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.     

Experimental validation of a lumped model of single droplet deformation,oscillation and detachment on the GDL surface of a PEM fuel cell

Proton Exchange Membrane Fuel Cell (PEMFC) performance significantly depends on electrodes water content. Liquid water emerging from the Gas Diffusion Layer (GDL) micro-channels can form droplets, films or slugs in the Gas Flow Channel (GFC). In the regime of droplets formation, the interaction with the gas flow leads to an oscillating mechanisms that is fundamental to study the detachment from the GDL surface. In this work, a numerical model of a droplet growing on the GDL surface is developed to describe the interaction between droplet and gas flow. Therefore, a lumped force balance is enforced to determine the center of mass motion law. Oscillation frequencies during growth and at detachment are found as a function of droplet size. The model is also exploited to find the relationship between droplet critical detachment size and gas velocity. The numerical results are compared with the experimental data previously published by the authors as well as with other experimental results available in the literature. The matching between the numerical and experimental data is very good. The low computational burden and the conciseness of the proposed approach make the model suitable for applications such as control and optimization strategies development to enhance PEMFC performance. Additionally, the model can be exploited to implement monitoring and diagnostic algorithm as well.     

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.     

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.     

Experimental investigation of water droplet–air flow interaction in a non-reacting PEM fuel cell channel

It has been well documented that water production in PEM fuel cells occurs in discrete locations, resulting in the formation and growth of discrete droplets on the gas diffusion layer (GDL) surface within the gas flow channels (GFCs). This research uses a simulated fuel cell GFC with three transparent walls in conjunction with a high speed fluorescence photometry system to capture videos of dynamically deforming droplets. Such videos clearly show that the droplets undergo oscillatory deformation patterns. Although many authors have previously investigated the air flow induced droplet detachment, none of them have studied these oscillatory modes. The novelty of this work is to process and analyze the recorded videos to gather information on the droplets induced oscillation. Plots are formulated to indicate the dominant horizontal and vertical deformation frequency components over the range of sizes of droplets from formation to detachment. The system is also used to characterize droplet detachment size at a variety of channel air velocities. A simplified model to explain the droplet oscillation mechanism is provided as well.     

The analysis of adhesion force at the interface of gas diffusion layer and channel in polymer electrolyte membrane fuel cell

The adhesion force of water droplet on the gas diffusion layer (GDL) is modeled based on the droplet deformation. The deformed droplet is represented as an ovoid shape. The adhesion force is calculated based on it and verified by the surface tilting experiment. The model predicts the shape of deformed droplet and adhesion force within 30% error, whereas previous models predict adhesion force with error larger than 30%. The modified model is used to compare the adhesion force among 3 types of GDL having pore gradient. The comparison result is well matched with the water distribution in polymer electrolyte membrane fuel cell (PEMFC) and water detachment phenomena at the GDL. High adhesion force makes more water accumulation at the interface of GDL and gas supplying channel. This makes different boundary condition and changes the water distribution in PEMFC.     

Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels

Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.     

Numerical investigation of water droplet removal characteristics in novel block channels of PEMFC using dynamic wettability model

Liquid water transport and removal is one of the critical issues in the proton exchange membrane fuel cell (PEMFC) for achieving good performance and durability. In this study, two novel channels with different blocks are designed to study their effects on water removal using the volume of fluid (VOF) model considering the dynamic contact angle effect. It is found that compared with the conventional straight channel, both the one-block and two-block channels can promote liquid water removal. The one-block channel leads to faster water movement and removal on the gas diffusion layer (GDL) surface, but results in a much higher pressure drop. The separated two-block channel directly drags water away from the GDL surface by the capillary wicking effect of the block surface, achieving both faster water removal and smaller pressure drop. Effects of the droplet size, air velocity and static contact angle of GDL surface on water removal are investigated comprehensively in both the novel channels, as well as the conventional straight channel, with particular attention on the variations of water removal time, water coverage ratio and pressure drop.     

