Cryogenic helium subsurface pressurization in terrestrial and low-gravity: experiments and flow visualization

Pressurization of cryogenic propellant storage tanks is required for propellant thermodynamic conditioning and for enabling propellant transfer from a supply tank to another tank or an engine. In microgravity, the pressurization gas may be injected directly into the liquid propellant if the nozzle is submerged in liquid. Known as helium subsurface pressurization (HSP), the incoming warm helium gas causes immediate evaporation of the propellant into the helium bubbles which subcools the liquid. For HSP of cryogenic liquids in reduced gravity environments, there are many unknowns such as the rate of evaporation, change in temperature of helium after injection, bubble growth rate, and boil-off and pressurization rates. The purpose of this paper is to present new experimental data and flow visualization of subsurface gaseous helium injection into liquid nitrogen in terrestrial gravity and low-gravity. Varied parameters include injector size (0.25, 1.0 mm), injection flow rate (10^-9-10^-5 kg/s), and temperature (170-260 K). The new data clearly shows the role of gravity, buoyancy, and inertial forces on multiple cryogenic subsurface parameters of interest.