Reversible Pump Turbine Research Articles

RPT Runner Flow Structures Dependence on Guide Vane Opening Angle: A CFD Numerical Simulation

Pumped storage power plants are now honored for different novelties mostly to do with large energy storage ability and electrical grid stabilization capacity. However the control of Reversible pump Turbines (RPT) operations within these plants is still an issue, where the so-called S-shape flow instabilities cause different problems especially when under low discharge operating conditions. Taking from the grounds that these instabilities have been found to mostly be based within the vaneless space between the guide vanes and the runner, this paper intends to investigate the effect of guide vanes opening (GVO) on runner flow characteristics. CFD-backed numerical simulations were carried out on a RPT complete flow passage under different GVOs; namely 17mm, 21mm and 25mm openings. Instabilities were found to take source from low flow conditions where resulting hydraulic losses maybe the trigger of s-shape characteristics appearance. The GVO however, showed the ability to tame the severity of these flow instabilities and associated pressure pulsations.

Energy Storage at Groundwater Banks

This study investigated adding energy storage (pumped storage) at a groundwater bank, Willow Springs Water Bank, in Southern California. Two different technologies were evaluated. Aquifer pumped hydro (APH) uses reversible pump turbines, the aquifer as the lower storage reservoir, and a surface storage reservoir as the upper reservoir. APH and peak hour pumped storage (PHPS) can be used with or without demand response (DR), which deliberately curtails pumping to limit energy use at times when energy use and prices are high and shifts pumping to times when energy use and prices are lower. PHPS uses a surface storage reservoir as the lower reservoir, an upper elevation surface reservoir as the upper reservoir, and pumps and a hydroelectric generator to connect the two. Three different water bank operating modes were assessed (wet year—recharge, dry year—extraction, and neutral year—pumped storage). The results of the analysis are that APH was not cost‐effective. PHPS was, but the ability to curtail/adjust pumping and participate in DR was the most valuable resource, more valuable than the ability to generate electricity.

Analysis on the internal flow field in vaneless space and draft tube of one reversible pump turbine during load rejection under turbine mode

The development of pumped storage plants presents quicken tendency since the deregulation of energy market, for balancing the unstable grid caused by electricity generated by renewable energy. Whilst pumped storage plants are increasingly operating at off-design conditions in order to meet the requirement of electrical grid, therefore performance of unit at off-design conditions has become more important. In this work, three dimensional unsteady calculations were carried out for simulating pump turbine load rejection period based on the dynamic sliding mesh method and turbulent model of detached eddy simulation, also weakly compressible fluid was considered. The results was analyzed in both time domain and frequency domain to study the internal flow field characteristics, while flow field evolution in vaneless space between guide vane and runner was also investigated, reasons of vortex rope formation and characteristics of vortex rope in draft tube were studied.

Numerical simulation and experimental research the characteristic at rated guide vane opening on pump-turbine

The performance of a reversible pump turbine with S-shaped (dQ11/dn11>0) characteristics is of great harmful to the transient processes such as start-up and load rejection. When working point transition is required, then process in turbine mode must be fast, efficient and reliable. Especially during load rejection, the pressure increasing at spiral case and negative pressure taking place at draft tube. This phenomenon has great relation with the characteristic curve with pronounced S shape. In this paper, the characteristic curve is improved with runner optimization. And hydraulic performance results are compared with runner geometry parameters changing. The runner optimization is an effective method to solve this problem.

Performance prediction and flow analysis of pump operation of tidal unit with bilateral vanes

Tubular reversible pump-turbine is a special machine used in tidal power, tidal fluctuation by affecting its working head was positive and negative periodic alternation, so the operating conditions of turbine power generation not only, also includes forward and reverse pump operation. A lot of researches have been done on the turbine operation at home and abroad, and a relatively mature bi-directional “s” airfoil blade has been designed, and good power generation performance has been obtained in both forward and backward directions by adding bilateral guide vanes. However, the study on the conditions of pump operation has proved dissatisfactory. In this paper, the steady numerical simulation of a water pump-turbine with bilateral guide vanes is carried out, and the internal flow of the pump operation is analyzed. The results show that by adjusting the front and rear of guide vane and blade angle combination relationship, forward and reverse operation of the pump efficiency can be as high as 83.83% and 58.07%. The experimental research of increasing the unit pump condition is of great significance to the development of large tide unit.

