Heat pipes combine thermal conduction, phase change, and (at least in most designs) capillary flow to transport energy efficiently between a heat source and a heat sink. They can be used for cooling laptop microprocessors and graphics processors, keeping permafrost from thawing, and thermal management in spacecraft and satellites. Because the heat pipe’s fluid circulation is driven by interfacial forces, the devices operate without any moving parts, which makes them simple, light, and reliable.
Nonetheless, the humble heat pipe is still able to surprise when one looks at the fluid dynamics inside. In experiments conducted with a simply designed heat pipe on the International Space Station, we discovered, for example, that the pipe’s working fluid can condense at the heat-source end, where the temperature greatly exceeds the fluid’s boiling point.
Figure 1 illustrates a typical heat pipe, which consists of a sealed container with wicks on the internal walls. At...