Physics / mechanism
A heat pipe is a sealed evacuated vessel containing a working fluid that transfers heat via phase-change cycling: evaporation at the heat source (evaporator), vapour transport along a pressure gradient, condensation at the heat sink, and capillary-driven liquid return through a wick structure. Thermal conductivity effective values reach 10,000–100,000 W/m·K, dwarfing copper (~400 W/m·K). Key parameters: working fluid selection (water, ammonia, sodium for high-temp), wick permeability, vapour space pressure drop, and tilt sensitivity. State of the art includes sintered-powder and axially-grooved wicks operating at <0.1°C/W thermal resistance; loop heat pipes (LHPs) and oscillating heat pipes (OHPs) push performance in orientation-agnostic or ultra-compact geometries.
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