A novel sensor for monitoring acoustic cavitation. Part II: Prototype performance evaluation

This paper describes a series of experimental studies to evaluate the performance of newly developed sensors for monitoring broadband acoustic emissions generated by acoustic cavitation. The prototype sensors are fabricated in the form of hollow, open-ended cylinders, whose inner surface is made from a thin film of piezoelectric polymer acting as a passive acoustic receiver of bandwidth greater than 10 MHz. A 4-mm thick coating of special acoustical absorber forms the outer surface of the sensor. The layer functions as a shield to cavitation events occurring outside the hollow sensor body, allowing megahertz acoustic emissions emanating from within the liquid contained in the sensor to be monitored. Testing of the new sensor concept has been carried out within the cavitating field provided by a commercial ultrasonic cleaning vessel operating at 40 kHz whose power output is rated at 1 kW. It is demonstrated that the prototype cavitation sensors are able to record a systematic increase in the level of the high-frequency acoustic spectrum (> 1 MHz) as electrical power to the cleaning vessel is increased. Through careful control of the experimental conditions, reproducibility of the high frequency "energy" associated with the cavitation spectrum was found to be typically +25%.