Experimental investigation of water droplet emergence in a model polymer electrolyte membrane fuel cell microchannel

Using quantitative flow visualization and a transparent microfluidic platform replicating the salient features of the cathode channel of a polymer electrolyte membrane fuel cell (PEMFC), we investigate the dynamic evolution of water droplets emerging from a single 50 mu m pore of a gas diffusion layer (GDL) into a 250 mu m x 250 mu m air channel. The flow regimes are found to be primarily dependent on the air flow Reynolds numbers which ranged from 50 to 1200. At low Re, slug flow blocks the air flow through the channel. At higher Re, a periodic pattern of droplet emergence, growth and detachment appears. Further increase in air velocity induce wavy water film pattern. The characteristic frequencies and a flow map of the flow regimes as a function of superficial air and water velocities and presented. The significantly higher critical air velocities compared to previous results in the literature highlight the important impact of pore connectivity. Three different phases are identified in the dynamic evolution of the contact angles in the droplet regime. Both advancing and receding angles initially increase at the same rate: in the next phase only the advancing angle increases; and finally both angles increase monotonically under the combined effect of pressure and shear forces, resulting in a maximum hysteresis of similar to 110 degrees. A decrease in the droplet aspect ratio (height to chord ratio) and contact angle hysteresis is observed in the film flow regime, and is found to reduce water removal capacity. (C) 2012 Elsevier B.V. All rights reserved.