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.     

Development of bipolar plates with different flow channel configurations based on plant vein for fuel cell

In this paper, a kind of proton exchange membrane fuel cell (PEMFC) bipolar plate based on plant vein is developed through biomimetic analogy theory. Fluent software is employed to test the performance characteristics of this newly designed bipolar plate. It is found from the numerical simulation results that the PEMFC performance will be influenced by the number and location of the biomimetic flow channel branches. The distribution pattern of branches has great impact on the outlet velocity. The more the branch number is, the more favorable for water removal. Finally, different operating parameters, such as temperature, pressure, relative humidity and stoichiometric ratio, are chosen based on the optimal flow channel configuration to improve PEMFC performance. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

A three-dimensional full-cell CFD model used to investigate the effects of different flow channel designs on PEMFC performance

A three-dimensional “full-cell” computational fluid dynamics (CFD) model is proposed in this paper to investigate the effects of different flow channel designs on the performance of proton exchange membrane fuel cells (PEMFC). The flow channel designs selected in this work include the parallel and serpentine flow channels, single-path and multi-path flow channels, and uniform depth and step-wise depth flow channels. This model is validated by the experiments conducted in the fuel cell center of Yuan Ze University, showing that the present model can investigate the characteristics of flow channel for the PEMFC and assist in the optima designs of flow channels. The effects of different flow channel designs on the PEMFC performance obtained by the model predictions agree well with those obtained by experiments. Based on the simulation results, which are also confirmed by the experimental data, the parallel flow channel with the step-wise depth design significantly promotes the PEMFC performance. However, the performance of PEMFC with the serpentine flow channel is insensitive to these different depth designs. In addition, the distribution characteristics of fuel gases and current density for the PEMFC with different flow channels can be also reasonably captured by the present model.     

Three-dimension simulation of two-phase flows in a thin gas flow channel of PEM fuel cell using a volume of fluid method

This study investigates the two-phase flow in a thin gas flow channel of PEM fuel cells and wall contact angle's impact using the volume of fluid (VOF) method with tracked two-phase interface. The VOF results are compared with experimental data, theoretical solution and analytical data in terms of flow pattern, pressure drop and water fraction. Stable film flow is predicted, as observed experimentally, for the contact angle ranging from 5° to 40° including varying contact angles at different walls of a channel. The contact angle is found to have small impact on the gas pressure drop for the stratified flow regime, but it determines the meniscus of the two-phase interface, which affects the optical detection of the liquid thickness in experiment. The work is important to study of two-phase flow dynamics, multichannel design, experimental design and control of two-phase flows in thin gas flow channels for PEM fuel cells.     

Experimental study on non-uniform arrangement of 3D printed structure for cathodic flow channel in PEMFC

Enhancing mass transfer capability of flow channel is important subject to improve fuel cell performance. In this study, an experimental study about non-uniform arrangement of metallic structures in cathodic flow channel on polymer electrolyte membrane fuel cell is conducted. Metallic 3D printer is used to manufacture 3D mesh with complex geometry. Channel width and bottom-rear channel depth are selected to change the geometry of structure. Assuming that accelerating flow velocity and reinforcing flow direction to gas diffusion layer can improve fuel cell performance, eight basic arrangements are inserted into cathodic carbon bipolar plate to measure fuel cell performance. With I–V curve, the unit cells with non-uniform arrangements of width and tapered structure show performance enhancement of 12.8% in maximum power density, compared with conventional parallel flow channel. According to electrochemical impedance spectroscopy results, performance enhancement by non-uniform arrangement is mainly occur in high current density operation due to low mass transfer resistance.     

