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.     

Numerical investigation on the performance of proton exchange membrane fuel cells with channel position variation

Such factors as mole fractions of species, water generation, and conductivity influence the performance of proton exchange membrane fuel cells (PEMFCs). The geometrical shape of the fuel cells also should be considered a factor in predicting the performance because this affects the species' reaction speed and distribution. Specifically, the position between the channel and rib is an important factor influencing PEMFC performance because the current density distribution is affected by the channel and rib position. Three main variables that decide the current density distribution are selected in the paper: species concentration, overpotentials, and membrane conductivity. These variables should be considered simultaneously in deciding the current density distribution with the given PEMFC cell voltage. In addition, the inlet relative humidity is another factor affecting current density distribution and membrane conductivity. In this paper, two channel‐to‐rib models, namely, channel‐to‐channel and the channel‐to‐rib, are considered for comparing the PEMFC performance. Thorough performance comparisons between these two models are presented to explain which is better under certain parameters. A three‐dimensional numerical PEMFC model is developed for obtaining the current density distribution. Water transfer mechanism because of electro osmotic drag and concentration diffusion also is presented to explain the PEMFC performance comparison between the two models. Copyright © 2012 John Wiley & Sons, Ltd.     

Flow channel design in a proton exchange membrane fuel cell: From 2D to 3D

Flow channel design has attracted more and more attention with the evolution of fuel cell technology. Compared with conventional 2D flow channel, 3D flow channel has been proved to improve the performance of proton exchange membrane fuel cell with great enhancement of reactant transport in many researches. In this paper, flow fields of parallel 2D, simplified 3D and 3D with inclination are presented to study the transport and distribution characteristics of reactant and water inside a fuel cell, and efficiency evaluation criterion is proposed to evaluate the superiority of the flow channel design. It is found that 3D flow fields are superior compared with parallel 2D flow channel, with improved capacity of mass transfer, uniform water distribution and advanced water removal ability. The performance improvements of both 3D flow channel designs become significant at elevated current density, with the output voltage increasing to 4.4% at 1.6 A cm^?2 and up to 10% at 2 A cm^?2. Compared with 3D flow channel with inclination, simplified 3D flow channel shows smaller pressure drop, and it has better performance than that of 2D flow channel. Considering both the performance and flow resistance, simplified 3D flow channel performs the best with high efficiency and easy-processing, thus it is the future direction of flow design.     

Numerical investigation on the water saturation of proton exchange membrane fuel cells with channel geometry variation

Many factors, such as mole fractions of oxygen and hydrogen, help improve the performance of proton exchange membrane fuel cells. The variation of mole fractions can be achieved by changing the operating pressure and relative humidity of the fuel cells. The changes in operating conditions are directly related to the electrochemical reaction and water generation of the fuel cells. The geometrical shape of the fuel cells also should be considered a factor in predicting performance because this affects the species' reaction speed and distribution. The current study considers four geometrical cell shapes with varied lengths and electrode and gas channel numbers. The variation in inlet pressure is considered in analyzing the current density distribution of the fuel cells and, subsequently, of liquid water generation. A serpentine gas flow channel is assumed, and its two‐dimensional arrangement is considered in the different gas channel numbers and its length. Four inlet pressure variations and four geometrical shape variations also are considered in analyzing the fuel cells' current density and water generation distributions. The results obtained from this research can be utilized in identifying the fuel cells' optimal operating pressure and designing their gas channel number and arrangement. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Numerical simulation of three-dimensional gas/liquid two-phase flow in a proton exchange membrane fuel cell

Investigation into the formation and transport of liquid water in proton exchange membrane fuel cells (PEMFCs) is the key to fuel cell water management. A three-dimensional gas/liquid two-phase flow and heat transfer model is developed based on the multiphase mixture theory. The reactant gas flow, diffusion, and chemical reaction as well as the liquid water transport and phase change process are modeled. Numerical simulations on liquid water distribution and its effects on the performance of a PEMFC are conducted. Results show that liquid water distributes mostly in the cathode, and predicted cell performance decreases quickly at high current density due to the obstruction of liquid water to oxygen diffusion. The simulation results agree well with experimental data. Translated from J Tsinghua Univ (Sci & Tech), 2006, 46(2): 252–256 [译自: 清华大学学报]     

Measurements of serpentine channel flow characteristics for a proton exchange membrane fuel cell

Flow characteristics at Re = 660–3000 in a serpentine channel are measured. A scale-up model whose channel hydraulic diameter is 50 times as large as that for a proton exchange membrane fuel cell (PEMFC) is used for the measurements. The flow conditions correspond to operating conditions for PEMFCs of 25–40 cm² at current density of 1–3 A/cm² when the fuel utilisation ratio is 0.75 and air is used for the O₂ supply. Two different porous media are used to simulate the gas diffusion layer (GDL). The results suggest that although the leakage flow rate is rather insensitive to the total flow rate, it increases significantly depending on the increase of the GDL permeability. Increasing the flow rate or the permeability enhances the sectional secondary flows and is expected to enhance mass transfer on the GDL. It is confirmed that the flow becomes turbulent around the bend even at Re = 660.     

