Numerical simulation on partial coalescence of a droplet with different impact velocities*

Abstract

Partial coalescence is a complicated flow phenomenon. In the present study, the coalescence process is simulated with the volume of fluid (VOF) method. The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence. The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet changing into a prolate shape and the final pinch-off. The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off. With the droplet impact velocity increasing, the droplet shape before the pinch-off will get close to a sphere. When the shape gets close enough to a sphere the partial coalescence shifts to full coalescence. The effect of film thickness on the coalescence is also investigated. With large film thickness partial coalescence happens while with small film thickness full coalescence happens. In addition, the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient while decreases with the increase of viscosity and initial droplet diameter. And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.