A heat pipe provides very high effective conductivity — hundreds of times that of copper — because the working fluid evaporates at one end and condenses at the other. It is used in electronics cooling, spacecraft thermal control, and everywhere from laptops to power modules.
Practical modelling: equivalent conductivity
In industrial analyses the interior of a heat pipe is not solved. The pipe is represented as a solid with very high and anisotropic conductivity: very high along the axis, and the true wall material conductivity across it.
This simplification is correct at system scale and makes the computation an order of magnitude cheaper.
But it is not unlimited: transport limits
The power a heat pipe can carry is limited, and beyond that limit the evaporator dries out and the effective conductivity collapses. The main limits:
- Capillary limit. The wick cannot return the condensate. This is the most restrictive in orientations that work against gravity (evaporator above).
- Boiling limit. Boiling begins between wall and wick, cutting off the liquid return.
- Sonic and viscous limits. At low operating temperatures the vapour flow itself becomes limiting.
If you use a fixed equivalent conductivity in the model, you must check at the end of the solution that the power entering the pipe stays below the limit. Otherwise the model shows a component that has in fact collapsed as working perfectly.
Orientation and temperature range
Heat pipe performance depends on the installation angle; the same product is not the same lying on a desk and mounted on a wall. Each working fluid also has a useful temperature range — below it the vapour pressure is insufficient, above it the pressure becomes a structural problem.
The equivalent conductivity used, the assumed orientation and the limit check all belong in the report together.
References
- JEDEC Solid State Technology Association. JESD51 — Methodology for the Thermal Measurement of Component Packages (Single Semiconductor Device).
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605