Waterpower Laboratory Research Articles

Comparison of discharge characteristics of Pelton injector estimated by empirical relation, numerical simulation and experiment

Abstract It is difficult to define a general relationship for discharge characteristics of a Pelton injector due to high degree of variability in geometry of injector amongst different Pelton turbine designs. This study has been aimed to determine discharge characteristics of injector of Pelton rig at Waterpower Laboratory, NTNU. Three different approaches have been undertaken, viz. calculation based on empirical relations, numerical simulations, and experimental validation. For empirical calculation, the injector has been simplified into series of flow geometries with equivalent cross-sectional area and axial length for multiple stroke to constant aperture diameter ratio (s/D 0). Discharge coefficient has been calculated for a constant pressure difference between inlet and atmospheric outlet for each case of s/D 0, based on head loss estimated using empirical resistance coefficient (ζ - values) available in engineering handbooks. Discharge has been determined through numerical simulations for same s/D 0 values. Results from numerical and empirical calculations are compared and correction factor (CF) has been determined. It is found that the CF is almost constant, at value of 0.85, for all values of s/D 0. The discharge coefficient calculated by numerical simulation is around 0.02 lower than the values obtained by experimental observation.

A numerical study on the effect of spanwise installation of vortex generators on Francis runner blades

Abstract Hydro turbine operation far off from its best efficiency point introduces vortices and swirl that detaches flow from the blade surface. This break-off of flow accounts to loss in hydrodynamic lift accompanied by pressure fluctuations and vibration that leads to poor performance and mechanical failure of the turbine blades. Vortex generators (VGs) are efficient passive devices deployed to enhance aerodynamic lift of wind turbine blades and aircraft wings and can improve hydrodynamic lift on turbine runner blades as well. This study aims to investigate on the possibility of installation of VGs on the runner blade at leading edge, midspan and trailing edge of the runner blade. The study is carried out on a model Francis turbine developed at Waterpower Laboratory, NTNU. The numerical study is mainly focused on improving performance of the turbine at low speed as well as at the best efficiency speed. The operating head selected for the study is 11.94 m and the speed of the turbine is varied from 233 rpm to 433 rpm at an increment of 25 rpm. The turbine is simulated at different positions of guide vanes and hydraulic performance is calculated and the results of the analysis are compared with reference case without VGs. The results at deep-part load show major improvement in runner efficiency with increment in values up to 4%. The core purpose of the research is to develop effective techniques to operate traditional turbine runner at variable speed with cost-efficient minimal modifications in the geometry of the runner blades.

Numerical Investigation of Effects of Ripple Type Needle Erosion on Pressure Inside a Pelton Nozzle

Abstract Pelton turbines, a commonly used impulse turbine for hydroelectric generation which converts the potential energy of the water into kinetic energy through a needle structure within the nozzle system. However, the exposure of the Pelton turbine components such as needle, seat ring, buckets to the sediment laden water can erode these components. The erosion on the needle can also lead to the efficiency degradation. This study aims to investigate the effect of needle erosion on the pressure dynamics within the Pelton nozzle system. For the study purpose, the nozzle system from Pelton rig at the Waterpower Laboratory, NTNU is used and a general erosion pattern is developed through literature and field information. The research focuses on analysing the difference in pressure distribution between the normal and eroded needles under different opening conditions, with the objective of understanding how erosion affects the nozzle performance. Single-phase 3-D simulations are performed with an unstructured ANSYS mesh and the CFX solver. This study examines the static pressure at the inner surface of the nozzle casing wall. The results show that for a constant head at the inlet boundary condition, the static pressure with eroded needles was significantly lower than that with normal needles. The pressure difference was around 1.88 kPa for a 60% nozzle opening and around 3.57 kPa for a 40% nozzle opening. The reduction in pressure is due to the increased area in the needle orifice and nozzle outlet caused by erosion, which is more pronounced at lower openings. The findings reveal a direct correlation between needle erosion and alterations in pressure distribution, highlighting the necessity for vigilant monitoring and timely maintenance. The study provides valuable insights into optimizing turbine performance in sediment-laden water conditions and underscores the importance of addressing needle erosion to maintain efficiency.

