Mechanism study on the enhancement of silica gel regeneration by power ultrasound with field synergy principle and mass diffusion theory

Abstract

The enhancement of silica gel regeneration by power ultrasound has been validated by a series of experiments in our previous studies, but there still lacks theoretical basis for illustrating its mechanism. In this paper, the mechanism of ultrasound-enhanced regeneration has been explored. Firstly, the benefit of ultrasonic mechanical effect to the enhancement of regeneration has been illustrated by the field synergy principle. The average synergy degree (κ¯, only considering intersection angle between the velocity and the temperature gradient) and the average overall synergy degree (κ¯overall, considering local values of velocity and temperature gradient based on κ¯) in terms of the near wall region are suggested for analysis, and they are obtained based on the k–omega model which is suitable for the near wall free-shear flow velocity predicting. Results manifest that the ultrasonic mechanical effect can significantly enlarge the synergy degrees between the temperature and the velocity field around the particle, and this can be used to explain the enhancement of convective heat and mass transfer on the gas side due to the mechanical effect of ultrasound. Afterwards, a moisture diffusion model is developed to investigate spatial distributions of moisture ratio and temperature in a silica gel particle as well as its surface equilibrium humidity during the regeneration with and without ultrasound. Results show that ultrasonic heating effect can lead to an increase in the average temperature and moisture diffusivity in the silica gel particle, and this confirms the contribution of ultrasonic heating effect to the enhancement of silica gel regeneration.