Post on 07-Apr-2022
Title Study on Heat Transfer and Fluid Flow Characteristics ofRectangular Finned Surface in Duct( Abstract, Chapter 1-4 )
Author(s) Didarul, Islam Md.
Citation : 1-100
Issue Date 2007-09
URL http://hdl.handle.net/20.500.12000/5748
Rights
PhD・Dissertation
onHeatTransfcrandFluidFlowCharacteristicsStudy
ofRectangularFinnedSurfnceinDuct
September2007
UniversityoftheRyukyus
GraduateSchoolofEngmeermgandScience
Material,StrucmralandEnergyEngmeermg
IslamMdDIDARUL
Contents
Abstract
Nomenclature
l・Introduction
L1Whyheattransferaugmentationisimportant?
L2Differentmethodsofenhancingheattransfbr
1.2.1Vortexgenerator-anemergingtechnology
L2・ZExtendedsurftlces
l30bjectiveofthisresearch.
Pageno.
1
11136
ZExperimentalApparamsandProcedure
2、1Intmduction
2、ZFinpattemsandalTangemems
23Expelimemalsetupfbrheattransfermeasuremem
23・lFinsinsidetheduct
2、3.2Heatingsurfhcewithfins
23.31nffaredimagetechniqUe
2、3.41nsulationtechnique
8
0012345567
881111111111
2.4Expelimentalapparamsfbrpressuredropmeasuremem
2.5F1owvisualizationapparams
2、5.1Experimentalsetupfbrdienowinwaterchannel
25・zEXperimentalsetupfbrsmokeflowvisualization
2.5.3Expenmentalsetupfbroiltitamumoxidefilmflowvisualization,C
3.DataReductionandUncertaintyAnalysis
3」Intmduction
32Datareductio、
3.3Uncertaimyanalysis
18
18
20
4HeatTransfbrCharacteristicsandFlowAnalysisofFinnedSurfacesinCaseof
TallDuctandNarTowDuct、
4.1Introduction
4.2.Localheattransfbrcoefficiellts
4.2.1Smoothductresults
4ユZEffectofinclinationanglesonheattransfer
4.2.3Heattransfbrthrougdlfinofresinmaterial
4.2.4Effectoffinheightonheattransfercharacteristics
42.sEffectofvelocityonheattransfbrcharacteristics
43Detailedheattransfbranalysis
44.Area-averagedheattransfer
4・SFlowvisualizationbydyefIowinwaterchanne1
4.6.Frictionfactor
21
111367
222222
28
069
333
41
4247Sunmary
SF1uidFlowandHeatTransferAnalysisofDifferentFinPatternsand
ArrangementataNarrowDuct,
5.1Introduction
333
444
5.2F1owvisualization
5.2.1SurfaceoilfilmHowpattemsaroundfinsattheendwall
5.2.2Smokeflowvisualbration
43
46
4853Frictionfactor
5.4Detailedheattransferanalysis
5、5A1℃a-averagedheattransfbrCoefficient
5.6Thermalperfbnnancerati0
5.7Sunmary
50
835
566
6.TheEffectsofDuctHeightonHeatTransferE血ancementandFlow
CharacteristicsofFinnedSurface.
777
666
6.1.Introduction
6.2.Detailedheattransfer
6.3AreaaveragedheattransferandNusseltnumbels
64Frictionfactor
6,sVortexstructureanalysis
d6Thermalperfbnnanceratio
6,7Summary
7・Conclusions
81
85
87
90
93
94
Refbrences 97
Acknowledgements 100
Abstract
StudyonHeatTransfbrandFluidFIowCharacteristicsofRectangular
FinnedSurfiuceinDuct
AdetailedexperimentalinvestigationoftheheattransfbrandnuidHowcharacteristicsof
rectangLllarfhmedsurfacesoffburdiffbremfinpatterns,co-angular,zigzag,co-rotatmgandco-
counterrotatingwasconductedfbranairflow(Re=15700-104500)inaductShortrectangular
finsofeitheraluminumorresinmaterialwereattachedin7x7arraystoaheatmgsurface(base
plate)ofconstantheatnuxbydoublesidedthintapaT-typethennocouplesandanin丘ared
camera(TVS8000)withal60x120pointln-SbsensorwereusedtocapturetheinhPared
imagesaswellastomeasurethetemperatureandthedetailedheattransferattheendwallalong
withfinbase.DiffbremnowvisualizationtechniqUe,fluorescencedyeflowmwaterchannel,
smokeflowvisualizationandoiltitamumoxidefilmflowvisualizationwereusedtoanalysisthe
Howbehavioranditseffectonheattransfer、Inthisstudywehavefirstinvestigatedtheheat
transferandfluidflowcharacteristicsofco-angularandzigzagfinpatternataflatplate(200mm
ductheight)boundarylayerandnarrowductof20mmheight・Zigzagpattemwerefbundtobe
moreeffbctivemheattransfbratbothflatplateandnaxTowductcase・Incaseofco-angUlar
pattemdyeflowstagnatedmfiPontofthefinandfbnnedastronghorseshoevortexaroundthefin
whilethelongitudinalvortexesgeneratedbythesidetopedgestouchedthefinsurfaceandthe
endwalLOntheotherhandmcaseofzigzagpattemaweakhorseshoevortexappearedand
longimdinalvortexstrucktheendwallmainlyandasinusoidalwavyflowbehaviorwasobserved
WehavethenfhrtherinvestigatedheattransfbrandUowcharactelisticsofco-rotatingpatternand
co-counterrotatingpatternalongwithothertwopatternataductof50mmheight・Theheat
transferresultshowsthattheco-rotatillgpattemhasthehighestNusseltnumberandtheco‐
angularpatternhastheleastNusseltnumber、ConsideringthethermalperfbnnanCe,co-rotating
patternwithsmallerpitchratiowasfbundtobethemostrecommendedpattemastheheat
transferaugmemationwithco-rotatmgpatternismorethanthreetimesthefin-lessduct・
Horseshoevortex,mainlongitudinalvortexandrolledUpvortexwereagainconfinnedbysmoke
flowandoiltitamumnowvisualization・AmongthefburpatternslargestfiPictionfactorocculTed
fbrtheco-rotatingpatternatsmallerpitchratioowingtothestrongUowinteractionsand
combmedvortexattackontheendwallandfhlsurfacewhereastheleastfiPictiondevelopedfbr
co-angularpattenLFinallytheeffectsofducthei8lltonheattransferandflowcharacteristics
wereinvestigatedandthemostimportantinfbnnationaboutvortexstrucmresatseveral
streamwisepositionswasobtainedwhichshowsthereasonsofheattransfbrenhancement、
Vortexstructuresfbrco-angularandco-rotatingpatternwerefbunddifferemandthe
reattachmentpositionswereobtame。、Comparativelylargescalevortexrotationwasobservedin
co-rotatingcasewhichisobviouslyresponsiblefbrheattransferenhancemem.
