Heat Transfer Performance Enhancement of Gravity Heat Pipes by Growing AAO Nanotubes on

12 Pages·2017·3.87 MB·English

inventions
Article
Heat Transfer Performance Enhancement of Gravity
Heat Pipes by Growing AAO Nanotubes on Inner
Wall Surface
HueiChuWeng* ID andMeng-HsuehYang
DepartmentofMechanicalEngineering,ChungYuanChristianUniversity,Taoyuan32023,Taiwan;
[email protected]
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:31May2018;Accepted:28June2018;Published:30June2018
Abstract: Inthispaper,theheattransferperformanceofgravityheatpipeswithanodicaluminum
oxide (AAO) wall surface is studied. The main purpose is to study the effects of the length and
diameterofAAOnanotubesonthetemperaturedistribution,overallthermalresistance,anddry-out
occurrence of gravity heat pipes charged with acetone under different input heat powers. AAO
nanotubeswerefirstgrownbyanodizingtheinnerwallsurfaceoftheevaporatorsectionofaluminum
alloygravityheatpipes. TheinfluencesofAAOnanotubelengthanddiameteronthetemperature
distribution,overallthermalresistance,anddry-outoccurrenceweretheninvestigatedbyathermal
performance test system. Experimental results show that increasing the AAO nanotube length
couldresultinreducedtemperaturevariationbetweentheevaporatorsectionandthecondenser
section,leadingtoreducedthermalresistance,anddelayeddry-outoccurrenceathigherheatinputs.
In addition, increasing the AAO nanotube diameter could also cause decreases in temperature
variationandoverallthermalresistance,butitcouldnothaveasignificanteffectontheoccurrence
ofdry-outphenomenon. Basedontheseresults,itcanbeconcludedthat,iftheanodicoxidation
treatmentisappliedtotheinnerwallsurfaceoftheevaporatorsectionofagravityheatpipe,itsheat
transferperformancecouldbesignificantlyimproved. Themaximumtemperaturedifferenceand
overallthermalresistanceoftheprocessedheatpipeare46.12%and58.68%lowerthanthoseofthe
unprocessedheatpipe,respectively;moreover,comparedtotheunprocessedheatpipe,themaximum
applicableinputheatpowertoavoiddry-outoccurrencecanbeincreaseduptoabout40%. Sucha
studycouldbeusedforcoolingpurposesinawiderangeofapplicationssuchaspassivecoolingof
electronicdevices,highlyefficientheatrecovery,andcleanroomairconditioning.
Keywords: gravityheatpipes;thermalperformance;anodicaluminumoxide;nanotubesize
1. Introduction
Withtherapiddevelopmentofelectronicdevices, humanshavebeenpursuingproductsthat
arelighter,thinner,smaller,andbetterperforming. However,whenmanyelectroniccomponentsare
operatedinasmallspace,theamountofwasteheatgeneratedrisesgreatly. Howtodischargethe
generatedwasteheatmoreeffectivelyhasbecomeoneofthekeytechnologiesforthemorerecent
developmentofelectronictechnology. Iftheelectroniccoolingtechnologyisclassifiedaccordingto
theworkingfluidused,themaindevelopmentdirectionscanbedividedintothreecategories: (1)air
cooling; (2) liquid cooling; and (3) phase-change cooling. Phase change cooling is widely used in
electroniccoolingtechnology,andheatpipesarepassivetwo-phaseheattransferdevicesthattransfer
heatbyevaporationandcondensation.
There are many different types of heat pipes, including two-phase closed thermosyphons,
capillary-drivenheatpipes, annularheatpipes, pulsatingheatpipes, micro/miniatureheatpipes,
Inventions2018,3,42;doi:10.3390/inventions3030042 www.mdpi.com/journal/inventions
Inventions2018,3,42 2of12
etc.[1]. Thesimplestthermosiphonsarejustgravity-aidedheatpipes(orsimplygravityheatpipes),
whichfeaturestheuseofgravitytoflowdownthecondensedworkingfluidtotheevaporatorsection.
Theyhaveunidirectionalheattransfercharacteristicsandcanbeusedasthermaldiodes.Therevolution
inmanufacturingtechnologyhasmadeitpossibletomanufacturenanomaterialsandapplythemto
heatpipes. Theapplicationofnanomaterialsingravityheatpipesismainlyusingfluidsuspensionsof
nanostructures(nanofluids)asworkingfluidsormodifyingheatpipesurfacestohavenanostructures.
Nanofluids are expected to provide enhancements in heat transfer due to the adding of
nanoparticlesinbasefluid. Xueetal.[2]performedanexperimenttoinvestigatethecarbonnanotube
effectontheheattransferperformanceofgravityheatpipes. Theresultsindicatedthatthealterations
ofsolid–liquid–vaporinterfacialpropertiesduetotheadditionofcarbonnanotubeschangetheboiling
mechanismandthusdeterioratetheheattransferperformance. Liuetal.[3]investigatedtheCuO
nanoparticle effect on the heat transfer performance of miniature gravity heat pipes. The results
confirmed that the use of nanoparticle suspensions can significantly enhance the heat transfer
performance. Khandekaretal.[4]usedpurewaterandwater-basedAl O ,CuOandlaponiteclay
2 3
nanofluidsasworkingfluidstoinvestigatedtheoverallthermalresistanceofgravityheatpipesand
observedthatallthesenanofluidsshowinferiorheattransferperformancethanpurewaterduetothe
increasedwettabilityandentrapmentofnanoparticlesinthesurfaceroughnessgrooves. Noieetal.[5]
studiedthewater-basedAl O nanofluidsofdifferentparticleconcentrationsinagravityheatpipe.
