Water Slug Formation and Motion in Gas Flow Channels: The Effects of Geometry, Surface Wettability, And Gravity

Abstract

Water emerging from ∼100 μm pores into millimeter-size gas flow channels forms drops that grow and become slugs which span the flow channel. Flowing gas causes the slugs to detach and move down the channel. The effect of channel geometry, surface wettability, and gravity on the formation and motion of water slugs has been analyzed using high-speed video images of the drops and differential pressure-time traces. Drops grow and appear, assuming a sequence of shapes that minimize the total interfacial energy of the gas-liquid and liquid-solid interfaces. The drops are initially spherical caps centered on the pore (the liquid contacts one wall). Above a certain size, the drops move to the corner, forming "corner drops" (the liquid contacts two walls). Corner drops grow across the channel, evolving into partial liquid bridges (drops confined by three walls), and finally the drops span the channel cross-section forming slugs (contacting all four walls). Smaller slugs are formed in channels with hydrophobic walls than in channels with hydrophilic walls. Smaller slugs are formed in channels with curved walls than in square or rectangular channels. Slugs move when the differential gas pressure overcomes the force to move the advancing and receding gas-liquid-solid contact lines of the slugs. Residual water left behind in corners by moving slugs reduces the barriers for drops to form slugs, causing the steady-state slug volumes to be smaller than those seen at start-up in dry channels.

Residence time of water film and slug flow features in fuel cell gas channels and their effect on instantaneous area coverage ratio

“…It can be seen that both cases present a reasonable agreement in interface shape. Cheah et al [31,32] and Hellstern et al [33] experimentally studied the transitions from drop and film structures to water slugs and their motion through the gas flow channels of an ex situ test section. Their results indicate that the slugs achieve a constant shape and travel at the superficial gas velocity after detaching from the emergence/ coalescence point.…”

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

“…1, and h is the effective static contact angle of water with the channel walls. For Re G < 30, the critical drop volume for slug formation was shown to be independent of the gas flow rate 26,30…”

“…The time for slug formation only depends on the liquid flow rate and is independent of gas flow for gas phase Reynolds' number Re G < 30. 30 After slug formation, the differential pressures increases with time; the greater the gas flow rate the faster the differential pressure increased. Slugs detached and moved once the differential pressure increased by $90 Pa above background pressure for gas flow with no liquid flow.…”

“…At "steady state" the pressure trace in Channel 1 resembled the pressure traces for slug formation is a single isolated channel. 26,30,37 The differential pressure in Channel 1 was $10 Pa while drops grow at the inlet water pore. When drops grow large enough and become slugs, the differential pressure increases rapidly.…”

“…Cheah et al showed that the wettability of the flow channel walls plays a key role in the size of liquid slugs, but the power required to remove slugs was only weakly dependent on slug size. 30 Several other investigators have also observed slug flow in gas flow channels with emerging water drops. Ody observed regular periodic slug motion in microchannels with rectangular cross-sections.…”