Clarification of the surface wettability effects on two-phase flow patterns in PEMFC gas channels

•Design, realization of two ex situ experimental set-ups with different hydrophobicity.•Investigated water and gas flow ranges relevant to PEMFC applications.•Mixed slug/film flow is the dominant flow pattern in both wettability conditions.•For both conditions, residual droplets and films make the flow pattern unstable.•No significant difference in flow pattern according to the surface wall wettability.

The motivation for the present work was to demonstrate clearly the effects of surface wettability on the two-phase flow in channels for PEMFC applications. Actually, these wettability effects are still controversial in the literature. The experimental investigation was conducted in isothermal conditions with ex situ dedicated experimental apparatus. Two channel materials with two different wettabilities were tested.In both hydrophobic and hydrophilic configurations, starting from a dry channel, the first emitted droplets left a residual liquid film in the channel corners and/or on the channel walls. These liquid films that spread all along the channel made the two-phase flow unstable. Finally, no distinction could be made between the two-phase flow regimes in hydrophilic and hydrophobic configurations. Our conclusions explain why the effects of surface wettability on fuel cells have been so unclear in the literature. For flow rates representative of a fuel cell application, it was impossible to draw flow pattern maps because of a lack of reproducibility. Based on the present results, the strict limit between slugs and droplets or droplets and film does not seem justified.