In operation LHPs realize the same basic physical processes as conventional heat pipes. These are, first of all, absorption and emission of considerable latent heat during the evaporation and condensation of a working fluid, heat transfer in the vapor phase and the motion of a working fluid at the expense of capillary forces created in the wick. However, in LHPs these processes are organized in a different way, which corresponds to the distinctive features of the design of this device. Among these are:

— the shape and local disposition of the wick only in the evaporator;

— the wick structure;

— special organization of the evaporation zone on the principle of "inverted menisci”;

— separate pipe-lines for the motion of vapor and liquid.

These peculiarities in the LHP design make it possible to achieve a much higher heat — transfer capacity owing to a relatively low hydraulic resistance in all its sections, the high capillary pressure created by the wick and an efficient organization of heat exchange in the evaporation zone.

To describe the LHP operation, it is convenient to use its scheme shown in Fig. 2 and the diagram of the working cycle with respect to the saturation line of the working fluid in "pressure — temperature” coordination in Fig. 3.

Fig. 3. Diagram of the LHP working cycle

If an LHP is situated vertically, and the evaporator is above the condenser, the free level of the liquid L-L is located in the liquid line and the evaporator as in communicating vessels. The wick in this case is fully saturated. When a heat load is supplied to the evaporator, the liquid begins to evaporate from the wick, absorbing in doing so the latent heat of vaporization. The main evaporation takes place at the evaporator wall, where the vapor temperature is T1. At the exit of the evaporator vapor may be slightly superheated and have a temperature T2 > T1. Since the wick has a certain thermal resistance, the vapor temperature T7 in the inner space of the wick and in the compensation chamber is lower than the temperatures T1 and T2. This results in a difference of vapor pressures ДР=Р1-Р7, which displaces the liquid from the vapor line and the condenser into the compensation chamber and the inner space of the wick. From here the liquid soaks into the wick closing the LHP working cycle.

From analysis of the diagram of the working cycle it follows that there are two main conditions of LHP serviceability. The first condition may be written as the balance of the capillary pressure created in the wick and the pressure losses in all the sections of circulation of the working fluid:

=ZAP, (1)

i=1 …n

where ДРс is capillary pressure determined by the effective pore radius of the wick, the coefficient of surface tension at the liquid-vapor interface and the angle of wetting of the wick with the liquid, APi presents the pressure losses in the i-th section of an LHP.

The second condition relates the value of the pressure drop between the evaporating surface of the wick and the compensation chamber:

ДР1-7 ^ ATi_7 , (2)

dT

SP

where — is the derivative characterizing the slope of the saturation line at a point

dT

between T1 and T7.

Since the difference between the values of the temperatures T1 and T7 required for creating the necessary pressure drop ДР1-7 is quite insignificant, the condition (2) is sufficiently correct for some average value ЗРЛ9Т in the range between T1 and T7. Then an additional condition following from the main conditions (1) and (2) is the relation:

ДР7—8 =APC — AP1-7 , (3)

which shows that the pressure drop between the zone of the evaporator and the compensation chamber should not exceed the value of the capillary pressure created by the wick. Together with the condition (2) it also means that in LHPs the wick serves not only as "a capillary pump”, but also as "a thermal lock”, which makes it possible to create in the evaporator a temperature drop and the corresponding pressure drop required for displacing the working fluid into the compensation chamber.