Investigation of the Primary Breakup of Round Turbulent Liquid Jets Using LES/VOF Technique

Abstract

*The disintegration of a round turbulent liquid jet issuing from a coaxial type atomizer into a high pressure chamber is numerically studied. The liquid-gas interface is tracked using Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM). Large Eddy Simulation techniques are used to resolve the large scale motions in the flow while modeling the small scale statistics, required to predict the breakup mechanism. A one equation eddy viscosity model has been used to resolve the sub-grid scale stresses, which solves a transport equation for sub-grid scale kinetic energy. The advantage of using the one equation model is its inherent ability to predict backscattering, which is essential in predicting the energy exchange between the resolved and modeled scales in the currently considered multiphase regime. Numerical computations are performed using this combined LES/VOF method to investigate the primary breakup mechanism often encountered in round turbulent jets. Due to fine mesh requirements, simulations are limited to a downstream distance of 10 nozzle diameters. Our numerical study concerns the effect of relative velocity between liquid and gas phases on the turbulent jet disintegration characteristics. Injecting liquid with no co-flowing gases results in development of short wavelength perturbations on the liquid surface by the action of aerodynamic forces leading to stripping of liquid surface to form discs, ligaments and finally droplets. At the onset disc formation, the disc protrusion length to thickness (neck width near the liquid surface) ratio of the discs measured 1.9, while the ratio of disc width to jet diameter is observed to vary between 0.2-0.4. With co-flowing gas of equal velocity magnitude, suppression of liquid-gas interface instability occurs. The interface stretching and the ligament alignment configurations are modified due to decreased radial spread. Primary breakup mechanism involving disc base breakup and disc tip (ligament type) breakup have been discussed. The ligament characteristics arising from different flow configurations are clarified. I. Introduction The mechanism of primary breakup of turbulent liquid jets is of fundamental importance in various industrial processes such as spray coating, combustors, metal powder formation etc. The atomization process of liquid jets is thought to consist of two consecutive steps: primary and secondary breakup. During the primary breakup, the liquid jet exhibits large scale coherent structures that interact with the gas-phase and break into both large and small scale drops. Proceeding downstream, the drops formed due to primary breakup split further into much smaller drops. This mechanism is called secondary breakup process. The process of atomization occurs in the turbulent flow environment which results in the presence of wide range of length and time scales of motion. The foresaid variation in length and time scales differentiates the treatment of primary and secondary flow structure during atomization. The characteristics of primary breakup has significant influence on the properties of dispersed phase affecting the mixing rates with the surrounding gas, mechanisms of secondary breakup and droplet collisions, among others. Over the years, many researchers have identified that the disintegration characteristics of round turbulent liquid jets, is influenced by several parameters such as nozzle design (influence of internal turbulence, cavitation etc), injection conditions, liquid and ambient gas properties etc. The mechanism of primary breakup was earlier observed by De Juhasz et. al 1 and Spenser 2 . Further studies taken up by Grant and Middleman 3 , McCarthy and Malloy 4 identified liquid turbulence properties behind the jet instability and subsequent breakup. Later, Hoyt and Taylor 5 concluded that the drop formation due to turbulent primary breakup was associated with formation of discs and ligaments along the liquid surface and that aerodynamic effects were generally of secondary importance for turbulent primary