2 3
Theresultsrevealedthattheuseofnanofluidsasaworkingfluidcanincreasetheefficiencyofthe
heat pipe by 14.7% and make the temperature distribution of the heat pipe more uniform. Later,
Paramatthanuwatetal.[6]consideredwater-basedAgnanofluidsandfoundthattheheattransfer
capacitycanbeincreasedby70%. Tengetal.[7]consideredwater-basedAl O nanofluidsandfound
2 3
thattheheatpipeefficiencycanbeincreasedby16.8%. Huminicetal.[8]consideredwater-based
γ-Fe O /Fe O nanofluids and found that the heat transfer rate can be increased by 22.2%. Yang
2 3 3 4
and Liu [9] investigated the effect of stable nanofluid on the heat transfer performance of gravity
heatpipesusingawater-basednanofluidfunctionalizedbythecovalentbonding“Si-O-Si”asthe
workingfluid. Theresultsshowedthatthereisnomeaningfulstablenanofluideffect,andthemain
influenceonheattransferperformanceisthepresenceofthedepositionlayerontheheatedsurfaceof
theevaporatorduetotheprecipitationofthenanoparticlesinanunstablenanofluid. Chenetal.[10]
further used a new water-based SiO functionalized nanofluid as the working fluid to investigate
2
the stable nanofluid effect on the heat transfer performance of a loop gravity heat pipe without
consideringtheeffectsofheatedsurfacecharacteristics. Theresultswereconcludedthattheuseof
functionalizednanofluidcannotimprovetheheattransferperformance,andnoparticularnanoscale
effectswerefoundinthisstudy. Kamyaretal.[11]investigatedtheparticleconcentrationeffectson
theheattransferperformanceofaclosedgravityheatpipefilledwithwater-basedAl O andTiSiO
2 3 4
nanofluids. The results demonstrated that both nanofluids can improve the performance by 65%
forAl O and57%forTiSiO . BuschmannandFranzke[12]consideredwater-basedTiO andAu
2 3 4 2
nanofluids and found that the thermal resistance can be decreased by 24%. Asirvatham et al. [13]
furtherconsideredgraphene–acetonenanofluidsandfoundthatthethermalresistancecanbereduced
by70.3%. Morerecently,Zhaoetal.[14]usedawater-basedgraphenenanofluidastheworkingfluid
toinvestigatethethermalstart-upperformanceofsolargravityheatpipes. Theresultsindicatedthat
theuseofnanofluidsinsteadofwatercanreducethestart-uptimebyupto15.1%,therebyimproving
thethermalefficiencyofsolarenergycollection.
Inadditiontotheworkingfluid,modifyingthewallsurfacetohavenanostructuresisanother
novel method that is expected to enhance heat transfer. Solomon et al. [15] conducted a study to
investigate the thermal performance of a heat pipe with nanoparticles coated over the surface of
the screen mesh. The results revealed that the thermal resistance and heat transfer coefficient in
the evaporator of the heat pipe are, respectively, lower and higher than those in the conventional
evaporator;however,theyshowedtheoppositeresultsinthecondenser. Itwasalsofoundthat,inthe
evaporatorsection,thethermalresistancecanbereducedto40%andtheheattransfercoefficientcanbe
Inventions2018,3,42 3of12
increasedby40%. Solomonetal.[16]usedasimpleandcost-effectiveanodizingtechniquetoprepare
ananoporousfilmontheinnerwallofagravityheatpipechargedwithacetoneandtheninvestigated
itsheattransferperformance. Theresultsshowedthatthenumberofnucleationsitesoftheanodized
surfaceisatleast2–3timeshigherthanthatofthenormalsurface. Moreover,thehighernucleation-site
surfacehasasignificantpositiveeffectonthethermalperformanceoftheevaporator,butthethermal
performance of the condenser may be worse. Singh et al. [17] further studied the heat transfer
performanceofflatgravityheatpipeswithandwithoutanodizedinnersurfaces. Theanodizedsurface
hasaporedensitythatisnearly90%greaterthanthatofthenon-anodizedsurface. Theresultsshowed
thattheoverallthermalresistanceoftheanodizedheatpipeis20%lowerthanthatofthenon-anodized
heatpipe; moreover, comparedtothenon-anodizedheatpipe, themaximumenhancementinthe
heat transfer coefficients of the evaporator and condenser in the anodized heat pipe are 9% and
27%,respectively. Solomonetal.[18]usedanelectrochemicaldepositionprocesstostudytheheat
transfer enhancement of a gravity heat pipe with an inner thin porous copper coating, compared
withtheuncoatedheatpipe. Itwasfoundthattheheattransfercoefficientoftheevaporatorcanbe
increasedto44%whentheinclinationangleis45◦;moreover,themetal-coatedheatpipewasfoundto
performbetterthantheoxide-coatedheatpipe. Tharayiletal.[19]usedphysicalvapordeposition
asameansofproducingnanoparticlethinfilmstoinvestigatetheeffectofnanoparticlecoatingon
theperformanceofaminiatureloopheatpipechargedwithdistilledwater. Theirfindingsshowed
that,asthefilmthicknessincreases,thethermalresistancedecreasesandtheevaporatorheattransfer
coefficientincreases. Atathicknessof400nm,thethermalresistancecandecreaseby22.6%andthe
heattransfercoefficientcanincreaseby86%.
Althoughmanyresearchpapershaveaddednanoparticlesintheworkingfluidormodifiedthe
wall surface to improve the heat transfer performance of gravity heat pipes, the influence of inner
surfacemorphologyofgravityheatpipesontheheattransferperformancehasnotbestudied.Thiswork
aimedtoyieldafundamentalunderstandingoftheapplicationofanodicaluminumoxide(AAO)wall
surfaceinagravityheatpipe. TheexperimentswerecarriedoutfordifferentsizesofAAOnanotubes
grownbyanodizingtheinnerwallsurfaceoftheevaporatorsectionofaluminumalloygravityheat
pipes. TheeffectsofthelengthanddiameteroftheAAOnanotubesonthetemperaturedistribution,
overallthermalresistance,anddry-outoccurrenceweretheninvestigatedanddiscussedfortheheat
inputrangeof25–200W.
