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Fluid ow and heat transfer characteristics of nanouids in heat pipes: A review Omer A. Alawi, Nor Azwadi Che Sidik , H.A. Mohammed, S. Syahrullail Department of Thermouids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia abstract article info Available online 14 May 2014 Keywords: Heat pipe Nanouid Heat transfer enhancement Comprehensive research work on heat transfer in heat pipe using traditional working uids has been carried out over the past decade. Heat transfer in heat pipes using suspensions of nanometer-sized solid particles in base uids have been experimentally and theoretically investigated in recent years by various researchers across the world. The suspended nanoparticles effectively enhance heat transfer characteristics and the transport properties of base uids in heat pipes. The objective of this paper is to present an overview of literature dealing with recent developments in the study of heat transfer using nanouids in heat pipes and some important inferences from the various papers are also highlighted. It also discusses the mechanism of heat transfer enhance- ment or degradation, the existing problems for various heat pipes utilizing nanouids, and explores the possible application prospects. © 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 2. Preparation of nanouids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3. Fundamental studies of nanouids in heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.1. Micro-grooved heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.2. Mesh wick heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3.3. Sintered metal wick heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.4. Oscillating heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.5. Two-phase closed thermosyphon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4. Heat transfer characteristics of nanouids in heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.1. Experimental investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2. Theoretical investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 1. Introduction With the increase of work frequency and heat ux of electronic com- ponents, the dissipation problem of the high heat ux components becomes one of the key technologies of the electronic device design. Up to now, heat pipe technology has been widely applied in the eld of microelectronics cooling, as the improved construction of the general heat pipes, at heat pipe has now become a hotspot technology of heat pipe research and development [1,2] and has been widely applied in many elds, such as spacecraft thermal control, high heat ux electronic equipment cooling, medical and health undertakings, and household appliances. Heat pipe is a device used to transfer the heat from one place to the other. The heat pipe consists of evaporator section, adiabatic section and condenser section (Fig. 1). Heat absorption takes place in the evaporator section and heat rejection at the condenser section. Adiabatic section is fully insulated. The heat pipe is evacuated using a vacuum pump and is lled up with the working uid. The working uid absorbs the heat at one end of the heat pipe called evaporator and releases the heat at the other end called condenser. Due to the capillary action, the condensed working uid through the mesh wick structure returns to the evaporator, on the inside wall of the pipe. Nor- mally conventional uids are used in heat pipes to remove the heat [3]. For the time being, nanouids play an important role in heat pipes to increase the heat transfer compared to conventional uids. International Communications in Heat and Mass Transfer 56 (2014) 5062 Communicated by W.J. Minkowycz. Corresponding author. E-mail address: [email protected] (N.A.C. Sidik). http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.04.014 0735-1933/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
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Fluid flow and heat transfer characteristics of nanofluids in heat pipes

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Page 1: Fluid flow and heat transfer characteristics of nanofluids in heat pipes

International Communications in Heat and Mass Transfer 56 (2014) 50–62

Contents lists available at ScienceDirect

International Communications in Heat and Mass Transfer

j ourna l homepage: www.e lsev ie r .com/ locate / ichmt

Fluid flow and heat transfer characteristics of nanofluids in heat pipes:A review☆

Omer A. Alawi, Nor Azwadi Che Sidik ⁎, H.A. Mohammed, S. SyahrullailDepartment of Thermofluids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.

E-mail address: [email protected] (N.A.C. Sidik).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.04.010735-1933/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Available online 14 May 2014

Keywords:Heat pipeNanofluidHeat transfer enhancement

Comprehensive research work on heat transfer in heat pipe using traditional working fluids has been carried outover the past decade. Heat transfer in heat pipes using suspensions of nanometer-sized solid particles in basefluids have been experimentally and theoretically investigated in recent years by various researchers acrossthe world. The suspended nanoparticles effectively enhance heat transfer characteristics and the transportproperties of base fluids in heat pipes. The objective of this paper is to present an overview of literature dealingwith recent developments in the study of heat transfer using nanofluids in heat pipes and some importantinferences from the various papers are also highlighted. It also discusses themechanismof heat transfer enhance-ment or degradation, the existing problems for various heat pipes utilizing nanofluids, and explores the possibleapplication prospects.

© 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502. Preparation of nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513. Fundamental studies of nanofluids in heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.1. Micro-grooved heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.2. Mesh wick heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.3. Sintered metal wick heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543.4. Oscillating heat pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.5. Two-phase closed thermosyphon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4. Heat transfer characteristics of nanofluids in heat pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.1. Experimental investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.2. Theoretical investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

1. Introduction

With the increase ofwork frequency and heatflux of electronic com-ponents, the dissipation problem of the high heat flux componentsbecomes one of the key technologies of the electronic device design.Up to now, heat pipe technology has been widely applied in the fieldof microelectronics cooling, as the improved construction of the generalheat pipes, flat heat pipe has now become a hotspot technology of heatpipe research and development [1,2] and has been widely applied inmany fields, such as spacecraft thermal control, high heat flux electronic

4

equipment cooling, medical and health undertakings, and householdappliances. Heat pipe is a device used to transfer the heat from oneplace to the other. The heat pipe consists of evaporator section, adiabaticsection and condenser section (Fig. 1). Heat absorption takes placein the evaporator section and heat rejection at the condenser section.Adiabatic section is fully insulated. The heat pipe is evacuated using avacuum pump and is filled up with the working fluid. The workingfluid absorbs the heat at one end of the heat pipe called evaporatorand releases the heat at the other end called condenser. Due to thecapillary action, the condensed working fluid through the mesh wickstructure returns to the evaporator, on the inside wall of the pipe. Nor-mally conventional fluids are used in heat pipes to remove the heat [3].

For the time being, nanofluids play an important role in heatpipes to increase the heat transfer compared to conventional fluids.

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Fig. 1. Schematic diagram of heat pipe [3].

51O.A. Alawi et al. / International Communications in Heat and Mass Transfer 56 (2014) 50–62

Thermal conductivity is an important parameter in enhancing the heattransfer performance of a heat transfer fluid. Researchers have alsotried to increase the thermal conductivity of base fluids by suspendingnanometer-sized solid particles in fluids since the thermal conductivityof solid is typically higher than that of liquids, as seen from Table 1.Many researchers have presented the heat transfer characteristics ofheat pipe using nanofluids [3]. The concept of “nanofluid” has firstlyproposed by Choi and Eastman [4]. That is, adding nanoscale metal ormetal oxide particles in the liquid with a certain way and proportion,which forms a new class of heat transfer and cooling working fluid. Be-cause of its stability and high thermal conductivity, the nanofluid showsa promising prospect in the heat transfer enhancement. Keblinski et al.[5] made an interesting review to discuss the properties of nanofluidsand future challenges. Weerapun and Somchai [6] summarized thepublished experimental and numerical investigations of forced convec-tive heat transfer of nanofluids. Bahrami et al. [7] provided an overviewon the effective thermal conductivity of nanofluids. Cheng et al. [8] car-ried out an overview on the studies of nanofluids boiling and two phaseflow. The application of nanofluid research in heat pipes was firstlypublished by Chien et al. [9]. Over 20 relevant articles have been pub-lished since then, involving mesh wicked heat pipes [10] and [11],micro-grooved heat pipes [9,12–17], sintered metal wicked heat pipes[18] and so on.

