Recent developments of lightweight, high performance heat pipes

Applied Thermal Engineering 33-34 (2012) 1e14

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Recent developments of lightweight, high performance heat pipes X. Yang a, Y.Y. Yan a, c, *, D. Mullen b a

Energy & Suitability Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Thermacore Europe, Ashington, Northumberland NE63 8QW, UK c Key Laboratory of Bionic Engineering, Jilin University, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 January 2011 Accepted 6 September 2011 Available online 29 September 2011

Heat pipes, known as “super thermal conductors” have been widely used in many areas for more than 50 years. Currently, due to the various requirements put on cooling systems, such as lightweight, better heat transfer performance, and optimised appearance, heat pipes have been improved significantly in the past decades. This paper summarises the recent developments of lightweight, high performance heat pipes. Various methods or approaches to achieve the requirements of lightweight and high performance are introduced. The applications of lightweight materials can help reduce by up to 80% the weight of conventional copper heat pipes; however the lightweight material often has problems of corrosion. Although improving the design of wick structures and changing the size of conventional heat pipe assemblies can help to reduce weight and achieve high heat flux, there are still some limitations to the applications of lightweight materials such as magnesium due to its incompatibility with some working fluids. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Heat pipe Lightweight High performance

1. Introduction Heat pipes, which are well regarded as “super thermal conductors” and often the primary components of a heat transfer system, have been widely used in thermal devices and components for their efficient cooling and thermal management. The applications of heat pipes can be seen in many industrial areas such as the electrical and electronic, aerospace, telecommunications, food industries, etc. Over the past decades, much attention has been paid to the improvements of heat pipes including the appearance, design and optimisation, miniaturisation and weight reduction, and towards achieving higher heat flux. According to the report for NASA [1], reducing one pound of weight on a spacecraft can help save $10,000 US dollars in launch costs. Also, in terms of a telecommunication satellite, more than a hundred heat pipes are often required [2]. In addition, for current electronic device design, such as CPUs, graphic cards etc., it is necessary to minimise the size and accommodate much more heat generation than in previous products, so that current cooling devices must absorb more heat energy and be similarly more compact. Based on these requirements, “lightweight” and “high performance” become the key goals for

* Corresponding author. Energy & Suitability Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. Tel.: þ44 (0) 115 951 3168; fax: þ44 (0) 115 951 3159. E-mail address: [email protected] (Y.Y. Yan). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.09.006

current heat pipe design, especially for applications in the aerospace and electronic industries. In this paper, the materials and configurations that might be used in lightweight heat pipes to satisfy the demands of these applications are reviewed. 1.1. General heat pipe mechanisms A heat pipe is a high heat flux, passive heat transfer device which uses the evaporation, condensation, and surface tension of a working fluid to attain an extremely high thermal conductivity. Broadly speaking, in terms of a heat pipe, the vapour flow from the evaporator (hot side) to the condenser (cold side) is caused by the vapour pressure difference. Meanwhile the liquid flow from the condenser (cool side) to the evaporator (hot side) is produced by the forces, such as capillary force, gravitational force, electrostatic force, or other forces directly acting on it. Regardless of the orientation of the heat pipe (vertical or horizontal, for example), the basic principles are the same. Therefore, a simple horizontal heat pipe is taken as the example in order to explain the principle (see Fig. 1) [3]. In more detail, the heat pipe is in equilibrium with an isothermal environment. Moreover, the liquid in the wick and the vapour in the vapour space are at saturation. When heat is applied to the evaporator, the temperature raises, and the liquid in the wick evaporates. The vapour pressure over the hot liquid working fluid at

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X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14

According to literature [4,6], there are two primary ways to classify the heat pipes. These are based upon the working fluids’ operating temperatures, and the types of control. For each heat pipe application, there will be a temperature range for its particular operating conditions [7] (see Fig. 2). Therefore, choosing a suitable working fluid is necessary, which not only considers the operating temperature (along with the pressure condition), but also concerns the compatibility with heat pipe container and wick materials. Based on the operating temperature, the heat pipes can be classified by the following four different types (see Table 1):

Fig. 1. The structure of a heat pipe [3].

2. Development of lightweight and high performance heat pipes

the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe. The velocity of vibrating molecules in a gas is approximately the speed of sound (the sonic limit being one of the upper limits to heat pipe performance) and in the absence of non-condensing gases, this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end. The condensed working fluid then flows back to the hot end of the pipe [4]. Therefore, its two phase heat transfer mechanism results in heat transfer capabilities from “one hundred to several thousand times” that of an equivalent piece of copper. However, if non-condensing gas exists in the heat pipe, it impedes the gas flow, and reduces the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low [5].

With regard to lightweight heat pipes, the main applications are in electrical devices and the aerospace industry, which generally require room temperature heat pipes. Copper is generally the most popular material for heat pipe production. Since the 1970s, copper has been used widely to make both heat pipe containers and wick structures, due to its good thermal conductivity and compatibility with many working fluids. For instance, in the early stage, Fischer and Gammel [8] used copper to do research on early porous media, which was a sintered structure, with coarse and fine pores. Kessler and Hess [9] created a cylindrical mesh screen to improve the heat transfer performance. However, due to the compactness of electronic systems and the increase of heat fluxes, presently the limiting criteria in the microminiaturisation of electronic components and devices is determined by the maximum acceptable junction temperature [10]. To reduce the weight of cooling devices and improve the performance, generally, there are several primary methods: using lightweight metals, improving the wick structure and minimising the size of devices.

