Thermosyphons

Thermosyphons transfer heat in exactly the same way as the Heat Pipe by evaporation followed by condensation.
Howevere no capillary structure is present to aid liquid transport from the condenser back to the evaporator, and  thus the evaporator must be located vertically below the condenser, gravity will then ensure that the condensate returns to the evaporator.

To avoid the confusion because the term " Heat Pipe" is commonly used to describe both the Heat Pipe  and Thermosyphone the term "Gravity Assisted  Heat Pipe"  has been used to describe thermosyphons.