I'd never heard of this robotic orbital test vehicle but it's apparently just spent 780 days in orbit. Here's some links for anybody interested.
Apparently, one of its recent experiments was measuring the performance of an oscillating heat pipe...which, I gather, is a form of passive heat transfer.
Boeing X-37 - Wikipedia
en.wikipedia.org
Secretive X-37B Orbital Test Vehicle lands after 780 days in orbit
The US Air Force's X-37B Orbital Test Vehicle landed at NASA's Kennedy Space Center after 780 days in orbit.
ukdefencejournal.org.uk
Apparently, one of its recent experiments was measuring the performance of an oscillating heat pipe...which, I gather, is a form of passive heat transfer.