1.2. Classification of heat pipes

2.1. Lightweight materials used for heat pipes

Regarding the classifications of heat pipes, under the different conditions, they can be divided into different categories, which might depend on the geometries, applications, and so on.

2.1.1. For heat pipe container In the past decades, many of the improvements and investigations on terrestrial, as opposed to spacecraft, heat pipes were based

Fig. 2. Operating temperature range of common working fluids [7].

X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14 Table 1 Classifications of heat pipes by operating temperature [4,6]. Type

Temp. range

Specification

High temperature (liquid-metal)

>700 K

Medium temperature

550e700 K

Room temperature

200e550 K

Cryogenic (low temperature)

1e200 K

Using liquid metals, very high heat fluxes can be obtained due to the inherent properties of the fluid, namely, very large surface tensions and high latent heats of vaporization. Potassium, sodium, and silver are the examples of commonly used liquid metals. Some special organic fluids, such as naphthalene and biphenyl can be used for medium temperature applications. The working fluids typically used methanol, ethanol, ammonia, acetone, and water. With working fluids such as helium, argon, neon, nitrogen, and oxygen. Due to very low values of the latent heat of vaporisation, and low surface tensions of the working fluids, they usually have relatively low heat transfer capabilities.

on copper and copper alloys. Until now, copper is still the optimum choice for designing mid-temperature and room-temperature heat pipes, and copper/water heat pipes occupy the main market for these heat transfer devices. However, for conventional heat pipes, copper is no longer the best choice in some fields, where lightweight is required as a first priority. Under this condition, some researchers and engineers have chosen the direct way: using lightweight materials to replace the conventional material e copper. Therefore, many lightweight metals or their alloys have been involved in heat pipe research, such as aluminium and it alloys, beryllium-based alloys, epoxyimpregnated carbon fibre wound over thin aluminium shells, laminate materials, metal/matrix composites, titanium alloys and magnesium alloy [1]. 2.1.1.1. Investigations in the past decades. In previous investigations, aluminium alloys were chosen as desirable lightweight material to make the heat pipe envelops due to it being 25% the weight of copper. Although the thermal conductivities of aluminium alloys are about 50e60% of copper, they still present good heat transfer performance for room temperature and cryogenic heat pipes applications. More and more people realised the benefits of aluminium: high electric and heat conductivity, lightweight, excellent malleability, and suitable for continuous extrusion. Hata [11] made a comparison of lead and aluminium, and then found lead was too soft and poor in maximum tensile stress although it had high heat conductivity and good compatibility with many chemicals. Thus, aluminium seemed more suitable to be a heat pipe material based on their research. However, without any anti-corrosion protection, aluminium alloys could not be employed as a heat pipe container with water (gas and liquid) and methanol etc. Pointing at the corrosion problem, Hata [11] added a lead inner wall inside the aluminium pipe to prevent chemical reactions between the wick structure and working fluids. In addition, Baehrle et al. [12] claimed that adding a 10e15 mm nickel coating layer inside an aluminium heat pipe could help to solve the corrosion issue as well. Other researchers [13,14] preferred making aluminium alloy heat pipes without coating layers. Instead they changed the working fluids. For example, Freon 11, ammonia, acetone etc. were

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all mentioned in both early patents and papers [15e19]. However, considering the environmental issues, Freon was stopped being used as a working fluid for heat pipes; normally ammonia and acetone are common working fluids for aluminium heat pipes at present. However, other researchers [20] pointed out the problem of acetone: if acetone is used as a working fluid to manufacture an aluminium alloy heat pipes, then after 17,500 h life test, the aluminium/acetone heat pipe performance declined because of the self-reaction of acetone, which lead to the working fluid boiling temperature increasing to 160  C from 56  C. Meanwhile, the diacetone alcohol results from the aldol condensation of acetone, which may react to form water and was incompatible with aluminium alloy. 1 year

Acetone ƒ! Diacetone alcohol

(1a)

2(CH3eCOeCH3) / (CH3)2eC(OH)eCH2eCOeCH3

(1b)

