pp

a

f wndeffes ki. Thtedforctiv

Since the 1990s, researchers began to apply nano-material tech-e achieved many meaningfult. In 19ich isded inare pu, TiO2,ifferenarboncol anropriat

ticle than liquid, the liquid layer at the interface would reasonablyhave a higher thermal conductivity than the bulk liquid), Brownianmotion, ballistic transport of energy carriers (ballistic phonontransport through the nanoparticles, heat is carried by phonons,i.e., by propagating lattice vibrations), and thermophoresis (nano-particles can diffuse under the effect of a temperature gradient)[24].

Keblinski et al. [5] made an interesting review to discuss the ther-mophysical properties of nanouids and future challenges. Wang

As a new kind of heat transfer working uid, the nanouid is anew technology attempt to use the special properties of this func-tional uid to enhance the phase-change heat transfer in heatpipes, and will have wide application prospect. The fundamentalstudies of nanouids applied in heat pipes are still in its initialstage, most of which are experimental study and some experimen-tal results cannot be unied yet. The research on application ofnanouids in heat pipes was rstly published in 2003 [10]. Over30 relevant articles have been published since then as shown inTable 1 and Figs. 13, involving miniature micro-grooved heat

Corresponding author. Tel.: +86 21 34206568.

International Journal of Heat and Mass Transfer 55 (2012) 67866797

Contents lists available at

H

.eE-mail address: liuzhenh@sjtu.edu.cn (Z.-H. Liu).nanouids will exhibit high thermal conductivity and stabilityand are increasingly being used in many heat transfer applicationsin industrial elds. In recent years, the studies on nanouidsmainly focused on its thermal conductivity, and on forced convec-tion and boiling heat transfer mechanisms. Various mechanisms ofthe heat transfer enhancement have been proposed including theinterface effect (liquid layering around the nanoparticle makesthe atomic structure of the liquid layer more ordered than thatof bulk liquid, due to higher thermal conductivity of the nanopar-

Heat pipes are high-efcient heat transfer devices and havebeen widely applied in various thermal systems. Since heat pipesutilize the phase change of the working uid to transport the heat,the selection of working uid is of essential importance to promotethe thermal performance of heat pipes. Owing to the heat transferenhancement effect of nanouids in the single phase and phase-change heat transfer, some researchers have applied various nano-uids in heat pipes as the working uids to enhance their heattransfer performance.nology to heat transfer eld and havresults on heat transfer enhancemenposed the concept of nanouid, whof nanometer-sized particles suspenexamples of applied nanoparticlesFe), metal oxides (CuO, SiO2, Al2O3(SiC, TiC), Nitrides (AlN, SiN) and dmond, graphite, single/multi wall cliquids, such as water, ethylene glyexamples of base uids. Under app0017-9310/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.0695, Choi [1] rstly pro-a uid with some kindsto a base liquid. Somere metals (Au, Ag, Cu,ZnO, Fe3O4), Carbidest types of carbon (dia-nanotubes). Traditionald engine oil are somee operating conditions,

and Maunder [6] summarized the recent researches on ow andheat transfer characteristics of nanouids in forced and free con-vective ows. Weerapun and Somchai [7] summarized the pub-lished experimental and numerical investigations of forcedconvective heat transfer of nanouids. Bahrami et al. [8] providedan overview on the effective thermal conductivity of nanouids.Up to 200 literatures have been published so far [9]. Recently,the fundamental study of nanouids in heat pipes has been signif-icantly developed [1047].the latest reports of nanouids on various fundamental studies.A new frontier of nanouid research A

Zhen-Hua Liu , Yuan-Yang LiSchool of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, Chin

a r t i c l e i n f o

Article history:Received 1 April 2011Received in revised form 24 October 2011Accepted 27 October 2011Available online 21 July 2012

Keywords:NanouidHeat pipeHeat transfer enhancement

a b s t r a c t

Nanouid is a new kind opipes. This paper reviews auids in recent years. Themal performance in varioutions have been discussednanouids in the investigaof the total heat resistanceand also presents a perspe

1. Introduction

International Journal of

journal homepage: wwwll rights reserved..086lication of nanouids in heat pipes

orking uid with special properties to enhance the heat transfer of heatsummarizes the research done on heat pipes using nanouids as workingct of characteristics and mass concentrations of nanoparticles on the ther-nds of heat pipes with different base uids under various operating condi-e mechanism of enhancement or degradation of heat transfer utilizingheat pipes has been explained. The paper discusses the relative reductionvarious heat pipes with nanouids in comparison with the existing onese on possible future research applications.

2012 Elsevier Ltd. All rights reserved.

Also, some researchers contribute their efforts on summarizing

SciVerse ScienceDirect

eat and Mass Transfer

l sevier .com/locate / i jhmt

pipe, mesh wick heat pipe, sintered metal wick heat pipe, oscillat-

Heating heat pipe (OHP), closed two-phase thermosyphon. The appliednano-materials included metal, metal oxides, diamond, carbonnanotubes and several other materials. The current studies aremainly experimental, theoretical studies were very few [1113].The type, size of heat pipes and operating conditions of heat pipes,the kind of the base uids, the material and size of nanoparticles allvaried in very wide ranges among these experiments. Therefore, itis difcult to quantitatively make the comparison among differentexperimental data and then the most exiting research conclusionsare qualitative. This paper summarizes and reviews the latest re-searches of heat pipes using nanouids as working uids in recentyears. It also discusses the mechanism of heat transfer enhance-ment or degradation, the existing problems for various heat pipesutilizing nanouids, and explores the possible applicationprospects.Nomenclature

Cp,0 heat capacity of water, (kJ/kg/K)h heat transfer coefcient, (W/m2/K)I current, (A)m0 mass ow rate of water, (kg/s)P maximum heat removal capacity, (W)Q input power, (W)q heat ux, (W/m2)R total heat resistance of heat pipe, R = (Te Tc)/Q, (K/W)T temperature, (K)V voltage, (V)h nanoparticle volume fraction, (%)w nanoparticle mass concentration, (wt%)U lling ratio (for OHP and FHP, u = liquid volume/total

void volume; for other types of heat pipe, u = liquid vol-ume/the evaporator section void volume), (%)

Subscripts0 watern nanouids

Z.-H. Liu, Y.-Y. Li / International Journal of2. Fundamental studies of nanouids applied in heat pipes

2.1. Micro-grooved heat pipe

Chien et al. [10] rstly carried out an experimental study on theapplication of nanouids in FHP. They studied a disk-shaped alu-minum miniature micro-grooved heat pipe. The diameter and thethickness were 9 mm and 2 mm, respectively. A total number of18 micro-grooves were evenly distributed on the aluminum baseto provide the capillary force. The depth and the width of rectangu-lar micro-grooves were 0.4 mm and 0.35 mm, respectively. Thenanouid consisted of gold nanoparticles with a diameter of17 nm and DI water. The experimental data of the nanouids werecompared with those of DI water including the wall temperaturesand the total heat resistances of the heat pipe. Experimental resultsshowed that the total heat resistance of the heat pipe using nano-uids was less than that of the heat pipe using DI water at differentlling ratios as shown in Fig. 4. The use of the nanouids made theheat resistance reduce by an average of 40%. A laser diode with anemission power of 0.35 W was used as the applied heat source inthe measurement. The laser beamwas focused on the center of alu-minum base where was painted with black paint of 0.95absorptivity.

