An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

Analysis of an active tubular liquid-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model was developed for an active, tubular, liquid-feed direct methanol fuel cell (DMFC). The liquid-gas, two-phase mass transport in the porous anode and cathode was formulated based on the multi-fluid approach in the porous media. The two-phase mass transport in the anode and cathode channels was modeled using the drift-flux and the homogeneous mist-flow models, respectively. Water and methanol crossovers through the membrane were considered due to the effects of diffusion, electro-osmotic drag, and convection. The model enabled a numerical investigation of the effects of various operating parameters, such as current density, methanol flow rate, and oxygen flow rate, on the mass and heat transport characteristics in the tubular DMFC. It was shown that by choosing a proper tube radius and distance between the adjacent cells, a tubular DMFC stack can achieve a much higher energy density compared to its planar counterpart. The results also showed that a large anode flow rate is needed in order to avoid severe blockage of liquid methanol to the anode electrode due to the gas accumulation in the channel. Besides, lowering the flow rate of either the methanol solution or air can lead to a temperature increase along the flow channel. The methanol and water crossovers are nearly independent of the methanol flow rate and the air flow rate.     

Quantification and characterization of water coverage in PEMFC gas channels using simultaneous anode and cathode visualization and image processing

A new technique is presented to characterize and quantify the two-phase flow in the anode and cathode flow field channels using simultaneous anode and cathode visualization combined with image processing. In situ visualization experiments were performed at 35 °C with stoichiometric ratios (an/ca) of 1.5/2.5, 1.5/5, 3/8 to elucidate two-phase flow dynamics at lower temperature/low power conditions, when excess liquid water in the cell can be especially prevalent. Video processing algorithms were developed to automatically detect and quantify dynamic and static liquid water present in the flow field channels, as well as discern the distribution of water among different two-phase flow structures. The water coverage ratio was introduced as a parameter to capture the time-averaged flow field water content information through recorded high speed video sequences. The automated processing allows for efficient and robust spatial and temporal averaging of steady state channel water over very large visualization data sets acquired through high speed imaging. The developed algorithm calculates the water coverage ratio using the liquid water in the channels which is contacting the GDL surface, and selectively removes the superficial condensation on the visualization window from the coverage area. The water coverage ratio and distribution metrics techniques were demonstrated by comparing the performance of Freudenberg and Toray gas diffusion layers (GDLs) from a water management perspective, including direct anode to cathode comparisons of simultaneous water coverage data for each GDL sample. The anode water coverage ratio was found to exceed the cathode for both GDL samples at most operating conditions tested in this work. The Freudenberg GDL consistently demonstrated a higher water coverage ratio in the flow field gas channels than the Toray GDL, while the Toray GDL indicated a propensity for greater water retention within the membrane electrode assembly (MEA) based on performance, high frequency resistance (HFR), and water coverage metrics.     

Numerical modeling of three-dimensional two-phase gas–liquid flow in the flow field plate of a PEM electrolysis cell

Numerical simulations were performed for three-dimensional two-phase water/oxygen flow in the flow field plate at the anode side of a PEM electrolysis cell. The mixture model was used to simulate two phases for the purpose of examining flow features in the flow field plate in order to effectively guide the design of electrolysis cells. The water flow rate was maintained as a constant of 260 mL/min, while the flow rate of oxygen generation was assumed to change from 0 to 14 mg/s. The obtained results including the velocity, pressure, and volume fraction distributions are presented and discussed. It is found that the obtained results for single-phase flow cases cannot be linearly extrapolated into the two-phase flow cases. The irregular velocity profile (locally low velocity magnitude near the exit port section) is not observed when the flow rate of oxygen generation is relatively low. As the mass flow rate of oxygen generation increases, reverse flow develops inside the flow channels.     

Numerical analysis of improved water management of open-cathode proton exchange membrane fuel cells with a dead-ended anode by pulsating flow

Anode water management is critical for the efficient operation of proton exchange membrane fuel cells with a dead-ended anode. To clarify the mass transfer phenomenon in the anode flow channel under the dead-ended anode mode, and reveal the influence mechanism of pulsating flow on water management, a three-dimensional, two-phase, non-isothermal transient model is established in this study. The water content and species distribution in different layers are analyzed, and the internal relationship between water transport behavior and output performance of the proton exchange membrane fuel cell under different operating conditions is explored. The simulation results show that the output performance of the proton exchange membrane fuel cell in dead-ended anode mode is directly related to the gas diffusion layer's water saturation and the hydrogen mass transfer. Furthermore, pulsating flow can effectively suppress the back diffusion of water, and improve the mass transfer rate of hydrogen. Consequently, the water management and the operational stability of the proton exchange membrane fuel cell are significantly improved. The research results of this paper have important guiding significance for improving the water and gas management of fuel cells.     