Despite further investigation into aluminium heat pipes, a fact is unassailable that the commercial space industry uses a few thousand heat pipes annually in unmanned spacecraft, and similar devices are used for heat rejection from manned spacecraft. The standard material of construction for these devices is 6000 series aluminium alloy, with a density of 2700 kg/m3 and yield strength of about 2.413  108 Pa [1] Therefore, although aluminium alloys, are limited by working fluid compatibility and they need anticorrosion protection as well, aluminium has remained as the material for making heat pipes for spacecraft for nearly thirty years. Considering the limitation of working fluids for aluminium alloy heat pipes, many other candidates have been brought into the investigation for the heat pipe container materials. Based on the requirement of lightweight on aircraft, Wiacek and Mahtani [21] pointed out that using low expansion alloys to make heat pipes could achieve the lightweight requirement. Therefore, iron-nickel based alloys and beryllium-based alloys were promoted [22e24], such as the Beryl-castÔ alloys supplied by Starmet Corporation with a density of less than 2000 kg/m3, which reduced the weight a further 25% than aluminium alloys [1]. However, due to their toxicity, high cost, and questionable compatibility, beryllium-based alloys have not been applied widely. In the late 1980s, Rosenfeld and Zarembo [1] developed an epoxy-impregnated carbon fibre material wound over a thin aluminium liner. This composite proved to be of high strength and low mass and compatible with ammonia; however, the drawbacks include high fabrication cost and the fact that it cannot be bent, welded, or brazed. Hereafter, laminate material [25] (see Fig. 3) also became a candidate which has been developed by NASA. Regarding these materials, their advantages are flexibility, low-mass, and potentially low cost. In contrast, aluminium laminate sheet limits the application with ammonia due to its low specific strength and the requirement of high vapour pressure for ammonia [1,25]. In addition, one type of material called “matrix composite” has also found applications in spacecraft thermal control devices, especially when used to make a thermal control coating, such as the invention of Long [26] which is a ceramic matrix comprising a silica transformation product and a plurality of doped zinc oxide pigment particles. This kind of material offers high strength and lower mass,

Fig. 3. Laminate material: an invention by Ref. [25].

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X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14 Table 2 Overview of lightweight materials: magnesium, aluminium and titanium alloys [31e36].

Fig. 4. Wick structure of titanium/water heat pipe [28].

but typically cannot be bent, welded, extruded, thus narrowing the application of the “matrix heat pipe”. Therefore, reviewing the investigations of lightweight heat pipe materials over the past 40 years and considering the advantages and disadvantages, the fact is aluminium alloys are much more desirable than other candidates due to their low cost, ease of manufacture and good heat transfer performance, so that they can be widely applied into the aerospace area and the low temperature environment. 2.1.1.2. Investigations in recent years. Currently, investigations have continued into the development of titanium alloy and magnesium alloy heat pipes, and other products such as small titanium vapour chambers, titanium/water heat pipe (see Fig. 4) and magnesium/ ammonia heat pipes [27e29]. With regard to the advantages of titanium: lower mass and can take high temperature, titanium was selected as the container material of the heat pipes. Based on the requirements of space radiator and nuclear systems, the cooling devices should operate under the 400e550 K operating temperature range. A titanium vapour chamber manufactured by Thermacore can achieve an extremely small size with thickness of 1 mm and reduce the product weight significantly. Moreover, titanium powder can present very good performance when it is used for making porous media [27e29]. Maydanik [30] pointed out that sintered nickel and titanium wicks can obtain capillary structures with the small pore size, possess a high strength and are compatible with many lowtemperature working fluids. To achieve the 400e550 K operation temperature, the titanium/water heat pipes have been quite widely applied into the space radiator systems and space nuclear power system [27e29]. Due to the expensive costs for titanium alloy (see Table 2), it restricts the relevant applications of the titanium heat pipes or vapour chambers. Based on this situation, magnesium alloys seem to be more ideal for the current requirements. As shown in Table 2 [31e36], magnesium alloys are lightweight and low cost with good thermal and mechanical properties. It is well known that the pure magnesium has a very low density of 1700 kg/m3 which is only 20% the weight of pure copper and 63% the weight of pure aluminium, and has very similar properties to aluminium. Moreover, for its alloys, most of them have densities around 1800 kg/m3, and a range of thermal conductivity from 80 to 130 W/(m K) [37]. Therefore, magnesium alloys have been studied as a possible ideal material for heat pipes. However, due to the active chemical property of magnesium, it is more difficult for investigators to solve the corrosion problems and find any suitable working fluids compatible with the magnesium alloys. Magnesium alloys react with most of the liquids which contains hydroxyl, and generate hydrogen which is a non-condensable gas (see Fig. 5). The following equations (Eqns. 2aec) present the general process of the reaction.

Magnesium alloy

Aluminium alloy

Titanium a/b alloy

Density, kg/m3

1330e2400

16e3500

Tensile strength, Ultimate, MPa Tensile strength, Yield, MPa Elongation at break, % Modulus of elasticity, GPa

90.0e1070

0.7e1600

4420 e4840 825e1580

21.0e460 1.00e75.0 38.0e120

230e260 250e585

1.24e750 0.150e50.0 0.0480 e342 0.138 e3400 90.0e414 250e585

125e515

117e1090

Shear strength, MPa Specific heat capacity, J/kg  C Thermal conductivity, W/m K Melting point,  C

55.0e190 800e1450

0.138e420 690e1010

690e1550 1740e 2280 1350e 1790 550e760 368e670

44.3e159

148e255

6.10e10.9

330e650

204e1350

Solidus,  C

330e650

204e660

Liquidus,  C

585e650

543e674

Metal Price of raw material billet, $/lb

1.47

1.10

1300 e1680 1300 e1630 1640 e1680 12.36

Compressive yield strength, MPa Notched tensile strength, MPa Ultimate bearing strength, MPa Bearing yield strength, MPa

21.0e448

759e1410 4.00e18.0 105e123 860e1280

Anodic reaction: Mg / Mg2þ þ 2e

(2a)

Cathodic reaction:

2H2O þ 2e / 2H2þ2OH

(2b)

General reaction:

Mg2þ þ 2H2O / Mg(OH)2 þ H2[

(2c)

With regards to a basic heat pipe, the non-condensable gas will lead to the degradation of the heat pipe performance. Due to the

Fig. 5. Magnesium alloys with water.