Effects of needle orientation and gas velocity on water transport and removal in a modified PEMFC gas flow channel having a hydrophilic needle

Water management is critical to the performance and operation of the proton exchange membrane fuel cell (PEMFC). Effective water removal from the gas diffusion layer (GDL) surface exposed to the gas flow channel in PEMFC mitigates the water flooding of and improves the reactants transport into the GDL, hence benefiting the PEMFC performance. In this study, a 3D numerical investigation of water removal from the GDL surface in a modified PEMFC gas flow channel having a hydrophilic needle is carried out. The effects of the needle orientation (inclination angle) and gas velocity on the water transport and removal are investigated. The results show that the water is removed from the GDL surface in the channel for a large range of the needle inclination angle and gas velocity. The water is removed more effectively, and the pressure drop for the flow in the channel is smaller for a smaller needle inclination angle. It is also found that the modified channel is more effective and viable for water removal in fuel cells operated at smaller gas velocity.     

Evaluating breakthrough pressure in gas diffusion layers of proton exchange membrane fuel cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer （GDL） of a pro- ton exchange membrane fuel cell （PEMFC）. An ex-situ testing was conducted on a transparent test cell to visu- alize the water droplet formation and detachment on the surface of different types of GDLs through a CCD cam- era. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for the GDLs with different porosities and thick- nesses. The equilibrium pressure, which is defined as the minimum pressure required maintaining a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. The breakthrough pressure increases for the thick GDL with smaller micro-pore size.     

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.     

Numerical simulations of two-phase flow in a proton-exchange membrane fuel cell based on the generalized design method

The water management of proton-exchange membrane fuel cell (PEMFC) has a major impact on the performance of the cell system. In order to investigate the influence of air velocity and wettability on the whole process during penetration of liquid water, a generalized two-dimensional model in conjunction with the volume of fluid (VOF) method was used to simulate the whole processes from gas diffusion layer (GDL) to gas channel (GC). The results show that the wettability of the medium plays a significant role than flow rate for the penetration of liquid water in the GDL. It is shown that favorable hydrophobicity and high air velocity in GC is helpful to remove liquid droplets on the GDL surface. By contrast, the stable droplets spacing on GDL surface is more concentrated and the percentage of liquid area is more extensive under the hydrophilic and low-velocity case, which would aggravate the liquid water and hard to remove from 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.     

Investigating effect of different gas diffusion layers on water droplet characteristics for proton exchange membrane (PEM) fuel cells

In this work, advanced x-ray radiographic techniques available at the Canadian Light Source (CLS) were utilized to study water droplet dynamics in a serpentine flow channel mimicking a proton exchange membrane fuel cell (PEMFC). High spatial and temporal resolution coupled with high energy photons of an x-ray beam provided high-resolution images of water droplets. This technique solved the problem caused by the opaqueness of fuel cell materials including the gas diffusion layer by providing a unique way to study water droplet dynamics at different operating conditions. From the captured images, droplet emergence and formation on porous gas diffusion layers (GDLs) were analyzed. Three commercially available GDLs (Sigracet AA, Sigracet BA, and Sigracet BC) were used and droplet detachment height was found to decrease in the following order AA < BA < BC under the same flow condition. Increasing the superficial gas velocity was found to decrease the droplet detachment height for all GDLs tested. Average droplet cycle for various operating conditions was obtained. It was found that humidified air did not show a difference in droplet dimensions at detachment compared to dry air used at the inlet gas. However, it did show an impact on droplet cycle time, which might be due to condensation.     