K臼ywo伽JConvectiveheattransfer;HeattransferenhancememRectangularfin;Longimdinal
voltex;Horseshoevortex;Inffaredlmage
Nomenclature
DH:Hydraulicdiameteroftlleduct,m
DE:EqUivalentdiameteroftheduct,m
/:Frictionfactor
L:Finlength=Z0mn
Ha:Heightofduct20~Z00mm
Hi:Heigiltoffin=5~15,m
A:Averageheattransfbrcoefficient,W/m2.K
A:HeattransfercoefficiemW/m2.K
ハx:Streamwiselocalheattransfbrcoefficient,W7m2,K
hz:Spanwiselocalheattransfercoefficient,W/m2.K
jVil:Area-averagedNUsseltnumber(/i、,,/几)
MJE:Area-averagedNUsseltnumber(/JDB/几)
IVjJL:Area-averagedNusseltnumber(んL/几)
lViJ。…,ノ:Area-avemgedNusseltnumberonoverall surfacea1℃a(A…"、H/几)
Mイノi、:Area-averagedNusseltnumberonfi、base(/i/1,iDH//L)
RPbo:Streamwiseandatmosphericpressurerespectively,N/m2
jW:Walltemperamre,oC
16。:Mainstreamtemperature,。C
Re:Reynoldsnumber[UDH/v]
ReE:Reynoldsnumber[UDE/v]
ReL:Reynoldsnumber[UZ/v]
U:Meanairvelocity,m/S
PR:Ptchratio
Sx:Finspacingmsn℃amwisedirection(showninFig、2.1)
Sz:Fmspacinginspanwisedirection(showninFig、2.1)
Z:Streamwisecoordinate(X=OatthecemerofrectangularfinoffirstrDw)
Z:Spanwisecoordinate(Z=Oatthecenterofrectangularfinoffirstrow).
r:Streamwisecoordinateぽ=OatthecenterofrectangularfInofthirdrow)
Z.:Spanwisecoordinate(Z*=Oatthecenterofrectangularfinofthirdrow)
G”ekqy腕boKF
a:Inclinationangle
v:Kinematicviscosityofair,m2/s
/I:Themlalconductivity,W/(、.K)
Subsripts
s:Smoothsurface
訓Introduction
1.1Whyheattransferissoimportant?
HeattransferorThermalmanagememisanimportantfhctorinengineenngwhichhave
greatcomlibutiontothetechnologicaladvancementofthemodemcivilization・Thermal
managementortemperaturecontrolisagreatconcerntoheatgeneratingbodies,asfbrexample,
powerplants,nuclearreactors,engines,motors,heatexchangers,processindustries,generators,
electronicequipment,digitalcomputerseventhoughinspacecraft・Iftheheatgeneratedina
machineisnotremovedatasufficientrate,someproblems,eventhebreakdowncantakeplace
mthemachineduetooverheatmg・Thissortofproblemcanonlybeovercomebyamore
effectiveheattransfbnSoagreatdealofresearchworkisgoingonmthisfieldtodevelopa
moreeffbctivetechnology.
1.2.Differentmethodsofenhancingheattransfer
Therearemanywaysofenhancingheattransfbr、Coolingbyjetspray,especiallyathigh
temperatures,vortexgeneratorsandfinsorextendedsurfacesareveryimportanttechnologyand
commonlyencounteredinheattransfbraugmentation・Iwouldliketolimitmyliteraturereview
onvortexgeneratorsandextendedsurfaces.
1.2.1.Vortexgenerators-anemergingtechnology
Avortexgeneratorisanemergingtechnologytoenhanceheattransfbr・Smdies
concemingtheinfluenceoflongimdinalvorticesonheattransferareagreattopictothepresem
1
daydevelopmentofhighperfbrmancethennalsystems・Theuseoflongimdinalvortices
generatedbydifferentkindofvortexgeneratorstoenhancetheheattransferintheairsideof
heatexchangershasbeenconsideredpmmising・Umillastdecade,fewresearchworkshave
shownthepotentialuseofvortexgeneratorsinenhancingheattransferincompactheat
exchangers・Mostlyearlierworksdealtwitlllongimdinalvorticesandheattransferrelatedto
mrbinebladecooling,OneoftheearliestisreportedbyEdwardsandA1ker[19741who
investigatedtheeffectofbothcounterrotatmgandco-rotatingvorticesonheattransferof
boundarylayers、Spatiallyresolvedheattransiierratesweredeterminedbypassingflowovera
wallofunifbmlheatflux・Cubesandvortexgeneratorsbladeswereusedtocreatethe
longimdinalvorticesandfbundtllatthecUbespmducedthehighestlocalheattransfer
improvememofupto160%whilethegeneratorseffbctwasextendedatfartherdownstream・The
counterrotatingvortexpairsshowedmoreeffbctivenessthantheco-rotatmgpairswithan
increasemheattransferofmaximum65%thannatplatevaluesfbrthecounterrotatmgvortex
generatorswithl5oattackanglaTurkandJunkhan[1986]measuredthespanwiseheattransfer
downstreamofarectangularbladetypevortexgeneratorsmountedonaflatplateandthe
spanwiselocalheattransfercoefficientswerefbundvarymgwiththespanwisebladespacing
Fiebigandcoworkers(1986,1991)haveconductedaseriesofexperimemalandnumerical
simulationabouttheinfluenceofwmgtypevortexgeneratorsonheattransfbrandpressureloss
inductHows、Theytriedtoimprovetheperfbnnanceofplate-finheatexchangers、Heattransfer
infm-tubeconfigurationwasfirstsmdiedbyRusseletal[1982]andfbcusedontheuseofvortex
generatorstoenhanceheattransferinfintubeconfigurations・Theirexperimemalworkwasvery
promisingbecausetheyshowedthatthevortexgeneratorscouldenhanceheattransfbr・Inamore
applicationbasedeXperiment,FiebigetaL[1993,1994]studiedtheinfluenceofwinglettype
vortexgeneratorsinfinmbeconfigurationswiththreetuberowsandmeasuretheheattransfer
andpress、巴lossfbrdifferemfinSpacmg,tubearrangement(inlineorstaggered)andmbefbnn、
2
TheyfbundthattheheattransfercoefYicientwasenhancedby55%-65%fbrcirculartubewith
aninlinearrangementwhereasstaggeredarrangementcaused9%.