Wei and co-workers [14] used a cylindrical micro-grooved heatpipe with the inner diameter and the length of 6 mm and 200 mm,respectively. The width and the depth of the rectangular groovewere 211 lm and 217 lm, respectively. The working uid con-sisted of silver nanoparticles with an average particle size of10 nm and pure water. They mainly measured the total heat resis-tance of the heat pipe lled with pure water and nanouids at thesame lling volume of 0.51 mL (u = 10%). Nanoparticle volumefractions of 1 ppm to 100 ppm were used in the tests. The totalheat resistance of the heat pipe using nanouids could decreaseby 28%44% compared with that of the heat pipe using water.Researchers did not explain the mechanism of the heat transferenhancement.

Kang et al. [15] also carried out experiments with the same heatpipe [14] using nanouids consisting of silver nanoparticles andpure water. The silver nanoparticle sizes were 10 nm and 35 nm,respectively. The experimental results showed that the total heatresistance of the heat pipe using nanouids at the same lling vol-ume of 0.51 mL (u = 10%) decreased by 1080% comparing with

e evaporator section of heat pipec condenser section of heat pipep pressurev vaporw wallav averagein inputout outputAbbreviationCNT carbon nanotubesCPL capillary pumped loopDI di-ionizedFHP at heat pipeLHP loop heat pipeOHP oscillating heat pipe

and Mass Transfer 55 (2012) 67866797 6787that using water in the heating power range of 3060 W. The totalheat resistance decreased with the increase in both the nanoparti-cle concentration and the nanoparticle size. Fig. 5 shows that thetotal heat resistance of the heat pipe using nanouids can decreaseby 50% for 10 nm nanoparticles and 80% for 35 nm nanoparticlesrespectively comparing with that of the heat pipe using water.They considered that the improvement of thermal performance ismainly due to the reduction of uid temperature gradient innanouids.

Liu and Lu [16] and Yang et al. [17] carried out some steady heattransfer experiments under several constant operating tempera-tures to investigate the heat transfer performance of a cylindricalmicro-grooved heat pipe. Water-based CuO nanouids andwater-based carbon nanotubes (CNT) without dispersants wereused as the working uids. The length and the inner diameter ofthe heat pipe were 350 mm and 8 mm, respectively. Sixty rectan-gular grooves with the depth of 0.2 mm and the width of0.25 mm were uniformly fabricated on the inner wall of the heatpipe. The experiments were carried out at three xed operatingpressures of 7.45 kPa, 12.38 kPa and 19.97 kPa, with respectivelycorresponding operating temperatures of 40 oC, 50 oC and 60 oC.Data of evaporation and condensation heat transfer were investi-gated and the impacts of the nanoparticle mass concentration,the operating temperature on the heat transfer characteristicswas discussed. Fig. 6 indicates the effect of the nanoparticle massconcentration on the total heat resistance of the heat pipe using

Table 1Summary of researches of heat pipes using nanouids.

Type and shape of heat pipe Researcher Working liquid type (nanoparticle size and optimalconcentration)

Effect

Miniature micro-grooved heatpipe

Disk-shaped Chien et al. [10] (2003) Au-water (17 nm) +Cylindrical Wei et al. [14] (2005) Ag-water (10 nm, 0.01wt%) +Cylindrical Kang et al. [15] (2006) Ag-water (35 nm, 0.01wt%) +Cylindrical Yang et al. [17] (2008) CuO-water (50 nm, 1.0wt%) +Cylindrical Liu and Lu [16] (2009) CNT-water (diameter: 15 nm, length: 515 lm, 2.0wt%) +Flat Do KH and Jang SP [13] (2010) Al2O3-water (38.4 nm, 0.8wt%) +Cylindrical Shafahi et al. [11] (2010) CuOwater, Al2O3water, TiO2water +Disk-shaped Shafahi et al. [12] (2010) CuOwater,Al2O3water, TiO2water +Cylindrical Liu et al. [18] (2010) CuOwater (50 nm, 1.0wt%) +Cylindrical Wang et al. [19] (2010) CuOwater (50 nm, 1.0wt%) +

Mesh wick heat pipe Cylindrical Tsai et al. [20] (2004) Auwater (21 nm) +Cylindrical Liu et al. [21] (2008) CuOwater (50 nm, 1.0wt%) +Flat Chen et al. [22] (2008) Agwater (35 nm, 0.01wt%) +Cylindrical Do et al. [23] (2010) Al2O3water (30 nm, 2.4wt%) +Cylindrical Liu et al. [24] (2011) CuOwater (50 nm, 1.0wt%) +

Sintered metal wick heat pipe Loop heat pipe Riehl. [26] (2006) Niwater (40 nm, 3.5wt%) -Cylindrical Kang et al. [25] (2009) Agwater (10 nm, 0.01wt%) +

Oscillating heat pipe Closed loop OHP Ma et al. [27] (2006) Diamondwater (20 nm and 40 nm, 2.2wt%) +Closed loop OHP Ma et al. [28] (2006) Diamondwater (20 nm and 40 nm, 2.2wt%) +Closed loop OHP Shang et al. [29] (2007) Cuwater (25 nm, 0.45wt%) +Closed loop OHP Lin et al. [30] (2008) Agwater (20 nm, 0.1wt%) +Closed loop OHP Park and Ma. [31] (2007) CuNiwater (40150 nm, 8.8wt%) +Closed loop OHP Qu et al. [32] (2010) Al2O3water (56 nm, 0.9wt%) +Closed loop OHP Bhuwakietkumjohn and Rittidech [33]

(2010)Agethanol +

Flat-plate closed loopOHP

Cheng et al. [34] (2010) Diamondacetone (2 nm, 0.33wt%), +

Auwater (3 nm, 0.006wt%), +Diamondwater (2 nm, 2.6wt%) -

Closed two-phasethermosyphon

Cylindrical Peng et al. [35] (2004) Alwater (30 nm, 7.8wt%) +Cylindrical Xue et al. [36] (2006) CNTwater (15 nm, 2.2wt%) -Cylindrical Liu et al. [38] (2007) CuOwater (30 nm, 1.0wt%) +Flat Liu et al. [37] (2007) CuOwater (1550 nm, 1.0wt%) +Cylindrical Khandekar et al. [40] (2008) Al2O3water (4047 nm, 1.0wt%), CuOwater