Three-dimensional numerical simulation of water droplet emerging from a gas diffusion layer surface in micro-channels

The dynamic formation of water droplets emerging from a gas diffusion layer (GDL) surface in micro-channels was simulated using the volume of fluid (VOF) method. The influence of GDL surface microstructure was investigated by changing the pore diameter and the number of pore openings on the GDL surface. Simulation results show that the microstructure of the GDL surface has a significant impact on the two-phase flow patterns in gas flow channels. For a non-uniform GDL surface, three stages were identified, namely emergence and merging on the GDL surface, accumulation on the channel sidewalls and detachment from the top wall. It was also found that if the pore size is small enough, the flow pattern in the channel does not change with further reduction in the pore diameter. However, the two-phase flow patterns change significantly with the wettability of the GDL surface and sidewalls, but remain the same when the liquid flow rate is reduced by two orders of magnitude from the reference case.     

Water droplet detachment characteristics on surfaces of gas diffusion layers in PEMFCs

Water management is one of the key issues affecting the performance and stability of proton exchange membrane fuel cells (PEMFCs). Water detachment on the gas diffusion layer (GDL) surface is critically important to water management in PEMFCs. In this study, water droplet detachment characteristics under various GDL surface contact angles and channel heights are investigated, by using a customized transparent model cell for direct ex-situ water visualization. The droplet height, chord, height/chord ratio, and contact angle hysteresis at the instant of droplet detachment are quantitatively analyzed. The droplet detachment is easier for higher gas Reynolds number (Re_g). The height and chord of the droplet both decrease with Re_g for both GDLs with and without PTFE but their decrement rates become smaller in higher Re_g regions for all the channel heights investigated. Compared with droplets on the untreated GDL, the droplet height/chord ratio on the PTFE-treated GDL with larger static contact angle is increased by 36.7%, 64.1% and 76.0% and the contact angle hysteresis is reduced by 17.1%, 16.3% and 12.6% for the channel height H of 1 mm, 2 mm and 3 mm, respectively, which indicates that the PTFE-treated GDL improves water detachment. It shows that the water detachment is improved by reducing the channel height due to the smaller contact angle hysteresis at the instant of droplet detachment.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

Visualisation of water droplets during the operation of PEM fuel cells

A transparent proton exchange membrane fuel cell (PEMFC) has been designed to enable visualisation of water droplets during its operation. Images of the formation of droplets on the surface of the gas diffusion layer (GDL) on its cathode side, which result in water accumulation and blockage to the airflow channels, were recorded using a CCD camera. Measurement of the cell current and droplet characterisation have been carried out simultaneously and the effect of the airflow and external resistive load has been quantified. The droplet images show that water accumulation occurs first in the middle channels of a serpentine reactant-flow fuel cell design and that no droplets are formed at the bends of the flow channels. Water blockage to the airflow path was caused by the overlapping of two land-touching droplets developing on each side of the channel. Flooding was found to be more susceptible to the airflow than the other test operating conditions.     

Dynamic water transport and droplet emergence in PEMFC gas diffusion layers

The dynamics of liquid water transport through the gas diffusion layer (GDL) and into a gas flow channel are investigated with an ex situ experimental setup. Liquid water is injected through the bottom surface of the GDL, and the through-plane liquid pressure drop, droplet emergence and droplet detachment are studied. The dynamic behaviour of water transport in and on the surface of the GDL is observed through fluorescence microscopy, and the through-plane liquid pressure drop is measured with a pressure transducer. With an initially dry GDL, the initial breakthrough of liquid water in the GDL is preceded by a substantial growth of liquid water pressure. Post-breakthrough, droplets emerge with a high frequency, until a quasi-equilibrium liquid water pressure is achieved. The droplet emergence/detachment regime is followed by a transition into a slug formation regime. During the slug formation regime, droplets tend to pin near the breakthrough location, and the overall channel water content increases due to pinning and the formation of water slugs. Droplets emerge from the GDL at preferential breakthrough locations; however, these breakthrough locations change intermittently, suggesting a dynamic interconnection of water pathways within the GDL. The experiments are complemented by computational fluid dynamics (CFD) simulations using the volume of fluid method to illustrate the dynamic eruption mechanism.