Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of hydrophilic/hydrophobic properties of gas flow channels on liquid water transport in a serpentine polymer electrolyte membrane fuel cell

The droplet dynamics in the serpentine flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a serpentine channel with the aim of effectively preventing flooding. Three-dimensional two-phase flow simulations employing the volume of fluid (VOF) method have been performed. Liquid droplets emerging from four adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the serpentine flow channel. The effects of contact angle variation of the channel walls on liquid water removal have been tested in terms of liquid water saturation and coverage of liquid water on the gas diffusion layer (GDL) surface. The numerical results show that the hybrid case, which consists of hydrophilic channel walls at the straight part and hydrophobic walls at the turning part of the serpentine flow channels, enhances water removal compared with two other cases in which the channel wall is homogeneously hydrophilic or hydrophobic. The three-dimensional visualization of liquid water droplets reveals that the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.     

Numerical simulation of two‐phase cross flow in the gas diffusion layer microstructure of proton exchange membrane fuel cells

The cross flow in the under‐land gas diffusion layer (GDL) between 2 adjacent channels plays an important role on water transport in proton exchange membrane fuel cell. A 3‐dimensional (3D) two‐phase model that is based on volume of fluid is developed to study the liquid water‐air cross flow within the GDL between 2 adjacent channels. By considering the detailed GDL microstructures, various types of air‐water cross flows are investigated by 3D numerical simulation. Liquid water at 4 locations is studied, including droplets at the GDL surface and liquid at the GDL‐catalyst layer interface. It is found that the water droplet at the higher‐pressure channel corner is easier to be removed by cross flow compared with droplets at other locations. Large pressure difference Δp facilitates the faster water removal from the higher‐pressure channel. The contact angle of the GDL fiber is the key parameter that determines the cross flow of the droplet in the higher‐pressure channel. It is observed that the droplet in the higher‐pressure channel is difficult to flow through the hydrophobic GDL. Numerical simulations are also performed to investigate the water emerging process from different pores of the GDL bottom. It is found that the amount of liquid water removed by cross flow mainly depends on the pore's location, and the water under the land is removed entirely into the lower‐pressure channel by cross flow.     

Dynamics of liquid water in the anode flow channels of PEM fuel cells: A numerical parametric study

A 3D volume of fluid (VOF) model for an anode channel in a PEM fuel cell has been built. The effects of the initial position of the water droplet, its size as well as the wettability of the gas diffusion layer (GDL) are investigated under different operating conditions. It is found that the initial position of the relatively small water droplet in the channel has almost no effect on the pressure drop and the time taken for the liquid water to move out from the channel; however, such effects become more profound as the size of the water droplet increases. Also, when the droplet is placed at the side wall of the channel, then it develops into pockets of water that are mainly located at the upper corners of the channel, thus causing a smaller pressure drop compared to the cases in which the water droplet is placed either on the surface of the GDL or on the top wall of the channel. Furthermore, the hydrogen velocity is found to have a negligible effect on the dynamics of liquid water; however, the pressure drop and removal time are significantly influenced by the hydrogen velocity. Moreover, as the size of the water droplet increases, the pressure drop increases and the time required for the liquid water to move out of the channel decreases. Finally, the pressure drop in the channel decreases and the removal time of the liquid water increases as the contact angle of the GDL decreases.     

Numerical simulation of two-phase flow in a multi-gas channel of a proton exchange membrane fuel cell

Understanding the two-phase distribution characteristics within the multi-gas channel of a fuel cell is important for improving fuel cell performance. In the paper, the volume of fluid model is used to predict the dynamic behaviour of water in the multi-gas channel, analyze the pressure drop, velocity distribution, and flow resistance coefficient between different channels, and investigate the influence of operating conditions, surface wettability and channel structure on the two-phase distribution characteristics in the channel. The results show that water undergoes the processes of growth, separation, single droplet transport, wall impact, droplet collision, liquid film formation, and liquid film transport in the multi-gas channel. Inlet velocity and surface wettability significantly affect the pressure drop, water saturation, and surface water coverage. As the inlet velocity and gas diffusion layer surface wettability increase, the flow resistance coefficient and unevenness of the distribution decrease, indicating that the in-channel flow distribution homogeneity is enhanced. The rectangular channel has better water removal and flow distribution uniformity than the tapered channel, and the unevenness of distribution decreases significantly with decreasing rectangular width, from 0.15715 to 0.00315. The research work is a guide to understanding water transport in multi-gas channels, accelerating water removal, and improving inter-channel flow distribution uniformity.     

