Researchers at the Air Force Research Laboratory and a partner company have developed a way to cool the next generation of high-powered electronics —increasing capabilities of warfighters — with simple materials.
AFRL Space Vehicles Directorate Thermal Systems Lead Brent Taft said he expects the oscillating heat pipes to enter commercial systems within the next few years.
“We’re right on the cusp,” he said.
Companies such as Boeing are interested in using the technology.
The Japanese invented the concept in 1990, Taft began studying it in 2008 for his master’s thesis, and the Air Force and ThermAvant Technologies, a small company in Missouri, have led the research into the application since 2011.
Taft said as computer chips gain power and decrease in size, the heat they generate is concentrated into smaller areas. That makes it harder to keep them from overheating, failing and melting nearby components.
In 2004, he said, technology reached a point when it could no longer keep high-powered chips cool enough. So, industry began dividing the functions into multiple processors to spread out the heat.
AFRL wants to break that thermal limit so electronics can use fewer processors for the same work. Oscillating heat pipes help.
They have a variety of applications, from better combat communication systems to electronics boxes in tanks to Air Force satellites, which is the focus of Taft’s group.
Since electronics don’t function well in the frigid environment of space, he explained, satellites have to be insulated. The insulation puts electronics at risk of overheating.
The technology the Air Force has used in satellites for decades can absorb up to about 6 watts per square centimeter, roughly the same amount of heat as a stove burner set to “high” produces. Taft said the next generation of technology is expected to put out 1,400 watts per square centimeter, enough to quickly melt uncooled metal.
“The electronics would fail instantly if we didn’t have the thermal management,” he said.
Oscillating heat pipes have shown an ability to handle more than 1,250 watts per square centimeter.
While traditional heat-management technology, like heat pipes, uses a mesh screen called a wick to transport heat away against or without gravity, oscillating heat pipes don’t need that wick.
“It’s important because it allows us to manufacture these devices much more cheaply than traditional heat pipes,” Taft said.
Oscillating heat pipes can be made with a 3-D printer.
They consist of a metal plate with a tiny serpentine channel running through the center. The channel, sandwiched between the solid exterior layers of the plate, is filled with liquid slugs alternating with vapor bubbles.
The fluid on the hot side of the plate evaporates. The hot vapor moves to the colder side of the plate, releases the heat, turns back into fluid and goes back to the hot side of the plate to repeat the procedure.
That oscillation occurs 100 times a second and forces convective cooling, which is the same way a fan cools and very effective, Taft said.
His group usually makes oscillating heat pipes from aluminum, but they could use another metal.
Although a variety of liquids would work, the cooling fluids Taft plans to test soon are R134A, the same coolant as in car air-conditioners, and butane, normally used in lighters. ThermAvant discovered butane could be used as a coolant as long as it isn’t exposed to flame.
“It really is kind of a simple device,” Taft said. “But it gives us very complex performance from very simple materials.”
AFRL has worked with plates ranging in size from 2 inches by 6 inches to 4 inches by 6 feet. The size of the plate depends on the size of the electronics and the distance heat needs to be moved.
In 2012, researchers tested a plate-embedded oscillating heat pipe on a microgravity aircraft. The plane flies in curves that cause gravity on board to fluctuate from near zero to twice the normal pull of Earth’s gravity.
Taft said that experiment showed the oscillating heat pipes were reliable at both gravitational extremes and in between. However, it couldn’t show long-term dependability because the extremes only lasted for 20 seconds.
He is preparing for another test in which the technology will be tested in space for at least a year. The process will help researchers validate parts of their models they can’t validate on the ground, as well as demonstrating oscillating heat pipes are reliable in zero gravity for a long time.