Evaluation of the jet distortion by sediment erosion in the needle in the Pelton turbines

Abstract In order to succeed in the transition to a more renewable energy sector, the hydropower industry must overcome multiple challenges in the coming decades, including the need for faster and reversible turbines to support emerging renewable energy sources. In addition, the location of new projects in tropical and semi-tropical areas poses unique considerations, as does the increase in sediment. Evaluating sediment erosion in turbines is a critical challenge, which may partly be addressed by analyzing the behavior of water jets from eroded nozzles in Pelton turbines. This paper presents an evaluation of the quality of the jet under various hydraulic conditions, focusing on needle erosion. The study is part of a research project conducted in the Waterpower Laboratory at NTNU, focusing on Pelton turbines. Furthermore, this paper investigates erosion patterns caused by sediments on the needle and explores the impact on jet performance. Through high-speed visualization and image processing, the behavior of a jet from the needle tip to a representative distance downstream has been investigated. The investigation revealed discernible differences between jets with eroded and uneroded needles in size and edge variation.

Evaluating natural degassing in a river to create a baseline for comparison to technical degassing methods.

In Norway, a recent project used a risk matrix to identify about 200 hydropower plants ( 11 % of the total number) of being in high risk of producing air-supersaturated water. The combined installed capacity of these hydropower plants amounts to more than 45 % of Norway’s total installed hydropower capacity. Total dissolved gas (TDG) supersaturation in water is, dependent on the actual saturation value and the duration of the occurrence, lethal to certain types of fish and aquatic invertebrates. Natural degassing is highly dependent on the morphology of the downstream water system, but tends to be insufficiently slow with regard to lethality of TDG supersaturation. The Brokke power plant in southern Norway is known to create TDG supersaturation. Since 2012, continuous monitoring of the TDG level happens at the power plant outlet and several positions in the Otra river downstream. Measurement data has been evaluated and a method has been developed to calculate the liquid-gas mass transfer coefficient kLa as an indicator for the natural degassing efficiency within certain stretches of the river using assumptions for the river’s geometry. The results are in agreement with former evaluation of the measurements. At the Waterpower Laboratory at NTNU in Trondheim, Norway, a test rig has been designed and instrumented to prove the effectiveness of degassing methods. An in-house pressure system is used to produce air-supersaturated water, which is then channeled through an open flume. At the flume’s inlet, the degassing mechanism is installed. The TDG levels are monitored both upstream and at downstream the degassing device. The kLa will be used to compare natural degassing in the river downstream Brokke power plant to the experimental degassing methods in the Waterpower Laboratory.

Governing of a Francis Turbine Model Subjected to Load Variations

In this work, the problem of mechanical backlash in the governor system for the Francis test rig at the Waterpower Laboratory, NTNU Trondheim, is investigated. Through numerical simulations on a digital model of the test rig, done in the software SIMSEN, the maximum allowable backlash which still enables stable governing was detected. This was done by further developing the model used in the work by A. Ingranata [1], to perform load variation tests presented by Statnett [2]. From these frequency control tests it was concluded that the maximum mechanical backlash which still enables stable governing, is 1% of maximum stroke length of the servomotor present in the governor system. To design a system which can achieve the requirements of 1% backlash, a more conventional layout which utilizes links instead of the slots present in the existing governor ring, is suggested.

Review on erosion phenomenon, maintenance, and financial calculation of lifetime as an asset for Pelton turbines

