Fuel Cell Gas Channels Research Articles

Numerical analysis of transport phenomena in solid oxide fuel cell gas channels

The gas channel geometry in solid oxide fuel cells (SOFCs) influences the reacting thermo-fluid process and, thus, overall cell performance. This paper presents a dimensionless approach to the study of the transport phenomena in the gas channels of planar anode-supported proton-conducting SOFC. Out-of-scale modeling reduces the number of variables that should be investigated and offers generalized results, giving insight into similar fuel cells. A 2D numerical model for the multiphysics process in SOFC is developed. A dimensionless form of the governing equations is derived in order to identify the dimensionless quantities that characterize the transport phenomena in SOFC. Reynolds, Peclet, and Sherwood are the important parameter groupings of flow channels that influence mass and temperature distribution. The efficacy of the computational fluid dynamic model is confirmed by comparing simulated results with experimental data from the literature. The effect of fuel and air channels’ dimensionless parameters on cell performance is discussed. Similar changes in fuel and air channels exert various influences on SOFC electrical performance. It is found that reducing Pe in the fuel channel improves power generation. However, Sh and Re reduction effect neutralize the increase in power generation due to Pe reduction in the air channel.

Discrete-Particle Model to Optimize Operational Conditions of Proton-Exchange Membrane Fuel-Cell Gas Channels.

Operation of proton-exchange membrane fuel cells is highly deteriorated by mass transfer loss, which is a result of spatial and temporal interaction between airflow, water flow, channel geometry, and its wettability. Prediction of two-phase flow dynamics in gas channels is essential for the optimization of the design and operating of fuel cells. We propose a mechanistic discrete particle model (DPM) to delineate dynamic water distribution in fuel cell gas channels and optimize the operating conditions. Similar to the experimental observations, the model predicts seven types of flow regimes from isolated, side wall, corner, slug, film, and plug flow droplets for industrial temporal and spatial scales. Consequently, two-phase flow regime maps are proposed. The results suggest that an increase in water accumulation in the channel is related to the increase in the water cluster density emerging from the gas diffusion layer rather than the increased water flow rate through constant water pathways. From a modeling perspective, the DPM replicated well volume-of-fluid channel simulation results in terms of saturation, water coverage ratio, and interface locations with an estimated 5 orders of magnitude increase in calculation speed.

Water Distribution in Fuel Cell Gas Channels Using a Mechanistic Discrete Particle Model

Water removal and accumulation in fuel cell gas channels is a complex phenomenon which is a consequence of the strength of the capillary forces and inertial forces (1 < Re < 1000). The presence of water in the channel restricts the access of oxygen to diffuse through the gas diffusion layers to the catalyst layer. The air supplied exchanges momentum to the droplets and water clusters, causing detachment when the adhesion force is overcome.Water emerging from discrete locations as droplets interact with both the channel walls and each discrete water cluster, depending on the configuration and operating conditions. Complexity increases as multiple detachment and coalescence events occur over large channel lengths and time scales. Modelling these systems with traditional method computational fluid dynamic (CFD) methods is computationally expensive so we have developed a new mechanistic discrete particle model to handle the liquid water two-phase flow.Taking inspiration from interface resolving volume of fluid (VoF) simulations of water flow in gas channels, we use analytically equations determined by the equilibrium contact angle in different configurations on the channel wall to predict detachment times and coalescence events. We compare the prediction of water area coverage ratio and channel saturation to the VoF simulations and can also estimate dynamic pressure drop.Using this model, we are able to investigate the dynamic effects of liquid water accumulation and distribution in the channel, with often periodic detachment events arising from the formation of liquid plugs. Consequently, the model is able to investigate the effect of the channel wall wettability, from hydrophilic to hydrophobic and the impact that has on the distribution and accumulation of water in the system.We compare the speed of this model against the VoF simulations to show that this type of modelling of two-phase flow in the channel can enable higher resolution accuracy when coupled with continuum scale approaches. Furthermore, with this approach it is possible to investigate the effect of: channel dimensions, liquid water flow rate and spatial location of liquid water emergence. This should allow for designed fuel cell structures to be evaluated. Figure 1

Modeling of droplet detachment using dynamic contact angles in polymer electrolyte fuel cell gas channels

Climate change, energy security and air pollution are all motivators for the further development of fuel cells. A volume of fluid approach was used to investigate the impact of dynamic contact angle boundary conditions (Kistler model), mainly at the gas diffusion layer surface but also at the channel wall, of a polymer electrolyte fuel cell gas channel. From this study, it is clear that a dynamic contact angle boundary condition, with advancing and receding contact angles, influences the droplet detachment characteristics, for example, the detachment time and droplet size. Implementing dynamic contact angle boundary conditions for a thin channel causes the droplet, after being reattached to the wall on the side opposite the GDL, to flow very slowly when attached to the wall, until it is merged with a second droplet and they exit the channel (but remain attached to the wall) fairly quickly. Similar phenomena are not observed while using a static contact angle.