X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14

Fig. 6. Contaminated magnesium alloy sample.

non-condensable gas blocking the condensation section, less vapour can be condensed back to the evaporation section. Therefore the working fluid filled within the heat pipe will easily dry out. Meanwhile, if the magnesium alloy is contaminated, the alloy is easily oxidised by the liquid with strong oxidation ability (see Fig. 6). Moreover, as for the primary heat pipe working fluids for room temperature applications (see Fig. 2), such as methanol, water, acetone, ammonia etc., many of them cannot avoid a chemical reaction with magnesium, especially for water which is the most desirable working fluids for normal heat pipe applications. So far, few papers or patents relevant to Magnesium heat pipes have been published. The rare documents that could be found are all currently provided by Thermacore Inc. According to a report by Rosenfeld and Zarembo [1], axial-grooved magnesium/water heat pipe and magnesium/ammonia heat pipe prototype (see Fig. 7) with sintered porous wick structure passed tests at 85  C and 40  C respectively; but it seemed there was a protective layer inside the heat pipe container, which was not indicated clearly in that report. With regard to the recent works by Yang and Yan et al. [38e40] on magnesium heat pipes, acetone as the working fluid is compatible with magnesium alloys, and methanol and water are incompatible.

Fig. 7. The assembled axial-grooved magnesium/ammonia heat pipe prototype [1].

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Rosenfeld and Eastman [41] published a patent of “Chemically compatible, lightweight heat pipe”. In this patent, a new type magnesium alloy was used to make a vessel/flat heat pipe (see Fig. 8) with water as the working fluid, which only contained 1% (by weight) zirconium in the magnesium alloy. According to the authors mentioned in the patent, zirconium oxide could be created inside the vessel and form a protective layer between the working fluid and the magnesium alloy, so that the incompatibility between magnesium alloy and water was solved. Moreover, this type of magnesium alloy was tested with ammonia as well and gave satisfactory results at room temperature. However, as for the alloys containing zirconium, they are much stronger than the alloys without zirconium, which makes it difficult to bend the heat pipes. For efficient bending heat must be applied to the magnesium alloy, and then the manufacturing process started, but there is a high risk of breaking the wick structure inside the pipe. In addition, regarding the storage of the magnesium alloy and the manufacturing process, there is another essential issue requiring attention: contamination (see Fig. 9). The effects of some elements in the magnesium have been reported [42,43]. Generally, these corrosive elements can be categorized into three levels in terms of their severity. The most critical four elements are Fe, Ni, Cu, and Co. Even with less than 0.2% concentration of Ni, Fe, Cu, and Co in magnesium, the increase in corrosion rate can be very profound. The existences of Ag, Ca, and Zn, with concentrations from 0.5% to 5% have a modest effect of accelerating the corrosion rate. Other elements such as Al, Sn, Cd, Mn, Si, and Na have very little or no effect at concentration up to 5% [42,44e49]. Therefore, with these manufacturing issues in mind, considerable thought needs to be used to improve and overcome the contamination which will cause oxidation. 2.1.1.3. Lightweight material and working fluids. For a heat pipe design, choosing suitable materials with good compatibility is necessary. It is well known that a heat pipe is a completely sealed device, so that any chemical reactions between the working fluid and the wall or wick material will bring about negative performance of the heat pipe. In contrast to most corrosion problems, the structural integrity of the tube wall is not the primary consideration. One of the factors that is critical to the performance of a heat pipe is the amount of non-condensable gas that is generated. The gas could result from materials out-gassing or chemical reactions. This gas collects in the condensing region and causes condenser blockage. Certain combinations of materials, such as ammonia and copper, are known to react quickly with one another, and hence are not likely to be chosen. Another example of this is the hydrolysis of water which occurs in aluminium/water heat pipes. The compatibility and stability of working fluids and heat pipe materials at the intended operating temperatures must be

Fig. 8. Magnesium alloy vessel [41].

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X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14

Fig. 9. Contamination tests: (a) Mg with Cu; (b) Mg with Fe; (c) Mg with Al.

established by testing. A widely used approach to compatibility testing is to employ the actual heat pipe hardware and monitor the rate of gas generated. As mentioned previously, non-condensable gas generated within a heat pipe collects at the end of the condenser, blocking vapour flow and causing a local temperature drop. According to the previous experiences [50], methanol works well with stainless steel but reacts with aluminium; water could work well with copper, but not with Inconel; some short-term success has been achieved with carbon steel, but carbon steel pipes using methanol appear to be generating hydrogen gas, which diffuses through the pipe wall; this observation indicates an internal reaction is taking place. Several such compatibility tests have been performed by many different researchers and laboratories. Based on the review above and current investigations [1,13e19,25,27e29,38e40,42,49,50], Table 3 summaries the compatibility of common working fluids and lightweight materials. 2.1.2. Lightweight material for wick structures based on conventional heat pipes Considering the excellent heat transfer performance of copper heat pipes and its good compatibility with common working fluids, many people prefer copper as the heat pipe material of choice, but some have tried to optimise the design of the heat pipe cooling assemblies in order to achieve the lighter weight as well. Heat pipe performance depends on the heat transport limitations of the heat pipe, which govern the performance of the heat pipes, and mainly include capillary limit, boiling limit, entrainment limit, viscous limit, sonic limit and so on (see Fig. 10) [6]. Table 3 Generalised results of experimental compatibility tests. Aluminium alloy