Prevention of greenhouse emissions is the top priority for all countries, which urges them to switch to renewable energy as much as possible. Hydropower is one of the renewables that have high flexibility and at the same time compatibility to be used with any other renewable sources. Moreover, hydropower plants operating in the Himalayas, Andes, and Alps are facing operational challenges due to the high concentration of sediment loads in rivers. Although the arrangement of traditional sediment control mechanisms like dams and sand traps, the erosion tendency of hydro-turbine components operating in this sediment-laden water increases with the increased concentration of sediments. Much past research has been directed towards understanding sediment behaviors, investigation of flow, and effect of concentration, shape, and size, especially with Francis turbines. However, there are very fewer studies regarding sediment erosion and flow behavior in the case of the Pelton turbine. Hence, delving deeper into the flow characteristics, sediment behavior, and performance of the Pelton turbine is important to better understand the flow and sediment pattern of these types of turbines. The paper consists of the evaluation of studies conducted on the flow pattern in the Pelton turbine buckets and its validation with the numerical analysis models using image processing. It is being used in the Waterpower Laboratory at the Norwegian University of Science and Technology, NTNU. This paper also evaluates the scope of investigations about erosion by sediments in Pelton buckets using image analysis and state-of-the-art technology in the hydropower sector. In addition, a review is done about the predictability of erosion based on the measurements of the quantity of sediments that passes through the turbine. This research paper can build a background for quantifying sediment erosion in Pelton turbines with a certain degree of error, which can be utilized as a reference in future studies. The life cycle estimation of a turbine is also analyzed with the consideration of its location and financial return requirements together with the type of maintenance that it may have and the repair that is foreseen, in the case of a non-coated surface.

An experimental study regarding the effect of streamwise vorticity on trailing edge vortex induced vibrations of a hydrofoil

A set of experiments testing the effect on vortex induced vibrations from three different trailing edge designs on a generic hydrofoil has been carried out at the Waterpower Laboratory of NTNU. One of the designs, a Donaldson type semi-blunt trailing edge not a-typical for use in hydraulic machinery serves as a reference. The other two, a trailing edge with boundary layer vortex generators and a trailing edge with rounded serrations cut into the blade, both introduce streamwise vorticity in the wake of the hydrofoil, as shown by particle image velocimetry measurements. The trailing edge of the hydrofoil is made exchangeable, so that a direct comparison of the strain intensity levels measured with a structurally embedded strain gauge bridge located close to the trailing edge can be made. The results indicate that the vortex generators are effective at mitigating the lock-in state of resonance and significantly reduces the maximum strain intensity measured, compared to the more conventional reference design. Velocity measurements both at lock-off conditions as well as close to resonance reveal a de-correlation of the streamwise velocity fluctuations in the wake for this design, suggesting that the von Kármán type vortex shedding is weakened even at resonance conditions. A de-correlation of the streamwise velocity fluctuations in the wake of the serrated trailing edge is also observed at lock-off conditions, but the vortex shedding appear to become gradually more coherent, as well as strengthened, for this design as the foil vibrations increase closer to resonance.

Optimization of guide vanes for variable speed Francis turbines – Human controlled vs. automated approach

Guide vanes of hydraulic turbines can maximise turbine efficiency and directly influence on turbine operating characteristics. A hydraulic analysis is carried out on geometrically parameterised guide vane foils, to maximise efficiency and potentially observe which configuration gives operating range expansion. Variable-speed operated turbines in hydropower plants tend to run with improved performance at off-design operating points due to the speed variations. A combination of variable speed turbine operation and guide vane shape optimization gives a win-win situation in water to energy usage ratio. Human controlled developed designs and CFD calculations were previously done. The obtained results are compared with automated approach in ANSYS Workbench using Design of Experiments. The numerical tests were made on parametrically developed guide vane cascades, where the turbine efficiency is the evaluated parameter and it is observed in the range of several rotational speeds and calculated flow rates. The cascade design which produces less turbine efficiency deviations for the tested rotational speed range is later exported and hydraulically and geometrically compared with the developed design from the manual approach. The tested configurations of the guide vanes are developed on the basis of Francis 99 turbine model from the Waterpower Laboratory at the Norwegian University of Science and Technology in the existing geometric space for the guide vanes and blades axis positioning circumference.