1.2.2.Extendedsurfnces
Finsarewidelyusedastheprimarymeansofheatexchangingdevices・Theneedfbr
moreefficientcoolingtechniquesofdeviceshasrecemlypromptedsmdyintoheattransfbrand
nowcharacteristicsofvariousconfigurationsoffinnedsurftlces・Thepinfinisatypical
configurationthatisoftenusedtocoolthetrailingedgeregionofmrbineblades;theintemal
passageofturbinebladescanbeverynanPow,sothatthechoiceofcoolmgsCllemeislimited、
TheeffectiveheattransfbrofstaggeredalTaysofshortpinfinswasstudiedbyVanfbssen[1982]
andfbundtobehalfthatfbrlongerpinfins、However,thisstudydidnotdetailthelocal
characteristicsofpinfinarrays・SpalTowetal.[1976]investigatedthelocalheattransfer
coefficientoveraplatesurfacebythenaphthalenesublimationteclmiqUefbratwo-rowplatefin
andmbeheatexchangerlfindingthatthecoefficientwaslowestbehindthepin・Theeffectofpin
finarrangementonheattransferoveranendwall,theheatingsurfacewherethefinsareattached,
wasexaminedusingathennosensitiveliquidcrystalfilmbyMatsumotoetaL[1997],Who
concludedthattheheattransfbrfiomtheendwallwasenhancedbyflowaccelerationbetweenthe
pinfinsratherthanthehorseshoevortexaroundthepinRectangularfinisfbundtohavean
extensiveuseinheatexchangersurfacesandinmanyapplications,Geometncparameters,such
asaspectratio,finheighttoductblockageratio,finmclinationangle,orientationoffini.e・fin
pattern,finpitchratioinstreamwiseandspanwisedirectionandfInshapeshavepronounced
effectsonthelocalandoverallheattransfbrcoefIiciemsaswellasonthethennalperfbrmanceof
theheatexchangingdevices、Withtheincreasingdemandofcompact,lightweighthigh
perfbnnanceheatexchangingdevicesresearchersaretryinghardtoservethefinsasextended
3
surfaceaswellasvortexgeneratortoobtainthecombinedeffbctofextendedsurfaceandvortex
generatorwhichisanemergingtechnologyinthefieldofheattransfbr,
Recently,inclinedrectangularfinembeddedtotheendwallisfbundtobeaneffective
confIgurationfbrheattransfbrenhancemem,becausetheinclmedfinproducethelongitudmal
vortexofwhichintensityexistatfardownstream、Itisexpectedthattheheattransfbrffomthe
endwallandthefInsurfacecanbeimprovedbythismclinedfinsandhencethisconfigurationis
identifiedasbeingverypromising・ToriiandYanagihara[1997]hasreviewedonheattransfbr
enhancementbylongimdinalvorticesanddiscussedhowlongitudmalvorticesareproducedby
diffe泥ntkindofvortexgeneratorsandenhancesheattransfbr・Thatreviewfbcusedontheeffect
oflongimdinalvorticesgeneratedbywingsontheheattransferenllancementinboundarylayers,
ductsandplatefintUbegeometlies・HeattransferenhancementinalTaysofrectangularblocks
hasbeeninvestigatedbymanyresearchers、SpalTowetaL[1982,1983]studiedtheheattransfer
andpressuredropcharacteristicsofarraysofrectangularmodulesconⅡnonlyencoumeredin
electronicequipmentlntheirexperiment,heattransferenhancementsexceedingafactoroftwo
wereachievedbytheuseofmultiplefbnceslikebalTiers,withtheimerbarrierspacingandthe
balTierheightbeingvariedparalnetricallyalongwiththeReynoldsnumber・Igarashi[1983]
studiedheattransfbrfromasqUarepnsmwithdiffbrentinclinationanglesandobserved
reattachmentflowfbrinclinationanglesofl4o-35o、HeattransfbrfiomananFayofparallel
longimdinalfinstoamrbulemairstreampassmgthroughthemterfinspaceshasbeen
mvestigatedbothnumericallyandexperimemallybyKadleandSparrow[1986]・Theyfbundthat
thelocalheattransfercoefIicientsvariedbothalongthefinsandthesurfaceofthebaseplate・
OyakawaetaL[1993]smdiedtheheattransferofplatesettingrectangularfinswithincUnation
angleof20oandshowedtllatthisconfigurationcanenhanceheattransfbr幹MolkietaL[1995]
experimemallysmdiedtheheattransferattheentranceregionofanarrayofrectangularheated
blocksandpresemedempiricalcolTelationsoftheheattransfbrfbrthealTay・El-SaedetaL
4
[2002]investigatedheattransferandfluidUowinrectangularfinarraysandfbundthatthemean
NusseltnumberincreaseswithincreasingReynoldsnumber,inter-finspaceandfinthicknessbut
didnotexamineendwallheattransfbr、BilenandYapici[2002]investigatedheattransfer
enhancememfromasurfacefittedwithrectangularblocksatdiffbrentinclinationangleand
fbundthatthemaximumheattransferwasobtainedatanangleof45.whiletheeffectof
inclinationangleislittlefbrlargerthan225。,thoughthelocalheattransfercharacteristicswere
notreported・
IncontrasttorectangUlarfins,cylinder-likepinfinshavebeenfbundtoexhibitalarge
pressu頬dropduetothedragfbrceandflowaccelerationordecelerationthroughthefins、This
difIbrenceisduetothefactthatrectangulararraysdonotfbnnextremelynarrowpassages,
whichalsocausestheheattransferfbrrectangularfinstoberelativelylower・However,thepoor
heattransferregionbehindthepmmaybeimprovedbylongimdinalvortexesgeneratedbythe
fmFurthermore,WhenthefInarrayissetsuchthattheinclinationangleofeveryrowischanged
altemately>thelongimdinalvortexproducedbythetopedgeofthefinreattachestothatofthe
fbllowmgfinincreasingtheheattransferffomthefinsurhlce・Astheaboveliteraturereview
shows,previousstudieshaveinvestigatedheattransferenhancememfromsurfnceswitharraysof
shortrectangularfinsandafewsmdieshaveconcentratedonheattransferffomendwallfbrshort
rectangularfinswithdiffbrentinclinationangles、However,adetailedheattransferanalysisfrom
anendwallwitharraysofshortrectangularfInsofdiffbrentpattems(co-angular,zigzag,co-
rotatmgandco-counterrotatmg)intheductflowhasnotyetbeencompletedBilenandYapici
[2002]haveconductedasmdyfbrtheco-angularcaseonly,butdidnotclarifythelocalheat
transfercharactelistics・Inco-angularpattern,finsarrangedatsameangletothesamedirectionat
eachandeveryrow・Thisinclinationangleofthefinswillfbrmlongitudinalvortexwhichwill
stlikethenextfinsurfhceandendwalLF1owwilltouchbothsidesofthefinsurfaceandsoitis
expectedtheextendedsurfhceeffbctwillbehigherthanvortexeffectontheendwalLWhenthe
5
finsaresetatsameangleateachrowbutthedirectionsarealtematelychangedthenwedefine
thatpattemaszigzagpattern,Duetothealtematedirectionsofthefins,aslnusoidalwavyUow
behaviorisexpectedtoobservewhichwillincreasethevortexeffbctontheendwaUmorethan
theextendedsurfaceeffect、Inco-rotatmgandco-counterrotatingpattern,finsaresetinsucha
waythatitfbnnsconverginganddivergmgfinpairsinarow・Inco-rotatingpatternthe
convergmganddivergingfinpairsremainsameatcoITespondingfinrows・Inco-counterrotating
pattemtheconvergmganddivergingfinpairsarealternatelychanged、Thisdivergmgfinpairs
willfbnnacoupleoflongimdillalvortexwhichmightstrikestheendwallstronglyandthis
combineddownwashisexpectedtoenhanceheattransferhPomtheendwallsignificantly.
