Yu. Maydanik

Institute of Thermophysics, Ural Branch of the Russian Academy of Sciences Amundsen st. 106, Ekaterinburg, 620016, Russia

Phone: +7 (343) 2678791. Fax: +7 (343) 2678799. E-mail: maidanik@etel. ru

Loop heat pipes (LHPs) are hermetic heat-transfer devices operating on a closed evaporation-condensation cycle with the use of capillary pressure for pumping the working fluid [1]. In accordance with this, they possess all the main advantages of conventional heat pipes, but, as distinct from the latter, have a considerably higher heat-transfer capacity, especially when operating in the “antigravity” regime, when heat is transferred from above downwards. Besides, LHPs possess a higher functional versatility, are adaptable to different operating conditions and provide great scope for various design embodiments. This is achieved at the expense of both the original design of the device and the properties of the wick — a special capillary structure used for the creation of capillary pressure. The LHP schematic diagram is given in Fig. 1.

The device contains an evaporator and a condenser — heat exchanger connected by means of smooth-walled pipe-lines with a relatively small diameter intended for separate motion of vapor and liquid. The evaporator is equipped with a wick specially configured for providing in it a sufficiently low hydraulic resistance, despite a very small pore radius of the order of 1 micron. The evaporator is joined to a compensation chamber intended for accumulating the working fluid during the operation of the device.

The most efficient working fluids for LHPs in the range of operating temperatures from 20°C to 200°C are ammonia and water. It is also possible to use some other working fluids which possess the necessary thermophysical properties and are chemically compatible with LHP structural materials, which are copper, stainless steel, nickel, titanium.

Wicks sintered from fine-grained particles are capable of creating the pressure required for pumping a working fluid for a distance up to several tens of metres along pipe-lines 6-10 mm in diameter. In operation in the “antigravity” regime the heat-transfer distance may reach 5-7 m, and its value 1-5 KW.

The device may have one or several parallel evaporators and condensers, whose design varies in accordance with the conditions of heat exchange with the heat source and the heat sink.

At present loop heat pipes are most extensively employed in thermoregulation systems of spacecrafts. Miniature LHPs are used for cooling electronics and computers. At the same time there exists a considerable potential of using these devices for the recovery of low-grade (waste) heat from different sources, and also in systems of sun heat supply. In the latter case LHPs may serve as an efficient heat-transfer link between a sun collector and a heat accumulator, which has a low
thermal resistance and does not consume any additional energy for pumping the working fluid between them.