(8.613 nm, 1.0wt%),-

Laponite clay water (25 nm, 1.0wt%)Cylindrical Naphon et al. [41] (2008) Tialcohol (21 nm, 0.57wt%) +Cylindrical Naphon et al. [42] (2008) Tirefrigerant R11 (21 nm, 0.31wt%) +Cylindrical Noie et al. [43] (2009) Al2O3water (20 nm, 3.0wt%) +Cylindrical Liu et al. [39] (2010) CNTwater (15 nm, 2.0wt%) +Cylindrical Parametthanuwat et al. [44] (2010) Agwater (

CuO

-wat

er

Ag-

wat

er

Al2

O3-

wat

er

CNT-

wat

er

Au-

wat

er

Dia

mon

d-w

ater

Al-w

ater

Iron

oxi

de-w

ater

TiO

2-w

ater

Cu-w

ater

Ni-w

ater

CuN

i-wat

er

mag

netic

nan

opar

ticle

s-w

ater

Lapo

nite

cla

y -w

ater

Ti- a

lcoh

ol

Ti-re

frige

rant

R11

Ag-

etha

nol

Dia

mon

d-ac

eton

e

0123456789

10

Pape

r unm

ber

type of nanofluids

Fig. 3. Number of papers/ type of nanouids.

0 10 20 30 40 50 60 700

1

2

3

4

5

6

7

8

R (K

/W)

average heat resistances of DI water average heat resistances of Nanofluid

Fig. 4. Comparison of heat resistances between DI water and nanouid for differentlling ratios [10].

0 10 20 30 40 50 60 700.000

0.001

0.002

0.003

0.004

0.005

0.006

R (K

/W)

10ppm Pure Water 10nm 35nm

Fig. 5. Experimental data of heat resistances using both nanouid and water atdifferent heating powers [15].

Z.-H. Liu, Y.-Y. Li / International Journal of HeatCuO nanouids. It is shown in [1619] that there existed an opti-mal CuO nanoparticle mass concentration of 1.0 wt% and an opti-mal inclination angle of 75. In addition, the heat transferenhancement decreased with increasing operating temperature.And substituting nanouids for deionized water as the working li-quid can reduce the startup time of heat pipe. For both water andthe nanouid, the maximum heat removal capacity in the unsteadystartup procedure is much lower than that in the steady operatingprocedure. The mechanisms of the heat transfer enhancementwere thought to be the increase of the heat conductivity of thenanouids, the disturbance effect of nanoparticles in the base li-quid, as well as the reduction of solidliquid contact angle in theliquid lm. As reported, an extremely thin porous layer made ofnanoparticles formed on the wall surface. Liquid on this porouslayer could be more sufciently supplied. The existence of the por-ous layer could also reduce the solidliquid contact angle, whichled to the increase of the capillary force and the enhancement ofthe pumping effect and hence increased the maximum heat ux.

Shafahi et al. [11] modied a two-dimensional model to simu-late the thermal performance of a cylindrically grooved heat pipeutilizing nanouids. The mathematical model adopted in this work

0 20 40 60 80 100 1200.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Q (W)

R (K

/W)

Tv=40C water 0.5 wt% 1.0 wt% 2.0 wt%

Fig. 6. Effect of mass concentration of nanoparticles on the heat resistance [17].

and Mass Transfer 55 (2012) 67866797 6789was based on the following assumptions: the process was steadystate; radiative and gravitational effects were negligible and theuid was considered Newtonian and incompressible. Moreover,the injection and suction velocities at the liquid-vapor interfacewere considered to be uniform. Also, the wick was assumed tobe isotropic and saturated with the working uid. The liquid owwithin the porous wick was modeled using the generalizedmomentum equation. The analysis incorporated the presence ofnanouid within the heat pipe. Three of the most common nano-particles, namely Al2O3, CuO, and TiO2 were applied. The simula-tion found that the nanoparticles within the base liquid enhancethe thermal performance of the heat pipe by reducing the heatresistance while enhancing the maximum heat load. In theory,there exists an optimum nanoparticle mass concentration corre-sponding to the maximum heat transfer enhancement. In addition,smaller particles have a more pronounced effect on the tempera-ture gradient along the heat pipe. They also utilized some analyti-cal models to calculate the thermal performance of at-shapedheat pipes using nanouids. The model assumed that vapor and li-quid ow are steady and laminar and transport properties of thevapor and liquid are considered to be constant; the vapor injectionand suction rates are considered to be uniform in the evaporatorand condenser, respectively [12]. Operating temperature, liquidvelocity prole, the wall temperature distribution of the heat pipe,

heat resistance and maximum heat load were investigated for theheat pipes utilizing both the base liquid and nanouids. The calcu-lated results of at-shaped heat pipes were the same as the calcu-lated results of cylindrically grooved heat pipes [11].

2.2. Mesh wick heat pipe

Tsai et al. [20] performed an experiment concerning a cylindri-cal mesh wick heat pipe. The working uid was an aqueous solu-tion of various-sized gold nanoparticles. The inner diameter andthe length of the test copper tube were 6 mm and 170 mm, respec-tively. A 200 mesh screen was distributed on the inner wall. Thenumber of mesh layers was unknown. The experimental resultsshowed that the total heat resistance of the heat pipe reduced20%37% due to the addition of nanoparticles. Fig. 7 shows the to-tal resistance of the heat pipe for nanouids of various particlesizes. The mechanism of the heat transfer enhancement was ex-plained by the authors as follows: a major heat resistance of heatpipe was caused by the formation of vapor bubbles at the liquidsolid interface; the suspended nanoparticles tended to bombardthe vapor bubbles during the bubble formation; therefore, it wasassumed that the release diameter of vapor bubbles was much

200 mm, respectively. The size and the number of mesh layerswere unknown. As shown in Fig. 9, the total heat resistance ofthe heat pipe using nanouids is reduced compared with that ofthe heat pipe using pure water. In the volume concentration rangetested, the larger the volume concentration of nanoparticles, thehigher the reduction of the heat resistance will be. The authors as-sumed that the mechanisms of heat transfer enhancement were:(1) the increase of the wettability increases the critical heat ux;(2) the increase of the liquid thermal conductivity and the wickconductivity enhance the heat transfer.

Do et al. [23] and Liu et al. [24] experimentally observed thethin porous coating layer formed by nanoparticles suspended innanouids at wick structures. Based on the observation, it is shownthat the primary mechanism on the enhancement of the thermalperformance for the heat pipe is the coating layer formed by nano-particles at the evaporator section because the layer can not onlyextend the evaporation surface with high heat transfer perfor-mance but also improve the surface wettability and capillary wick-ing performance.