2. ResearchMethod
2.1. GrowthofAAONanotubes
Inthisstudy,ananodicoxidationtreatmentmethodwasusedtogrowanodicaluminumoxide
(AAO)nanotubesontheinnerwallsurfaceoftheevaporatorsectionofaluminumalloygravityheat
pipes.TheprocessofgrowingtheAAOnanotubeswasaccomplishedbyimmersinganaluminumor
aluminumalloysubstrateinanacidicelectrolytesolutionandpassingavoltagebetweentheanodeand
thecathode.Itcanbedividedintothreestages.Inthefirststage,duetothehighelectricalconductivityof
aluminum,aluminumions(Al3+)canbeeasilydissolvedonthewallsurfaceofthealuminumsubstrate
andreactwithoxygenionsfromtheelectrolytetogeneratealumina(Al O ),therebyrapidlyforming
3 2
a dense barrier layer. In the second stage, the resistance in the anodization circuit increases as the
barrierlayerthickens.Whenthethicknessofthebarrierlayerreachesacertainvalue,theelectricfield
startstoconcentrateonthelocalimperfectionsonthebarrierlayer,resultinginanon-uniformoxide
thickeningandformingindividualpenetrationpathsintotheinitialpores.Inthethirdstage,thebottom
oftheporescontinuouslydissolvesaluminumionsduetotheconcentrationoftheelectricfielduntilthe
dissolutionofaluminumionsandthegrowthofaluminareachadynamicequilibrium. Atthispoint,
thecurrentinthecircuitremainsalmostconstantandAAOnanotubesgrowsteadily. Byincreasing
the treatment time, the length of the AAO nanotubes can be extended; by increasing the anodizing
voltage, thediameteroftheAAOnanotubescanbeenlarged(duetotheconcentrationeffectofthe
Inventions2018,3,42 4of12
electric field). To form regularly arranged pore nanostructures, different electrolytes may be used.
Eachelectrolytehasitsapplicableoperatingvoltagerange. Fromlow-voltagerangetohigh-voltage
range,thesuitableelectrolytesaresulfuricacid(H SO ),oxalicacid(H C O ),andphosphoricacid
2 4 2 2 4
(H PO )[20].ItshouldbenotedthattheAAOnanotubelengthL(inunitofµm)anddiameterD(inunit
3 4
ofnm)areapproximatelyproportionaltothetreatmenttimet(inunitofhour)andanodizingvoltage
V (inunitofV)[20–22],respectively. Inourexperiments,usingoxalicacidelectrolytetogrowAAO
nanotubes,theempiricalformulasforpredictingthenanotubesizeare
L=2t (1)
D=2V−10 (2)
Theaccuraciesofthetwoformulasarewithin1.96%and7.78%relativeerrors,respectively.
2.2. ExperimentalDesignandSetup
In this section, the relevant experimental design and setup, including the anodic oxidation
treatment,heatpipeprocessing,andthermalperformancetest,arereported.
2.2.1. AnodicOxidationTreatment
6063-T5aluminumalloywasusedasthebasematerialforthegravityheatpipesinthepresent
study. Thealuminumalloypipesfabricatedbyamachiningprocesshaveanouterdiameterof19mm,
aninnerdiameterof17mm,andapipelengthof300mm,asshowninFigure1. Theratiobetween
evaporatorsection,adiabaticsection,andcondensersectionis1:1:1. TogrowAAOnanotubesonthe
innerwallsurfaceoftheevaporatorsectionofaluminumalloygravityheatpipes,thefollowinganodic
oxidationtreatmentprocedure,alsoshowninFigure2,wasdesignedforeachpipe:
1. Anopen-endedhollowaluminumalloypipewasannealedat450◦Cfor3htoreducetheinternal
stressconcentration.
2. Afirstmechanicalpolishingwasperformedonthepipeusing#600,#1200,and#2000sandpapers,
sequentially;then,asecondmechanicalpolishingwasperformedusingapolishingfleecewith0.1µm
aluminapowdertomaketheinnerwallsurfaceofevaporatorsectionsmoother.Thepipewasthen
immersedinacetone(CH COCH )andethanol(C H O),respectively,washedfor30minusingan
3 3 2 6
ultrasoniccleaner,andrinsedwithdeionized(DI)watertoremovesurfaceimpuritiesandoil.
3. Thepipewasconnectedtotheanode,acopperbarwasconnectedtothecathode,andbothwere
thenimmersedinanelectrolyticsolution(2:1:1ratioofphosphoricacid(H PO ),sulfuricacid
3 4
(H SO )andDIwater)andsubjectedtoelectrolyticpolishingatacurrentof15Atoensurethat
2 4
anyscratchesleftaftermechanicalpolishingwereremoved. After90sofpolishing,thepipewas
takenoutandallowedtocooltoroomtemperature. Thesamechemicalpolishingprocesswas
completedthreetimes. ItshouldbenotedherethataPETinsulationtapewasattachedtothe
outerwallofthepipetoblockthechemicalpolishingontheoutsideofthepipe. Finally,thepipe
wasrinsedwithDIwatertoensureresidualelectrolyteisremoved.
4. The pipe was reconnected to the anode, a 0.25-inch-diameter copper bar was placed in the
centerofthepipeandconnectedtothecathode,andbothwerethenimmersedinanoxalicacid
(H C O )electrolytewithaconcentrationof0.3M.Aparticularanodizingvoltagewasappliedto
2 2 4
thefirstanodizingtreatmentstepataworkingtemperatureof5◦C.Itshouldbenotedherethata
micropumpandamagnetstirrerareusedtoensurethattheconcentrationofelectrolyteduring
thetreatmentisrelativelyuniform. Thetreatmenttimewasmaintainedat2h, and, afterthe
treatmentwascompleted,theresidualelectrolytewasremovedbyrinsingwithDIwatertoobtain
aninitialAAOsurface.
5. Theanodizedpipewasthenimmersedinundilutedphosphoricacid(H PO )forsurfaceremoval
3 4
withthehelpofanultrasonicbath. After2hsurfaceremoval,aregularlyarrangedbarrierlayer
Inventions2018,3,42 5of12
onsurfacewasleft. Acetoneandethanolwereagainused,respectively,toultrasonicallywash
thepipefor30min,andDIwaterwasthenusedtoremovetheresidualphosphoricacidonthe
Inventions 2018, 3, x FOR PEER REVIEW 5 of 12
Inventions 2018, 3, x FOR PEER REVIEW 5 of 12
surface. TheAAObarrierlayercouldserveasabasisforfurthergenerationofAAOnanotubes.
