Investigation of interface stability during the filling of liquid into a tank under reduced gravity

Abstract

Long-range and long-term space exploration missions can be accomplished in the future with the help of in-space propellant depots. The spacecraft can dock at the orbiting propellant depot and refill their tanks. The spacecraft tanks can be filled with liquid either by venting or not venting the gas in reduced gravity. However, to effectively accomplish the orbital refueling, the fluid mechanics coupled with thermodynamics under reduced gravity conditions has to be fully understood.

In this thesis, the investigation of interface stability during the vented filling was carried out by performing experiments and 2D numerical simulations under isothermal conditions. A multi-species multiphase system of the storable test liquid HFE-7500 and air was used in the experiments and simulations. The vented filling experiments were conducted on the ground, in the Bremen Drop Tower and on a parabolic flight. The 2D numerical simulations were performed using the multiphase volume of fluid (VOF) model of ANSYS Fluent.

The drop tower experiments were carried out in the Bremen Drop Tower with the catapult mode, which offers a longer microgravity time of about 9 s. The initial liquid fill height was set to 30 mm in all the drop tower experiments. The liquid jet that entered the partially filled cylindrical tank interacted with the interface and formed different geyser patterns for different volumetric flow rates in the range between 1.00 mL/s and 1.50 mL/s. Subcritical, critical and supercritical flow regimes were observed and the interface was termed unstable when the geyser disintegrated into droplets. The critical flow rate was found to be 1.30 mL/s and the corresponding critical Weber number was 1.04. The gas-free liquid filling into a rectangular experiment tank under variable accelerations was demonstrated by performing parabolic flight experiments on the Airbus A310 Zero-G aircraft. Each parabolic maneuver provided a reduced gravity time of 22 s. The filling of liquid into an initially empty tank was predominantly tested during the parabolic flight experiments for different volumetric flow rates. For tests with a pre-filled tank, a violent sloshing of the bulk liquid caused by variable accelerations was observed.

The 2D numerical simulations over-predicted the geyser height and did not capture the unsteady movements of the geyser, in comparison to the drop tower experiments. Nevertheless, a parametric study of different initial liquid fill heights and refined volumetric flow rates was performed using the numerical simulations and the corresponding critical Weber numbers were found. The critical Weber numbers from the simulations and drop tower experiment were compared with the existing literature. It was found that the centreline velocity, which is dependent on the velocity profile of the incoming liquid jet that enters the tank, directly affects the stability of the interface. Some more dimensionless numbers that describe the filling problem were defined and their influence on the interface stability was also discussed. All these results lead to the design of an international space station (ISS) experiment to demonstrate the filling and transfer of a storable liquid under microgravity conditions.