Self-sensing cavitation detection in ultrasound-induced acoustic cavitation

The generation of cavitation by ultrasound is used in a large number of processes in different scientific and industrial applications. Chemical reactions are made possible or accelerated by locally occurring high temperatures and pressures generated by the collapse of cavitation bubbles. Mixing or separating substances, emulsification, ultrasonic cleaning, degassing, and microbiological treatment of fluids are some more applications for ultrasonic generated cavitation. In most of these applications, an optical examination of the events within the cavitating medium is not possible. Non-transparent media or closed containers prevent an optical process monitoring. In addition, the use of sensors is often impossible for cost reasons, limited construction space or disturbance of the process. In order to still enable process monitoring, the authors follow a novel approach: the analysis of the electrical signals of the ultrasound transducer used for cavitation generation. The current signal of the ultrasound transducer is inspected for frequency components, known as acoustic cavitation indicators. For this, the time signal is recorded and transferred to the frequency domain for further processing and evaluation. In previous studies, acoustical sensors like hydrophones or microphones were used as reference for the self-sensing technique. In order to link cavitation events inside the fluid container to cavitation indicators in the current signal, a photo study of the cavitation events inside a transparent water container is conducted. In contrast to previous self-sensing attempts, the ultrasound transducer's transfer characteristic is also considered. The evaluation of the acquired data shows that a frequency component which is 3/2 times the driving frequency (∼30 kHz) can be used to determine the onset of transient cavitation. Once the transient cavitation threshold has been exceeded, broad band noise levels show a good correlation with cavitation intensity.