Three-dimensional coupling numerical simulation of two-phase flow and electrochemical phenomena in alkaline water electrolysis

Alkaline water electrolysis has the advantage of scalability for industrial-scale mass production of hydrogen; however, it is operated under a lower current density than other methods of water electrolysis because a high overpotential resulting from ion transport limitations will occur at high current density. Bubble dynamics can both prevent ion transport by its existence and accelerate it by bubble-induced flow. In this study, we conduct three-dimensional coupling numerical simulations of two-phase flow and electrochemical phenomena to elucidate the mechanisms by which microscale bubble dynamics influence ion transport and the cell overpotential. We find that the flow induced by rising microbubbles enhances ion transport to the anode and suppresses the cell overpotential. Moreover, bubble atomization further suppresses the overpotential because smaller bubbles approach the anode more closely than larger ones and accelerate ion transport to the anode surface.     

A two-phase flow and transport model for the cathode of PEM fuel cells

A unified two-phase flow mixture model has been developed to describe the flow and transport in the cathode for PEM fuel cells. The boundary condition at the gas diffuser/catalyst layer interface couples the flow, transport, electrical potential and current density in the anode, cathode catalyst layer and membrane. Fuel cell performance predicted by this model is compared with experimental results and reasonable agreements are achieved. Typical two-phase flow distributions in the cathode gas diffuser and gas channel are presented. The main parameters influencing water transport across the membrane are also discussed. By studying the influences of water and thermal management on two-phase flow, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures.     

Modeling of water transport through the membrane electrode assembly for direct methanol fuel cells

In this work, a one-dimensional, isothermal two-phase mass transport model is developed to investigate the water transport through the membrane electrode assembly (MEA) for liquid-feed direct methanol fuel cells (DMFCs). The liquid (methanol–water solution) and gas (carbon dioxide gas, methanol vapor and water vapor) two-phase mass transport in the porous anode and cathode is formulated based on classical multiphase flow theory in porous media. In the anode and cathode catalyst layers, the simultaneous three-phase (liquid and vapor in pores as well as dissolved phase in the electrolyte) water transport is considered and the phase exchange of water is modeled with finite-rate interfacial exchanges between different phases. This model enables quantification of the water flux corresponding to each of the three water transport mechanisms through the membrane for DMFCs, such as diffusion, electro-osmotic drag, and convection. Hence, with this model, the effects of MEA design parameters on water crossover and cell performance under various operating conditions can be numerically investigated.     

Self-compensating characteristic of steam–water mixture at low mass velocity in vertical upward parallel internally ribbed tubes

This paper presents an experimental investigation on Self-Compensating Characteristic (SCC) in vertical upward parallel tubes with low mass velocity of steam–water two-phase mixture. A physical model was built up using parallel internally ribbed tubes. A method called Differential Pressure Substitute was used to measure two-phase flow parameters. The results indicated that the SCC of vertical upward parallel tubes is caused by combined action of frictional pressure drop and gravitational pressure drop. The mass velocity in the tube with lower heat flux decreases first, and then increases with an increase in quality. The uneven heat fluxes among tubes are the main reasons that cause mass velocity differentials. Greater uneven heating ratio enhances the SCC in low quality region and weakens it in high quality region. The SCC has different variation rules in different pressure region. In the sub-critical pressure region, rising pressure weakens the SCC when quality is low and enhances it when quality is high. In near-critical pressure region, the mass velocity varies monotonically and slowly with the increase in quality because the difference between water and steam is minor in this pressure region. The results provide some instructive advices to improve the design and operation safety of once-through boiler.     

Numerical investigation of PEM electrolysis cell with the new interdigitated-jet hole flow field

The flow field structure has important influences on the mass and heat transfer and the distribution uniformity in the proton exchange membrane electrolysis cell (PEMEC). In this paper, the application and operation modes and the structural parameters of the new interdigitated-jet hole flow field (JHFF) are explored, to guide the processing of the JHFF and provide references for experimental testing. A three-dimensional and two-phase model is established to simulate the effect of JHFF on the performance of PEMEC. The results demonstrate that compared with the application of JHFF only on the anode side, the application of JHFF on both sides of the anode and cathode can increase the temperature distribution uniformity and polarization performance by 41.78% and 16.25%, respectively. By increasing the number of inlet flow channels and using the counter-flow water supply mode, the temperature distribution can be more uniform. The lower the height of jet holes, the better the normal mass transfer and polarization performance, while the worse the temperature distribution uniformity. Reducing the diameter of the inlet jet holes can improve the normal mass transfer performance in the porous electrode. Synthetically, the hole height of 0.2 mm and the hole diameter of 0.4 mm are recommended. The findings provide theoretical guidance for the practical application of JHFF in PEMEC so that the positive role of JHFF in improving electrolysis performance can be fully realized.     

