Dynamics of Submerged Diffuser Pressurization

Ullage gas diffusers are used for pressure control within cryogenic propellant tanks by distributing high pressure, high temperature pressurant into the gas volume and minimizing mixing with the cryogen. Occasionally, diffusers may be submerged within a propellant due to the motion of a spacecraft or simply due to the minimum energy state in microgravity environments. Pressurization of a tank in this configuration can result in condensation of the injected gas, higher average liquid temperatures, and a lower-than-expected pressure rise. Moreover, since a diffuser is likely not designed for this mode of operation, it may unsettle the liquid even further when gas is injected. This off-nominal behavior is traditionally avoided by first applying a settling force from reaction control systems or propulsive vents, aggregating the liquid away from the diffuser. Conversely, applying a settling force before pressurization can be costly for mission operations, especially for multiple engine starts. Attitude control mass budgets must be increased significantly to both provide settling and counteract the added thrust to maintain trajectory.

The dynamics of submerged diffuser pressurization, especially in low gravity, were assessed using computational fluid dynamics (CFD) analysis. The CFD solver Loci/STREAM with a volume of fluid (VoF) method to distinctly represent fluid phases was used to simulate gas injection into a liquid volume. Validation of bubble detachment in low gravity was performed to provide confidence in the predictive capability of the modeling method for submerged pressurization. Predictions of pressurization dynamics were made for a large-scale propellant tank as a function of pressurization rate and diffuser design.

The absence of relevant test data makes analysis results valuable as future missions to the Moon and Mars are in development now. Thus, the analysis results will be used to help inform future trade studies of submerged and unsubmerged pressurization system operations and define optimal operating conditions for future low gravity submerged pressurization tests. The present analysis is the first step of a concerted, larger scope effort to better understand if submerged pressurization is viable or optimal for any applications or parameter spaces in future NASA missions.