Experimental investigation of droplet wall-film interaction of binary systems

Abstract

A variety of industrial applications depend on impact dynamics of droplets on liquid surfaces of a different fluid, e.g. fuel post-injection in diesel engines. Hereby, the injected fuel impacts directly on the oil-wetted cylinder wall. Thus, the oil’s lubrication performance is diminished and secondary droplets are ejected back into the combustion chamber, increasing engine emissions. Here a better understanding of binary splashing dynamics is beneficial for optimizing post-injection strategies. In this work, the understanding of the underlying physical mechanisms is improved by conducting a systematic experimental study on the basic process of a single droplet impinging on a thin wall-film. The employed measurement techniques are the two-perspective and top-view high-speed shadowgraphy. Contrary to previous studies, the effect of two miscible fluids, role of wall-film viscosity, and effect of wall-film thickness on the splashing dynamics are analysed. Two mechanisms of secondary droplet ejection, crown-type splashing and crown bottom breakdown (CB), are comparatively analysed. The one-component interactions of the test fluids, Hyspin and hexadecane, are also investigated to establish reference cases. This study allows proposing a unified treatment of the splashing phenomenon. The splashing threshold is determined, showing that an increase in film thickness inhibits splashing. The crown kinematics are strongly linked to vorticity production in the crown’s neck region and dissipative losses in the boundary layer close to the solid wall. Both effects are enhanced by thin films and high fluid viscosities and lead to the ejection of the crown wall with lower angles. For thicker films these effects are quenched since viscous effects are confined to a thin sub-layer within the wall-film, which leads to cylindrical crowns. These changes strongly affect the crown top diameter and height. For V-shaped crowns the expansion of the top diameter is promoted, while for cylindrical crowns the vertical growth is enhanced. The number of secondary droplets increases with increasing impact energy and with decreasing film thickness. The volume of ejected droplets mostly exceeds 1, which indicates their contamination with wall-film liquid. CB occurs only for very thin wall-films. It results in a considerably higher number of much smaller droplets. This effect diminishes with increasing impact energy and film thickness until crown-type splashing becomes the dominant mechanism.