Modeling of two-phase transport in the diffusion media of polymer electrolyte fuel cells

A three-dimensional model of polymer electrolyte fuel cells (PEFCs) is developed to investigate multiphase flows, species transport, and electrochemical processes in fuel cells and their interactions. This two-phase model consists of conservation principles of mass, momentum, species concentration and charges, and elucidates the key physicochemical mechanisms in the constituent components of PEFCs that govern cell performance. Efforts are made to formulate two-phase transport in the anode diffusion media and its coupling with cathode flooding as well as the interaction between single- and two-phase flows. Numerical simulations are carried out to investigate multiphase flow, electrochemical activity, and transport phenomena and the intrinsic couplings of these processes inside a fuel cell at low humidity. The results indicate that multiphase flows may exist in both anode and cathode diffusion media at low-humidity operation, and two-phase flow emerges near the outlet for co-flow configuration while is present in the middle of the fuel cell for counter-flow one. The validated numerical tools can be applied to investigate vital issues related to anode performance and degradation arising from flooding for PEFCs.     

循环流化床烟气脱硫气固两相流动特性的试验研究

采用三准则相似理论设计了循环流化床烟气脱硫气固两相流动试验台.通过对循环流化床脱硫反应器试验装置内沿高度方向阻力分布和不同高度截面上局部颗粒质量通量的测量,详细地研究了脱硫反应器内气固两相流动规律和内循环特性.结果表明:脱硫反应器阻力主要集中在文丘里管段,而且随着循环物料量和气体流量的增加,系统阻力显著增加;脱硫反应器内气固两相流动呈典型的环核流动结构,边壁下降流颗粒浓度高,中心区域上升流颗粒浓度低,且固体质量回流比率随着脱硫反应器高度的上升而下降.研究结果为循环流化床烟气脱硫系统的设计与放大提供了依据.     

Water management studies in PEM fuel cells,Part I: Fuel cell design and in situ water distributions

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.     

Performance evaluation of a polymer electrolyte fuel cell with a dead-end anode: A computational fluid dynamic study

The operation of polymer electrolyte fuel cell (PEFC) with a dead-end anode requires careful gas and water management to achieve optimal operating performance. The amount of water accumulated in the anode and nitrogen crossover are particularly important factors. To ascertain (i) the behavior of a PEFC with a dead-end anode, (ii) the accumulation of water and nitrogen in the anode cell with time, and (iii) efficient purging strategies to manage the gas and water, a transient PEFC model with a dead-end anode was developed and analyzed. The model assumes a two-phase flow and solves the governing equations of conservation of mass, momentum, species, energy, charge, coupled with a phenomenological membrane model and agglomerate model for catalyst layer. The model results indicate that water and nitrogen can accumulate in the anode region with time, such that the amount of available hydrogen decreases and hence the cell performance drops. The accumulation rate is found to be closely linked to the current that is drawn from the cell. Further, it is found that to alleviate the problem of build-up of nitrogen and water, the purge frequency and duration of the purge play important roles in affecting cell performance. The transient behavior and impact of the relevant operating conditions obtained from the simulation results can be used for development of efficient purging strategies.     

Boiling two-phase flow and efficiency of co- and counter-current microchannel heat exchangers with gas heating

To learn how to utilize the exhaust heat from a high-temperature gas product of a methanol reformer, the present study experimentally investigates the boiling two-phase flow in co- and counter-current microchannel heat exchangers (MCHE) with gas heating. Boiling two-phase flow patterns, two-phase flow instability, and efficiency are explored. The working fluid on the hot and cold sides are helium and liquid methanol, respectively. The silicon-based MCHE, which has dimensions of 20 mm × 20 mm, is designed with 18 parallel microchannels on both sides and is prepared using microfabrication processes. Four types of two-phase flow patterns – bubbly-elongated slug flow, annular flow, annular flow with liquid film breakup, and dryout are identified in both types of MCHE that are studied. A flow pattern map is then constructed on the plane of the methanol mass flux versus heat flux for both types of MCHE. In the counter-current MCHE, the efficiency increases significantly with an increase in the mass flux in both the single- and two-phase flow regions, while the effect of mass flux is insignificant in the co-current MCHE. In the two-phase flow region, the efficiency of both types of MCHEs gradually increases with an increase in the hot-side thermal power until the CHF is approached. The highest efficiency obtained in the present study is about 0.85 and 0.90 for the co- and counter-current MCHEs, respectively.     

