Atomization of undulating liquid sheets

Journal of Fluid Mechanics, 2012

The aerodynamically driven annular liquid sheet exhibits a complex nonlinear instability. Novel interfacial velocimetry experiments suggest that two distinct physical sources of instability may be present. The first is the well-known free shear layer instability, which is quasi-sinusoidal and nonlinear. The second is a distinct nonlinear rupturing instability, modulated on the previous one. It may be directly driving primary atomization. This instability has not been previously observed in isolation and is inherently nonlinear and non-sinusoidal. Novel application of Koopman analysis and the Hilbert transform permit investigation of these distinct instabilities. A greater understanding of the rupturing instability may lead to a better understanding of atomization phenomena.

We report a set of experimental investigations on the break-up of a liquid drop when falling in a miscible solvent, with the density difference being positive, or negative, or zero. Non-dimensional numbers, derived from the characteristic times of the drop evolution, account for the hydrodynamic instabilities and the self-similar character of the fragmentation process. The role of the initial surface tension at the air-drop interface is explored, leading to scaling laws for the drop volume V and the various height h reached by the drop before it fragments into smaller droplets. From the first break-up to the onset of diffusion, the fragmentation process is shown to have a fractal structure, which is associated to universal power laws for h and V during the dynamical processes associated to the break-up phenomena.

International Journal of Multiphase Flow, 1997

A study was performed of the distortion and breakup mechanisms of liquid drops injected into a transverse high velocity air jet at room temperature and atmospheric pressure. The investigation included the use of ultra-high magnification, short-exposure photography to study the three drop breakup regimes previously referred to as the bag breakup regime, the shear or boundary-layer stripping breakup regime, and the ‘catastrophic’ breakup regime. In the experiments the initial diameters of the injected diesel fuel drops were 69, 121 and 198 μm, and the transverse air jet velocity was varied from 68 to 331 m/s. The experimental conditions correspond to drop initial Weber numbers of 56, 260 and 463 for the three breakup regimes. The drop Reynolds numbers (based on gas properties) ranged from 509 to 2488. It was found that the drop breakup process occurs in two stages. During the first stage, under the action of aerodynamic pressure, the drop distorts from its undisturbed spherical shape and becomes flattened, or disk shaped, normal to the air flow direction. This feature exists in all three drop breakup regimes. A dynamic drag model that is a modified version of the DDB (Dynamic Drag and Breakup) model and accounts for the increase of both the drop's frontal area and its drag coefficient as a function of its distortion was used to analyze the drop trajectory and its distortion during the first stage of the drop breakup process. During the second stage of the drop breakup process, the three drop breakup regimes display different breakup features. In the bag breakup regime the appearance and growth of holes on the bag sheet blown out of the center of the flattened drop is the dominant reason for the breakup; in the so-called shear or boundary-layer stripping breakup regime the results indicate that bending of the flattened drop's edge under the action of aerodynamic pressure, followed by production of folds on the bent sheet results in production of ligaments aligned in the direction of the air flow; and in the ‘catastrophic’ breakup regime the growth of capillary waves on the flattened drop surfaces, combined with the bending and folding of the sheet edge makes the breakup process demonstrate ‘catastrophic’ breakup characteristics. In addition, the experimental results confirm that for drops with different sizes, the same breakup regimes appear when the Weber number is held constant, and the Reynolds number does not play a dominant role. These results thus cast considerable doubt on the validity of the widely used ‘shear’ or ‘boundary-layer stripping’ drop breakup theories in which viscous effects would be important.

Experiments in Fluids, 2007

We describe the characteristics of a radially spreading unstable liquid sheet in quiescent air via optical measurement techniques and linear instability theory. A high speed CCD camera system and a complimentary laser refraction method were employed to measure the intact sheet diameter, unstable wave lengths, wave speed, wave frequency spectrum and spatial wave growth rates. Linear instability models for thinning, viscous and inviscid liquid sheets, which are available from the literature, allow for a comparison of experimental data and predicted sheet behaviour. The last section evaluates the differences and similarities between the current liquid sheet experiment and industrial spray applications such as fuel atomisation via pressure-swirl nozzles.

