2.1. Heat pipe construction

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

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

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

evaporator section, an adiabatic section, and a condenser section) [4].

heat transport ability of the heat pipe and loop heat pipe.

2. Heat pipe

Figure 1. Schema of heat pipe.

142 Porosity - Process, Technologies and Applications

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 gravity [5].
