Vortices catapult droplets in atomization

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.

A two-phase mixing layer between parallel gas and liquid streams: multiphase turbulence statistics and influence of interfacial instability

“…(1.1)) which is in between the gas and liquid injection velocities (see the right column of figure 2). This is consistent with experiments by Raynal (1997), Hoep↵ner et al (2011 and Jerome et al (2013). The amplitude of the wave grows in time.…”

Influence of Gas Turbulence on the Instability of an Air-Water Mixing Layer

“…We have demonstrated the robustness of this effect via two independent forcing techniques: each show up to a doubling in frequency when turbulent intensity in the incoming gas stream increases from 2% to 10%. The break-up of the instability waves has been recognized as central in drop formation [4,9,17]: the present results therefore reassert the relevance of internal flow characteristics on assisted atomization, beyond the already established role of δ G . It will next be crucial for improving applications to assess how upstream turbulence, via its effect on the shear instability, impacts drop sizes.…”

Multiscale simulation of atomization with small droplets represented by a Lagrangian point-particle model

“…16. The recirculation zone formed at the downstream of the interface wave has also been observed experimentally and numerically by Jerome et al (2013). The interaction between the wave and the gas stream becomes more complex as the wave further grows and propagates downstream, and the wave crest breaks up in a more pronounced way and generates a large amount of droplets, as shown in Fig.…”

“…The clockwise circulation at the downstream of the wave grows in time and pushed downstream as the wave moves. These circulation regions are observed to have a significant impact on droplet formation (Jerome et al, 2013). The ligaments, stretched by the gas stream, finally break up into many droplets due to a Plateau-Rayleigh instability, as shown in Fig.…”