L30bjectiveofthisresearch・
Therefbre,wehaveinvestigatedtheheattransferenhancementfbrfinarraysoffbur
diffbrentpattemsatnalTowandtallductbysettingshortrectangularfinsonaductwalLWehave
chosenthefburfInpatterns,becausetheorientationsofthefinpattemgreatlyaffbcttheflow
fleldandasaresultheattransferisaugmented、Inthissmdy,shortrectangularfinswithfbur
differempatterns,areinvestigatedbecauseofitssignificantheattransfbrenhancementcausedby
thelargescalevoltexeffbctandextendedsurfaceeffect・Inadditionto,determiningtheoverall
heattransfbroftherectangularfInarray,therelativemagnitudesofheattransferbetweenthefins
andendwallcanbedetemlinedinthisstudy、Thissmdyattemptstoanalyzethedetailedheat
transfercharacteristicsoffinbaseandendwallsurfhcefbrdiffbrentfinpatternsandexaminethe
extendedsurfaceeffectfiPomthefinsurfacesandthevortexeffbctontheendwallfbrvarylng
pitchratioinstreamwisedirection、Detailedheattransfbrs,aswellasaverageheattransfer
coefficientsareestimatedffomtheinffaredimagesoftherepresentativefinregion・Theflow
behavioramundthefinsisfUrtheranalyzedtodetermmethelclationbetweentheheattransfer
andfluidflowcharacteristics,Furthelmorewediscussedthethermalperfbrmanceofthefbur
6
differentfInpattemsundertheconstantpumpingpowercondition,becausetheheattransfer
enhancementalwaysaccompaniesthedragfbrceandpressuredropsimultaneously・Sowemight
comparetheheattransfbrenhancementrateofthissystemwithtjhatofsmoothduct.
7
Z’ExperimentalApparamsandProcedure
2.1Introduction
Inthissectionwehavemainlydiscussedthediffbrentfinpatternsandtheexpenmental
setUpfbrtheheattransfermeasurements,pressuredropmeasurememsandflowvisualization
technique・Heattransfercoefficientsweremeasuredbytwotypesofapparams;heattransfbr
measurementbyTtypesthermocouplessolderedonthebacksideoftheheatmgsurfaceandthe
othertypeisheattransfermeasurementbymfiaredcamera.
2.2Finpatternsandarrangements
FourtypesoffinpattemsaresmdiedmthisresearchworkasshowninFi9.2.1
Thefinsweremadeofeitheraluminumorresinmaterialsandwererectangularwithdimensions
20mmlong,Smmthickandthreeheights,5,10andl5mm・Thefinsweresetinlinesandrows
ofseveneachFinspacmg,fincentertocenterdistanceSx,mstreamwisedirectionwere
maintained40mm,60mmand70nⅡnwhereasqinspanwisedirectionwas40mm、Theratios
SyLandS/Laredefinedaspitchratio,PRhereafter・Theanglesofthefinstotheflowdirection
weremaintainedatcL=0.,20.and25..Intheco-angularpattemthefinsweresetatthesame
angleandinthesamedirection・Inthezigzagpattemthefinshavethesameanglebutaresetin
alternatedirectionsaftereachrow、Incaseofco-rotatingpatternfinpairsarerotatedinsame
directionandfbnnconvergentanditbecomessameateveryfinrows・Contrarily,incaseofco‐
counterrotatingpatternfinpalrsaresetasconvergentanddivergentaltematelyatrow-wise
direction.
8
Aとらら鼻うらみBつつつつヴグP
C色-Vぞa且2-色_色_PamidlineDグーヅー少・ヴー・グーダーゲcenterline
EPPPPPPP-
FつPPPPPP-Qa
nP-P尊P(a)Co-angularpattem
7Pづづ多づづP
】・
6,勺、一、、、、
一一
5つPつ一錫つググ趣a
4勺。、一も、。、秤Z
3グググー惨PPP醜
2,,,-、、、、⑥
』’
1グジグーづつPP
ABCDEFG
7ppq夢ppq
6勺P負やQPQm
5勺ロロラロリ、脚
4勺つ勺稜qPQ煙a
3勺ヴローサロつQmo
2勺つQ-PQつ勺加
lロヮロラロつQ
ABClDEFG
7勺つ9づQPQme
6つ。つ一句つ。つ剛
』』
5勺つローヴQPQ・唾a
4。Qつ一句PQPme
3勺。。-ザQPQmO
2つ。P一句つQPK
--
C
lQつQ-PQpQ⑥
ABClDEFG
Fig.2.1Diffbrenttypesoffinpattems
9
2.3ExperimentalsetupfDrheattransfermeasurement
2.3.1Finsinsidetheduct
Expelimemswereconductedinlongrectangularductsbothof20and200mmheight,
230mmspanwidthand784mmlengthThefirstrowoffinswaslocated200mmawayfromthe
ductentrance、TheeXperimentalapparamsandfinsettingsareshowninFig、2.1.AsshownmFig
2・ZfbraductheightofHa=200,Ⅱn,thevelocityfielddoesnotdevelopoverthetestsection,
butratheritassumesfinsaremflatplateboundarylayerOntheotherha、。,atHa=20mmthe
flowisfUllydevelOped・ThefinswereattachedtothelowerwalloftlleductbymeansofalOO
似mdoUblesidedthintape・Theupperplateofductwasmadeofatransparemacrylicresinplate
fbrobservationoftheimenoroftheduct.◆
目L1rllc
~>'二
Fig.2.2Finsinsidetheduct(alldimensionsareinmm)
10
2.3.2Heatingsurfacewithfnns
Tomeasurethelocalheattransfbrcoefficients,thelowerwallwasfblmedofaBakelite
plateofthicknesslOmmanda30umthickstainlesssteelfbil(200mm×784mm)asaheating
Surface・Aconstantheatflux9wasfbnnedattheheatingsurfaceusingadircctcurrentsource.■
Topreventheatlossfiomtheheatingsurfacethroughthelowerwall,alayerofglasswoolwas
placedundertheBakeliteplate,Thewalltemperamre恥wasmeasuredusing70ⅡmdiameterT-
typethennocouples,solderedonthebacksideofthelowerwallat58discretelocationsalongthe
centerlineoftheheatingsurfaceandthemidlinebetweenthefinsinthespanwisedirection,as
showninFig.2.3.
