Vortices catapult droplets in atomization

TL;DR

This study investigates a droplet ejection mechanism in two-phase flows where vortex shedding causes large droplet ejections at high angles, combining experiments and simulations to reveal the vortex-liquid interaction process.

Contribution

It uncovers a novel vortex-induced droplet ejection mechanism driven by recirculation zones and vortex shedding in planar two-phase mixing layers.

Findings

Vortex shedding creates recirculation zones that eject droplets at large angles.

The ejection process involves a sequence of vortex formation, detachment, and reattachment.

Droplets are propelled by vortices similar to a bag-breakup event.

Abstract

A droplet ejection mechanism in planar two-phase mixing layers is examined. Any disturbance on the gas-liquid interface grows into a Kelvin-Helmholtz wave, and the wave crest forms a thin liquid film that flaps as the wave grows downstream. Increasing the gas speed, it is observed that the film breaks up into droplets which are eventually thrown into the gas stream at large angles. In a flow where most of the momentum is in the horizontal direction, it is surprising to observe these large ejection angles. Our experiments and simulations show that a recirculation region grows downstream of the wave and leads to vortex shedding similar to the wake of a backward-facing step. The ejection mechanism results from the interaction between the liquid film and the vortex shedding sequence: a recirculation zone appears in the wake of the wave and a liquid film emerges from the wave crest; the recirculation region detaches into a vortex and the gas flow over the wave momentarily reattaches due to the departure of the vortex; this reattached flow pushes the liquid film down; by now, a new recirculation vortex is being created in the wake of the wave - just where the liquid film is now located; the liquid film is blown up from below by the newly formed recirculation vortex in a manner similar to a bag-breakup event; the resulting droplets are catapulted by the recirculation vortex.