Volker P. Schulz

Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach

A full morphology (FM) model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell (PEFC). The three-dimensional (3D) fibrous microstructure for the nonwoven gas diffusion layer (GDL) microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.

Numerical Determination of Two-Phase Material Parameters of a Gas Diffusion Layer Using Tomography Images

In this paper, we give a complete description of the process of determining two-phase material parameters for a gas diffusion layer: Starting from a 3D tomography image of the gas diffusion layer the distribution of gas and water phases is determined using the pore morphology method. Using these 3D phase distributions, we are able to determine permeability, diffusivity, and heat conductivity as a function of the saturation of the porous medium with comparatively low numerical costs. Using a reduced model for the compression of the gas diffusion layer, the influence of the compression on the parameter values is studied.

Pore-Morphology-Based Simulation of Drainage in Porous Media Featuring a Locally Variable Contact Angle

Since the first publications by Hazlett (Transp Porous Med, 20:21–35, 1995) and Hilpert and Miller (Adv Water Res, 24:243–255, 2001), the pore-morphology-based method has been widely applied to determine the capillary pressure–saturation curves of porous media. The main advantage of the method is the simulation of a primary drainage process for large binary images using moderate computational time and memory compared to other two-phase flow simulations. Until now, the pore morphology model was restricted to totally wetting materials or those with a constant contact angle. Here, we introduce a similarly computationally efficient extension of the model that now enables the calculation of capillary pressure–saturation curves for porous media, where the contact angle varies locally within, due to a composite structure.

Numerical Evaluation of Effective Gas Diffusivity - Saturation Dependence of Uncompressed and Compressed Gas Diffusion Media in PEFCs

Electrochemical Engine Center, Pennsylvania State University, University Park, Pennsylvania 16802, USA In the current work, we present a comprehensive modeling framework to predict the effective gas diffusivity, as a function of liquid water saturation, based on realistic 3-D microstructures of the uncompressed as well as compressed gas diffusion layer (GDL). The presented approach combines the generation of a virtual microscopic GDL and different physical modeling. We develop a reduced model in order to simulate the compression of the GDL layer since its compression has a strong impact on the material properties such as the water transport or its gas diffusion. Then, we determine the two-phase distribution of a non-wetting fluid, i.e. water, and a wetting fluid, i.e. air, within the GDL for different saturations. This is done using a full morphology (FM) model. Finally, solving the Laplace equation for the partly saturated medium we determine the relative gas diffusion, i.e. the gas diffusion depending on the saturation. In the present work, our approach is applied to a typical GDL medium, a SGL10BA carbon paper.

Characterization of gas diffusion electrodes for metal-air batteries

Gas diffusion electrodes are commonly used in high energy density metal-air batteries for the supply of oxygen. Hydrophobic binder materials ensure the coexistence of gas and liquid phase in the pore network. The phase distribution has a strong influence on transport processes and electrochemical reactions. In this article we present 2D and 3D Rothman-Keller type multiphase Lattice-Boltzmann models which take into account the heterogeneous wetting behavior of gas diffusion electrodes. The simulations are performed on FIB-SEM 3D reconstructions of an Ag model electrode for predefined saturation of the pore space with the liquid phase. The resulting pressure-saturation characteristics and transport correlations are important input parameters for modeling approaches on the continuum scale and allow for an efficient development of improved gas diffusion electrodes.

Pore Size Distribution and Soil Water Suction Curve from Micro-tomography Measurements and Real 3-D Digital Microstructure of a Compacted Granular Media by Using Direct Numerical Simulation Technique

Predictive measurement of capillary pressure – saturation relationship of a porous media is obtained based on the actual microstructure obtained from high resolution tomography data. X-ray micro-tomography provided a high contrast for silica phase, and actual geometry of sand particles and void distribution. The morphological opening (erosion + dilation) is used to get a pore size distribution using the concept of granulometry. Full-morphology approach is used to model the quasi-static wetting and non-wetting phase distribution of a primary drainage process. Predicted soil water suction curves for a compacted silica sand sample is presented along with the effects of assumed contact angle between water and silica surface.

