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Sep 19, 2012

On 1:01 PM by Lalith Varun   3 comments

       
INTRODUCTION

          The aim of atomization is to substantially increase the surface area of the liquid to enhance vaporization, mixing and combustion. The end result is that the liquid jet becomes unstable which leads to the disintegration of the liquid surface into droplets. This surface area increase can be achieved in various ways and shear coaxial jet injector atomization process is one of them. The breakup of the liquid jet is a result of complex interactions between inertial, viscous and surface tension forces. Aerodynamic forces promotes disturbances on the surface while viscous forces have a damping effect. Surface tension tends to pull the liquids together. Turbulence and pressure oscillations in the injected fluids affect atomization. The non dimensional parameters such as Reynolds number, Weber number, Mixture ratio and Ohnesorge number help in characterizing the overall process. Le Visage, D. showed that both the momentum and density ratios determine the breakup length of the liquid core. By plotting the Ohnesorge number vs the Reynolds number, one can distinguish between
a) low-velocity region, where the breakup is due to the action of surface tension forces and
b) high-velocity region, where the influence of aerodynamic forces increases exponentially with Reynolds number at constant Ohnesorge number.


THE PROCESS OF DISINTEGRATION

1) The Primary Atomization zone: In the near field of the injector nozzle, the huge difference in the velocity between the gas and liquid, leads to a surface instability and formation of filaments or drops from the jet surface. This is the primary atomization zone.
2) The Secondary Atomization zone: In the far field of the injector nozzle, the fluid velocity decreases due to mixing with the external atmosphere which leads to instability. The large droplets and ligaments produced in the primary atomization zone and in the jet breakup zone, breakup further into smaller and more stable droplets, depending on the local weber number. The breakup time of the droplets can be expressed as a function of local relative velocity and ratio of gas to liquid density. The breakup time and initial droplet velocity determines the distance from the injector where secondary atomization takes place to the flame front.

CLASSIFICATION OF BREAKUP REGIMES

i) based on Weber number

Extensive experimental study on round liquid jets under conditions of with and without co-flowing gas stream were carried out by Farago, Z. and Chigier, N. They observed

1) a Rayleigh type breakup, which is further divided into two subgroups
a) axisymmetric breakup (We < 15)
b) non axisymmetric breakup (15 < We < 25) and

2) a Membrane type breakup (25 < We < 70), where the round jet develops into a thin sheet, which forms Kelvin-Helmholtz waves and breaks up into drops

3)a fiber type breakup (100 < We < 500).

ii) based on Mixture ratio

Based on previous studies and experimental work carried out by Gomi, N., the breakup regime is classified into three categories depending on the mixture ratio (MR).

1) MR < 0.2, relative velocity determines the drop size
2) 0.2 < MR < 1, relative velocity and mixture ratio determines the drop size
3) MR > 1, many parameters affect the drop size

          The basic atomization phenomena that converts primary liquid jets into droplets is not yet fully understood. No unified theory is currently available and experimental investigations are the best way to characterize a given injection element.

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