Liquid and vapor transport phenomena in microstructured porous media for high heat flux cooling applications
Keywords: Mechanical Engineering.

Rising power densities have presented thermal management challenges to a number of industrial applications such as power electronics, laser diodes, microprocessors, and defense and space systems. Many micro- and power-electronics systems are facing difficult challenges of dissipating extremely high heat flux over 1 kW cm-2 while maintaining the component temperature below an application-driven maximum temperature. Capillary-driven evaporation and boiling from porous metal wicks has attracted increased attention due to its passive liquid delivery via capillary wicking and the potential of dissipating concentrated heat fluxes. Advances in fabrication technology have enabled the design of capillary wicking structures with microscale features. This work examines the fundamental liquid and vapor transport phenomena in microstructured porous wicks during liquid-to-vapor phase-change processes, which is essential to wick design optimization in high-performance two-phase heat sinks and heat exchangers. First, we utilize a template-assisted copper electrodeposition method to synthesis copper inverse opal structures that consist of uniform pores with three-dimensional periodicity. We demonstrate the ability to tailor the fluid permeability of microporous inverse opal structures by varying the sintering process of the sacrificial template. This template modification allows more than an order of magnitude increase in permeability associated with the widening of necks between adjacent pores of the copper structure. The ability to tune permeability of copper inverse opals for fixed feature sizes provides a useful design parameter for two-phase electronic cooling systems using porous metals as wicking structures. Second, we investigate capillary-driven boiling in copper inverse opal wicks with passive delivery of working fluid via a capillary feed. We develop a semi-analytical model to correlate key transport mechanisms to the maximum heat flux before severe heat transfer degradation occurs, including capillary wicking of liquid supply and vapor departure from the porous structures. Both experimental results and the model show a capillary-limited regime where the maximum heat flux increases as the liquid wicking distance decreases and a boiling-limited regime where further decrease in liquid wicking distance fails to significantly improve the heat dissipation capability. An optimal wick thickness can be determined based on competing transport mechanisms between liquid wicking and vapor venting within the microporous wicks. Our model assists to better understand liquid and vapor transport of capillary-driven boiling from microstructured porous metals and provide useful guidelines to wicking structure design of high-performance two-phase cooling systems. Third, we utilize copper inverse opal wicks as a platform to examine evaporation phase-change process from free liquid menisci formed on top of the wicks prior to boiling incipience. We demonstrate that evaporation in an air ambient is dominated by vapor diffusion in vapor-air mixture driven by concentration gradients and convective bulk flow. The heat and mass transfer across the liquid-vapor interface is diffusion limited as governed by the Maxwell-Stefan equation. Reducing the evaporator feature size is shown to be effective in improving evaporation flux by decreasing the diffusion boundary layer thickness. To reduce vapor diffusion resistance and promote interfacial transport during evaporation, eliminating the presence of air to create pure vapor environment should be considered.