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Flow-Induced Pulsation and Vibration in Hydroelectric Machinery

Engineer’s Guidebook for Planning, Design and Troubleshooting

verfasst von: Peter Dörfler, Mirjam Sick, André Coutu

Verlag: Springer London

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

Since the 1970’s, an increasing amount of specialized research has focused on the problems created by instability of internal flow in hydroelectric power plants. However, progress in this field is hampered by the interdisciplinary nature of the subject, between fluid mechanics, structural mechanics and hydraulic transients. Flow-induced Pulsation and Vibration in Hydroelectric Machinery provides a compact guidebook explaining the many different underlying physical mechanisms and their possible effects.

Typical phenomena are described to assist in the proper diagnosis of problems and various key strategies for solution are compared and considered with support from practical experience and real-life examples. The link between state-of the-art CFD computation and notorious practical problems is discussed and quantitative data is provided on normal levels of vibration and pulsation so realistic limits can be set for future projects. Current projects are also addressed as the possibilities and limitations of reduced-scale model tests for prediction of prototype performance are explained.

Engineers and project planners struggling with the practical problems will find Flow-induced Pulsation and Vibration in Hydroelectric Machinery to be a comprehensive and convenient reference covering key topics and ideas across a range of relevant disciplines.

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Inhaltsverzeichnis

Frontmatter

Chapter 1. Basic Concepts

Abstract

Some basic engineering and physical concepts are introduced and discussed in this chapter. These concepts are important to characterize the performance, pulsations and vibrations, and other related phenomena presented in this book. First, the representation of an operating point and the operating range by non-dimensional parameters is described. Many different parameters may be defined and these are briefly outlined. The occurrence of many vibration and pulsation phenomena in turbines is restricted to specific operating conditions and the use of non-dimensional parameters allows different machines to be directly compared with each other in a dimensionless hill chart. Second, the quantitative description of an oscillation process in terms of amplitudes or frequency spectra is explained. Third, the fundamentals of hydraulic resonance and stability are briefly outlined as many phenomena, in particular intense low-frequency fluctuations, often concern the whole water conduit and ducting rather than just the interior of the hydraulic machine.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 2. Low-Frequency Phenomena in Swirling Flow

Abstract

The presence of swirl is often the cause of separations and pressure fluctuations in the draft tube of hydraulic reaction turbines, in particular Francis turbines. At the design point, water turbines generally operate with little swirl entering the draft tube and no flow separations, but at off-design, at both high and low load, the flow leaving the turbine has a large swirling component. The present chapter describes a number of physical mechanisms that work to produce the pulsation. Their occurrence and impact depend mainly on the actual flow rate of the turbine, but also on the local pressure level, on the exit velocity field of the particular runner, the shape of the draft tube, and the dynamic response of the whole hydraulic circuit. In medium- and large-sized turbines, the frequency of these ‘draft tube surges,’ which are often approximately periodic, is of the order of 1 Hz, and therefore they may also produce significant electrical power swings. Given a sufficiently strong vortex flow, comparable phenomena may also occur in other locations, for example in the runner channels, or in penstock manifolds.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 3. Periodic Effects of Runner-Casing Interaction

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 4. High-Frequency Vortex Phenomena

Abstract

As a straightforward consequence of dimensional analysis, the frequency of vortex phenomena increases proportionally with the flow velocity, and is inversely proportional to its size. Thus, while draft tube surges are in the Hertz range—the characteristic length being the runner diameter, Von Kármán eddies are basically determined by a trailing edge thickness, and due to the smaller size their frequency is well in the audible range. The Von Kármán eddies have to be minimized by using particular profiles for the trailing edges, in particular in the stay vanes and runner blades, or otherwise the lock-in effect and the narrow-band character of the phenomenon may put the structural safety at risk.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 5. Cavitation-Related Phenomena

Abstract

In hydroelectric equipment, there are many flow situations where the static pressure drops far enough to reach the vapor pressure of the fluid. Vapor- or gas-filled cavities, or clouds of vapor bubbles develop in the regions concerned, and their dynamics has a strong influence on the vibration behavior and flow stability. Due to the wide range of bubble sizes involved, their oscillation and collapse create a wide band spectrum of intense vibration and noise

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 6. Stability-Related Unsteady Phenomena

Abstract

This chapter presents a number of common oscillation phenomena which are not due to a direct external force but due to a feedback mechanism which may depend on the response of a larger system. Accordingly, the frequency of this kind of fluctuation is dictated by a natural frequency rather than a forcing frequency. For example, if a small displacement in a turbine runner creates a labyrinth force that tends to increase the displacement, then this may suddenly start a whirling motion at a natural bending frequency of the shaft line.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 7. Model Tests, Techniques, and Results

Abstract

In hydropower projects, model tests using a reduced-scale model are well-established practice regulated by international standards. Measurement of unsteady phenomena, like pulsation of pressure or guide vane torque in the model is also widely used. Systematic comparison of test results from many projects in a physically sensible way has permitted ‘generic’ data describing the normal behavior of a certain type of machine to be established. Such data, which have not been publicly available before, are presented here to enable planners to formulate realistic specifications, and to make the assessment of measured pulsations more rational.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 8. Selected Field Experience

Abstract

Measurement of vibration or pressure pulsation data on a prototype machine may be performed in order to verify compliance with guarantee, or in a monitoring process or, in some cases, to provide data for root cause analysis if the dynamic behavior is deemed unsatisfactory. It may occur that the nature of a problem is not immediately clear and testing has to check for several possible aspects. This chapter therefore presents a number of practical cases separated from the pertinent chapters. The logic of finding the facts and possible solution strategy based on the underlying physical phenomena is demonstrated.

Peter Dörfler, Mirjam Sick, André Coutu

Chapter 9. Practical Guidelines

Abstract

In the successive stages of a hydroelectric project, the risk of flow-excited pulsations leading to problematic behavior of the plant should be accounted for by the various participants in a suitable manner. This starts with a good technical specification, and continues with a good design practice that avoids all kinds of possible pitfalls. The first physical test of the laboratory model has to be correctly assessed and finally the dynamic performance has to be correctly measured during commissioning of the machine, in some instances also in special tests, if improvements have to be found. Caution is required because results may be distorted under field conditions.

Peter Dörfler, Mirjam Sick, André Coutu

Errata to: Flow-Induced Pulsation and Vibration in Hydroelectric Machinery

Peter Dörfler, Mirjam Sick, André Coutu

Backmatter

Titel: Flow-Induced Pulsation and Vibration in Hydroelectric Machinery
verfasst von: Peter Dörfler
Mirjam Sick
André Coutu
Copyright-Jahr: 2013
Verlag: Springer London
Electronic ISBN: 978-1-4471-4252-2
Print ISBN: 978-1-4471-4251-5
DOI: https://doi.org/10.1007/978-1-4471-4252-2