CHAIR OF

THERMAL PROCESS ENGINEERING

Haashir Altaf, Tamara Miličic, Felix Faber, Tanja Vidaković-Koch, Evangelos Tsotsas, and Nicole Vorhauer-Huget. Available at: https://doi.org/10.3390/pr13040943

The efficiency of an electrolyzer is significantly influenced by mass, heat, and charge transport within its porous transport layer (PTL). The infeasibility of measuring them in-situ makes it challenging to study their influence experimentally, leading to the adoption of various modeling approaches. This study applies pore network (PN) modeling to investigate mass transport properties and capillary invasion behavior in three commercial titanium felt PTLs commonly used in proton exchange membrane water electrolyzers (PEMWEs). One PTL has a graded structure. Reconstructed PNs were derived from microcomputed X-ray tomography (µ-CT) data, allowing for a detailed analysis of pore size distributions, absolute and relative permeabilities, capillary pressure curves, and residual liquid saturations. The results from the PN approach are compared to literature correlations. The absolute permeability of all PTLs is between 1.1 × 10⁻¹⁰ m² and 1.5 × 10⁻¹⁰ m², with good agreement between PNM results and predictions from the Jackson and James model and the Tomadakis and Sotirchos model, the two latter involving the fiber diameter as a model parameter. The graded PTL, with fiber diameters varying between 25 µm and 40 µm, showed the best agreement with literature correlations. However, the capillary pressure curves exhibited significant deviations from the Leverett and Brooks–Corey equations at low and high liquid saturations, emphasizing the limitations of these correlations. In addition, residual liquid saturation varied strongly with PTL structure. The thicker PTL with a slightly narrower pore size distribution, demonstrated a lower residual liquid saturation (19%) and a more homogeneous invasion compared to the graded PTL (64%), which exhibited significant gas fingering. The results suggest that higher gas saturation could enhance gas removal, with much higher relative permeabilities, despite the greater PTL thickness. In contrast, the graded PTL achieves the highest relative liquid permeability (~70%) while maintaining a relative gas permeability of ~30%. These findings highlight the impact of microstructure on invasion and transport properties and suggest PN modeling as a powerful tool for their study.

Andrea Dernbecher, Supriya Bhaskaran, Nicole Vorhauer-Huget, Jakob Seidenbecher, Suresh Gopalkrishna, Lucas Briest and Alba Dieguez-Alonso. Available at https://doi.org/10.1016/j.partic.2025.01.006

In the present study, the influence of the dynamic and anistropic pore microstructure of wood and char samples on the intra-particle flow permeability and tortuosity was investigated. To this end, a beech wood sphere was pyrolysed at different temperatures (100 °C, 200 °C, 300 °C, 400 °C, and 500 °C) and characterised, after each pyrolysis step, by X-ray micro-computed tomography (μ-CT). From the μ-CT images, the structural geometry of the particle at the different conversion degrees achieved at each temperature level was extracted. The porosity evolution was characterised, accounting for pores larger than 15 μm, which was the limit of resolution for μ-CT imaging in this study. The structural geometry was divided in subdomains and used for CFD (computational fluid dynamics) simulations, where the pressure loss at different velocities and in different directions with respect to the main pores (vessel cells) was determined and used to estimate the dynamic and anisotropic permeabilities. The permeabilities differed by an order of magnitude in the direction of the main pores (vessel cells) in comparison to the perpendicular directions, supporting the need to develop permeability tensors for improved simulations of the pyrolysis process at particle level, accounting for the coupled effects of microstructure, transport, and reaction.

As part of the „Kinder-Uni“ series at OVGU, Dr.-Ing. Nicole Vorhauer-Huget and her team have given a special lecture for children in the age of 8 to 12 years. The topic was related to her research in the CRC/TRR 287 BULK-REACTION, namely microwave heating, but with a special focus on Christmas.

The children were very excited to see an experiment with a large piece of chocolate, demonstrating uneven heating inside household microwave apparatuses. The children further participated in the lecture by trying to reproduce high-frequency waves with their scarfs and ribbons.

