Break-up conditions of a flapping liquid jet

In air assisted atomization, small droplets arise from the stripping of the liquid jet (or film) by the fast gas stream [1,2,3,4,5]. Yet, the incoming liquid is never fully atomized by the stripping process alone. Instead, the remaining jet experiences a flapping instability [2], similar to the instability observed on liquid sheet configurations [6]: the resulting large scale structures (see fig.1a) break into large liquid lumps at some distance downstream the injection. Little is known on the underlying mechanisms of this instability, and on the characteristics of the large drops it produces, though these large drops most probably control flame extent in combustion devices. We therefore examine the break-up conditions as a function of two control parameters, namely the liquid and gas exit velocities. The break-up distance, given as the longitudinal extent of the liquid structure still connected to injection, happens to smoothly decrease with M=GUG2/LUL2 (fig.1c). We also investigate the break-up mode, by considering the time evolution of the number of daughter drops versus time, and we sought to relate the size of the very first drops with the wavelength. The flapping frequency is a constant for given control parameters [2] but the wavelength increases with the distance to the injector. This behavior arises from the acceleration experienced by the large scale structures as they are convected downstream (fig.1b). This acceleration is found to be larger than the one due to gravity, indicating that the co-flowing air stream plays an active role in the spatial evolution of these structures. According to available observations, it happens that the size of the largest detached drop is directly controlled by the wavelength, and besides, that the number of daughter inclusions is a smooth function of the gas and liquid velocities. These findings will be discussed in more detail during the presentation