An experiment concerning a cylindrical mesh wicked heat pipe wasperformed by Tsai et al. [10]. The working fluid was an aqueous sus-pension of various-sized gold nanoparticles. The inner diameter andthe length of the tested copper tube were 6 mm and 170 mm, respec-tively. A 200mesh screen was distributed on the inner wall. The exper-imental results showed that the total thermal resistance of the heatpipe reduced a lot due to the addition of nanoparticles under thesame cooling condition. The experiment also found that the best wayto use nanofluids in the heat pipe was using a well dispersed nanofluid.The mechanism of the heat transfer enhancement was explained asfollows: a major thermal resistance of heat pipe was caused by the

Table 1Thermal conductivities of various solids and liquids [3].

Thermal conductivity (W/m-K) Material

401 Metallic solid copper237 Aluminum148 Nonmetallic solid silicon40 Alumina (Al2O3)72.3 Metallic liquid sodium (644 K)0.613 Nonmetallic liquid water0.253 Ethylene glycol (EG)0.145 Engine oil (EO)

formation of vapor bubbles at the liquid–solid interface; the suspendednanoparticles tended to bombard the vapor bubbles during the bubbleformation; therefore, it was expected that the nucleation size of vaporbubbles was much smaller for the fluid with suspended nanoparticlesthan that without them. Chen et al. [11] studied the performance ofaxially flat mesh wicked heat pipe (FHP) using water-based silvernanofluids with different nanoparticle concentrations under theinput power of 20–40 W. The average diameter of nanoparticles was35 nm. The height and the length of the FHP used in the experimentwere 3 mm and 200 mm, respectively. It was found that the totalthermal resistance of the heat pipe using nanofluids was reduced com-pared with that of the heat pipe using deionized water under the samecooling condition. In the volume concentration range tested, the largerthe volume concentration of nanoparticles was, the more reductionof the thermal resistance could be. The mechanisms of heat transferenhancement were given by authors as: (1) the increase of the wetta-bility increased the critical heat flux; (2) the mutual increases of theliquid thermal conductivity and the wick conductivity increased theheat transfer.

Some steady heat transfer experiments under several steady opera-tion pressures conducted to investigate the heat transfer performanceof a cylindrical micro-grooved copper heat pipe. Water-based CuOnanofluids and water-based carbon nanotubes without dispersantwere used as the working fluids [15]. All experiments show that addingnanoparticles into the base liquid can enhance both the heat transferperformance and the maximum input power of heat pipes [9,12–17].

Analytical models carried out to investigate the thermal perfor-mance of rectangular and disk-shaped heat pipes using nanofluids.Some of the more widely utilized nanoparticles, such as Al2O3, CuOand TiO2 with a range of nanoparticle diameters were considered.Results show that the presence of nanoparticles in the working fluidleads to a reduction in the speed of the liquid, smaller temperature dif-ference along the heat pipe and the possibility of reduction in size underthe same operational conditions. It is similar to what has been observedexperimentally that using a nanofluidwill reduce the thermal resistanceof the flat-shaped heat pipe. The maximum heat removal capability ofthe flat-shaped heat pipe was displayed for a range of wick thicknessesand nanoparticle concentration levels. The existence of an optimumnanoparticle concentration level and wick thickness in maximizingthe heat removal capability of the flat-shaped heat pipe was established[19]. Alizad et al. [20] studied the thermal performance, transientbehavior and operational start-up characteristics of flat-shaped heatpipes using nanofluids. Three different nanofluids (CuO, Al2O3, andTiO2) were utilized in their analysis. A comprehensive analyticalmodel, which accounts in detail the heat transfer characteristics withinthe pipe wall and the wick within the condensation and evaporationsections, was utilized. The results illustrate the enhancement in theheat pipe performancewhile achieving a reduction in the thermal resis-tance for both flat-plate and disk-shaped heat pipes throughout thetransient process. It was shown that a higher concentration of nanopar-ticles increases the thermal performance of either the flat-plate or disk-shaped heat pipes. The study has also established that for the same heatload a smaller size flat-shaped heat pipe can be utilized when usingnanofluids.

The papers presented on the study of heat transfer and flow charac-teristics of the heat pipewith nanofluids have rarely been reported. Theobjective of this paper is to present an overview of literature dealingwith recent developments in the study of heat transfer using nanofluidsin heat pipes and some important inferences from the various papersare also highlighted by the following studies.

2. Preparation of nanofluids

The powder form nanoparticles which disperse in host liquids arecalled nanofluids. Nanofluids can be produced by two techniques; thetwo-step (double-step) method, and one-step (single-step) method.

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These methods have been utilized using different types of chemical andphysical techniques to make sure that the solid–liquid mixture is stableto avoid agglomeration, additional flow resistance, possible erosion andclogging, poor thermal conductivity, and poor heat transfer. The two-step method is done by producing the nanoparticle powder initially asintroduced in the previous section, and then disperses them into ahost liquid. However, in one-stepmethod the nanoparticles are simulta-neouslymade anddirectly dispersed into the basefluid [21]. It is noticedin the literature that nanofluids with oxide nanoparticles and carbonnanotubes are produced well by the two-step method, while it is notsuitable for nanofluids with metallic nanoparticles. The summary of re-sults reported by various researchers in the area of nanofluid prepara-tion is provided in Table 2.

3. Fundamental studies of nanofluids in heat pipes

3.1. Micro-grooved heat pipe

An experimental study on the application of nanofluids in FHP firstlycarried out by Chien et al. [9]. They studied a disk-shaped aluminumminiature micro-grooved heat pipe. The diameter and the thicknesswere 9 mm and 2 mm, respectively. A total number of 18 micro-grooves were evenly distributed on the aluminum base to provide thecapillary force. The depth and the width of rectangular micro-grooveswere 0.4 mm and 0.35 mm, respectively. The nanofluid consisted ofgold nanoparticles with a diameter of 17 nm and DI water. The experi-mental data of the nanofluidswere comparedwith those of DIwater in-cluding the wall temperatures and the total heat resistances of the heatpipe. Experimental results showed that the total heat resistance of theheat pipe using nanofluids was less than that of the heat pipe using DIwater at different filling ratios. The use of the nanofluids made theheat resistance reduces by an average of 40%.

Wei et al. [38] used a cylindrical micro-grooved heat pipe withthe inner diameter and the length of 6 mm and 200 mm, respectively.The width and the depth of the rectangular groove were 211 μmand 217 μm, respectively. Theworkingfluid consisted of silver nanopar-ticles with an average particle size of 10 nm and pure water. Theymainly measured the total heat resistance of the heat pipe filledwith pure water and nanofluids at the same filling volume of 0.51 mL(φ = 10%). Nanoparticle volume fractions of 1 ppm to 100 ppm wereused in the tests. The total heat resistance of the heat pipe usingnanofluids could decrease by 28%–44% compared with that of the heatpipe using water. Researchers did not explain the mechanism of theheat transfer enhancement.

Table 2Summary of nanofluid preparation methods.

System Synthesis process Particlesize (nm)

Heat transferenhancement (%)

Ref.

Cu/EG Single-step 10 40 [22]Cu/H2O Single-step 75–100 23.8 [23]Cu/H2O Two-step 100 78 [24]Fe/EG Single-step 10 18 [25]Ag/toluene Two-step 60–80 16.5 [26]Cu2O/H2O Single-step 200 24 [27]Au/ethanol Two-step 4 1.3 ± 0.8 [28]Fe3O4/H2O Single-step 10 38 [29]TiO2/H2O Two-step 15 30–33 [30]Al2O3/H2O Two-step 20 20 [31]CuO/H2O Two-step 33 11.5 [32]SiC/H2O Two-step 25 15.9 [33]NCTs/engine oil Two-step 20–50 30 [34]NCTs/poly oil Two-step 25 160 [35]NCTs/EG Two-step 15 19.6 [36]NCTs/H2O Two-step 15 7.0 [36]NCTs/decene Two-step 15 12.7 [36]H2O/FC-72 Two-step 9.8 52 [37]

Kang et al. [13] also carried out experiments using nanofluidsconsisting of silver nanoparticles and pure water. The silver nanoparti-cle sizes were 10 nm and 35 nm, respectively. The experimental resultsshowed that the total heat resistance of the heat pipe using nanofluidsdecreased by 10–80% comparing with that using water in the heatingpower range of 30–60 W. The total heat resistance decreased with theincrease in both the nanoparticle concentration and the nanoparticlesize. Fig. 2 shows that the thermal resistance of a heat pipe containing10 nm nanoparticles was 52% lower than that using DI-water at 50 W.They considered that the improvement of thermal performance ismainly due to the reduction of fluid temperature gradient in nanofluids.