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 on the effect of flow field and landing to channel ratio on the performance of PEMFC

Three-dimensional numerical investigation of PEMFC with landing to channel ratio (L:C) of 2:2 for 25-cm² serpentine-parallel channel has been simulated, and the obtained results have been validated with the polarization curve obtained through experiments. It is found that the maximum error in the polarization curve is less than 4%, and thus a very good deal exists between the simulation study and experimentation. Upon validation, the study has been extended for various flow path designs with different L:C ratio numerically. The prediction reveals that the L:C ratio of 2:2 exhibits the better performance for all the flow channels considered, and it is found that the straight-zigzag flow field with L:C ratio of 2:2 attributes the maximum power density of 0.3250 W/cm² for an optimum open circuit voltage of 0.4 Volts with minimal pressure drop. Oxygen consumption in the cathode flow channels of serpentine-parallel, serpentine-zigzag, and straight-parallel are 77.08%, 10.41%, and 42.70% lesser than that of straight-zigzag PEMFC, respectively. The pressure drop in the flow channel of serpentine-parallel, serpentine-zigzag, and straight-parallel with landing to channel ratio 2:2 are 78.18%, 95.81%, and 48.33% higher than that of straight-zigzag flow field, respectively. The polarization curve, hydrogen (H₂), oxygen (O₂), water content along the flow channel and the proton conductivity, H₂O content across the membrane electrolyte, and current density contour at the GDL/catalyst interface of the anode side for all flow channel configurations have been presented and discussed.     

Numerical simulation and experimental study on the effect of symmetric and asymmetric bionic flow channels on PEMFC performance under gravity

In proton exchange membrane fuel cell (PEMFC), bionic flow field design is to apply the biological characteristics of nature to the structure design of flow field. The flow field designed by bionics can improve the water balance of the fuel cell and make the fuel distribute uniformly in the flow field. In order to study the PEMFC performance of symmetric and asymmetric bionic flow channel under gravity, the simulation and visualization experiments are used to study the bionic flow channel in different orientations. Under the influence of gravity, the distribution characteristics of liquid water are changed in the flow channel, and the difference of the transport process of liquid water in two different bionic flow channel under gravity is obtained. The results of the simulation and visualization experiments show that the gravity has a significant effect on the transport process of liquid water in the bionic flow channel, and the water transport process in the two types of bionic flow channel is obviously different. Meanwhile, the performance of the fuel cells with two bionic flow channel at different orientations is tested by experiments. The results show that gravity has a significant effect on the performance of PEMFC with bionic flow field. And there are significant differences between symmetrical and asymmetric bionic flow channel on PEMFC performance. The results of I–V curve show that when the PEMFC with asymmetric bionic flow channel has the best performance in the orientation of perpendicularity.     

Numerical investigation of the impact of two-phase flow maldistribution on PEM fuel cell performance

Flow maldistribution usually happens in PEM fuel cells when using common inlet and exit headers to supply reactant gases to multiple channels. As a result, some channels are flooded with more water and have less air flow while other channels are filled with less water but have excessive air flow. To investigate the impact of two-phase flow maldistribution on PEM fuel cell performance, a Volume of Fluid (VOF) model coupled with a 1D MEA model was employed to simulate two parallel channels. The slug flow pattern is mainly observed in the flow channels under different flow maldistribution conditions, and it significantly increases the gas diffusion layer (GDL) surface water coverage over the whole range of simulated current densities, which directly leads to poor fuel cell performance. Therefore, it is recommended that liquid and gas flow maldistribution in parallel channels should be avoided if possible over the whole range of operation. Increasing the gas stoichiometric flow ratio is not an effective method to mitigate the gas flow maldistribution, but adding a gas inlet resistance to the flow channel is effective in mitigating maldistribution. With a carefully selected value of the flow resistance coefficient, both the fuel cell performance and the gas flow distribution can be significantly improved without causing too much extra pressure drop.     

Modification of carbon aerogel supports for PEMFC catalysts

Nitrogen enriched carbon aerogels and Co-based non-noble metal catalysts supported on carbon aerogels have been synthesized and tested using XPS, HRTEM, XRD and RDE techniques. XPS spectra of unmodified carbon aerogels indicated a presence of two oxygen O(1s) groups and five carbon C(1s) groups in deconvoluted spectra. XPS spectra of chemically modified samples indicated nitrogen N(1s) introduced in the carbon aerogel structure by acidic (HNO₃) or basic (NH₄OH) chemical treatment.Synthesis of aerogel supported Co catalysts performed by using Co-methoxy-tetra-phenylporfirin as a macrocyclic compound incorporated into the aerogel structure, and sintered at 700 and 900 °C in N₂, revealed the presence of Co-metal nano-particles with 20 nm diameter. HRTEM and diffraction patterns show a β-Co FCC structure with many {111}<110> micro-twins in the Co nano-particles. The electrochemical properties of the synthesized catalysts in O₂-free and O₂-saturated sulfuric and perchloric acid solutions, evaluated by a rotating disc electrode (RDE) technique, demonstrated catalytic activity in hydrogen oxidation and oxygen reduction reactions.     

