Browsing by Author "Michalkowski, Cynthia"

Now showing 1 - 4 of 4

Open Access
Droplet formation, growth and detachment at the interface of a coupled free-flow-porous medium system : a new model development and comparison
(2023) Veyskarami, Maziar; Michalkowski, Cynthia; Bringedal, Carina; Helmig, Rainer
Coupled free-flow-porous medium systems are of great importance in various natural and industrial applications. Modeling of such systems is always challenging, especially when droplets form at the interface between the two domains. We propose a new concept to take droplet formation, growth and detachment at the interface into account. In this concept, we use pore-network modeling to describe the porous medium and the Navier-Stokes equations for the free-flow domain. New coupling conditions are developed which include droplet interactions with the free flow and the porous medium. Impacts of using different descriptions of the forces acting on the triple contact line and contact angle hysteresis on the predicted onset of the droplet detachment are examined. In addition, we compare the new approach with another model built using ANSYS Fluent based on the volume of fluid method. The results show that the new model is able to describe the droplet formation, growth and then detachment by the free flow. The proposed model provides a base for further developments to handle formation of multiple droplets at the interface between a free flow and a porous medium as well as to include the evaporation in future works.
Open Access
Modeling of two phase flow in a hydrophobic porous medium interacting with a hydrophilic structure
(2022) Michalkowski, Cynthia; Weishaupt, Kilian; Schleper, Veronika; Helmig, Rainer
Fluid flow through layered materials with different wetting behavior is observed in a wide range of applications in biological, environmental and technical systems. Therefore, it is necessary to understand the occuring transport mechanisms of the fluids at the interface between the layered constituents. Of special interest is the water transport in polymer electrolyte membrane fuel cells. Here, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. This is crucial to choose optimal operating conditions and improve the overall cell performance. Pore-scale modeling of gas diffusion layers (GDLs) and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the hydrophobic porous GDL and the hydrophilic gas distributor, a particular challenge is the combination and interaction of the different material structures and wetting properties at the interface and its influence on the flow. In this paper, a modeling approach is presented which captures the influence of a hydrophilic domain on the flow in a hydrophobic porous domain at the interface between the two domains. A pore-network model is used as the basis of the developed concept which is extended to allow the modeling of mixed-wet interactions at the interface. The functionality of the model is demonstrated using basic example configurations with one and several interface pores and it is applied to a realistic GDL representation in contact with a channel-land structured gas distributor.
Open Access
Modeling water transport at the interface between porous GDL and gas distributor of a PEM fuel cell cathode
(Stuttgart : Eigenverlag des Instituts für Wasser- und Umweltsystemmodellierung der Universität Stuttgart, 2022) Michalkowski, Cynthia; Helmig, Rainer (Prof. Dr.-Ing.)
Operating vehicles with polymer electrolyte membrane (PEM) fuel cells is a promising technology for reducing traffic-related greenhouse gas emissions. In a PEM fuel cell, hydrogen and oxygen react producing water, electric energy, and heat. Oxygen is consumed on the cathode side of the cell, while the excess water must be removed to prevent the so-called flooding (blockage of the transport paths). A sophisticated water management is crucial for improved operating conditions of a PEM fuel cell. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents, where an intelligent use and drainage of the water buffer can be used to enhance the performance of the fuel cell. Pore-scale modeling of gas diffusion layers (GDLs) and the gas distributor has been established as a favorable technique to investigate the ongoing processes. A particular challenge is the investigation of the interface between the GDL and the gas distributor. Here, multi-phase flow in the porous material of the GDL is combined with the free flow in the gas distributor resulting in strong interaction. Different interface processes occur based on the pore-local structural properties, such as surface wettability and interaction with the gas flow in the gas distributor. At the interface between hydrophobic porous GDL and the hydrophilic side walls of the gas distributor, the fluids interact with the differently wetting surfaces. This results in complex pore-scale transport processes in the pores located at the interface. In the channels of the gas distributor, drops emerging from the porous domain at the interface have a strong influence on the exchange of mass, momentum, and energy between the two flow regimes. Additionally, we also consider transport processes of the gas phase between the GDL and the gas distributor, where no water breakthrough occurs.
Open Access
Two-phase flow dynamics at the interface between GDL and gas distributor channel using a pore-network model
(2022) Michalkowski, Cynthia; Veyskarami, Maziar; Bringedal, Carina; Helmig, Rainer; Schleper, Veronika
For improved operating conditions of a polymer electrolyte membrane (PEM) fuel cell, a sophisticated water management is crucial. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. Pore-scale modeling of diffusion layers and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the cathode layers, a particular challenge is the combination and interaction of the multi-phase flow in the porous material of the gas diffusion layer (GDL) with the free flow in the gas distributor channels. The formation, growth and detachment of water droplets on the hydrophobic, porous surface of the GDL have a major influence on the mass, momentum and energy exchange between the layers. A dynamic pore-network model is used to describe the flow through the porous GDL on the pore-scale. To capture the droplet occurrence and its influence on the flow, this dynamic two-phase pore-network model is extended to capture droplet formation and growth at the surface of the GDL as well as droplet detachment due to the gas flow in the gas distributor channels. In this article, the developed model is applied to single- and multi-tube systems to investigate the general drop behavior. These rather simple test-cases are compared to experimental and numerical data available in the literature. Finally, the model is applied to a GDL unit cell to analyze the interaction between two-phase flow through the GDL and drop formation at the interface between GDL and gas distributor channel.