Experimental study on the vibrational performance and its physical origins of a prototype reversible pump turbine in the pumped hydro energy storage power station

Abstract In the present paper, the vibrational performance and its physical origins of a prototype reversible pump turbine in the pumped hydro energy storage power station is experimentally investigated. Specifically, the vibrations of the unit in the X, Y and Z directions of the top cover, the upper and the lower brackets were all measured for three water heads (from 48% to 90% in terms of non-dimensional values) and nine load conditions (from 34% to 96% of the rated power) together with the pressure measurement at several typical monitoring points. Based on our analysis, it was found that the vibrations of the top cover are mainly induced by the fluid flow inside the reversible pump turbine while the vibrations of the upper and the lower brackets are generated by the mechanical aspects of the rotor. For the top cover, three regions are proposed with their characteristics and its physical origins fully demonstrated with the aid of several typical examples. In region I (with the low partial loads), the vibrational level of the unit is the highest and its physical origin is the pressure fluctuation in the vaneless space with the blade passing frequency. In region II (with the medium loads), the vibrational level of the unit is medium and its physical origin is the swirling vortex in the draft tube. In region III (with the high partial load), the vibrational level of the unit is the lowest and its physical origin is twofold: the pressure fluctuation in the vaneless space with the harmonics of blade passing frequency and the mechanical aspects of the rotor. For different water heads, transitions between aforementioned categorized regions could be observed with the primary characteristics maintained. At last, comparing with the cases of the top cover, the vibrations of the upper and the lower brackets are less affected by the load and the water head variations.

Current Research in Hydraulic Turbines (CRHT) VIII

PrefaceCurrent Research in Hydraulic Turbines (CRHT) is an international symposium dedicated to research activities related to hydraulic turbines. The event has been taking place in Kathmandu University since 2010 and since 2018, the publication of the short-listed papers from the event in IOP Journal of Physics: Conference Series has been provisioned. This year, the topic of the papers/presentations was divided into six thematic areas: i) Modeling and simulation in hydro turbines ii) Simulation and experimental techniques in hydropower iii) Sediment erosion and cavitation in hydraulic turbines iv) Renewable energy, devices, energy and efficiency measurement v) Pumps, FSI in pipe and net metering and vi) Reversible pump turbines and Hill chart of Francis turbine.In this symposium, the technical session was represented by six Universities in three parallel sessions. In total, 24 papers were presented in the symposium, out of which, 10 papers were selected for publication in IOP. The selection was based on the evaluation and thorough review process by respective session chairs and the organizing committee. The event was organized by Turbine Testing Lab of Kathmandu University and supported by International Center for Hydropower (ICH).The papers included in this Journal contain a wide range of research activities conducted in the sector of hydropower and hydraulic turbines. The problems of sediment erosion in the context of Indian Sub-continent have been discussed with the challenges of modeling and simulating the erosion in CFD. Theoretical approach for generating hill charts of Francis turbines using shroud-streamline modeling has been presented. Similarly, the problems of pressure pulsations in Francis turbines, experimental and numerical techniques of measurement and modeling of the power plants have been emphasized. Some papers also include design optimization using CFD for better efficiency and minimum erosion in turbines.

Simplified hydrodynamic analysis on the general shape of the hill charts of Francis turbines using shroud-streamline modeling

The paper presents a simplified one-dimensional calculation of the efficiency hill-chart for Francis turbines, based on the velocity triangles at the inlet and outlet of the runner’s blade. Calculation is done for one streamline, namely the shroud streamline in the meridional section, where an efficiency model is established and iteratively approximated in order to satisfy the Euler equation for turbomachines at a wide operating range around the best efficiency point (BEP). Using the presented method, hill charts are calculated for one splitter-bladed Francis turbine runner and one Reversible Pump-Turbine (RPT) runner operated in the turbine mode. Both turbines have similar and relatively low specific speeds of nsQ = 23.3 and nsQ = 27, equal inlet and outlet diameters and are designed to fit in the same turbine rig for laboratory measurements (i.e. spiral casing and draft tube are the same). Calculated hill charts are compared against performance data obtained experimentally from model tests according to IEC standards for both turbines. Good agreement between theoretical and experimental results is observed when comparing the shapes of the efficiency contours in the hill-charts. The simplified analysis identifies the design parameters that defines the general shape and inclination of the turbine’s hill charts and, with some additional improvements in the loss models used, it can be used for quick assessment of the performance at off-design conditions during the design process of hydraulic turbines.

