CFD Model Predictions of Zero Boil‑Off Unvented and Vented Tank Filling in Microgravity

Cryogenic Fluid Management (CFM) of propellant fluids in microgravity presents an important challenge for NASA’s future long-duration space missions, which will require safe and efficient storage and transfer of cryogenic liquid fuels. The Zero-Boil-Off Tank Filling and Transfer Experiment (ZBOT-FT) or Transfer and Refilling Experiment (TREX), through collaboration between the University of Bremen and Case Western Reserve University, aims to study two-phase tank-to-tank transfer operations in microgravity using the simulant fluid Perfluoro-n-pentane (PNP). As part of this effort, this work presents two-phase Computational Fluid Dynamics (CFD) models for no-vent and vented cylindrical tank filling operation in microgravity for filling rates of 1.0-1.65 mL/s. 2D axisymmetric and 3D parametric CFD simulation case studies are performed to show the detailed physics and evolution of tank pressure. A zero-dimensional homogeneous thermodynamic model is developed to predict the final steady-state pressure. The CFD models demonstrate two stable geyser behaviors (low stable geyser and wobbly geyser) and two unstable behaviors (droplet shedding and jet column). When Weber number is adjusted for inlet velocity profile, a threshold between 1.5 and 1.7 divides stable and unstable regimes, consistent with previous experimental findings. The final pressure in the CFD models converges to the thermodynamic limit. 2D and 3D results show broad agreement for most cases, though 3D modeling is required for wobbly geyser cases. The models show that geyser behavior significantly affects the early transient pressure evolution, with stable geyser cases displaying higher pressure peaks, and suggests that lower fill rates may sometimes result in higher peak pressure.