دانلود Numerical simulation of the breakup of a round liquid jet by a coaxial flow of gas with a subgrid Lagrangian breakup model

Abstract

Motivation and objectives

One of the challenging problems in computational physics is to devise an integrated simulation of multiscale and multiphysics phenomena commonly found in nature. One important example is the multiphase phenomenon typically encountered in gas-turbine engines or internal combustion engines. Combustion of liquid fuels involves various physical processes such as liquid film atomization into droplet, evaporation, and turbulent mixing with the ambient gas when the combustion occurs. The atomization of a liquid film or jet can be divided into two subsequent processes: i.e., primary atomization followed by secondary atomization. The primary atomization is the initial breakup of the liquid jet into large and small liquid structures close to the injection nozzle. It involves complex interface topology of large coherent liquid structures. The secondary atomization is the subsequent breakup into smaller drops forming sprays. As distinct physical processes are involved in each regime, different modeling strategies are needed. The objective in the present study is to apply an appropriate physical model that captures each flow regime and to integrate the resulting model into a computational simulation of the whole combustion process.The primary atomization is dominated by the interaction of the liquid film with the surrounding gas phase, giving rise to liquid surface instability waves. These interfacial instabilities are important in the overall spray evolution and droplet formation process.However, the dynamics of the phase interface is highly complex, poorly understood,and remains an unresolved problem in the area of atomization simulation. Atomizationsimulation is generally performed based on Lagrangian particle tracking and secondary breakup mechanisms, assuming that the typical size of the liquid drops is much smaller than the available grid resolution and that the shapes of the individual drops are similar to spheres or ellipsoids (Reitz 1987; Tanner 2004; Apte et al. 2003). Although these models can successfully predict secondary breakup, they are not applicable to the primary breakup regime because their underlying assumptions do not hold for the primary breakup regime where the length scale is an order of injector nozzle size.In the primary breakup regime, the liquid fluid interacts with the surrounding turbulent gas phase, which results in complex topological shapes of the interface. The large-scale coherent structures that interact with the gas phase will disintegrate into filaments and breakup into droplets. Thus, if we are to capture the physical process that occurs at the phase interface, it must first be identified and tracked. To this end, the level set method coupled to the Navier-Stokes equation is employed in the present study. In order to correctly capture the breakup phenomenon of phase interface, the smallest length scale of the phase interface should be larger than the grid size in the level set grid.