In this paper we present an overview of the experimental work carried out as part of research geared towards the understanding of two-phase flow in microchannels. The greater scope of the project is to use the knowledge gained towards the development of strategies to improve water management in fuel cell applications. We have conducted pressure versus flow rate experiments in microchannels with contrasting hydrophobic characteristics and under different liquid water injection conditions. These measurements have been complemented with flow visualization studies using white light and fluorescence. As expected, parameters associated to surface energy such as hydrophobicity have a big influence on the flow. Under hydrophobic conditions the formation of slugs or blobs of size comparable to that of the microchannel greatly impedes the flow of air, especially at low pressure drops. On the other hand liquid water effects under hydrophilic conditions are only noticeable at large injection rates (100 μL/min). In contrast to their hydrophobic counterparts, two-phase flow in hydrophilic microchannels is characterized by the formation of a thin film of liquid. Only when the thickness of the film becomes substantial does it have an effect on the air flow.
This work designs and fabricates a microchannel structure for measurement of wall temperature fields in two-phase flow. The microchannel with hydraulic diameter of 100 micrometers is etched into a suspended beam of silicon with three independently heated regions and integrated doped silicon resistors sensitive to channel temperature. Doped silicon resistors are also sensitive to strain in the silicon caused by pressure transients in the channel, so sensors are designed with two different orientations and thus two different piezoresistive coefficients to allow decoupling of pressure and temperature effects. Use of a 400 micrometer wide suspended beam reduces side-wall conduction compared to a bulk sample and provides better opportunities to measure the influence of flow regimes on heat transfer coefficients in future work. Use of the central heater reduces fluid preheating in the inlet plenum. The measured temperature distributions at flowrates up to 0.25 ml/min with heat fluxes into the silicon beam up to 78 W/cm 2 show initial capabilities of the structure.
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