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Journal of Hazardous Materials 273 (2014) 183–191

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

s metal nanofluid reliable as heat carrier?

d.J. Ninea,b,∗, Hanshik Chunga, Md. Riyad Tanshena,.A.B. Abu Osmanc, Hyomin Jeonga,∗

Department of Energy and Mechanical Engineering, The Institute of Marine Industry, Cheondaegukchi-Gil 38, Tongyeong,yeongnam 650-160, South KoreaSchool of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, AustraliaDepartment of Biomedical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

i g h l i g h t s

A pre- and post experimental analysis of metal nanofluids charged in OHP.Characterization of metal nanofluids as heat carrier.Degradation in quality of metal nanofluids is analyzed.The risks of using metal nanofluids in heat exchanger are reported.

r t i c l e i n f o

rticle history:eceived 3 January 2014eceived in revised form 27 February 2014ccepted 20 March 2014vailable online 2 April 2014

eywords:etal nanofluids

a b s t r a c t

A pre- and post experimental analysis of copper–water and silver–water nanofluids are conducted toinvestigate minimal changes in quality of nanofluids before and after an effective heat transfer. A sin-gle loop oscillating heat pipe (OHP) having inner diameter of 2.4 mm is charged with aforementionednanofluids at 60% filling ratio for end to end heat transfer. Post experimental analysis of both nanofluidsraises questions to the physical, chemical and thermal stability of such suspension for hazardless usesin the field of heat transfer. The color, deposition, dispersibility, propensity to be oxidized, disintegra-tion, agglomeration and thermal conductivity of metal nanofluids are found to be strictly affected by

epositionxidationhermal conductivityeat transfer

heat transfer process and vice versa. Such degradation in quality of basic properties of metal nanofluidsimplies its challenges in practical application even for short-term heat transfer operations at oxidativeenvironment as nano-sized metal particles are chemically more unstable than its bulk material. The useof the solid/liquid suspension containing metal nanoparticles in any heat exchanger as heat carrier might

ole sy

be detrimental to the wh

. Introduction

Corrosion and fouling are common issues experienced duringhe lifetime of heat exchanger that can be initiated with theeposition of extraneous materials on the surface of heat transferevice. Such problem of heat exchanger is often the consequencesf sedimentation, lack of solubility, physical and chemical changesf unwanted solid particles into heat transfer fluid [1]. Neverthe-

ess researchers accepted the challenge of using the suspensionf nanoparticles (<100 nm) into conventional fluids to use aseat carrier because of its effectiveness in enhancing thermal

∗ Corresponding author at: Department of Energy and Mechanical Engineering,he Institute of Marine Industry, Cheondaegukchi-Gil 38, Tongyeong, Gyeongnam50-160, South Korea. Tel.: +82 1024603444; fax: +82 557729119.

E-mail address: mdjulker9@gmail.com (Md.J. Nine).

ttp://dx.doi.org/10.1016/j.jhazmat.2014.03.055304-3894/© 2014 Elsevier B.V. All rights reserved.

stem.© 2014 Elsevier B.V. All rights reserved.

conductivity of conventional heat transfer fluids [2]. This newlydeveloped fluid with enhanced thermal conductivity and upgradedrheological properties becomes more energy-efficient in advancedheat transfer applications [3]. Different metal particle in nanoscalesize have been introduced in the past decade to synthesis suchenergy-efficient metallic solid/liquid suspension. Nanoparticlesof copper (Cu) [4–11], silver (Ag) [12–20,28], iron (Fe) [21–25],aluminum (Al) [26–28], gold (Au) [12,29–32], titanium (Ti) [33],as well as metal alloys namely; aluminum copper (Al2Cu) [34,35],silver aluminum (Ag2Al) [35,36], have been proposed to disperseinto different conventional fluids namely; water, ethylene glycol,engine oil to work as heat carrier fluids to perform better thanoxide nanofluids [37]. These metallic suspension were proposed

to charge in compact heat exchanger [38], double pipe heatexchangers [39], shell and tube heat exchangers [19], micro chan-nel heat exchanger [40], heat pipe [9,33,41–43], to work underlaminar and turbulent flow regime [44] in convective heat transfer

