Thermofluidic Visualization of the Capillary-Driven Boiling for Design Optimization of Silicon-Embedded Evaporators

Abstract

Two-phase embedded microcoolers emerge as a promising solution to high-performance and highly efficient cooling for microprocessors and power electronics. We characterized capillary-driven embedded microcoolers with silicon microchannels (width = 120 $\mu $ m, height = 200 $\mu $ m, and spacing = 480 $\mu $ m) coated with 20- $\mu $ m-thick, porous hydrophilic copper dendritic microstructure and a heated area of $5^{2}$ mm². We implemented a two-way (1-D) and four-way (2-D) liquid delivery scheme to the evaporator, achieving improved critical heat flux (CHF) levels of 370 versus 450 W/cm² and superheat level of ~10 °C, leading to a heat transfer coefficient (HTC) of 300 000 versus 450 000 W/m ${}^{2} {\cdot}^{\circ }$ C (or R ${}^{\prime \prime } \sim ~0.027$ versus 0.022 cm ${}^{2} {\cdot}^{\circ }$ C/W). To provide additional insight into the physics of capillary-driven boiling, bubble dynamics, and dry-out formation, we conducted infrared (IR) thermal and high-speed flow visualization imaging. An important finding is related to the delayed dry-out of ~300 ms in capillary-based microcoolers compared to a much shorter timescale of 210 ms for the two-phase pumped flow boiling in smooth silicon microchannels. The result demonstrates the potential of the capillary-based two-phase microcooler for transient thermal management. For future work, we will consider optimizing the microchannel dimensions, in particular, width and spacing, and introducing hydrophilic copper dendritic microstructure at the walls of the microchannels.