Atomization and Sprays, 2017

that liquid jets in the first wind-induced regime may be destabilized by non-axisymmetric deformations to the Rayleigh regime without any aerodynamic influence. We present models for the mean size of drops formed by the breakup of capillary axisymmetric viscoelastic liquid jets and of plane sheets. The former is based on Weber's equation for the optimum disturbance wavenumber for a Newtonian liquid jet, applying the correspondence principle. The latter is derived from the dispersion relation for a plane liquid sheet and used for predicting the Sauter-mean drop size in viscoelastic liquid sprays from pressure-swirl atomizers. Proper account for the influence from the liquid viscoelasticity on the formation of drops is essential. Influences from the molecular weight and flexibility or rigidity of the polymeric solute on the solution behavior upon deformation are represented by rheometrically accessible stress relaxation and deformation retardation times of the liquid. Mean drop sizes predicted by our models are in good agreement with experimental data.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018

We consider the fluid splitting dynamics of a small fluid drop entrapped between two parallel solid plates, which are rapidly pulled apart such that a free liquid sheet is formed between them for a few milliseconds. Velocities, time scales and shear rates correspond to the lift-off of a printing cylinder from the substrate in an industrial printing process. The formation and collapse of the liquid sheets, and the filaments forming at their rim are studied. Driven by the surface tension of the liquid sheet, the filaments are accelerated towards each other, and finally collide and decay. We show that surface waves that travel on the liquid sheet have a critical impact on this collision, which may either result in a smooth unification or in a highly dynamical collapse, ending in a random emission of fluid drops. Furthermore, we discuss the relevance of the Laplace number on the propagation of these waves. We also consider the liquid sheet decay by capillarity-driven expansion of spontaneously forming holes and discuss how sheet thickness and its local variation can be detected by an anisotropic hole expansion rate measurement.

Proceedings of the National Academy of Sciences of the United States of America, 2020

Soft Matter, 2015

The coexistence of multiple droplet breakup instabilities in a Step-emulsification geometry is studied. A liquid filament, which is confined in one dimension by channel walls and surrounded by a co-flowing immiscible continuous phase, decays into droplets when subject to a sudden release of confinement.

International Journal of Multiphase Flow, 2018

Highlights  Effect of external acoustic excitation on the liquid sheet breakup is investigated experimentally in the context of impinging jet injectors used in liquid rocket combustors.  Visualization studies reveal sheet distortion, violent sheet flapping, wave amplification and the local increase in droplet density.  Reduction in the sheet breakup length and width occurs at few selected frequencies, not multiples of each other, ruling out the resonance phenomenon.  The mean drop size decreases in the presence of acoustic field without altering the universal behavior of drop size distribution.

Journal of Fluid Mechanics, 2002

A round liquid jet with density ρ, surface tension σ and diameter D 0 impacting a solid circular surface at normal incidence with velocity U 0 takes the form of a radially expanding sheet whose thickness decreases with distance from the impact point. When the sheet develops in a still environment with density ρ a = αρ, it destabilizes, provided the impacting Weber number We = ρU 2 0 D 0 /σ is larger than about 40α −1/2 , as a result of a shear instability with the surrounding medium, in a sinuous, flag-like motion. We show how the instability properties set both the radial extent of the liquid sheet and the drop formation process at its rim. The shear instability gives the liquid a flag-like motion, ultimately triggering a Rayleigh-Taylor instability at the rim of the sheet which disintegrates, at the radial location R, into disjointed droplets of size d such that R/D 0 ∼ α −2/3 We −1/3 and d/D 0 ∼ α −2/3 We −1 . The features of the sheet instability, its radius and the droplet sizes are determined experimentally for a broad range of control parameters, using different liquids and ambient-medium densities.