Heating CenterlineMidline
thcrmocouplethermPcouplcsurlace
露》雫一
ニニニエニジニー
ニジニニニー
蟇霄二一
窕露》画一
LX
Z
A
784
Sidetopontside、
Fig.2.3Heatmgsurfacewiththennocouplelocations(Dimensionsinmm)
11
2.3.31nfraredimagetechnique
lnordertoobservethedisnibutionsofthetemperatureandtheheattransfbrcoefficients
aroundthefinsattheendwall,infraredimagesweretakenusinganinffaredcamerawitha
indium-antimony(In-Sb)sensor,whichcanmeasurcthetemperatureatl60x120pointswitha
resolutlonofOO25oCfbrablackbody、Hereitshouldbementionedthatanotherheatingsurftlce
similartothepreviousoneismadeexceptthethemlocouplesattachments・Inthiscase,the
temperaturewasmeasuredatthebackoftheheatingsurfacethroughawindowcovercdby
polyvinyledenefilmwithatransmissivityfbrinfraredenergyofnearlyunity、Thebackofthe
heatingsurfacewaspaintedblackandthewholeexperime、talapparatuswascoveredbyblack
clothtoensurethatthesurroundingswerecompletelydarkasshowninFi924.
囮]E窪匡團巨塞曇QLD
Blackoutcurtain
、A 。,LLocalheattmnsfe
coefficients
繍糟w…T>|<耐多し
Foam
MW,。.、。NozzleIA
Bakeliteplate
÷f1ii箒篶予
CつN1つNⅡ百田
〆
Fig.2.41nfraredimagetechnique
12
First,wemeasurcdtheheattransferusingthethennocouplesinthelargerregiontodeterminethe
effectofthefinsetting・WethenestimatedthedetailedheattransferandaveragedNusselt
numbersintherepresentativeregionbythemfraredimagetechnique、
2.3.41nsulationTechnique
Figure25(a)and(b)showsthethelmalinsulationoftheheatingsurfacewith
thermocouplesandwithoutthermocouplesrespectively・Thestainlesssteelfbil,attachedtothe
bakeliteplatewasusedastheheatmgsurftlceandthatbakeliteplatewascoveredwith60mm
thickmsulator,fbamtopreventheatlosshomthebackoftheheatingsurface,Therefbreheat
losseshPombackoftheheatmgsurfaceexceptwindowmayassumetobeverylittleThe
measurementwindowwascoveredwithtwolayersofpolyvinyledenefnmwithlmmspace
clearance・Heatlosseswere1.3%oftheheatfluxsuppliedtothesurfhcebymeasurementof
temperature difference betweensurfaceheatingandbacksurface.
S
0.1
plate
〃'''BakcliteF
fbil
ape
(b)Thennalinsulationoftheheatingsurfaceexceptwmdow(a)Thennalmsulationoftheheatmgsurftlce
attachedwiththennocouples.
Fig25Thennalinsulationtechnique(dimensionsareinmm)
13
2.4ExperimentalapparatusfOrpressuredropmeasurement
TomeasurethepressuredlDpresultingfromthepresenceofthefins,0.5mmdiameterstatic
pressuretapswereplacedatl3vanouslocationsofupstreamanddownstreamofthefinsonthe
pressureplatewith60-l20mmspaceintervaLExperimentalapparatusfbrpressuredrop
measurememisshowninFig2.6.Herethepressureplateisapolishedaluminumplateandthe
finsweresetontheplatebydoublesidedthintape・Pressurewasmeasuredbyadigitalmicro
manometerwithanaccuracyof±O1mmH20.
に一一一一
O~200
Fig.2.6Experimentalsetupfbrpressuredropmeasurement(dimensioninmm)
14
2.5Flowvisualizationapparatus
InordertoObservethebasicUowpatternandtheflowbehavioraroundthefin,How
visualizationisveryimportant・Therehavebeenuseddiffbrentmethodstovisualizetheflow
behavior・Inourexpenment,thef1uidnowwasvisualizedasfbllows:Dyeflowmwaterchanne1,
Titaniumoxideoilfilmflowvisualizationandsmokeflowvisualiズation.
2.5.1Experimentalsetupfbrdieflowinwaterchannel
Dyeflowmwaterchanneliso、eofthegoodvisualizationtechniquestogetaclearview
oftheflowpattelnFigure27showstheexperimentalsempfbrdyeflowinwaterchannel・Flow
wasvisualizedbyinducingfluorescentdyeinthewaterchannelatlowerReynoldsnumberto
observetheflowpatternclearly・Thewaterchannelwasmadeofatransparentacrylicplateof
Overflowtank
Hallidelight
Nozzle
トーラz5-l
T<一面一>
|’雲霧富
Testsection
era
Fig.2.7Experimentalapparatusfbrdyeflowinwaterchannel
15
dimension50mmheightandZOOmmwidthFinswereattachedtothesidewallasshownin
Fi92.7.FluorescentdyewasinducedjustattheffontofthefinsbyanalTowtube・Ametalhalide
lightsetatthetopofthechannelandadigitalcamera(NikonD70)setpelpendiculartochannel
sidewallwereusedtoobserveandshotthephotographsatpropertime.
2.5.2Experimentalsetupfbrsmokef1owVisualization
lnordertodetelminethenowbehaviorindetailsaroundtherectangularfInofdiffbrentpattern,
thenuidflowwasvisualizedbyflashlightsheetmethodusingsmokeasatracersuppliedat
upstreamsideatlowerReynoldsnumber・SmokeUowvisualizationapparatusisshownin
Fi9.2.8.ItconsistsofarectangularductmadeoftranSparentacrylicsheet,greenNd:YAGlaser
light,digitalcamera(NikonD70)andasmokegeneratorthatproducesmokeoffinedropletof
diameter0.3~1Um・ForsmokefIowvisualizationfinsweremadeoftransparentacrylicsheet・
TheNd:YAGlasersheetwhichwasusedhasanemittedradiationofwavelengthof532mn.
