Recensement et examen critique des méthodes de détermination de la résistance à l'érosion de cavitation

Review and critical examination of methods for the determination of resistance to cavitation erosion The production of a state of erosion by cavitation

E.D.F., Centre de Recherches et d'Essais de Chatou.

Résumé

The trouble cavitation erosion has always been to hydraulic machine manufacturers and users explains why so much laboratory research has been carried out on the subject. Methods of studying the problem fall within two categories, one in which erosion effects are reproduced by cavitation, and the other in which the actual amount of erosion is measured. I __ The production of a state of erosion by cavitation. A) Review of methods__Methods used to reproduce cavitation are either static with a fixed "wearing" sample, or dynamic with a mobil test piece. The first is used in the following cases : 1. For tests in an experimental hydrodynamic cavitation research tunnel, to study erosion of a profile, or behind an body in the flow (Figs. 1, 2, 3 and 4). 2. For ultrasonic cavitation research. The second type of method is used as follows (Fig. 7) : __ 1. In turbine plants or with immersed test pieces in rotary machines (Figs. 5 and 6). 2. In magnetostrictive apparatus, which subjects the test piece to intense vibration. 3. In shock erosion tests in which test pieces on the periphery of a disc cut across a fluid jet. B) Critical examination.- In order to be able to compare cavitation states produced by these various methods and 10 discuss their value, it is necessary to accurately define characteristic quantities and parameters for the considered effect. Numerous parameters are available for this purpose, for instance Leroux number, Thoma coefficient, Stepanoff thermal criterion, Strouhal of weber numbers, etc. In fact, a complete definition should allow for the value of all the various physical factors affecting erosion, e.g. flow velocity, pressure gas content of the cavitating fluid, its temperature, flow geometry, etc. As the role of these factors varies depending on the method considered, however, the mechanisms involved are obviously bound to undergo very deep modification. II __ Measurement of cavitation erosion. The various methods used for this are based on the choice of a characteristic criterion for the degree of destruction suffered by the material. Thus, the following can be measured : __ (i) Loss of mass. (H) Loss of volume. (iii) Changes in radiation from a radioactive surface. (iv) Changes is the resistance of a metal blade under cavitation. All these procedures suffer from certain disadvantages, either because the considered criterion is only of limited significance, or because of measurement inaccuracy, or else because of difficult application. The advantage of electrical gauges must be emphasised, however, for they provide a means of continuously observing the propagation of cavitation erosion. This study shows that, above an, it is necessary to completely define a state of cavitation. Furthermore, as different mechanisms are associated with the various experimental procedures, the choice of a cavitation research method will, therefore, depend on the aim in view. Where the measurement of erosion is concerned, an additional difficulty arises in the comparison of results, due to the great diversity of the criteria involved. It follows, therefore, that an effort should be made towards more comprehensive analysis of the cavitation phenomenon while attempting to achieve a satisfactory degree of standardisation of the research methods used.