2. Heat pipe

The heat pipe is heat transfer device using the phase-change of working fluid to transfer heat from a heat source to heat sink and capillary forces generated in the wick structure to working fluid circulation. The heat pipe consists of hermetically closed container with wick structure on the inner surface and working fluid near its saturation temperature. Heat absorbed through the container to the liquid in the evaporator causes liquid evaporation and vapor flow through the open core of the heat pipe evaporator. The vapor flows out of the evaporator through the adiabatic section into the condenser. There the vapor condenses and released heat is transfer through wick structure and container wall into condenser ambient. Condensed liquid saturates the wick structure and creates capillary pressure; thus the liquid is pumped back into the evaporator. The operation of the heat pipe depends mainly on the parameters of container, working fluid and wick structure. Proper selection and design of the main heat pipe parts influence its operational characteristics defined by the heat transfer limitations, effective thermal conductivity, and axial temperature difference. The two-phase heat transfer of the working fluid makes a heat pipe ideal for transferring heat over long distances with a very small temperature drop due the temperature stabilization of the almost isothermal surface created during the operation. Almost isothermal condition of the heat pipe operation are related to working fluid operation in thermodynamic saturated state when the heat is transported using the latent heat of vaporization instead of sensible heat or conduction. Heat transported using the latent heat of vaporization is several times greater than heat transported by sensible heat for a geometrically equivalent system. The two-phase capillary-driven working fluid system allows efficiently to transfer large amounts of heat without additional mechanical pumping systems, decreasing the heat transfer area and thus saving the material, cost and weight. Wide range of the usable working fluid, high efficiencies, low dimensions and weights, and the absence of external pumps make heat pipes attractive options in a wide range of heat transfer applications [4].
