Microscale layering of liquid and vapor phases within microstructures for a new generation two-phase heat sink.

In this study, a new heat sink architecture is introduced that operates at two different phase change heat transfer modes. At low wall superheat temperatures, the heat sink operates under the thin film evaporation heat transfer mode and then transitions to the boiling heat transfer mode when the wall superheat temperature increases. This unique function is enabled through constraining the liquid and vapor phases into separate domains using a capillary-controlled meniscus formed within a hierarchical 3D structure. The structure is designed to form thin liquid layers between vertically oriented menisci across which the liquid is evaporated into neighboring vapor channels. The entire structure is then capped by a hydrophobic vapor-permeable membrane to fully confine the liquid layers while allowing vapor to pass through the membrane. At low wall superheats, the liquid layers directly evaporate into the vapor channels. In this operation mode, the heat flux was linearly increased to a maximum of 54 W/cm 2 at approximately 8 °C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 62 kW/m 2 K. The heat transfer coefficient only slightly declined with increasing the wall superheat temperature but substantially improved as the liquid supply pressure was increased. Increasing the superheat temperature beyond 7–9 °C resulted in transition to the boiling heat transfer mode with a pronounced increase in surface temperature fluctuations. This transition to boiling results in a decline in the heat transfer coefficient because the meniscus formed between the liquid and vapor spaces breaks down. However, the heat removal capacity is significantly increased, and a critical heat flux of about 300 W/cm 2 is reached at <30 °C wall superheat temperature, corresponding to a heat transfer coefficient of approximately 100 kW/m 2 K. [ABSTRACT FROM AUTHOR]