2.1.3. Wick structures

The wick structure and working fluid generate the capillary forces that are required to pump liquid from the condenser to the evaporator and keep liquid evenly distributed in the wicking material. Heat pipe wicks can be classified as either homogeneous wicks or composite wicks. Homogeneous wicks are composed of a single material and configuration. The most common types of homogeneous wicks are wrapped screen, sintered metal and axial groove. Composite wicks are composed of two or more materials and configurations. The most common types of composite wicks are variable screen mesh, screen-covered groove, screen slab with grooves, and screen tunnel with grooves. Regardless of the wick configuration, the desired material properties and structural characteristics of heat pipe wick structures are a high thermal conductivity, high wick porosity, small capillary radius, and high wick permeability [6].


Table 1. Typical heat pipe working fluids.

#### 2.2. Operation of heat pipe

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

The wick structure and working fluid generate the capillary forces that are required to pump liquid from the condenser to the evaporator and keep liquid evenly distributed in the wicking material. Heat pipe wicks can be classified as either homogeneous wicks or composite wicks. Homogeneous wicks are composed of a single material and configuration. The most common types of homogeneous wicks are wrapped screen, sintered metal and axial groove. Composite wicks are composed of two or more materials and configurations. The most common types of composite wicks are variable screen mesh, screen-covered groove, screen slab with grooves, and screen tunnel with grooves. Regardless of the wick configuration, the desired material properties and structural characteristics of heat pipe wick structures are a high thermal con-

ductivity, high wick porosity, small capillary radius, and high wick permeability [6].

Boiling point at atmospheric pressure (C)

Helium 271 269 21 271 to 269 Nitrogen 210 196 198 203 to 160 Ammonia 78 33 1360 60 to 100 Acetone 95 57 518 0 to120 Methanol 98 64 1093 10 to 130 Ethanol 112 78 850 0 to 130 Water 0 100 2260 30 to 200 Mercury 39 361 298 250 to 650 Caesium 29 670 490 450 to 900 Potassium 62 774 1938 500 to 1000 Sodium 98 895 3913 600 to 1200 Lithium 179 1340 19,700 1000 to 1800 Silver 960 2212 2350 1800 to 2300

Latent heat of vaporization (kJ kg<sup>1</sup>

)

( C)

Useful range

typical heat pipes working fluids sorted by operating temperature range.

2.1.3. Wick structures

144 Porosity - Process, Technologies and Applications

Working fluid

Melting point at atmospheric

pressure (C)

Table 1. Typical heat pipe working fluids.

In order of the heat pipe operation, the maximum capillary pressure must be greater than the total pressure drop in heat pipe.

Total pressure drop in heat pipe consist of three sections:


The correct operation of heat pipe must meet condition of:

$$
\Delta \mathbf{P}\_{\mathbf{c}, \max} \ge \Delta \mathbf{P}\_{\mathbf{l}} + \Delta \mathbf{P}\_{\mathbf{v}} + \Delta \mathbf{P}\_{\mathbf{g}} \tag{1}
$$

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 a boiling limit [7].

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

$$Q = m\_{\text{max}}^{\cdot}.L.\tag{2}$$

$$m\_{\text{max}}^{\cdot} = \left[\frac{\rho\_l \cdot \sigma\_l}{\mu\_l}\right] \cdot \left[\frac{\text{K.A}}{l}\right] \cdot \left[\frac{2}{r\_e} - \frac{\rho\_l \cdot \text{g.l.}}{\sigma\_l} \cdot \sin\theta\right] \tag{3}$$
