Basic aspects of noise sampling, signal processing and analysis, and data processing, analysis, and interpretation in vibro-acoustic diagnostics of turbine cavitation are investigated in a series of prototype and model experiments. Several weak points of the practice are identified, and improvements and new techniques are developed. These techniques enable extraction of data on cavitation details and early detection of detrimental effects met in turbine exploitation. A brief review of weak points of the practice, developed improvements, and new techniques, as well as examples of application, are presented in the paper.
RÉSUMÉDes aspects de base de Tenregistrement du bruit, des traitements et analyses analogiques et numériques des signaux. avec leur interpretation dans le diagnostic vibro-acoustique de la cavitation de turbine, sont étudiés dans une série d'expériences de prototype et de modèle. Plusieurs points faibles de la pratique sont identifies, et des ameliorations et de nouvelles techniques sont développées. Ces techniques permettent l'extraction de données sur des détails de cavitation et la detection précoce des effets néfastes rencontres dans l'exploitation de turbine. Un bref examen des points faibles de la pratique, des ameliorations développées, et de nouvelles techniques, aussi bien que des exemples d'application, sont présentés dans le papier.
A novel technique for vibro-acoustical diagnostics of turbine cavitation is introduced and its use demonstrated on a Francis turbine. The technique enables identification of different cavitation mechanisms functioning in a turbine and delivers detailed turbine cavitation characteristics, for each of the mechanisms or for the total cavitation. The characteristics specify the contribution of every critical turbine part to the cavitation intensity. Typical diagnostic results: (1) enable optimization of turbine operation with respect to cavitation erosion; (2) show how a turbine’s cavitation behavior can be improved; and (3) form the basis for setting up a high-sensitivity, reliable cavitation monitoring system.
Full-scale vibro-acoustical diagnostic measurements of cavitation in four Francis 6 MW double runner turbines were performed. Two types of sensors were used—a hydrophone sensing waterborne noise at the pressure side of a runner and an accelerometer mounted at various points at the outer turbine casing, facing the runner’s pressure side. The correlation of noise and acceleration intensity with suction-side pressure fluctuations and runner position was checked. A simple but efficient method of spectrum normalization, which rejects the influence of the measurement set characteristics and vibro-acoustical characteristics of a turbine, was developed. The resulting spectra reveal the dependence of cavitation source strength on the turbine power as a function of noise or acceleration frequency.
Mathematical model of the dynamic frequency characteristics of the heterodyne spectrum analyzer with a rectangular bandpass filter of small relative bandwidth is constructed. The model is investigated numerically and proved experimentally.
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