Fragmentation of falling liquid droplets in bag breakup mode

Using direct numerical simulations, the fragmentation of falling liquid droplets in a quiescent media is studied. Three simulations with different Eotvos numbers were performed. An adaptive, volume of fluid (VOF) method based on octree meshing is used, providing a notable reduction of computational cost. Results are presented in three main parts; droplet deformation and bag formation, bag breakup mechanism, and variations in the number and size of fragments. It is clearly shown that the droplet initially deforms into a thin hollow bag. After that, due to growth of instabilities over the thin liquid sheet, several punctures are formed. The holes further grow, generating a web of ligaments. In this stage, the fragmentation domain consists of a liquid torus left above the bag, a number of ligaments and droplets, and the parent droplet core. The liquid torus, the core, and the unstable ligaments disintegrate further. The breakup processes continue until the hydro-dynamic and the surface tension forces reach an equilibrium condition for each fragment. The growth of fragment numbers during the atomization stage is described and the size distributions between the simulations are compared. The analysis performed here precisely demonstrates the mechanism of the bag breakup and depicts an accurate history for the small fragments generated in the secondary atomization process. The outcomes can be used for development of the secondary atomization models. Moreover, the results can be directly used for better understanding of rain drop atomization during precipitation, as well as water droplet atomization in cooling towers. (C) 2012 Elsevier Ltd. All rights reserved.