Analysis of the vortices in the inner flow of reversible pump turbine with the new omega vortex identification method

Reversible pump turbines are widely employed in the pumped hydro energy storage power plants. The frequent shifts among various operational modes for the reversible pump turbines pose various instability problems, e.g., the strong pressure fluctuation, the shaft swing, and the impeller damage. The instability is related to the vortices generated in the channels of the reversible pump turbines in the generating mode. In the present paper, a new omega vortex identification method is applied to the vortex analysis of the reversible pump turbines. The main advantage of the adopted algorithm is that it is physically independent of the selected values for the vortex identification in different working modes. Both weak and strong vortices can be identified by setting the same omega value in the whole passage of the reversible pump turbine. Five typical modes (turbine mode, runaway mode, turbine brake mode, zero-flow-rate mode and reverse pump mode) at several typical guide vane openings are selected for the analysis and comparisons. The differences between various modes and different guide vane openings are compared both qualitatively in terms of the vortex distributions and quantitatively in terms of the areas of the vortices in the reversible pump turbines. Our findings indicate that the new omega method could be successfully applied to the vortex identification in the reversible pump turbines.

Cavitation behavior study in the pump mode of a reversible pump-turbine

Abstract Cavitation is an important issue of reversible pump-turbines especially in the pump mode. It usually causes noise, vibration, material-damage and operation stability on the pump-turbine unit. To diminish the bad influences of cavitation, the cavitation behavior in the pump mode of a pump-turbine is experimentally and numerically investigated. Results show that the best range of inception cavitation number and the best range of critical cavitation number have no intersection. Influenced by the incidence angle on the leading edge, the best inception cavitation range occurs around the impeller design condition. However, the best critical cavitation range is found at partial-load. To find a proper cavitation criterion, the development of cavitation is studied in detail. The relationship among the critical cavitation, the vapor volume and the fluid volume below the vaporization pressure is analyzed. At partial-load, cavitation incepts at a higher cavitation number than under the impeller design condition. During the cavitation number's decreasing, the vapor volume under impeller design condition transcend it at partial-load. Finally, the impeller design condition has a higher critical cavitation number than the partial-load. Considering the existing cavitation before critical cavitation, the inception cavitation standard is strongly recommended for the pump-turbine and other high-energy hydraulic turbomachineries.

Francis-Type Reversible Turbine Field Investigation During Fast Closure of Wicket Gates

Flexible electricity demand and variability of the electricity produced by wind turbines and photovoltaic affect the stable operations of power grids. Pump-turbines are used to stabilize the power grid by maintaining a real-time electricity demand. Consistently, the machines experience transient conditions during the course of operation, such as start-up, load acceptance, load rejection, and shutdown, which induce high amplitude pressure pulsations and affect operating lifespan of the components. During the closure of the wicket gates, the transient flow characteristics is analyzed for a Francis-type reversible pump-turbine in generating mode by three-dimensional (3D) numerical simulation with a moving mesh technique and using detached eddy simulation (DES) turbulent model. Mesh motion is carried out in the region of wicket gates during the load rejection by a moving, sliding mesh, which makes dynamic flow simulation available, instead of building various steady models with different guide vanes angles. The transient flow characteristics are illustrated by analyzing the flow, torque, and pressure fluctuations signals by frequency and time–frequency analyses. The flow field analysis includes the onset and strengthening of unsteady phenomena during the turbine load reduction. The flow pattern in return channel maintained a quite stable flow field, whereas the flow pattern in the runner and draft tube emphasized its instability with the flow rate decreased. Influence of 3D unsteady flow structures on runner is determined, and its evolution is characterized spectrally during fast closure of wicket gates.