6. Thsseuusrrffeaacccoeen.. dTThhaeen AAodAAiOOzi nbbaagrrrrtirieeerra lltaamyyeeernr ctcoowuulaldds sspeeerrvrvfeeo arasms aae bdbaasosiinss fftoohrre ffupurritpthheeerrw g gieetnhneertrahatetioioAnn Ao ofOf A AbAAaOrOr n inearannolaotuytuebbreesis.n . the
66..p reTTvhhieoe ussseeccmoonnaddn naaennrootddoiizzoiibnntgga ittnrreeAaattAmmOeennntta wwnoaastsu ppbeeerrsffooorrnmmtehedde oionnnn tethhreew ppaiiplplees u wwrifitathhc e tthhoeef AtAhAeAOeOv b abaparorrrireiaertr ol alraysyeeercr ti inion nt hthoeef the
previous manner to obtain AAO nanotubes on the inner wall surface of the evaporator section of
hepatrepviipoeu.sI nmtahninsetrr etoat ombetanitns AtaAgeO, tnhaenoletnugbtehs oonf AthAe Oinnnearn wotaullb seusrifnaccree oafs ethde wevitahpothraettorre asetmctieonnt otifm e.
the heat pipe. In this treatment stage, the length of AAO nanotubes increased with the treatment
the heat pipe. In this treatment stage, the length of AAO nanotubes increased with the treatment
7. Inourexperiments,theanodizingvoltageappliedwasintherangeof20–50V,andthetreatment
time.
time.
7.t imIne ocounr seuxpmeeridmwenatssi, nthteh aenroadnigzeinogf v3o–l9tahg.eT ahpeplaiveder wagaes idni athme ertaenrgaen odf 2le0n–g50th Vo, fatnhde tAheA trOeantamneonttu bes
7. In our experiments, the anodizing voltage applied was in the range of 20–50 V, and the treatment
metiamsue rceodnsbuymsecda nwnaisn ign tehlee crtarnogne mof i3c–r9o shc. oTphee a(SvEerMag)ei dmiaamgientegr manedt hleondgtahr eofa tlhme oAsAtOp rnoapnoorttuiboensa lto
time consumed was in the range of 3–9 h. The average diameter and length of the AAO nanotubes
themaenaosudriezdin bgy vsocaltnangienga neldectthroent rmeaictrmosecnotptei m(SeE,Mre) sipmeacgtiinvge lmy,eatshosdh oawren alimnoEsqt uparotipoonrsti(o1n)aal ntod (2).
measured by scanning electron microscope (SEM) imaging method are almost proportional to
Figthuer ean3osdhizoiwngs vtholetaSgEe Manidm thage etsreoatfmAeAntO timnaen, roetsupbeectsivuerlfya, caess sohbotwanin iend Eaqtupatairotnics u(1la) ranand o(d2)i.z ing
the anodizing voltage and the treatment time, respectively, as shown in Equations (1) and (2).
voFFltiiaggguuerrsee (332 0sshhVoowwanss dtthh5ee0 SSVEE)MMa n iidmmapaggaeersts i coouff lAAarAAtOOre anntaamnnoeotntuutbbteei m ssueursrffa(a3cceehss aoonbbtdtaai7innehed)d. aatt ppaarrtitcicuulalarr aannooddiziziningg
voltages (20 V and 50 V) and particular treatment times (3 h and 7 h).
voltages (20 V and 50 V) and particular treatment times (3 h and 7 h).

Figure 1. Aluminum alloy pipe: (a) Schematic drawing; and (b) photograph.
Figure1.Aluminumalloypipe:(a)Schematicdrawing;and(b)photograph.
Figure 1. Aluminum alloy pipe: (a) Schematic drawing; and (b) photograph.

Figure 2. Anodic oxidation treatment procedure for each pipe.
Figure 2. Anodic oxidation treatment procedure for each pipe.
Figure2.Anodicoxidationtreatmentprocedureforeachpipe.
Inventions2018,3,42 6of12
Inventions 2018, 3, x FOR PEER REVIEW 6 of 12
Inventions 2018, 3, x FOR PEER REVIEW 6 of 12

Figure 3. (a) AAO nanotube diameters obtained at anodizing voltages of 20 V and 50 V; and (b) AAO
Figure3.(a)AAOnanotubediametersobtainedatanodizingvoltagesof20Vand50V;and (b)AAO
nanotube lengths obtained at treatment times of 3 h and 7 h.
nanoFtiugubreel e3.n (gat)h AsAoObt naianneodtuabtet dreiaamtmeteenrst otibmtaeisneodf 3ath anaonddiz7inhg. voltages of 20 V and 50 V; and (b) AAO
2.2.2n.a Pnrootucebses lienngg athnsd o Pbtearifnoerdm aat ntrceea tTmesetn t times of 3 h and 7 h.
2.2.2. ProcessingandPerformanceTest
2.2.2. PTrhoec mesasiinn gb oadnyd oPfe trhfoe rhmeaatn pceip Tee usst ed in this study is an anodized open-ended hollow aluminum
Talhloeym paipine. bTohde ypiopfe twheash feirastt pimipmeeursseedd inin etthhaisnostl uadndy cisleaanneadn woditihz eand uolptreanso-ennicd celdeahnoerl lfoowr 2a0l ummini num
The main body of the heat pipe used in this study is an anodized open-ended hollow aluminum
alloyapnidp eth.eTnh reinpsiepde wwitahs DfiIr swtaitmerm toe resnesdurien ietst hinatneornlaaln sdurcfalecaen weads wclietahn aanndu lftrreaes oofn iimcpclueraitnieesr. fNoerx2t0, min
alloy pipe. The pipe was first immersed in ethanol and cleaned with an ultrasonic cleaner for 20 min
andtbhoetnh ritisn soepdenw einthdsD wIewrea tseeralteode bnys uwreelditisngin, taenrdn aolnseu ernfadc ewwasa esqculiepapneda nwditfhr eaen oafnitmi-lpeaukr ivtiaelvs.e.N ext,
and then rinsed with DI water to ensure its internal surface was clean and free of impurities. Next,
Evacuation and working fluid (acetone) filling were then performed down to a pressure of 5 kPa and
both its open ends were sealed by welding, and one end was equipped with an anti-leak valve.
buopth t oit as no apmeno uenntd osf w40e mreL s etharleodu gbhy t hwise lvdailnvge, toan cdo mopnlee teen adn walausm einquumip paelldo yw hiethat apnip aen htia-vleinakg AvaAlvOe .