Dynamic behaviour of liquid water emerging from a GDL pore into a PEMFC gas flow channel

A numerical investigation of the dynamic behaviour of liquid water entering a polymer electrolyte membrane fuel cell (PEMFC) channel through a GDL pore is reported. Two-dimensional, transient simulations employing the volume of fluid (VOF) method are performed to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to air flow in the bulk of the gas channel. The modeled domain consists of a straight channel with air flowing from one side and water entering the domain from a pore at the bottom wall of the channel. The channel dimensions, flow conditions and surface properties are chosen to be representative of typical conditions in a PEMFC. A series of parametric studies, including the effects of channel size, pore size, and the coalescence of droplets are performed with a particular focus on the effect of geometrical structure. The simulation results and analysis of the time evolution of flow patterns show that the height of the channel as well as the width of the pore have significant impacts on the deformation and detachment of the water droplet. Simulations performed for droplets emerging from two pores with the same size into the channel show that coalescence of two water droplets can accelerate the deformation rate and motion of the droplets in the microchannel. Accounting for the initial connection of a droplet to a pore was found to yield critical air inlet velocities for droplet detachment that are significantly different from previous studies that considered an initially stagnant droplet sitting on the surface. The predicted critical air velocity is found to be sensitive to the geometry of the pore, with higher values obtained when the curvature associated with the GDL fibres is taken into account. The critical velocity is also found to decrease with increasing droplet size and decreasing GDL pore diameter.     

Modeling of liquid water transport in a proton exchange membrane fuel cell gas flow channel with dynamic wettability

Liquid water transport in the gas flow channel is significantly important for the water removal and management in proton exchange membrane fuel cells. Previous numerical studies consider a single and constant static contact angle for the liquid water transport on the channel surface, which is insufficient to account for the dynamic wettability behavior of the flow. In this study, a dynamic wettability model is developed that incorporates the sliding angle and dynamic contact angles for the simulation of water transport in the flow channel. It is found that both the sliding and dynamic contact angles have significant impact on the characteristics of the water transport and dynamics in the flow channel. Water spreading on the channel surface is elliptic, and its minor and major axes oscillate out of phase with the droplet height. The pressure loss for the 2‐phase flow in the channel is directly related to this oscillation and deformation of the droplet shape. Flow channel surface with a small sliding angle facilitates the water transport and removal and reduces the associated pressure loss in the channel. The conventional static wettability model would overpredict droplet deformation and breakup as well as the pressure loss in the channel.     

Numerical investigation of liquid water dynamics in wave‐like gas channels of PEMFCs

For the air feed in proton exchange membrane fuel cells (PEMFCs), the wave‐like gas channel (GC) shows obvious advantages over the straight GC because the former enhances collision of secondary flow and diffusion in the gas diffusion layer (GDL). However, it is prone to water flooding, which brings greater pressure drop, larger pressure oscillation, and blocking of reaction area. In the present study, numerical models of the water dynamic processes, including water droplets emerging from micropores on the GDL surface and removing through the GC, are established based on the volume of fluid (VOF) method. Water coverage ratio and pressure drop are calculated to evaluate the water flooding. The effects of the dimensional parameters of wave‐like GC and contact angle of channel walls on the water accumulation are studied. The emergence and removal of liquid water is a quasiperiodic and oscillating process. Multicycle simulations show that channel pressure drop increases linearly with greater growth rate than channel length. The equilibrium position of water droplet is strongly dependent on the relative wettability of the GDL and bipolar plate (BPP) surfaces. And the geometric parameters of GC have a significant impact on the pressure, water removal behavior and detachment time. Smaller bent angle brings bigger pressure drop, and larger cycle length is helpful for relieving the oscillation of pressure.     