1 ardous Materials 273 (2014) 183–191

[ensa

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2

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2

hadwo

1 wt% Cu/Water; 60% FR

0 20 40 60 80 100 120 140 160 180 200

Wal

l Tem

pera

ture

, o C

20

40

60

80

100

120Condenser Wall Temp. a t 8 0 WEvaporator Wall Temp. a t 8 0 WCondenser Wall Temp. a t 1 20 WEvaporator Wall Temp. a t 1 20 W

84 Md.J. Nine et al. / Journal of Haz

44–46], and in boiling heat transfer modes [47–49]. However, thenvision of noble application of metallic nanofluids is still haveot been of interest for practical applications due to problems ofedimentation, erosion, fouling and increased pressure drop, costnd difficulties in production [50,51].

This article reveals the post experimental changes in qualityf heat carrier metal nanofluids for the first time. The changesre identified in color of nanofluids, modification of heat trans-er surface, dispersibility, disintegration and agglomeration of solidarticle into solvent, thermal conductivity and the most impor-antly the propensity to be oxidized of metal nanoparticles. Suchesults made metallic heat carrier nanofluids questionable to theepeated heat transfer operation. In addition the oxidation of metalanoparticles by hydrolysis can be detrimental to valuable elec-ronics and other heat exchanges where the metal nanofluids aremployed to carry heat.

. Experiments

A single loop OHP is chosen as heat transfer device to exam-ne such quality analysis of nanofluid before and after an effectiveeat transfer. OHP is a sensitive heat transfer device having nega-ive pressure inside capillary pipe that assists to grow bubbles andelease bubbles at a temperature lower than boiling temperaturef working fluids and transfers heat from one end to another endithout pumping power [52]. The reason of choosing such heat

ransfer device is to take into account chaotic behavior of fluid flownside OHP [53] that allows solid particles remaining in dancing

ood to resist growing quick sedimentation. In addition OHP letsanofluid be experienced to work under boiling heat transfer andlaying with bubbles and liquid slug.

.1. Experimental device and operating condition

The experiment, as shown in Fig. 1, consists of a single loopeat pipe with an evaporative section coiled with electric heater,

condensing section connected to a constant cooling bath and aata acquisition system. The heat pipe is made by copper joinedith an acrylic tube of 2.4 mm inner diameter having a loop length

f 230 mm. However, the same inner diameter (2.4 mm) of OHP

Fig. 1. Schematic diagram o

Time (Sec )

Fig. 2. Temperature distribution recorded on OHP wall for Cu/water nanofluids.

was used by previous researchers to study effects of nanofluidsinside OHP [54,55]. The setup was oriented vertically where thedownward U-loop section of 100 mm was coiled by a silicon rubberheater and insulated with 2 cm thick glass wool. Evaporative heatload was varied from 20 W to 120 W (DC power system) measuredby the digital power meter (YOKOGAWA WT200, YOKOGAWA Elec-tric Corporation, Japan). The 100 mm of upper loop section waspushed inside specially made condensing tank and kept sealed.Condenser temperature was maintained 16 ◦C by using isothermalcooling unit. In the middle of evaporative and condensing section ofOHP, 130 mm acrylic adiabatic section was exposed to observe valorplug and bubble formation. T-type thermocouple was soldered tothe outer wall of OHP in both condensing and evaporative sectionsto measure the wall temperature distribution.

Wall temperature distribution of both condenser and evapora-tive section of OHP is shown in Fig. 2 while the experiment wasin operation. Both Cu–water and Ag–water nanofluids were indi-

vidually charged in OHP to work under a maximum temperaturerange 93 ◦C with corresponding evaporative heat load of 120 W.Each experiment for an individual heat input (e.g. 20 W or 40 W)was repeated three times to confirm the reproducibility of current

f experimental setup.

Md.J. Nine et al. / Journal of Hazardous Materials 273 (2014) 183–191 185

F ater nA or in fi

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ig. 3. Color of nanofluids, (A) Cu/Water nanofluids (before experiment); (B) Cu/Wg/Water nanofluids (after experiment). (For interpretation of the references to col

xperimental device. Post experimental sample of metal nanofluidsere extracted from OHP and collected for further analysis.

