LOOP THERMOSYPHONS

Gravity-Driven Two-Phase Cooling for the 21st Century

Two-phase cooling has been utilized in the electronics cooling industry for many decades, with possibly the most wellknown adaptation being the heat pipe. Heat pipes are capillary-driven, twophase devices that rely on the boiling and condensation of a working fluid to transfer heat significant distances with minimal temperature gradient. The flow of the working fluid inside of a heat pipe is facilitated by a capillary wick structure that relies on surface tension to return the condensed liquid to the heat generating components. Heat pipes have found their way into a large number of industries and applications because of their high performance, high reliability, and low cost. Unfortunately, as the electronics industry's insatiable quest for smaller, higher-powered devices soldiers on, the discrete cooling power of the heat pipe approaches obsolescence.

Cue the heat pipe's lesser-known cousin; the loop thermosyphon (or thermosiphon). Loop thermosyphons (LTS) are gravity-driven, two-phase devices that operate in a similar manner to a heat pipe in so far as a working fluid is evaporated and condensed in a closed loop to transfer heat over a given distance. Some readers may be more familiar with a traditional thermo sy phon, shown in Figure 1a, where the liquid and vapor occupy a single tube. Loop thermosyphons, as shown in Figure 1b (and as the name suggests), operate in more of a loop fashion where the liquid and vapor travel more independently.