power plant type, that does not require a large dam or water discharge, but rather diverts the flow rate of water into a channel. Vortex turbine utilizes whirlpool of water obtained from the spiral basin shape to push the turbine to rotate. The reason of this research was to decide the design of the MHPP prototype using a vortex turbine. This prototype is a miniature or laboratory scale. This research is designed to model the MHPP prototype using a Vortex turbine which is helpful for learning before it is implemented directly to the river. In this study, the prototype MHPP with vortex turbine was designed with a 0.3 m head, 1.5 dim input pipe, 4.5 L/s water flow, basin with an upper diameter of 40 cm, 40 cm height and 12 cm outlet and 41° cone angle. The runner blade design has 9 blades with a runner outer diameter of 15 cm, 5 cm neck diameter, and 5 cm runner width. This prototype is able to generate power of 0.387 Watt and has an efficiency of 2.93%.

Cavitation detection in a Kaplan turbine based on multifractal detrended fluctuation analysis of vibration signals

Cavitation generally causes a decrease in efficiency and vibrations to a Kaplan turbine, but effectively identifying cavitation state is quite difficult. In this paper, a comprehensive test system consisting of a high-speed camera and a vibration testing system has been constructed based on a high-precision universal hydraulic machinery test stand. The vibration signals of the runner and the draft tube and cavitation images on the runner blade have been obtained for a Kaplan turbine under different cavitation states based on the system. In addition, the multifractal detrended fluctuation analysis (MF-DFA) method has been introduced to analyse the obtained vibration signals, in order to investigate the influence of runner blade cavitation on turbine vibration. Results show that the vibration signals of both the runner and the draft tube have multifractal characteristics. The Hurst exponent, the scaling exponent and the multifractal spectrum by MF-DFA are strongly dependent on turbine cavitation state. It is found that cavitation states including the incipient cavitation of the turbine could be effectively identified by using characteristic parameters extracted from the multifractal spectrum obtained by the MF-DFA of vibration signals. This work provides an effective method for cavitation identification of Kaplan turbines.

Analysis and reduction of the pressure and shear damage probability of fish in a Francis turbine

The fish mortality rate for a Francis turbine mainly depends on the runner. To reduce fish injury and mortality caused by Francis runners, the effects of the number and wrap angle of runner blade on the pressure and shear damage probability are investigated in this paper. The pressure and shear damage probability of different runner schemes are evaluated quantitatively by the computational fluid dynamics (CFD) simulation, and the accuracy of CFD simulation has been verified. The results show that both the pressure and shear damage probability induced by runner have a positive correlation with the discharge. But the shear damage probability is much less than the pressure damage probability. Pressure damage probability should be the main factor to be considered in the design of fish-friendly Francis runner. Reducing the blade number and increasing the warp angle of the runner blade could effectively reduce the damage probability caused by low pressure and high shear stress. But it has little effect on the damage probability caused by high pressure change rate and also results in slight decrease in hydraulic efficiency. This work is beneficial to the fish-friendly turbine design and optimization.

Hybridization in Kaplan turbines. Wear and tear assessment

In the current energy market, hydraulic turbines are increasingly demanded to work in Frequency containment reserve (FCR) mode to compensate the constant frequency fluctuations in the electrical grid. To do so, hydraulic turbines change their generating power continuously which implies to regulate the flow rate. Kaplan turbines are double regulated machines that change the position of both guide vanes and runner blades to regulate the flow rate maximizing their efficiency. Therefore, guide vanes and runner blades are continuously moving when they provide FCR, leading to high wear and tear of the regulation system components.Within the frame of the European project XFLEX Hydro, a new technology to reduce the wear and tear of the regulation system in FRC have been implemented. This technology consists in the hybridization of the unit with a battery system. In that way, the battery is the one in charge of providing part of the power fluctuations to the grid, reducing the movements of guide vanes and runner blades of the turbine. The battery system was successfully installed in August 2021 in one unit of the Vogelgrun powerplant, in France. Since that moment, the unit has been working in hybrid mode.A monitoring system was installed in the power plant in two different units, the one hybridized and another without hybrid system. Several sensors were installed and different parameters were measured simultaneously to calculate the wear and tear of the different components. In this paper, a comparison between the hybrid mode and the standard mode (non-hybrid) is performed in terms of mileage and wear and tear of the guide vanes and runner blades servomotors.

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

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

On the correlation between runner blade dynamic stresses and pressure fluctuations in a prototype Francis turbine

Francis turbines operating in off-design conditions are subject to pressure fluctuations resulting from the development of hydrodynamic instabilities in the draft tube. Depending on the nature of the flow-induced pressure fluctuations (synchronous or convective), this may induce dynamic stresses on the runner blades, increasing fatigue and the risk of crack propagation. This paper proposes to identify the impact of draft tube flow instabilities on the dynamic stresses of Francis turbine runners. Measurements are conducted on a prototype Hydro-Québec Francis turbine from low-load to full-load, including pressure and strain measurements on the stationary and rotating components, respectively. It is first noted that the convective component of the part-load vortex is the main source of excitation for the runner blades. The amplitude of the corresponding dynamic stresses is however reduced at locations closer to the leading edge, for which the dominant fluctuations result from the propagation of synchronous pressure fluctuations. Finally, correlations between runner dynamic stresses and pressure fluctuations measured in water passages are tentatively established for flow instabilities observed at both deep part-load and part-load conditions. This aims to evaluate the feasibility of estimating runner blade dynamic stresses based on signals measured in the stationary components for further investigation.