Comparative analysis of two-phase flow in sinusoidal channel of different geometric configurations with application to PEMFC

The droplet dynamics inside a sinusoidal channel for PEMFC (polymer electrolyte membrane fuel cell) are investigated numerically using the VOF (volume of fluid) method. This study is done for three geometrically different channels corresponding to various non-dimensional sinusoidal distances (50, 25, 12.5, 16.7 and 8.3). The effects of key parameters like sinusoidal distance (pitch-amplitude ratio), radius of curvature and wall contact angle on the droplet removal in the flow channel are investigated. The performance of the sinusoidal as compared to the conventional channel is studied based on droplet removal rate and GDL (gas diffusion layer) surface water coverage. It is found that the droplet removal rate increases with increasing sinusoidal distance and wall contact angle. In addition, decrease in the sinusoidal distance results in a significant reduction in the average droplet speed and gas diffusion layer surface water coverage. It was also observed that broken bits of the droplet stuck on the wall corners accrued with a reduction in the wall contact angle. The curvy nature of the side walls generally induces a secondary flow effect which would be most beneficial in enhanced reactant diffusion and cell performance. It is suggested that the sinusoidal distance and wall contact angle effect on two-phase flow in a channel is highly significant. As such, needs to be considered for water management in sinusoidal channels.     

Effect of air flow on liquid water transport through a hydrophobic gas diffusion layer of a polymer electrolyte membrane fuel cell

The transport of liquid water through an idealized 2-D reconstructed gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell is computed subject to hydrophobic boundary condition at the fibre–fluid interface. The effect of air flow, as would occur in parallel/serpentine/interdigitated type of flow fields, on the liquid water transport through the GDL, ejection into the channel in the form of water droplets and subsequent removal of the droplets has been simulated. Results show that typically water flow through the fibrous GDL occurs through a fingering and channelling type of mechanism. The presence of cross-flow of air has an effect both on the path created within the GDL and on the ejection of water into the channel in the form of droplets. A faster rate of liquid water evacuation through the GDL (i.e., more frequent ejection of water droplets) as well as less flooding of the void space results from the presence of cross-flow. These results agree qualitatively with experimental observations reported in the literature.     

Numerical simulation of liquid water emerging and transport in the flow channel of PEMFC using the volume of fluid method

Three-dimensional numerical simulation of liquid water emerging from the gas diffusion layer (GDL) surface to the gas flow channel in the proton exchange membrane (PEM) fuel cell (PEMFC) is carried out using the volume of fluid (VOF) method. The effects of the water velocity in the GDL hole, the airflow velocity and the wettability of the channel surfaces on the water emerging process and transport in the flow channel are investigated. It is found that at low water velocity, the water detaches from the water hole, forming discrete water droplets on the GDL surface, and is transported downstream on the GDL surface until removed from the GDL surface by the U-turn part of the flow channel; whereas at high water velocity, the continuous water column impinges the hydrophilic channel surface counter to the GDL surface, being directly removed from the GDL surface. The airflow velocity affects water detachment and impact process in the channel corner, and water droplet breakup is observed under high airflow velocity. The channel surface wettability influences water droplet shape and its transport in the channel. Rather than forming corner water films at the U-turn for hydrophilic channel surface, water maintains the droplet shape and smoothly passes through the U-turn for hydrophobic channel surface. The importance of the U-turn to the water removal is also discussed. The U-turn promotes water removal from the GDL surface at low water velocity and water breakup at high airflow velocity.     

