Large Eddy Simulation of Atomizing Sprays
Abstract: Liquid-gas two-phase flows occur frequently in daily life, in nature e.g. falling rain drops or waves in the sea, and in industrial applications, e.g. fuel injection in Diesel engines or gas turbines. For many of these applications the interaction between liquid and gas is crucial for their efficient operation. In this work liquid-gas interaction is studied numerically, specifically for atomizing flows. Large Eddy Simulations (LES) are performed to obtain time-dependent results. The disintegration of a liquid jet and the formation of small spherical droplets is simulated using the Volume of Fluid (VOF) method. The development of liquid sprays, the breakup into smaller droplets, the evaporation and mixing with the surrounding gas is simulated using the Lagrangian Particle Tracking (LPT) method. This thesis focuses a) on improving the understanding of the underlying physics and b) improving the numerical modelling techniques of atomizing two-phase flows. Concerning physics of two-phase flows, it is studied how variations of the liquid properties, such as density, viscosity or surface tension influence the primary breakup of the liquid jet, the spray development and the liquid-gas mixing. It is shown how viscosity and surface tension stabilise the liquid structures, while higher liquid density leads to faster liquid breakup. Also pulsed injection and its effect on primary breakup and the spray development is studied. The formation of a entrainment wave is indicated which propagates faster than the jet itself and enhances liquid-gas mixing. Concerning numerical modelling of two-phase flows, a novel method is developed to couple a VOF simulation of the jet atomization with a LPT simulation of a dispersed spray. The method is based on a statistical coupling which shows to be accurate and to improve significantly the computational efficiency compared to existing VOF-LPT coupling methods. Also, a novel approach to correlate spray data obtained from measurements to numerically obtained spray data is proposed. Instead of only global spray parameters, a complete field of a variable, the light extinction coefficient, is compared.
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