The hydrodynamic details of droplet-droplet and droplet-liquid film interactions on solid surfaces are believed to have a significant role in spray impingement phenomena, yet details of this interaction have not been clearly identified. The interaction among the droplets during impact affects their residence time on the surface, spreading, and droplet and liquid film stability. After impact, droplet interactions affect droplet collisions, coalescence and liquid splashing, This interaction affects secondary atomization and the droplet dispersion characteristics of the impingement process. In this study, details of droplet-droplet and droplet-liquid film interactions in solid surface impingement have been visualized using high speed photography. The effects of these interactions on secondary atomization and droplet dispersion have been quantified.
Parcels of accurately spaced droplets were impinged on a heated surface and the parcels were characterized by time and length scales and by their void fraction. The impingement surface temperature was varied to examine different heat transfer regimes, from saturation through nucleate boiling to film boiling. Single droplets and multiple droplet impingements were used to show the difference in the characteristics of an isolated droplet impingement compared to the simultaneous impingement of more than one droplet. This arrangement was meant to provide some information to bridge the gap between isolated droplet behavior and full sprays.
This study concludes that for the liquid films formed in the saturation regime, liquid film breakup depends on the impinging droplet(s) energy content, the level of the liquid film surface disturbance and on the liquid film thickness. In the transition and film boiling regimes three major variables affect the interaction process: parcel configuration (droplet number and spacing), Weber number and surface superheat. In these boiling regimes multiple droplet impingement generally increases the number of ejected droplets, but also enhances coalescence due to the increased secondary atomized droplet density. Film boiling conditions produce a reduced normal rebound velocity. Due to vapor bubble formation and blow-off in the transition boiling regime, the droplet normal rebound velocity for transition boiling is an order of magnitude higher than that in the film boiling regime.
