Characterization of Impingement Dynamics of Single Droplet Impacting on a Flat Surface 2019-01-0064

The liquid fuel spray impingement onto surfaces occurs in both spark ignited and compression ignited engines. It causes a fundamental issue affecting the preparation of air-fuel mixture prior to the combustion, further, affecting engine performance and emissions. To better understand the underlying mechanism of spray interaction with a solid surface, the physics of a single droplet impact on a heated surface was experimentally investigated. The experimental work was conducted at four surface temperatures where a single diesel droplet was injected from a precision syringe pump with a specific droplet diameter and impact velocity. A high-speed camera was used to visualize the droplet impingement process. Images from the selected test condition (We = 52 to 925, Re = 789 to 3330 based on initial droplet impingement parameters) were analyzed to qualify the impinging outcomes and quantify the post-impingement characteristics. In particular, splashing characteristics and the effect of the surface temperature on the temporal evolution of the droplet spreading factor, dynamic contact angle, contact line velocity, as well as heat flux were studied. The hydrodynamics of single droplet impingement on a heated surface was also studied in terms of the transition conditions. Building on the experimental results of droplet impingement on a solid surface, a further comprehensive understanding on the splashing criteria was given and a new correlation of splashing threshold was found. The results comparison between the low and high surface temperatures showed that surface temperature introduced complexity to the analysis of droplet-surface impingement phenomena due to the change of liquid properties.