2.3. Sintered metal wick heat pipe

10

20

30

40

50

60

70

80

90

100

Te-T

c (K

)

DI water Rav=0.27 E Rav=0.215 D Rav=0.206 C Rav=0.2 A Rav=0.17

0.6

6790 Z.-H. Liu, Y.-Y. Li / International Journal of Heat0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

Q (W)0 5 10 15 20 250

Q (W)

Fig. 7. Measured value of heat resistance of a mesh wick heat pipe using nanouidsof various particle sizes (Rav = average heat resistance of heat pipe) [20].

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2Tv=60C

Water =0.5% CuO =1.0% CuO =2.0% CuO

R ( K

/W)Fig. 8. Effect of mass concentration of particles on the total heat resistance for amesh wick heat pipe using CuO nanouids [21].smaller for the uid with suspended nanoparticles than that with-out them.

Liu and Shu [21] investigated the heat transfer characteristics ofa cylindrical mesh wick heat pipe using CuOwater nanouids. Theinner diameter and the length of the test tube were 10 mm and350 mm, respectively. Two layers of 160 mesh screen were distrib-uted on the inner wall. It was found that the nanoparticle massconcentration had signicant impact on both the heat pipe evapo-ration and condensation heat transfer. There was an optimal massconcentration of 1.0% under a variety of operating temperature. Anenhancement of the evaporation and condensation heat transferand the maximum heat ux was obtained at lower operatingtemperature. Fig. 8 shows that the total heat resistance of theheat pipe using nanouids is signicantly smaller than withoutnanoparticles.

Chen et al. [22] studied the performance of at mesh wick heatpipe using water-based silver nanouids with different nanoparti-cle concentrations in the input power range of 2040W. The aver-age diameter of nanoparticles was 35 nm. The height and thelength of the FHP used in the experiment were 3 mm and

15 20 25 30 35 40 450.00

0.01

0.02

0.03

0.04

0.05

0.06

Q (W)

R (K

/W)

Water 5ppm 50ppm 100ppm

Fig. 9. Inuence of particle concentration on the heat resistance of FHP undervarious input powers [22].0.07

and Mass Transfer 55 (2012) 67866797Kang et al. [25] studied the total heat resistance of a cylindricalsintered wick heat pipe with the outer diameter and the length of

6 mm and 200 mm, respectively. The heat pipe contained a 1 mm-thick sintered-wick made of copper powders. The nanouids weremade of pure water and silver nanoparticles with the particle sizesof 10 nm and 35 nm, respectively. The tested nanouids concentra-tions were 1 mg/L, 10 mg/L and 100 mg/L. The investigated powerrange was 30W70W. The condenser section of the heat pipe wasmaintained at 40 C in all runs. The experimental results showedthat the maximum heat loads of the heat pipe using nanouids in-creased by 40% and the temperature distributions of the evapora-tor section were more uniform compared with those of the heatpipe using water. The total heat resistance decreased by 88% forthe 60 W heat load. Authors considered that the reason for the heattransfer enhancement could be explained as follows. The maxi-mum heat ux could be enhanced by higher wettability; nanopar-ticles could atten the transverse temperature gradient of theworking uid and reduce the boiling limit because of the increaseof the effective liquid conductance in the heat pipe. The heat resis-tance of the heat pipe was reduced for the same reason.

and the viscosity of the nanouids would increase the ow drag

50 nm. Fig. 10 shows the comparison of the total heat resistancebetween a water charged OHP and a nanouids charged OHP. Itis evident that diamond nanoparticles signicantly increase theheat transport capability. The enhanced heat transfer mechanismwas considered as below: higher thermal conductivity, lower vis-cosity of nanouids, and stronger oscillating motion of nanoparti-cles might be the primary factors enhancing the heat transportcapability in nanouids charged oscillating heat pipe.

Shang et al. [29] investigated the heat transfer characteristics ofa closed loop OHP with Cuwater nanouids as the working uidunder different lling ratios. The results were compared with thoseof the same heat pipe with distilled water as the working uid. Theexperimental results conrmed that the use of Cuwater nano-uids in the heat pipe could enhance the maximum heat removalcapacity by 83%. It was conrmed that directly adding nanoparti-cles into distilled water without any stabilizing agents had greaterheat transfer enhancement compared to the case where a stabiliz-ing agent was added to the distilled water.

Z.-H. Liu, Y.-Y. Li / International Journal of Heat and Mass Transfer 55 (2012) 67866797 6791and reduce the capillary force in the sintered metal wick channels

2.4. Oscillating heat pipe

The studies of nanouids used in a vertical closed loop oscillat-ing heat pipes (OHP) were performed by the Mas research team[27,28]. They used alloy 122 copper tube with an inside diameterof 1.65 mm, an outer diameter of 3.18 mm and 12 turns. The exper-iment was carried out with the heat load ranging between 0 and336W, the ambient temperature maintained at 1070 C and theinternal lling ratio xed at 50%. The nanouids consisted of thehigh-performance liquid chromatography (HPLC) grade waterand 1.0 vol% diamond nanoparticles with the diameter of 5

0 50 100 150 200 250 300 3500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Q (W)

R (K

/W)

Water, Vertical Nanofluid, VerticalRiehl [26] performed an experimental study on the thermal per-formance of the sintered metal wick miniature loop heat pipe(LHP) using nickel-water nanouid. A simple wettability test ofthe nanouids was rstly carried out in various wick materials,which were hydrophilic polyethylene, sintered nickel and copper.It was found that the waternickel nanouids did not presentedgood wettability in the sintered copper; and solid nanoparticleswere separated even if the wick pore radius was greater than thatof the nanoparticles. Only sintered nickel wick could be applied inthe LHP using waternickel nanouids. However, the thermal per-formance was reduced. The authors explained the mechanism ofheat transfer reduction as follows: the increase of the densityFig. 10. Heat resistance comparison between a water charged OHP and a nanouidcharged OHP, lling ratio = 50%, vertical, Top = 20 C [27].Lin et al. [30] investigated experimentally the thermal perfor-mance of a closed loop oscillating heat pipe using nanouids. Theyapplied water-based silver nanouids at different volume fractions(100 ppm and 450 ppm) and various lling ratios (20%, 40%, 60%,and 80%). The silver nanoparticle had a diameter of 20 nm. Resultsshowed that the thermal performance of the oscillating heat pipeusing nanouids was better than that using water. The best llingratio was reported to be 60%. As shown in Fig. 11, there exists abest volume concentration of 100 ppm. When the input powerwas 85W, the average temperature difference between the innerwall of evaporator and the saturated vapor decreased by 7.8 K,which is equivalent to a decrease of the total heat resistance ofthe heat pipe by 15%. The authors considered that the mechanismfor the existing optimal volume fraction of silver nanouids couldbe explained as follows: although the nanouids with higher con-centration had higher thermal conductivity, higher nanoparticleconcentration resulted in higher viscosity; this caused more dif-culty to the bubbles growing and generated larger obstruction ofthe liquid slug, hence an optimal concentration would exist.

