Influence of ultrasonic frequency on sonochemical reactions

Abstract

Numerical simulations of the pulsation of an air bubble have been performed for various ultrasonic frequencies (20 kHz to 1 MHz) and pressure amplitude taking into account the effect of nonequilibrium chemical reactions inside a bubble and that of nonequilibrium evaporation and condensation of water vapor at the bubble wall. The calculated results have revealed that as ultrasonic frequency decreases, the mole fraction of water vapor inside a bubble increases at the end of the bubble collapse. While the main oxidant created inside a vaporous bubble, which is defined as a bubble consisting mostly of water vapor at the end of the bubble collapse, is OH radical, that inside a gaseous bubble, which is defined as a bubble consisting mostly of noncondensable gas (air) at the end of the bubble collapse, is H2O2 or O atom when the bubble temperature is less than or more than 7000 K, respectively. Although the optimum acoustic amplitude for the oxidant’s creation inside a bubble increases as the ultrasonic frequency increases, the range of the suitable acoustic amplitude for the oxidants creation becomes wider. It possibly results in the optimum ultrasonic frequency for the oxidant’s creation reported in the literatures (200–600 kHz).