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By Jerome Lacapere, Air Liquide DTA, Sassenage, France; and Bruno Vieille, CNES, Evry Cedex, France
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A few seconds before the lift-off of European Space Launchers, cryogenic
tanks containing propellants are pressurized in order to have
the necessary pressure conditions for the engine. If the Synchronized
Sequence (a list of procedures to be performed before launch) is stopped
(i.e. if the launch is aborted), the tanks must be returned to atmospheric
pressure in order to control the temperatures and perform a new Synchronized
Sequence later on. For future space launchers, this depressurization process
will also occur during the long ballistic phase in microgravity conditions.
In normal gravity, the pressure relief valve, positioned on the top of the
tank, releases only gas to the environment, acting to lower the temperature
of the liquid and ensuring the structural integrity of the vessel. In a
microgravity environment, however, the liquid and gas phases will not separate
by virtue of the gravitational body force, so it will be more difficult
to deal with pressure relief by a release of gas alone. Since experimental
measurements are not feasible in this regime, numerical methods are the
most promising option for understanding the process. Experimental measurements
of a normal gravity depressurization process performed at Air Liquide/DTA
have been compared to simulations using FLUENT. Validation of this work
has allowed engineers to put confidence in other depressurization simulations
performed under microgravity conditions.
The onset of boiling during depressurization of the cryostat in the experiment; the
liquid level increases slightly before boiling begins
Volume fraction of liquid nitrogen (red) and nitrogen gas (blue) during
depressurization in normal gravity conditions
Volume fraction of liquid nitrogen (red) and nitrogen gas (blue) during
depressurization in microgravity conditions
Experiments were performed using liquid nitrogen in a glass cryostat,
800 cm in height. Pressure and temperature probes were positioned inside
the vessel. Several rates of depressurization were examined for their impact
on the thermal stratification and subsequent boiling patterns. For one rate
studied, the level of liquid was found to increase slightly following depressurization,
with bubbles observed to form, coalesce, and rise to the top of
the liquid after 1.4 seconds, distorting the surface.
CFD simulations of this case were performed using the VOF model in
FLUENT. Both surface tension and contact angle were specified, and source
terms were added to simulate mass transfer during boiling1. Predictions of
volume fraction showed that the results are qualitatively the same as those
observed in the experiments. Both an increase in the liquid level and the onset
of boiling were predicted. The CFD solution predicted more of an increase
in the liquid level than that observed in the experiments, however. This is
because in the 2D axisymmetric model, bubbles correspond to torii, so they
take up more space than the spherical bubbles in the actual experiment, thereby
forcing the liquid to rise more as a result of their presence.
After validation of this depressurization case, engineers have used CFD
to explore depressurization in microgravity conditions. The same rate of
depressurization has been studied, with g = 0.001 m/s3, and the results show
a very different scenario. Bubbles form more rapidly, and the free surface
is destabilized more easily. Rather than rise and coalesce, as in the normal
gravity case, bubbles expand and entrain liquid throughout the entire domain.
In one of the most significant findings of the study, the gas bubbles entrain
droplets of liquid, and a mixture of gas and liquid, rather than pure gas,
escapes out the pressure release valve at the top of the vessel.
Reference:
- S.W.J. Welch and J. Wilson, A Volume of Fluid-Based Method for Fluid Flows with Phase
Change, J. Comput. Phys., 160, pp.662-682, 2000.
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