Modal testing of the Francis-99 runner

The Francis-99 low specific speed Francis runner has undergone much study in the years since it was first made available to the public. The High Head Francis(HiFrancis) project, hosted by the Waterpower Laboratory at NTNU, have conducted multiple experimental campaigns to supply validation data for numerical simulations of the RSI phenomenon for the runner. Due to the amount of instrumentation on board the runner an asymmetry exists in the runner structure that naturally affects the structural response of the runner. From numerical analysis the runner has been found to exhibit multiple natural modes of vibration, within narrow frequency bands. In order to validate the identified natural frequencies, with their corresponding mode of vibration and the amplitude, an experiment has been conducted. The experiment utilises piezo electric patches mounted on the runner hub to excite the runner in a defined mode of vibration and the response at the trailing edge of all blades is measured with a 1D Laser Doppler Vibrometry scanner. Preliminary results are shown that demonstrate the possibilities with the presented experimental method.

How to Avoid Total Dissolved Gas Supersaturation in Water from Hydropower Plants by Employing Ultrasound

In Norway, more than 10 hydropower plants are known to have caused biologically relevant levels of total dissolved gas supersaturation in the rivers downstream power plants. This phenomenon is causing fish kills due to gas bubble disease and have large impacts on the biodiversity. The gas supersaturation is often caused by undersized or blocked brook intakes creating turbulent flows and resulting in large amounts of air dragged into the tunnel. One possible solution to this problem is employing power ultrasound (20 kHz - 1 MHz) for degassing water prior to releasing it back into the river system. Acoustic cavitation is known to have a positive effect on the degassing mechanism, and this paper is investigating whether ultrasound can be applied to create acoustic cavitation and avoid biologically relevant levels of total dissolved gas supersaturation from hydropower plants. The objective is to develop background knowledge for constructing an experimental setup in the Waterpower Laboratory at NTNU to investigate whether gas supersaturation can be decreased from power plants by application of ultrasound. Preliminary experiments, carried out at the NTNU Hydrogen Energy and Sonochemistry Laboratory, exploring the behaviour of the degassing process at different ultrasonic frequencies and amplitudes conclude that the most effective degassing occur at high acoustic amplitude and a frequency of 24 kHz.

Signature Investigation of Typical Faults on Francis Turbines

Hydropower is facing new operational strategies as the increasingly competitive power market demands for higher flexibility. Consequently, turbines are forced to handle tougher operation and are prone to more frequent degradation. By implementing a real-time monitoring system, a better understanding of the behavior of components may contribute to detect faults at an earlier stage, reducing potential downtime. This paper will present the work done in the preliminary work of the author’s master thesis. The focus has been to characterize the normal behaviour of a Francis turbine through amplitude and frequency analysis. Steady-state measurements of pressure pulsations have been conducted on the Francis test-rig at the Waterpower laboratory at NTNU. Different hydraulic phenomena and respective frequencies were identified. In addition, the results revealed several unexpected frequencies and suspicious observations, and potential sources of these are discussed. Possible fault detection schemes based on peak-peak and frequency analysis are suggested. Alarm should be raised in case of mismatches in sensor relations, magnitude variations or if new harmonics or frequencies appears.

Vortex generator’s effect on trailing edge vortex shedding and fluid structure interaction

An initial study of the effects vortex generators (VGs) have on the mitigation of vortex-induced vibrations (VIVs) of hydrofoils have been performed at the Waterpower laboratory of the Norwegian University of Science and Technology. The VGs are placed close to the trailing edge of a blunt hydrofoil. Reasonable design parameters of the VGs were found from the literature with regard to which would induce the strongest vortices with the lowest drag. Vibration frequencies of the hydrofoil were measured using flush mounted strain gauges located close to the trailing edge. This paper presents the design of the VGs and some initial results of the experiments performed with these devices. The preliminary results indicate a possible mitigation of VIVs. This could be because of the interference between the primary vortices generated by the hydrofoil and the longitudinal vortices generated by the VGs, but further work is necessary to make a conclusion.

