With an increasing number of missions in space, cryogenic fluid supply
systems become necessary as mission duration is extended. Most of the
system operations involve multiple-phase phenomena. Accurate simulati
on of the operations is needed to predict the physics and efficiency o
f these systems. One operation of interest is the expulsion of liquid
cryogens from a partially filled depot; the challenge concerns finding
ways to expel the liquid while preventing vapour from entering the tr
ansfer line. The problem involves tracking the motion of a floating, m
obile and expanding vapour ullage as fluid is expelled from a tank. Th
e simulation of the interface movement of the vapour ullage in a volum
e of confined liquid under zero gravity is carried out by solving the
Navier-Stokes equations in the transformed boundary-fitted curvilinear
co-ordinate system using a finite difference approach. An alternating
expulsion and pressurization scheme is presented for the operation ba
sed on mass and momentum conservation.