Silicon Nickel Inconel Titanium Magnesium alloy

Water Ammonia

I C

C

C C

Methanol Acetone R-134a C6F6 n-butane n-butane n-butane Benzene Sodium Potassium Caesium

I C C

C C

C

I

C

C

I I C

Fig. 10 indicates that the separate performance limits define an operational range represented by the region bounded by the combination of the individual limits. Moreover, this operational range defines the region or combination of temperatures and maximum transport capacities at which the heat pipe will function. In terms of capillary limit, it is the most important factor for heat pipe design and also plays a necessary role in the wettability of heat pipes. If there is insufficient driving capillary pressure providing an adequate liquid flow from the condenser to the evaporator, dry-out of the evaporator wick will occur. A critical heat flux exists and balances the capillary pressure with the pressure drop associated with the fluid and vapour circulation. For the capillary pressure [51], it definitely depends on pore radius of the wick and the surface tension of the working fluid.

DP ¼

2sl R

(3)

where DP is the capillary pressure drop, Pa, sl is the surface tension N/m, and R is radius of the wick, mm. Moreover, the capillary pressure must be greater than the sum of the gravitational losses, liquid flow losses through the wick (pressure drop from condenser to evaporator), and vapour flow losses (pressure drop from evaporator to condenser).

DPc;max  DPg þ DPl þ DPv

Under current requirements, many people carried out research on wick structures in attempt to get a higher capillary limit and then achieve a higher performance. Based on the common wick structure (groove, sintered and mesh (see Fig. 11 [52])) and the requirements of lightweight, some advanced wicks have been invented.

I Ca,b (100% concentration) I Ca,b

C C C C C C C

C ¼ Compatible; I ¼ incompatible. a Sensitive to Cleaning. b results are based on 3 years tests.

(4)

Fig. 10. Typical heat pipe performance map [6].

X. Yang et al. / Applied Thermal Engineering 33-34 (2012) 1e14

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Fig. 11. Common wick structure: (a) sintered; (b) mesh; (c) groove [52].

Unlike the conventional pure metal wicks, the fibre wick structure is the typical optimised wick structure, which is lightweight and arranged on an inner wall of the pipe envelop (see Fig. 12). Take an invention as the example [53], the fibre wick structure has at least two kind of fibres with different melting point. Sintering metal powders is an efficient way to make a better wick. For this invention, due to the melting point difference, the fibre with lower melting point will reach the sintering temperature first and then will be made the first wick structure adhered on the inner wall. After that the fibre with higher melting point will provide a support force for the whole of the fibre wick. This design uses lightweight material to achieve a better porous structure, and also ensures the wick structure can adhere on the heat pipe wall permanently, so that the heat pipe can have a higher performance for a longer time. Later the inventor improved the fibre wick which became more efficient. Woven mesh [54,55] structure adhered to the heat pipe internal wall first, and then the fibre wick added later. This kind of composite wick structures (see Fig. 13) draws the advantages from both sintered and mesh wicks, which significantly modifies the movement of liquid and vapour phases to and from the heat surface and a higher value of heat flux nucleation occurs. Moreover, this advanced wick aims to be used on embedded heat pipes assemblies and improves the heat transfer performance for smaller size heat pipes. However, as for the fine fibre bundle wicks, they have a good capillary pumping head, but have low permeability and high effective thermal resistance (low thermal conductivity) across the wick. Therefore, this kind of wick is normally combined with other types of wick. In terms of the grooved wicks, they have a large pore radius and high permeability, but its fluid pumping pressure is low.

Although Lin and Ponnappan [56] designed partially opened groove structures which can dissipate heat up to 140 W/cm2, sintered wick structures or mesh wicks are much more popular to make the combined lightweight wick in order to achieve a better performance. Moreover, with regard to the applications for cooling high heat flux electronics, when combined with sintered wicks they are much more efficient, and can handle 250 W/cm2. Certainly, the improvements on the wick structures can reduce some weight compared with conventional copper heat pipes; however, it cannot give the significant reductions of the total heat pipe weight. Additionally, the complex wicks also increase the difficulty for manufacture.

Fig. 12. A type of fibre wick structures [53].

Fig. 13. Combined wick [54].

2.2. Miniature high performance heat pipes Since 2000, with regards to electronic engineering applications, the main trend of heat pipe design has been focused on “high performance”, “lightweight” and “low cost”. On one side, the investigations of lightweight heat pipes are still continuing; on the other side, the people who prefer the conventional materials (such as copper), have made many improvements on the traditional copper heat pipes and their assemblies, especially in the electrical devices applications. 2.2.1. Flat heat pipes Flat heat pipes (FHPs) are the transmutation of the conventional heat pipes, which aims to apply the heat pipes into narrow or limited space. Due to its configuration flexibility and effective performance, FHPs are widely applied into electrical device cooling. Compared with 4 mm or ¼ inch small size round heat pipes, flat heat pipes can be easily fixed onto PCB cooling devices, because of the thickness shrinking. According to the current Thermacore heat pipe manufacturing techniques [57,58], the thickness of the FHP product can be made in the range from 1.6 mm to 3.6 mm depending on the wick structure type, which can reduce the space of cooling devices significantly.