.2. Nanofluid preparation

Copper (Dark brown, 99.9% purity, mean diameter of <100 nm)nd silver (Gray, spherical, 99.9% purity, <100 nm) nanoparticlescollected from NTi Npowder, Daejeon, South Korea) were dis-ersed in de-ionized (DI) water having the particle concentrationf 1 wt%. No pH changers or surfactants were used to disperseanoparticles. Ultrasonic vibrator (Sonic Vibra-Cell VC-130PB) withltrasonic pulses of 130 W at 20 kHz was used for 1 h to disperse

ndividual sample. OHP was evacuated by a vacuum pump and thenharged with the prepared nanofluids at 60% filling ratio followingack-filling method [56].

.3. Characterization and instrumentation

The characterization of used nanofluid starts with the visualbservation of dispersion state. The changes in color of bothu–water and Ag–water nanofluids in post experimental sessionere easily distinct as shown in Fig. 3. In addition, nanofluid and

ts adjacent heat transfer surface of OHP (in evaporative section) isound rough formed by micro-scale discontinuous deposition ofanoparticles observed by Optical microscope OLYMPUS, modelX60MF. Transmission electron microscopy (TEM) (JEM-2100F,

EOL) was used to observe the morphological characteristics ofanofluids before and after the heat transfer experiment.

In order to justify the particle size measured by TEM, the parti-le sizing system (PSS, NICOMP 380, and Santa Barbara, California,SA) was used to measure sizes of particle in suspension beforend after heat transfer experiment. The zeta-potential (Malvernetamaster, Malvern Instrument Ltd., UK) measurement system issed to characterize the colloidal dispersibility of the both nanoflu-

ds. The zeta potential of nanofluids were measured at neutralpH = 7.0) zone. In addition, X-ray diffraction (XRD) was used toerify the crystalline nature of Cu and Ag nanoparticles before andfter heat transfer experiment. The measurements were taken with

Bruker AXS, D8 advance powder diffractometer, using Cu Ka radi-tion (l ¼, 1.5406 A). The elemental analysis of sample was doney Energy dispersive X-ray (EDX) spectroscopy measurements bydditional attachment of JEOL JSM 6390. Subsequently, the thermal

anofluids (after experiment); (C) Ag/Water nanofluids (before experiment) and (D)gure legend, the reader is referred to the web version of the article.)

characteristics of the Cu–water and Ag–water nanofluids wereexamined by a thermal conductivity analyzer (LAMBDA, F5Technologies GmbH, Willingshausen, Germany) operating at tem-peratures of 20–40 ◦C in intervals of 5 ◦C. The LAMBDA systemmeasures the thermal conductivity of fluids according to ASTMD2717. The operation of the instrument was based on the workingprinciples of the transient hot wire method used for nanofluids [57].The sample temperature was managed by a special double-jacketheating/cooling device, which provides a homogeneous tempera-ture distribution.

3. Results

3.1. Color of nanofluids before and after experiments

This is the first resultant evidence that made researchers sus-picious to the quality sustainability of metal nanofluids in heattransfer application. Suspension of dark-brown (Fig. 3(A)) Cunanoparticles into DI water is found to be turned into reddish-yellow (Fig. 3(B)) after few repeated heat transfer experimentsunder the temperature of 90 ◦C. In the same way, gray suspension ofAg nanoparticles was found slightly lighter than the original graycolor after the experiment subjected to the similar heat transferconditions. However, the color observation of nanofluids before andafter experiments suggested to do further analysis. Interestingly,such observation of post experimental color changing phenomenaof nanofluids is reported by previous researchers as well [58]. Theydescribed that the deposition of nanoparticles on heat transfersurface made the overall concentration less than its original con-centration and found to be light in color. In addition, such colorchanges are also sometimes observed by chemical instability ofnanofluids reported by Witharana et al. [59]. They found that sunlight triggered chemical reactions in propylene glycol fluids in thepresence of TiO2 nanoparticles which may have acted as a cata-lyst. However, the changes in color of Cu nanofluids in the currentexperiments are found to be irregular and uncommon comparing toAg nanofluids. This is assumed that the discoloration of Cu nanoflu-ids is because of low temperature oxidation of Cu nanoparticles at

oxidative environment by hydrolysis that converts dark-brown Cuparticles into yellow-reddish Cu2O.