An experimental study was performed by Zhen-hua Liu et al. [39] tounderstand the nucleate boiling heat transfer of water–CuO nanoparti-cle suspension at different operating pressures and different nanoparti-cle mass concentrations. The experimental apparatus is a miniatureflat heat pipe (MFHP) with micro-grooved heat transfer surface of itsevaporator. The experimental results indicate that the heat transfercoefficient and the critical heat flux (CHF) of nanofluids increase greatlywith decreasing pressure as compared with those of water. The heattransfer coefficient and the CHF of nanofluids can increase about 25%and 50%, respectively, at atmospheric pressure whereas about 100%and 150%, respectively, at the pressure of 7.4 kPa. The heat transfercoefficient and the CHF increase slowly with the increase of the nano-particle mass concentration at low concentration conditions. However,when the nanoparticle mass concentration is over 1.0 wt.%, the CHFenhancement is close to a constant number and the heat transfer coef-ficient deteriorates.

Liu and Lu [15] and Yang et al. [17] carried out some steady heattransfer experiments under several constant operating temperaturesto investigate the heat transfer performance of a cylindrical micro-grooved heat pipe. Water-based CuO nanofluids and water-based car-bon nanotubes (CNTs) without dispersants were used as the workingfluids. The length and the inner diameter of the heat pipe were350 mm and 8 mm, respectively. Sixty rectangular grooves with thedepth of 0.2 mm and the width of 0.25 mm were uniformly fabricatedon the inner wall of the heat pipe. The experiments were carried out atthree fixed operating pressures of 7.45 kPa, 12.38 kPa and 19.97 kPa,with respectively corresponding operating temperatures of 40 °C,50 °C and 60 °C. Data of evaporation and condensation heat transferwere investigated and the impacts of the nanoparticle mass concentra-tion, the operating temperature on the heat transfer characteristicswas discussed. Fig. 3 indicates the effect of the nanoparticle mass con-centration on the total heat resistance of the heat pipe using CuOnanofluids. It is shown in [15,17,40] and [41] that there existed an opti-mal CuO nanoparticle mass concentration of 1.0 wt.% and an optimal

Fig. 2. Experimental data of heat resistances using both nanofluid and water at differentheating powers [13].

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Fig. 3. Effect of mass concentration of nanoparticles on the heat resistance [17].

Fig. 4. Measured value of heat resistance of a mesh wick heat pipe using nanofluids ofvarious particle sizes [10].

53O.A. Alawi et al. / International Communications in Heat and Mass Transfer 56 (2014) 50–62

inclination angle of 75. In addition, the heat transfer enhancementdecreased with increasing operating temperature. And substitutingnanofluids for deionized water as the working liquid can reduce thestartup time of heat pipe. For both water and the nanofluid, the maxi-mum heat removal capacity in the unsteady startup procedure is muchlower than that in the steady operating procedure. The mechanismsof the heat transfer enhancement were thought to be the increase ofthe heat conductivity of the nanofluids, the disturbance effect of nano-particles in the base liquid, as well as the reduction of solid–liquid con-tact angle in the liquid film.

Amathematical model was developed by Do and Jang [16] for quan-titatively evaluating the thermal performance of a water-based Al2O3

nanofluid heat pipe with a rectangular grooved wick. For the purpose,the axial variations of the wall temperature, the evaporation and con-densation rates are considered by solving the one-dimensional conduc-tion equation for the wall and the augmented Young–Laplace equationfor the phase change process. From the comparison of the thermal per-formance using both DI water and nanofluids, it is found that the thinporous coating layer formed by nanoparticles suspended in nanofluidsis a key effect of the heat transfer enhancement for the heat pipeusing nanofluids. Also, the effects of the volume fraction and the sizeof nanoparticles on the thermal performance are studied. The resultsshows the feasibility of enhancing the thermal performance up to100% although water-based Al2O3 nanofluids with the concentrationless than 1.0% are used as working fluid. The thermal resistance of thenanofluid heat pipe tends to decrease with increasing the nanoparticlesize, which corresponds to the previous experimental results.

A two-dimensional model modified by Shafahi et al. [19] to simulatethe thermal performance of a cylindrically grooved heat pipe utilizingnanofluids. The mathematical model adopted in this work was basedon the following assumptions: the process was steady state; radiativeand gravitational effects were negligible and the fluid was consideredNewtonian and incompressible. Also, the wick was assumed to be iso-tropic and saturated with the working fluid. The liquid flow within theporous wick was modeled using the generalized momentum equation.The analysis incorporated the presence of nanofluid within the heatpipe. Three of the most common nanoparticles, namely Al2O3, CuO,and TiO2 were applied. The simulation found that the nanoparticleswithin the base liquid enhance the thermal performance of the heatpipe by reducing the heat resistance while enhancing the maximumheat load. In theory, there exists an optimumnanoparticlemass concen-tration corresponding to themaximumheat transfer enhancement. Themodel assumed that vapor and liquid flow are steady and laminar andtransport properties of the vapor and liquid are considered to be con-stant; the vapor injection and suction rates are considered to be uniformin the evaporator and condenser, respectively. Operating temperature,

liquid velocity profile, wall temperature distribution of the heat pipe,heat resistance and maximum heat load were investigated for the heatpipes utilizing both the base liquid and nanofluids.

Zhen-hua Liu et al. [42] carried out an experimental investigation tostudy the heat transfer performance of a cylindrically micro-groovedheat pipe using aqueous nanofluids as the working fluids. The baseliquid was distilled water, while, the five kinds of nanoparticles: Cuwith two mean diameters of 40 nm and 20 nm, CuO with two meandiameters of 50 nm and 20 nm and SiO with a mean diameter of30 nmwere added respectively into thebase liquid to composedifferentkinds of nanofluids. Experiments were performed under three steadyoperating pressures of 7.45 kPa, 12.38 kPa and 19.97 kPa, respectively.Effects of nanoparticle kind, nanoparticle size, nanoparticle mass con-centration and operating pressure on the evaporation and condensationheat transfer coefficients, the maximum heat flux and the total heat re-sistance of the heat pipe were investigated, compared and discussed.Experimental results show that adding Cu and CuO nanoparticles intothe base fluid can apparently improve the thermal performance of theheat pipe and there is an optimal nanoparticle mass concentration toachieve the maximum heat transfer enhancement. However, addingSiO nanoparticles into the base fluid will contrarily deteriorate theheat transfer performance.

3.2. Mesh wick heat pipe

Tsai et al. [10] performed an experiment concerning a cylindricalmesh wick heat pipe. The working fluid was an aqueous solutionof various-sized gold nanoparticles. The inner diameter and the lengthof the test copper tube were 6 mm and 170 mm, respectively. A200 mesh screen was distributed on the inner wall. The experimentalresults showed that the total heat resistance of the heat pipe reduced20%–37% due to the addition of nanoparticles. Fig. 4 shows the total re-sistance of the heat pipe for nanofluids of various particle sizes. Themechanism of the heat transfer enhancementwas explained as follows:a major heat resistance of heat pipe was caused by the formation ofvapor bubbles at the liquid–solid interface; the suspended nanoparticlestended to bombard the vapor bubbles during the bubble formation;therefore, it was assumed that the release diameter of vapor bubbleswas much smaller for the fluid with suspended nanoparticles thanthat without them.

