Abstract
Numerical simulations of chemical reactions in a pulsating air bubble have been performed for various ultrasonic frequencies and pressure amplitudes in order to study the influence of ultrasonic frequency on sonochemical reactions. Below the threshold acoustic amplitude for cavitation, there is a range of the acoustic amplitude optimum for oxidants production, which corresponds to the optimum bubble temperature [K.Yasui et al., J.Chem.Phys. 119, 346 (2003)]. The range becomes wider and shifts toward higher acoustic amplitude as the ultrasonic frequency increases. The rate of oxidants production at the optimum acoustic amplitude increases as the ultrasonic frequency increases up to around 300kHz because it is proportional to the number of the bubble collapses per second. At 1MHz, however, the rate is lower than that at 300kHz due to the smaller ambient bubble radius than that at 300kHz. It may result in the optimum ultrasonic frequency of around 300kHz for the oxidants production reported in many experiments. According to the present numerical calculations, however, the optimum condition for the oxidants production by a bubble is at 20kHz when the pressure amplitude is larger than about 3 bar. It may be realized using pulsed ultrasound or in strongly degassed water as the continuum ultrasound is strongly attenuated by cavitation bubbles.
