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>> ZBOT is a space experiment.
It's a science experiment performed
in the International Space Station in support
of cryogenic fluid management of propellant tanks in space.
It's a science experiment
because it studies fundamental issues of phase change.
Fluid flow.
Heat transfer that affects preserving
and keeping fuels in depots in space.
ZBOT is important because it's a step towards designing this zero
boil-off tank control system without which you won't be able
to do the long-duration expedition.
For long-duration missions you have
to be able to fuel on orbit.
Preserve these fuels.
Which could be hydrogen.
Methane. Oxygen.
All of these liquids have to be preserved in space
under very low temperatures.
Hydrogen, for example, has to be preserved at 20 degrees Kelvin.
That's very low temperature.
So small heat leaks from the environment,
radiation on the tank surface can penetrate the heavy
insulation that's installed on the tank.
And once the heat goes in, it starts evaporating the liquid.
These tanks are also supported by some support structure.
Conduction through these support structures also can cause
localized heating of the tank.
And that localized heating can instigate boiling
and evaporation in the tank.
Which, again, means the tanks would be pressurized.
The physical phenomena that happens
in the storage tank is very complicated.
It requires solving the energy.
The fluid flow.
The species.
And the mass equations in the tank under conditions
where the flow is very orderly, which we call laminar flow.
Or very chaotic, which we call turbulent flow.
This requires solving these equations at many,
many points in a computational grid.
So as a result, for simulating
like a pressure control situation, a mixing control
in ZBOT, you might be solving 50 million simultaneous equations
with 800 processes working.
Some of the simulations take over a month to perform.
We had a few interesting results for ZBOT.
Let's start with self-pressurization.
Because those were the first series
of experiments that we made.
When we self-pressurize the tank, heat goes from the wall
into the tank and causes the temperatures to rise
in the liquid and the vapor.
And evaporation takes place.
The pressure rises.
Sometimes during self-pressurization,
boiling can occur at a hot spot on the wall.
This causes undesirable pressure spikes in the tank.
Because of microgravity thermal stratification,
boiling happens more easily in space than on Earth.
It was interesting to note that the trend in microgravity,
although a lot of things, including the ullage shape,
which is the bubble-shape the vapor shape is different.
And the amount of convection is different.
The trend followed the trend that we observe on Earth.
That's an important finding.
Which, because it says that we can use the Earth data
as a conservative estimate
of what pressurization would be in space.
And for the first time ZBOTs was able
to obtain this self-pressurization data,
in microgravity.
And we were able to validate our models against that data.
During the pressure controlled studies with the forced jets,
we found out that actually we can use the forced jet
at very low velocities, at very low speeds
to position the liquid where we want it to be.
The follow-on experiment to ZBOT-1 is called ZBOT-NC.
It stands for ZBOT non-condensible.
What happens in a real tank situation is, when you want
to feed the engine, you want to pressurize the tank very fast.
So that's often done by injecting a foreign gas.
Like in a hydrogen tank, you inject helium inside.
Helium doesn't condense at the temperatures of hydrogen.
So it's referred to as a non-condensible gas.
Then you can feed your engine the pressure drops.
Now you have to store the fluid, which is a mixture
of hydrogen and helium.
When the tank pressurizes again because of heat leaks
from the surroundings and the pressure goes up.
With the helium present there, you will find
out that you don't have the same capability to bring the pressure
of the tank down by cooling the liquid.
The non-condensible forms a barrier to that process.
ZBOT-NC is studying the effect of non-condensible
under the pressure control strategy.