During the operation of a proton exchange membrane fuel cell, water is produced in the cathode electrode. Accumulation of produced water in the flow channel can block the transport of reactants, which ultimately lowers the performance of the cell. The water content in the flow channel can be efficiently removed from the channel with an external excitation. Previously, the author reported utilization of acoustic pressure waves in order to remove the water content from the flow channel [1]. However, the dynamics of liquid water droplets during this removal process were not investigated. The current study investigates dynamics of water droplets on the surface of the gas diffusion layer (GDL) when acoustic pressure waves are superimposed on the core gas flow. Two different modes of superimposition were implemented; (i) continuous, and (ii) on demand. Study of droplet dynamics was achieved by visualizing the droplet from the side-view with a high-speed camera. When the superimposition was done in the continuous mode at 20 Hz, the droplet went through the rocking motion on the surface of the GDL. For 60 and 80 Hz of superimposition, in addition to the rocking motion, droplets underwent the prolate mode of oscillation, which was characterized by vertical oscillations. For higher frequencies of acoustic pressure waves, in addition to rocking and prolate modes of oscillation, droplets underwent the oblate mode of oscillation, which featured horizontal oscillations. The on on demand experiments demonstrated that the liquid water droplet detached from the surface of the GDL only when the droplet size was large enough.
During the operation of proton exchange membrane (PEM) fuel cells, water is produced in the cathode side. The produced water passes through the porous structure of the electrode and emerges from the surface of the gas diffusion layer (GDL) within the flow channel. The emerged droplet is constantly fed through liquid columns which are formed underneath the droplet within the GDL. This study focuses on dynamics of growing droplets on the surface of the GDL which are exposed to shear gas flow. High-speed imaging was implemented to visualize droplet dynamics from emergence to detachment as the pressure drop across the droplet was measured simultaneously. Images were processed with MATLAB code which was developed in-house to obtain droplet lateral area and the location of the droplet centroid. Results clearly demonstrated that droplets underwent an oscillatory mode for both superficial gas velocities tested in this study. While the oscillatory motion was observed both in horizontal (i.e. stream-wise direction) and vertical directions, the amplitude of the oscillation was greater in the horizontal direction. In addition, the oscillation amplitude was observed to increase with droplet size and reached the maximum value upon droplet detachment. For a superficial gas velocity of 10.76 m/s, the oscillation amplitude upon detachment was as high as around 0.16 mm in x direction while the corresponding oscillation in y direction was around 0.06 mm. Study of contact angles revealed that while the advancing and receding contact angles for the superficial gas velocity of 4.17 m/s are higher than angles for the superficial gas velocity of 10.76 m/s, the contact angle hysteresis for both velocities were almost identical upon detachment.
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