Now the perception from aforementioned evidence on discol-oration of nanofluids might be significant to reassess longer lasting

186 Md.J. Nine et al. / Journal of Hazardous Materials 273 (2014) 183–191

rticles

ccc

3

sihtfseiesi

F(

Fig. 4. Inner Surface of evaporative section of OHP, (A) Cu nanopa

hemical stability of metal nanoparticles corresponding to its appli-ation field. Particularly such degradation of nanofluids is realhallenge to the future application of heat carrier metal nanofluids.

.2. Particles deposition on heat transfer surface

Deposition of nanoparticles, as shown in Fig. 4, on heat transferurface is very common phenomena particularly where nanofluids a heat transfer fluid. Even the deposition of nanoparticles oneat transfer surface is found to be very effective in boiling heatransfer performance due to change in surface roughness and sur-ace wettability. Generally, the bubble generation is related to theurface roughness and wettability. Wang and Dhir reported theffect of surface wettability on active nucleation site density dur-

ng pool boiling and correlated their experimental data to give thexpression of surface active nucleation site density [60]. Previouslyimilar deposition of nanoparticles was observed by Qu et al. [61]n OHP evaporative section. They observed that the deposition of

ig. 5. TEM micrographs, (A) Cu nanoparticles (before experiment); (B) Cu nanoparticles (aafter experiment).

(before experiment) and (B) Cu nanoparticles (after experiment).

nanoparticles on evaporative surfaces increases roughness and sur-face wettability that accelerates nucleate boiling to create morebubbles inside OHP charged with nanofluids. In addition, Do et al.[62] and Kole and Dey [63] observed the formation of coating layeron screen mesh wick of heat pipe and they reported that the depo-sition of nanoparticles on screen mesh wick assists to enhance heattransfer rate.

Surprisingly, none of previous researchers reported that suchdeposition of nanoparticles on heat transfer surface can be per-manent and assist to grow fouling and corrosion issues that areconsidered as major loss for heat exchanger in the world.

3.3. TEM analysis of nanofluids before and after experiments

The morphology of particles in nanofluids before and afterheat transfer experiment was studied by transmission electronmicroscope (TEM). Nanofluids containing Cu nanoparticles shownin Fig. 5(A) and (B) are before and after heat transfer experiment

fter experiment); (C) Ag nanoparticles (before experiment) and (D) Ag nanoparticles

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stmbbndoonhtttef

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Md.J. Nine et al. / Journal of Haza

espectively. The changes in morphology of Cu nanoparticles areound to be related to sizes, shapes and particle surface that madehe nanofluid unstable and agglomerated. Spherical Cu particlesre found to be irregular in shape and smaller in sizes after heatransfer experiment. Particles are found clustered and splitted aftereat transfer. Similar results are found for Ag–water nanofluidshown in Fig. 5(C) and (D) before and after heat transfer experimentespectively. Particles are found smaller than the original sizes andn agglomerated state. Agglomeration is known as the function ofime in nanofluids, but splitting into smaller particles are not reg-lar phenomena we observed in this experiment after an effectiveeat transfer. These splitting of particles in smaller and in irregularhape imply the changes of particle surfaces; hence the way ofhemical and thermal instability of metal nanoparticles are traced.

.4. Dispersion quality and particle size analysis of nanofluids

Better dispersion and stability is still a challenge of solid/liquiduspension especially for heat carrier nanofluids. Ultrasonic vibra-or, surfactant, wet and dry ball milling and hybridization are com-

on techniques employed to make the solid/liquid suspension sta-le for a while. Quick or slow sedimentation of nanoparticles at theottom of suspension are the ultimate result of these heat transferanofluids. Such sedimentation of nanoparticles causes unwantedeposition on heat transfer surface; reduces thermal conductivityf fluids, and changes viscous properties of fluids followed by theverall degradation of nanofluid’s noble properties. It is found thatanofluids are more prone to make agglomeration after couple ofeat transfer experiments and get quick sedimentation comparingo the unused nanofluids. Fig. 6 with sample A–D shows sedimenta-ion after 24 h of Cu nanofluids and silver nanofluids follows similarrends respectively. Fig. 5 (TEM micro graph) justified the postxperimental physical instability of nanofluids as nanoparticles areound in agglomerated after a heat transfer experiments.

