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.

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.