Fluid dynamic and autoignition characteristics of early fuel sprays using hybrid atomization LES

Abstract

Early fuel spray development and subsequent autoignition are investigated using our turbulent atomization model that has enabled physically-closed primary atomization modeling. Thanks to its modeling methodology, no parameter tuning or adjustment is needed to simulate a turbulent spray. Due to turbulent atomization, a dense droplet layer is formed over the injected liquid core. With evaporation, this region has a triple layer structure, which is composed of the inner layer with many droplets, the vaporization layer and the outer layer exposed to the hot air. With combustion, hot spots are generated in the outer layer, while the inner layer is too cold and hinders the hot gas to penetrate inside. The group combustion number indicates that this is sheath evaporation/combustion mode and its structure is preserved down to the liquid core length. The liquid core thinning is determined by atomization characteristics and under evaporative/reactive conditions where the outer gas temperature is high and thus the kinematic viscosity is also high, the flow motion is larger and the core is unsteadily affected by this motion. Due to this, the Rayleigh–Taylor (RT) atomization mode is further excited and core breakup occurs relatively faster. It is also observed that the unsteady core head flapping motion leads to non-uniform distributions of droplets and vapor. Due to this non-uniformity, the initial autoignition in the outer layer does not occur uniformly. After core breakup, the droplet layer rolls up in the front region and the group combustion number becomes lower, where external group combustion behavior is observed. Multistep chemistry improves the ignition delay estimation compared with the global chemistry, but the spray dynamics is mostly determined by the above fluid dynamic effects. Therefore, the present study indicates that the accurate simulation capability of fluid dynamics, i.e. primary atomization and early spray development, is important in predicting spray combustion behavior.