An overview of Nanofluids: A new media towards green environment

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INTERNATIONALJOURNALOFENVIRONMENTALSCIENCESVolume3,No1,2012

Copyrightbytheauthors-LicenseeIPA-UnderCreativeCommonslicense3.0

ReviewarticleISSN09764402

ReceivedonJune2012PublishedonJuly2012433

AnoverviewofNanofluids:AnewmediatowardsgreenenvironmentGuptaH.K,AgrawalG.D,MathurJ

DepartmentofMechanicalEngineering,MalaviyaNationalInstituteofTechnology,[email protected]

doi:10.6088/ijes.2012030131042

ABSTRACT

Recent advancements in nanotechnology have originated the new emerging heat transfer

fluids called nanofluids. Nanofluids are prepared by dispersing and stably suspending

nanometer sized solid particles in conventional heat transfer fluids. Past researches have

shownthataverysmallamountofsuspendingnanoparticleshavethepotentialtoenhancethe

thermo physical, transport and radiative properties of the base fluid. Due to improved

properties, better heat transfer performance is obtained inmany energy and heat transferdevicesas compared totraditional fluidswhichopen the door for anewfieldof scientific

researchandinnovativeapplications.Theaimofthispaperistopresentthebroadrangeof

nanofluid based current and future applications. Some barriers and challenges are also

focusedforimplementingthesenewclassofworkingfluids.Atlastfutureopportunitiesin

nanofluidresearchareidentifiedanddirectionsaregivensothatthevisionofnanofluidcan

becompleted.

Keywords:Nanomaterials,Nanofluids,Properties,HeatTransferEnhancement.

1.Introduction

Nanotechnologyprovidesnewareaofresearchtoprocessandproducematerialswithaverage

crystallitesizesbelow100nmcallednanomaterials.Thetermnanomaterialsencompasses

a wide range of materials including nanocrystalline materials, nanocomposites, carbon

nanotubes and quantumdots.Xuan andLi (2000) explained that due to its nanostructural

features,nanomaterialsexhibitenhancedproperties(mechanical,thermal,physical,chemical),

phenomenonandprocesses than conventionalmaterials. Ingeneral,there arefourtypesof

nanomaterials: Carbon based nanomaterials (eg: Carbon nanotubes), Metal based

nanomaterials(metal oxides such asaluminiumoxides),Dendrimers (nanosizedpolymers)

andComposites(nanosizedclays).

Figure1:Schematiccrosssectionofnanofluidstructure


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AnoverviewofNanofluids:Anewmediatowardsgreenenvironment

GuptaH.KetalInternationalJournalofEnvironmentalSciencesVolume3No.1,2012

434

When thesenanopariclesaresuspended in conventionalfluids (water,oil,ethyleneglycol)

called nanofluids. A study of Kakac and Pramuanjaroenkij (2009) resulted that the

nanolayerworksasathermalbridgebetweentheliquidbasefluidandthesolidnanoparticles

andananofluidconsistsoftheliquidbasefluid,thesolidnanoparticlesandthenanolayersas

seeninfigure1.

Nanofluidsclearlyexhibitimprovedthermo-physicalpropertiessuchasthermalconductivity,

thermaldiffusivity,viscosityandconvectiveheattransfercoefficient.Thepropertychangeof

nanofluids depends on the volumetric fraction of nanoparticles, shape and size of the

nanomaterialsasshownbyYangetal.(2005).Increasedthermalconductivityofnanofluidin

comparisontobasefluidbysuspendingparticlesisshowninfable1.

Table1:Thermalconductivityofsomematerials,basefluidsandnanofluids

Materials Thermalconductivity(W/mk)

Copper 401MetallicMaterials

Silver 429

Silicon 148NonmetallicMaterials

Alumina(Al2O3) 40

Carbon CarbonNanoTubes(CNT) 2000

Water 0.613

Ethyleneglycol(EG) 0.253

Basefluids

Engineoil(EO) 0.145

Water/Al2O3(1.50) 0.629

EG/Al2O3(3.00) 0.278

EG-Water/Al2O3(3.00) 0.382

Water/TiO2(0.75) 0.682

Nanofluids

(Nanoparticle

concentration%)Water/CuO(1.00) 0.619

Due to novel properties of nanofluid it can be widely used for various heat transfer

applications of engineering including, automotive and air conditioning cooling, solar and

power plant cooling, cooling of transformer oil, improving diesel generator efficiency, in

nuclearreactoranddefenseandspaceasreportedbyXiangandArun(2008).