A Three-Dimensional Model for Evaluating the Performance of Tubular-Shaped PEM by CFD Simulation

The CFD simulation of a tubular-shaped proton exchange membrane (PEM) fuel cell in the patterns co-current flow for evaluation of the effects of different influencing parameters on fuel cell performance is presented. The model considers transport phenomena in a fuel cell involving mass and momentum transfer, electrode kinetics, and potential fields. The governing equations coupled with the CFD model are then solved using the finite element method. The predicted cell potentials are in good agreement with the available experimental data. The parametric studies have been conducted to characterize the effects of the gas diffusion layer (GDL) porosity and length, the inlet velocity of gases, and the hydrogen channel diameter on various cell performance parameters such as the concentration of reactants/products and cell current densities. The effect of the length and diameter of the channel on cell current density and the optimum gas diffusion layer porosity at a given hydrogen channel diameter to obtain the maximum current density is determined. In this work, a systematic procedure to optimize PEM fuel cell gas channels in the systems bipolar plates with the aim of globally optimizing the overall system net power performance was carried out.

Numerical study of droplet dynamics in a polymer electrolyte fuel cell gas channel using an embedded Eulerian-Lagrangian approach

An embedded Eulerian-Lagrangian formulation for the simulation of droplet dynamics within a polymer electrolyte fuel cell (PEFC) channel is presented. Air is modeled using an Eulerian formulation, whereas water is described with a Lagrangian framework. Using this framework, the gas-liquid interface can be accurately identified. The surface tension force is computed using the curvature defined by the boundary of the Lagrangian mesh. The method naturally accounts for material property changes across the interface and accurately represents the pressure discontinuity. A sessile drop in a horizontal surface, a sessile drop in an inclined plane and droplets in a PEFC channel are solved for as numerical examples and compared to experimental data. Numerical results are in excellent agreement with experimental data. Numerical results are also compared to results obtained with the semi-analytical model previously developed by the authors in order to discuss the limitations of the semi-analytical approach.

On the application of the PFEM to droplet dynamics modeling in fuel cells

The Particle Finite Element Method (PFEM) is used to develop a model to study two-phase flow in fuel cell gas channels. First, the PFEM is used to develop the model of free and sessile droplets. The droplet model is then coupled to an Eulerian, fixed-grid, model for the airflow. The resulting coupled PFEM-Eulerian algorithm is used to study droplet oscillations in an air flow and droplet growth in a low-temperature fuel cell gas channel. Numerical results show good agreement with predicted frequencies of oscillation, contact angle, and deformation of injected droplets in gas channels. The PFEM-based approach provides a novel strategy to study droplet dynamics in fuel cells.

Water emergence from the land region and water–sidewall interactions in Proton Exchange Membrane Fuel Cell gas channels with microgrooves

Liquid water produced in a Proton Exchange Membrane Fuel Cell (PEMFC) can adversely affect the fuel cell performance in two ways: (a) reduction in surface area available for reactant transport at the channel–gas diffusion layer (GDL) interface, and (b) increase in two-phase pressure drop in channels leading to flow maldistribution and increased pumping power. Further, the channels blocked by water reduce reactant availability at reaction sites. Most of the earlier water transport studies were focused on water droplet formation on the gas diffusion layer (GDL) in the channel and its removal from the gas flow without considering the sidewall interactions. In an actual fuel cell, water under the land emerges in the channel and fills the corner, drawing in additional water from the GDL surface. The present work explores water droplet–sidewall interactions and the transport of water from the corner region. Transverse micro-grooves are introduced on the sidewalls and their effect on water removal from the corner region, flow patterns, area coverage ratio and pressure drop are investigated. The micro-grooves are also seen to introduce a wetting regime that facilitates removal of water at the channel exit without causing blockage at the manifold region.