Evacuationandworkingfluid(acetone)fillingwerethenperformeddowntoapressureof5kPaand
Envaancoutautbioens aonnd th we oinrknienrg w flaulli dsu (arfcaecteo noef )t hfiel leinvga pwoerareto trh seenc tpioernf,o arsm sehdo wdonw inn Ftoig au rper e4s. sure of 5 kPa and
uptoanamountof40mLthroughthisvalvetocompleteanaluminumalloyheatpipehavingAAO
up to an amount of 40 mL through this valve to complete an aluminum alloy heat pipe having AAO
nanontaunboetusboens tohne thinen inernewr awllasllu srufarfcaeceo foft htheee evvaappoorraattoorr sseeccttiioonn, ,aass shshoowwn nini nFiFguigrue r4e. 4.

Figure 4. Gravity heat pipe: (a) Schematic drawing; and (b) photograph.

Figure 4. Gravity heat pipe: (a) Schematic drawing; and (b) photograph.
Figure4.Gravityheatpipe:(a)Schematicdrawing;and(b)photograph.
Inventions 2018, 3, x FOR PEER REVIEW 7 of 12
Inventions2018,3,42 7of12
In this study, a thermal performance test system was set up, as shown in Figure 5. The test system
can be divided into three zones: the heating zone, the transportation zone, and the cooling zone. Each
Inthisstudy,athermalperformancetestsystemwassetup,asshowninFigure5. Thetestsystem
zone was covered with three layers of insulation material (fireproof cotton, calcium silicate board,
can be divided into three zones: the heating zone, the transportation zone, and the cooling zone.
and polyethylene foam board) to ensure that no heat exchange with the surrounding environment
Eachzonewascoveredwiththreelayersofinsulationmaterial(fireproofcotton,calciumsilicateboard,
occured during the test, resulting in heat loss. The evaporator section of the heat pipe was placed
andpolyethylenefoamboard)toensurethatnoheatexchangewiththesurroundingenvironment
vertically in the heating zone, where the heater was connected to a DC power supply to provide the
occuredduringthetest, resultinginheatloss. Theevaporatorsectionoftheheatpipewasplaced
required thermal power. The condenser section of the heat pipe was placed vertically in the cooling
verticallyintheheatingzone,wheretheheaterwasconnectedtoaDCpowersupplytoprovidethe
zone, and the temperature inside the cooling zone was maintained at 25 °C by a circulating water
requiredthermalpower. Thecondensersectionoftheheatpipewasplacedverticallyinthecooling
bath. Three K-type thermocouples were embedded in each area with equal distance (30 mm) and
zone, andthetemperatureinsidethecoolingzonewasmaintainedat25 ◦Cbyacirculatingwater
connected to a temperature recorder to fully record the exact temperatures during the test. Through
bath. ThreeK-type thermocoupleswere embedded ineach area withequal distance(30 mm)and
the measured temperature data, the thermal performance of the gravity heat pipe can then be further
connectedtoatemperaturerecordertofullyrecordtheexacttemperaturesduringthetest. Through
analyzed. This study used the overall thermal resistance of the gravity heat pipe (R) as one of the
themeasuredtemperaturedata,thethermalperformanceofthegravityheatpipecanthenbefurther
key parameters to investigate the heat transfer performance of gravity heat pipes, defined as the ratio
analyzed. Thisstudyusedtheoverallthermalresistanceofthegravityheatpipe(R)asoneofthekey
of the difference between the average temperature of the evaporator section (T ) and the average
parameterstoinvestigatetheheattransferperformanceofgravityheatpipes,definedeastheratioofthe
temperature of the condenser section (T ) to the input heat power from the heating zone (Q ). The
differencebetweentheaveragetemperaturceoftheevaporatorsection(T )andtheaveragetempeirnature
e
ofrethlaeticoonnsdheipn sies rasse fcotliloonw(sT: )totheinputheatpowerfromtheheatingzone(Q ). Therelationshipis
c in
asfollows:
T T
R =RTe−eQTcc ((33))
Q in
in
TThehea basboslouluteter erlealtaitvieveu uncnecretratianitnytyo fotfh teheq uquanatnittiytyd defiefniendedin inth tehea baobvoevee qeuquataitoinonw wasase setsitmimataetdedto to
bebele lsesssth tahnan2 .27.77%7%. .

Figure5.Cont.
Inventions2018,3,42 8of12
Inventions 2018, 3, x FOR PEER REVIEW 8 of 12

Figure 5. Thermal performance test system: (a) Schematic drawing; and (b) photograph.
Figure5.Thermalperformancetestsystem:(a)Schematicdrawing;and(b)photograph.
3. Results and Discussion
3. ResultsandDiscussion
Now, we consider the problem of heat transfer performance enhancement of gravity heat pipes
Now,weconsidertheproblemofheattransferperformanceenhancementofgravityheatpipesby
by growing AAO nanotubes on inner wall surface. The influences of AAO nanotube length and
growingAAOnanotubesoninnerwallsurface. TheinfluencesofAAOnanotubelengthanddiameter
diameter on the temperature distribution, overall thermal resistance, and dry-out occurrence of the
onthetemperaturedistribution,overallthermalresistance,anddry-outoccurrenceoftheprocessed
processed gravity heat pipes under different input heat powers (symbol: Qin) were analyzed,
gravity heat pipes under different input heat powers (symbol: Q ) were analyzed, respectively.
respectively. Parameter studies were carried out within thei n ranges: 6μmL18μm ,
Parameterstudieswerecarriedoutwithintheranges: 6µm≤ L ≤18µm,30nm≤ D ≤90nm,and
2350Wnm≤QDin≤9200n0mW, .and 25WQin 200W.
