The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method . The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface.
Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number (We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range (We = 400500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crowns rim. At high droplet Weber number (We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values.
The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing (We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.