Flapping instability of a liquid jet (original) (raw)
Related papers
Flapping Instability of an Atomized Liquid Jet
ASME 2010 3rd Joint US-European Fluids Engineering Summer Meeting: Volume 1, Symposia – Parts A, B, and C, 2010
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.
Study of a liquid–gas mixing layer: Shear instability and size of produced drops
Comptes Rendus Mécanique, 2013
We study experimentally the atomization of a liquid sheet by a parallel gas flow, in order to understand the conditions of destabilization of the liquid sheet and the conditions of drop creation. We study in particular the regimes at low M (ratio of gas/liquid dynamic pressures), to test the scaling law proposed and validated in previous studies at large M (M = 16). The inviscid stability analysis of the system is carried out with a new velocity profile taking into account the wake of the splitter plate (zero speed at the level of the splitter plate): the influence of liquid velocity on the shear instability frequency turns out to be significantly stronger for this type of velocity profile than for classical ones. An asymptotic study of the dispersion relation leads to a new scaling law giving the wavenumber of the shear instability as a function of gas velocity U g , with a corrective term in M. Frequency measurements carried out by a spectral method show a good agreement with this corrected law. We investigate by way of optical probe measurements the size distribution of produced drops downstream. The difficulty of these measurements resides in the decrease of the numerical drop flux at low M. Results obtained for the mean chord are consistent with previous studies. Diameter distributions are obtained from chord distributions with a numerical conversion procedure.
Turbulent primary breakup of round and plane liquid jets in still air
40th AIAA Aerospace Sciences Meeting & Exhibit, 2002
The formation of drops at the surface of turbulent liquids, e.g., turbulent primary breakup, was studied experimentally. Pulsed shadowgraphy and holography were used to observe the properties of the liquid surface and the drops formed by turbulent primary breakup. Measured properties included liquid surface velocities, conditions at the onset of ligament and drop formation, ligament and drop sizes, ligament and drop velocities and rates of drop formation. Phenomenological theories were used to help interpret and correlate the measurements. Present results show that the onset of ligament formation occurs once the kinetic energy of the turbulent eddies that form the ligaments exceeds the required surface tension energy of a ligament of comparable size. Subsequently, the onset of drop formation occurs once drops form at the tips of ligaments due to Rayleigh breakup. This same mechanism controls the subsequent variation of drop sizes due to turbulent primary breakup as a function of distance from the jet exit. In addition, ligament and drop velocities were associated with mean and fluctuating velocities of the liquid, and rates of drop formation could be expressed by surface efficiency factors defined as the fraction of the maximum cross stream liquid mass flux. B let-Water 13.5 Water 7.1 Round free jet: Water 8.0 Water 4.8
The breakup and atomization of a viscous liquid jet
Acta Mechanica Sinica, 1996
Based on the linear analysis of stability, a dispersion equation is deduced which delineates the evolution of a general 3-dimensional disturbance on the free surface of an incompressible viscous liquid jet. With respect to the spatial growing disturbance mode, the numerical results obtained from the solution of the dispersion equation reveal that a dimensionless parameter Jr exists. As Jr > 1, the axisymmetric disturbance mode is most unstable; and when Jr < 1, the asymmetric disturbances come into being, their growth rate increases with the decrease of Jr, till one of them becomes the most unstable disturbance. The breakup of a low-speed liquid jet results from the developing of axisymmetric disturbances, whose instability is produced by the surface tension; while the atomization of a high-speed liquid jet is brought about by the evolution of nonaxisymmetric disturbance, whose instability is caused by the aerodynamic force on the interface between the jet and the ambient gas.
Chemical Engineering Science, 2015
This work investigates the effects of multiple jet interactions and single jet instability on jet breakup and droplet size using experimental and computational techniques. In particular, the jet separation distance, jet breakup length and droplet diameter were measured as a function of initial nozzle separation distance and jet volumetric flow rate. It was found that the two jets moved closer to each other to reach an equilibrium separation distance that was approximately 70% of the spacing between the two nozzles. The distance at which the instabilities were first observed on the surface of the jet was also a function of the initial separation distance. However, it was weakly dependent on the jet velocity. The jet breakup length and resultant droplet diameter were both influenced by flow rate and nozzle separation distance. The jet breakup length was found to decrease with reduction in nozzle spacing at the high flow rates. Interestingly, a linear relationship between droplet diameter and breakup length was found that was largely independent of nozzle spacing and consist with conventional Rayleigh jet breakup theory. The implications of the experimental observations on the design of multi-jet systems are discussed. Furthermore, computational fluid dynamics simulations were also used to identify the mechanism and dynamics of jet instability in the single jet systems. The simulation results were analysed to study the effect of instability on various parameters such as jet breakup, droplet formation and size of emulsion droplets. It was found that at higher volumetric flow rates, the droplets size increased during the jet breakup due to an asymmetric instability. The asymmetric instability was caused by the pressure gradient in the continuous phase and was prevented in double jet systems.
Liquid breakup at the surface of turbulent round liquid jets in still gases
2002
An experimental study of liquid column breakup lengths and turbulent primary breakup properties at the surface of turbulent round liquid jets in still air at standard temperature and pressure is described. Jet exit conditions were limited to non-cavitating water and ethanol flows, long length/diameter ratio (greater than 40:1) constant-diameter round injector passages, jet exit Reynolds numbers of 5000-200,000, jet exit Weber numbers of 235-270,000 and liquid/gas density ratios of 690 and 860, at conditions where direct effects of viscosity were small (e.g., liquid jet Ohnesorge numbers were smaller than 0.0053). Three liquid column breakup modes were observed, as follows: a weakly turbulent Rayleigh-like breakup mode observed at small jet exit Weber and Reynolds numbers, a turbulent breakup mode observed at moderate jet exit Weber numbers, and an aerodynamic bag/shear breakup mode observed at large jet Weber numbers. The turbulent primary liquid column breakup mode was associated with conditions where drop diameters resulting from turbulent primary breakup along the liquid surface were comparable to the diameter of the liquid column itself. The bag/shear liquid column breakup mode was observed when liquid turbulence caused large deformations of the liquid column so that portions of it were in a gaseous cross flow; this resulted in bag or shear liquid column breakup, very similar to the breakup of non-turbulent liquid jets in gaseous cross flow. Mean streamwise drop velocities after breakup were comparable to mean streamwise velocities within the jet whereas mean cross stream drop velocities after breakup were comparable to cross stream velocity fluctuations within the liquid. Rates of primary breakup at the liquid surface are reported as surface efficiency factors (the fraction of the maximum possible cross stream drop liquid flux at the surface based on the mean relative cross stream velocity of the drops at the surface and the liquid density). The resulting surface efficiency factors varied from small values near the onset of liquid surface breakup to values of the order of unity as conditions near breakup of the liquid column as a whole were approached. Ó
1997
The behaviour of unsteady liquid jets in a gas atmosphere is mainly governed by the conservation of momentum and the interaction with the environment. In this article it will be shown that many of the particular effects in the propagation and desintegration of unsteady jets are simply explained by the conservation of initial momentum. Many of the distortions and peculiar shapes of the liquid elements of the jet can be explained by a time and space development of weak initial distortions of momentum in travelling waves during propagation.
Reports on Progress in Physics, 2008
Jets, i.e. collimated streams of matter, occur from the microscale up to the large-scale structure of the universe. Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids. Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology. Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup. In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology. They also arise from the last but one topology change of liquid masses bursting into sprays. Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium. The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects.
Instability of a liquid jet emerging from a droplet upon collision with a solid surface
Physics of Fluids, 2000
A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed.