Experimental Study Of Droplet Impact On Heated Surfaces

The impact of droplets is a ubiquitous phenomenon,and reducing the residence time of the impact process is important for many potential applications,such as antiicing,self-cleaning and corrosion-resistance.The understanding of droplet impact on heated surface in the film boiling regime is not complete.In this study,by using highspeed imaging technique,the impact dynamics on heated surface is captured,and the reduced residence time induced by heat and the multiple contact of the droplet bottom liquid with the surface are the main concerns.In the first part,a mode of droplet bouncing(bouncing-with-spray mode)is identified,it can reduce the residence time significantly compared with the traditional retraction-bouncing mode.Comparing with other strategies to reduce the residence time,this approach induced by heat is simple and reliable.The reduction in the residence time is due to the burst of vapor bubbles in the liquid film,which results in the formation of holes in the liquid film and consequently the recoiling of the liquid film from the holes.A scaling law is proposed for the transition boundary between the retractionbouncing mode and the bouncing-with-spray mode in the film boiling regime,and it agrees well with the experimental data.This model can also explain the transition between these two modes in the transition boiling regime.In the second part,a detailed study on the film boiling regime is made.Generally speaking,in the film boiling regime,as the droplet retracts its periphery,its bottom liquid will not contact the surface because of the existence of vapor layer beneath the droplet.In the study,for a higher surface temperature in the film boiling regime,a special situation where the bottom interface of the droplet can oscillate multiple times during the droplet retracting is identified,and with the increase of impact velocity,a single interface oscillation is observed.Further more,the more detailed phase diagram is provided and the parameter range of multiple oscillation is identified.The dimple depth beneath the droplet decide whether there is multiple interface oscillation.Finally,the oscillation period is studied,it shows that as the times of the interface oscillation increase,the oscillation period decreases,with the decrease of droplet diameter,the oscillation period can decrease overall,and it can increase with the decrease of the surface tension.