GRK 2160 DROPIT: Upscaling of coupled free-flow and porous-media-flow processes

(Zeitschriften-) Aufsätze

1. Ackermann, S., Fest-Santini, S., Veyskarami, M., Helmig, R., & Santini, M. (2023). Experimental validation of a coupling concept for drop formation and growth onto porous materials by high-resolution X-ray imaging technique. International Journal of Multiphase Flow, 160. https://doi.org/10.1016/j.ijmultiphaseflow.2022.104371
   • Zusammenfassung
   • BibTeX
   • Link

   Zusammenfassung

   Droplets formed on the hydrophobic surface of a porous medium exposed to a cross-flowing gas stream have a strong influence on the mass, momentum, and energy exchange across the porous interface, being of importance in several physical systems and engineering applications, like fuel cells. However, the fluid displacement front is highly complex at the pore-scale, since it is characterized by the surface wettability and a contact angle variation along the triple line. Furthermore, the droplet formation is affected by the pore geometry and the surface wettability. These result in a challenging multi-phase scenario with controversial literature, and few studies consider the detailed mechanism of droplet formation on the surface of the porous medium. In this work, two multi-scale simulations for the emerging water droplet from a single hydrophobic capillary pore opening into the gas phase are validated by a benchmark experiment with the help of a high-resolution X-ray imaging technique to characterize the droplet growing. A three-domain approach (gaseous free flow, interface and porous medium, FIP) was compared with a two-domain approach (based on a free-flow and the porous medium, FP). Whereas the three-domain approach introduces two sets of coupling conditions, one for the free flow-interface and one for the interface-porous medium, the two-domain approach introduces only one set of coupling connections for describing the interaction between the domains. In both approaches, droplet formation and growth alter the coupling conditions between the domains. While the FIP approach treats the porous medium as a continuous matter, it is resolved discretely with the help of a pore network in the FP approach. Both approaches show good agreements with the experimental observations and high correspondences for the drop volume, radii and contact angle evolution. The proposed models provide a base for future developments and help to gain a better insight about the impact of the droplet formation on the interface in a coupled free flow-porous medium system.

   BibTeX

   @article{paper-2023-ackermann-experimentalvalidation, abstract = {Droplets formed on the hydrophobic surface of a porous medium exposed to a cross-flowing gas stream have a strong influence on the mass, momentum, and energy exchange across the porous interface, being of importance in several physical systems and engineering applications, like fuel cells. However, the fluid displacement front is highly complex at the pore-scale, since it is characterized by the surface wettability and a contact angle variation along the triple line. Furthermore, the droplet formation is affected by the pore geometry and the surface wettability. These result in a challenging multi-phase scenario with controversial literature, and few studies consider the detailed mechanism of droplet formation on the surface of the porous medium. In this work, two multi-scale simulations for the emerging water droplet from a single hydrophobic capillary pore opening into the gas phase are validated by a benchmark experiment with the help of a high-resolution X-ray imaging technique to characterize the droplet growing. A three-domain approach (gaseous free flow, interface and porous medium, FIP) was compared with a two-domain approach (based on a free-flow and the porous medium, FP). Whereas the three-domain approach introduces two sets of coupling conditions, one for the free flow-interface and one for the interface-porous medium, the two-domain approach introduces only one set of coupling connections for describing the interaction between the domains. In both approaches, droplet formation and growth alter the coupling conditions between the domains. While the FIP approach treats the porous medium as a continuous matter, it is resolved discretely with the help of a pore network in the FP approach. Both approaches show good agreements with the experimental observations and high correspondences for the drop volume, radii and contact angle evolution. The proposed models provide a base for future developments and help to gain a better insight about the impact of the droplet formation on the interface in a coupled free flow-porous medium system.}, author = {Ackermann, Sina and Fest-Santini, Stephanie and Veyskarami, Maziar and Helmig, Rainer and Santini, Maurizio}, doi = {https://doi.org/10.1016/j.ijmultiphaseflow.2022.104371}, journal = {International Journal of Multiphase Flow}, month = {01}, page = {104371}, title = {Experimental validation of a coupling concept for drop formation and growth onto porous materials by high-resolution X-ray imaging technique}, url = {https://www.sciencedirect.com/science/article/pii/S0301932222003305}, volume = 160, year = 2023 }