Dispersion quality depends on different physical criteria,amely, particle concentration, particle sizes and purity, particledhesive property, pH of solution etc. The zeta potential indicateshe degree of repulsion between adjacent, similarly charged parti-

les in dispersion. A high zeta potential value confers the stabilityor molecules and particles that are small enough. When theotential is low, attraction exceeds repulsion and the dispersionill break and flocculate [64]. Dispersion state of Samples-A–D into

ig. 6. Sedimentation after 24 h, and corresponding zeta potential value (a) Sedimentaanofluids after experiment; (c) sedimentation Ag/Water nanofluids (before experiment)

Materials 273 (2014) 183–191 187

bottles is shown in Fig. 6 (left) to understand the dispersion qualityof Cu–water and Ag–water nanofluids after 24 h of Ultrasonica-tion. The all four samples in this experiment were taken underthe inspection of zeta potential measurement system at neutral(pH ∼ 7.0) zone and found supportive value to the agreement withthe visual observation of dispersion state shown in Fig. 6 (left).

Free-standing particle (Fig. 5(a) and (c)) and agglomerated parti-cle (Fig. 5(b) and (d)) are found different in mean diameter analyzedby particle size analyzer (PSA) shown in Fig. 7. Both Cu and Agparticles in base fluids are found larger in size after heat transferexperiment in spite of disintegration of particles. Such agglomer-ation causes quick sedimentation that results physical instabilityof solid/liquid suspension. Before heat transfer experiment Cu andAg particles wear identical in sizes that changes in sizes and inshapes observed in post experimental analysis. Particles are founddisintegrated with appreciable surface changes in post experimen-tal analysis (shown in Fig. 5) that changes particles surface adhesiveproperties and shapes. Surface changes with irregular shapes ofparticles are prone to make agglomeration into suspension.

3.5. Chemical stability of metal nanofluids before and afterexperiments

The doubt on chemical instability of metal nanoparticles underboiling heat transfer condition is justified by elemental analysis ofparticles from Cu–water and Ag–water nanofluids before and afterheat transfer experiment. Energy-dispersive X-ray spectroscopy(EDX) indicates the presence of Oxygen (O) element in Cu parti-cles retrieved post experimental Cu–water nanofluids (shown inFig. 8(B)). The atomic percentage ratio of Copper (Cu) and Oxy-gen (O) is found as 2.21:1. Such percentage ratio indicates the firstdegree oxidation of copper that implies cuprous oxide (Cu2O) orincomplete oxidation of Cu particles. Similar EDX pattern for Cu2Owas reported by Ritasalo et al. [65] where they studied micro struc-tural and mechanical characteristics of Cu–Cu2O composites. ForAg–water nanofluids oxygen (O) element is also noticeable (shownin Fig. 8(D)) but the atomic percentage ratio of Ag and O (56.80:1)does not assure oxidation of Ag nanoparticles.

The study and analysis of Cu particle hydrolysis under lowtemperature is not new. Researchers synthesized Cu2O by low tem-perature controlled oxidation of Cu particles [66,67]. In addition,the relevant mechanism of low temperature oxidation of copper

tion of Cu/Water nanofluids before experiment; (b) sedimentation of Cu/Water and (d) sedimentation Ag/Water nanofluids after experiment.

