Phase Separation with a Porous Screen in Zero Gravity: CFD Modeling and Validation

Cryogenic Fluid Management (CFM) in zero gravity represents an important area of research at NASA. The ability to store and transfer the cryogenic liquid fuels efficiently and safely impacts the success of future long-duration space missions. Vapor bubbles entering a transfer line can lead to disruption of the flow of cryogenic fuel or even engine failure. A screened Liquid Acquisition Device (LAD) can be installed at the outlet of the supply tank to promote vapor-free flow into the line. Passive separation of the gaseous phase from the liquid is enabled by the small pores of the LAD screen, which resist dry-out via surface tension. This phase separation is possible until the pressure drop across the porous region is large enough to overcome the capillary pressure due to surface tension, which occurs at the critical bubble point pressure. The Transfer Refueling Experiment (TREX) is the third experiment in the Zero-Boil-Off Tank (ZBOT) Experiment series that is undertaken by the University of Bremen in collaboration with the Case Western Reserve University. The TREX project aims to study two-phase flow tank-to-tank transfer operations in microgravity. As part of this effort, in this work, we present the development and validation of a 3-dimensional Computational Fluid Dynamics (CFD) model for LAD screen phase separation in zero-gravity using Ansys Fluent. We begin with a model of the single-phase pressure drop across the porous region, and we study the relationship between this pressure difference and the flow velocity. The CFD predictions for this case are validated against experiments performed by Armour and Cannon (1968). We also present a two-phase study of the bubble point pressure, and we analyze the fluid flow behavior and the limitations of the LAD screen for phase separation. In these simulations, the effect of capillary pressure in the porous region is captured through a modification of the momentum equation that is implemented into a customized version of ANSYS Fluent via a User Defined Function (UDF). Finally, we model the behavior of a LAD screen during liquid transfer from a supply tank in microgravity and compare the results to the recent drop tower experiments performed by the University of Bremen. Strengths and shortcomings of the numerical approach in capturing this phenomenon are also discussed.