Geometry-driven coalescence of water droplets in a highly viscous fluid

This work is dedicated to numerical studies of the coalescence of water droplets moving in bitumen. The Navier-Stokes equations coupled with the volume of fluid model (VOF) using adaptive grids techniques, available in the commercial computational fluid dynamics (CFD) software Ansys Fluent 20R2, were solved numerically to investigate the behaviour of water droplets with diameters from 1 to 100 mu m moving in rapidly converging and diverging microchannels of different sizes. The model has been validated against 3-D experimental data published in the literature. A good agreement has been demonstrated. The results of simulations revealed that the main parameter influencing coalescence is the bulk flow velocity in the channel. Analysis of unsteady simulations showed the existence of a critical flow velocity, above which no coalescence occurs, corresponding to capillary number Ca < 0.5 for droplets Reynolds number Re < 0.1. Besides, image processing analysis has been used for a mean droplet size estimation in different geometries. A mean size significantly increased due to the late coalescence occurring in a wider constriction.