Erosion de cavitation : méthodes pratiques de prédiction au stade du projet

Abstract

Click to increase image sizeClick to decrease image sizeCavitation erosion is the final consequence of a number of highly complex physical phenomena.The geometry of the hydraulic equipment, its operating conditions including the flow velocity and cavitation as well as the nature and history of the fluid are the key parameters which govern the size and type of vapor cavities. These cavities are entrained by the flow and collapse against the neighboring solid boundaries. When strong enough, each of these collapses may create a small pit in the material. This pit formation is called the incubation period. The cumulative effect of these numerous impacts is responsible of fatigue at the surface of materials and this finally causes material removal. It is practically imposssible to simulate all these phenomena by available computer models or correlations. The only possible way to forecast cavitation damage is to use reduced scale models and apply proven scaling laws. The most complex part of the scaling concerns the hydrodynomic phenomena. It has been shown that they were satisfactorily represented on models provided the scaling parameters are within a reasonable range and the test fluid is correctly prepared. This may imply nuclei injection. This hypothesis has been used to derive scaling laws on the flow geometry, the rate of occurence of vapor cavities and their capability to produce pits on solid materials during the so-called incubation period. The histogram of the pits produced can be a measure of the damage capability of the flow. Finally, it has been shown experimentally that this histogram of pits, or at least part of it could be transposed to other operating conditions of a similar flow.