Residence time of water film and slug flow features in fuel cell gas channels and their effect on instantaneous area coverage ratio

Water in the gas channels of a Proton Exchange Membrane Fuel Cell is modeled as slugs and film, and removal mechanisms for these flow patterns are numerically investigated. The removal of excess liquid water is simulated using computational fluid dynamics (CFD) through the volume of fluid (VOF) model. The computational domain consists of a gas flow channel appropriate for commercial stacks for automotive applications. The effects of superficial air velocity, channel surface wettability, and channel cross-section geometry are investigated through quantitative comparison of two-phase pressure drop, area coverage ratio (ACR) over the gas diffusion layer (GDL) and liquid removal time. Top wall film flow was identified as a desirable feature since it did not cover the GDL and facilitated transport of oxygen to the reaction sites while removing the water. A range of hydrophilic channel walls in combination with a hydrophobic GDL is proposed to promote this behavior while reducing the fluctuations in two-phase pressure drop for different contact angles. Additional enhancements to liquid water removal were associated with the channel cross-section geometry. An alternative trapezoidal shape is suggested for improved top wall film flow while improving the manufacturability of the bipolar plates for mass production.

Experimental investigation of liquid water droplet removal in a simulated polymer electrolyte membrane fuel cell gas channel with gas diffusion layer characteristics

Since the accumulation of liquid water droplets in gas flow channels significantly affects the performance of polymer electrolyte membrane (PEM) fuel cells, a comprehensive understanding of liquid water removal from the cell is of great importance to achieve optimal water management. This study is mainly concerned with the ex situ measurement of water droplet growth and its removal in a non-reacting simulated cathode channel for PEM fuel cells. To this end, the dynamic behavior of a liquid water droplet at the gas flow channel/gas diffusion layer (GDL) interface was investigated experimentally using a specially designed transparent cell device with different GDLs. The effects of the GDL parameters (GDL thickness and micro-porous layer (MPL) inclusion) on water droplet removal are mainly investigated by analyzing the contact angle hysteresis with inlet air flow rate (Reynolds number). The critical Reynolds number is employed to evaluate the droplet removal from the GDL surface. It is observed that the water droplet on a thinner GDL tends to be removed under a higher critical Reynolds number. In addition, it is shown that the GDL with an MPL requires a higher critical Reynolds number for the water droplet removal under the same droplet height. The results suggest that the structure of the GDL sample has considerable influence on the water droplet removal. It is thought that the data obtained from this study can provide useful insight into the design of a GDL with enhanced water removal capability for PEM fuel cells.

Contact line characteristics of liquid–gas interfaces over grooved surfaces

Surface wetting is an important phenomenon in many industrial processes including micro- and nanofluidics. The wetting characteristics depend on the surface tension forces at the three-phase contact line and can be altered by introducing patterned groove structures. This study investigates the effect of the grooves on the transition in the wetting behavior between the Cassie to Wenzel regimes. The experiments demonstrate that the wettability on a patterned surface depends on the spacing factor (S = channel depth/channel width). The spacing factor influences the contact angle, contact angle hysteresis, and the transition characteristics between the Cassie and Wenzel states. It was noted that under certain conditions (S > 1) the droplet behaved as a Cassie droplet, while exhibiting Wenzel wetting the rest of the time on the silicon microchannels tested. This criterion was used to design the groove structures on the sidewall of the proton exchange membrane fuel cell gas channel to remove the water effectively. The water coming from the land region into the gas channel is pulled by the grooves to the top wall where the airflow aided in its removal. Also, the contact angles measured on the surfaces were compared with the classical models that use wetted area, and the contact line model that uses the three-phase contact line length. It was found in our experiments that the contact line model predicts the contact angle on the patterned groove surfaces more accurately than the classical models.

Effect of channel materials and trapezoidal corner angles on emerging droplet behavior in Proton Exchange Membrane Fuel Cell gas channels

Ex situ experiments were conducted to study water droplet dynamics in Proton Exchange Membrane Fuel Cell (PEMFC) gas channels with different trapezoid channel open angles and GDL materials under controlled air velocities (0.48–7.23 m s−1). High speed videos revealed that the droplets interacted with the channel walls at lower air velocities (0.48–4 m s−1) and led to channel corner filling depending on the channel open angle. However, at higher air velocities (4.1–7.23 m s−1), the droplets did not contact the channel walls and slid off from the GDL surface. For higher air velocities, a correlation was obtained for the minimum velocity required to remove the droplet from the channel without interacting with the channel walls. For lower air velocities, the minimum pressure drop required for removing the droplet from the gas channel for both corner filling and non-filling scenarios was obtained. The study clearly establishes the important role played by the channel sidewalls and the channel angle on the water droplet transport characteristics.

