Thermal Design, 2nd Edition

3Heat Pipes

3.1 OPERATION OF HEAT PIPE

A heat pipe is a simple device with no moving parts that can transfer large quantities of heat over fairly large distances essentially at a constant temperature without requiring any power input. A heat pipe is basically a sealed slender tube containing a wick structure lined on the inner surface and a small amount of fluid such as water at the saturated state, as shown in Figure 3.1. It is composed of three sections: an evaporator section at one end, where heat is absorbed and the fluid is vaporized; a condenser section at the other end, where the vapor is condensed and heat is rejected; and the adiabatic section in between, where the vapor and the liquid phases of the fluid flow in opposite directions through the core and the wick, respectively, to complete the cycle with no significant heat transfer between the fluid and the surrounding medium.

The operation of a heat pipe is based on the thermodynamic properties of a fluid vaporizing at one end and condensing at the other end. Initially, a wick of the heat pipe is saturated with liquid and the core section is filled with vapor, as shown in Figure 3.1. When the evaporator end of the heat pipe is brought into contact with a hot surface or is placed into a hot environment, heat will flow into the heat pipe.

Being at a saturated state, the liquid in the evaporator end of the heat pipe will vaporize as a result of this heat transfer, causing the vapor pressure there to rise. This resulting ...