A heat pipe with no wick structure is often referred to as a thermosyphon. Some heat pipes can operate in any orientation, since they use the capillary pumping action of the wick structure to return working fluid from the condenser back to the evaporator. In contrast, a thermosyphon uses gravity to return liquid, so the evaporator must be located below the condenser.
One possible advantage is that a thermosyphon can also act as diode heat pipes. When heat is applied to the condenser, there is no condensate present, and hence no way to form vapour and transfer heat to the evaporator other than conduction through the envelope material.
These heat pipes are typically used in applications with radial heat fluxes up to 5 W/cm2.