Even if the use of nanofluids will improve the overall properties and heat transfer

characteristics of base fluid and overcome the problems of poor suspension stability and

channelcloggingofmilliandmicroparticleseventhenthedevelopmentandapplicationsofnanofluidsmaybelimitedbyseveralfactorsduetouseofverysmallsizesolidparticleswith

verysmallconcentration.Thispaperfirstcoversvariouspotentialapplicationsofnanofluids

thenidentifiesparameterswhichchallengesfortheuseofnanofluidstovariousapplications

andinlastsuggestdirectionsforfutureresearchofnanofluids.

2.Preparationofnanofluids

To prepare nanofluids by suspending nanoparticles into base fluids, some special

requirements are necessary such as even suspension, durable and stable suspension, low

agglomerationofparticlesandnochemicalchangeoffluid.Therearethreegeneralmethods

usedforpreparationofstablenanofluid:(1)AdditionofacidorbasetoChangethepHvalueofsuspension(2)Addingsurfaceactiveagentsand/ordispersants todisperseparticles into


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fluid(3)Usingultrasonicvibration.Themostcommontwosteppreparationprocessisshown

inFigure2.

Figure2:Twosteppreparationprocessofnanofluid

Thesemethodscanchangethesurfacepropertiesofthesuspendedparticlesandcanbeused

tosuppresstheformationofparticleclustersinordertoobtainstablesuspensions.Theuseof

these techniques depends on the required application of the nanofluid. A summary of

preparationprocessofnanofluidsusedbydifferentresearchersisshownintable2.

Table2:Preparationmethodsofdifferentnanofluids

S.No. Nanofluid Method Surfactant Stability Reference

1. Al2O3-Water Two-Step No 24h Eastmanetal

(1997)

2. TiO2-Water Two-Step OleicAcid

andCTAB

Murshedetal

(2010)

3. Cu-Water Two-Step Lauratesalt 30h XuanandLi

(2000)

4. MWCNT-

Water

Two-Step SDS Hongetal

(2000)

5. Ag-Water Two-Step No 24h Godsonetal

(2005)

2.1Benefitsofuseofnanofluids

Nanofluids possess the following advantages as compared to conventional fluids which

makesthemsuitableforvariousapplicationsinvolvingheatexchange.

1. Absorptionofsolarenergywillbemaximizedwithchangeofthesize,shape,material,andvolumefractionofthenanoparticles.

2. Thesuspendednanoparticlesincreasethesurfaceareaandtheheatcapacityofthefluidduetotheverysmallparticlesize.

3. The suspendedNanoparticles enhance the thermal conductivitywhich resultsimprovementinefficiencyofheattransfersystems.


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4. Heating within the fluid volume, transfers heat to a small area of fluid andallowingthepeaktemperaturetobelocatedawayfromsurfaceslosingheatto

theenvironment.

5. Themixingfluctuationandturbulenceofthefluidareintensified.6. Thedispersionofnanoparticlesflattensthetransversetemperaturegradientof

thefluid.7. Tomakesuitablefordifferentapplications,propertiesoffluidcanbechanged

byvaryingconcentrationofnanoparticles.

3.ApplicationsofNanofluids

Nanofluidscanbeusedinbroadrangeofengineeringapplicationsduetotheirimprovedheat

transferandenergyefficiencyinavarietyofthermalsystems.Thefollowingsectiongivesa

briefideaofdifferentareasofnanofluidapplicationsbasedonavailableliteratures.

3.1Applicationsinautomotive

In automobile arena, nanofluids have potential application as engine coolant, automatic

transmissionfluid, brake fluid, gearlubrication, transmissionfluid, engineoilandgreases.

ThefirstapplicationincoolingautomaticpowertransmissionsystemdonebySenthilrajaet

al(2010)showthatCuOnanofluidshavethelowesttemperaturedistributionandaccordingly

thebestheattransferperformance.

