Investigation of the Cavitation Aggressiveness Using PVDF Sensors
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Abstract
The assessment of aggressiveness of cavitation phenomena through direct measurement of impact forces produced by bubble collapse is studied. This was done by recording impact forces from a commercially available Polyvinylidene Fluoride (PVDF) film sensor which is exposed to cavitation generated by ultrasonic transducer equipment. The ideal number of runs which will better represent the cavitation phenomena was first identified. Results showed good repeatability of every run and that a minimum of 3 experimental trials with time duration of 15 ms are enough to get a consistent data. The preparation of the PVDF films was optimized by selecting among four PVDF films that were setup differently, namely, a PVDF film as obtained from the supplier, a PVDF film topped with one layer of tape as an added protection, PVDF film with two layers of protective tape, and lastly a PVDF film which is folded into half. The PVDF film with one layer of tape is found to be the most suitable film for aggressiveness tests in terms of sensitivity and durability from cavitation damage. This film was then subjected to three different vibration amplitudes and it showed that increasing the oscillation amplitude leads to stronger impacts. Although most impacts were seen to occur at each horn vibration period, there were prominent high impact forces observed to arise after some cycles of horn vibration. This suggests that bubbles coagulate to form large cavity and then collapse more violently at a frequency lower than the frequency of ultrasonic horn vibration. Moreover, these strong impacts occur at a frequency that decreases as the vibration amplitude is increased. The obtained impact force signal was then compared to a previously obtained pitting test data which utilized the same experimental setup as used in this study. A remarkably high difference in the order of magnitude was seen between the cumulative peak rate and pit rate indicating that not all impacts cause pits on the surface of the material. Additionally, if it is assumed that a pit is formed from a single impact force during the incubation period, then a load of 319 N is necessary to create a 0.9 m diameter pit on aluminum alloy.
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Cavitation, ultrasonic cavitation, PVDF Sensor, Cavitation, ultrasonic cavitation, PVDF Sensor