Camerawaspositionedattopoftheductandthelaserlightwassetatthesideoftheductparallel
tothelowerwalloftheductwherethefinset、Oncethecameraandlasersheetwerepositioned,
thesmokegeneratorsuppliedthesmokeattheentranceoftheduct,thenimagesofflowpattem
wererecorded.
〃Air
F1ow aser
へ
SmokeGenerator Imageprocessor
Fig.2.8EXperimentalapparamsfbrsmokeflowvisualization
16
2.5.3ExperimentalsetupfUroiltitaniumoxidefilmfIowvisualization
ExpenmentalsetupfbrtheoiltitaniumoxidefilmflowisshowninFig2.9.Titanium
oxideoilfI1mflowpattemsatthelowerwallaroundthefinswereobservedatthesameReynolds
numberofthatofthcmainexperlmentnow・Inthisexpenment,athinblackfIlmwasattachedto
thelowerwallandblackpaintedfinswereattachedtothefilmThelowerwallandthefin
surfacewerepaimedbyamixmreoftitaniumoxide,lmseedoilandoleicacidofsuitabledensity
andwereeXposedtoairflowtoobservetheoiltitaniumoxidefilmflow・Aftercompletionofthe
run,photogPaphsweretakenbyadigitalcamera(NikonD70).
Camem
NikonD70:蝋、/
’一
4円FII e
Fig.2.9Expenmentalapparatusfbroiltitaniumoxidefilmflowvisualization●
17
3DataReductionandUncertaintyAnalysis
3.11ntroduction
Inthischapterdatareduction,heatbalanceanduncertaintyanalysisaredescribed,because
theheatbalanceanduncertamtyanalysisareveryimportantfbrthereliabilityofthe
experimemalsmdy.
3.2.Datareduction
InthissmdywehaveinvestigatedthefluidUowandheattransfer・Theessentialquamities
determinedwerethefiictionfactorandtheheattransfercoefficientsfbrthedifferentfinpattems.
TI1etotalheatgeneratedfiomtheheatingsurfaceisdistributedintotheheattransfelTedby
convectiontotheflowingair,heatlossesfbroughtheinsulationandthewindow、Heatlosses
hombackoftheheatmgsurfaceexceptwindowmayassumetobeverylittleasthebakeliteplate
wasinsulatedby60mnthickinsulator,fbamtoprevemheatloss・Itwasfbundthattheheatloss
ffomwindowwas1.3%oftheheatUuxsuppliedtotheheatingsurface・Soitsvalueissosmall
comparedtotheheatinputvalueanditcanbeneglectedConseqUentlythetotalheattransfbrred
byconvectiontotheUowingairequalstheheatfluxsuppliedtotheheatingsurface,
Thehictionfactor/wasevaluatedfromthepressuredifferencebetweenpointsjust
upstreamanddownstreamofthefinsattachedtotheheatingsurfaceusmgtheEq.(3.1)
/=Ploss(DH/O/(paU2/2),(3.1)
whereノisthedistancetotherefbrenceregion,i、e・thedistancebetweenjustupstreamofD1fin
anddownstreamofD7fmandDHisthehydraulicdiameteroftheduct.
18
Theheattransfercoefficiems,
ノz=9/('鵬一t。。),(32)
wereobtainedatrepresentativeregion,wheretwisthewalltemperatureandtllet,cisthe
mainstreamtemperature・
WeconsideredthreekindsofReynoldsnumbersandexaminedtheireffbctsontheheat
transfercharacteristicsattheflatplateboundarylayerandthenalTowductwithfinarrays.
Reynoldsnumberbasedonthemainstreamvelocityandfinlengthcanbeexpressedas
(3.3)ReL=UZ〃
Reynoldsnumberbasedonthemainstreamvelocityandtheducthydraulicdiameterandcanbe
expressedas
Re=DDH〃 (3.4)
Reynoldsnumbersisbasedonthemeanvelocityandtheductequivalentdiameterandcanbe
expressedas
ReE=UDE" (3.5)
Thearea-averagedNusseltnumberbasedontheducthydraulicdiametercanbeexPressedas
(3.6)M=ハDH/几
Thearea-averagedNusseltnumberbasedontheductequivalentdiametercanbeexpressedas
(3.7)/VjlE=ADE/几
Thearea-averagedNusseltnumberbasedonthefInlengthcanbeexpressedas
jVZZL=AL/ス (3.8)
19
3.3Uncertaintyanalysis
Thepressurewasmeasuredbyadigitalmicromanometerofaccuracy土0.1mmofH20・
Theheatingsurfaccwascoveredwith60mmthickinsulator,fbamtopreventheatlossffomthe
heatingsurface・Therefbreheatlossesfrombackoftheheatingsurfacemayassumetobevery
little・Windowwascoveredwithtwolayersofpolyvinyledenefilmwithlmmspaceclearance
andheatlosseswere1.3%oftheheatfluxsuppliedtothesurface、Asaresult,heattransfer
coefficientcontainedlessthan3%uncertamty、Thetemperaturewasmeasuredbyboththe
themlocouplesusingadataloggerandtheinfiParedcamera・Thepercemagerelativeuncertainty
mthemeasuredtemperaturefbrthethermocouplesandtheinfiaredcamerawere±0.25%and±
1.3%respectively、Thepe1℃entagerelativeuncertamtymthemeasuredelectricpowerinputwas
±1.4%、Thepercemagerelativeunceltaintyinthecompoundvariableswerefbundtobeless
than±5%bothfbrthevelocityandtheReynoldsnumberand±3%bothfbrtheheattransfer
coefficientandNusseltnumber.
20
(11伽司『HeatTransferCharacteristicsandFlowAnalysisofFinnedSurfacesinCaseof
TallDuctandNarrowDuct.
4.1Introduction
Inthischapterwewouldliketodiscusstheheattransfercharacteristicsandfluidflow
analysisoffinnedsurfacesofco-angularandzigzagpatternExperimemswereconductedfbr
finssettingataHatplateboundarylayeri.e・intallductandnarrowductcolTespondingto200
mmand20mmductheight.
