Erosion and impact intensity of vibratory, jet and turbine cavitation

The ratio of the cavitation erosion resistance of the new austenitic stainless steel Hydroloy to the one of the standard 308 has been found to vary from 11 to 16 in vibratory cavitation tests to about 5 in large hydroturbines. Cavitation erosion tests in a high pressure water jet show also the same range of resistance ratio between 4 and 6 as in large turbines. It has been postulated that this behavior was related to the impact intensity of individual cavitation collapses and the deformation stress of these steels. To verify that hypothesis simultaneous erosion rate and impact intensity have been measured in vibratory and jet cavitation tests. An electrochemical titanium erosion detector coupled to a high frequency accelerometer is used. These measurements coupled with those of pit size distribution on aluminum confirm that the impact energy in jet cavitation can be more than two orders of magnitude larger than in the 20 kHz vibratory cavitation. The inferred forces of these impacts are then compared with those measured on a cavitation tunnel NACA profile, a turbine mode! and its prototype. It is concluded that vibratory cavitation produces mainly fatigue elastic deformation on higher resistance alloys whereas high velocity flow cavitation produces more plastic deformation and/or brittle fracture.