Hydrodynamic and heat transfer characteristics of droplet train spreading-splashing transition on heated surface

Abstract

Heat transfer and hydrodynamic characteristics of droplet-induced spreading-splashing transition have been investigated experimentally and numerically. Specifically, the study involved the characterization of a train of monodispersed micron-sized droplets impinging on a heated surface covered by a thin liquid film at different droplet Weber number values. A single stream of mono-dispersed droplets 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 was coated on a translucent sapphire substrate, which was used as a heating element. High-speed optical imaging and infrared thermal imaging techniques were used to characterize the hydrodynamics and heat transfer of droplet train impingement, respectively. It was observed that at low Weber number (We = 280), droplet impingements resulted in smooth spreading of the droplet-induced crown while splashing was observed at high Weber number (We > 489) cases. The effect of spreading-splashing transition on surface heat transfer has also been investigated at fixed flow rate conditions. Time-averaged temperature measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature was observed when dry-out appeared on the heater surface due to phase change. It was also found that strong splashing is unfavorable for heat transfer at high surface temperatures due to the instabilities seen in the liquid film, which leads to the onset of dry-out. Numerically, the Coupled Level Set-Volume of Fluid approach was used successfully to capture the hydrodynamic and thermal aspects of droplet train impingement on a heated surface. Furthermore, the dynamic mesh adaption technique was used in all simulations, which is capable of capturing the propagation of droplet-induced crown with high spatial and temporal resolutions. A good agreement was reached between experimental and numerical data in terms of droplet-induced crown morphology and surface temperatures. Thermal boundary layer displacement thickness profiles from numerical analysis were used to understand the coupled nature of hydrodynamics and heat transfer within the thin liquid film. In summary, the results indicate that droplet Weber number is a significant factor in spreading-splashing transition and droplet-induced liquid film heat transfer.