Yosuke Matsukuma

Evaluation of gas diffusion performance in wet GDL with 3D pore network model

INTRODUCTION Polymer Electrolyte Fuel Cell (PEFC) is expected as automotive power supply and stationary cogeneration system. In PEFC, the diffusion inhibition by liquid water in gas diffusion layer (GDL) has to be reduced. GDL is fibrous porous media which consists of carbon fibers and has a heterogeneous structure. In order to comprehend the two-phase condition in actual GDL, the calculation models considering actual heterogeneous structures are needed. In our past studies, the simulated GDL structure was developed by numerical analysis, and its structural properties were validated. In addition, two-phase condition in GDL was calculated by network model [1-3]. However, this method had many assumptions. There was the difference between actual condition and the calculation condition. In this study, various structures were developed, for example, carbon paper, paper with binders and cloth. Next, our past two-phase network model was improved and we evaluated the gas diffusion performance in wet various GDL with this model.

Evaluation of Liquid and Mass Transfer in GDL by Direct Network Simulation

INTRODUCTION Polymer Electrolyte Fuel Cell (PEFC) is expected as stationary power supply and automotive power supply. In PEFC, the diffusion inhibition by liquid water in gas diffusion layer (GDL) has to be reduced. GDL is fibrous porous media which consists of carbon fibers and has a heterogeneous structure. It is expected that this structural property affects the transfer of liquid water extremely. In this study, the simulated GDL structure was developed by numerical analysis, and its structural properties were validated. In addition, the two-phase flow model based on an actual structure was developed. And the influence of multi-layer GDL structure on the condition with accumulated water was evaluated, and effective diffusion coefficient in GDL with liquid water was calculated.

Numerical Analysis of Liquid and Heat Transfer in Heterogeneous GDL

INTRODUCTION Polymer Electrolyte Fuel Cell (PEFC) is expected as stationary power supply and automotive power supply. In PEFC, the diffusion inhibition by liquid water in gas diffusion layer (GDL) has to be reduced. GDL is fibrous porous media which consists of carbon fibers and has a heterogeneous structure. It is expected that this structural property affects the transfer of liquid water extremely. In this study, the simulated GDL structure was developed by numerical analysis with the effect of adhesion of PEFC, and its structural properties were validated. In addition, the heat and mass transfer model was added to the ordinary two-phase flow model in heterogeneous GDL structure which was developed in our past study. And the influence of various GDL structure on the condition with accumulated water was evaluated.

Numerical Simulations of Droplets on Porous Structures by Use of Lattice Boltzmann Method

This paper demonstrates numerical simulations of droplet on gas diffusion layer of the polymer electrolyte fuel cell. The lattice Boltzmann method for incompressible two-phase flows at high density ratios were applied for the simulations in order to precisely predict the shape and moving velocity of water droplet surrounding by the air in the gas channel. Simulations were conducted in 2D and 3D and height and moving velocity of droplet were compared with experiment as a function of mean gas velocity in the gas channel. The droplet heights by the simulations were qualitatively agreed with the experimental data, while the simulation results of moving speed of droplet somewhat overestimated the experimental results.© 2011 ASME

Modeling Carbon Black Aggregate Structure and Ionomer Coat for Optimum Design of PEFC Catalyst Layer

In present Polymer Electrolyte Fuel Cell (PEFC), the cathode oxygen reduction reaction (ORR) is dominant at high current density condition. In order to accelerate this reaction, oxygen, proton and electron, which move in void space, ionomer and carbon black (CB) or CNT respectively, have to be transferred at Pt surface smoothly. Accordingly, it is very important to know the transport phenomena in catalyst layer (CL) to design the optimum structure and to develop new materials. In order to investigate the transport phenomena in CL by applying calculation technique, in this study, heterogeneous CB aggregate structure was simulated by computer calculation as the first examination. In addition, ionomer coating condition and proton conductivity were examined. Furthermore oxygen, proton and electron transfer in 3D porous catalyst layer were calculated, and the effective reaction field was examined in various structure and ionomer condition.Copyright

Simulation of liquid water evaporation in gdl for pemfc under gas purge condition

