A heat pipe is a closed evaporator-condenser system consisting of a sealed, hollow tube whose inside walls are lined with a capillary structure or wick.

Thermodynamic working fluid, with substantial vapor pressure at the desired operating temperature, saturates the pores of the wick in a state of equilibrium between liquid and vapor.

When heat is applied to the heat pipe, the liquid in the wick heats and evaporates. As the evaporating fluid fills the heat pipe hollow center, it diffuses throughout its length.

Condensation of the vapor occurs wherever the temperature is slightly below that of the evaporation area. As it condenses, the vapor gives up the heat it acquired during evaporation.

This effective high thermal conductance helps maintain near constant temperatures along the entire length of the pipe.

Attaching a heat sink to a portion of the heat pipe makes condensation take place at this point of heat transfer and establishes a vapor flow pattern.

Capillary action within the wick returns the condensate to the evaporator (heat source) and completes the operating cycle.

The internal liquid absorbs thermal energy conducted through the wall of the pipe and evaporates. 

The pressure forces this vapor away from the heated portion of the heat pipe to the cooler region(s).

 Thermal energy is absorbed by the cooler heat pipe region away from the heat source, where the vapor condenses back into liquid. 

Capillary action from the wicking structure transports the liquid back to the area where evaporation occurs, starting the cycle again.