33..11.. TTeemmppeerraattuurree DDiissttrriibbuuttiioonnss
WWhheenn tthhee eelleeccttrroonniicc ccoommppoonneennttss iinn aa ccoommppuutteerr ppeerrffoorrmm rreellaatteedd ccaallccuullaattiioonnss,, tthhee wwaassttee hheeaatt
ggeenneerraatteedd wwiillll aaffffeecctt tthhee ooppeerraattiinngg ssppeeeedd,, aanndd,, aass tthhee tteemmppeerraattuurree rriisseess,, eexxcceessssiivvee tteemmppeerraattuurreess wwiillll
ccaauussee tthhee hhaarrddwwaarree ttoo ccrraasshh oorr eevveenn ccaauussee ppeerrmmaanneenntt ddaammaaggee.. IInn FFiigguurree 66,, tthhee tteemmppeerraattuurree TT vveerrssuuss
tthhee ppoossiittiioonn XX iiss pplolotttetedd fofor rththe eAAAAOO nannaontoutbueb elelnegntght hLL=6μ6mµm, ,1010μmµm, ,1144μµmm,, aanndd 1188μµmm wwiitthh
tthhee AAAAOO nnaannoottuubbee ddiiaammeetteerr DD=3300nnmm((FFiigguurree 66aa)) aanndd DD=9900nnmm ((FFiigguurree 66bb)) uunnddeerr aa hheeaatt ppoowweerr
conditionofQ =100W.Theresultsshowthat,ataparticulartubediameterD,intheevaporator
condition of Qin 100W. The results show that, at a particular tube diameter D, in the evaporator
in
section,thetemperaturedecreaseswiththeincreaseinthetubelengthL.Thereasonforthedecreasein
section, the temperature decreases with the increase in the tube length L. The reason for the decrease
temperaturemaybethatthegrowthofthetubelengthcanincreasethespecificsurfacearea,sothatthe
in temperature may be that the growth of the tube length can increase the specific surface area, so
workingfluidcantransfermorethermalenergyfromtheheatsourcethroughthesolidsurface,thereby
that the working fluid can transfer more thermal energy from the heat source through the solid
improvingtheheattransferperformanceoftheheatpipe. Inthecondensersection,thetemperature
surface, thereby improving the heat transfer performance of the heat pipe. In the condenser section,
increasesasthetubelengthincreases. Thereasonisthatthethermalenergyoftheheatsourceismore
the temperature increases as the tube length increases. The reason is that the thermal energy of the
broughtfromtheevaporatorsectiontothecondensersection,sothatthecondensersectionhasthe
heat source is more brought from the evaporator section to the condenser section, so that the
opportunitytoremovemorewasteheat,therebyimprovingtheheattransferperformance. Inaddition,
condenser section has the opportunity to remove more waste heat, thereby improving the heat
comparingFigure6a,b,itcanbeseenthatthetemperatureintheevaporatorsectiondecreaseswiththe
transfer performance. In addition, comparing Figure 6a,b, it can be seen that the temperature in the
increaseinthetubediameterD.
evaporator section decreases with the increase in the tube diameter D.
Inventions2018,3,42 9of12
Inventions 2018, 3, x FOR PEER REVIEW 9 of 12

FFiigguurree 66.. TTeemmppeerraattuurree ddiissttrriibbuuttiioonn aalloonngg hheeaatt ppiippee aatt QQin == 110000 WW ffoorr ddiiffffeerreenntt vvaalluueess ooff tthhee ttuubbee
in
length L with: (a) D30nm; and (b) D90nm.
lengthLwith:(a)D=30nm;and(b)D=90nm.
The reason may be that the increase in the tube diameter makes the nucleation sites larger,
The reason may be that the increase in the tube diameter makes the nucleation sites larger,
resulting in larger bubbles, and, when the working fluid boils, the mutual interference between the
resultinginlargerbubbles,and,whentheworkingfluidboils,themutualinterferencebetweenthe
generated bubbles is more severe, thereby promoting the convective boiling effect to enhance the heat
generatedbubblesismoresevere,therebypromotingtheconvectiveboilingeffecttoenhancetheheat
transfer performance of the heat pipe. In the condenser section, the temperature increases as the tube
transferperformanceoftheheatpipe. Inthecondensersection,thetemperatureincreasesasthetube
diameter increases. The reason for this is that the convective boiling effect is greatly enhanced in the
diameterincreases. Thereasonforthisisthattheconvectiveboilingeffectisgreatlyenhancedinthe
evaporator section, so that the condenser section has the opportunity to remove more waste heat,
evaporator section, so that the condenser section has the opportunity to remove more waste heat,
thereby improving the heat transfer performance. The maximum percentage decrease in temperature
therebyimprovingtheheattransferperformance. Themaximumpercentagedecreaseintemperature
for the heat power of 100 W is 18.44% and in the temperature difference between the evaporator
fortheheatpowerof100Wis18.44%andinthetemperaturedifferencebetweentheevaporatorsection
section and the condenser section is 46.12%, compared to the unprocessed heat pipe (dashed line).
andthecondensersectionis46.12%,comparedtotheunprocessedheatpipe(dashedline).