In automotive Polymer Electrolyte Fuel Cell (PEFC) system, dry gas purge operation is needed at shutdown condition in order to remove the liquid water in gas diffusion layer (GDL) and to reduce the oxygen diffusion inhibition by liquid water in GDL. However, exceed drying operation leads to degradation of electrolyte membrane because of little water content. Therefore, drying process has to be optimized. In this study, various GDL structure with unique fiber orientation were simulated by numerical analysis, and the real GDL structure was reconstructed by X-ray CT image of carbon paper GDL. Next, our past two-phase network model was improved to include phase change effect. The multi-block two-phase network model based on an actual structure was developed by a direct 3D networking porous structure. As results, the evaporation interface area depended on the porous structure of GDL, and the overall evaporation rate of homogeneous GDL which has uniform structure was 1.5 time higher than that of heterogeneous GDL because of the difference of this interface area. In addition, in the case of rib and channel, liquid water under channel evaporated faster than that under Rib. It is very important to control the drying operation in order to prevent the excess membrane drying.Copyright

Numerical simulations of droplets on the hydrophobic and hydrophilic walls by lattice boltzmann method

Gas-liquid flows in/on porous structures are simulated by using of the two-phase Lattice Boltzmann method (LBM), in which the wetting boundary conditions on solid wall with complex geometry are incorporated. The complex geometry simulating the packed bed is numerically constructed by the discrete element method (DEM). It is confirmed that structure of the simulated packed bed is similar to the actual bed by comparison of wall friction factor. Next the behaviors of droplet on the porous structures are simulated with different wetting properties. For hydrophilic cases, the droplets set on the porous structure at initial stage penetrated into the porous structure as time marching on and spread in the bed. It was shown that the droplet behavior depends on the surface tension and its viscosity. From these numerical simulations, the applicability of LBM to Gas-liquid flows in/on porous structures was confirmed.Copyright

The Direct Numerical Simulation of the Rising Gas Bubble With the Volume of Fluid (VOF) Method

A gas-liquid two phase flow is complicated and it has not been understood well thus far, in spite of extensive investigation. Numerical simulation is a potential approach to understand this phenomenon. Although a number of studies have been conducted to understand the behavior of bubbles on the basis of computational fluid dynamics (CFD), it is difficult to completely simulate a complicated three-phase flow, including coalescence and breakup of bubbles. Although the two-fluid model based on the semi-empirical model can well estimate the actual behavior of the system in which the equations are derived, the estimation over the applicable region of equations does not always agree with the actual result. Since the 1960s, various procedures have been proposed to directly track the free surface between two phases, for example, the adaptive mesh method and the particle method. Although each of these methods has certain advantages and disadvantages, the volume of fluid (VOF) method is the most acceptable method for capturing the free surface accurately and clearly. However, a concern related to this method is the maintenance of a constant volume of the fluid. In this study, a simulation code using the VOF method is developed in order to estimate the behavior of bubbles in a vertical pipe. Further, an offset of the volume fraction is introduced to stably calculate and minimize the volume fluctuation. The effect of the surface tension is also built into the program in order to estimate the behavior of the bubbles rising through the liquid medium. The simulations of the collapsed water column and a single rising bubble are conducted with the proposed simulation code. Consequently, we confirm that these results fairly agree with the experimental ones.© 2011 ASME

Numerical Simulation of Flow Around Melting Object by Lattice Gas Automata Method

From the view point of effective use of energy resources and reduction of greenhouse gases, methane hydrate has received considerable attention as a promising alternative energy resource. It is important to study effective recovery system of the methane hydrate, since it exists on the seabed at a depth of more than 1000m. The hot water injection method has been proposed as a promising methane hydrate recovery system. In this method, hot water is injected into methane hydrate layer through a pipe, and then molten methane is recovered. In this study, as the first step of the numerical analysis of the multiphase flow through complex boundary changing geometry, a new technique to generate a deformable solid boundary is proposed based on the lattice gas automata method. By using this technique, fundamental numerical simulations are demonstrated for the immiscible two-component flow in two-dimensional systems. Comparisons between simulation and experimental results clarified that the present technique is applicable to the flow of hot water and liquid methane and the disassociation of methane hydrate wall.Copyright

NUMERICAL SIMULATION OF THE VAPOR FILM COLLAPSE BEHAVIOR AT VAPOR EXPLOSION WITH THREE-DIMENSIONAL LATTICE GAS THERMAL-HYDRAULIC AUTOMATA METHOD

In order to clarify the dominant driving force of complex vapor film collapse behavior, numerical simulation is performed with three-dimensional fifteen-velocity lattice gas automata method. As the result, numerical result is qualitatively different from the experimental result. On the other hand, numerical simulation of vapor film collapse behavior is performed with three-dimensional fifteen-velocity lattice gas automata method including phase-change effect. As the result, numerical result is qualitatively similar to the experimental results. Comparison between the experimental result and the numerical result confirms that experimentally observed vapor film collapse behavior is dominated not by fluid motion but by phase change.