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Bizarre Boiling

Watching liquids boil in low gravity is an out-of-this-world experience. It has plenty of entertainment value, but it's teaching scientists some important physics lessons, too.

by Patrick Barry

The next time you're watching a pot of water boil, perhaps for coffee or a cup of soup, pause for a moment and consider: what would this look like in space? Would the turbulent bubbles rise or fall? And how big would they be? Would the liquid stay in the pan at all?

Until a few years ago, nobody knew. Indeed, physicists have trouble understanding the complex behaviour of boiling fluids here on Earth. Perhaps boiling in space would prove even more baffling.... It's an important question because boiling happens not only in coffee pots, but also in power plants and spacecraft cooling systems. Engineers need to know how boiling works. 

In the early 1990's a team of scientists and engineers from the University of Michigan and NASA decided to find out. Using a freon coolant as their liquid, they conducted a series of boiling experiments on the space shuttle during 5 missions between 1992 to 1996. And indeed, they found some intriguing differences between what happens to boiling fluids on Earth and what happens to them in orbit. For example, a liquid boiling in weightlessness produces - not thousands of effervescing bubbles - but one giant undulating bubble that swallows up smaller ones!

"Think of it: no one had really seen boiling in space before these experiments - in the whole world, ever!" says Dr. Francis Chiaramonte, who was the NASA Project Scientist for the Pool Boiling Experiment. Already, he says, the series of experiments has come to be regarded as "classic" by today's researchers.

Image courtesy NASA Glenn Research Centre .

Without buoyancy or convection, boiling fluids behave quite differently in space.

Despite its entertainment value, this research is much more than a simple curiosity. Learning how liquids boil in space will lead to more efficient cooling systems for spacecraft, such as the ammonia-based system on the International Space Station. Knowledge of boiling in space might also be used someday to design power plants for space stations that use sunlight to boil a liquid to create vapour, which would then turn a turbine to produce electricity.

The research could also have applications here on Earth. The weightless environment gives scientists a new "window" into the phenomenon of boiling. Scientists can use this perspective to improve their understanding of the fundamentals of boiling, which might be used to improve the design of terrestrial power plants.

The International Space Station uses a "2-phase" cooling system in which ammonia changes from liquid to vapour and back, which involves boiling. Engineers designing the ISS cooling system used information gleaned from microgravity boiling experiments.

"The phenomenon of boiling is so complex that most of our understanding is empirical, rather than based on the solutions to fundamental equations," Chiaramonte says.

In the free-fall of orbit, boiling is simpler than it is on Earth. Weightlessness effectively removes two of the variables in boiling - convection and buoyancy. This difference explains why boiling liquids behave so differently in space. It also provides a powerful tool for scientists who want to unravel the tangled physics of boiling.

"As an example, imagine you were trying to study the Earth, which has such complex ecosystems. You would also want to look at a simpler planet with fewer variables. One thing space does for us is simplify the problem that we're studying," Chiaramonte says.

When a pool of liquid is heated on Earth, gravity causes hotter regions in the liquid to rise, and cooler, more dense parts to sink - a process called "convection." This motion spreads the heat around inside the liquid. Once it begins to boil, buoyancy sends bubbles hurling upward, creating a "rolling boil."

All of this motion within the liquid makes the physics of the situation much more complex.

Without convection or buoyancy, the process unfolds differently. Heated fluid doesn't rise, and instead just sits next to the heater surface and continues to get warmer. Regions of liquid away from the heater remain relatively cool. Because a smaller volume of water is being heated, it comes to the boil much more quickly. As bubbles of vapour form, though, they don't shoot to the surface - they coalesce into a giant bubble that wobbles around within the liquid.

Without buoyancy, the vapour produced by boiling simply floats as a bubble inside the liquid after the heating has stopped. Surface tension effects cause the many small bubbles produced to coalesce into one large sphere.

Much of this could be predicted from existing theory, but to learn the fine details of the process and to look for unexpected behaviours, a real experiment was necessary.

"There were many fundamental issues that were still not understood well," says Dr. Herman Merte, the Principal Investigator for the experiments. Merte, who some see as a kind of "founding father" of microgravity pool boiling research, devised the experiments.

Merte and other scientists had performed earlier research on weightless boiling using "drop towers," which could simulate zero-G for a few seconds by simply dropping samples inside a tall tower. These early experiments provided some guidance for designing the shuttle-based experiment, but these brief glimpses don't really compare to the minutes-long observation provided by the shuttle.

One important product of that early research, though, was a method for building a boiling chamber that let scientists look through the heater surface and watch the liquid right where it contacts the heater.

"The action is right at the solid-liquid interface at the heater, and you can't look down from the top because you have the refraction of the fluid's upper surface that interferes," says Merte, who recently retired as Emeritus Professor of Mechanical Engineering at the University of Michigan.

Image courtesy NASA Glenn Research Centre

One way to simulate the weightlessness of space is to simply let an experiment freefall inside a "drop tower," such as this one at NASA's Glenn Research Centre in Cleveland, Ohio. Other methods for simulating weightlessness include flying parabolic arcs in aircraft - such as NASA's KC-135 "vomit comet" used to train astronauts - and using sounding rockets.

Merte used quartz to make a smooth, hard, transparent bottom for the boiling chamber. Then he coated that quartz with an ultra-thin layer of gold. Less than 400 angstroms thick (an angstrom is one ten-billionth of a meter), this layer was so thin that it allowed visible light to pass through it, yet it still conducted electricity like bulk gold.

Using this apparatus, Merte and his colleagues made some interesting discoveries. For example, depending on the temperature conditions of the experiment, the giant bubble would sometimes float in the centre of the liquid and sometime remained attached to the heater surface. When the bubble remained attached - which Merte calls a "dry out" - it effectively insulated the liquid from the heater, preventing further boiling and causing the heater temperature to soar.

Knowing precisely the conditions when this occurs is vital for designing spacecraft systems that might rely on boiling.

"If you understand a phenomenon better, then you can design for it closer to its limits for optimisation," Merte says. "If you have an uncertainty, then you're going to design conservatively."

Today's researchers continue to expand on the foundation of knowledge laid by these experiments. With a better understanding of the physics of boiling fluids, engineers will be able to design improved cooling and power systems to serve people in the future - both in space and here on Earth.

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First Science 2014