3.2. Thermal Resistance versus Nanotube Size
3.2. ThermalResistanceversusNanotubeSize
Figure 7 shows the overall thermal resistance R versus the tube diameter D for the AAO
Figure7showstheoverallthermalresistanceRversusthetubediameterDfortheAAOnanotube
lneanngothtuLbe= le6nµgmth, 1L0µm6,μ1m4µ, m1,0aμnmd1, 81µ4mμmwi,t hanthde i1n8pμumth ewatitpho twheer iQnpu=t 2h5eaWt p(Foiwguerre 7Qain)an2d5QW
in in
=(F1ig0u0rWe 7(Fa)ig aunred 7bQ)i.nIn1F0ig0uWre 7(,Faitgaurgei v7ebn).h Iena tFpigouwreer ,7t,h eato av egriavlletnh ehremata lproewsisetra, ntcheeR odveecrarella stehserwmitahl
irnecsrisetaasninceg Rth deetcurbeeasdeisa mwietthe rinDc.reTahseinrge atshoen tumbaey dbiaemtheatetrt hDe. eTnhlea rrgeeamsoenn tmoafyt hbee ttuhbaet tdhiea meneltaerrgmemakeenst
tohfe thneu ctluebaeti odniasmiteetselra rmgaerk,easn tdh,ew nhuecnletahteiown osrikteins glaflrgueidr, baonidls,, wthheemn othree swevoerrkeinthge flmuiudt ubaolilisn,t ethrfee rmenocree
bseevtwereee nthteh meugetunaelr aintetedrfbeurebnbclees b,ethtweesetnro tnhgee rgethneercaotendv ebcutibvbelebso, itlhineg sterfofencgte,ra tnhde tchoenbveetctteivret hbeoihlienagt
terfafencstf,e arnpde rthfoer mbeattnecre thoef thheeaht etraatnpsifpere .pIenrfaodrmdiatniocne, othf ethoev heeraatl lptihpeer.m Ina lardedsiitsitoann,c tehRe odveecrraelals tehsewrmitahl
irnecsrisetaasninceg Rth edetucrbeealseensg wthitLh. iTnhcereraesaisnogn tihset htuatbteh leenggrothw tLh. oTfhteh reetausboen leisn gththatc tahne ingrcorewatshe tohfe thspee tcuifibce
sleunrgfathce caarne ian,ctrheeaswe othrkei snpgeflciufiicd scuarnfatcrea anrsefae,r tmheo wreotrhkeinrmg afllueidn ecragny trfraonmsfetrh mehoerea tthseorumrcael ethnreorugyg hfrtohme
sthoeli dhesautr sfoacuer,cae nthdrtohuegnhe tnhhea snocliedt hsuerhfaecaet, tarnands tfheernp eenrfhoarnmcaen tchee ohfetaht etrhaenastfepri ppee.rfIotrcmananacleso obf ethfeo huenadt
tphiapte.a lIot ncganw aitlhsot hbee gfroouwntdh tohfatth aelotunbge wleinthg tthheL ,gtrhoewotvhe roaf llththe etrumbael lreensgisttha nLc,e tRhed ooveesrnalolt tchhearnmgael
sriegsnisitfiacnacnet lRy wdoitehs tnhoetc hchananggeeo fsitghneidfiicaamnteltye rwDit;ht htahtei sc,htahnegree iosfn tohsei gdniaifimcaentetrc hDa; ntgheati nist,h tehvearer iaist ionno
tsriegnndifiocfaRnta nchdaDng.Ien iand tdhieti ovna,rciaotmiopna trrinengdF iogfu rRe a7na,db iDt.c aInn baeddfoiutinodn,t hcoamt,wpaitrhintgh eFiingcurreea s7eao,bf tiht ecainnp buet
hfoeuantpdo twhaetr, Qwiint,ht htheev ianrciaretiaosne torfe nthdeo ifnRpuatn hdeDat tpenowdsetro Qbine, gtheen tvlea,raiantdiotnh etriennfldu oefn cRe sanodf tDub teenddiasm toe tbeer
agnednttlue,b aenlden tghteh inbfelcuoemnceelse sosf otubbveio duisa.mTehteerr eaansdo ntufboer ltehnisgtish tbheactohmigeh leerssin opbuvtiohuesa.t Tphoew reerawsoinll floear dthtios
aisn tihnactr ehaisgehienr tihnepouvte hraelaltt epmowpeerra twuirlel olefatdhe tow oanrk iinngcrfleauside iinn tthhee hoevaetrpailpl ete,mleapdeirnagtutroel eosfs tphreo nwoournkcinedg
sfluurifda cien stthreu chteuarte peifpfeec, tlse.aTdhinegm toa xleimssu pmropneorucenncetadg seudrefaccreea ssteruinctouvree reaflfletchtse.r Tmhael mresaixsitmanucme fpoerrtcheenhtaegaet
pdoewcreerasoef 2in5 Woviesr5a8ll. 6t8h%er,mcoaml preasriesdtatnocteh efourn tphreo cheesaset dphoewaetrp iopfe 2(d5 aWsh eids l5in8e.6).8%, compared to the
unprocessed heat pipe (dashed line).
Inventions2018,3,42 10of12
Inventions 2018, 3, x FOR PEER REVIEW 10 of 12
Inventions 2018, 3, x FOR PEER REVIEW 10 of 12

FigFuigreur7e. V7.a Vriaartiiaotnioonf othf ethoev oevraerllaltlh tehremrmalarle rseissitsatnacneceR Rw witihtht htheet utubbeed diaiammeeteterrD Df foorrd diiffffeerreenntt vvaalluueess of

theotfu tbhee tluenbge tlhenLgwthi tLh w:(iath):Q (ain) =Q2in5W2;5anWd(; ba)ndQ i(nb)= 1Q0in0W1.00W.
Figure 7. Variation of the overall thermal resistance R with the tube diameter D for different values
3.33..O3.c Ocucrcorufer ntrhecene tcouefb oeDf lrDeynr-gyOt-hOu Ltu wPt iPhtheh:ne (noam)o meQneinnoonn 25W; and (b) Qin 100W.