Park and Ma [31] carried out an experiment on the heat trans-port capability in a closed loop well-balanced oscillating heat pipeusing nanouids. Here, the well-balanced OHP was dened as aperfectly round ring with no turns. The OHP was fabricated fromcopper tube with an inside diameter of 1.6 mm. It had six heatingsections and six cooling sections spaced along the circumference.Between the heating section and the cooling section was adiabaticsection. The nanouids consisted of HPLC water and 1.0 vol% CuNi

0 20 40 60 80 1000.5

1.0

1.5

2.0

2.5

3.0

3.5

R (K

/W)

=60%450ppm water 100ppmQ (W)

Fig. 11. Heat input vs. average heat resistance for 60% lling ratio [30].

nano-ethanol mixture were used as working uids with a lling ra-tio of 50%. Results show that the main ow pattern changes from abubble ow with slug ow and annular ow to a dispersed bubbleow.

Cheng et al. [34] experimentally and theoretically investigatedthe heat transfer performance of at-plate oscillating heat pipes,which were created by machining grooves on both sides of a cop-per plate. Acetone, water, diamondacetone, goldwater, and dia-mondwater nanouids were tested as working uids. Thethermal resistance was further decreased when the nanouidwas used as the working uid. It was observed that high-volumefraction diamondwater nanouids was not stable but settled withtime and reduce thermal performance.

2.5. Closed two-phase thermosyphon (gravity-supported heat pipe)

Xue et al. [36] carried out an investigation about the interfaceeffect of carbon nanotube (CNT) suspension with surfactant on

eat and Mass Transfer 55 (2012) 67866797nanoparticles with the diameter of 40 nm to 150 nm. The heat pipewas tested in two situations: (1) without oscillating motions and(2) with oscillating motions. Although the heat transfer perfor-mance was improved after substituting nanouids for the baseuid, the heat transfer enhancement effect was not signicant forthe rst situation when the input power increased from 0 to20 W. The primary reason for this was that the CuNi nanoparticlesin the HPLC water settled on the bottom of the heat pipe due to thelack of oscillating motion. For the second situation, oscillating mo-tions occurring in the heat pipe were very irregular and were dif-ferent from those occurring in common OHP. The nanouidscould signicantly enhance the heat transfer in the heat pipe whenthe oscillating motions existed. Also, the impact of the nanouidson the heat transport capability depended on the lling ratio. Asshown in Fig. 12, the thermal performances of the OHP using bothnanouids and water have no differences for a power input of30 W and a lling ratio below 40% and above 70%. The heat pipehas its best heat transfer performance when the lling ratio is 50%.

Qu et al. [32] performed an experimental investigation on thethermal performance of a closed loop OHP charged water-basedAl2O3 nanouids. The OHP was fabricated by bending a stainless

20 30 40 50 60 70 80

80

90

100

110

120

130

140

Q (W)

Te-T

c (K

)

Q=30W Water Nanofluid

Fig. 12. Thermal performances of the OHP using both nanouid and water [31].

6792 Z.-H. Liu, Y.-Y. Li / International Journal of Hsteel capillary tube with an inner diameter of 2 mm and an outerdiameter of 3 mm. It had 6 turns with a total length of 3 m. TheOHP, consisting of evaporation, adiabatic and condensation sec-tions with 50, 105 and 70 mm in length, respectively, was verti-cally oriented. The effects of lling ratios, mass concentrations ofalumina nanoparticles, and power inputs on the total heat resis-tance of the OHP were investigated. Experimental results showedthat the adding alumina nanouids to the base uid signicantlyimproved the thermal performance of the OHP, with an optimalmass concentration of 0.9 wt% for maximal heat transfer enhance-ment. Compared with pure water, the maximal heat resistance wasdecreased by 32.5% when the power input was 58.8 W at 70% ll-ing ratio and 0.9 wt% mass concentration. The major reason for theenhanced thermal performance was found to be the nano-rough-ness surface state of the evaporator due to settlement of nanopar-ticles. The irregular nanopores formed between the depositedalumina nanoparticles which created the nano-roughness withinthe micrometer-roughened surface would dramatically increasethe active nucleation site density and the bubble release frequency,but decrease the bubble release diameter.

Bhuwakietkumjohn and Rittidech [33] investigate the internalow patterns and heat transfer characteristics of a closed-looposcillating heat-pipe with check valves. The ratio of number ofcheck valves to meandering turns was 0.2. Ethanol and a silver

0.025 water filled200 300 400 500 600 700 8000.005

0.010

0.015

0.020

R (K

/W)

Q (W)the thermal performance of a closed two-phase thermosyphon.The test section was a copper tube with an inner diameter of20 mm. The lling ratio of the closed two-phase thermosyphonwas 20%. The experimental results in Fig. 13 show the total heatresistance of the heat pipe using CNT is higher than those of theheat pipe using water. It is obvious that in Fig. 13 adding CNT inthe base liquid deteriorated the thermal performance of the heatpipe. It was found in this experiment that the CNT was broken tochips due to the addition of some acid liquids in the CNT suspen-sion to improve the stability of the suspension. The chips of CNTsettled on the evaporator surface formed a coating layer and signif-icantly diminished the density and number of the active nucleationsites, the release frequency and the coalesced patches of vaporbubbles. Authors considered that these factors would weaken theheat transfer of the heat pipe.

Liu et al. [38] investigated the effect of nanoparticle parameterson the thermal performance in a miniature closed two-phase ther-mosyphon using CuO nanouids without surfactant. The test tubediameter, the lengths of the evaporation section, the insulationsection and the condensation section were 8 mm, 100 mm,100 mm and 150 mm, respectively. The experiment was carriedout at three operating pressures of 7.45 kPa, 12.38 kPa and19.97 kPa, respectively and the corresponding saturation operatingtemperatures were 40 C, 50 C and 60 C, respectively. The exper-imental results showed that adding nanoparticles in the heat pipecould enhance both the heat transfer and the critical heat ux. The

0.030 suspension filledFig. 13. Heat resistances of closed two-phase thermosyphon using both CNTsuspension and water at different heating powers [36].

than water in the copper closed two-phase thermosyphon andthe improvement in wettability along with entrapment of nano-particles in the grooves caused the decrease of Peclet number inevaporator section. These factors nally leaded to poor thermalperformance.

Naphon et al. [41] researched the heat transfer performance ofthe closed two-phase thermosyphon using titaniumethanol nano-uids and titaniumwater nanouids. The closed two-phase ther-mosyphon was fabricated from the straight copper tube with anouter diameter of 15 mm and a length of 600 mm. The nanoparti-cles used in the experiments were titanium nanoparticles with21 nm in size. The evaporation heat transfer coefcient of the ther-mosyphon using nanouids increased by 10.6% compared withthat of the heat pipe using ethanol.

