Coalescence and break-up of large droplets in turbulent channel flow

The behaviour of large, deformable and coalescing droplets, released in a turbulent channel flow, has been numerically investigated with a Phase Field approach; focus has been put on droplet-droplet interactions and droplet fragmentation, enhanced by turbulent fluctuations. Two different dynamics of the dispersed phase may be observed depending on the Weber number ($We$). For small $We$ surface tension balances turbulent shear; slightly deformed droplets, transported by the carrying fluid, may only coalesce. The analysis of the droplet pair distance shows that the geometrical separation of the droplets is a leading factor for the coalescence regime determination. On the contrary, if $We$ is larger than a critical value, a dynamic equilibrium between coalescences and breakups is shown. In this regime, in line with seminal work of Hinze (1955) and recent numerical simulations of Perlekar et al. (2012), $We$ controls the critical stable diameter of droplets as well as the average distance between droplet pairs.