4.ZLocalheattransfercoefficients
4.2.1Smoothductresults
ThedistributionsofthelocalheattransfbrcoefficientsArattheendwallwithout
rectangularfinsareshowninFi9.4.1.Tochecktheaccuracyoftheheattransfermeasurements
andtodetenninethecharacteristicsofbothductswithoutfins,wemeasuredthelocalheat
transfercoefficientonthesmoothductsurface・Fig.4.1(a)presemsthestreamwisedistributions
ofthelocalheattransfercoefficiemalongtheendwallfromtheductentranceintheductofHd=
20OmmfbrU=5,10and20m/s・Thedistributionsgraduallydecreaseinthestreamwise
direction・Sincetlleductheightwassolarge,boththehydmdynamicandthennalboundary
layerswerestilldeveloPmg,evenatthetestsection・Duetotheroughsurfaceattheducti、let,
actlngasatriPPingsurface,theflowwasmrbulent,ThelocalNusseltnumber,basedoMrand
thedistance,Xfmmentranceagreeswiththeequation
M↓漣=0.0296Rejro8Pノ"06 (4.1)
21
fbramrbulentboundarylayeronanatplate,asshownbythesolidlinesinFig4l(a).Inthe
figure,theopenandsolidsymbolsdesignatethecenterlineandthemidlme,respectivelyBoth
setsofdataareequalregaldlessofspanwisepositionindicatingthetwodimensionalityofthe
150
国100m
g彦一
二i<50
0
-10-505101520
XL
(a)Hb=200mm
150
口100GJ
日~
芦一
旦<50
0
-10505101520
X/L
(b)Hu=20mm
Fig41Localheattransferdistlibutionswithoutfinonsmoothsurfhce
heatingsu1face、ThedatashowsgoodagreementwithEq.(4.1).
TheresultsfbrthenarrowductareshowninFig4.1(b).Thedistributions獣adually
decreasetowardthedownstreamdirectionatlowerReynoldsnumbers,AtRe=Z34x104and
22
35lx104,thedistributionslowlyincreasesandcoillcideswiththefbllowingequationfbrafUlly
developedhydrodynamicboundarylayerfbrairflowatthedownstreamregionasshownbythe
solidlines:
jVjl=0.O19Req8 (4.2)
HencetheductheightsofHa=Z00mmandHti=20mmrepresentsaflatplateturbulem
boundarylayerandaMlydevelopedductHowintestsection,respectively.
4.2.2EffCctofinclinationanglesonheattranSfer
Theeffectoftheinclinationangleoftherectangularfinonthelocalheattransfer
coefficientispresentedinFigs、42.(a),(b),and(c)fbru=0.,20.and25゜,respectivelyEach
graphrepresemsthestreamwiselocalheattransferCoefficientatthecemerlmeandatthemidline・
Cemerlmeandmidlinedataarecalculatedalongthecenterofthefinrowsandmiddlebetween
finlinsrespectivelyasshowninFi9.2.LTY1edatafbrthecemerlinedecreasesgraduallyuptothe
firstrowoffIns,whereitexhibitsarapidincreaseandattainsamaxⅡnumvaluebefbrerapidly
decreasingtoaminimumvalue,withthepatternrCpeatedperiodically,Maximumvaluesare
obtamedatfinpositionsandtheminimaareobtamedbetweenthefinsalongthestreamwise
direction・Themax1mumvalueatthefincentermaybethecombinedeffectoftheextended
surfaceandlongimdinalvortex・Eventheminimumvaluesarehigherthanthosewithoutfinsand
henceitmaybeconsideredthatheattransfbrisenhancedbyturbulenceorlargescalevortexes
generatedbythefins・Thesolidsymbolsinthefiguredescribetheheattransfbrenhancementsat
theendwallcausedbythelongimdinalandothervortexes・Asthelongitudinalvortexfbmledby
thetopsurfaceofthefinstrikestheendwallbetweenfinssotheheattransferisaugmentedof
thatにgion、Atthesametimetulbulencecausedbythecorneredgealsoincreasestheflow
mixing;asaresultheattransferisincreasedbehindthefins、Fig.42(a)showsthepeliodical
23
distributionfbroL=0.whiletheheattransfbrincreasesbetweenthefinsinthespanwisedirection
anddecreasesatotherplaces・Thoughtheheattransfercoefficientsalongthecenterlineposition
ofthefinfbru=20。(Fig.4.2(b))and25。(Fig.42(c))mcreasesinthesamemannerasibru=
0.,theheattransfbrcoeffIcientsalongthemidlinearesomewhatdifferent・Alongthemidlme,the
localheattransfercoefficientsareaffectedbythelongimdinalvortex,maimaminglargevalues
fbru=20.and25゜comparedtoOo・TheheattransfbrcoefIicientfbru=Z0oiscomparatively
higheramongthethreeinclinationangles,whichagPeeswiththedataobtainedbyBilenetaL
[2002]whofbundthattheeffectofinclinationangleonheattransferenhancementissmallfbr
inclinationanglesfbrlargerthan22.5..Aninclinationangleof20ocausedconsiderable
enhancementinheattransfer,Thisisbecauseoftheedgeeffectoftherectangularfin,which
increasestheHowmixmg・Whenthefinsaremmedaroundthemidpoint,thesideedgescan
producemoreturbulence・Becauseofmrningaroundmidpoint,thefinwillactasvortex
generatorandproducethelongimdinalvortexconstructingtrailingvortex,comervortexand
inducedvortex・Thelongitudinalvortexisalsocalledthestreamwiseonewhichaxisisinsame
directionasthestreamwise,andpersistsoveralongstreamwisedistance,distulbingtheentire
velocityfieldandtemperaturefIeld・Asaresultheattransfbrratemcrcases・Alsothecomeredges
increasesthemrbulenceleveLThissituationhasbeendiscussedbyHennan[1994]andHelman
andKang[2001]Therefbre,aninclinationangleof20oispreferredfbrfUrtheranalysis.