F3i.3gF.u iOgreuccr8uer er8xe neaxcmea ominf iDensreyts-h Otehueot voPevhreeanrlaolmllt hetnheoernmr maallr reessiissttaannccee RR vveerrssuuss tthhee iinnppuutt hheeaat tppoowwere rQQin ifnofro trhteh AeAAOA O
nannoantuobtueblee nlgenthgtLh =L6µ6mμ,m10, µm10,μ1m4µ, m1,4aμnmd1, 8aµnmd w18ithμmth ewAitAhO thnea nAoAtuOb endainaomtuebteer dDia=m3e0tenr m
(FigDure380aFn)imagnu(drFei Dg8 ue=rxea9 m08ainn)m eas n(tFhdie g ouDvreera89lb0l )tn.hmIenr mF(aFigli gureurseries8 t,a8ntbhc)ee. RoInv v eeFrriasgluulsrt heth ee8r ,mi ntaphluer te ohsveisaettra apnlolc wetheRer rQgmeinan fleo rrra etlhsliyes tAdaAneccOree aRs es
nanotube length L6μm , 10μm , 14μm , and 18μm with the AAO nanotube diameter
asgtheneDeirnapl3lu0y tndmhece(raFetiagpsueorsew a 8sea rt)h Qea niinndpi nuDctr heea9as0te npsm,oaw n(eFdri gQtueirnn eid n8scbrt)eo.a Isbnee sF, giagenundrt etl ee8n., dHtsh oteo w obveeve greaerln,l twtlheh.e rHemnoawtlh ereveseqirsu,t aawnlihcteey nRi tn heth e
evaqpuoarliattyo irns tehceti oevnarpeoarcahteosr 1se0c0t%io,nt hreeatcehmesp 1e0r0a%tu,r tehoe ftethmepeevraatpuorrea otof rthsee cetvioanposruadtodre snelcytiroinse ssu,dredseunlltyin g
inarissegused,n dreeersanullliytni dncrgeec iranes aeas esinsu datshd teehnoe viinnecprraueltal hsteeh aietnr p mtohawel eorrve Qesirinsa tilanl ncthrceeearsRme.sa,Al a rtnedtsh itsiestnapdnosc eitno Rt b,. eiAt gtce tanhntilseb .pe Hojiounwdt,ge ivet edcra,t nwh abhtee anju tddhrgey e-do ut
quality in the evaporator section reaches 100%, the temperature of the evaporator section suddenly
phethnaortim sae esd,n rryoe-nsouuhlttai npsgho iecnnc aou msrureednddo.ennT hhinaecsrd oeracysc-euo riunre ttdhp.e hT oehvneeo rdamrllye t-nhooeurntm poahcl ecrunesorismstwaennhcoeen nR o.t chAcetu thrhsei saw tphoieninnpt ut, hitte hc haeenaa tbtp ei nojupwdugeter hdQe aint is
grepaotewthreatrth aQa ndinr 1yis2- o5gurWte ap,th1e5ern0 tohWma,ne1n 17o25n5 W hWa,s,1 o17c55c0uW Wrr,ea,d n1. 7dT5h1 We7 5d, rW1y7-5oin uWtt ph, heaencndaos m1e7se5no Wofntu ionbc ectuhleresn cwgathsheessn L othf= et u0hbeµeam tl ien(nupgunthtp hsr oeLac te= s 0se d
µm (unprocessed heat pipe), 6 µm, 10 µm, 14 µm, and 18 µm, respectively. This means that the dry-
heatppiopwe)e,r6 Qµinm is, g1r0eµatmer, t1h4anµ m12,5a Wnd, 11580 µWm, ,1r7e5s pWe,c 1ti7v5e Wly., Tanhdis 1m75e Wan sint hthaet tchaeseds royf- otuubteo lcecnugrthresn Lc e= c0a nbe
out occurrence can be delayed due to the surface structure. This reason is that the growth of the tube
delayµemd d(uuneptrooctehsesesdu rhfeaacte psitpreu)c, t6u µrem., T1h0 iµsmre, a1s4o µnmis, athnadt 1t8h µemgr, orwespthecotifvtehlye. tTuhbies mleneagntsh tchaant tihnec drerays-ethe
length can increase the specific surface area, so that the working fluid can transfer more energy from
specifioucts oucrcfuarcreenacree ac,anso bteh daetlathyeedw dourek tion gthfle usuidrfaccaen sttrraunctsufreer. mThoirse reeanseorng iys tfhroatm thteh gerhoewatths oofu thrcee tuthbreo ugh
thel ehnegatth s coaunr icnec rthearsoeu tghhe tshpee csiofilci dsu srufarcfaec aer,e tah, esroe tbhya tr ethdeu wcionrgk itnhge fhlueiadt caacnc utrmanuslfaetri omno arne den deerglayy firnogm t he
the solid surface, thereby reducing the heat accumulation and delaying the occurrence of dry-out
occtuherr heenacte s oouf rdcery t-horuotu gphh ethneo smoelindo snu.r fIanc ea, dthdeitrieobny, rceodmucpinargi nthge Fhiegaut raec c8uam,bu, liatt icoann a nbde dfoeulanydin gth tahte by
phenomenon. Inaddition,comparingFigure8a,b,itcanbefoundthatbyincreasingthetubediameter
incorcecausrinregn tchee o tfu dbrey d-oiaumt petheern Dom, tehneo onv. eIrna lald tdhietriomna, lc roemsipstaarnincge RFi gcaunre b 8ea f,ub,r tiht ecra rne dbeu cfeodu,n bdu tth tahte btyu be
D,theoverallthermalresistanceRcanbefurtherreduced,butthetubediameterdoesnotseemto
diaimncerteears idngo etsh en toutb es edeimam teot ehr aDv,e t hteh eo veefrfaelcl tt hoefr mdealla ryeisnisgt atnhcee Rd rcya-no ubte foucrctuhrerre rnecdeu. cCedo,m bupta trheed tutob et he
havetheeffectofdelayingthedry-outoccurrence. Comparedtotheunprocessedheatpipe(dashed
undpiraomceestseer dd oheesa tn poti psee e(mda stoh ehda vlien eth),e tehfefe mct aoxfi mdeulmay ianpgp tlhicea bdlrey -ionuptu ot chcuearrte pnocew. eCro tmo paavroedid tod rtyh-eo ut
linoe)c,cutuhnrperermonccaeexs sciemadnu hbmeea iatn ppcpripelaiec sa(edbdal seuhpien dtpo lu iantbeho),eu atth t4ep0 m%owa. xeirmtuoma vaopipdlicdarbyl-eo iuntpouct chuerarte pnocweecra nto baevoinidc rderays-eodutu pto
abouto4cc0u%rr.ence can be increased up to about 40%.

FigFuigrFeuigr8eu. r8eV. a8Vr. iaVartiaiarotiianotinoo nfo tfoh fte hteho evo eovrvearelarlalllt lht hteherermrmmaaalll r rereesssiiissstttaaannnccceee RRR wwwiiittthhh tththhee e iniinnppuputu tht hehaeeta atp toppwoowewre Qre riQn Qfino rf odfroi fdrfeidfrfeienfrftee nret nt
in
valvuaelsu eosf othf eth teu tbueb lee lnegntght hL Lw witihth: :( a(a) ) DD3300nnmm;; aanndd ((bb)) DD9900nnmm. .
valuesofthetubelengthLwith:(a)D=30nm;and(b)D=90nm.