High flexibility in Francis turbine operation and design philosophy: A review

This paper examines and discusses how various operation schemes and design philosophies can prolong the expected lifetime of a hydro turbine subjected to many start-stop cycles and high ramping rates. With the proliferation of renewable and non-dispatchable energy sources in Europe, a higher demand for flexible operation is put on the rest of the system. Given the short response time of hydro power, there is a huge potential for the Norwegian power sector to act as a stabiliser for the rest of mainland Europe. With a more flexible operation however, the turbines will experience a higher load variation which can lead to premature fatigue and failure of existing turbines. This research reviews the current startup and ramping schemes, as well as design aspects, which might affect and reduce the mechanical stresses experienced by the hydro turbine.publishedVersio

A Review on Sediment Erosion Challenges in Hydraulic Turbines

Sediment constitutes several mineral compositions depending upon the geological formation and geography. In many of the rivers in Himalayas and Andes, Quartz is found as a main constituent (more than 50%), along with feldspar and other hard minerals. These particles have hardness more than 5 Moh’s scale, which is capable to erode turbine components. In hydraulic turbines, flow is highly turbulent and unsteady, which can aggravate the erosion problems. Depending upon the nature of the flow, different components of turbines are eroded with different mechanisms. This chapter will provide a review on how various flow phenomena is responsible for particular types of erosion in turbines and their potential consequences. Some examples of the effect in existing power plants will be shown. This chapter will also discuss about some preventive measures that have been proposed and implemented to reduce the impact of the sediment particles in hydraulic machineries

Quantum Chaos and Quantum Randomness - Paradigms of Entropy Production on the Smallest Scales

Quantum chaos is presented as a paradigm of information processing by dynamical systems at the bottom of the range of phase-space scales. Starting with a brief review of classical chaos as entropy flow from micro- to macro-scales, I argue that quantum chaos came as an indispensable rectification, removing inconsistencies related to entropy in classical chaos: Bottom-up information currents require an inexhaustible entropy production and a diverging information density in phase space, reminiscent of Gibbs' paradox in Statistical Mechanics. It is shown how a mere discretization of the state space of classical models already entails phenomena similar to hallmarks of quantum chaos, and how the unitary time evolution in a closed system directly implies the ''quantum death'' of classical chaos. As complementary evidence, I discuss quantum chaos under continuous measurement. Here, the two-way exchange of information with a macroscopic apparatus opens an inexhaustible source of entropy and lifts the limitations implied by unitary quantum dynamics in closed systems. The infiltration of fresh entropy restores permanent chaotic dynamics in observed quantum systems. Could other instances of stochasticity in quantum mechanics be interpreted in a similar guise? Where observed quantum systems generate randomness, that is, produce entropy without discernible source, could it have infiltrated from the macroscopic meter? This speculation is worked out for the case of spin measurement.Comment: 41 pages, 17 figure

GPS Synchronization and EMC of Harmonic and Transient Measurement Equipment in Offshore Wind Farms

The challenges of harmonic and transient measurements in wind farms are described in this paper. It is shown that appropriate measurements of harmonic and transient phenomena in offshore wind farms are essential for data analysis and model creation/validation of components or subsystems. The GPS synchronization, electromagnetic compatibility (EMC) and interference (EMI) challenges during the development, construction, testing and installation of a measurement system for multi-point, high-speed and long-term data logging is described in this paper. The presented measurement system was tested in a rough offshore environment at Avedøre Holme and Gunfleet Sands offshore wind farms.The paper will describe the application of GPS technology in synchronised measurements carried out at Avedøre Holme and Gunfleet Sands wind farms. Different aspects of software development and hardware configuration in order to optimise measurement systems reliability during offshore measurements will be presented. Also real-life examples of results from both offshore measurement campaigns will be described. Some limitations, improvements and results of the measurement system will be explained from both harmonic and transient measurements. The paper clearly presents possible electromagnetic interference in wind turbines that can affect measurements. Also the application of appropriate mitigation techniques such as data acquisition board configuration, coaxial cable leading, as well as usage of EMC-proof boxes for high frequency measurements is described

Prediction of Wave Loads on Tidal Turbine Blades

AbstractWave loads are one of the main contributors to fatigue loads of tidal turbine blades. Because of this, they are a determinant parameter for calculation of turbine blade life time. To avoid cost associated with oversizing blades or replacing a damaged blade, it is essential to evaluate the loads acting on the turbine, and especially wave loads on the blades, with the best possible accuracy.Experience from wind industry is valuable for horizontal axis tidal turbine design and loads acting on the blade can be estimated using the same methods, even if the loads are different. This article presents the main features of a code written with Matlab, able to predict thrust force and torque on each blade while the turbine is operating in a regular wave field. The quasi-static Blade Element Momentum theory is combined here with an added mass force modeling. The linear wave theory is employed to describe the water particle velocity due to waves. This velocity is, as a first approximation, simply added to a uniform stream velocity to account for wave-current interaction.The analytical results are validated by comparing them with experimental data obtained by testing a 1.475m-diameter rotor towed in a 260m-long wave tank, for different combinations of current speeds and wave characteristics. This emphasizes the importance of wave effects and dynamics in the design of tidal turbine blades

