This paper investigates phenomena related to water condensation behavior inside a polymer electrolyte membrane fuel cell (PEMFC), and analyzes the effects of liquid water and gas flow on the performance of the fuel cell. A method for simultaneous measurements of the local current density across the reaction area and direct observation of the phenomena in the cell are developed. Experimental results comparing separator types indicate the effect of shortcut flow in the gas diffusion layer (GDL) under the land areas of serpentine separators, -2 -and also show the potential of straight channel separators to achieve a relatively-uniform current density distribution. To evaluate shortcut flows under the land areas of serpentine separators, a simple circuit model of the gas flow is presented. The analysis shows that slight variations in oxygen concentration caused by the shortcut flows under the land areas affect the local and overall current density distributions. It is also shown that the establishment of gas paths under the water in channels filled with condensed water is effective for stable operation at low flow rates of air in the straight channels.

The performance of free-breathing polymer electrolyte membrane fuel cells (PEMFCs) was studied experimentally and the effect of the cathode separator structure on the cell performance was investigated. The result showed that it is difficult to realize a uniform contact pressure across the cell layers for the open type separator, and this results in higher contact resistance and poorer cell performance than the channel type separator.The channel type separator can maintain a low contact resistance, and the cell performance is strongly affected by the natural convection inside the channel. Optimization of the channel design of the channel type separator achieves good performance and this type of separator is superior for a free-breathing PEMFC. A computational three-dimensional analysis for the free-breathing channel type PEMFC with the different channel depths was performed, and it identified the influence of natural convection.

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In a polymer electrolyte membrane fuel cell, the condensed water in the separator-channel prevents the supply of reactants to electrodes, which deteriorates the cell performance. The Lattice Boltzmann simulation has been conducted to understand the behavior of condensed water in the separator-channels. The scheme for the two-phase flow with large density difference was applied and the boundary condition for wettability at the corner inside the channel was examined. The present simulation demonstrates the effects of the cross-sectional shape, the wettability of channel and the volume of condensed water on the liquid water behavior. In the hydrophilic separator-channels, the liquid water spreads along the channel wall to form film and, in a specific condition, the water draws away from the gas diffusion layer, which suppresses the flooding. On the other hand, the liquid water forms sphere, covering larger area of the surface of gas diffusion layer in the hydrophobic separator-channels, but the drain performance of liquid water is superior.

Rapid mixing of fuel and air is an essential factor in improving combustion and emissions of diesel engines. Thus extensive investigation has been made to increase turbulence in the diesel combustion process by increasing swirl ratios, injection velocity, and modifying combustion chamber configurations. However, the relationship between the microscopic structure of the heterogeneous distribution of fuel clouds and the local turbulence structure is not well understood. Additionally there is no appropriate index for the analysis of the degree and size scale of heterogeneity. This paper investigates local diffusion phenomena with focusing on scales of fuel cloud and eddies based on a newly developed entropy method.The entropy analysis is based on the concept of statistical entropy, and it identifies the degree of homogeneity in the fuel concentration. The entropy values increase with the progress of uniformity in the diffusion process. Figure A1 is an example of entropy values comparing two different states of heterogeneity. The pictures show tracers mixed in fuel jets and were taken by a laser sheet. The entropy value of the N 2 jet image is higher than the value of the C 2 H 4 image, indicating the higher degree of diffusion. By analyzing the speed of change in the entropy values, the diffusion intensity of the fuel cloud can be estimated, and it is also possible to identify size scales of the heterogeneity.Using the entropy analysis, the microscopic structure in turbulent jets and diesel sprays was investigated. Figure A2 is a picture of a turbulent jet simulating a diesel spray. In the experiment, a fluorescent compound is injected in to water. The PIV method was used for the analysis of the velocity distribution, and the diffusion intensity was obtained by the analysis of the local entropy.The results show that the diffusion intensity is the highest in the vicinity of the nozzle exit, and the heterogeneity scale is the smallest here. The heterogeneity scale increases gradually along the spray axis towards the downstream, with smaller size scales in the large clouds. In the downstream region, small-scale structures diffuse and become unclear, while large scale structures clearly remain.The paper details the microscopic structure of the heterogeneity in diesel sprays, and it demonstrates availability of the entropy method.

This paper observes phenomena related to water production behavior inside a fuel cell and analyzes the effect on the current and temperature distribution across the reaction area. A fuel cell permitting direct observation of the phenomena in the cell, 2-D temperature measurements in the cathode channels, and local current density measurements on the anode side was manufactured. The experimental results showed the production and flow of liquid water in the cell, and there were good correlations among the distributions of current density, temperature, and water amounts in the channels. The behavior of current, voltage, water distribution, and pressure differences in the cathode channels were used to hypothesize about the possibility of gas paths deep in the gas diffusion layer in the flooded condition and a positive feedback mechanism in the drying-out condition.

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