Experimental investigation on the spreading progress after droplets impacting on to a vertical vibrating plate at low frequencies

The impingement of fuel droplets on the piston surface would greatly affect the combustion process and emission generation of direct injection diesel engines. In this paper, to mimic the interaction between the droplets and vibrating engine body, the dynamic behaviors of droplets impacting on a vertically vibrating plate were explored experimentally. The main target of this paper is the spreading process from the droplet attaching the plate to it reaching the maximum diameter. Several groups of impacting heights and plate vibrating frequencies were set to obtain a broad range of Weber numbers. The temporal evolutions of the inner lamella and rim after impact were traced by high-speed photography. The results show that the maximum spread time of the inner lamella maintains a relatively stable value, and the whole spreading time depend on the growth time of the rim. The fluctuation range of the growth time of the rim increases under increased vibrating frequency, while decreases with the increase of impacting height. The fluctuation range of the thickness of rim would be smaller with higher impacting height, and the relationship between growth time of rim and impact phase is cosinoidal at the same impacting Weber number. The effect of acceleration plays a key role in the development of the liquid layer. In addition, the maximum spread radius would be smaller for droplets with an impacting height of 15 cm, because the ring-shaped rim would receive more resistance from viscous forces and surface tension than fingers. Finally, an empirical formula based on Srivastava's model was built to accurately predict the maximum spread factor of droplets after impacting on the vibrating plate, in which the influence of the impact phase, vibration frequency, vibration amplitude and Weber number were incorporated.