2.1.1. Container

The container of the heat pipe can have different shapes for different applications, but the most often is in the form of a closed pipe of a circular, flat or triangular cross-section. The main function of the heat pipe container is to isolate the working fluid from the outside environment. The container of the heat pipe should be strong enough to prevent internal dimension and internal pressure in case of compression or bending. The choice of the container material depends on many properties and should have the most appropriate combination (working fluid and environment compatibility, strength-to-weight ratio, thermal conductivity, porosity, wettability, machinability, formability, weldability or bondability).The container material should have a high thermal conductivity, solid and tough but easily machined, formable and easily soldered and welded. The surface of the material should be well-wetted, but at least as porous as possible to avoid gas diffusion. The materials of heat pipes are most commonly made of steel, copper, aluminum and their alloys. Various coatings of steel materials are also used [6].

#### 2.1.2. Working fluid

Since the operation of the heat pipe is based on evaporation and condensation of the working fluid, its selection is an important factor in the design and manufacture of the heat pipe. The working fluid is chosen in particular according to the working temperature range of the heat pipe. Therefore, when selecting a working fluid, it is necessary to be careful if the operating temperature range of the working fluid lies in the operating temperature range of the heat pipe. The heat pipe can operate at any temperature that is in the range between the triple and the critical point of the working fluid. The decision criterion at working fluid selection, in case of working fluids with the same operating temperature, is an appropriate combination of working fluid thermodynamics properties. The recommended features that working fluid should have are compatibility with the capillary structure material and the heat pipe container, good thermal stability, wettability of the capillary structure and heat pipe container, vapor pressure in the operating temperature range, high surface tension, low viscosity of the liquid and vapor phase, high thermal conductivity, high latent heat of vaporization, acceptable melting point and solidification point [6]. Table 1 shows typical heat pipes working fluids sorted by operating temperature range.

2.2. Operation of heat pipe

total pressure drop in heat pipe.

denser to the evaporator.

positive or negative.

condenser.

a boiling limit [7].

Total pressure drop in heat pipe consist of three sections:

The correct operation of heat pipe must meet condition of:

In order of the heat pipe operation, the maximum capillary pressure must be greater than the

1. ΔPl is pressure drop in the wick structure necessary to return the liquid from the con-

2. ΔPv is pressure drop in the vapor core necessary to vapor flow from the evaporator to the

3. ΔPg is pressure drop due gravity, depending on the heat pipe inclination that may be zero,

If heat pipe does not meet this condition, it will not operate due to the dry out of the wick in the evaporator section. This condition is referred as the capillary limit which determines the maximum heat flux of majority heat pipe operating range. The vapor velocity of liquid metal heat pipes may reach sonic values at start-up and with certain high-temperature. Then, heat pipe performance is limited by speed of sound, and compressibility effects must be taken into account in the calculation of the vapor pressure drop. Other most important limitations are the vapor pressure or viscous limit which occur at heat pipe stat-up when the heat pipe operates at low temperature. However the condenser pressure cannot be less than zero, the low vapor pressure of the liquid in the evaporator cause that the vapor pressure difference between evaporator and condenser of the heat pipe is insufficient to overcome viscous and gravitational forces. When the heat pipe operates at high heat fluxes, vapor flow may entrain liquid returning to the evaporator and cause dry out of the evaporator. This condition is referred as an entrainment limitation. Above mentioned limitations of the heat pipe relate to axial flow. During the heat pipe operation, temperature difference of radial heat flux is relatively small. When the heat flux reaches a critical value, the vapor blankets surface of evaporator wall results in an increase in temperature difference in evaporator. Limitation related to the radial flow of the heat pipe is referred as

If stable liquid properties along the pipe, uniform wick structure along the pipe and neglect of pressure drop due to vapor flow are assumed, the total heat flux of heat pipe is given by

Q ¼ mmax

l : <sup>2</sup>

re � rl :g:l σl

mmax :

<sup>¼</sup> <sup>r</sup><sup>l</sup> :σl μl : <sup>K</sup>:<sup>A</sup>

ΔPc,max ≥ΔPl þ ΔPv þ ΔPg (1)

Porous Structures in Heat Pipes

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http://dx.doi.org/10.5772/intechopen.71763

: :L: (2)

(3)

: sin θ