188 Md.J. Nine et al. / Journal of Hazardous Materials 273 (2014) 183–191

F ; (B) Ca

nM

fia

FA

ig. 7. Particle sizes analysis in colloid, (A) Cu/Water suspension before experimentnd (D) Ag/Water suspension after experiment.

anoparticles subjected to oxidative environment was reported by

ansour et al. [68] and Nine et al. [66].X-Ray Diffractions of all four samples are analyzed to be con-

rmed about the possible changes in material structure particularlyfter heat transfer experiment as shown in Fig. 9. XRD analysis

ig. 8. EDX micrographs, (a) Cu/Water nanofluids (before experiment); (b) Cu/Water nag/Water nanofluids (after experiment).

u/Water suspension after experiment; (C) Ag/Water suspension before experiment

reconfirmed the oxidation of Cu particles that resulted Cu–Cu2O

cermets (shown in Fig. 9(B)). The cermets of Cu–Cu2O is formed bythe incomplete oxidation of Cu nanoparticles inside OHP at oxida-tive environment. All the diffraction peaks are perfectly indexed toCu2O or Cu phases justified by the result reported by Feng et al. [69]

nofluids (after experiment); (c) Ag/Water nanofluids (before experiment) and (d)

Md.J. Nine et al. / Journal of Hazardous Materials 273 (2014) 183–191 189

F ofluidn

Oid(rA

3

dTegmb

FC

ig. 9. XRD analysis, (A) Cu/Water nanofluids (before experiment); (B) Cu/Water nananofluids (after experiment).

n the other hand Ag particles does not show any phase changesdentified by the diffraction peaks shown in Fig. 9(C) and (D). Theiffraction peaks indicates to those of pure face-centered cubicFCC), corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planesespectively, in good agreement with reported literature for pureg crystalline [70].

.6. Degradation in thermal conductivity of nanofluids

In order to validate transient hot wire device, thermal con-uctivity of DI water was measured at temperatures of 15–40 ◦C.hen the measured data of water were compared with the ref-

rence data presented in standard book [71]. The measured dataave reasonable agreement with the reference data with maxi-um deviation of 1.67% as shown in Fig. 10 (left). However the

ar-graph in Fig. 10 (right) shows changes in thermal conductivity

Working fluid : Water

Temperature, oC

10 15 20 25 30 35 40 45

Ther

mal

Con

duct

ivity

, (W

/mK

)

0.50

0.55

0.60

0.65

0.70Measured DataRefere nce Data [71 ]

ig. 10. Left: Comparison of thermal conductivity between measured and standard datu/Water nanofluids (after experiment); (C) Ag/Water nanofluids (before experiment) an

s (after experiment); (C) Ag/Water nanofluids (before experiment) and (D) Ag/Water

of collected samples. The degradation in thermal conductivity ofnanofluids (Cu–water and Ag–water) is noticed after heat trans-fer experiments. Cu–water nanofluid is affected severely thanAg–water nanofluid in this degradation of thermal conductivity.Post experimental Cu–water nanofluid shows 9% less thermal con-ductivity than the pre-experimental Cu–water nanofluids whereasAg–water nanofluid shows only 1.75% degradation from pre-experimental thermal conductivity value of that nanofluid. Lossof particles due to unwanted particle deposition (shown in Fig. 4)on heat transfer surface and oxidation of metal nanoparticles(shown in Fig. 8) are two reasons identified for such thermaldegradation of nanofluids. In addition, post experimental agglom-

eration and particle disintegration influences thermal conductivityof these two types of nanofluids. Concentration of nanoparticles ischanged because of particle loss by deposition that affects both ofCu–water and Ag–water nanofluids. However, the oxidation of Cu

SampleA B C D

Nor

mal

ized

The

rmal

Con

duct

ivity

[Knf

/Kbf

]

1.00

1.05

1.10

1.15

1.20Cu/Water Before ExperimentCu/Water Aft er ExperimentAg/Water Before ExperimentAg/Water After Experiment

Concentration 1 wt%At 2 5 oC

a. Right: Thermal conductivity, (A) Cu/Water nanofluids (before experiment); (B)d (D) Ag/Water nanofluids (after experiment)

190 Md.J. Nine et al. / Journal of Hazardous Materials 273 (2014) 183–191

Concentration 1 wt%

10 15 20 25 30 35 40 45

Ther

mal

Con

duct

ivity

[W/m

.K]

0.60

0.65

0.70

0.75Measure d Data _DI WaterCu/Water _Before Exp._Heating Up Cu/Water _Before Exp._Cooli ng Down Cu/Water_After Exp._Heating Up Cu/Water _After Exp._Cooli ng Down

