Two-phase microchannels promise an efficient method to dissipate heat from high performance electronic systems by utilizing the latent heat of vaporization during the phase-change process. However, phase-change in microchannel heat sinks leads to challenges that are not present in macroscale systems due to the increasing importance of surface tension and viscous forces. In particular, flow instabilities often occur during the boiling process, which lead to liquid dry-out in the microchannels and severely limits the heat removal capabilities of the system. We propose a microscale breather device consisting of an array of hydrophobic breather ports which allow vapor bubbles to escape from the microchannels to improve flow stability. In this study, we use the combination of microfabricated structures and surface chemistry to separate vapor from the liquid flow. We designed test devices that allow for cross-sectional optical visualization to better understand the governing parameters of a breather design with high vapor removal efficiencies and minimal liquid leakage. We examined breather devices with average liquid velocities ranging from 0.5 cm/s to 4 cm/s and breather vacuum levels between 1 kPa and 9 kPa on the maximum gas removal rate through the breather. We demonstrated successful breather performance. In addition, a model was developed that offers design guidelines for future integrated breathers in microchannel heat sinks. The breathers also have significant promise for other microscale systems, such as micro-fuel cells, where liquid-vapor separation can significantly enhance system performance.
A review on heat transfer and hydrodynamic characteristics of nano/microencapsulated phase change slurry (N/MPCS) in mini/microchannel heat sinks