1. Introduction

Two-phase heat transfer systems with capillary-driven offer important advantages over traditional single-phase systems. The most significant advantage associated with the phase change of a working fluid is higher heat transfer coefficient that results in enhanced heat transfer. Comparing with the single-phase liquid system, smaller mass flow rates are required to transport equivalent heat flux amounts for a given temperature range. Better thermal characteristics and lower mass flow rates offer the two-phase system benefit of smaller and lighter construction and increased performance. Single-phase system requires a high temperature gradient or a high mass flow rate to transfer high amount of heat flux, because thermal capacity of a single-phase system depends on the temperature change of the working fluid. The two-phase system provides essentially isothermal operation regardless of the heat load.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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

Porous Structures in Heat Pipes

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

The heat pipe may have several basic parts depending on its type. During the heat pipe development, the main components and materials remained the same. The simplest type of heat pipe consists of two basic parts, the body (container) and the working medium. A capillary structure (wick) can be placed inside the heat pipe body to allow the condensed liquid phase of the working fluid wicking against the vapor flow due the capillary action. Such a heat pipe is called a wick heat pipe. The heat pipe without capillary structure, is called gravitational heat pipe because it returns the liquid phase from the condenser part to the evaporator part which is due to

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,

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

aluminum and their alloys. Various coatings of steel materials are also used [6].

of heat transfer applications [4].

2.1. Heat pipe construction

gravity [5].

2.1.1. Container

2.1.2. Working fluid

Figure 1. Schema of heat pipe.

Additionally, the single-phase systems need to working fluid circulation mechanical pumps or fans, while in the capillary-driven two-phase systems the working fluid circulate without any additional mechanical devices, which make such systems more reliable and free of electric power. The best known capillary-driven two-phase system is heat pipe and its schema is shown in Figure 1. The concept of the heat pipe was first suggested by Gaugler in 1944 [1] and by Trefethen [2] but was not widely publicized until serious development work by Grover and his co-worker in Los Alamos laboratory [3]. Heat pipes are passive heat transfer devices that transport heat from one point (heat source) to another (heat sink) with extremely high thermal conductance due the latent heat of vaporization of a working fluid. As shown in Figure 1, it consists of container, working fluid, wick structure and has three sections (an evaporator section, an adiabatic section, and a condenser section) [4].

Since one of the most important parts of the heat pipe HP and loop heat pipe LHP is the porous wick structure, this work is focused on experiments influencing porous wick structure on the heat transport ability of the heat pipe and loop heat pipe.