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.     

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 study of optimized three-dimension novel Key-shaped flow field design for proton exchange membrane fuel cell

Key-shaped three-dimension (3D) flow field channel is designed to improve the performance and mass transfer of proton exchange membrane fuel cell (PEMFC). This study comprehensively analyses the impacts on the performance and mass transfer of the flow channel from multiple dimensions such as the size, shape, and placement of the blocks. In comparison with the conventional straight single flow field channel, the new channel with rectangular blocks can effectively improve performance by 30%. Semi-elliptical and quarter-elliptical blocks are designed to make forced convection and increase the diffusion area of oxygen. The results indicate that the flow velocity in the Z-axis direction can be increased to 0.08–0.2 m/s due to the narrow space formed by variable cross-sections. In conclusion, the Key-shaped design has a potential to improve the performance of mass transfer in the cathode channel, providing a new strategy for the development of flow field design in PEMFC field.     

Optimization and evaluation criteria of water-gas transport performance in wave flow channel for proton exchange membrane fuel cell

Flow channel optimization is an important method to improve the water-gas transport capacity and enhance the performance of proton exchange membrane fuel cells. In this study, the size ratio of the opposite sinusoidal wave flow channel (OSWFC) is optimized, and three-dimensional numerical models are developed to evaluate the comprehensive performance. The results show that OSWFCs obtain performance enhancement compared to the parallel flow channel. 1:1 wave channel has a maximum improvement of 19.53% in mass transfer capacity by the most frequent flow disturbance, while 1:3 has a maximum increasement of 37.5% in water removal performance by the virtue of gentle wave structure. Large pressure loss decreases the mass transfer efficiency, although it with the maximum mass transfer capacity, the mass transfer evaluation criterion of 1:1 wave channel is less than that of 1:3. Moreover, a significant correlation between water-gas transport capacity and electrochemical performance is demonstrated by Pearson correlation coefficient.     

Evaluation of flow field design effects on proton exchange membrane fuel cell performance

Fluid distribution, conduction, and heat control are important phenomenon in the fuel cell fraternity, therefore it is crucial to develop a state-of-the-art bipolar plate (BP) to attain optimum cell performance. Metal foam (MF) and fine mesh have attracted a lot of attention in mitigating some of the challenges associated with straight, and serpentine channels. In this study, MF, 3D fine mesh, fine wire mesh (FWM) flow fields are compared with triple serpentine flow field to develop an optimum design for improved PEMFC performance. Two different foam designs are studied to attenuate the existing drawback associated with MF, mainly caused by high water retention. The 3D fine mesh is leading in performance under anodic and cathodic stoichiometry of 1 and 3 respectively. On increasing the anodic and cathodic stoichiometry to 1.2 and 3.5 respectively, the FWM took the lead. This is brought by the improved water drainage under high stoichiometry. Because FWM is already in mass production, although for other purposes, it is cost competitive over the other designs. The fine mesh and the MF have the potential to break down large water droplet making them easy to drain. They also showed symmetric fluid flow, compared to the serpentine design.     

Mass transfer in proton exchange membrane fuel cells with baffled flow channels and a porous‐blocked baffled flow channel design

In proton exchange membrane fuel cells, baffled flow channels enhance the reactant transfer from flow channels to gas diffusion layers. However, the reactant transfer depends on both the diffusive transfer and convective transfer, and how the baffles in flow channels affect them is still unknown. Therefore, in this work, a two‐dimensional, two‐phase, nonisothermal, and steady‐state model of proton exchange membrane fuel cells is developed, and these two transfer processes from flow channels to gas diffusion layers are comparatively studied. Simulation results show that first of all, the reactant transfer from flow channels to gas diffusion layers mainly depends on the diffusive transfer. Therefore, if the desire is to enhance the mass transfer from flow channels to gas diffusion layers, the diffusive mass transfer should be enhanced firstly. Being guided by this goal, a porous‐blocked baffled flow channel is developed. This flow channel design can further enhance the reactant transfer from flow channels to gas diffusion layers, and the cell performance can be improved. Moreover, when the porosities of porous blocks at the front place of flow channels are lower, the cell power is also increased but the pumping power can be reduced a lot.     

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.     

