Modal Interactions in High-Power Ultrasonic Processing Transducers

High-power ultrasonic processing is based on the exploitation of nonlinear phenomena linked to large amplitude ultrasonic waves and includes a wide range of processes. Such processes require the development of specific devices for the right generation of the high-intensities needed. Stepped-plate transducers, piezoelectrically driven, represent a new efficient power ultrasonic technology for applications in fluids and multiphase media. Such devices are multimode systems which operate in one single selected mode at high power and low ultrasonic frequencies. However, nonlinearity gives rise to a series of phenomena which are not present under linear excitation. In particular, the appearance of harmonics, fractional harmonics, combination resonances, etc., as well as the interaction among different modes represent important limitations for the use of high-power ultrasonic transducers. Energy exchange among the modes may generate high-amplitude vibration of non-tuned modes causing heating, noise and even component failures. This paper deals with the experimental evidence of the production of undesired vibration modes and their interaction in high-power stepped-plate ultrasonic transducers and suggests some practical procedures in the design and use of such devices to avoid or to palliate such important practical problem.