Mechanisms of Liquid Stream Breakup: Vorticity and Time and Length Scales

Abstract

The 3D, temporal instabilities on a planar liquid jet are studied using DNS with level-set and VoF interface-capturingmethods. The λ2 method has been used to relate the vortex dynamics to the surface dynamics at different stages of the jet breakup. The breakup character depends on the Ohnesorge number (Oh) and gas-to-liquid density ratio. At high Reynolds number (Re) and high Oh, hairpin vortices form on the braid and overlap with the lobe hairpins, thinning the lobes, which then puncture creating holes and bridges. The bridges break, creating ligaments that stretch and break into droplets by capillary action. At low Oh and high Re, lobe stretching and thinning is hindered by high surface tension and splitting of the original Kelvin-Helmholtz vortex, preventing early hole formation. Corru- gations form on the lobe edges, influenced by the split vortices, and stretch to form ligaments. Both mechanisms are present in a transitional region in the We-Re map. At lower Re and not-too-large Weber number (We), lobe stretching occurs but with longer and larger ligaments in this third domain which has a hyperbolic transition to the hole formation domain as We increases. The three domains with differing breakup behaviors each occupy distinct portions of a plot of We based on gas density versus Re based on liquid properties. Characteristic times for the hole formation, as well as the lobe and ligament stretching are different - the former depending on the surface tensionand the latter on liquid viscosity. In the transitional region, both times are of the same order.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4616