Experimental investigations of a prototype reversible pump turbine in generating mode with water head variations

Influences of water head variations on the performances of a prototype reversible pump turbine are experimentally studied in generating mode within a wide range of load conditions (from 25% to 96% of the rated power). The pressure fluctuations of the reversible pump turbine at three different water heads (with non-dimensional values being 0.48, 0.71 and 0.90) are measured and compared based on the pressure data recorded in the whole flow passage of the turbine. Furthermore, effects of monitoring points and load variations on the impeller-induced unstable behavior (e.g. blade passing frequency and its harmonics) are quantitatively discussed. Our findings reveal that water head variations play a significant role on the pressure fluctuations and their propagation mechanisms inside the reversible pump turbine.

Numerical simulation of hydraulic force on the impeller of reversible pump turbines in generating mode

The hydraulic force on the reversible pump turbine might cause serious problems (e.g., the abnormal stops due to large vibrations of the machine), affecting the safe operations of the pumped energy storage power plants. In the present paper, the hydraulic force on the impeller of a model reversible pump turbine is quantitatively investigated through numerical simulations. It is found that both the amplitude of the force and its dominant components strongly depend on the operating conditions (e.g., the turbine mode, the runaway mode and the turbine brake mode) and the guide vane openings. For example, the axial force parallel with the shaft is prominent in the turbine mode while the force perpendicular to the shaft is the dominant near the runaway and the turbine brake modes. The physical origins of the hydraulic force are further revealed by the analysis of the fluid states inside the impeller.

Numerical Study on the Internal Flow Field of a Reversible Turbine during Continuous Guide Vane Closing

The unsteady flow field in a reversible pump-turbine is investigated during the continuous load rejection using a 3D computational fluid dynamic analysis. Numerical calculations are carried out using the detached eddy simulation (DES) turbulence model and a new approach involving automatic mesh motion. In this way, the instability of the flow field is analyzed by continuously changing the guide vane openings from the best efficiency point (BEP). Unsteady flow characteristics are described by post-processing signals for several monitoring points including mass flow, torque, head and pressure in the frequency and time-frequency domains. The formation of vortices of different scales is observed from the origin to further enlargement and stabilization; the effect of the rotating structures on the flow passage is analyzed, and the influence of unsteady flow development on the performance of the turbine is investigated. Finally, the evolution during the period of load rejection is characterized in order to determine the hydrodynamic conditions causing the vibrations in the machine.

Numerical analysis of non-axisymmetric flow characteristic for a pump-turbine impeller at pump off-design condition

The present paper analyzed the inner flow characteristic of a reversible pump-turbine at pump mode with the optimal guide vane opening. An unusual asymmetrical feature of both pressure fluctuation and flow pattern inside the rotating impeller was detected under a pump off-design condition, in which the flow rate is smaller than that of hump district. The steady and unsteady simulations were performed in ANSYS 16.0 with the SAS SST-CC turbulence model. The simulation results of pressure fluctuations show that there exists a noteworthy difference of pressure amplitudes in each rotating flow passage, which is not obvious under other conditions. Flow pattern analysis indicates that the impeller flow fields are asymmetrical in different passages under this pump off-design condition, with significant flow separation and vortexes. When exploring the possible causes for this particular flow pattern, it is revealed that the specific elbow structure has an impact on the flow distribution in the draft tube. Flow separation and reverse flow are discovered in the conical and elbow part of the draft tube, which leads to the uneven flow pattern at the impeller inlet. It is demonstrated that there is a close relationship between unsteady flow structures and asymmetrical pressure fluctuation characteristic.