质子交换膜燃料电池建模及水热传输特性分析

针对质子交换膜燃料电池(PEMFC)水管理开展了研究,建立了一维非等温两相流解析模型,研究了不同电流密度、微孔层接触角和不同加湿方案对电池内部水分布和温度分布的影响,提出了更好的进气加湿方案。结果表明:电流密度增大会导致阳极拖干、阴极水淹加剧,导致电池各部分温度上升。因各层材料亲水性不同,在交界面处能观察到液态水阶跃现象。增大微孔层接触角促进阴极液态水反扩散到阳极,一定程度上缓解阳极变干,但过大的接触角可能导致阴极水淹加剧。通过采取"阳极充分加湿、阴极低加湿"的进气加湿方案可以有效提高电池性能,并且能在一定程度改善电池内部受热,提高电池使用寿命。     

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs.     

Numerical simulation of water and heat transport in the cathode channel of a PEM fuel cell

Cathode channel of a PEM fuel cell is the critical domain for the transport of water and heat. In this study, a mathematical model of water and heat transport in the cathode channel is established by considering two-phase flow of water and air as well as the phase change between water and vapor. The transport process of the species of air is governed by the convection-diffusion equation. The VOSET (coupled volume-of-fluid and level set method) method is used to track the interface between air and water, and the phase equilibrium method of water and vapor is employed to calculate the mass transfer rate on the two-phase interface. The present model is validated against the results in the literature, then applied to investigate the characteristics of two-phase flow and heat transfer in the cathode channel. The results indicate that in the inlet section, water droplets experience three evolution stages: the growing stage, the coalescence stage and the generation stage of dispersed water drops. However, in the middle and outlet sections of the channel, there are only two stages: the growth of water droplets, and the formation of a water film. The mass transfer rate of phase change in the inlet section of the channel varies over time, exhibiting an initial increase, a decrease followed, and a stabilization finally, with the maximum and stable values of 1.78 × 10^?4 kg/s and 1.52 × 10^?4 kg/s for Part 1, respectively. In the middle and outlet sections, the mass transfer rate increase firstly and then keeps stable gradually. Furthermore, regarding the distribution of the temperature and vapor mass fraction in the channel, near the upper surface of the channel, the temperature and vapor mass fraction first change slightly (x < 0.03 m) and then rapidly decrease with fluctuations (x > 0.03 m). In the middle of the channel, the temperature and vapor mass fraction slowly decrease with fluctuation.     

Effect of anode diffusion layer fabricated with mesoporous carbon on the performance of direct dimethyl ether fuel cell

In this study, we present the novel membrane electrode assembly (MEA) for direct dimethyl ether fuel cell (DDFC). The anode gas diffusion layer (AGDL) of the MEA is fabricated with mesoporous carbon to facilitate the anode mass transport and enhance the performance of DDFC. The major differences of mesoporous carbon AGDL (MAGDL) and XC-72 AGDL (XAGDL) are the BET surface, the pore volume, and the pore size distribution. The MAGDL provides many more passageways for mass transport than XAGDL. The MAGDL possesses hydrophilic small and hydrophobic middle pores, which benefit the liquid and gas transport simultaneously. The maximum power density of DDFC increases by 20% when using MAGDL instead of XAGDL at 60 °C. The electrochemical measurements indicate that the promotion of the anode two-phase mass transport is the main reason for the significant improvement of DDFC performance.     

Two-dimensional numerical pore-scale investigation of oxygen evolution in proton exchange membrane electrolysis cells

Oxygen blocking the porous transport layer (PTL) increases the mass transport loss, and then limits the high current density condition of proton exchange membrane electrolysis cells (PEMEC). In this paper, a two-dimensional transient mathematical model of anode two-phase flow in PEMEC is established by the fluid volume method (VOF) method. The transport mechanism of oxygen in porous layer is analyzed in details. The effects of liquid water flow velocity, porosity, fiber diameter and contact angle on oxygen pressure and saturation are studied. The results show that the oxygen bubble transport in the porous layer is mainly affected by capillary pressure and follows the transport mechanism of ‘pressurization breakthrough depressurization’. The oxygen bubble goes through three stages of growth, migration and separation in the channel, and then be carried out of the electrolysis cell by liquid water. When oxygen breaks through the porous layer and enters the flow channel, there is a phenomenon that the branch flow is merged into the main stream, and the last limiting throat affects the maximum pressure and oxygen saturation during stable condition. In addition, increasing the liquid water velocity is helpful to bubble separation; changing the porosity and fiber diameter directly affects the width of pore throat and the correlative capillary pressure; increasing porosity, reducing fiber diameter and contact angle can promote oxygen breakthrough and reduce the stable saturation of oxygen.