Flapping Instability of an Atomized Liquid Jet

We present an experimental study of the flapping instability which appears when a coaxial liquid jet is atomized by a cocurrent fast gas stream. When primary atomization does not lead to a total break-up of the liquid jet, it undergoes a largewavelength instability, characterized by very large amplitude oscillations, and can break into large liquid fragments whose typical size is the jet diameter. These large liquid fragments, and consequently the flapping instability, are to be avoided in applications related to combustion where liquid droplets need to be as small as possible. We carried out experiments with air and water coaxial jets, with a gas/liquid velocity ratio of order 50. We studied the consequence of the flapping instability on the break-up of the liquid jet. Measurements of the frequency of the instability were carried out. We suggest a mechanism where the flapping instability could be triggered by non axisymmetrical KH modes. INTRODUCTION Airblast, or assisted atomization of a liquid jet is fundamental to a large number of applications. In this process the liquid is stripped from a cylindrical jet by a fast co-current air-stream, and a spray is produced 1 . Applications range from injectors in turboreactors, to cryotechnic rocket engines with LOX/H2. This process is widely used, and has proven reliable, but the mechanism by which the liquid bulk is broken into droplets is still subject to controversy. A better understanding of the different stages of the atomization process could help improve the efficiency of combustion, and decrease the amount of emissions. Experiments carried out by coworkers on a plane mixing layer 2,3 and on a coaxial injector 4 have shown that the liquid break up is the result of two successive instabilities 5 . The first instability is analogous to a Kelvin-Hemholtz instability, and leads to the formation of waves at the interface between the liquid and the fast gas stream. However, while KelvinHelmholtz instability involves a discontinuity of the velocity profile between the gas and liquid phases, the instability involved here has been shown to rely on the smoothness of the velocity profile, namely on the finite thickness of the gas vorticity layer. Within an inviscid approximation, it predicts that the wavelength λ of the surging waves will be given by 2,5 : λKH = CKH (ρL/ρG)δG (1) with δG thickness of the gas boundary layer, ρL and ρG the liquid and gas densities and CKH ≈ 4 a dimensionless coefficient. The axisymmetric waves resulting from this instability are next accelerated by the fast gas stream, and undergo a Rayleigh-Taylor transverse instability, leading to the formation of liquid ligaments . These ligaments grow and will eventually break into droplets, whose size is therefore controlled by the thickness of ligaments, i.e. the wavelength of the R-T instability. If the liquid intact length is larger than the potential cone, as is the case in our experiment, atomization of the liquid jet is incomplete: while small droplets are still produced in the potential cone region, far downstream the liquid jet ends up breaking into large liquid lumps. Just before its break-up, the liquid jet downstream the potential cone exhibits a striking “oscillating” aspect, in which the jet undergoes oscillations of a