The modeling of particle aggregation under a simple shear flow and the extension of the model to a stirred vessel is described. The model quantitatively demonstrates the change of the number of aggregates with time for each shear rate. This number increased with higher shear rate and, conversely, the aggregate size became small when raising the shear rate. This was because aggregates were broken by the stronger shear force. The number of aggregates for different impellers was determined. The shear rate was back‐calculated from the experimentally obtained aggregate size and the model equation. This shear rate was different from that estimated from the Metzner‐Otto method, consequently, some revisions of the Metzner‐Otto equation might be necessary for its application to particle aggregation.
Serpentine-interdigitated hybrid pattern gas channels were developed for polymer electrolyte fuel cells (PEFCs). The performance of conventional (triple-serpentine channel), interdigitated and serpentine-interdigitated hybrid pattern gas channels were compared. The performance of the interdigitated channel is significantly higher than that of the conventional channel. However, the interdigitated channel causes flooding under high air utilization. The hybrid pattern channels have the highest performance of the three patterns. A through-pore ratio (A/A 0 ) was also developed. This ratio indicates the level of flooding in the gas diffusion layer (GDL) under the rib. According to calculated values of this ratio, flooding is suppressed when using the hybrid pattern channels.
Polymer electrolyte fuel cells (PEFCs) generally have external humidifiers to feed humidified hydrogen and air for membrane hydration. The external humidifiers should be omitted to develop a very simplified PEFC system with increased total efficiency and reduced cost. In the present study, a water vapor exchange flow channels installed in the PEFC thus has been developed to enhance the performance without cathode humidification. A gas diffusion layer (GDL) coated with a hydrophilic microporous layer (MPL) consisting of polyvinyl alcohol (PVA) and carbon black is employed for the cathode water exchange flow channels to promote water transport from the cathode outlet wet gas to the anode inlet dry gas. This is effective for the membrane hydration, enhancing PEFC performance. Addition of hydrophilicity to the MPL is effective to water transport from the cathode to the anode.
Polymer electrolyte fuel cells (PEFCs) generally have external humidifiers to supply humidified hydrogen and oxidant gases, preventing dehydration of the membrane electrode assembly (MEA). If a PEFC could be operated without humidification, external humidifiers may be removed, resulting in a very simplified PEFC system with increased total efficiency and reduced cost. One of most important issues to advance the commercial viability of PEFCs is to develop high performance PEFCs that can operate without humidification. In the present study, a water vapor exchange system installed in the cell was developed to enhance the PEFC performance without humidification. A gas diffusion layer coated with a hydrophilic microporous layer, which consists of carbon black and polyvinyl alcohol (PVA), used at the cathode exchange area increases water transport from the wet cathode outlet gas to the dry anode inlet gas. This prevents dehydration of the MEA, thereby reducing the IR (ohmic) overpotential. The exchange area using interdigitated flow channels is effective to achieve further enhancement of water transport from the cathode to the anode, which significantly enhances the ability to prevent dehydration of the MEA. This results in a much higher output power density compared with that for a PEFC without the water vapor exchange area.
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