3. The comparison of the existing experimental data for theheat resistance and the maximum heat-transfer capacity

0 20 40 60 80 100 120 140 160 180 2000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

R ( K

/ W )

q ( kW/m2 )

Tv=40C water Nanofluid with =1.0 wt%

Fig. 14. Total heat resistances of closed two-phase thermosyphon using water andnanouid [38].

Z.-H. Liu, Y.-Y. Li / International Journal of Heat and Mass Transfer 55 (2012) 67866797 6793operating temperature could signicantly affect the heat transferenhancement. The enhancement effect of nanouids increasedwith the decrease of the operating temperature. For the CuO nano-uids heat pipe, the heat transfer coefcient increased by a maxi-mum of 160%, and the critical heat ux increased by 120% when anoptimal nanoparticle mass concentration of 1% was applied. Fig. 14shows that the total heat resistance can decrease about 30%90%by substituting the nanouids for water as the work liquid. Atlow heat uxes, the heat transfer enhancement is especiallyremarkable. They also [39] investigated the thermal performancein the same miniature closed two-phase thermosyphon using car-bon nanotube (CNT) suspensions without surfactant. The experi-mental results are similar to those using CuO nanouids.

Khandekar et al. [40] investigated the total heat resistance of aclosed two-phase thermosyphon using pure water and variouswater-based nanouids containing nanoparticles of Al2O3 (4047 nm), CuO (8.613.5 nm) and laponite clay (discs of diameter25 nm and thickness 1 nm). The length and the inner diameter ofthe test closed two-phase thermosyphon were 720 mm and16 mm, respectively. All of the experimental results shown inFig. 15 illustrate that the heat transfer performance of the closedtwo-phase thermosyphon using nanouids was worse than thatof the closed two-phase thermosyphon using pure water. The

authors considered that nanouids had better surface wettability

40 45 50 55 60 650.00

0.02

0.04

0.06

0.08

0.10

0.12

Q (W)

R (K

/W)

Laponite CuO Al2O3 Water

Fig. 15. Heat resistance of closed two-phase thermosyphon with different nano-uids as compared to pure water [40].

1.4 Au-water, 17 nm [10]0 10 20 30 40 50 60 70 80 90 100 1100.00.10.20.30.40.50.60.70.80.91.01.11.21.3

Q (W)

(R0-

R n)/R

0

Ag-water, 10nm, 0.01 wt% [14] Ag-water, 35nm, 0.01 wt% [15] CuO-water, 50nm 1.0 wt% [17] CuO-water, 50nm 1.0 wt% [18] CuO-water, 50nm 1.0 wt% [19]In the existing literatures, both the mass concentration and vol-ume fraction were used for indicating the nanoparticle concentra-tion in the base uid. In order to arrange the data using the sameparameter of the nanoparticle concentration, the mass concentra-tion is used to describe the nanoparticle concentration in the pres-ent review.

The volume fraction h can be estimated by following correlation

1ww

qnq0

1 hh

1

Fig. 16 gives out comparison of the relative reduction of the to-tal heat resistance of grooved heat pipes among the existing exper-imental results. Here, Rn is the total resistance of heat pipes usingnanouids and R0 is that of heat pipes using the base uid. Sincethe mass concentration of nanoparticles in nanouids changed ina wide range in these experiments, and then the relative reductionof the total heat resistance also varied with the mass concentrationof nanoparticles. Therefore, in Fig. 16, only the data correspondingto the optimal mass concentration are plotted for every literature.All experimental data show an improvement of the total heat resis-tance when adding nanoparticles into the base uids. The relativereduction of the total heat resistance lies between 0.4 and 0.8 andchanges greatly for different experimental conditions. Althoughthe heat transfer enhancement of nanouids depends strongly onthe operating temperature [17], most experiments were carriedout under constant coolant temperatures; they did not give out

1.5Grooved heat pipeFig. 16. Relative reduction of total heat resistance of micro-grooved heat pipesusing nanouids.

fect of the total heat resistance when substituting nanouids for

0 20 40 60 80 100 120 1400.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Q (W)

(R0-

R n) /

R 0

Mesh heat pipe Au-water, 21 nm[20] CuO-water, 50nm,1.0 wt% [2124] Ag-water, 35nm, 0.01 wt% [22] Al2O3-water, 30nm, 2.4wt% [23]

0 50 100 150 200 250 300 350 400 450

-0.25-0.20-0.15-0.10-0.050.000.050.100.150.200.250.300.350.400.45

Q (W)(R

0-R n

)/R0

Oscillating Heat Pipe Diamond-water,50 nm,2.2 wt% [27] Diamond-water, 20 nm,2.2 wt%, [27] CuNi-water, 40-150 nm, 8.8 wt% [31] Al2O3, 56 nm,0.9 wt% [32] Diamond-acetone, 2nm, 0.33wt% [34] Au-water, 3nm, 0.006wt% [34] Diamond-water, 2nm, 2.6wt% [34]

Fig. 19. Relative reduction of total heat resistance of oscillating heat pipes usingnanouids.

Al-water, 30nm, 7.8 wt% [35] CNT-water, 15nm, 2.2 wt% [36] CuO-water, 15-50nm, 1 wt% [37] CuO, 30nm, 1 wt% [38]

6794 Z.-H. Liu, Y.-Y. Li / International Journal of Heat and Mass Transfer 55 (2012) 67866797the operating temperatures or the operating temperatures corre-sponding to the varied input powers. Therefore, it is difcult tosummarize the effect of nanouids parameters on the heat transferenhancement at the unclear operating conditions using the exist-ing data.

Fig. 17 shows a comparison of the relative reduction of the totalheat resistance of the mesh wick heat pipes of the three availablereferences. As in the case of grooved heat pipe, only the data cor-responding to the optimal mass concentration are plotted inFig. 17. All experimental data show an improvement of the totalheat resistance when substituting nanouids for water as the baseuids. The relative reduction of the total heat resistance lies be-tween 0.1 and 0.75 and changes greatly for different experimentalconditions. The total inuence of nanouids parameters on thethermal performance of mesh wick heat pipe is quite similar tothat of grooved heat pipe.

Fig. 18 shows comparison of the relative reduction of the totalheat resistance between only two experimental studies of sinteredmetal wick heat pipes. For the case using 10 nm Agwater nano-uids, the reducing ratio of total heat resistance can increase to 0.5.However, for the case using 40 nm Niwater nanouids, the reduc-

Fig. 17. Relative reduction of total heat resistance of mesh wick heat pipes usingnanouids.ing ratio of total heat resistance changes dramatically from plus 0.2to minus 1.5 with the increase of the input power from 10W to40 W. The reason is assumed to be due to the fact that large nano-

10 15 20 25 30 35 40 45 50 55 60-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

(R0-

R n)/R

0

Q (W)

Sintered metal wicked heat pipes Ag-water, 10 nm, 0.01 wt%[25] Ni-water, 40 nm, 3.5 wt % [26]

Fig. 18. Relative reduction of total heat resistance of sintered metal wick heat pipesusing nanouids.particles get deposited in some small cavities and thence would in-crease the ow drag and reduce the capillary force in the sinteredwick channels.

