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Abstract: Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas–liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas–liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas–liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas–liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented. [Copyright &y& Elsevier]

Abstract: In the present work, instantaneous gas flow rates in each of two parallel channels of gas–liquid two-phase flow systems were investigated through measurements of the pressure drop across the entrance region. Liquid flow rates in two branches were pre-determined through liquid injection independently into each channel. Experiments were conducted in two different manners, i.e., the gas flow rate was varied in both ascending and descending paths. Flow hysteresis was observed in both gas flow rate distributions and the overall pressure drop of two-phase flow systems. Effects of liquid flow rates on gas flow distributions were examined experimentally. The presence of flow hysteresis was found to be associated with different flow patterns at different combinations of gas and liquid flow rates and flow instability conditions. A new and simple method was developed to predict gas flow distributions based on flow regime-specific pressure drop models for different experimental approaches and flow patterns. In particular, two different two-phase pressure drop models were used for slug flow and annular flow, separately. Good agreement was achieved between theoretical predictions and our experimental data. The developed new method can be potentially applied to predict gas flow distributions in parallel channels for fuel cells. [Copyright &y& Elsevier]

Abstract: In the present study, gas–liquid two-phase flow in a parallel square minichannel system oriented horizontally and at an incline is studied under operating conditions relevant to fuel cell operations. Flow mal-distribution in parallel channels occurs at low gas and liquid flow rates. In general, high superficial gas velocities are required to ensure even flow distribution, and the minimum gas flow rates required to achieve even distribution depend on the liquid flow rates, channel orientation and experimental procedures. As the inclination angle is increased, a higher gas flow rate is required to ensure even gas–liquid flow distribution while flow channels inclined downward seems to help in improving the even flow distribution. The presence of flow hysteresis phenomena indicate that multiple flow distributions exist at the same given flow conditions when the gas flow rates are varied in ascending and descending manners. Flow mal-distribution and flow hysteresis are directly linked with flow stability. More specifically, the actual gas and liquid distribution in parallel channels is determined by the stability of mathematical solutions of mass and momentum balance equations and also the flow history. For the first time, the present work investigates flow distributions in fuel cell flow fields by accounting for two-phase flow conditions. In addition, a novel approach is introduced to ensure flow distributions and their stability through contour construction of isobars where unstable flow region can be identified, which can be used in the design of parallel channel flow fields, especially for fuel cells. [Copyright &y& Elsevier]

Abstract: Two-phase flow in horizontal parallel channels has been experimentally investigated under fuel cell related operating conditions. Pronounced hysteresis is observed in the pressure drop versus flow characteristic curve when starting from either flooded or dry conditions. When gas is introduced into channels initially filled with water (flooded initial condition), both gas and liquid tend to flow predominantly in one channel at low gas or liquid flow velocities. As the gas flow velocity increases, even distribution of gas and liquid flow in both channels is observed, accompanied with a sudden decrease in the pressure drop. On the other hand, even gas and liquid flow distribution between both channels is found at comparatively lower gas flow velocities when starting with dry-gas flow conditions with liquid introduced into channels filled with gas (stratified flow regime). The flow regimes of this system are visualized in plots of the pressure drop against gas and liquid flow velocities. However, this phenomenon tends to vanish at high gas and liquid flow velocities, suggesting that high gas and liquid flow velocities are required to ensure even flow distribution in parallel channels. The hysteresis points appear at the same level of the pressure drop, reflecting intrinsic characteristics of the parallel channels used in this study. These results have important implications for PEM fuel cell operational strategies. In order to avoid reactant mal-distribution in parallel flow channels in the flow field in the two-phase flow regime, fuel cells should be operated at sufficiently high gas flow velocities. [Copyright &y& Elsevier]