Experimental investigation of self-regulating capability of open-cathode PEMFC under different fan working conditions

Self-regulation capability of the open-cathode PEMFC generally means that the stack itself can adjust its state to response to different operating conditions to achieve better performance when the external control strategy remains unchanged. In this paper, self-regulating capability of the stack are analyzed when its cooling fan works under blow or suction mode at different voltages. The result of output voltage shows that the stack achieves better self-regulation when the fan operates at 8.5 V in both blow mode and suction mode. Analysis of impedance spectra reveals that the stack can realize self-regulating function by adjusting activation resistance and ohmic resistance, and the cathode activation resistance is dominant. Furthermore, the result of a cycle load test indicates that the stack can better reflect the self-regulating capability in fan suction mode than in blow mode, and the stack can achieve better water and heat regulation in suction mode. Finally, according to the air velocity distribution and temperature change, it is found that self-regulating capability in suction mode play a better role due to more uniform heat remove. A suitable fan operating voltage and mode are critical for the self-regulating capability of the open-cathode PEMFC stack to maintain a water-heat balance.     

Impact of channel geometry on two-phase flow in fuel cell microchannels

An important function of the gas delivery channels in PEM fuel cells is the evacuation of water at the cathode. The resulting two-phase flow impedes reactant transport and causes parasitic losses. There is a need for research on two-phase flow in channels in which the phase fraction varies along the flow direction as in operating fuel cells. This work studies two-phase flow in 60 cm long channels with distributed water injection through a porous GDL wall to examine the physics of flows relevant to fuel cells. Flow regime maps based on local gas and liquid flow rates are constructed for experimental conditions corresponding to current densities between 0.5 and 2 A cm⁻² and stoichiometric coefficients from 1 to 4. Flow structures transition along the length of the channel. Stratified flow occurs at high liquid flow rates, while intermittent slug flow occurs at low liquid flow rates. The prevalence of stratified flow in these serpentine channels is discussed in relation to water removal mechanisms in the cathode channels of PEM fuel cells. Corners facilitate formation of liquid films in the channel, but may reduce the water-evacuation capability. This analysis informs design guidelines for gas delivery microchannels for fuel cells.     

Experimental analysis of internal gas flow configurations for a polymer electrolyte membrane fuel cell stack

The internal gas distribution system utilised for supplying fresh reactants and removing reaction products from the individual cells of a fuel cell stack can be designed in a parallel, a serial or a mixture of parallel and serial gas flow configuration. In order to investigate the interdependence between the internal stack gas distribution configuration and single cell as well as overall stack performance, a small laboratory-scale fuel cell stack consisting of identical unit cells was subject to operation with different gas distribution configurations and different operating parameters. The current/voltage characteristics measured with the different gas distribution configurations are analysed and compared on unit cell- as well as on stack-level. The results show the significant impact of the internal stack gas distribution system on operation and performance of the individual unit cells and the overall stack.     

A critical review of two-phase flow in gas flow channels of proton exchange membrane fuel cells

Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas-liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas-liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas-liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas-liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented.     

