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Two-phase Flow in High Aspect-ratio Microchannels

  • Author(s): Lewis, James
  • Advisor(s): Wang, Yun
  • et al.

Two-phase flow in a microscale channel with a characteristic pressure drop, flow pattern, and liquid saturation occurs in a variety of applications. Understanding the behavior of two-phase flows and predicting the two-phase pressure drop can aid in the development and assessment of engineering designs; PEM fuel cells serve as a test case for this work. The experimental investigation of this work consists of measuring the two-phase pressure drop in a high aspect-ratio microchannel of dimensions 3.23mm wide by 0.304mm high combined with visualization of the two-phase flow. The tested superficial velocities represent common fuel cell operating conditions. In the first case, the microchannel consists of hydrophilic surfaces and the flow forms a stratified pattern. The two-phase pressure measurements led to the determination of a new relative permeability exponent (Wang, 2009) for stratified flow equal to 1.159, which predicts the two-phase pressure with a mean absolute percent error of 3.25%. The X-model and the correlations proposed by Lee & Lee (2001) and Kim & Mudawar (2012) well predict the experimental data with mean absolute percent errors of 3.32%, 4.1%, and 4.2%, respectively. Measurements of the water film thickness extracted from images of the flow follow the analytical result of Steinbrenner (2011). The new relative permeability exponent reasonably predicts the liquid saturation (dimensionless water film thickness).

In the second case, the base of the channel becomes hydrophobic while the other walls remain hydrophilic. This configuration forms a mixed-wettability microchannel, similar to the gas-supply channels found in PEM fuel cells. The change of the base wettability results in primarily rivulet flow along the hydrophobic surface and an increase in the two-phase pressure drop compared to the hydrophilic case. Researchers have presented inconsistent trends of the two-phase pressure drop with changing contact angle. This work proposes a critical liquid Capillary number in the range of 1.38×10 −4 to 9.63×10 −4 to clarify the trend for adiabatic flow in a single mixed-wettability microchannel. Furthermore, the work demonstrates the variability of the flow in the mixed-wettability channel. This suggests an instability of the rivulet flow, which prevents existing two-phase pressure models from collapsing the data. Optimizing the relative permeability exponent in the two-fluid model for rivulet flow led to a value of 1.747, which predicts the two-phase pressure measurements with a mean absolute percent error of 14.9%. The existing correlations of English & Kandlikar (2006) and Sun & Mishima (2009) predict the entire experimental data set with mean absolute percent errors of 22.7% and 23.7%, respectively.

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