Boiling a liquid does something in near-zero gravity that even scientists didn’t expect would happen: ScienceAlert

In the dark depths of space, electronics and fuel can still get very hot.

Plans have also been made for Longer missions and more advanced technical systems, one of the key areas that scientists want to better understand is what happens to ultra-cold and cryogenic liquids When it boils.

These liquids are used as rocket fuel and to cool electronics, for example, but currently, we don’t know enough about their boiling behavior in microgravity.

A new study looking at this, in a series of airborne experiments, has shown some surprising results. The results are published in Microgravity npj.

We know boiling liquids You’re acting strange in space; We’ve seen it happen before.

With low gravity, convection does not distribute heat through fluids as easily. Bubbles do not separate from surfaces easily; They are less buoyant, so they do not float as they do on land.

So it is reasonable to assume so Low gravity It would also reduce the cooling capacity of those space bubbles because they cannot carry heat away quickly.

But the researchers found the opposite: lower gravity led to improved heat removal under certain conditions.

That is, until the temperature threshold is exceeded and boiling becomes unstable.

“What we found is that to some extent, boiling became more effective, which is the opposite of what we expected.” He says Mechanical engineer and senior author Youngseob Song, from the University of Florida.

“The problem is that the safety limit drops sharply. Both halves are important if you are designing real devices.”

Liquid nitrogen It was used as a safe and reliable alternative to all cryogenic liquids here, and experiments were carried out via parabolic flights simulating weightlessness.

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The researchers compared the data from these experiments with similar tests conducted in the laboratory, on solid ground under the influence of normal gravity.

“As we lose buoyancy in low gravity, we expected boiling to become less effective across the board.” He says song.

But that’s not what they found. Bubbles began to form sooner, and heat transfer improved under near-zero gravity conditions.

The researchers believe this is because in microgravity, the bubbles stick close to the hot surface, improving heat removal efficiency.

Our hypothesis is that bubbles stop floating, so they stay on the surface. He says song.

“When the bubbles are on the surface, there is a small liquid gap between the bubble and the heater, and this liquid layer is so thin that it can improve heat transfer.”

Although this appears to be beneficial, the cooling systems reached their maximum faster in the simulation Space conditions.

The maximum amount of heat the liquid nitrogen coolant can handle is reduced by 65 percent in microgravity compared to laboratory testing.

Essentially, suspended bubbles begin to bind together, drying out the surface faster, and causing the cooling mechanism to break down. Low gravity is an advantage for heat transfer, so it runs out.

There are limitations to consider here: only nitrogen was tested, and in short flight equivalents.

However, the data collected will be useful for future modeling as researchers create equipment for more efficient and safer space travel.

More importantly, in their experiments, Song and his colleagues used a silicon wafer coated with silicon dioxide as a heating surface, polished to be essentially free of microscopic defects and cavities.

Previous experiments relied on standard metal surfaces that contain small defects, which affect the formation of bubbles, leading to… Conflicting results Which cannot isolate the effects of gravity.

Therefore, the study here highlights the importance of material surface design when developing fuel conservation systems Electronics are cool.

Related to: A new ground study suggests that gravity can exist without mass

For propellants, for example, the further we want to explore beyond Earth, the longer the propellant will need to stay below the boiling point, something surface design can help us with.

“We want to engineer the surfaces to see if we can delay boiling in the storage tank.” He says song.

“That means [tuning] Surface properties, such as structures and chemistry, prevent boiling.”

The research was published in Microgravity npj.