The heat transfer characteristics of a cylindrical meshwick heat pipeusing CuO–water nanofluids investigated by Liu and Shu [43]. The innerdiameter and the length of the test tube were 10 mm and 350 mm,respectively. Two layers of 160 mesh screen were distributed on theinner wall. It was found that the nanoparticle mass concentration had

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Fig. 6. Influence of particle concentration on the heat resistance of FHPunder various inputpowers [11].

54 O.A. Alawi et al. / International Communications in Heat and Mass Transfer 56 (2014) 50–62

significant impact on both the heat pipe evaporation and condensationheat transfer. There was an optimal mass concentration of 1.0% under avariety of operating temperature. An enhancement of the evaporationand condensation heat transfer and the maximum heat flux wasobtained at lower operating temperature. Fig. 5 shows that the totalheat resistance of the heat pipe using nanofluids is significantly smallerthan without nanoparticles.

Chen et al. [11] discussed the performance of flat mesh wick heatpipe using water-based silver nanofluids with different nanoparticleconcentrations in the input power range of 20–40 W. The averagediameter of nanoparticles was 35 nm. The height and the length of theFHP used in the experiment were 3 mm and 200 mm, respectively.The size and the number of mesh layers were unknown. As shown inFig. 6, the total heat resistance of the heat pipe using nanofluids isreduced compared with that of the heat pipe using pure water. In thevolume concentration range tested, the larger the volume concentrationof nanoparticles, the higher the reduction of the heat resistance willbe. The authors assumed that themechanisms of heat transfer enhance-mentwere: (1) the increase of the wettability increases the critical heatflux; (2) the increase of the liquid thermal conductivity and the wickconductivity enhance the heat transfer.

Do et al. [44] and Liu and Zhu [45] experimentally observed the thinporous coating layer formed by nanoparticles suspended in nanofluidsat wick structures. Based on the observation, it is shown that the prima-ry mechanism on the enhancement of the thermal performance for theheat pipe is the coating layer formed by nanoparticles at the evaporatorsection because the layer can not only extend the evaporation surfacewith high heat transfer performance but also improve the surface wet-tability and capillary wicking performance.

Putra et al. [46] manufactured and tested screen mesh wick heatpipes were to determine the thermal resistance of nanofluids such asAl2O3–water, Al2O3–ethylene glycol, TiO2–water, TiO2–ethylene glycoland ZnO–ethylene glycol charged in the screen mesh wick heat pipes.The concentration of the nanoparticles was varied from 1% to 5% ofthe volume of the base fluid. The screen mesh wick heat pipe with thebest performance was that which used Al2O3–water nanofluid with 5%volume concentration. Using nanofluids in the heat pipes resulted inthe formation of a thin coating on the screen mesh surface from theelement of the nanoparticles. However, the thin coating promotesgood capillary structure. The higher thermal performance of heat pipescharged with nanofluids proved the potential of nanofluids as a substi-tute for conventional working fluids. This finding makes nanofluids at-tractive as working fluids in screen mesh wick heat pipes.

Kole and Dey [47] prepared fairly stable surfactant free copper-distilledwater nanofluids by usingprolonged sonication andhomogeni-zation. Thermal conductivity of the prepared nanofluid displays a

Fig. 5. Effect ofmass concentration of particles on the total heat resistance for ameshwickheat pipe using CuO nanofluids [43].

maximum enhancement of ~15% for 0.5 wt.% of Cu loading in distilledwater at 30 °C. The wall temperature distributions and the thermalresistances between the evaporator and the condenser sections of acommercial screen mesh wick heat pipe containing nanofluids are in-vestigated for three different angular positions of the heat pipe. The re-sults are compared with those for the same heat pipe with water as theworking fluid. The wall temperatures of the heat pipes decrease alongthe test section from the evaporator section to the condenser sectionand increase with input power. The average evaporator wall tempera-tures of the heat pipe with nanofluids are much lower than those ofthe heat pipe with distilled water as shown in Fig. 7. The thermal resis-tance of the heat pipe using both distilled water and nanofluids is highat lowheat loads and reduces rapidly to aminimumvalue as the appliedheat load is increased. The thermal resistance of the vertically mountedheat pipe with 0.5 wt.% of Cu-distilled water nanofluid is reduced by~27%. The observed enhanced thermal performance is explained inlight of the deposited Cu layer on the screen mesh wick in the evapora-tor section of the heat pipe.

Asirvatham et al. [48] presented the improvement in heat transferperformance of a heat pipe using silver nanoparticles dispersed in DI(De-Ionized) water. The nanoparticles suspended in conventional fluidshave superior heat transfer capability due to improved thermal conduc-tivity. The heat pipes are tested for heat inputs ranging from 20 W to100 W in five steps, which is suitable for removing heat from powertransistors in electronics and processors in computers. The effect ofvarious operational limits and test parameters such as heat inputs,volume fraction, vapor temperature on the thermal resistance, evapo-ration and condensation heat transfer coefficients, are experimentallyinvestigated. The tested silver nanoparticle volume concentrationranged from 0.003% to 0.009% with average nanoparticle diameterof 58.35 nm. The experimental results are evaluated in terms of per-formance metrics by direct measurement of vapor temperatures inthe center core of heat pipe. A substantial reduction in thermal resis-tance of 76.2% is observed for 0.009 vol.% concentration of silver nano-particles. Further an enhancement in the evaporation heat transfercoefficient of 52.7% is observed for the same concentration. The useof nanoparticles enhances the operating range of heat pipe by 21%compared with that of DI water.

3.3. Sintered metal wick heat pipe

Riehl [49] performed an experimental study on the thermal perfor-mance of the sintered metal wick miniature loop heat pipe (LHP)using nickel–water nanofluid. A simplewettability test of the nanofluidswasfirstly carried out in variouswickmaterials, whichwere hydrophilic

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Fig. 7. Distribution of wall temperatures in heat pipes for an input power of 100 W in the evaporator containing (a) distilled water and (b)–(d) different nanofluids inclined at differentangles [47].

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polyethylene, sintered nickel and copper. It was found that the water–nickel nanofluids did not present good wettability in the sinteredcopper; and solid nanoparticleswere separated even if thewick pore ra-dius was greater than that of the nanoparticles. Only sintered nickelwick could be applied in the LHP using water–nickel nanofluids. How-ever, the thermal performance was reduced. The authors explainedthe mechanism of heat transfer reduction as follows: the increaseof the density and the viscosity of the nanofluids would increasethe flow drag and reduce the capillary force in the sintered metal wickchannels.

Kang et al. [18] studied the total heat resistance of a cylindricalsintered wick heat pipe with the outer diameter and the length of6 mm and 200 mm, respectively. The heat pipe contained a 1 mm-thicksintered-wick made of copper powders. The nanofluids were madeof pure water and silver nanoparticles with the particle sizes of 10 nmand 35 nm, respectively. The tested nanofluid concentrations were1 mg/L, 10 mg/L and 100 mg/L. The investigated power range was30 W–70 W. The condenser section of the heat pipe was maintainedat 40 °C in all runs. The experimental results showed that themaximumheat loads of the heat pipe using nanofluids increased by 40% and thetemperature distributions of the evaporator sectionweremore uniformcompared with those of the heat pipe using water. The total heat resis-tance decreased by 88% for the 60 W heat loads. They considered thatthe reason for the heat transfer enhancement could be explained as fol-lows. Themaximum heat flux could be enhanced by higher wettability;nanoparticles could flatten the transverse temperature gradient of

the working fluid and reduce the boiling limit because of the increaseof the effective liquid conductance in the heat pipe. The heat resistanceof the heat pipe was reduced for the same reason.

