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Allie Yuxin Lin
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The trials of climate change and humanity’s desire to mitigate our carbon footprint is motivating the research and development of renewable technologies for the transportation and energy sectors. Hydrogen is a promising technology, with the potential to address issues in energy security, pollution, emissions reduction, and sustainability. Hydrogen is carbon free, abundant, and can be stored as a gas or a liquid, making it an important player in the transition toward a cleaner planet.
However, the challenges associated with devising safe and reliable storage methods delay the increase of hydrogen production. Hydrogen’s highly diffusive and corrosive nature makes it prone to leaking, while its unique thermodynamic properties, such as the negative Joule-Thompson effect, require engineers to rethink traditional storage infrastructure. Hydrogen’s low compressibility and extreme sensitivity to the environment mean tanks should both be strong enough to withstand high pressures and flexible enough to handle large temperature fluctuations. Tank design should also account for hot pockets formed due to the increased pressure when hydrogen is compressed, which could damage the tank’s structural integrity.
Computational fluid dynamics (CFD) can help overcome some of the hurdles associated with hydrogen storage. CFD provides insight into the behavior of various fluids and gasses in different environments, so it can be used to optimize the design of hydrogen fuel systems, including fuel cells, storage tanks, and delivery systems. CFD can be used to identify areas of improvement in the fuel system design and help pinpoint potential safety hazards.
CONVERGE is a powerful CFD software whose unique capabilities make it advantageous for simulating hydrogen storage. Autonomous meshing removes the mesh generation bottleneck, while Adaptive Mesh Refinement (AMR) continuously adjusts the mesh throughout the simulation. Conjugate heat transfer (CHT) modeling solves for the heat exchange between the fluids and solids in the system. Additionally, CONVERGE provides multiple turbulence models to efficiently capture the flow dynamics within the storage unit.
The HyTransfer project1 was funded by the European Union to study the physics in the hydrogen filling process, with the hopes of providing guidelines on how to achieve an efficient filling strategy. Using the framework and the experimental data publicly available for the HyTransfer project, we performed a validation study to showcase the value of CONVERGE in hydrogen storage.
The Hexagon 36 L Type IV tank consists of a polymer liner encased in a composite wrapping, providing the necessary structural length to withstand large pressures up to 70 MPa. A liner thickness of 4 mm and an injector diameter of 10 mm were chosen for the study. In line with existing literature,2,3,4 we simulated half the horizontal tank domain, reducing overall computational cost. We provided the inlet mass flow rate profile and monitored the development of the tank pressure from the initial state.
CONVERGE’s graphical user interface, CONVERGE Studio, allows users to take advantage of a wide variety of geometry manipulation and repair tools during case setup. Since hydrogen’s behavior is known to deviate from ideal gas representations, CONVERGE provides the option to use real gas properties.
With the density-based PISO solver, we were able to achieve rapid convergence while simultaneously capturing the heat fluxes between different regions with CONVERGE’s CHT analysis.
As the injector diameter used for hydrogen tanks typically ranges from 3–10 mm, the incoming flow velocity can reach up to 300 m/s. High jet penetration plays a critical role in maintaining circulation within the tank, and mesh embedding used in conjunction with CONVERGE’s AMR can capture the jet profile with a high degree of accuracy.
To get the appropriate jet penetration and reduce the spreading over-prediction, the RNG k-epsilon constant in the turbulence model was modified according to past research.2,3,4
Figure 1 shows the velocity contours of the hydrogen jet during the filling process. The velocity decreases rapidly as the filling progresses due to the compression of hydrogen. The flapping motion of the jet is related to flow circulation within the tank, which is important when considering the redistribution of thermal gradients. In our study, this circulation caused hot pockets to form near the injection side of the tank.
The temperature profiles within the tank agree with the experimental data (Figure 2), demonstrating CONVERGE can capture the complex thermodynamics of hydrogen. The reading at thermocouple 5 (TT5) is higher than thermocouple 1 (TT1) because long filling times can cause thermal stratification. As the filling progresses, the jet velocity decreases and the circulation within the tank stabilizes.
The ideal material for a hydrogen storage tank is lightweight with excellent thermal integrity and strength. CONVERGE’s CHT analysis enables engineers to assess the thermal gradients within the tank’s structure and helps in the proper selection of tank materials. Figure 3 shows the predicted temperature at the liner-composite interface compared to the experiment, demonstrating CONVERGE can also be used to capture the tank’s internal thermal behavior.
Using CONVERGE, we simulated the flow dynamics and thermal behavior during the hydrogen filling process; our results aligned well with previous experimental data.1 We assessed major flow features and identified recirculating vortical structures caused by the fluctuating behavior of the jet. CONVERGE accurately captured temperature profiles inside the tank, and its CHT capabilities predicted the liner-composite interface temperature.
The ease of use, flexibility, accuracy, and rich set of features make CONVERGE a highly effective tool for studying hydrogen tank storage. Check out our white paper, “Exploring Hydrogen Tank Filling Dynamics,” to learn more about how CONVERGE is helping engineers tackle an important challenge of the modern era!
[1] Ravinel, B., Acosta, B., Miguel, D., Moretto, P., Ortiz-Cobella, R., Janovic, G., and van der Löcht, U., “HyTransfer, D4.1 – Report on the experimental filling test campaign,” 2017.
[2] Melideo, D. and Baraldi, D., “CFD analysis of fast filling strategies for hydrogen tanks and their effects on key-parameters,” International Journal of Hydrogen Energy, 40, 735-745, 2015.
[3] Melideo, D., Baraldi, D., Acosta-Iborra, A., Cebolla, R.O., and Moretto, P., “CFD simulations of filling and emptying of hydrogen tanks,” International Journal of Hydrogen Energy, 42, 7304-7313, 2017.
[4] Gonin, R., Horgue, P., Guibert, R., Fabre, D., and Bourget, R., “A computational fluid dynamic study of the filling of a gaseous hydrogen tank under two contrasted scenarios.” International Journal of Hydrogen Energy, 47(55), 23278-23292, 2022.
