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International Space Station

Boiling at zero-gravity. That’s hot

U. MARYLAND (US) — Shuttle Discovery recently launched carrying an experiment designed to figure out how boiling is altered in zero-gravity.

The experiment—known as the Microheater Array Boiling Experiment—has been tested on NASA’s “Vomit Comet” and the European Space Agency’s Parabolic Flight Campaign in France. It will be installed in the Microgravity Science Glovebox once aboard the International Space Station.

“In space, boiling may be required to generate vapor to power turbines in some advanced concepts for power generation, for temperature control aboard spacecraft, and for water purification,” says Jungho Kim, a mechanical engineering professor at the University of Maryland.

When a liquid is boiled on Earth, vapor, which is less dense than liquid, is removed from heated surfaces through the action of buoyancy. In zero-gravity, the buoyancy force becomes negligible and vapor can blanket the heated surfaces rather than moving away, potentially leading the surfaces to a state known as critical heat flux.

Critical heat flux occurs when a heater or plate becomes too hot, restricting the flow of liquid to the surface and causing the plate to overheat and potentially burn out. Since liquids boil differently in space, an understanding of how these fluids behave can improve the reliability and expand the applications of space exploration hardware.

The experiment that will take place on the Space Station will use two arrays of platinum microheaters bonded to a quartz plate. The arrays measure 7 mm and 2.7 mm across. The heaters are warmed when electricity is applied, and spaces between the heaters lines will allow the boiling process to be visualized through the transparent quartz.

Boiling of a refrigerant-like fluid (FC-72) will be filmed at high speed and the video sent back to Earth along with the heater data in real-time for analysis.

“We have calibrated these heaters as a function of a temperature, and we measure the power level required to keep each of the heaters in the array at a constant temperature” Kim says.

“Using a camera that looks through these microheaters, we can examine the relationship between the power level and the state of the fluid above that microheater.”

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