Numerical simulation of ultrasonic enhancement by acoustic streaming and thermal effect on mass transfer through a new computation model

Abstract

Mass transfer coefficient is an important parameter in the process of mass transfer to reflect the degree of enhancement of mass transfer process in liquid–solid reaction and in non-reactive systems. In the present paper, a new computational model including both acoustic streaming and ultrasonic thermal effect is established to quantitatively calculate the ultrasonic enhancement on mass transfer coefficient in liquid–solid reaction. A nonlinear Helmholtz equation containing the cavitation effect is used in the model to calculate the acoustic pressure distribution in the reactor, and a fluid-thermal coupling CFD involving mass transfer process for KOH solution is conducted to obtain the distribution and time-dependence of the mass transfer coefficient in silicon–KOH reaction. Mass transfer coefficient on silicon surface with a transducer at frequencies of 100 kHz has been numerically simulated, indicating that the mass transfer coefficient has the maximum value near the center of the silicon wafer, and increases with reaction time. The mass transfer coefficient in the center of the silicon wafer is increased by 14.3% from 6.043 × 10−5 m/s to 6.908 × 10−5 m/s under ultrasound power of 50W during reaction time from 0.1 hour to 1 hour (the mass transfer coefficient without ultrasound changes from 2.525 × 10−6 m/s to 7.615 × 10−7 m/s). A set of control simulations shows that the enhancement of mass transfer coefficient comparing to the situation without ultrasound is mainly resulted from the acoustic streaming, while the increasing of mass transfer coefficient with time is due to the thermal effect of ultrasound. The mass transfer coefficients under transducer power of 10W and 30W are also calculated, indicating that the mass transfer coefficient is positively correlated with the ultrasonic power.