A flow channel design procedure for PEM fuel cells with effective water removal

Proper water management in polymer electrolyte membrane (PEM) fuel cells is critical to achieve the potential of PEM fuel cells. Membrane electrolyte requires full hydration in order to function as proton conductor, often achieved by fully humidifying the anode and cathode reactant gas streams. On the other hand, water is also produced in the cell due to electrochemical reaction. The combined effect is that liquid water forms in the cell structure and water flooding deteriorates the cell performance significantly. In the present study, a design procedure has been developed for flow channels on bipolar plates that can effectively remove water from the PEM fuel cells. The main design philosophy is based on the determination of an appropriate pressure drop along the flow channel so that all the liquid water in the cell is evaporated and removed from, or carried out of, the cell by the gas stream in the flow channel. At the same time, the gas stream in the flow channel is maintained fully saturated in order to prevent membrane electrolyte dehydration. Sample flow channels have been designed, manufactured and tested for five different cell sizes of 50, 100, 200, 300 and 441 cm². Similar cell performance has been measured for these five significantly different cell sizes, indicating that scaling of the PEM fuel cells is possible if liquid water flooding or membrane dehydration can be avoided during the cell operation. It is observed that no liquid water flows out of the cell at the anode and cathode channel exits for the present designed cells during the performance tests, and virtually no liquid water content in the cell structure has been measured by the neutron imaging technique. These measurements indicate that the present design procedure can provide flow channels that can effectively remove water in the PEM fuel cell structure.     

Three dimensional numerical simulation of gas-liquid two-phase flow patterns in a polymer-electrolyte membrane fuel cells gas flow channel

Water management in polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on fuel cell performance and durability. To investigate the two-phase flow patterns in PEMFC gas flow channels, the volume of fluid (VOF) method was employed to simulate the air-water flow in a 3D cuboid channel with a 1.0 mm × 1.0 mm square cross section and a 100 mm in length. The microstructure of gas diffusion layers (GDLs) was simplified by a number of representative opening pores on the 2D GDL surface. Water was injected from those pores to simulate water generation by the electrochemical reaction at the cathode side. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. The water injection rate was also amplified 10 times, 100 times and 1000 times to study the flow pattern formation and transition in the channel. Simulation results show that, as the flow develops, the flow pattern evolves from corner droplet flow to top wall film flow, then annular flow, and finally slug flow. The total pressure drop increases exponentially with the increase in water volume fraction, which suggests that water accumulation should be avoided to reduce parasitic energy loss. The effect of material wettability was also studied by changing the contact angle of the GDL surface and channel walls, separately. It is shown that using a more hydrophobic GDL surface is helpful to expel water from the GDL surface, but increases the pressure drop. Using a more hydrophilic channel wall reduces the pressure drop, but increases the water residence time and water coverage of the GDL surface.     

Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach

With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.     

Numerical investigation of water dynamics in a novel wettability gradient anode flow channel for proton exchange membrane fuel cells

The effective removal and transport of water in flow channels play an important role in the water management of proton exchange membrane fuel cells (PEMFCs). In this paper, a novel design of anode serpentine flow channel with the wettability gradient wall is discussed and numerically investigated by utilizing the volume-of-fluid (VOF) method. The effects of the contact angle and the wettability gradient of channel walls, as well as hydrogen flow velocity and water droplet size, on the droplet dynamic behavior are studied. The results indicate that compared with the conventional flow channel, the water droplet can be more effectively removed from the turning part in the wettability gradient flow channel. And the water removal ability in the turning part is improved with the increase of the wettability gradient. Moreover, the wettability gradient flow channel can also improve the water removal performance for the cases with different hydrogen flow velocities and water droplet sizes. This study provides ideas for guiding the design of flow channel to effectively enhance anode water management.     

Treatment of two-phase flow in cathode gas channel for an improved one-dimensional proton exchange membrane fuel cell model

It has been reported recently that water flooding in the cathode gas channel has significant effects on the characteristics of a proton exchange membrane fuel cell. A better understanding of this phenomenon with the aid of an accurate model is necessary for improving the water management and performance of fuel cell. However, this phenomenon is often not considered in the previous one-dimensional models where zero or a constant liquid water saturation level is assumed at the interface between gas diffusion layer and gas channel. In view of this, a one-dimensional fuel cell model that includes the effects of two-phase flow in the gas channel is proposed. The liquid water saturation along the cathode gas channel is estimated by adopting Darcy’s law to describe the convective flow of liquid water under various inlet conditions, i.e. air pressure, relative humidity and air stoichiometry. The averaged capillary pressure of gas channel calculated from the liquid water saturation is used as the boundary value at the interface to couple the cathode gas channel model to the membrane electrode assembly model. Through the coupling of the two modeling domains, the water distribution inside the membrane electrode assembly is associated with the inlet conditions. The simulation results, which are verified against experimental data and simulation results from a published computational fluid dynamics model, indicate that the effects of relative humidity and stoichiometry of inlet air are crucial to the overall fuel cell performance. The proposed model gives a more accurate treatment of the water transport in the cathode region, which enables an improved water management through an understanding of the effects of inlet conditions on the fuel cell performance.     