Experimental Study of Load Variations on Pressure Fluctuations in a Prototype Reversible Pump Turbine in Generating Mode

The characteristics of pressure fluctuations in a prototype reversible pump turbine (RPT) is investigated within a wide range of load conditions with a focus on the low-load condition (e.g., 25% of rated power) with the aid of pressure signals obtained at several typical recording points (i.e., spiral casing, vaneless space, draft tube cone, and draft tube elbow). Our findings reveal that at the low-load condition, the pressure fluctuation is quite significant (e.g., above 12% in terms of nondimensional values), especially in the vaneless space and spiral casing with the dominant frequency being the blade passing frequency. Furthermore, based on the characteristics of pressure fluctuation, the investigated load range is divided into three zones. For zone I (with low load), the amplitude of pressure fluctuation is highest and the dominant mechanism is the rotor–stator interaction in the vaneless space with the blade passing frequency. For zone II (with medium load), the amplitude of pressure fluctuation is less prominent (below 5%) and the dominant mechanism is the low-frequency fluctuations induced by the swirling vortex rope. For zone III (with high load), the amplitude of pressure fluctuation is quite limited (less than 3%) and the dominant mechanism is still the rotor–stator interaction but with the dominant frequency being the harmonics of blade passing frequency. Detailed examples for all three zones are given and discussed with quantitative descriptions of propagation mechanism of fluctuation.

Simulation and Experimental Investigation of the Stay Vane Channel Flow in a Reversible Pump Turbine at Off-Design Conditions

This work presents the assessment of the mean flow field and low frequency disturbances in the stay vane channel of a model pump turbine using transient numerical simulations and LDV-based measurements. The focus is laid on transient CFD simulations of characteristic flow states in the stay vane channel when operating at off-design conditions in pump mode. Experimental and numerical investigations obtained a shifting velocity distribution between the shroud and hub of the distributor when continuously increasing the discharge in the part-load range. Simulations captured the occurrence of this changing flow state in the stay vane channel reasonably well. A further increase of the discharge showed a uniformly redistributed mean flow of both hub and shroud side. Monitoring points and integral quantities from measurements and transient simulations were used to interpret the development of transient flow patterns in the stay vane channel at the operating point of strongest asymmetrical flow. During simulation and measurement, a dominant rotating stall inception was observed near the design flow of the pump turbine. At this point where the stall becomes severe, a high level of correlation between the signals of the upper and lower stalled flow in the stay vane channel was calculated. Further simulations for different guide vane positions predicted a strong influence of the guide vane position on the structure of rotating stall.

Modeling and static optimization of a variable speed pumped storage power plant

Pumped storage power plants are key components to stabilize electric distribution networks with high amount of intermittent power sources as, e.g., solar and wind power plants. Tailored mathematical models are important for the transient and the stationary analysis of such plants. A comprehensive mathematical model of a variable speed operated pumped storage power plant, which incorporates reversible pump turbines in combination with doubly fed induction machines, is developed in this paper. Special emphasis is laid on an accurate description of important dynamic effects (e.g., water hammer) and of the energy losses of the system. Based on this model, optimal stationary operating points are determined, which minimize the overall system losses and systematically take into account the operating constraints.

Cavitating vortices in the guide vanes region related to the pump-turbine pumping mode rotating stall

Reversible pump-turbines used in Pumped Storage Power Plants are among the most cost-efficient solutions for storing and recovering large amount of energy in short time. Presented paper is focused on the pump-turbine pumping mode part-load instabilities, among them the rotating stall and the cavitating vortex in the distributor region. Rotating stall can be described as a periodic occurrence and decay of the recirculation zones in the distributor with its own rotational characteristics frequency. Unstable behaviour can result in high radial forces, high pressure fluctuations and local velocity fluctuations that can in some cases lead into the occurrence of the cavitating vortex in the distributor region, even though the distributor is located in the high pressure region. Computationally demanding calculations have been performed using commercial CFD code. Analysed results have been compared to the experimental data obtained in the ČKD Blansko Engineering hydraulic laboratory.

1D simulation of pump-turbine transition

Variable speed motor-generators based on full size converter offer new options for operation of reversible pump-turbines. To explore the challenges related with this new technology a specific test rig configuration is developed to perform such transition with a PT reduced scale model. Transition from pump to turbine and vice versa are performed with various guide vanes opening and transition time. In order to extrapolate the results obtained at model scale to prototype, it is important to understand the system behaviour including the waterways and its influence on the flow in the pump-turbine. For this purpose, a 1D model of the test rig is realised. After validation with measurement results, the 1D model has been used to test the influence of the waterway on the pump-turbine transition.