Interaction between trailing edge wake and vortex rings in a Francis turbine at runaway condition: Compressible large eddy simulation

The present study aims to investigate the unsteady flow phenomenon that produces high energy stochastic fluctuations in a highly skewed blade cascade. A complex structure such as a turbine is operated at runaway speed, where the circumferential velocity is dangerously high, and the energy dissipation is so significant that it takes a toll on the operating life of a machine. Previous studies showed that a large vortical structure changes spatial location very quickly and interacts with the secondary flow attached to the blade pressure-side. The temporal inception of the rings dissipates the energy of a wide frequency band and induces heavy vibration in the mechanical structure. The focus of the present study is to experimentally measure and numerically characterize the time-dependent inception of vortical rings in the blade cascade. The experimental data are used to verify and validate the numerical results obtained from the large eddy simulation. Flow compressibility is considered to obtain more accurate amplitudes of unsteady pressure pulsations associated with the wave propagation and reflection. The following three aspects are of particular focus: (1) How the wake from a guide vane interacts with the stagnation point of a blade, (2) How vortex rings are developed in a blade cascade, and what are the temporal characteristics, and (3) How decelerating flow at the outlet interacts with the secondary flow in the blade cascade

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

A reference pelton turbine - design and efficiency measurements

The Pelton turbine has been subject to a varying degree of research interest since the debut of the technology over a century ago. Despite its age there are gaps in the knowledge concerning the flow mechanisms effecting the flow through the turbine. A Pelton turbine has been designed at the Waterpower Laboratory at NTNU. This has been done in connection to a Ph.D. project focusing on the flow in Pelton turbine buckets. The design of the turbine has been conducted using in-house knowledge in addition to some comments from a turbine producer. To describe the geometry multiple Bezier curves were used and the design strategy aimed to give a smooth and continuous gradient along the main flow directions in the bucket. The turbine has been designed for the operational conditions of the Pelton test rig installed at the Waterpower Laboratory which is a horizontal single jet test rig with a jet diameter(ds) of 35 mm. The diameter(D) of the runner was set to 513 mm and the width(W) of a bucket 114 mm, leading to a D/W ratio of 4.5. Manufacturing of the turbine has been carried out in aluminium and the turbine has undergone efficiency testing and visual inspection during operation at a head of 70 m. The turbine did not performed as expected and the maximum efficiency was found to be 77.75%. The low efficiency is mainly caused by a large amount of water leaving the bucket through the lip and hence transferring close to zero of its energy to the shaft. The reason for the large lip loss is discussed and two possible causes are found; the jet is located too close to the lip, and the inner surface of the bucket does not lead the water away from the lip. The turbine geometry and all data from both measurements and simulations will be available upon request in an effort to increase the amount of available data concerning Pelton turbines

Evaluation of runner cone extension to dampen pressure pulsations in a Francis model turbine

Today's energy market has a high demand of flexibility due to introduction of other intermittent renewables as wind and solar. To ensure a steady power supply, hydro turbines are often forced to operate more at part load conditions. Originally, turbines were built for steady operation around the best efficiency point. The demand of flexibility, combined with old designs has showed an increase in turbines having problems with hydrodynamic instabilities such as pressure pulsations. Different methods have been investigated to mitigate pressure pulsations. Air injection shows a significant reduction of pressure pulsation amplitudes. However, installation of air injection requires extra piping and a compressor. Investigation of other methods such as shaft extension shows promising results for some operational points, but may significantly reduce the efficiency of the turbine at other operational points. The installation of an extension of the runner cone has been investigated at NTNU by Vekve in 2004. This has resulted in a cylindrical extension at Litjfossen Power Plant in Norway, where the bolt suffered mechanical failure. This indicates high amplitude pressure pulsations in the draft tube centre. The high pressure pulsation amplitudes are believed to be related to high tangential velocity in the draft tube. The mentioned runner cone extension has further been developed to a freely rotating extension. The objective is to reduce the tangential velocity in the draft tube and thereby the pressure pulsation amplitudes

Numerical study of a Francis turbine over wide operating range: Some practical aspects of verification

Hydropower plays an essential role in maintaining energy flexibility. Modern designs focus on sustainability and robustness using different numerical tools. Automatic optimization of the turbines is widely used, including low, mini and micro head turbines. The numerical techniques are not always foolproof in the absence of experimental data, and hence accurate verification is a key component of automatic optimization processes. This work aims to investigate the newly designed Francis runner for flexible operation. Unsteady simulations at 80 operating points of the turbine were conducted. The numerical model consisted of 16 million nodes of hexahedral mesh. A SAS-SST (scale adaptive simulation-shear stress transport) model was enabled for resolving/modeling the turbulent flow. The selected time-step size was equivalent to one-degree angular rotation of the runner. Global parameters, such as efficiency, torque, head and flow rate were considered for proper verification and validation. (1) A complete hill diagram of the turbine was prepared and verified with the reference case. (2) The relative error in hydraulic efficiency was computed and the over trend was studied. This allowed us to investigate the consistency of the numerical model under extreme operating conditions, far away from the best efficiency point. (3) Unsteady fluctuations of runner output torque were studied to identify unstable regions and magnitude of torque oscillations