Date of Award
Summer 8-15-2022
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
The rapid development of electronic systems with ever-higher power densities in a wide range of applications requires new advanced thermal management methods. Droplet-based two-phase thermal management technologies are considered promising solutions to conquer the cooling challenges in the electronic industries. The heat transfer behavior of droplets is based on several important fundamental processes, such as droplet evaporation, droplet impact on heated surfaces, and molten droplet impact. In this dissertation, four research projects are completed to explore the insights of these fundamental processes. For droplet evaporation, I introduce an investigation of diffusion confinement of droplets evaporating on a supported pillar structure. We investigate the diffusion confinement effect imposed by the bottom substrate and the side wall of the micropillar through numerical simulations and experimental investigation. We find that raising the micropillar height from 0 to 2R, 5R, and 20R led to 26%, 35%, and 42% enhancements, respectively, in the total evaporation rates of a hemispherical droplet under an isothermal condition. Moving from the static evaporation process to a dynamic process, I experimentally study the low-Weber number droplet impact on heated hydrophobic surfaces in the film evaporation regime. Denoting the drop diameter and impact velocity as D and v, we find that the total transferred heat Q scales as ~D1.25v, which is validated using experiments. A unique feature of low-We droplet impact on non-wetting surfaces is the formation of a sub-millimetric entrapped bubble that forms during receding. We find that the overall heat transfer is reduced by 5.6% and 7.1% at surface temperatures of 50℃ and 65℃, respectively, as the entrapped bubble reduces the total liquid-solid interface area. After studying the droplet impact under non-boiling conditions, I continue to explore the droplet impact on a heated post in the nucleate boiling and Leidenfrost regimes to investigate the influence of the surface structure on the heat transfer and hydrodynamics. We find that the post substrate leads to a shorter droplet lifetime and a 20℃ higher dynamic Leidenfrost temperature compared to a flat substrate, attributed to mixed boiling modes along the height of the post and additional pinning. In the nucleate boiling regime, the droplet impact on the post substrate shows an up to 24% larger cooling capacity due to the additional liquid-solid interface area. Finally, I experimentally investigate the heat transfer and solidification mechanisms during molten paraffin droplet impact using synchronized high-speed optical and infrared (IR) imaging. The contact line heat transfer is found to be nearly constant during the spreading process after impact. The overall heat transfer is increased by higher impact Weber numbers due to a larger spreading area. In addition, when replacing the low-conductivity paraffin droplet with Field’s metal, we find a higher local heat flux near the center region compared to the contact line region due to the higher conductivity of the metal droplet. Overall, this dissertation presents new fundamental insights into droplet-based two-phase systems, including droplet impact, droplet evaporation and boiling, and droplet solidification. The research outcomes enhance our understanding of static and dynamic phase change processes and provide fundamental theories for developing droplet-based two-phase thermal management systems.
Language
English (en)
Chair
Patricia Weisensee
Committee Members
Ramesh Agarwal, David Peters, Richard Axelbaum, Mark Meacham,