A numerical study of the acoustic and turbulent agglomeration processes by a coupled computational fluid dynamics–population-balance model

Acoustic agglomeration has been widely recognized as a novel technology for fog elimination and artificial precipitation. In this study, computational fluid dynamics coupled with the population-balance model was applied to investigate the agglomeration process of micro-scale droplets under the combined effects of acoustic waves and turbulent flows. The dynamic processes and distribution of turbulence, vortex characteristics, orthokinetic and hydrodynamic interactions of cloud droplets, as well as agglomeration caused by Brownian motion and turbulent flows, were studied. It was found that there was an eigenfrequency and a threshold sound intensity for acoustic agglomeration. The eigenfrequency of cloud droplets was 50–200 Hz. The threshold intensity for acoustic intervention was in the range of 100–110 dB. When the sound pressure level (SPL) was greater than 110 dB, the median size of the droplets and the spectral width of the droplet volume distribution increased with SPL. The frequency band corresponding to the optimal agglomeration effect of large droplets was relatively low. In such studies involving agglomeration processes, the effect of turbulence on droplet agglomeration should not have been neglected, and the combined effects of sound and flow on the droplet number concentration were more significant than those of pure flow. The effect of acoustic waves on agglomeration became weaker when the flow velocity increased. These insights inform future developments in particle-control technologies and atmospheric precipitation enhancement as well as guide further fundamental research on droplet agglomeration under acoustic fields.