Three-dimensional numerical simulations of water droplet dynamics in a PEMFC gas channel

The dynamic behavior of liquid water emerging from the gas diffusion layer (GDL) into the gas flow channel of a polymer electrolyte membrane fuel cell (PEMFC) is modeled by considering a 1000 μm long air flow microchannel with a 250 μm × 250 μm square cross section and having a pore on the GDL surface through which water emerges with prescribed flow rates. The transient three-dimensional two-phase flow is solved using Computational fluid dynamics in conjunction with a volume of fluid method. Simulations of the processes of water droplet emergence, growth, deformation and detachment are performed to explicitly track the evolution of the liquid–gas interface, and to characterize the dynamics of a water droplet subjected to air flow in the bulk of the gas channel in terms of departure diameter, flow resistance coefficient, water saturation, and water coverage ratio. Parametric simulations including the effects of air flow velocity, water injection velocity, and dimensions of the pore are performed with a particular focus on the effect of the hydrophobicity of the GDL surface while the static contact angles of the other channel walls are set to 45°. The wettability of the microchannel surface is shown to have a major impact on the dynamics of the water droplet, with a droplet splitting more readily and convecting rapidly on a hydrophobic surface, while for a hydrophilic surface there is a tendency for spreading and film flow formation. The hydrophilic side walls of the microchannel appear to provide some benefit by lifting the attached water from the GDL surface, thus freeing the GDL-flow channel interface for improved mass transfer of the reactant. Higher air inlet velocities are shown to reduce water coverage of the GDL surface. Lower water injection velocities as well as smaller pore sizes result in earlier departure of water droplets and lower water volume fraction in the microchannel.     

Lattice Boltzmann simulations of liquid droplet dynamic behavior on a hydrophobic surface of a gas flow channel

Using the multiphase free-energy lattice Boltzmann method (LBM), the formation of a water droplet emerging through a micro-pore on the hydrophobic gas diffusion layer (GDL) surface in a proton exchange membrane fuel cell (PEMFC) and its subsequent movement on the GDL surface under the action of gas shear are simulated. The dynamic behavior of the water droplet emergence, growth, detachment and movement in the gas flow channel is presented. The size of the detached droplet and the time of the droplet removing out of the channel under the influence of gas flow velocity and GDL surface wettability are investigated. The results show that water droplet removal is facilitated by a high gas flow velocity on a more hydrophobic GDL surface. A highly hydrophobic surface is shown to be capable of lifting the water droplet from the GDL surface, resulting in more GDL surface available for gas reactant transport. Furthermore, an analytical model based on force balance is presented to predict the droplet detachment size, and the predicted results are in good agreement with the simulation results. It is shown that the LBM approach is an effective tool to investigate water transport phenomena in the gas flow channel of PEMFCs with surface wettability taken into consideration.     

Numerical simulation for metal foam two-phase flow field of proton exchange membrane fuel cell

Metal foam (MF) flow field has been the potential reactant gas distributor to improve the water management, gas reactant transport and enhance the performance of proton exchange membrane fuel cells (PEMFCs) owing to its unique porous structure. In this study, the full morphology of MF flow field is reconstructed by geometry representation method, and a two-phase volume of fluid (VOF) model is employed to investigate the gas transport and liquid water dynamics in the MF flow field. The present model is validated with the previous experimental and theoretical studies. The single-phase and two-phase flow behaviors in MF flow field and conventional parallel channel are discussed and compared. The results show that a more uniform and convective-to-electrode gas flow can be obtained in MF flow field. Although the water hold-up phenomenon, i.e., water droplets trapped in pores, is observed and slows down the water transport in MF flow field, the porous structures with favorable connectivity and numerous gas pathways still reduce the “water flooding” in the flow field. In addition, hydrophobic walls (or ligaments) are proved necessary for the water management of a MF flow field.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.