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Fig. 14. Flat heat pipe with porous wick model [59].

More than ten years ago, Khrustalev and Faghri [59] designed a group of flat heat pipes (see Fig. 14). Among these samples, was one with dimensions of 2 mm  7 mm  120 mm which achieved a heat flux of 200 W/m2 in the horizontal orientation and 150 W/m2 in the vertical orientation based on an operating temperature of 120  C. In their studies, the wick structure was made of axial capillary grooves covered with a porous plate, and water was selected as the working fluid compatible for the copper container. In recent years, more and more research has been carried out relevant to FHPs. Due to the similar manufacturing process as conventional heat pipes, to improve the performance of the FHPs, there are several methods that have been used in the investigations. Tan and Ooi et al. [60,61] indicated the relationship between the pressure drop across the wick and the wick structure. According to their studies, the optimum performance of the FPH can be achieved when the minimum pressure drop is attained across the wick structure. Esarte and Domígues [62] carried out analysis of a flat heat pipe operating against gravity based on their 258 mm  176 mm prototype, which contains 60 mm copper powder plate as wicks. According to the results, they found that compared with a conventional aluminium heat dissipating plate, the COP of FHP was 51.4% higher. Qin and Liu [63] studied the liquid flow in the anisotropic wick of the FHP. They pointed out that by using an anisotropic wick, the maximum pressure difference in the wick will increase with the permeability decrease in the transverse direction. Meanwhile the capillary limit of the FHP dropped down as well. Rightley et al. [64] made the FHP model of 1.33 mm thickness and 20 mm width. The FHP prototype was made of anisotropic wick as well, which was named as “Laplace wick” (see Fig. 15) and significantly improved the heat transfer performance. Their testing results indicated that the conductivity value was over 760 W/(m K) due to the optimal operation conditions. Meanwhile

the dry-out point of the methanol FHP was measured as well, when the heat flux achieved 64 W/cm2. From Lin’s review [56], he mentioned that Plesch et al. made flat heat pipes with dimensions of 7 mm of width, 2 mm of thickness and 120 mm of length, and achieved maximum heat flux of 35 W/cm2, if the flat heat pipes with longitudinal grooves. Also, Plesch and co-workers created a water-based micro-heat pipe capable of dissipating 60 W/cm2 prior to dry-out using longitudinal grooves over the entire interior surface of the device [56]. As for the flat groove heat pipe, Lips et al. [65] used methanol as a working fluid to make a 70 mm  90 mm  3 mm FHP. With regard to the nucleate boiling experiment, they investigated the thermal behaviour of the FHP with 400 mm micro grooves and especially focused on the boiling section (see Fig. 16) From the experiments, they concluded that “the heat flux at onset of nucleate boiling depends on the filling ratio” and their FHP achieved heat flux ranges from 3 to 12 W/cm2 based on using gravity. Compared with conventional heat pipes, the investigations into FHPs need to continue in the future, as there is less understanding of their performance, such as the stability, power dissipation power limits. Although FHPs give the obvious advantage of small size, the current applications in the markets are still not prevalent. Therefore, for the current and the next heat pipe generation, the developments of FHPs will carry on. 2.2.2. Miniature vapour chamber Similar to the FHPs, the vapour chamber (VC) is one of the typical advanced types of heat pipe that are available and are very popular in electronics cooling applications. The main difference between the FHPs and VCs is seen as their fabrication method. FHPs can be stamped from originally round material, whilst vapour chambers are formed through a welding or brazing operation and originally consist of a separate evaporator and condenser where only the evaporator has a wick structure. Vapour chambers can have a number of different shell materials and working fluid combinations. The selection of these materials depends mainly on the operating temperature of the cooling system. The most common combination in the electronics cooling field is copper and water due to its operating temperature of about 10e250  C [5], but other liquids and materials can be used for extreme temperature ranges. To dissipate heat from electronic devices, such as chips, the size of VCs can be reduced significantly. Haddad et al. [66] and Boukhanouf et al. [67] made square VC samples of 40 mm  40 mm and the thickness of 3 mm (see Fig. 17). The performance of these miniature VCs was determined by a heat input range of 10e100 W with a constant condensation temperature. According to Haddad’s and Boukhanouf’s results, the VCs with mesh wicks appeared to

Fig. 15. (a) Flat heat pipe prototype; (b) Laplace wick structure taken by SEM [64].

