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Flow boiling instabilities in Single Microchannels


Boiling in microchannels is considered an asset in the removal of heat to the microscale level, as it provides an efficient way of fluid movement, due to the smaller necessity of pumping power required when compared with single-phase liquid flow to achieve a certain amount of heat removal. Also, the higher ability of heat removal when compared with single-phase liquid flow is of major importance.


Nevertheless, two-phase flow boiling is still poorly understood by researchers, namely, flow instabilities and their influence on heat transfer. The processes of heat transmission and hydrodynamics are different from those recorded for the macroscale, and as such, only some of the knowledge can be applied directly. An important aspect of flow boiling in microchannels is the fluctuation of pressure, because it can lead to instabilities.


Two-phase flow instabilities become undesirable as they promote temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations, channel dry-out and reverse flow, all with highly adverse effect on heat transfer in the microchannel cooling system, hence the need for these phenomena to be well understood and predicted.

Design, Fabrication and Test of an Integrated

Multi-microchannel Heat Sink


Improvement of state-of-the-art experimental capabilities is a requisite to further progress current knowledge of phase changeheat transfer in channels which can contribute to enhance the use of microchannel heat sinks as a viable cooling technique for high power density devices.


In this paper, MEMS fabrication techniques are used to assemble an integrated multi-microchannel evaporative cooling system (iMMECo) with optical access to the coolant flow on a 400 ± 5  micrometer thick <100> oriented 6 in. p-type silicon wafer(Si-Mat®). The double-side polished wafer has 1000 Å thermal SiO2on both sides, which serves two purposes: to protect the wafer sur-face during processing and to insulate it electrically.


The fabrication process follows innovative procedures to integrate Ru thermal sensors and an Al heater on the chip front side and microchannels on the backside at the same that guaranteeing a uniform distribution of the heat flux and precise alignment of the sensors with the microchannels.


The thermo-fluid-dynamic performance of the integrated multi-microchannel evaporative cooling device (iMMECo) is assessed heating the substrate with uniform wall heat fluxes up to10.8 W cm−2 and making use of a hydrofluoroether coolant (HFE-7000) with Reynolds number, Re, up to 640. Furthermore, the anodic bonding has been submitted to pressure cycles by flowing compressed air in the microchannels.

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