蛇形流场结构质子交换膜燃料电池的性能研究

建立包括催化层、扩散层、质子膜在内的三维质子交换膜燃料电池模型,通过Fluent软件模拟4种不同结构的蛇形流场,通过对速度、膜中水含量以及功率密度等分析得出蛇形流场的最优结构,并对最优结构进行参数优化。研究表明,4种不同蛇形流场结构中,Multi-serpentine II为最优,随着温度、压强的增加,这种流场结构的燃料电池呈现出良好的性能,从而为质子交换膜燃料电池双极板的设计提供依据。     

Improving two-phase mass transportation under Non-Darcy flow effect in orientated-type flow channels of proton exchange membrane fuel cells

Orientated-type flow channels of proton exchange membrane fuel cells cause non-Darcy effect occurring in flow regions. Therefore, the species transportation is affected by inertial effect. However, how the inertial force affects convection and diffusion of different species has not been discussed before. Thus, a modified two-dimensional, non-isothermal, two-phase and steady state model considering non-Darcy effect is employed in this study, and reactants and products transportations through diffusion and convection under inertial effects are quantitatively analyzed for the first time. Simulation results reveal that the convective transportation of reactants increases more under the influence of inertial force; water vapor transportation through convection increases the water content in porous regions. At the same time, liquid water expels more rapidly from gas diffusion layers under baffle regions, and enlarging baffle volumes increases the regions where the liquid water is rapidly removed under the inertial effect.     

A CFD model with semi-empirical electrochemical relationships to study the influence of geometric and operating parameters on DMFC performance

A three-dimensional computational fluid dynamics (CFD) model is developed to investigate the influence of geometric and operating parameters on performance of a direct methanol fuel cell (DMFC). Semi-empirical relationships are introduced to describe the electrochemical behaviors required in the CFD governing equations. Coefficients in these semi-empirical relationships are fitted using experimental data. Two geometric configurations with serpentine channels at the anode and cathode are considered in this work. Temperature, methanol concentration, and methanol flow rate are selected as the operating parameters. Due to the computational effort of CFD, an adaptive metamodeling method is developed to reduce the number of data-fitting iterations for obtaining the coefficients in the semi-empirical relationships. The effectiveness of the method is demonstrated by fitting the model using the experimental data collected from the first geometric configuration of the DMFC and comparing the predicted performance of the second configuration with its experimental performance. A commercial CFD system, Fluent 12.0, was used in this research.     

The optimal parameters estimation for rectangular cylinders installed transversely in the flow channel of PEMFC from a three-dimensional PEMFC model and the Taguchi method

The study first applies a three-dimensional model to analyze the cell performance of PEMFCs using rectangular cylinders with various numbers transversely inserted at the axis in the channel, and finds the higher performance with reasonable pressure drop. The Taguchi optimization methodology is then combined with the three-dimensional PEMFC model to determine the optimal combination of five primary operating parameters for the best arrangement of the rectangular cylinders in the channel. The results indicate that the optimal combination factor is a cell temperature of 313 K, an anode humidification temperature of 333 K, a cathode humidification temperature of 333 K, a hydrogen stoichiometric flow ratio of 1.9, and an oxygen stoichiometric flow ratio of 2.7. This study also examines the pressure drop for the channels with rectangular cylinders transversely inserted. Using experimental data verifies the numerical results of the flow field design with rectangular cylinders.     

The effects of relative humidity on the performances of PEMFC MEAs with various Nafion ionomer contents

For PEMFC operation, water management is very important to provide both sufficient proton conductivity and mass transport. Therefore, in this study, the effect of the relative humidity (38–87%) on cell performance is examined for PEMFC MEAs with various Nafion^® ionomer contents. The MEAs were fabricated using a CCM (catalyst-coated membrane) spraying method. As the relative humidity of the cathodes (RHC) increases, the cell voltages at 0.4 and 1.2 A/cm² increased for a MEA with 20 wt% ionomer. This can be explained in terms of the expansion of active sites with enhanced ionic conductivity (activation overpotential). In contrast, with a higher RHC value, the cell voltages for 35 wt% ionomer (more hydrophilic) gradually decreased as a result of slower gas transport (concentration overpotential). For MEAs with intermediate ionomer contents, 25 and 30 wt%, the cell voltages at 0.4 A/cm² showed maximum values at a RHC of 67%, at which point the mass transport begins to be the more dominant factor. The highest unit cell performance was observed in a MEA with an ionomer content of 25 wt% at a RHC of 59% and in a MEA at 20 wt% ionomer content at a higher humidity of 87%.