Concentration 1 wt%

10 15 20 25 30 35 40 45

Ther

mal

Con

duct

ivity

[W/m

.K]

0.60

0.65

0.70

0.75Measure d Data _DI WaterAg/Water _Before Exp._Heating Up Ag/Water _Before Exp._Cooli ng Down Ag/Water _After Exp._Heating Up Ag/Water _After Exp._Cooli ng Down

nduct

napah

ppdnhpaatsFtnnatcailppcfssm

4

pcdmtpmn

i

Temperature [oC]

Fig. 11. Heating up and cooling down effects on thermal co

anoparticles made the Cu–water nanofluids thermally unstablend resulted sharp degradation in thermal conductivity as oxidearticles possess low thermal conductivity itself. pH of solution isnother thermo sensitive factor that might be changed because ofydrolysis of Cu nanoparticles [72].

Another interesting phenomenon is observed for both pre- andost experimental nanofluids that were not reported by contem-orary researchers. Heating up from low temperature and coolingown from high temperature affect thermal conductivity of steadyanofluid as shown in Fig. 11. Thermal conductivity recorded ineating up and cooling down mode, does not follow the similarath. Brownian motion of particles and time factor are identifieds two functions of such changes in thermal conductivity. Gener-lly, viscosity of base fluids reduces when temperature raises uphat increases the Brownian motion of nanoparticles into suspen-ion to enhance thermal conductivity linearly upward as shown inig. 11. Conversely, cooling down from high temperature increaseshe viscosity of base fluids that decreases Brownian motion ofanoparticles into suspension and reduces thermal diffusion intoanofluids. The deviation recorded between these two modes isttributed to the non-uniform Brownian motion of particles athe same temperature while cooling down and heating up. Suchontinuous process of thermal conductivity measurement takesbout 3 h by the system used in this experiment. Particles sed-mentation by the time might be another reason of achievingower thermal conductivity while cooling down from high tem-erature fluid. Previously Nguyen et al. [73] observed hysteresishenomenon in nanofluids viscous properties that raised seriousoncerns regarding the reliability of using nanofluids for heat trans-er enhancement purposes. The results from our experiment alsoupport previous investigation about the reliability and thermaltability of nanofluids as thermal properties of nanofluid are veryuch inter-related with rheological properties.

. Conclusion

Fresh nanofluid initially acts effectively with robust thermalroperties. However the sudden degradation in physical, chemi-al and thermal properties is the challenge to its industrial andomestic heat transfer applications. In addition, there are manyore challenges to overcome beyond the physical, chemical and

hermal instability of nanofluids. Cost, difficulties in productionrocess, viscosity, pumping power, availably, and reliability are theain reasons why nanofluids are not commercialized even after

early the 2 decades of its first introduction.

i) Heat carrier metal nanofluids are found completely unstablein boiling heat transfer application. Quick sedimentation of

Temperature [oC]

ivity, (A) Cu/Water nanofluids and (B) Ag/Water nanofluids.

particles causes unwanted deposition on heat transfer surfacethat may rise fouling and corrosion issues.

ii) Post experimental metal nanoparticles are found different insizes and shapes than that of original. Particles are more proneto disintegrate and agglomerate together after heat transferexperiment.

ii) Thermal conductivity of nanofluids severely affected by thephysical instability (aggregation, sedimentation, particle shapechanges) of nanofluids. Thermal properties of heat carrier metalnanofluids are found different in heating up and cooling downmode. The results support the hysteresis phenomena previouslyobserved in rheological properties of nanofluids.

iv) Metal nanoparticles are more prone to be oxidized at even lowtemperature. The hydrolysis of metal nanoparticles is identifiedto be responsible of physical, chemical and thermal instability.In addition, this can be detrimental to the sensitive and expen-sive heating and cooling electronics as well as to industrial heatexchangers.

However, such solid/liquid heat transfer fluids (nanofluids)apparently seem uniformly dispersed into solution for a while, butinstability is the real and final destination of these suspensions.

Acknowledgements

The research was supported by Basic Science Program throughthe National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (NRF-2011-0021376).

In addition, this research was partially supported by theMalaysia UM/MOHE/HIR grant (Project No: D000010- 16001).

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