   Link

   https://www.sciencedirect.com/science/article/pii/S0301932222003305
2. Veyskarami, M., Michalkowski, C., Bringedal, C., & Helmig, R. (2023). Droplet Formation, Growth and Detachment at the Interface of a Coupled Free-FLow--Porous Medium System: A New Model Development and Comparison. Transport in Porous Media, 1--31.
   • BibTeX

   BibTeX

   @article{veyskarami2023droplet, author = {Veyskarami, Maziar and Michalkowski, Cynthia and Bringedal, Carina and Helmig, Rainer}, journal = {Transport in Porous Media}, pages = {1--31}, publisher = {Springer}, title = {Droplet Formation, Growth and Detachment at the Interface of a Coupled Free-FLow--Porous Medium System: A New Model Development and Comparison}, year = 2023 }
3. Michalkowski, C., Veyskarami, M., Bringedal, C., Helmig, R., & Schleper, V. (2022). Two-phase Flow Dynamics at the Interface Between GDL and Gas Distributor Channel Using a Pore-Network Model. Transport in Porous Media, 144(2), Article 2. https://doi.org/10.1007/s11242-022-01813-4
   • Zusammenfassung
   • BibTeX
   • Link

   Zusammenfassung

   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.

   BibTeX

   @article{michalkowski_two-phase_2022, abstract = {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.}, author = {Michalkowski, Cynthia and Veyskarami, Maziar and Bringedal, Carina and Helmig, Rainer and Schleper, Veronika}, doi = {10.1007/s11242-022-01813-4}, issn = {1573-1634}, journal = {Transport in Porous Media}, month = {09}, number = 2, pages = {429--458}, title = {Two-phase Flow Dynamics at the Interface Between GDL and Gas Distributor Channel Using a Pore-Network Model}, url = {https://doi.org/10.1007/s11242-022-01813-4}, volume = 144, year = 2022 }

   Link

   https://doi.org/10.1007/s11242-022-01813-4
4. Ackermann, S., Bringedal, C., & Helmig, R. (2021). Multi-scale three-domain approach for coupling free flow and flow in porous media including droplet-related interface processes. Journal of Computational Physics, 429, 109993. https://doi.org/10.1016/j.jcp.2020.109993
   • Zusammenfassung
   • BibTeX
   • Link

   Zusammenfassung

   Drops on a free-flow/porous-medium-flow interface have a strong influence on the exchange of mass, momentum and energy between the two macroscopic flow regimes. Modeling droplet-related pore-scale processes in a macro-scale context is challenging due to the scale gap, but might be rewarding due to relatively low computational costs. We develop a three-domain approach to model drop formation, growth, detachment and film flow in a lower-dimensional interface domain. A simple upscaling technique allows to compute the drop-covered interface area fraction which affects the coupling fluxes. In a first scenario, only drop formation, growth and detachment are taken into account. Then, spreading and merging due to lateral fluxes are considered as well. The simulation results show that the impact of these droplet-related processes can be captured. However, extensions are necessary to represent the influence on the free flow more precisely.

   BibTeX

   @article{ackermann-bringedal-helmig-2021, abstract = {Drops on a free-flow/porous-medium-flow interface have a strong influence on the exchange of mass, momentum and energy between the two macroscopic flow regimes. Modeling droplet-related pore-scale processes in a macro-scale context is challenging due to the scale gap, but might be rewarding due to relatively low computational costs. We develop a three-domain approach to model drop formation, growth, detachment and film flow in a lower-dimensional interface domain. A simple upscaling technique allows to compute the drop-covered interface area fraction which affects the coupling fluxes. In a first scenario, only drop formation, growth and detachment are taken into account. Then, spreading and merging due to lateral fluxes are considered as well. The simulation results show that the impact of these droplet-related processes can be captured. However, extensions are necessary to represent the influence on the free flow more precisely.}, author = {Ackermann, Sina and Bringedal, Carina and Helmig, Rainer}, doi = {https://doi.org/10.1016/j.jcp.2020.109993}, issn = {0021-9991}, journal = {Journal of Computational Physics}, month = {03}, pages = 109993, publisher = {Elsevier BV}, title = {Multi-scale three-domain approach for coupling free flow and flow in porous media including droplet-related interface processes}, url = {http://www.sciencedirect.com/science/article/pii/S0021999120307671}, volume = 429, year = 2021 }

   Link

   http://www.sciencedirect.com/science/article/pii/S0021999120307671