Liquid Hydrogen Sloshing under Microgravity in Superheated Vessels

We consider the influence of superheated walls on the axial sloshing motion of liquid hydrogen in an upright circular cylinder. The sloshing motion is induced by the capillary driven reorientation of the free surface after a step reduction of gravity. The step reduction is achieved in a drop tower facility which provides 4.7 s of compensated gravity conditions (Drop Tower Bremen). We conducted 15 experiments with two vessel radii of 26.2 mm and 20.15 mm, and various wall temperature gradients from isothermal up to 101.4 K/m. The vessels were filled with hydrogen only (single species system), and operated at a pressure of approximately 100 kPa. The saturation temperature of hydrogen at this pressure is 20.7 K. Right after the step reduction of gravity, the perfectly wetting liquid rises at the superheated wall and overshoots the steady state position of the new equilibrium, thus causing an axial sloshing motion which persists throughout the whole experiments time. It is shown that the wall superheat reduces the initial rise velocity and decreases the period of oscillation. A single sided model is presented which computes the transient temperature distribution in the wall. This model allows us to compute the heat flux into the liquid film at the wall, and therefor to determine the evaporated mass. We present a relation between the evaporated mass and the sloshing half-periods. The results contribute to the understanding of the behavior of cryogenic liquids, such as hydrogen, oxygen, and methane, in launchers, spacecrafts and future orbital propellant depots.