Effects of High Frequency Droplet Train Impingement on Spreading Splashing Transition, Film Hydrodynamics and Heat Transfer

Abstract

The objective of this study is to investigate the effects of dropletinduced crown propagation regimes (spreading and splashing) on liquid film hydrodynamics and heat transfer. In this work, the effects of high frequency droplet train impingement on spreadingsplashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE7100 droplet train was generated using a piezoelectric droplet generator at a fixed flow rate of 165 mL/h. Optical and IR images were captured at stable droplet impingement conditions to visualize the thermal physical process. The dropletinduced crown propagation transition phenomena from spreading to splashing were observed by increasing the droplet Weber number. The liquid film hydrodynamics induced by droplet train impingement becomes more complex when the surface was heated. Bubbles and microscale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dryout was observed outside the impact craters under high heat flux conditions. IR images of the heater surface show that heat transfer was most effective within the droplet impact crater zone due to high fluid inertia including high radial momentum caused by highfrequency droplet impingement. Timeaveraged heat transfer measurements indicate that the heat fluxsurface temperature curves are linear at low surface temperature and before the onset of dryout. However, a sharp increase in surface temperature can be observed when dryout appears on the heater surface. Results also show that strong splashing (We = 850) is unfavorable for heat transfer at high heat flux conditions due to instabilities of the liquid film, which lead to the onset of dryout. In summary, the results show that droplet Weber number is a significant factor in the spreadingsplashing transition, liquid film hydrodynamics and heat transfer.