Abnormal heating peak of cavitation clouds deviating from their resonance point

This work investigates the physics of thermally relevant acoustic cavitation in a liquid. A nonlinear sound wave equation in bubbly liquid is applied together with an equation for bubble pulsation to calculate sound wave propagation and bubble radii in a cavitation cloud. The solutions are used to determine the heat generation caused by viscous dissipation at bubble walls and by liquid absorption. The results show that for an ultrasonic source frequency of 1 MHz, the resonant bubbles respond violently to acoustic waves and generate considerable heat owing to the liquid viscosity at bubble walls. The heating effect decreases as the ambient bubble radius deviates from the resonant size. In addition, acoustic waves emitted from the oscillating bubbles are absorbed by the liquid and convert into heat. However, it plays a minor role during the heating process compared with the heat generation at the bubble walls. For an ultrasonic source frequency of 20 kHz, the heating characteristics change significantly because a forbidden band appears in the frequency spectrum. Generally, as the ambient bubble radius close to or larger than its resonant size the forbidden band appears, when the sound wave with low frequency may be blocked and a little can incidence into the cavitation cloud and the heat generation due to the viscosity at bubble walls, which is usually dominant, is reduced, though the heating effect caused by liquid absorption is enhanced. Thus, there is no maximum heat generation caused by viscous dissipation at the bubble wall, as normal heating occurs when the ambient radius is at its resonant size.