3.4. Oscillating heat pipe

Nanofluid studies used in a vertical closed loop oscillating heat pipes(OHPs)were performed byMa's research team [50] and [51]. They usedalloy 122 copper tube with an inside diameter of 1.65 mm, an outer di-ameter of 3.18 mm and 12 turns. The experiment was carried out withthe heat load ranging between 0 and 336 W, the ambient temperaturemaintained at 10–70 °C and the internal filling ratio fixed at 50%. Thenanofluids consisted of the high-performance liquid chromatography(HPLC) grade water and 1.0 vol.% diamond nanoparticles with thediameter of 5–50 nm. The comparison of the total heat resistancebetween water charged OHP and nanofluid charged OHP is shown inFig. 8. It is evident that diamond nanoparticles significantly increasethe heat transport capability. The enhanced heat transfer mechanismwas considered as below: higher thermal conductivity, lower viscosityof nanofluids, and stronger oscillating motion of nanoparticles mightbe the primary factors enhancing the heat transport capability innanofluid charged oscillating heat pipe.

Shang et al. [52] investigated the heat transfer characteristics ofa closed loop OHP with Cu–water nanofluids as the working fluidunder different filling ratios. The results were compared with thoseof the same heat pipe with distilled water as the working fluid. The

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Fig. 8.Heat resistance comparison between a water charged OHP and a nanofluid chargedOHP, filling ratio = 50%, vertical, top = 20 °C [50].

Fig. 9. Thermal performances of the OHP using both nanofluid and water [53].

Fig. 10. Heat input vs. average heat resistance for 60% filling ratio [54].

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experimental results confirmed that the use of Cu–water nanofluids inthe heat pipe could enhance the maximum heat removal capacity by83%. It was confirmed that directly adding nanoparticles into distilledwater without any stabilizing agents had greater heat transfer enhance-ment compared to the case where a stabilizing agent was added to thedistilled water.

An experiment on the heat transport capability in a closed loopwell-balanced oscillating heat pipe using nanofluids carried out by Park andMa [53]. The well-balanced OHP was defined as a perfectly round ringwith no turns. The OHP was fabricated from copper tube with an insidediameter of 1.6 mm. It had six heating sections and six cooling sectionsspaced along the circumference. Between the heating section andthe cooling section was adiabatic section. The nanofluids consisted ofHPLC water and 1.0 vol.% CuNi nanoparticles with the diameters of40 nm to 150 nm. The heat pipe was tested in two situations: (1) with-out oscillating motions and (2) with oscillating motions. Although theheat transfer performance was improved after substituting nanofluidsfor the base fluid, the heat transfer enhancement effect was not signifi-cant for the first situation when the input power increased from 0 to20 W. The primary reason for this was that the CuNi nanoparticles inthe HPLC water settled on the bottom of the heat pipe due to the lackof oscillating motion. For the second situation, oscillating motionsoccurring in the heat pipe were very irregular and were different fromthose occurring in common OHP. The nanofluids could significantlyenhance the heat transfer in the heat pipe when the oscillating motionsexisted. Also, the impact of the nanofluids on the heat transport capa-bility depended on the filling ratio. As shown in Fig. 9, the thermalperformances of the OHP using both nanofluids and water have no dif-ferences for a power input of 30 W and a filling ratio below 40% andabove 70%. The heat pipe has its best heat transfer performance whenthe filling ratio is 50%.

Lin et al. [54] investigated experimentally the thermal performanceof a closed loop oscillating heat pipe using nanofluids. They appliedwater-based silver nanofluids at different volume fractions (100 ppmand 450 ppm) and various filling ratios (20%, 40%, 60%, and 80%). Thesilver nanoparticle had a diameter of 20 nm. Results showed that thethermal performance of the oscillating heat pipe using nanofluids wasbetter than that using water. The best filling ratio was reported to be60%. As shown in Fig. 10, there exists a best volume concentration of100 ppm. When the input power was 85 W, the average temperaturedifference between the inner wall of evaporator and the saturatedvapor decreased by 7.8 K, which is equivalent to a decrease of thetotal heat resistance of the heat pipe by 15%. The authors consideredthat the mechanism for the existing optimal volume fraction of silvernanofluids could be explained as follows: although the nanofluids

with higher concentration had higher thermal conductivity, highernanoparticle concentration resulted in higher viscosity; this causedmore difficulty to the bubbles growing and generated larger obstructionof the liquid slug, hence an optimal concentration would exist.

Bhuwakietkumjohn and Rittidech [55] investigated the internal flowpatterns and heat transfer characteristics of a closed-loop oscillatingheat-pipe with check valves. The ratio of number of check valves tomeandering turns was 0.2. Ethanol and a silver nano-ethanol mixturewere used as working fluids with a filling ratio of 50%. Results showthat the main flow pattern changes from a bubble flow with slug flowand annular flow to a dispersed bubble flow.

An experimental investigation was performed by Jian Qu et al. [56]on the thermal performance of an oscillating heat pipe (OHP) chargedwith base water and spherical Al2O3 particles of 56 nm in diameter.The effects of filling ratios, mass fractions of alumina particles, andpower inputs on the total thermal resistance of the OHP were investi-gated. Experimental results showed that the alumina nanofluids signif-icantly improved the thermal performance of the OHP, with an optimalmass fraction of 0.9 wt.% for maximal heat transfer enhancement. Com-pared with pure water, the maximal thermal resistance was decreasedby 0.14 °C/W (or 32.5%)when the power inputwas 58.8Wat 70%fillingratio and 0.9% mass fraction. By examining the inner wall samples, itwas found that the nanoparticle settlement mainly took place at theevaporator. The change of surface condition at the evaporator due tonanoparticle settlement was found to be the major reason for the en-hanced thermal performance of the alumina nanofluid-charged OHP.

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An experimental investigation has been carried out by Qu and Wu[57] to compare and analyze the different thermal performances ofthe oscillating heat pipes, which were charged with SiO2/water andAl2O3/water nanofluids, respectively. Both the average evaporator walltemperature and the overall thermal resistance of the OHPs at differentnanoparticle mass concentrations (0–0.6 wt.% for silica nanofluidsand 0–1.2 wt.% for alumina nanofluids) and at the volume filling ratioof 50% were tested and compared. Results showed that differentnanofluids caused different thermal performances of OHPs. Within theexperimental range, using the alumina nanofluid instead of pure waterenhanced the heat transfer of the OHP (reductions in the evaporatorwall temperature and thermal resistance of the OHP of about 5.6 °C (or8.7%) and 0.057 °C/W (or 25.7%), respectively, were obtained), whileusing the silica nanofluid instead of pure water deteriorated the thermalperformance of the OHPs. A preliminary analysis was conducted for thedifferent effects induced by the addition of different nanoparticles topure water, and it was found that the change of surface condition atthe evaporator and condenser due to different nanoparticle depositionbehaviors was the main reason for the thermal performance improve-ment or deterioration of the OHPs charged with different nanofluids.

