Classification of Cavitation in Hydraulic Machines using Vibration Analysis

Mohamed Farhat, Michaël Kaye
International Association For Hydraulic Research, 2002
Article de conférence

This paper describes vibration analysis techniques that have been adapted and used to classify cavitation in a model Francis turbine. This analysis can be used to compliment standard cavitation tests. Cavitation can be classified in various ways using vibration analysis. Cavitation attached to the blades of the runner is modulated by instabilities caused by the interaction with the blades and the guide vanes, as well as by the outlet swirl. The shape of this modulation is not sinusoidal. Cavitation on the runner is non-uniform; i.e. some blades are, on average, experiencing more aggressive cavitation than others are. Analysis techniques employed include a demodulation technique that considers the envelope auto-power spectrum, joint time-frequency analysis and phase averaging.

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Mohamed Farhat, Michaël Kaye
International Association For Hydraulic Research, 2002
Article de conférence

Résumé

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https://infoscience.epfl.ch/record/59022?ln=fr

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Cavitation test of hydraulic machines : procedure and instrumentation

François Avellan

Model testing of hydraulic turbines, storage pumps and pump-turbines is of great importance especially for large units involved in power generation. Among all the aspects of machine operation, cavitation plays a fundamental role with respect to the possible alteration of the efficiency and to the erosion risk. This paper describes the procedures of cavitation tests and the related instrumentation, depending both on the type of hydraulic machine and its operation range. Discussion is mainly directed toward the unavoidable distortions of the similitude requirements between model and industrial scales.

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Vortex shedding from blunt and oblique trailing edge hydrofoils

Philippe Ausoni, François Avellan, Mohamed Farhat, Amirreza Zobeiri

The phenomenon of vortex shedding behind a hydrofoil is an important issue from both scientific and technical point of view. The resulting fluctuating forces may lead to excessive vibrations and premature cracks in the hydraulic machines. According to previous studies, it is well known that an oblique trailing edge, also called "Donaldson cut", reduces the vibration in comparison with a blunt trailing edge; however the physics of the problem is still poorly understood. The purpose of the present work is the experimental investigation of vortex shedding dynamics in the wake of oblique and blunt trailing edge NACA0009 hydrofoils. Experiments are performed at zero incidence angle and high Reynolds numbers, ReL =5 10 5 - 2.9 10 6. The wake velocity profile is measured by two-component Laser Doppler Velocimetry. Cavitation occurrence in the core of the vortices is used as a mean of wake visualization with the help of a high speed camera. We have found that vortex induced vibration is significantly reduced for oblique trailing edge hydrofoil in comparison with the truncated one, which is in agreement with former reports. A disorganization of the Karman vortex street in the near wake is believed to be the reason of this vibration reduction. The high speed movies clearly show that the alternate shedding of the vortices turns into almost simultaneous shedding at the hydrofoil trailing edge. As a result, the upper and lower vortices pair with a significant thickening of the lower vortex core and a reduction of its strength. Consequently, the fluctuating lift, which is the cause of the structural vibration, is also reduced by the oblique truncation. We believe that this result stands for a basis to better optimize the trailing edge of turbine blades in order to further decrease the flow induced vibration.

Brno university of technology2009

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Experimental investigation of the local wave speed in a draft tube with cavitation vortex rope

François Avellan, Arthur Tristan Favrel, Christian Landry, Andres Müller, Christophe Nicolet, Keita Yamamoto

Hydraulic machines operating in a wider range are subjected to cavitation developments inducing undesirable pressure pulsations which could lead to potential instability of the power plant. The occurrence of pulsating cavitation volumes in the runner and the draft tube is considered as a mass source of the system and is depending on the cavitation compliance. This dynamic parameter represents the cavitation volume variation with the respect to a variation of pressure and defines implicitly the local wave speed in the draft tube. This parameter is also decisive for an accurate prediction of system eigen frequencies. Therefore, the local wave speed in the draft tube is intrinsically linked to the eigen frequencies of the hydraulic system. Thus, if the natural frequency of a hydraulic system can be determined experimentally, it also becomes possible to estimate a local wave speed in the draft tube with a numerical model. In the present study, the reduced scale model of a Francis turbine was investigated at off-design conditions. In order to measure the first eigenmode of the hydraulic test rig, an additional discharge was injected at the inlet of the hydraulic turbine at a variable frequency and amplitude to excite the system. Thus, with different pressure sensors installed on the test rig, the first eigenmode was determined. Then, a hydro-acoustic test rig model was developed with the In-house EPFL SIMSEN software and the local wave speed in the draft tube was adjusted to obtain the same first eigen frequency as that measured experimentally. Finally, this method was applied for different Thoma and Froude numbers at part load conditions.

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