3.1.1Nanofluidascoolant

Theuseofnanofluidsascoolantswouldallowforsmallersizeandbetterpositioningofthe

radiators. Therewould be less fluid due to the higher efficiency, coolant pump could be

shrunk and truck engines could be operated at higher temperatures allowing for more

horsepower.InastudydonebySaiduretal(2011)haveshownthattheuseofnanofluidsinradiators can lead to a reduction in the frontal area of the radiator by up to 10%. This

reductioninaerodynamicdragcanleadtoafuelsavingofupto5%.

3.1.2NanofluidinFuel

Itwas shownthat the combustion ofdiesel fuelmixedwith aqueous aluminumnanofluid

increasedthetotalcombustionheatwhiledecreasingtheconcentrationofsmokeandnitrous

oxideintheexhaustemissionfromthedieselengine.Itisduetothehighoxidationactivityof

pure Al which allows for increased decomposition of hydrogen from water during the

combustionprocess.

3.1.3NanofluidinBrakeFluids

During theprocessofbraking,theproducedheatcausesthebrakefluidtoreachitsboiling

point, a vapour lock is created that retards the hydraulic system from dispersing the heat

causedfrombraking.Itwillcreateabrakemalfunctionandposesasafetyhazardinvehicles.

Nanofluidswithenhancedcharacteristicsmaximizeperformanceinheattransferaswellas

removeanysafetyconcerns.

3.2Applicationsofnanofluidindomesticrefrigerator


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manyfactorsinwhichlongtermstabilityofnanofluidinsuspensionismajorreason.So,this

paper focuses many important challenges that should be solved in the near future. The

followingarethemostpressingissues.

4.1Poorlongtermstabilityofsuspension

Long term physical and chemical stability of nanofluids is an important practical issue

becauseofaggregationofnanoparticlesduetoverystrongvanderwallsinteractionssothe

suspension is not homogeneous. Physical or chemical methods have been applied to get

stable nanofluids such as (i) an addition of surfactant; (ii) surface modification of the

suspendedparticles;(iii)applyingstrongforceontheclustersofthesuspendedparticles.Lee

andChoi (1996) found that Al2O3 nanofluids kept after 30 days exhibit some settlement

comparedtofreshnanofluids.Particlessettlingmustbeexaminedcarefullysinceitmaylead

tocloggingofcoolantpassages.

4.2Increasedpressuredropandpumpingpower

Pressure drop development and required pumping power during the flow of coolant

determines the efficiency of nanofluid application. It is known that higher density and

viscosityleadstohigherpressuredropandpumpingpower.Therearemanystudiesshowing

significant increase of nanofluids pressure drop compared to base fluid. One of the

experimentalstudybyChoi(2009)calculated40%increaseofpumpingpowercomparedto

waterforagivenflowrate.

4.3Lowerspecificheat

An ideal heat transfer fluid should possesshigher valueof specific heat so the fluid can

exchangemoreheat.Previousstudiesshowthatnanofluidsexhibitlowerspecificheatthan

basefluid.Itlimitstheuseofnanofluidapplication.

4.4Highcostofnanofluids

Nanofluids are prepared by either one step or two step methods. Both methods require

advancedandsophisticatedequipments.Thisleadsto higherproductioncostofnanofluids.

Thereforehighcostofnanofluidsisdrawbackofnanofluidapplications.

4.5FutureResearch

As far as for better understanding ofnanofluids, further research isneeded.Animportant

focus for future research should be determining the key energy transport mechanism innanofluids.MostlyheattransferdependsonThermalconductivityofnanofluid.Thethermal

conductivityofnanofluids can bea functionofparameters such asparticle shape,particle

agglomeration etc. therefore future research should be focused on finding out the main

parametersaffectingthe thermalconductivityofnanofluids.Theoretical predictions should

beevaluatedintermsofagreementwithexperimentsregardingconcentration,particlesize

and temperature dependence. Currently, the available nanoparticles are limited and their

specificationsarenotaccurate.Thechallengingpoint istoobtainthedesirablenanoparticle

product.Thedevelopmentof thenanoparticleproductiontechniquewillbeveryhelpfulfor

thenanofluidresearch.Wehavetofacepublicconcernabouttheirsafetybothinproduction

and in use. Nanofluids engineerswould be prudent to pursue green designs by choosing

nontoxicorbiodegradable nanoparticles. Soin lastwecansay that low cost, highvolume


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An overview of Nanofluids: A new media towards green environment