The Al2O3 particle effect on the heat transfer performance of an oscil-lating heat pipe (OHP) was investigated experimentally by Ji et al. [58].Four size particles with average diameters of 50 nm, 80 nm, 2.2 μm, and20 μmwere studied, respectively. Fig. 11 shows that the Al2O3 particlesadded in the OHP significantly affect the heat transfer performance andit depends on the particle size. When the OHP was charged with waterand 80 nm Al2O3 particles, the OHP can achieve the best heat transferperformance among four particles investigated herein. In addition, it isfound that all particles added in the OHP can improve the startup per-formance of the OHP even with 20 μm Al2O3 particles.

Tanshen et al. [59] investigated an influence of multi-walled carbonnanotube (MWCNT) based aqueous nanofluids with different concen-trations on the heat transport and the relevant pressure distributionin oscillating heat pipe (OHP). They described the heat transfer phe-nomena in terms of thermal resistance, pressure and frequency of pres-sure fluctuation in multi-loop oscillating heat pipe (OHP) charged byaqueous nanofluids with MWCNT loadings of 0.05 wt.%, 0.1 wt.%,0.2 wt.% and 0.3 wt.%. The multi-loop OHP with 3 mm inner diameterhas been conducted in the experiment at 60% filling ratio. Experimentalresults show that thermal characteristics are significantly inter-relatedwith pressure distribution and strongly depend on the number of pres-sure fluctuations with time. The investigation shows that the 0.2 wt.%MWCNT based aqueous nanofluids obtain maximum number of thefluctuation frequency and low thermal resistance at any evaporator

Fig. 11. Particle size effect on (a) temperature difference

power input. Based on the experimental results, we discuss the reasonsfor enhancement and decrement of thermal characteristics of thenanofluids.

3.5. Two-phase closed thermosyphon

Xue et al. [60] carried out an investigation about the interface effectof carbon nanotube (CNT) suspension with surfactant on the thermalperformance of a closed two-phase thermosyphon. The test sectionwas a copper tube with an inner diameter of 20 mm. The filling ratioof the closed two-phase thermosyphon was 20%. The experimental re-sults in Fig. 12 show that the total heat resistance of the heat pipeusing CNT is higher than those of the heat pipe using water. It is alsoobvious that adding CNT in the base liquid deteriorated the thermalperformance of the heat pipe. It was found in this experiment that theCNT was broken to chips due to the addition of some acid liquids inthe CNT suspension to improve the stability of the suspension. Thechips of CNT settled on the evaporator surface formed a coating layerand significantly diminished the density and number of the activenucleation sites, the release frequency and the coalesced patches ofvapor bubbles.

Liu et al. [61] investigated the effect of nanoparticle parameters on thethermal performance in a miniature closed two-phase thermosyphonusing CuO nanofluids without surfactant. The test tube diameter, thelengths of the evaporation section, the insulation section and the con-densation section were 8 mm, 100 mm, 100 mm and 150 mm, respec-tively. The experiment was carried out at three operating pressures of7.45 kPa, 12.38 kPa and 19.97 kPa, respectively and the correspondingsaturation operating temperatures were 40 °C, 50 °C and 60 °C, respec-tively. The experimental results showed that adding nanoparticles inthe heat pipe could enhance both the heat transfer and the criticalheat flux. The operating temperature could significantly affect theheat transfer enhancement. The enhancement effect of nanofluidsincreased with the decrease of the operating temperature. For the CuOnanofluids heat pipe, the heat transfer coefficient increased by a maxi-mum of 160%, and the critical heat flux increased by 120% when anoptimal nanoparticle mass concentration of 1% was applied. The totalheat resistance can decrease about 30%–90% by substituting thenanofluids for water as the work liquid as shown in Fig. 13. At lowheat fluxes, the heat transfer enhancement is especially remarkable.They also [62] investigated the thermal performance in the samemini-ature closed two-phase thermosyphon using carbon nanotube (CNT)suspensions without surfactant. The experimental results are similarto those using CuO nanofluids.

and (b) thermal resistance (filling ratio: 50%) [58].

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Fig. 12. Heat resistances of closed two-phase thermosyphon using both CNT suspensionand water at different heating powers [60].

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Khandekar et al. [63] investigated the total heat resistance of aclosed two-phase thermosyphon using pure water and various water-based nanofluids containing nanoparticles of Al2O3 (40–47 nm), CuO(8.6–13.5 nm) and laponite clay (disks of diameter 25 nmand thickness1 nm). The length and the inner diameter of the test closed two-phasethermosyphonwere 720mmand 16mm, respectively. The experimen-tal results illustrate that the heat transfer performance of the closedtwo-phase thermosyphon using nanofluids was worse than that of theclosed two-phase thermosyphon using pure water. They consideredthat nanofluids had better surface wettability than water in the copperclosed two-phase thermosyphon and the improvement in wettabilityalong with entrapment of nanoparticles in the grooves caused the de-crease of Peclet number in evaporator section. These factors finallyleaded to poor thermal performance.

Noie et al. [64] combined twomentioned techniques for heat transferenhancement. Nanofluids of aqueous Al2O3 nanoparticle suspensionswere prepared in various volume concentrations of 1–3% and used in atwo-phase closed thermosyphon (TPCT) as working media. Differentvolume concentrations of nanoparticles (1–3%) in suspension withinthe TPCT were experimentally examined and results were comparedwith pure water. Nanofluids in all concentration studied showed betterthermal performance than pure water. They improved efficiency of theTPCT up to 14.7% as shown in Fig. 14. Temperature distributions on theTPCT were lower level using nanofluid compared to pure water. Tem-perature differences between the evaporator and condenser sectionswith nanofluids were less that pure water, i.e. thermal resistance of

Fig. 13. Total heat resistances of closed two-phase thermosyphon using water andnanofluid [61].

the TPCT when charged with nanofluids was less. The higher thermalperformance TPCTs loadedwith nanofluid proved its potential as substi-tute for conventional ones with pure water. This finding makesnanofluid attractive as working fluid in heat pipe and thermosyphontechnologies noting further investigation are needed.

Parametthanuwat et al. [65,66] investigated the effect of using silvernanofluid (de-ionized water mixed with silver nano and particles lessthan 100 nm) on the thermal characteristics of a two-phase closedthermosyphon. The thermosyphon was made with copper tubes with7.5, 11.1 and 25.4mm ID. The filling ratios of 30%, 50% and 80% by evap-orator length and aspect ratios of 5, 10, and 20with an inclination angle90°. Temperatureswere controlled so that the temperaturemeasured atthe adiabatic sectionwas constant at 40±4 °C, 50±4 °C and 60±4 °C.It was found that the filling ratio has no effect on the ratio of heat-transfer characteristics in the vertical position, but the properties ofthe working fluid affected the heat-transfer rate. In addition a correla-tion for predicting the heatflux for the two-phase closed thermosyphonin the vertical position has been established.

Huminic and Huminic [67] and Huminic et al. [68] presented theheat transfer characteristics of two-phase closed thermosyphon(TPCT) with iron oxide-nanofluids. The TPCT is fabricated from the cop-per tube with the outer diameter and length of 15 and 2000 mm, re-spectively. The TPCTs with the de-ionic water and nanofluids (waterand nanoparticles) are tested. The iron oxide nanoparticles with meandiameter of 4–5 nm were obtained by the laser pyrolysis techniqueand the mixtures of water and nanoparticles are prepared using an ul-trasonic homogenizer. Effects of TPCT inclination angle, operating tem-perature and nanoparticle concentration levels on the heat transfercharacteristics of TPCT are considered. The nanoparticles have a signifi-cant effect on the enhancement of heat transfer characteristics of TPCT.In Fig. 15, the heat transfer characteristics of TPCT with the nanofluidsare compared with that the based fluid.

