Experimental study of a loop heat pipe with a copper capillary structure and ammonia as a working fluid

Аuthors

Panasenko . S., Maydanik Y. F.^*, Vershinin S. V.

Institute of Thermal Physics of the Ural Branch of the Russian Academy of Sciences , Amundsena St., 107a Ekaterinburg, 620016, Russia

*e-mail: maidanik@itp.uran.ru

Abstract

Loop heat pipes (LHPs) are passive two-phase heat transfer devices that operate on a closed evaporation-condensation cycle and use a “capillary mechanism” to ensure circulation of the working fluid. The LHPs are designed as a closed loop, including an evaporator equipped with a fine-pored capillary structure – a wick and a condenser, which are connected by means of smooth-walled pipelines for separate movement of vapor and liquid, the diameter of which usually varies from 2 to 8 mm. This makes it easy to configure them, which is often necessary to place the device in highly constricted conditions. The evaporator can have both cylindrical and various flat shapes depending on the conditions of practical application. The wick, located only in the evaporator, has a special design that allows it to simultaneously perform the functions of a capillary pump as well as a hydraulic and thermal lock, which ensures the operability of the heat transfer device in a wide variety of conditions. The condenser can have different shapes and sizes adapted to the operating conditions of the LHP. Various liquids can be used as working fluids depending on their operating temperature range, chemical compatibility with structural elements, and the required operating characteristics of the LHPs. The most effective working fluid for LHPs, used in particular in electronics cooling systems, is ammonia.

The paper presents for the first time the results of the development and study of the operating characte-ristics of a loop heat pipe (LHP) made of stainless steel with a cylindrical evaporator with a diameter of 12 mm and a length of 90 mm, equipped with a copper wick with a breakdown radius of pores of 3,4 μm. A vapor line with a length of 395 mm had a diameter of 2,5 mm, while a liquid line measured 605 mm and 2 mm, respectively. A U-shaped tubular condenser with a length of 299 mm and a diameter of 4 mm was coupled with a flat copper interface with dimensions of 150×100×5 mm, which was pressed against a liquid heat exchanger during tests. Ammonia was used as a working fluid. The feature of this device is the combination of copper and ammonia, which has not previously been used in heat pipes. This combination makes it possible to significantly expand the technological capabilities in the production of LHPs, as well as to increase their efficiency. The LHP tests were carried out in a horizontal orientation at heat loads from 10 to 350 W at different temperatures of the circulating water cooling the condenser. The thermal resistance of the device in the range of nominal heat loads from 100 to 350 W was at a level of 0,2 °С/W. It has been shown that the “copper-ammonia” combination is quite accep-table, which will make it possible in the future to significantly reduce the thermal resistance of loop heat pipes made entirely of copper.

Keywords:

cooling system, loop heat pipe, heat load, thermal resistance, chemical compatibility

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