Fig. 19 shows a comparison of the relative reduction of the totalheat resistance for OHPs. As in the case of grooved heat pipe, onlythe data corresponding to the optimal mass concentration are plot-ted in Fig. 19. All experimental data also show an improvement ef-0 100 200 300 400 500 600 700 800 900 1000-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Q (W)

( R0-

R n)/ R

0

CNT-water, 15nm, 2 wt% [39] Al2O3-water, 40-47nm, 1 wt% [40] CuO-water, 8.6-13nm, 1 wt% [40] Laponite clay-water,25nm,1 wt% [40] Ti-ethanol, 21nm, 0.57wt% [41] Ti-R11, 21nm, 0.31wt% [42]

Fig. 20. Relative reduction of total heat resistance of closed two-phase thermosy-phon using nanouids.

200 Ag-water in sintered metal

Heatwater as the base uids. The relative reduction of the total heatresistance lies in 0.04 to 0.43 and has very great change for differ-ent nanoparticle kinds and sizes. It should especially be noticedthat the optimal nanoparticles mass concentration also varied ina very wide range for various metal and metal oxidesnanoparticles.

Fig. 20 shows a comparison of the relative reduction of the totalheat resistance of closed two-phase thermosyphon. Only the datacorresponding to the optimal mass concentration are plotted inFig. 20. Contrary to other experimental results from grooved andmesh wick heat pipes using nanouids, the existing experimentalresults of closed two-phase thermosyphon using nanouids canbe divided into two categories. In the all eight studies, the majorityof experiments (six papers) show that adding nanoparticles intothe base uids can decrease the total heat resistance, but there isalso a part of opposite experimental results. For example, as shownin Fig. 20, for the same CNT suspension, in experiment of Liu andco-workers [39], using the CNTwater suspension without surfac-tant well enhances the thermal performance of closed two-phasethermosyphon. However, use of the CNTwater suspension withsurfactant deteriorates thermal performance in the experiment ofXue et al. [36]. In addition, the optimal nanoparticles mass concen-tration varied also in a quite wide range for various nanoparticles.

Fig. 21 shows the inuence of nanouids on the enhancement ofthe maximum heat removal capacity. Since most literatures have

0

100

Kang [25] Liu [17] Liu [21] Qu [32] Liu [41]

wick heat pipe

Fig. 21. The maximum heat removal capacity of nanouid heat pipes.300

400

500

600

700

800

P (W

)

Increased maximum heat removal capacity by nanofluid water

15nm 2%wt CNT-water in closed two-phase thermosyphon

56 nm 0.9wt% Al2O3-water in oscillating heat pipe

50 nm 1.0 wt% CuO-water in mesh wick heat pipe

50nm 1.0wt% CuO-water in micro-grooved heat pipe 10nm 0.1wt%

Z.-H. Liu, Y.-Y. Li / International Journal ofnot given out relevant data, only data of ve papers are arrangedin Fig. 21. Only the data corresponding to the optimal mass con-centration are plotted in Fig. 21 for each paper. All experimentaldata show an improvement of the maximum heat removal whenadding nanoparticles into the base uids. The enhancement differsgreatly for different experiments due to the greatly different exper-imental conditions.

4. Mechanism of transfer, existing problems and futureresearch prospects

The current studies on nanouids applied in heat pipes can bedivided into three categories. The rst category is the heat pipeswith micro-grooves, meshes and sintered metal porous materialswhich provide the capillary force. The basic heat transfer modefor this category belongs to the convective evaporation and con-vective condensation of the uid lm. The boiling heat transfermay occur at high heat uxes in heat pipes with micro grooves,but it cannot occur in the mesh and sintered metal heat pipes.The ordinary cylindrical heat pipe, at-shaped axial heat pipeand capillary pumped loop (CPL) all are such heat pipes. The sec-ond category is oscillating heat pipes. The basic heat transfer modefor this category belongs to temperature gradient making a differ-ent volumetric distribution of the working uid and causing pres-sure waves and uid pulsations in each of the individual tubesections, which interact with each other generating secondary/ter-nary reections with perturbations [48,49]. Bubble generation pro-cesses in the heater tubes sections and condensation processes atthe other end create a sustained non-equilibrium state as theinternal pressure tries to equalize within the closed system. Thus,a self-sustained thermally driven oscillating ow is obtained[50,51]. The third category is the closed two-phase thermosyphon.Its heat transfer mechanism is similar to that of steady pool nucle-ate boiling. The driving force of the uid ow is the buoyancy gen-erated by the boiling two-phase ow.

For the heat pipe with micro-grooves, meshes and sintered me-tal porous materials, all the present experiments show that addingnanoparticles in the base liquid can greatly enhance both the heattransfer performance and the maximum heat removal capacity ofheat pipes. However, for the sintered metal wick heat pipes, theenhancement effect has a great uncertainty. Up to now, only veresearches were reported. And one of them reported that the per-formance was weakened.

Mechanism of enhanced heat transfer of the micro-groove wickheat pipe and the mesh wick heat pipe using nanouids can be ex-plained by three reasons. The rst reason is that the effective ther-mal conductivity of nanouids increases. The second reason is thatis that nanouids decrease the solidliquid contact angle; it makesthe liquid extending in micro-grooves and mesh. Also, the reduc-tion of solidliquid contact angle increases the capillary force inthe heat pipe. The above two reasons result from the changes ofthe physical properties of nanouids. The third reason is that nano-particles deposited on the wall form a thin porous layer, which in-creases the solidliquid wettability and the capillary force. Thisreason results from the changes of the heating surface characteris-tics. In addition, it is guessed that the random motion (Brownianmotion) of nanoparticles could enhance the heat transfer of thebase uid itself.

Reasons for the exiting optimal concentration of nanoparticlescan be estimated as follows by a numerical simulation [1113]:The capillary force inducted by the porous coating layer formedby nanoparticles on evaporation section will increase with increas-ing the nanoparticle concentration, and nally reaches a certainextent. Meantime, the liquid density, viscosity and ow resistancewill also increase with increasing the nanoparticle concentration.Therefore, there exists an optimal nanoparticles concentration thattakes a balance between the capillary force and the ow drag force.

Nanouids applied in OHPs mainly show positive effects in theexisting studies. Adding nanoparticles into the base uids can evi-dently enhance the heat transfer of the OHP. With increasing theoperating temperature of the OHP with nanoparticles, the heatresistance decreases and the maximum heat removal capacity in-creases greater than that without nanoparticles. The enhancedheat transfer mechanism was considered as below: when the oper-ating temperature increased, higher thermal conductivity andstronger oscillating motion of nanoparticles might be the primaryfactors enhancing the heat transport capability in nanouids. Inaddition, a thin layer of porous sediments on the wall of OHP willalso affect strongly the thermal performance of OHP as in closedtwo-phase thermosyphon.