Lu at al. [69] designed an especial open thermosyphondevice used inhigh-temperature evacuated tubular solar collectors. The indoor exper-imental research was carried out to investigate the thermal perfor-mance of the open thermosyphon using respectively the deionizedwater and water-based CuO nanofluids as the working liquid. Theeffects of filling rate, kind of the base fluid, nanoparticle mass concen-tration and operating temperature on the evaporating heat transfercharacteristics in the open thermosyphon were discussed. Experimentresults show the optimal filling ratio to the evaporator is 60% and thethermal performance of the open thermosyphon increase generallywith the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly

Fig. 14. Efficiency of TPCT versus input power and concentration of nanofluid [64].

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Fig. 15. Heat transfer rate distributions for different concentration levels at inclination angle of 90 [67].

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enhance the thermal performance of the evaporator and evaporatingheat transfer coefficients may increase by about 30% compared withthose of deionized water. The CuO nanoparticle mass concentrationhas remarkable influence on the heat transfer coefficient in the evapo-ration section and the mass concentration of 1.2% corresponds to theoptimal heat transfer enhancement.

Yang and Liu [70] carried out an experimental study to understandthe flow boiling heat transfer of water based CuO nanofluids in theevaporator of a thermosyphon loop under steady sub-atmosphericpressures. Experimental results show that both the heat transfer coeffi-cient (HTC) and the critical heat flux (CHF) of flow boiling in the evap-orator of the thermosyphon loop could be enhanced by substitutingnanofluids for water. The operating pressure has apparent impact onthe HTC enhancement of nanofluids. However, the operating pressurehas negligible effect on the CHF enhancement. There exists an optimalmass concentration of nanoparticles corresponding to thebest enhance-ment effect. Experimental results show that the CHF enhancementresults mainly from the existing of the coating layer on the heated sur-face formed by the sediment of nanoparticles. However, the HTC en-hancement results from the effects of both the existing of the coatinglayer and the change of thermophysical properties of the working fluid.

Kamyar et al. [71] examined the effects of using nanofluids on theperformance of a two-phase closed thermosyphon. Different concentra-tions (0.01%, 0.02%, 0.05% and 0.075%) of Al2O3 aswell as TiSiO4 particleswere dispersed in distilled water as base fluid. They focused on the re-sulted changes in temperature distribution, overall thermal resistanceof the thermosyphon and the heat transfer coefficient of the evaporatorsection. Various input powers (40–210W)were applied in the evapora-tor to see the behavior of nanofluid-filled thermosyphon in low andhigh heat loads. Compared with pure water, both nanofluids showedlower temperature distribution along the heat pipe. Reductions of upto 65% in thermal resistance were obtained for Al2O3 at the optimumvalue of 0.05 vol.%. However, for TiSiO4 the best performance was ex-plored to be at 0.075 vol.% where a reduction of 57% was found. Evapo-ration heat transfer coefficient also increased after using nanofluids. Therelative enhancement in boiling heat transfer coefficient was more sig-nificant at low powers. Although heat transfer coefficient improved byincreasing particle concentration for TiSiO4/water, it had the highestvalue at 0.05 vol.% for Al2O3/water showing a limit for increments inparticle concentration.

Huminic and Huminic [72] a 3D analysis is used to investigate theheat transfer of thermosyphon heat pipe using water and nanofluidsas the working fluid. The study focusedmainly on the effects of volumeconcentrations of nanoparticles and the operating temperature onthe heat transfer performance of the thermosyphon heat pipe usingthe nanofluids. The analysis was performed for water and γ-Fe2O3

nanoparticles, three volume concentrations of nanoparticles (0 vol.%,2 vol.% and 5.3 vol.%) and four operating temperatures (60, 70, 80 and90 °C). The numerical results show that the volume concentration

of nanoparticles had a significant effect in reducing the temperaturedifference between the evaporator and condenser. Experimental andnumerical results show qualitatively that the thermosyphon heat pipeusing the nanofluid has better heat transfer characteristics than thethermosyphon heat pipe using water.

Buschmann and Franzke [73] presented study aims to makenanofluids applicable for thermosyphons. Experiments employing avertical thermosyphon are carried out utilizing deionized water, waterbased titanium dioxide and gold nanofluids with different concentra-tions as working fluids. A maximal reduction of the thermal resistanceof about 24% can be achieved when nanofluids are employed. An opti-mum is reached at concentrations between 0.2 vol.% and 0.3 vol.%,whereas at higher concentrations the thermal resistance remains eitherunchanged or increases again. A nanoparticle layer on the evaporatorsurface seems to cause the found changes. Experiments with the goldnanofluid indicate that no nanoparticles are transported with thevapor phase and deposited on the condenser surface. Long-term exper-iments carried out with 0.3 vol.% indicate a massive aging of the porouslayer built of nanoparticles on the evaporator surface.

Over 70 relevant articles have been published since then involvingminiature micro-grooved heat pipe, mesh wick heat pipe, sinteredmetal wick heat pipe, oscillating heat pipe (OHP), and two-phase closedthermosyphon (TPCT).

4. Heat transfer characteristics of nanofluids in heat pipes

4.1. Experimental investigations

Many researchers have reported experimental studies on the ther-mal conductivity of nanofluids in heat pipes, thermal resistance andthermal efficiency of heat pipe. The heat pipe thermal efficiency canbe calculated from the ratio of cooling capacity rate of water at the con-denser section and supplied power at the evaporator section. The resultsfrom all the available experimental studies indicated that nanofluidscontaining a small amount of nanoparticles have substantially higherthermal conductivity than those of base fluids and also there is an in-crease in the thermal efficiency of heat pipe.

Naphon et al. [74] investigated the enhancement of heat pipe ther-mal efficiency with TiO2-alcohol nanofluids. The test section is fabri-cated from the straight copper tube with the outer diameter 15 mmand length 600 mm. In this, working fluids of heat pipe such as de-ionized water, alcohol, and nanofluids (alcohol and TiO2 nanoparticles)are tested. The diameter of TiO2 nanoparticles with 21 nm are used, inwhich the mixtures of alcohol and nanoparticles are prepared usingan ultrasonic homogenizer. The parameters considered are the effectsof percentage charge amount of working fluid, percentage nanoparticlevolume concentrations, and heat pipe tilt angle on the thermal efficien-cy of heat pipe. The nanoparticles added with the base fluid have a sig-nificant effect on the enhancement of thermal efficiency of heat pipe.

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The variation of heat pipe thermal efficiency with heat pipe tilt angle at66% charge amount of de-ionic water and alcohol has been calculated.The absorption heat capacity depends on the charge amount of workingfluid andmore space for the vapor ofworkingfluid. Due to that, the heatpipe thermal efficiency increases with increasing charge amount ofworking fluid. Maximum heat pipe thermal efficiency is attained atthe optimum condition of 45° tilt angle and 66% charge amount of alco-hol. The thermal efficiency of heat pipe is 10.60% higher than the baseworking fluid, with 0.10% nanoparticle volume concentration.

Chen [75] investigated the effect of flat heat pipe thermal perfor-mance using silver nano-fluid. The silver nanoparticles of size 35 nmand the pure-water as baseworkingfluidwere used. At the same chargevolume, the thermal resistance of heat pipe filled nano-fluid was lowerthan DI water. The reason for enhancement in the thermal performanceof FHP by using nano-fluid is higher wettability that enhances the capa-bility and flattens the temperature difference of FHP. The temperaturedifference and the thermal resistance of FHP with silver nano-particlesolution were lower than that with pure water. The result showedthat the silver nano-fluid not only enhanced the thermal performanceof traditional circular heat pipes but also increased the thermal perfor-mance flat heat pipe. This investigation concluded that further studieswill focus on the effect of the thickness of FHP, nano-fluid concentrationand the wettability effect of the nano-fluids on various geometry of theheat pipe wick to get the optimum thermal performance of heat pipe.