Besides that, the children learned why process engineering and conversion of raw materials is necessary for the production of Christmas presents like a chocolate or Christmas wrapping paper.

At the 23rd International Drying Symposium (IDS 2024), which took place from 22 to 25 November 2024 in Wuxi, China, Dr.-Ing. Nicole Vorhauer-Huget has obtained the Young Drying Scientist Award, in recognition of her excellent contribution to the advancement of drying science and technology.

Prof. Arun S. Mujumdar, McGill University (Canada) and Honorary Chairman of the IDS 2024, presents the Award to Dr.-Ing. Nicole Vorhauer-Huget (© TPM research group).

Dasika Prabhat Sourya, Pardha S. Gurugubelli, Supriya Bhaskaran, Nicole Vorhauer-Huget,  Evangelos Tsotsas, Vikranth Kumar Surasani. Avaialbe at https://doi.org/10.1016/j.ijhydene.2024.10.340

The optimization of Polymer Electrolyte Membrane (PEM) electrolyzers necessitates an intimate knowledge of the oxygen flows within the anodic porous transport layers (PTLs) to determine any possible reduction in performance. In this field, as experimental studies are cumbersome and expensive, numerical modeling has arisen as a viable alternative for studying the oxygen transport within the Anodic PTLs of a PEM electrolyzer. Amongst the various numerical modeling techniques, the Lattice Boltzmann Method (LBM) is gaining prominence for its effectiveness in analyzing fluid transport within porous media due to its mesoscopic nature and ease of implementation. This study utilizes the Shan-Chen LBM methodology to model the flow of oxygen within the Anodic PTL of a PEM electrolyzer and compares it against the Volume-of-Fluid-based Continuum Model. The results show that LBM can not only replicate the experimental studies accurately, but can also maintain its high accuracy at progressively shrinking length scales of PTLs, even at length scales where the VoF-based Continuum Model would run into accuracy issues. The high accuracy of the LBM model, combined with the simplicity of the LB algorithm makes LBM a powerful technique for simulating the microfluidic flows such as the flow of oxygen within the Anodic PTL of a PEM electrolyzer.

Felix Faber, Nicole Vorhauer-Huget, Maximilian Thomik, Sebastian Gruber, Petra Foerst, Evangelos Tsotsas. Available at https://doi.org/10.1080/07373937.2024.2407062

This study presents a pore network (PN) model with transient heat transfer and quasi-steady transitional vapor transport that is for the first time applied to irregular porous structures that are obtained by reconstruction of X-ray tomography image data. In contrast to previous studies, the irregular pores are not approximated by spheres but implemented in their original shape. Secondly, instead of assuming cylindrical throats as pore connections, the actual distance between pore centers as well as the pore cross sections are used for the computation of the vapor transport coefficient. The control volume elements of the computational model are matched with the cells obtained by Voronoi tessellation. The improvements have clear advantages over former approaches where the reconstructed void space is usually strongly simplified by balls and sticks confined in a regular lattice structure. A freeze-dried sample of maltodextrin DE12 with 20% (w/w) solid content is used for benchmarking the new methodology. Its morphological and thermal properties are determined by the novel PN model. The simulation results of primary freeze-drying (FD) are compared to reference cases in two ways. First, the differences in heat and mass transfer kinetics as compared to regular PNs are emphasized. Secondly, the PN simulation results are confronted with a simple literature model that neglects pore size distribution (PSD) and transient heat transfer. It is shown that already in small domains with relatively narrow PSD, the variation of the mass transfer coefficient affects the computed sublimation fluxes and yields a significant deviation from the simpler literature model. Moreover, it is revealed in this study that the structure has a significant impact on the sublimation front temperature. This is demonstrated by the comparison of FD in the irregular PN to a regular PN with almost identical PSD but different porosity. The development, verification, and benchmarking of the new PN model can be seen as an important step for studies of the structure dependence of FD.