Two-Phase Behavior and Compression Effect in a PEFC Gas Diffusion Medium

A key performance limitation in the polymer electrolyte fuel cell (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. A key contributor to the mass transport loss is the cathode gas diffusion layer (GDL) due to the blockage of available pore space by liquid water thus rendering hindered oxygen transport to the active reaction sites in the electrode. The GDL, therefore, plays an important role in the overall water management in the PEFC. The underlying poremorphology and the wetting characteristics have significant influence on the flooding dynamics in the GDL. Another important factor is the role of cell compression on the GDL microstructural change and hence the underlying two-phase behavior. In this article, we present the development of a pore-scale modeling formalism coupled with realistic microstructural delineation and reduced order compression model to study the structure-wettability influence and the effect of compression on two-phase behavior in the PEFC GDL.

Lattice Boltzmann Simulation of Two Phase Flow through Porous Media and Verification Using High Resolution X-ray and Neutron Tomography Data

Two phase flow simulations are predicted for a compacted silica sand specimen whose microstructure is obtained from three-dimensional X-ray microtomography. Direct numerical simulations using a simple full morphology method based on the measured three-dimensional geometry of the pore space is demonstrated. Additionally, an advanced two-phase lattice-Boltzmann based model is applied solving the Navier-Stokes equation numerically to simulate multi-phase flow. Experiments utilized a micro-focus X-ray system at Helmholtz-Zentrum-Berlin (HZB). Using a cold neutron imaging beam line (CONRAD) at HZB, in-situ imaging of water flow through compacted sand is performed. Water is allowed to flow from bottom to top of compacted sand specimen under controlled conditions. The flow of water is precisely controlled by using a syringe pump to a flow rate that corresponds to capillary fingering flow regime. Two-dimensional neutron radiography based cinematography was performed during the injection of water into porous media, and three-dimensional neutron tomography was performed at target states after reaching flow equilibrium. The simulation results are directly compared with the experimental results obtained from neutron tomography data. Preliminary results of the latticeBoltzmann simulation in 2D did not provide a close match to the measured water phase distribution and advancing front. This paper introduces the innovative concept of direct numerical simulations of complex multi-phase flow using realistic microstructures measured non-destructively using radiation based imaging. The technical approach used in this study shows promise for applications in diverse fields involving fluid transport and porous media.

Nondestructive Visualization and Quantification of 3-D Microstructure of Granular Materials and Direct Numerical Simulations

This paper summarizes the key concepts from the recent published work of the authors on using both neutron and X-ray imaging techniques to study partially saturated sand and water flow through compacted sand. The goal of the manuscript is to serve as a review paper, building on discrete contributions from cited publications for geomechanics community as the topic is rather new and concepts are connected. For this study, neutron and micro-CT-based X-ray imaging was performed at Helmholtz-Zentrum-Berlin (HZB) in Germany. Due to different attenuation characteristics of neutrons and X-rays to three phases (silica, air, and water) of partially saturated sand, radiation-based images provide unique but complementary information in a nondestructive fashion. Water phase is very precisely identified with neutron radiation-based images, and sand (silica) phase is well identified with X-ray images. An automatic image registration technique is implemented to combine neutron and X-ray images in the same coordinates for a detailed quantitative evaluation of microstructural features in three dimensions. In situ imaging experiment of flow through compacted sand was performed based on the dual modality imaging concept. The initial 3-D pore geometry was obtained from dry compacted sand specimen by using x-ray. The water flow pattern was monitored by using time-lapsed neutron radiography and tomography after a target water injection step. The initial microstructure obtained with X-ray tomography is also used to perform direct numerical simulations. Experiments based on using neutron and X-ray imaging technique thus providing a unique opportunity to characterize partially saturated sand and investigate multiphase flow behavior through porous media. Direct numerical simulation based on realistic geometry can account for complex pore geometry including heterogeneity of the pore structure.