Ji et al. [76] experimentally investigated the alumina nanoparticleshape effect on the heat transfer performance of an OHP. A binary mix-ture of ethylene glycol (EG) and deionized water (50/50 by volume)was used as the base fluid for the OHP. Four types of nanoparticleswith shapes of platelet, blade, cylinder, and brick were studied. The re-sults showed that the alumina nanoparticles used in the OHP signifi-cantly enhance the heat transfer performance and it depends on theparticle shape and volume fraction. In the four types, cylinder-like alu-mina nanoparticles with EG can give the best heat transfer performanceof OHP. The previous research found that these alumina nanofluidswere not beneficial in laminar or turbulent flow mode; they can en-hance the heat transfer performance of an OHP.

Mousa [77] experimentally studied the effect of Al2O3–water basednanofluid concentration on the performance of a circular heat pipe.The operating parameters considered are working fluid filling ratio,volume fraction of nano-particle in the base fluid, and heat input. Ther-mal resistance decreases with increasing Al2O3–water based nanofluidcompared to that of pure water. The results showed that the optimumfilling ratio of charged fluid in heat pipe was about 0.45 to 0.50for both pure water and Al2O3–water based nanofluid, respectively,and that the thermal performance of heat pipe can be decreased by in-creasing concentration of the nanofluid.

Yang and Liu [78] investigated the thermal performance of function-alized nanofluid (silica nanoparticles) and traditional nanofluid (waterand same silica nanoparticleswithout functionalized) in a thermosyphonand observed that functionalized nanofluid canmaintain long-term sta-bility and without any sedimentation. Traditional nanofluids enhancethe maximum heat flux. Further, it was found that both functionalizedand traditional nanofluids have no effects on the condenser of thethermosyphon. It can be concluded that there are no meaningfulnanofluid effects on the thermal performance of thermosyphon.

Liu and Li [79] studied the effect of characteristics andmass concen-trations of nanoparticles on the thermal performance of heat pipes. theeffect of different nanofluids on the thermal performance of differentheat pipes like micro-grooved heat pipe, mesh wick heat pipe, sinteredmetal wick heat pipe, oscillating heat pipe and closed two-phasethermosyphon have been carried out. In miniature micro-groovedheat pipe, the effect of different nanoparticle sizes and nanoparticleconcentrations enhances the thermal performance of heat pipe. Theboiling heat transfer may occur at high heat fluxes in heat pipes withmicro-grooves, but it cannot occur in the mesh and sintered metalheat pipes. In oscillating heat pipes, the temperature gradient makes

a different volumetric distribution of the working fluid and causes pres-surewaves andfluid pulsations in each of the individual tube sections. Inclosed two-phase thermosyphon, the driving force of the fluid flow isthe buoyancy generated by the boiling two-phase flow. They concludedthat in majority of micro-grooved heat pipe, mesh wick heat pipe, oscil-lating heat pipe and most closed two-phase thermosyphon addition ofnanoparticles to the working liquid significantly enhances the heattransfer, reduces the total heat resistance and increases maximumheat removal capacity.

Saleh et al. [80] used a straight copper heat pipe with an outer diam-eter of 8 mm, an inner diameter of 7.44 mm and the length of 200 mm.A stainless steel wire screenmeshwith a diameter of 56.5 μmand 67.42strands per mmwas used. ZnO nanofluids were prepared using a two-step procedure with base fluid ethylene glycol (EG). They mainly mea-sured the temperature distribution and thermal resistance of the heatpipe filled with pure EG and ZnO nanofluids at concentrations from0.025 vol.% to 0.5 vol.%. The experimental data revealed that nanofluidscontaining a small fraction of nanoparticles had higher thermal conduc-tivities compared to the base fluid. The conductivity ratio could be en-hanced by approximately 5.3% until 15.5%. In addition, it was observedthat the temperature distribution and the heat pipe thermal resistancewere varied with the particle volume fraction and the size of the ZnOparticles.

Kole and Dey [47] experimentally investigated the thermal perfor-mance of screen mesh wick heat pipes using water-based coppernanofluids. In this study, no surfactant is added to the copper-distilledwater nanofluids. The different concentrations of Cu nanoparticles like0.0005 wt.%, 0.005 wt.%, 0.05 wt.% and 0.5 wt.% were prepared byusing ultrasonicator followed by magnetic stirring process. The dimen-sions of heat pipe are length 300 mm, outer diameter 10 mm and wallthickness 0.6 mm and material used is copper. The evaporator section,adiabatic section and condenser section of the heat pipe are 70 mm,80 mm and 150 mm, respectively. Thermal conductivity shows an en-hancement of approximately 15% with 0.5 wt.% loading of Cu nanopar-ticles. The results show that vertical heat pipes are found to performbetter than other inclinations. Cu-distilled water nanofluid of 0.5 wt.%reduced the thermal resistance by approximately 27%.

4.2. Theoretical investigations

Do and Jang [16] numerically investigated the effect of water-basedAl2O3 nanofluids as working fluid on the thermal performance of a flatmicro-heat pipe with a rectangular grooved wick. The axial variationsof the wall temperature and the evaporation and condensation ratesare considered by solving the one dimensional conduction equationfor the wall and the augmented Young–Laplace equation for the phasechange process. The thermophysical properties of nanofluids as wellas the surface characteristics formed by nanoparticles such as a thinporous coating are considered. The thin porous coating layer formedby nanoparticles suspended in nanofluids is a key effect of the heattransfer enhancement for the heat pipe using nanofluids. The effectsof the volume fraction and the size of nanoparticles on the thermalperformance were studied and the results showed that the feasibilityof enhancing the thermal performance up to 100% although water-based Al2O3 nanofluids with the concentration less than 1.0% is usedas working fluid. Finally, it could be concluded that the thermal resis-tance of the nanofluid heat pipe tends to decrease with increasing thenanoparticle size compared with the previous experimental results.

Shafahi et al. [81] theoretically studied the thermal performanceof cylindrical heat pipe with Al2O3, CuO and TiO2 by using two dimen-sional analyses. When using nanofluids, there is substantial changein the heat pipe thermal resistance, temperature distribution andmaximum capillary heat transfer of the heat pipe observed. By utilizingnanofluid resistance decreases as the concentration increases or asthe particle diameter decreases for the smaller size of the cylindricalheat pipe. In this study, the influence of nanofluid and the geometrical

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characteristics of the wick on the maximum heat load carrying capa-bility of the cylindrical heat pipe are investigated. The existence ofan optimum mass concentration and smaller particle in size providingthe highest thermal performance had been established.

The observations based on the reviewed literature on the heat pipesshowed that the thermal performance of heat pipe using nanofluidis higher than that of the base fluid. A majority of the results that areavailable are of experimental findings and the theoretical investigationsare limited. It is obvious that more research is needed in future in orderto validate the simulation model with the experimental findings.

5. Conclusions

This review describes the research results of heat transfer character-istics of various types of heat pipes using nanofluids as working fluids.The limited number of available references has shown that nanofluidshave great application prospects in various heat pipes. For the majorityof micro-grooved heat pipes, mesh wick heat pipes, oscillating heatpipes and most closed two-phase thermosyphon, adding nanoparticlesto theworking liquid can significantly enhance the heat transfer, reducethe total heat resistance and increase the maximum heat removalcapacity. At the same time, there are still some problems and challengeson the mechanisms of the heat transfer enhancement and the actualapplications. The present research of nanofluids in heat pipes is still atits initial stage and needs further development.

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