Dynamics of liquid structures in turbulent, moderately dense spray jets

Spray jets of intermediate optical density (between dilute sprays and hard liquid cores) are of interest due to their practical as well as conceptual relevance. Such flows may span broad regions from the primary atomization zones near the injector tip to the downstream parts dominated by dispersed sprays. Understanding the dynamics of the liquid structures in this transitional domain is important, since it is likely to impact the quality of the flow further downstream. Conceptually, sprays jets of intermediate optical density are also intermediate in complexity and form a relevant model problem for advancing the computations of such flows. This paper uses a well-designed, air-assisted atomizer, where the liquid is injected from a needle that may be recessed upstream relative to the jet exit plane into the surrounding air-blast co-flow. The spray at the jet exit plane is fully dense when the needle is flush, but transitions to intermediate density as the needle is gradually recessed until it is dilute at suffi-ciently large recess distances. Sprays induced by various recess locations of the injection needle are studied. High-speed, microscopic shadowgraph imaging is employed to monitor the evolution of liquid fragments in the flow. These are classified into three categories: spherical droplets, ligaments, and irregular shapes. The statistics of each of these shapes are monitored at various axial locations in the jet and it was confirmed that the size and probability of their occurrence depend on the initial Weber number, fuel to air mass ratio and recess distance.8 page(s