Nonlinear behaviour of power ultrasonic transducers for food processing

Power ultrasonic systems at laboratory and semi-industrial scale are currently investigated to demonstrate the suitability of ultrasonic waves of high-intensity to industrial applications. It has been shown that intense ultrasonic fields trigger a series of mechanisms in the irradiated media that may enhance and/or accelerate a variety of processes in the food sector. Ultrasonic radiators driven by piezoelectric vibrators have been specifically developed for assisting in drying and extraction operations. Successful industrial scale-up of such tuned systems significantly depends on the control of their nonlinear vibration behaviour at high operational power levels. In this paper we investigated experimentally the nonlinear dynamics of two power ultrasonic transducers: a grooved-plate transducer and a cylindrical radiator transducer. Nonlinear mechanisms affecting the dynamic behaviour of both assemblies such as the appearance of harmonics, combination of resonances, or modal interactions, and response saturation are presented. In particular, energy transfers among system modes that may produce the excitation of nontuned resonant frequencies causing heating, noise and even failures of the transducers are identified and characterised.