Onset of Nucleate Boiling and Critical Heat Flux with Boiling Water in Microchannels

This paper focuses on experimental determination of onset of nucleate boiling (ONB) and critical heat flux (CHF) at the microscales, and comparison of these with available correlations. The working fluid is deionised water and microchannel of four different hydraulic diameters: 65, 70, 107 and 125 m, have been tested. Effect of hydraulic diameter (65-125 m), mass flux (60-1410 kg/m2s) and heat flux (0-910 kW/m2) on ONB and CHF has been studied in detail. The heat flux for onset of nucleate boiling increases with hydraulic diameter and mass flux. The critical heat flux tends to increase with a decrease in hydraulic diameter and with increasing mass flux. The effect of surface roughness on CHF has also been tested to a limited extent; no clear change in the CHF value was observed upon changing the surface roughness by an order of magnitude. The empirical correlations tested in this study predict the experimental data to varying extent. These results may help better determine the lower and upper limits of heat flux while designing heat sink for electronic cooling

Metal-based microchannel heat exchangers : manufacturing and heat transfer testing

This dissertation focuses on improving the functionality of metal-based microchannel heat exchangers (MHEs), as well as pushing this technology toward real-world applications. Design optimization was carried out on MHEs for performance maximization. Double-layered microchannel layout was experimentally studied, and a significant reduction on liquid flow pressure drop penalty was achieved. Other than water, another commonly-used coolant, ethylene glycol, was applied to MHEs, and flow and heat transfer characteristics were quantified. Transient Liquid Phase (TLP) bonding was used for joining Cu structures. For further understanding of the MHE heat transfer, a detailed examination was carried out on the TLP bonding interface region. In real applications, an MHE is likely to work with a heat rejection device. Therefore, further study was done on MHEs in the context of a close-loop recirculating-liquid cooling system. An alternative roll molding method suitable for continuous fabrication of metal-based microchannel arrays was studied. This technology may serve as an enabler for large-scale manufacturing of metal-based microchannel devices in an economical fashion

Use of a porous membrane for gas bubble removal in microfluidic channels: physical mechanisms and design criteria

We demonstrate and explain a simple and efficient way to remove gas bubbles from liquid-filled microchannels, by integrating a hydrophobic porous membrane on top of the microchannel. A prototype chip is manufactured in hard, transparent polymer with the ability to completely filter gas plugs out of a segmented flow at rates up to 7.4 microliter/s per mm2 of membrane area. The device involves a bubble generation section and a gas removal section. In the bubble generation section, a T-junction is used to generate a train of gas plugs into a water stream. These gas plugs are then transported towards the gas removal section, where they slide along a hydrophobic membrane until complete removal. The system has been successfully modeled and four necessary operating criteria have been determined to achieve a complete separation of the gas from the liquid. The first criterion is that the bubble length needs to be larger than the channel diameter. The second criterion is that the gas plug should stay on the membrane for a time sufficient to transport all the gas through the membrane. The third criterion is that the gas plug travel speed should be lower than a critical value: otherwise a stable liquid film between the bubble and the membrane prevents mass transfer. The fourth criterion is that the pressure difference across the membrane should not be larger than the Laplace pressure to prevent water from leaking through the membrane