The heat transfer characteristics of nanouids in closed two-phase thermosyphon are very complex. The existing experimentalresults can be divided into two categories. The majority of experi-ments believe that nanoparticles are able to enhance heat transfer,

and Mass Transfer 55 (2012) 67866797 6795but there are also parts of opposite experimental results. The heattransfer characteristics of nanouids in the closed two-phase ther-mosyphon are similar to the pool boiling characteristics of

heat transfer and deteriorated heat transfer. It is difcult to clearlyexplain the boiling mechanism of nanouids in closed two-phase

[14] W.C. Wei, S.H. Tsai, S.Y. Yang, S.W. Kang, Effect of nano-uid concentration on

175.[17] X.F. Yang, Z.H. Liu, J. Zhao, Heat transfer performance of a horizontal micro-

eatthermosyphon, because the boiling heat transfer characteristicsof nanouids are related to not only of the thermophysical proper-ties, but also related to the surface characteristics of the heater, thestability of nanouids. Especially in the boiling process, a very thinlayer of porous sediments will form on the heater and affectstrongly the boiling characteristics. The nanoparticles, the surfac-tant, the afnity between the base liquid and the heater and theafnity between the base liquid and the nanoparticles will all affectthe nature of porous sediment layer, and hence affect the boilingheat transfer characteristics. All these effects may lead to the greatdifferences among the present experimental results based on thesediments structure. It is well known that the existence of poroussediments can increase the number of the active nucleation sites,but slurry sediments will decrease the half cone angle of a cavityor decrease the mouth radius of the cavity and decrease the num-ber of the active nucleation sites. Thus the sediment layer maybeenhance the boiling heat transfer, especially the critical heat ux,but also maybe decrease the number of the active nucleation sitesand weakens the heat transfer.

The above explanations of the mechanisms are qualitative.There are still a lot of problems not being clearly dened for theheat pipes using nanouids. One is the impact of operating temper-ature on the thermal performance. A series of experiments of Liusresearch team have shown that the operating temperature has asignicant impact on the heat transfer enhancement of the nano-uids in the closed two-phase thermosyphon. A better heat transferenhancement is obtained at lower operating temperature. But theexisting boiling theory cannot make a reasonable explanation forthis. Another problem is the optimal nanoparticle concentration.In general, there is an optimal nanoparticle concentration in vari-ous types of heat pipes. For the grooved and mesh heat pipes,the perspective of the balance of ow resistance and capillary forcecan simply explain this phenomenon. However, it is difcult togive a clear explanation for the oscillating heat pipe and the closedtwo-phase thermosyphon dominated by boiling heat transfer. Thethird problem is the sediment layer formed on the wall. Experi-ments have found that regardless of the types of heat pipes, a sed-iment layer will form on the heating surface after heating test. Willthe sediment layer become thicker or maintain a certain thicknessduring the operation? This has signicant impacts on both the heattransfer mechanism and the practical engineering application ofnanouids in heat pipes. However, presently there seems to beno related literature available.

The preparation of nanouids with better stability in engineer-ing application is also a practical problem. The life expectancy, reli-ability, economy of nanouids heat pipes and the compatibilitywith the practical application should also be considered.

The direction of the future research on the application of nano-uids in heat pipes is mainly to nd the optimal types of nano-uids, nanoparticle size and nanoparticle concentration to achievethe best thermal performance. Also, it is also a hot spot for futureresearch to nd the impacts of various operating parameters, suchas the operating temperature, the heat ux and the ambient tem-perature on the heat transfer enhancement of nanouids in heatpipes.

5. Conclusionsnanouids. In fact, the pool boiling heat transfer characteristics ofnanouids can also be divided into the same two groups: enhanced

6796 Z.-H. Liu, Y.-Y. Li / International Journal of HThis paper describes the research results of heat transfer char-acteristics of various types of heat pipes using nanouids as work-ing uids. Results of the limited number of available referencesgrooved heat pipe using CuO nanouid, J. Micromech. Microeng. 18 (2008)035038.

[18] Z.H. Liu, Y.Y. Li, R. Bao, Thermal performance of inclined grooved heat pipesusing nanouids, Int. J. Thermal Sci. 49 (2010) 16801687.

[19] G.S. Wang, B. Song, Z.H. Liu, Operation characteristics of cylindrical miniaturegrooved heat pipe using aqueous CuO nanouids, Expt. Thermal Fluid Sci. 34(2010) 14151421.

[20] C.Y. Tsai, H.T. Chien, P.P. Ding, B. Chan, T.Y. Luh, P.H. Chen, Effect of structuralcharacter of gold nanoparticles in nanouid on heat pipe thermal performance,Mater. Lett. 58 (2004) 14611465.

[21] Z.H. Liu, T. Shu, Application of nanouids in thermal performanceenhancement of horizontal screen heat pipe, J. Aerospace Power 23 (2008)16231627.

[22] Y.T. Chen, W.C. Wei, S.W. Kang, C.S. Yu (Effect of nanouids on at heat pipeheat pipe thermal performance, IASME Trans. 2 (2005) 14321439.[15] S.W. Kang, W.C. Wei, S.H. Tsai, S.Y. Yang, Experimental investigation of silver

nano-uid on heat pipe thermal performance, Appl. Thermal Eng. 26 (2006)23772382.

[16] Z.H. Liu, L. Lu, Thermal performance of axially microgrooved heat pipe usingcarbon nanotube suspensions, J. Thermophys. Heat Transfer 23 (2009) 170have shown that nanouids have great application prospects invarious heat pipes. For the majority of micro-grooved heat pipes,mesh wick heat pipes, oscillating heat pipes and most closedtwo-phase thermosyphon, adding nanoparticles to the working li-quid can signicantly enhance the heat transfer, reduce the totalheat resistance and increase the maximum heat removal capacity.At the same time, there are still some problems and challenges onthe mechanisms of the heat transfer enhancement and the actualapplications. The present research of nanouids in heat pipes isstill at its initial stage and needs further development.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China under Grant No. 51076092.

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Z.-H. Liu, Y.-Y. Li / International Journal of Heat and Mass Transfer 55 (2012) 67866797 6797

A new frontier of nanofluid research Application of nanofluids in heat pipes1 Introduction2 Fundamental studies of nanofluids applied in heat pipes2.1 Micro-grooved heat pipe2.2 Mesh wick heat pipe2.3 Sintered metal wick heat pipe2.4 Oscillating heat pipe2.5 Closed two-phase thermosyphon (gravity-supported heat pipe)

3 The comparison of the existing experimental data for the heat resistance and the maximum heat-transfer capacity4 Mechanism of transfer, existing problems and future research prospects5 ConclusionsAcknowledgmentsReferences