Date of Award

8-16-2024

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

The rapidly increasing power density of electronic systems has raised challenges for thermal management. Efficient heat spreading plays an important role in distributing high-density heat to the large-area heat sink and maintaining the junction temperature below the temperature limit during operation. Vapor chambers, a type of flat heat pipe utilizing two-phase heat transfer and fast travel of vapor flow, can achieve far more efficient two-dimensional heat spreading than most conduction-based thermal ground plane materials. By promoting dropwise condensation in vapor chambers, the thermal performance and scalability can be further improved due to the enhanced condensation heat transfer and unique droplet dynamics. However, an industrial-ready dropwise vapor chamber has yet to be developed mainly due to the lack of feasible dropwise coating. Recently, the few-layer graphene nanostructure on nickel substrate has demonstrated remarkable sustainability, indicating the great potential for implementing dropwise condensation in two-phase heat spreaders.

This dissertation demonstrated a feasible approach for enhancing two-phase heat spreading in vapor chambers through sustainable dropwise condensation enabled by the nickel-graphene coating. Initially, a demountable vapor chamber structure was built to visualize the fluid dynamics and demonstrate the working mechanism within a dropwise vapor chamber (DWVC). Two liquid pathways enabled by droplet dynamics were identified in visualization. Subsequently, a hermetically sealed dropwise vapor chamber was designed and fabricated using brazing. The structure integrity and non-wetting property of the graphene coating were sustained after the high temperature brazing process, demonstrating its high feasibility for two-phase heat spreaders.

Comprehensive experimental investigations were conducted to characterize the thermal performance of DWVC. The results showed minimal impact of orientation on the vapor chamber performance. Benefitting from enhanced condensation and liquid return mechanisms, the DWVC achieved a notable 34% reduction in thermal resistance compared to a filmwise vapor chamber (FWVC). Additionally, unique temperature distribution and fluctuation were observed due to the periodic liquid return and nonuniform condensation in DWVC. The conductance-based vapor chamber models were developed by incorporating filmwise and dropwise condensation models. Key parameters and assumptions were discussed based on the comparison between the simulation results and experimental data.

Furthermore, the dropwise vapor chamber was integrated with liquid cooling in a hybrid cold plate, and the thermal performance was evaluated experimentally. Owing to the integration structure and highly efficient dropwise condensation, this compact hybrid cold plate demonstrated high efficiency in dissipating high-density heat, achieving a remarkable heat flux of 167.0 W/cm2 with a modest cooling water supply of only 10 GPH. Lastly, perspective and discussion of future work were presented with a focus on the improvement of evaporator performance and liquid removal to address the nonuniform condensation phenomena in DWVC. This work demonstrated the enhanced two-phase heat spreading achieved through the implementation of sustainable dropwise condensation via the nickel-graphene coating, offering a promising solution for efficient thermal management in high-performance electronics systems.

Rights

© 2024, Kai Luo

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