Experimental Investigation and Visualization of Two-Phase Flow and Water Transport in Microchannels

Abstract

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.

Two-Phase Microfluidics for Semiconductor Circuits and Fuel Cells (Keynote)

“…The wealth of experimental and modeling work has allowed the characterization of some interesting phenomena in twophase flows for both microchannel cooling and fuel cell water management applications. Axial temperature distribution [4], stratified flow film thickness [22,24], and water slug detachment criteria [29] are among the critical parameters investigated for these applications. Figure 10 shows a comparison of the experimental and simulated temperature profiles for a single 50 µm-wide by 70 µmdeep by 20 mm-long microchannel under different input powers and a flow rate of 0.1 mL/min [4].…”

“…Thus, samples are fabricated with distributed water injection slots or channels (see Figure 2). The architecture of the injection geometry allows for varied studies, where a single injection slot sample can be used to isolate the mechanics of droplet entrainment, while more involved geometry incorporating multiple injection slots are useful in understanding the global interactions of the liquid with the gas [22].…”

“…A Roper Scientific 12-bit CoolSNAP ES CCD camera is used for image capture with 4X and 10X objectives. A metal halide lamp and appropriate filter cubes are used for fluorescence purposes (mainly Fluorescein) [22]. Figure 4 shows a schematic of the microscope configuration in fluorescence mode.…”

1D Homogeneous Modeling of Microchannel Two-Phase Flow With Distributed Liquid Water Injection From Walls

“…For the friction factor, we use the value obtained by experiments [8], which is: Equation 14applies to the average pressure drop across the channel but is applied locally in the current model. It should be pointed out that we are using an experimental correlation and there is discrepancy with the expected theoretical value (which would be…”

“…In other words, there is an inverse relationship between the inlet pressure and the effects of liquid water injection on air flow velocity and pressure drop. The model predictions are compared with experimental results obtained by testing a set of microchannels with a varying number of water injection slots on the side walls (see [8] for more details). A series of microchannels with water injection slots in the side walls are used for this purpose.…”