On the auto-oscillation effect of Kaplan rotors during load rejection

High vibrations due to auto-oscillation effects can occur at power units with Kaplan type runner during load rejection. These vibrations may be present in the turbine braking mode when the wicket gates are almost closed, and the rotating runner pushes the water towards the draft tube resulting in higher pressure below the runner blades. The upward leakage flow between runner blades and discharge ring is suspected to cause the vibration phenomenon.Within this contribution, a fluid-structure-interaction (FSI) technique is applied to identify the mechanism of self-excitation. The investigation is carried out on a disk that represents the simplified geometry of a Kaplan runner. Computational fluid dynamics (CFD) simulations are used in order to obtain parameters in terms of stiffness, damping and inertia of the fluid to be applied in the following shaft line analysis. Damped modal analyses of the shaft line allow for evaluation of the stability of natural frequencies and corresponding mode shapes of the shaft line. A resulting negative damping indicates the occurrence of auto-oscillation behavior of the shaft line. For the operating condition under investigation, mode shapes with negative damping are obtained with this approach and a dynamic bending moment on the runner due to lateral movement in combination with a tilting at the runner in these mode shapes is identified as source of instability.Based on these findings, an analytical approach for modeling the relevant effect is derived and compared to CFD result as well as results from acoustic finite element modelling. Finally, conclusions for avoidance of self-excited vibrations at Kaplan runners are proposed.

Application of wireless measurement technology in hydraulic turbine test

At present, the measurement of internal characteristics of hydraulic machinery runner (blade pressure pulsation, pressure field and dynamic stress) and blade torque is difficult to be realized by the existing wired measurement methods due to the measurement object is a high-speed rotating part. This paper will introduce the system composition and application in hydraulic machinery of the wireless measurement technology one by one. In the measurement of model hydraulic machinery, it is mainly used to measure the rotating part. The sensors work in water, including measuring the internal characteristics of the runner (blade pressure pulsation, pressure field and dynamic stress) and blade torque. The wireless measurement technology that is by laying the corresponding measurement unit at the measured part, the signal sensed by the sensor is collected and stored by the node, and then wirelessly transmitted through the node, and the data is received through the wireless gateway and enters the computer acquisition system.

Solving the challenging puzzle of the Grimsel II runner vibration

After 30 years of operation, the runners of the four 100 MW high-head Francis turbines successively developed fatigue cracks starting from the hub-side trailing-edge fillets. Assessment of alternating stress in normal turbine operation revealed that stress levels were not nearly large enough to explain the damages. Consequently, a test campaign comprising start and stop manoeuvres in addition to normal turbine operation was conducted in 2017. Strain gauge signals obtained on-board of the runner blades were recorded together with casing vibration and acoustic signals. Intense high-frequency vibration centred around 611 Hz was detected in three transient conditions of operation: at speed-no-load (SNL), during coast-down and during back-to-back pump start. Common properties of these conditions were very low to no discharge, strong secondary flow in the runner and speed between 50 and 100 %. Due to the extremely narrow bandwidth, three possible ways of excitation were considered and checked: rotor-stator interaction (RSI), rotor-dynamic instability and Von Kármán excitation. RSI was excluded due to speed-independent frequency. Strong sidebands in the stationary-frame frequency spectra - obviously caused by runner shroud deformation shapes ND1 through ND5 - excluded any feedback from runner side chamber pressure; thus leaving Von Kármán excitation as the only possible diagnosis. Accordingly, a very safe profile was selected and implemented for the runner blade trailing edges. Subsequent field-testing at two modified units confirmed that the vibration problem at the harmful operation points had hereby been eliminated.

An engineering approach for the prediction of fatigue damage in the part load regime

Due to the volatile electricity received from solar and wind power plants, the energy market requires highly variable hydropower machines which are able to cope with fast changes of their operating regimes. With the need for flexible operation, Francis runners are exposed to various operating conditions outside the traditional operating range. Hence, the runners must be specifically designed for long time part load operation to meet hydraulic and structural requirements. Therefore, it is inevitable to have a reliable but also practical engineering approach in the design phase of the projects in order to ensure sufficient fatigue life of the Francis runners. Deep part load operation mainly consists of stochastic loads acting on the runner blades. Previous publications show that it is reasonable to normalize the Rainflow matrix of stress amplitudes to represent the characteristic behaviour at deep part load operation. Hence, it is feasible to utilize the normalized Rainflow matrix to predict the fatigue damage of Francis runner designs without prototype measurements. Within this paper, a scaling method to derive project specific fatigue results for deep part load is introduced and subsequently validated through an intercomparison of available strain gauge measurement data and prediction. Based on the results, the accuracy of the proposed engineering approach and possible impacts of different boundary conditions are discussed.

Mechanism of extraordinary stress on blade during start-up of the Francis turbine

This paper describes the mechanism of extraordinary transient stress that occurs during start-up of a Francis turbine by investigating the detailed analysis with measured stress of prototype runner and transient response calculation. According to the dynamic stress measurement with high sampling frequency, it has been clear that a unique phenomenon occurs during start-up especially at the low rotation speed. From the macro perspective, impulsive stress occurred at blade outlet instantaneously and sporadically. Time interval of the event can be considered close to random, and there is no synchronization with the rotation speed. From the micro perspective, the strain occurs owing to the entire vane vibration, and the impulsive loads bump blade outlet from the side of the pressure surface. A Blade shows dumped free vibration in its natural mode in water. As the conclusion, the phenomenon that runner blades are struck by impulsive load on the pressure surface during start-up is compared to a drum beaten by drumsticks randomly in short time, and we named the unique phenomenon “Hydro Drum”