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Cavitation damage scale effects: sudden enlargements


The present study was aimed at investigating the cavitation damage downstream from sudden enlargement energy dissipators. Tests were conducted on geometrically similar circular orifices of five different orifice to pipe diameter ratios in three different pipe sizes of: 3-, 6-, and 12-inches. Highly polished 1100-0 aluminum specimens were mounted in the downstream pipe wall to detect the cavitation damage. Two different stages or levels of cavitation damage were defined for study: (1) "incipient damage" level based upon maintaining a maximum pitting rate of 1 pit/in.2/min. on 1100-0 aluminum, (2) cavitation damage regime where the maximum pitting rate was greater than 1 pit/in.2/min. Previously defined incipient damage scaling equations based upon damage data taken in the 3-in. pipe accurately predicted the incipient damage condition in the 6- and 12-in. pipes. Pressure scale effects on the incipient damage condition were constant for all pipe sizes tested. There were no size scale effects found for the incipient damage condition based upon maintaining a maximum pitting rate of 1 pit/in.2/min. The incipient damage condition was investigated in greater detail by studying the variation in the volume of the damage pits in the soft aluminum. The volume of the damage pit was related to the energy expended in format ion of the pit which was assumed to be a measure of the intensity of the cavitation impact blow forming the pit. It was found that at the incipient damage condition the intensity of cavitation impact blows varied with pipe size and orifice to pipe diameter ratio. The damage in the cavitation damage regime was found to be a function of both cavitation pitting rate and intensity of cavitation impact blows (energy of pit formation). A cavitation intensity parameter, defined as the product of cavitation pitting rate and energy of pit formation, was used to measure cavitation damage. Data was presented to show the general variation of cavitation intensity under conditions of varying cavitation index, varying upstream pressure, and varying pipe size. In addition, the cavitation damage scaling evaluations introduced by Thiruvengadam were used along with experimentally measured flow field data to predict variations in cavitation pitting rate and intensity of impact blows. The experimental results indicate that it is impossible to simulate total prototype cavitation loading conditions in terms of both cavitation pitting rate and intensity of impact blows in a hydraulic model of reduced size. An example is introduced demonstrating this fact. An alternative method using the cavitation intensity parameter is proposed for modeling prototype cavitation loading conditions in a model of reduced size.


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Print version deaccessioned 2023.

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