Francis-99 Workshop 3: Fluid structure interaction

Francis-99 is a series of three workshops, which provides an open platform to the hydropower researchers. It gives the possibility to explore capabilities/skills on futuristic turbine design and development. Under the Francis-99 test cases, complete design and data of a Francis turbine are provided. The measurements are conducted on state-of-the-art facility at the Waterpower Laboratory, NTNU. The first workshop was organized during 15-16 December 2014, which focused on the steady state operating condition of the Francis turbine, i.e., best efficiency point, part load and high load. The second workshop was organized during 14-15 December 2016, which focused on the transient operating conditions, i.e., load variation and start-stop. The third workshop was organized during 28-29 May 2019, which focused on fluid structure interactions. In the third workshop, two test cases were provided: (1) Hydrofoil and (2) Francis turbine. The hydrofoil test case aimed to investigate fundamental research, and the turbine test case aimed to investigate applied research.https://www.ntnu.edu/nvks/f99-third-workshop

Pressure pulsations during a fast transition from pump to turbine mode of operation in laboratory and field experiment

The reversible pump turbine (RPT) is a suitable machine to meet fluctuations in the energy market. The usage of RPTs for this purpose will increase the number of operational mode changes of the machine. In order to reduce the response time of the machine, fast transitions in the mode of operation are desired. Therefore, increased knowledge of how the machine operates during these fast transitions is needed. This paper presents measurements done both at the NTNU Waterpower Laboratory and Tevla Pump Power Plant. The two machines are not geometrically similar, and part of the result focus on the similarities in the measurements despite the differences in the two RPTs. Tevla is owned by the Norwegian company Nord-Trøndelag Energi (NTE) and consists of two reversible pump turbines. The installed capacity is 50 MW. The focus is on the pressure pulsations during the fast transition from pump to turbine mode of operation. Results from both the laboratory and field measurements show few but high pressure amplitudes during the fast transition.

PIV measurements and CFD simulations of a hydrofoil at lock-in

As part of an ongoing investigation into the mitigation of vortex induced vibrations of hydrofoils, a combined experimental and numerical study of the fluid-structure interactions and wake of a hydrofoil at lock-in has been conducted at the Waterpower laboratory of the Norwegian University of Science and Technology. The hydrofoil has a blunt trailing edge and Von Karman vortex shedding induces a lock-in effect at a chord based Reynolds number of about 2,7·106. The present paper presents the initial measurements of vortex shedding frequencies going through lock-in, along with CFD simulations at lock-off conditions as well as some empirical estimates of vortex shedding. Experimentally the hydrofoil wake was studied in detail using particle image velocimetry (PIV). Hydrofoil vibration frequencies were measured by both a strain gauge positioned near the trailing edge of the foil as well as by a laser doppler vibrometer (LD-V). Numerically the phenomena was simulated using ANSYS CFX. Several different turbulence models was tested, from the two-equation standard k − ϵ model to the scale adaptive SST-SAS model, with considerably different results. It is observed that the vibrations induced at lock-in considerably shifts and reduces the hydrofoil wake velocity deficit. Further, the CFD results suggest that the driving parameter influencing the shedding frequency is the cross flow separation distance at the trailing edge.

A Reference Pelton Turbine—Three-Dimensional Flow Front Tracking Within a Rotating Pelton Bucket

A postprocessing method has been developed to enable the extraction of quantifiable data from images captured from within the rotating frame of reference of a Pelton turbine. The turbine tested was the reference Pelton runner, designed at the Waterpower Laboratory, Norwegian University of Science and Technology (NTNU). The method relies on interpolation to three-dimensional (3D) map the inner hydraulic surface of the bucket. Interpolation has been conducted with two different schemes, i.e., Barycentric triangular and biharmonic spline, where the latter showed significant increase in accuracy. The 3D mapping provides the world coordinates of the pixels within the bucket and enables the tracking of the water front as it propagates through the bucket. The method has been described and the uncertainties have been estimated in the order of 0.4 mm for most of the hydraulic surface. The results follow expected, and previously observed, behavior and show great promise with regard to validation of numerical simulations. Results obtained by the method will be of great interest for the computational fluid dynamics (CFD) community as it can be used as direct validation data for flow propagation found with numerical methods. The method relies heavily on manual input due to the high noise and low contrast of the available images, which causes an increase in both uncertainty and time consumption. Suggestion for reducing uncertainty and time consumption are presented and will be implemented in future publications.

