Validation of Non-condensable Pressurant Gas Requirement During Liquid Propellant Expulsion

In cryogenic liquid-fueled propulsion systems, propellant is typically expelled from the tank bottom, while high-temperature helium gas or gaseous propellant is injected into the ullage to maintain tank pressure and prevent cavitation at the pump inlet. This process involves complex thermodynamic phenomena, including interfacial heat and mass transfer, ullage–liquid–wall thermal interactions, vapor–liquid mass exchange, and multi-species transport in the ullage gas. Accurate modeling of these coupled processes is essential for effective design and optimization of pressurization systems.

This study presents a validation of computational tools for simulating thermodynamic behavior in a cryogenic propellant tank undergoing sloshing and draining-induced thermal de-stratification. The Loci-STREAM-VOF multiphase computational fluid dynamics (CFD) code developed at the NASA Marshall Space Flight Center (MSFC) is employed to predict non-condensable pressurant gas requirements during the ramping, holding, and draining phases of tank operation with liquid methane.

Experimental data from NASA’s K-site facility are used for validation, in which both helium and hydrogen were used as non-condensable pressurants. Simulation results from Loci-STREAM-VOF successfully capture key experimental observations:

1. The non-condensable gas mass needed to maintain ullage pressure increases with expulsion time, attributed to increased heat transfer from the ullage to the tank wall over longer expulsion times.
   
   The required gas mass decreases with increased inlet gas temperature, due to improved thermodynamic efficiency.

Overall, good agreement is observed between simulation and experiment across a range of expulsion durations and pressurant gas inlet temperatures, demonstrating the capability of Loci-STREAM-VOF to support high-fidelity modeling of cryogenic tank pressurization systems for current and future NASA missions.