ReviewLoop heat pipes
Introduction
The history of development of loop heat pipes (LHPs) originates from 1972, when the first such device with a length of 1.2 m and a capacity of about 1 kW, with water as a working fluid, was created and tested successfully by the Russian scientists Gerasimov and Maydanik from the Ural Polytechnical Institute [1], [2].
The appearance of LHPs was a response to the challenge connected with the acute demand of aerospace technology for highly efficient heat-transfer devices with all the main advantages of conventional heat pipes [3], but at the same tune much less sensitive to the change of orientation in the gravity field. The problem here lies in the fact that conventional heat pipes, in which the capillary structure (wick) is situated along the whole length, abruptly decrease their heat-transfer capacity when the evaporation zone is above the condensation one. First of all, it refers to low-temperature devices, where use is made of working fluids with a relatively low value of the surface-tension coefficient. This circumstance is connected with the fact that the maximum value of the capillary head providing the circulation of a working fluid in a heat pipe is directly proportional to this coefficient and inversely proportional to the effective pore radius of the wick. To compensate for the additional pressure losses during the liquid motion to the evaporation zone against the gravity forces, it is necessary to increase the capillary head. It is evident that in this case it can be done only at the expense of decreasing the effective pore radius of the wick. However, here one can observe an increase in the hydraulic resistance of the latter approximately proportional to the square of the pore radius. As a result of this contradiction, attempts to create a heat pipe of sufficient length capable of operating efficiently against gravity forces do not meet with success. Even when the working fluid is water, which is the “strongest” working fluid in the low-temperature range, heat pipes 0.5 m long decrease their capacity almost by an order when it is necessary to transfer heat from above downwards at a vertical position of the device [4]. Numerous well-known variants of solution of this problem by using various additional means for reinforcement or even replacement of the capillary mechanism of pumping of the working fluid lead only to an inadequate loss of these or those valuable properties typical of heat pipes [5], [6], [7], [8], [9], [10], [11]. Therefore, they have not gained extensive distribution and practical implementation.
The LHP conception to a considerable extent makes it possible not only to overcome this drawback of conventional heat pipes, but also to obtain some additional advantages, remaining in the framework of the capillary mechanism and using all its advantages. It includes the following main principles:
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the use of fine-pored wicks;
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maximum decrease in the distance of the liquid motion in the wick;
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organization of effective heat exchange during the evaporation and condensation of a working fluid;
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maximum decrease in pressure losses in the transportation (adiabatic) section.
For realization of this conception special capillary–porous materials were created on the basis of sintered nickel, titanium and copper powders with an effective pore radius of 0.7–15 μm and a porosity of 55–75%. Such materials possess a sufficient strength, which allows complex mechanical processing in making wicks, are capable of creating a very high capillary pressure with the use of low-temperature working fluids and are chemically compatible with most of them.
Minimization of the distance for the liquid motion in the capillary structure limited, as a rule, by several millimeters is achieved at the expense of the wick design, whose extent corresponds to the dimensions of the evaporation zone and does not depend on the total length of the device. The motion of vapor and liquid flows in such a wick proceeds mainly in the radial direction and has a counter character at which the evaporating menisci are inverted towards the wall being heated.
The evaporation zone, which is formed here by a ramified system of vapor-removal channels located at the “wall-wick” boundary ensures an effective heat exchange even during evaporation from a fine-pored capillary structure. Depending on the wick material and the kind of the working fluid, the intensity of heat exchange in such a zone may reach values from 10,000 to 100,000 W/m2 K.
The design and the dimensions of the condensation zone in LHPs may be quite different, which makes it possible to easily adapt it to the conditions of heat exchange with an external heat sink. Condensation here takes place, as a rule, at a smooth surface, in constrained conditions and is of film character. Where it is possible to organize a film suction, the intensity of heat exchange during condensation may be considerably increased.
A decrease in pressure losses in the adiabatic section of LHPs is ensured by the fact that for the motion of a working fluid here use is made of separate smooth-walled pipe-lines, which exclude both the thermal and the viscous interaction between counter flows of vapor and liquid.
Fig. 1 shows the schematic diagram in which the LHP conception is realized. Besides attaining the main aim formulated above, it allows wide variety for different design embodiments and, at the same time, extends considerably the sphere of functional potentialities of two-phase heat-transfer devices with capillary pumping of a working fluid. At present it forms the basis for large and powerful LHPs operating efficiently at any orientation in 1-g conditions, and also flexible, ramified, controllable, reversible, miniature and other devices, many of which have already found application in space technology and electronics and have good prospects for application in some other spheres. The wide variety of designs and functional indications of LHPs known to-day makes it possible to perform their classification, which is given in Table 1.
The paper presents the fundamentals of the theory of LHPs, various examples of their designs classified in the table, and also the results of tests and actual application of these highly efficient heat-transfer devices.
Section snippets
LHP theory
Theoretical analysis of different aspects of operation of loop heat pipes is presented in numerous publications [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Below are given some basic elements of this theory.
The operation of LHPs is based on the same physical processes as those used in conventional heat pipes. However, they are organized in quite a different way. First of all, it concerns the functions of the wick, which plays here a more complicated role. To determine these
Materials and working fluids for LHPs
The main structural material for making elements of the LHP body is stainless steel, which is amenable to different kinds of vacuum-hermetic welding and possesses a sufficiently high strength. The drawback of stainless steel is its relatively low thermal conductivity. More seldom use is made of more heat-conducting, but considerably less durable aluminium alloys. Copper is also quite a promising material for use in LHPs.
Sintered nickel and titanium powders are most widely distributed as wicks.
Large and powerful LHPs
The advantages of LHPs are best manifested at a large capacity and heat-transfer distance. This especially concerns the cases when it is necessary to ensure the efficient operation of the device at any orientation in the gravity field. The most suitable design for this purpose is that of an LHP with two compensation chambers situated at the evaporator butt-ends [27]. As an example Fig. 5a presents the scheme of a flexible ammonia LHP 2 m long made of stainless steel [28]. The evaporator 24 mm in
Thermoregulation systems of spacecraft
At present the main area of application of loop heat pipes is space technology. The first flight experiment in conditions of microgravitation was conducted in 1989 aboard the Russian spacecraft “Gorizont” [31]. The experimental module included an LHP with three parallel evaporators 24 mm in diameter and a condenser of the collector type joined to the panel of a radiator. The vapor and the liquid line 6 mm in diameter had a length of about 0.6 m. The evaporators were combined with a common plate
Conclusion
Loop heat pipes are highly efficient heat-transfer devices capable of transferring considerable heat flows for great distances at any orientation in 1-g and 0-g conditions. On their basis it is possible to create ramified, reversible, controllable systems for heat-transfer possessing mechanical flexibility and high adaptability to various operating conditions. A new generation of these devices––miniature heat pipes––can solve the problem of cooling of promising electronics and computer
Acknowledgments
The author is grateful to his collaborators Mr. Valery Dmitrin and Ms. Sofia Olemskaya for their great help in preparing this paper.
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