Is
the tank half empty or half full?
Even
optimists have a tough time predicting the pressure
within a cryogenic tank, much less sizing pressurant
bottles and designing pressure control systems based
on complex transient scenarios.
CFD
tools can predict liquid positioning and, with lesser
confidence, slow recirculations in the nearly-stagnant
ullage (vapor/pressurant) and liquid zones. They often
short-change thermodynamics (e.g., boiling, dissolution)
and are too slow to simulate important transient events
like fill, drain, and pressurization. Flow network
codes, on the other hand, have difficulty with thermal
stratification: fluid that is not well mixed (at the
same pressure but with a gravity- or environmental
heating-induced temperature gradient).
CRTech
has therefore developed unique methods in SINDA/FLUINT
for handling the special needs of stratified ullage
and liquid modeling, including treatment of the large
uncertainties involved and all the important physics,
such as:
1.
liquid level-dependent variations in heat transfer
(boiling vs. condensation, for example)
2. pool boiling, subcooled boiling, film and transition
boiling (exceeding critical heat flux and Leidenfrost
temperatures while filling a tank with warm walls)
3. diffusion-limited condensation, diffusion within
the ullage, and dissolution/evolution of the pressurant
into and out of the liquid
A
sample problem is available which may be used
as a starting point for a more customized case.
Click on the picture (requires Windows Media
Player) to see an animation of a vertical liquid
oxygen tank as it fills, holds for 30 minutes,
is pressurized by a high-pressure helium bottle,
then drains rapidly. Combined with other SINDA/FLUINT
features, such a model can be the terminus for
a more detailed pressurization system model.
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Thermally Stratified Supercritical Tank with Boundary Layer
How full is a tank that has no liquid/vapor interface?
The above example focuses on treating the interactions of both phases with each other an the wall, including motions of the liquid level. It uses "free floating" control volumes, but neglects boundary layers.
A counterpoint example is available that uses fixed control volumes, and which includes boundary layer effects. In this sample, a tank has been filled 90% full of liquid hydrogen, and is then pressurized beyond the critical point such that the distinction between liquid and vapor disappears ... along with any concerns regarding treatment of individual phases, boiling, condensation, etc.
Download Thermally Stratified Supercritical Tank sample model.
CRTech
also provides consulting and
custom software solutions to specifically meet your
needs. |