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Fig. 16. Heat transfer behaviour of a methanol flat groove heat pipe in the evaporation section [65].

have a dry-out phenomenon at 70 W power input, but for sintered wick VCs there was no dry-out even under 80 W power input. 2.2.3. Miniature loop heat pipe Based on the current requirements of high efficiency and miniature size, some optimised loop heat pipes have been produced as well. Yu.F. Maydanik et al. [30,68,69] have carried out many investigations on miniature loop heat pipes (mLHP) (see Fig. 18). The advanced design of the evaporator avoids the negative effect of a thin wick, which is difficult to create the temperature and pressure drop. Maydanik’s [68,69] experiments were based upon the mLHP prototypes with ammonia and water as working fluids respectively. Both of these mLHPs have an effective length of 300 mm and condensation fin stack length of 62 mm. As for the ammonia mLHP, the evaporator contained a titanium wick

Fig. 18. (a) Evaporator of mLHP; (b) the sketch of the whole mLHP [30].

Fig. 19. Embedded heat pipe assembly: (a) condensation section; (b) evaporation section [22].

Fig. 17. A test assembly of a miniature vapour chamber [66].

Fig. 20. Embedded heat pipes into the heatsink [71].

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2.5 mm diameter lines, which can take a maximum capacity of 130 W under the same operation conditions. In Chen et al.’s works [70], they also mentioned the minimum thermal resistance of those mLHPs: 0.12 K/W for methanol mLHP and 0.1 K/W for water mLHP.

Fig. 21. Advanced heat pipe assembly with different fin shape [73].

Fig. 22. Miniature heat pipe applications in electronics cooling [75].

structure with a diameter of 5 mm and lines for vapoureliquid with a diameter of 2 mm, which can take a maximum capacity of 95 W when the temperature of the evaporation wall reached 93  C. As for the water mLHP, it consisted of a 6 mm diameter evaporator and

2.2.4. Typical miniature heat pipe assemblies Besides improving and minimising the heat pipes, miniature heat pipe assemblies are also a kind of lightweight cooling device, which contain a miniature heat pipe and a small, but efficient fin stack. A popular heat pipe assembly method is to embed heat pipes into a metal block (normally aluminium or copper alloys) (see Fig. 19) [21]. The evaporation section of heat pipes is embedded into a metal plate/block, and the condensation section connects with a cooling part, such as a fin stack or cooling material. Due to the metal block fully touching the heat source, it increases the heat dissipation surface area, so that the assembly enhances the heat transfer performance of heat pipes. Compared with the early stage design, the main advantage of embedded heat pipe assemblies is increasing the surface area of the heat transfer media, and then dissipating much more heat from the heat sources. To achieve the reliability and low cost of cooling devices, some inventions put heat pipes within the heatsink base [71,72], in order to improve the reliability and thermal performance (see Fig. 20). Meanwhile, this kind of advanced embedded heat pipe technology also helps to reduce the cost due to heatsink size optimism. Due to the heat pipes fully embedded into the heatsink base, the heat from heat sources can be efficiently transferred by heat pipes and then dissipated from the heatsink via the fins. Owing to the high efficiency and miniature size, these kinds of devices are widely used for cooling CPU or smaller electronic components now. More advanced products have been invented which focus on a fin stack design, and also aim to improve the performance and reduce the weight of cooling devices. One unit [73] is designed for the miniaturised electronic equipment to dissipate heat at high temperature. The middle part of the heat pipe is the evaporation section, and the two ends become the condensation section (see Fig. 21). Due to the performance of the condensation section being improved more than a conventional design (such as Fig. 19), it

Fig. 23. Sketch of miniature heat pipe application for small electronics cooling [77].

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Fig. 26. Structure of axially grooved heat pipes [1].

Fig. 24. Miniature vapour chamber e heatsink prototype for CPU cooling [78].

actually accelerates the phase change in the heat pipes so that more heat can be taken away from the heat source. In addition, vertical orientation of the heat pipe allows operation with gravity and also achieves the best performance under the vertical orientation. Meanwhile, different designs of fin stack also help to create better turbulence of the air flow between the fins, and this in turn reduces the fin pressure drop. The benefits of this kind of design are lightweight, low cost and high performance, especially for higher temperature applications. All of these assemblies are targeting to reduce the size of cooling devices, in other words, lightweight heat pipes become the necessary requirement at present. Meanwhile much attention has been paid to research on how to minimise the weight of heat pipes and also maintain the better performance. 3. Applications of lightweight heat pipes An overview of the current heat pipe market suggests that most of the lightweight heat pipes or miniature heat pipes are applied into the fields below: Electronic device cooling and aerospace applications. 3.1. Electronic device coolers The efficient, reliable cooling of today’s semiconductors, with their steadily increasing heat loads and decreasing electronic component size, requires innovative and compact thermal solutions that go beyond traditional electronic heat sinks, optimise the heat dissipating devices size and enable products to perform at higher levels.

