Depressurization induced vapor bubble growth in liquid methane during microgravity

Abstract

Depressurization and re-pressurization maneuvers are used for cryogenic propellants in spacecraft tanks to mitigate the incoming heat flow and to maintain an appropriate net positive suction head (NPSH). A depressurization maneuver can be used to transfer thermal energy from liquid to vapor and thus allow for subcooling of the liquid by a following, quick pressurization. The depressurization will lead to phase change, caused by the superheat relative to the initial saturation conditions, and the following pressurization will subcool the bulk liquid. Phase change will occur at any of the free surfaces of the bulk liquid or at nucleation sites in cavities at a wall. The mechanism which governs phase change at nucleation sites differs from nucleate boiling due to the temperature distribution in the liquid. In this study, we investigate the behavior of a single nucleation site during a depressurization under microgravity. We observed the bubble growth and evaluated the bubble radius with time. A total of five experiments were performed. The thermodynamic conditions are known and therefore can be correlated to the vapor bubble growth behavior. The superheat in the system varies with time, thus simple models are not applicable but can be used to estimate the bubble size. The data can be used as a reference for further analytical or numerical investigations.