Numerical investigation into transient response of proton exchange membrane fuel cell with serpentine flow field

The transient response of a proton exchange membrane fuel cell (PEMFC) with a serpentine flow field design is investigated using a three‐dimensional numerical model. The simulations consider three different flow field designs with 7, 11, and 15 bends, respectively. For the flow field design with 11 bends, three different channel width ratios are considered, namely 25%, 50%, and 75%. The channel width ratio is defined as the ratio of the channel width to the total channel/rib width. The simulation results show that for all of the flow field designs, an overshoot in the local current density occurs when the voltage is reduced instantaneously from 0.7 to 0.5 V because of the high and uniform oxygen mass fraction. Conversely, a significant undershoot occurs when the voltage is increased instantaneously from 0.5 to 0.7 V because of the low and nonuniform oxygen mass fraction. The overshoot and undershoot phenomena are particularly evident in the PEMFC with a 15‐bend flow field. For the flow field design with 11 bends, the channel width ratio has little effect on the current density at an operating voltage of 0.7 V. However, at an operating voltage of 0.5 V, the oxygen concentration into the catalyst and diffusion layers increases with the increasing channel width ratio, which leads to higher current density. As a result, a more significant overshoot phenomenon is observed in the flow field with a width ratio of 75%. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical simulation of thermal–hydraulic characteristics in a proton exchange membrane fuel cell

The thermal–hydraulic characteristics of a proton exchange membrane fuel cell (PEMFC) are numerically simulated by a simplified two‐phase, multi‐component flow model. This model consists of continuity, momentum, energy and concentration equations, and appropriate equations to consider the varying flow properties of the gas–liquid two‐phase region in a PEMFC. This gas–liquid two‐phase characteristic is not considered in most of the previous simulation works. The calculated thermal–hydraulic phenomena of a PEMFC are reasonably presented in this paper, which include the distributions of flow vector, temperature, oxygen concentration, liquid water saturation, and current density, etc. Coupled with the electrochemical reaction equations, current flow model can predict the cell voltage vs current density curves (i.e. performance curves), which are validated by the single‐cell tests. The predicted performance curves for a PEMFC agree well with the experimental data. In addition, the positive effect of temperature on the cell performance is also precisely captured by this model. The model presented herein is essentially developed from the thermal–hydraulic point of view and can be considered as a stepping‐stone towards a full complete PEMFC simulation model that can help the optima design for the PEMFC and the enhancement of cell efficiency. Copyright © 2003 John Wiley & Sons, Ltd.     

An investigation of channel blockage effects on hydrogen mass transfer in a proton exchange membrane fuel cell with various geometries and optimization by response surface methodology

A 2D mathematical modeling was developed to analyze the mass transport in a proton exchange membrane fuel cell. The pin fins were inserted in the flow channel to improve reactant gas distribution in the gas diffusion layer (GDL). The effect of rectangular and triangular shape of fins and different title angles of 4, 6 and 8° on the reactant gas transport were examined. The results showed that performance of rectangular fins are better than triangular fins due to increasing reactant spread over the GDL. The effect of three independent factors including length and width of blocks and hydrogen gas velocity on the response (hydrogen gas diffusion to GDL and pressure drop in anode channel) was investigated using analysis of variance (ANOVA). The results showed that block height and hydrogen gas velocity are the most important factors affecting the responses. Also, response surface methodology (RSM) method was used to predict the optimal conditions to achieve the minimum the pressure drop and maximum the total flux magnetic H₂ to GDL in anode channel. The result of the optimization process shows that a gas velocity of 4.22 m/s and the block with height and width of 3 mm are the optimal conditions.     

Performance investigation on a novel 3D wave flow channel design for PEMFC

Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel.     

Effects of anode and cathode perforated flow field plates on proton exchange membrane fuel cell performance

The effects of both anode and cathode perforated flow field configurations on proton exchange membrane fuel cell performance are studied herein through electrochemical polarization techniques, electrochemical impedance spectroscopy, and cyclic voltammetry. The results demonstrate that serpentine flow field configuration in both anodes and cathodes is the best arrangement for cell performance (serpentine/serpentine, perforated/perforated, and serpentine/perforated). An electrochemical impedance spectroscopy examination shows that the serpentine/serpentine flow plate configuration results in a significant reduction in charge transfer resistance in a high current density (low voltage) regime. It further indicates that in a serpentine/serpentine flow pattern, a maximum electrochemical area is obtained with a higher Pt utilization of about 70% and is secured with full hydration at a cell temperature of 80°C. Finally, energy and exergy efficiencies analyses were also made. Data have been extracted and presented. Copyright © 2013 John Wiley & Sons, Ltd.