Based on current requirements, in the past ten years, miniature heat pipes are used in 80% of electronic devices [74], such as cooling telecom boots, personal computers (PCs), and laptops (see Fig. 22 [75]). Also these heat pipes have been appreciated by thermal designers for their small size and effective cooling capacity. For example, a heat pipe with a diameter of 4 mm was applied in a small electronic components cooling device [76,77], which is the typical miniature heat pipe in the market (see Fig. 23 [77]). As for this application, a sintered powder wick is the optimum choice, because of the high fluid pumping pressure, low effective thermal resistance. As for the vapour chamber, its applications can be an efficient way to manage heat in today’s small, yet high-power electronic devices where effective cooling helps ensure long component life and reliability. Fig. 24 shows a “miniature vapour chamber (mVC) e heatsink” assembly [78], which is designed for dissipating 130 W heat from a CPU and increase in performance by 21% compared with conventional heatsink design. The dimensions of this mVC are 40 mm  40 mm  4 mm. Although the main application of loop heat pipes is in the space industry, due to their ability to overcome gravitational effects and their high cost, there are still a few of applications for cooling PCs. Chen, Pastukhov, Vershinin and Maydanik [30,68e70] used miniature loop heat pipes for cooling PCs with a Athlon XP processor, which at a maximum loading dissipates about 70 W (see Fig. 25 [70]). The main advantage is seen in the superior power handling ability of the loop heat pipe in comparison to normal heat pipes. Therefore, they are capable of transporting the higher power provided by modern CPUs within the same space envelop. 3.2. Aerospace As we known, heat pipes have been widely used in aerospace since 1970s [2], which mainly have two functions: (1) used for satellite thermal control as well as isothermalisation of the effects of solar radiation from the sun around the structure; (2) used to overcome the effects of distortion through overheating and extend the lifetime of laser mirrors [79]. In recent years, thermal control and heat rejection for aerospace and avionics is more challenging than ever, due to advances in

Fig. 25. Miniature loop heat pipe application for cooling CPU [70].

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Fig. 27. U-shape axially grooved heat pipe for microsatellite application [80].

4. Conclusions

Fig. 28. Thermal control system scheme of the spacecraft “Obzor” [30].

space electronics (miniaturisation, complexity and integration). Based on this situation, axially grooved low temperature heat pipes and loop heat pipes (LHPs) provide efficient and lightweight thermal management solution. As for satellite applications, axially grooved heat pipes (AGHP) are currently used as passive exchangers, which are mainly made of aluminium alloys and filled with ammonia [2] as working fluid. Fig. 26 presents the structure of the aluminium/ammonia axially grooved heat pipe, which can operate from 40  C to 80  C [1,76] and achieve lightweight, high thermal conductivity, uniform temperature, long life, etc. In the past few years, Baturkin et al. [80] improved conventional AGHPs to form a U-shape and applied them to microsatellites Magion 4, Magion 5 and BIRD for elaboration and flight exploitation of thermocontrol systems (see Fig. 27). Both Ushape heat pipes were filled with acetone or ammonia and, and they could operate within the temperature range of 50  C to 60  C based on specific requirements. In addition, LHPs have actually been applied in space applications since 1994, one being incorporated in the Russian satellite “Obzor” (see Fig. 28) [30]. At present LHPs are widely used in aerospace, such as on the Russian spacecraft “Mars-96”, American satellites Hughes-702, American spacecraft ICESar [30].

The manufacture of lightweight heat pipes is a very important objective for current heat pipe industries and researchers, and further investigations are needed. The review above indicated that the major developments of lightweight heat pipes and the primary methods of achieving the lightweight and high performance can be summarised as follow: The easiest way to reduce weight is to use lightweight materials to manufacture the heat pipes, which can reduce the weight by up to 80% compared to copper heat pipes. However, most of these materials come with corrosion issues. To use water (the most desirable working fluids for normal heat pipes) as a working fluid, aluminium alloys, titanium alloys and magnesium alloys must have additional protection incorporated during the production process in order to avoid non-condensable gas generation. For other compatible working fluids, such as ammonia and acetone, their applications are limited for room temperature environment due to their low operating temperature. An alternative direct method to reduce weight is to improve the wick structure of the heat pipes. Using light material (fibre) to make a mixed wick structure, such as mixed mesh and sintering, can help to increase the heat flux in boiling process and also reduce the pressure drop inside the heat pipe which leads to a much lower temperature change in the adiabatic section. By using this method, the performance of the heat pipe is definitely enhanced; however it doesn’t give much of a visible weight reduction. Due to the low permeability and high effective thermal resistance of fibre wicks, they must be combined with other wicks to make prototypes. Besides using the lightweight materials, minimising the size of standard heat pipe cooling devices is an alternative way. To date, different types of miniature heat pipes are applied into different markets, such as flat heat pipe, vapour chamber, loop heat pipe and relevant miniature assemblies. The main improvement for those heat pipes is focused on optimal wick structures. As for the challenging investigations above: developing new lightweight materials for heat pipes still require a long term effort to prove their performance. Although many lightweight materials have been investigated, such as aluminium and its alloys, beryllium-based alloys, epoxy-impregnated carbon fibre wound over thin aluminium shells, laminate materials, metal/matrix composites, titanium alloys and magnesium alloy, only one or two materials were applied to the heat pipe envelop based on the life tests.

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Therefore, with regards to future research on heat pipes, lightweight, good heat transfer performance and low costs should be the main requirements. As the key component in many heat transfer devices, heat pipes, especially lightweight heat pipes will be increasingly needed by the electronic device industry, telecoms and aerospace areas. Nomenclature

DP DPc, max DPg DPl DPv R

pressure drop, kPa maximum capillary head, kPa pressure drop due to gravity, kPa pressure drop in liquid, kPa pressure drop in vapour, kPa radius of curvature of liquid surface, mm

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