Taolue Zhang

Effects of High Frequency Droplet Train Impingement on Crown Propagation Dynamics and Heat Transfer

The objective of this study is to investigate the hydrodynamics and heat transfer phenomena due to high frequency droplet train impingement on a pre-wetted solid surface for electronic cooling applications. The effects of crown propagation dynamics and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of mono-dispersed HFE-7100 droplets was generated using a piezoelectric droplet generator at a frequency ( f ) of 6000 Hz with a droplet Weber number (We) of 280. Droplet-induced crater and crown were imaged using a high speed camera system. Numerically, the ANSYS Fluent CFD tool was used to simulate the droplet train impingement process. A reasonable agreement was reached between experimental and numerical data in terms of crown propagation dynamics. Numerical simulations reveal that at the instant of initial spot formation, the magnitude of droplet velocity is almost identical to the crown’s radial velocity. The instantaneous temperature field obtained by numerical simulations shows that heat transfer was most effective within the crown propagation region due to the radial momentum generated by the droplets, which leads to a large velocity gradient within the liquid film. A significant increase in surface temperature was observed beyond a radial position of 500 . In summary, high frequency droplet impingement leads to a very small temperature gradient in the radial direction within the droplet-induced impact crater. This study will benefit in understanding the relationship between the droplet parameters and surface heat transfer for different cooling applications involving impinging droplets.

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

The objective of this study is to investigate the effects of droplet-induced 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 spreading-splashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE-7100 droplet train was generated using a piezo-electric 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 droplet-induced 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 micro-scale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dry-out 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 high-frequency droplet impingement. Time-averaged heat transfer 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 can be observed when dry-out 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 dry-out. In summary, the results show that droplet Weber number is a significant factor in the spreading-splashing transition, liquid film hydrodynamics and heat transfer.

Effects of Screen Laminates on Droplet-Induced Film Hydrodynamics and Surface Heat Transfer

Formation of dry-out area could lead to a sharp increase in surface temperature during droplet impingement cooling. The objective of this study is to figure out an effective way of suppressing dry-out formation in droplet impingement cooling. In this work, HFE-7100 droplet train was produced using a piezo-electric droplet generator at a frequency of 6000 Hz with a droplet Weber number of 280. A translucent substrate was coated with a thin film ITO, which was used as a heater in the experiments. A copper screen laminates with a single punched hole (diameter = 3 mm) was placed over the heater surface at a distance of 0.3 mm to enhance surface heat transfer. Optical images showed that screen laminates effectively suppressed the formation of the dry-out area. It was also found that heat transfer was greatly improved when screen laminates were used. The heat transfer improvement could be attributed to the enhanced surface tension effects, which keep the whole surface wet at high surface temperatures. In summary, the results show that screen laminates effectively suppress the formation of dry-out area and greatly improve surface heat transfer.