Phase-change systems of thermal control - closed loop thermosyphons and capillary loops

Conventional capillary heat pipes and two-phase thermosyphons, described in the [BE 9 545] dossier, are well-established thermal control systems. They have undergone considerable development since the 1960s-1970s, were used in many practical applications in the 1990s, and can now be considered mature technologies. Nevertheless, they present a number of limitations linked to the capillary structures or the characteristics of the liquid-vapor flows that govern their operation.

With the development of electronics, the miniaturization of the systems to be cooled becomes ever more demanding, while the thermal power to be dissipated increases. The result is an extremely rapid increase in the flux densities to be transferred, which are now reaching values that are no longer compatible with conventional heat pipe technology. What's more, their performance in terms of temperature field homogenization is sometimes too low to meet industrial requirements.

In response to these new challenges, new concepts for two-phase cooling systems have emerged, such as capillary-pumped two-phase loops or two-phase gravity loops. These loops differ from heat pipes in that steam and liquid circulate in separate pipes, rather than together (as in the case of liquid in equilibrium with steam). Their technologies are currently at the development, prototype validation and short production run stages, particularly in the space sector. At the same time, two-phase microheat pipes and thermal diffusers have enjoyed a boom, including on an industrial scale, while oscillating heat pipes have been the focus of academic research. These last two types of two-phase cooling systems are discussed in the following dossier [BE 9 547] .