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.     

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.     

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.     

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.     

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.     

Liquid water transport between graphite paper and a solid surface

We studied the interaction of a water droplet with a solid wall on a hydrophobic gas diffusion layer (GDL). Of particular interest is the stability of the droplet as a function of plate wetting properties and the potential for liquid entrapment in the GDL/land contact area. Such transport is of relevance to breakthrough dynamics and convective liquid droplet transport in polymer electrolyte membrane (PEM) fuel cell cathode gas channels. While a variety of complex coupled transport phenomena are present in the PEM fuel cell gas channel, we utilize a very simplified experimental model of the system where a droplet originally placed on a hydrophobic GDL is translated quasistatically across the GDL surface by a solid surface. Transport and entrapment are imaged using fluorescence microscopy. This work provides new insights into droplet behaviour at the GDL/land interface in a PEM fuel cell and suggests that hydrophobic land areas are preferable for mitigating the accumulation of liquid water under the land area of the gas flow channels.     

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 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.     

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.     

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.     

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.     

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.     

Droplet dynamics in a proton exchange membrane fuel cell flow field design with 3D geometry

Water management is crucial to achieve both high-performance and durability of proton exchange membrane fuel cell (PEMFC). Therefore, it is necessary to investigate the dynamic behavior of droplets in PEMFC channel for water management. In this paper, we explore the kinetics of droplets in a 3D flow field by experimental and theoretical analysis. More specifically, we examine the following four perspectives: 1) the movement and falling of droplets, and their force and deformation, 2) the superiority of 3D flow field drainage, 3) the pressure and viscous force under different scenarios including varying droplet sizes and velocities, and 4) the expression describing the shape change of droplets. The results show that the 3D flow field has a greater driving force on droplets and that their deformation affects the discharge of liquid water. Throughout the study, we provide better understanding of droplet dynamic in PEMFC gas channels. It enables to optimize the design and working conditions of these channels.     

A droplet size dependent multiphase mixture model for two phase flow in PEMFCs

A droplet size dependent multiphase mixture model is developed in this paper, and the droplet size in the gas channel can be considered as a parameter in this multiphase mixture model, which includes the effect of gas diffusion layer (GDL) properties and the gas drag function and cannot be considered in the commonly used multiphase mixture model in the references. The three-dimensional two phase and non-isothermal simulation of the PEMFCs with a straight flow field is performed. The effect of droplet size on the liquid remove, the effect of liquid water on the heat transfer and the effect of gas flow pattern on the heat and mass transfer are mainly investigated. The simulation results show that the large droplet is hard to be dragged by the gas, so it produces large water saturation. The results of the heat transfer show that the liquid water hinders the heat transfer in the GDL and catalyst layer, so it produces the large relative high temperature area, and there are large temperature difference and water saturation in the PEMFCs operated with coflow pattern compared with counter flow pattern.     

Visualization of water transport in GDL and flow channel of PEMFC

Liquid water penetrating the gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) was studied. The gas diffuse layer (GDL) has a great impact on PEMFC's performanc3e, and is an important component of a PEMFC. An ex‐situ test was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera. 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 GDLs with different porosities and thicknesses. The equilibrium pressure, which is defined as the minimum pressure required for 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. Also the drain performance using three kinds of different bipolar plates were compared in this paper. According to the result of experiment, the average diameters of porous GDLs were found determining the penetrating pressure. Serpentine flow channel proved the best pattern for drainage. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20330     

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.     

Investigating the effect of operating temperature on dynamic behavior of droplets for proton exchange membrane fuel cells

The present work depicts advanced understanding of dynamic behavior of droplets in serpentine channels for proton exchange membrane (PEM) fuel cells using a synchrotron-based X-ray radiographic technique at the Canadian Light Source (CLS). Due to high photon flux, the synchrotron-imaging technique provides high temporal and spatial resolution of images of water droplets in gas flow channels for PEMFCs. The whole cell and in particular, the gas diffusion layer (GDL) were operated under elevated temperatures through a self-designed heating channel around the GDL area (25–75 °C). In the experiments, a Sigracet 35 BC GDL was employed, and droplet height, chord and cycle time were measured over a series of images collected. Experimental results show that the droplet cycle time had been significantly reduced to less than 0.5 s from 14 s with an increase in the operating temperature from 25 °C to 75 °C. In addition, it is also found that increasing the operating temperature decreases the critical dimension of water droplets upon detachment i.e. from 0.46 mm for 25 °C to 0.33 mm for 75 °C. at a superficial gas velocity of 5.98 m/s.     

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.     

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 proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.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...