Non-invasive measurement techniques to monitor acoustic cavitation activity

• Main Author: Promasa, Kornpatsitt
• Published: University of Strathclyde 2014
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.635523

High power ultrasound has been used in a number of biomedical and industrial applications. In particular, such systems, when used in chemical processing, modify the course of a chemical reaction. The use of high power ultrasound induces cavitation in the load medium, which then leads to certain mechanical and chemical effects. These effects can cause damage to objects, increase local temperatures and can accelerate chemical reactions. Thus, in order to evaluate an efficient ultrasonic system and to quantify the cavitation activity, it is necessary to measure the cavitation in the reaction under the influence of a high power ultrasonic field. Conventional hydrophone techniques and sensor technologies are not suitable for this type of measurement because the sensor can be damaged and change the acoustic field in the presence of cavitation generated by the high power ultrasound. This Thesis describes the development of a non-invasive technique used to monitor acoustic cavitation activity within a reactor vessel. Both Laser Doppler Velocimetry (LDV) and broadband ultrasonic transducer approaches are considered as potential measurement techniques to measure the acoustic emission (AE) signals associated with a cavitating field. The LDV approach uses laser detection to provide information about cavitation intensity and distribution occurring within a reactor vessel. Next, finite element analysis (FEA) is used to provide a simulation platform to investigate the detection of AE sources from within the reactor vessel. This provides the necessary information to support the design of a broadband transducer appropriate for detection of cavitation generated AE. FEA is then used to design a piezoelectric ceramic composite transducer to develop a non-invasive transducer measurement system. In both approaches, a broadband integrated energy (BIE) approach is used to determine the intensity of the cavitation activity. This technique has been evaluated in different frequency ranges and at various power levels. Interestingly, the BIE in the frequency ranges of 1-5MHz was shown to be sufficient in monitoring the cavitation. Overall, the results reveal that both non-invasive techniques can be used to monitor cavitation activity. Although, neither approach provided adequate spatial resolution to accurately map the cavitation field. These techniques are most appropriate to use when the region of cavitation activity in the reactor vessel is known and the external, non-invasive technique can be targeted at this known position to provide information on the regional cavitation intensity.