Pressure Pulsation in a High Head Francis Turbine Operating at Variable Speed

This paper presents the preliminary work of the master thesis of the author, written at the Norwegian University of Science and Technology. Today, many Francis turbines experience formations of cracks in the runner due to pressure pulsations. This can eventually cause failure. One way to reduce this effect is to change the operation point of the turbine, by utilizing variable speed technology. This work presents the results from measurements of the Francis turbine at the Waterpower Laboratory at NTNU. Measurements of pressure pulsations and efficiency were done for the whole operating range of a high head Francis model turbine. The results will be presented in a similar diagram as the Hill Chart, but instead of constant efficiency curves there will be curves of constant peak-peak values. This way, it is possible to find an optimal operation point for the same power production, were the pressure pulsations are at its lowest. Six points were chosen for further analysis to instigate the effect of changing the speed by ±50 rpm. The analysis shows best results for operation below BEP when the speed was reduced. The change in speed also introduced the possibility to have other frequencies in the system. It is therefore important avoid runner speeds that can cause resonance in the system.

Francis-99 Workshop 2: transient operation of Francis turbines

Francis-99 is a set of workshops jointly organized by the Norwegian University of Science and Technology (NTNU), Norway and Luleå University of Technology (LTU), Sweden in the same spirit as the previous Turbine-99 workshops. The Francis-99 workshops aim to determine the state of the art of high head Francis turbine simulations (flow and structure) under steady and transient operating conditions as well as promote their development and knowledge dissemination openly. Three workshops are initially planned:- Workshop 1: steady operation of Francis turbines (performed in December 2014)- Workshop 2: transient operation of Francis turbines (December 2016)- Workshop 3: FSI of Francis turbines (December 2018)A high head Francis turbine model, named the Tokke model, has been designed and experimentally investigated at the Water Power Laboratory, NTNU. The complete geometry of the model and mesh are now freely available on the site http://www.ntnu.edu/nvks/francis-99 together with a large set of experimental pressure and velocity measurements. The organisers expect this geometry to become with time a reference test case to the hydraulic community for research and development on high head Francis turbines and the workshops a meeting place to discuss developments, potentials, issues… on a common and open test case.The present proceeding contains the papers presented at the second workshop at NTNU the 14th and 15th of December 2015. A total of 10 papers were presented. A large variety of codes and models were used highlighting different issues in the simulation of high Francis turbines.The editors:Prof. Michel J. Cervantes (LTU, NTNU),Dr. Chirag Trivedi (NTNU)Prof O.G. Dahlhaug (NTNU)Prof. T. Nielsen (NTNU)

Experimental Investigation of a High Head Model Francis Turbine During Steady-State Operation at Off-Design Conditions

Francis-99 is a set of workshops aiming to determine the state of the art of high head Francis turbine simulations (flow and structure) under steady and transient operating conditions as well as promote their development and knowledge dissemination openly. The first workshop (Trondheim, 2014) focused on steady state conditions. Some concerns were raised regarding uncertainty in the measurements, mainly that there was no clear vortex rope at the Part Load (PL) condition, and that the flow exhibited relatively large asymmetry. The present paper addresses these concerns in order to ensure the quality of the data presented in further workshops.To answer some of these questions, a new set of measurements were performed on the Francis- 99 model at Waterpower Laboratory at the Norwegian University of Science and Technology (NTNU). In addition to PL, two other operating conditions were considered, for further use in transient measurements, Best Efficiency (BEP) and High Load (HL). The experiments were carried out at a head of 12 m, with a runner rotational speed of 333 revolutions per minute (rpm). The guide vane opening angle were 6.72°, 9.84° and 12.43° for PL, BEP and HL, respectively. The part load condition has been changed from the first workshop, to ensure a fully developed Rotating Vortex Rope (RVR). The velocity and pressure measurements were carried out in the draft tube cone using 2D PIV and six pressure sensors, respectively. The new PL condition shows a fully developed rotating vortex rope (RVR) in both the frequency analysis and in the phase resolved data. In addition, the measurements confirm an asymmetric flow leaving the runner, as was a concern in the first Francis-99 workshop. This asymmetry was detected at both design and off-design conditions, with a stronger effect during off design.