Numerical simulations of water droplet dynamics in hydrogen fuel cell gas channel

The droplet dynamics in the cathode gas flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a flow channel to effectively prevent flooding. Three-dimensional two-phase flow simulations employing the volume of fluid method have been performed. Liquid droplets emerging from two adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the gas flow channel. The effects of various parameters (pore distance, locations, sidewall contact angle, and airflow rate) on the liquid water removal from the gas channel have been investigated in terms of liquid water saturation, coverage of liquid water on the gas diffusion layer (GDL) surface, and change in the pressure drop in the channel. The numerical results show that the coalescence of two adjacent droplets enhances the water removal as compared to two separate, small droplets. It is also observed that droplets generated near the hydrophilic sidewall can be attached to the upper corner of the channel walls, which prevents the liquid water from covering the GDL surface, whereas the hydrophobic sidewall may cause clogging of the gas channel with liquid water at a low airflow rate.

A Two-Phase Pressure Drop Model Incorporating Local Water Balance and Reactant Consumption in PEM Fuel Cell Gas Channels

A two-phase 1-D pressure drop modeling scheme that incorporates the effect of local channel saturation on flow regime is introduced. The model provides a basis for extending two-phase pressure drop correlations from fundamental studies to more complex conditions within PEMFC reactant channels. The need for additional studies beyond the fundamental level is highlighted and key factors that separate PEMFC reactant channel conditions are explained. A crucial concern in characterizing two-phase flow within PEMFC reactant channels is the local flow pattern transitions throughout the channel. The application of an elemental step-wise calculation scheme approach for the prediction of localized two-phase conditions allows for a more complete prediction of conditions within the channels. Additionally through the application of a control volume approach, the effect of water addition and mass consumption of reactants on pressure drop within PEMFC flow channels is investigated. The resulting modeling scheme shows the relative effects of relaxing several of the common assumptions made when modeling PEMFC reactant channels.

A Two-Phase Pressure Drop Model Incorporating Local Water Balance in PEM Fuel Cell Gas Channels

not Available.

Two phase flow simulation in a channel of a polymer electrolyte membrane fuel cell using the lattice Boltzmann method

Water management in polymer electrolyte membrane (PEM) fuel cells is important for fuel cell performance and durability. Numerical simulations using the lattice Boltzmann method (LBM) are developed to elucidate the dynamic behavior of condensed water and gas flows in a PEM fuel cell gas channel. A scheme for two-phase flow with large density differences was applied to establish the optimum gas channel design for different gas channel heights, droplet initial positions, droplet volume and air flow velocity for both hydrophobic and hydrophilic gas channels. The discussion of optimum channel height and drain performance was made using two factors “pumping efficiency” and “drainage speed”. It is shown that deeper channels give better draining efficiency than shallower channels, but the efficiency dramatically decreases when the droplet touches corners or the top of gas channel's walls. As the droplet velocity, i.e. the drainage flow rate becomes higher and the drainage efficiency becomes less dependent on droplet locations with shallower channels, shallower channels are better than deeper channels. Introducing a new dimensionless parameter, “pumping efficiency”, the investigation discusses the effect of the various parameters on the drainage performance of a PEM fuel cell gas channel.

Liquid filling in a corner with a fibrous wall—An application to two-phase flow in PEM fuel cell gas channels

The Concus–Finn condition has been established as a useful criterion to determine the rise height of a body of liquid bounded by a container in which two sides create a vertex. In the work presented here, the same principles are applied in the microscopic region of the contact line at the vertex created by the intersection of the gas channel (GC) wall and the fibrous gas diffusion layer (GDL) in a polymer electrolyte membrane (PEM) fuel cell. Recent efforts on water management issues in fuel cells have provided valuable insight into the water emergence and two-phase flow in gas channels. One of the main areas of focus is the accumulation of water in the reactant gas channels. Several works have focused on transport through the GDL or the growth of a droplet in an open area without taking into account the channel wall interactions. This issue is addressed in this work at a fundamental level by investigating droplet formation and accumulation in a corner. This work utilizes an experimental approach that visualizes the dynamics of water at the intersection of two interfaces of varying surface energies and surface topographies. It has been determined that the local microscopic contact angle is a critical parameter when determining the behavior of a growing droplet as it interacts with the channel wall and the values were found to exceed the measured contact angles for the surfaces due to the severely strained interface of the droplet over the fibrous matrix. Also, the Concus–Finn condition can be successfully used to determine the conditions of the droplet in the corner, based on the local microscopic contact angle, which can then be used to determine optimal angle to promote water management. For typical fuel cell materials, this angle has been determined to be less than about 52° to avoid filling of the corner with water.

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.