Wickless thermosiphons are a viable alternative to heat pipes for certain thermal applications. While the form-factor, two-phase operating principles, and working fluid choices make them nearly identical, there are some critical distinctions.
Key Benefits Versus Heat Pipes
1) Wickless thermosiphons are less expensive to produce than heat pipes. Once the tube is extruded, cut, and cleaned the device is evacuated and sealed after a working fluid is added.
Wickless Thermosiphon
2) For a given diameter, thermosiphons have a higher power carrying capacity than heat pipes. Because they are wickless, more vapor space exists to transport heat.
Qmax of Thermosiphon vs. Heat Pipe
3) Thermosiphons carry heat farther as the device does not rely on capillary action to return the liquid to the evaporator. These devices can operate in lengths tens of meters long while heat pipes are limited to around 300mm or so for -90 degree (against gravity) applications and a meter or two for horizontal applications.
Key Drawback Versus Heat Pipes
1) Thermosiphons rely on gravity to move liquid from the condenser to the evaporator. With no wick, the evaporator must be below the condenser for the working fluid to return, limiting thermosiphon design flexibility.
2) Wickless thermosiphons have a higher evaporation resistance that increases its total delta-t versus heat pipes. Something to consider when thermal budgets are tight.
3) Water based thermosiphons are susceptible to rupture if subjected to repeated freeze/thaw cycles. Alternative fluids can be used but Qmax suffers.
Related: Types of Heat Pipes