24
150
OCCs
[】對三]然函
0
-5 0 510
X/L
(a)CFO。
15
-0-centerline
+midlme
0000
s05
1[】判:β]詫二P ̄F’■■FロP ̄F ̄
-5 0 S10
X/L
○
(b)GI=20
15
-O-centerline
-十midline
150
0005
画週」宴]耳
戸一■■ ̄ ̄ ̄■■ ̄P ̄■■ ̄ ̄ ̄
0
-5 0 510
X/LO
(c)u=25
15
Fig.4.2EffectofinclinationangleonlocalheattransfercoefficientsfbrHf=10mm,Hb=Z00mnandU=10m/s
25
4.2.3Heattrans化rthroughfinofresinmaterial
Wenowwanttodiscussthefinmaterialwithregardsestimatingtheeffectofavortex
generatorontheendwall,butnotfbrtheextendedsurfaceThelocalheattransfercoefficientwas
measuredfbraresinmaterialfin,ratherthananaluminumfin・Theresultsareprcsentedin
Fig、4.3.Inthisfigur巳cemedmedistributionshowsthattheheattransfercoefficientatthefin
positionfbrtheresinmaterialincontrasttothealuminumfinasshowninFig42(b)issmaller,
becauseofnoextendedsurfaceeffect・Thecoefficientatthemidlinealsoappearstobeequalto
thatofthecenterline・Sotheoverallheattransfermaybeenhancedbyafactorofabouttwotimes
thanthatofwithoutfins、ThisresultsintheeffectofvortexgeneratorwithnoeffecthPom
150U(m/S)
一○--●-5
-匹-■-10
一○一一ローcenterline
一一midline0005
[】M…F]ゴ
-4 20246 8101214X/L
◎
Fig.43Variationoflocalheattrallsfercoefficientsfbrresinmaterialwithα=20,HIP=10mm,Ba=200mm.
extendedsurface・BecausetheBiotnumberfbrtheresinmaterialwasestimatedlOandthe
conductionresistanceofresinmaterialwasnearlyequaltothethermalresistanceTherefbre,the
heattransfere血ancementbyanaluminumfinonlywillbediscussedinthepresentexpenment
asthealuminumfinswillhavetheeffectofextendedsurfaceandthevortexgeneratoratthe
sametime・Asmentione。,thindouble-sidedtapeofthicknesslOO1unwasusedtoattachthefin
totheheatingsurface・Forthealunnnumfin,theconductionresistanceofthetapewas5%ofthe
26
totalresistance(fbranaverageheattransfercoefficienthassumedtobe200W7m2K).Therefbre,
thetapedidnothavemuchafTbctontheestimationoftheheattransfbrCoefficient、Also,theBiot
number,Bjofthealuminumfinwasabout0.009,whichissmallenough、Therefbre,theheat
transferenhancementbyaluminulnfinonlywillbediscussedhereafter.
4.2.4EffectoffinheightonheattransfbrcharacterMics
Theeffectoftheheightofthefinonthelocalheattransfercoefficientisnowdiscussed・
Fig.4.4showsthestreamwiselocalheattransfercoefficientdistributionfbrvaryingfinheightin
caseofco-angularpatternataUatplateboundarylayerasatypicalexample、Itwasfbundthatas
thefinheightmcreases,theextendedsurfaceareaalsoincreasesandasaconsequencetheheat
transferratelncreases・Thelocalheattransfbrcoefficientshowsaperiodicdistributionregardless
offinhei甑t・ThediffbrenceintheheattransferenhancementbetweenfinheightsoflOmmand
l5mmwasverysmall,whilethediffbrencebetweenfinsof5mmheightandotherfinswas
larger,Consideringthelargerpressurepenaltyfbrl5mmheightfin,10mmheightflnwas
chosenaSatypicalfi、.
200
000
505
引凶同自刮傷]×畠
0
-4-2024681012X/L
Fig.4.4EffectoffinheightonlocalheattransfbrcoefficiemalongcenterlineatcopattemfbrHd=Z00mmandU=10m/s
angular
27
4.2.SEffCctofvelocityonheattransfercharacteristics
Theeffbctofvelocityonthelocalheattransfbrcoefficientinaductof200mlnheightfbr
aco-angUlarpattemisshowninFig.4.5(a)Thelocalheattransfercoefficientslowlydecreases
inboththeupstreamanddownstrcamregion,whileatthefinnedregionaperiodicdistribution
appearswhichisindependentofvelocity・Thedistributionfbrthezigzagpatternissimilartothat
oftheco-angularpattem,asshowninFig、4.5(b),thoughwithsomedifferences・Forthezigzag
pattem,theheattransfercoefficientprofilenearthelowerpeaksappearsconcaveandshows
hi8hervaluesthanthatoftheco-angularpattem・Thepeakvaluesmtheupstreamreglonare
higherfbrtheco-angularpattemthanfbrthezigzagpatternThisisbecauseinthecaseoftheco‐
angularpattemtheflowtouchesbothsidesofthefinsstronglyandthencontinuessmoothly
whereasfbrthezigzagpattemtheUowstmnglytouchestheendwallandthefPontsideofthefin
whiletheothersideremainsuntouchedThus,theextendedsurfaceismoreeffectiveinthecase
oftheco-angularpatternfbraflatplateboundarylayer・Fig.4.5(c)showsthedistributionofthe
heattransfbrcoefficientfbrthezigzagpatteminanalTowduct・Inthisfiguretheupperpeak
valuesarefbundtobethehighestbecausetheupperwallofthenalmwductcausestheflowto
strikctheendwallandthefinmorestlDnglyDatafbrtheco-angularpatternfbrthenalrowduct
isnotshownbutthedistlibutionwasalmostthesameasthatfbrtallduct・I、Fig.4.5(a)-(c),
themaximumheattransfercoefYicientshowssomediscrepancyatsomestreamwisevelocities
duetosmalldeflectionsofthethermocouplesfromthecenterline.
28
13050
伽Sll2
uO8士桿
000000
50505
22[昏一副]潔昌
-4‐20246810121416
X/L
(a)Co-angularpattemHd=200mm
Oo0000
5o505
22[乢訊目一図]×二
-4-20246810121416
X/L
(b)Zigzagpatiem,Hd=200mm
000000
50505
22[乢引日へヨ]〆呂
-4‐20246810121416
X/L
(c)ZigzagpattemHd=20mm
Fig4SVariationoflocalheattransfercoefYicientwithvelocityalongcenteriinefbrHT=10mm
29
4.3Detailedheattransferanalysis
DetailedheattransfercoeffIcientdistributionsattheendwallsurfacesfbrrectangularfinsof
bothco-angularandzigzagpatternsfbraflatplateboundary(tallducOandnaITowductfloware
shownmFigs、4.6-4.9.AsshowninFigs、4.6(a)and47(a),theheattransfercoefficiemvalueis
higharoundthefinrowsfbrbothpatterns・Thesehigherheattransferregionsarethecombination
ofextendedsulfaceeffbctandvortexeffect,Atflatplateboundarylayer,Leintallductcase,
infraredimageviewoftheco-angularpattemshowsthediffbrenceofheattransfervaluaAt
narrowductcase,co-angularpattemexpenenceshigherheattransfbrthanthatoftallductcase・
Thisdifferenceisapparentlyshownbothatthefinbaseandendwallsurface・Becauseupperwall
ofthenarrowductmfluencestheflowbehaviorandasaresultflowstrikesthefinsurfaceaswell
asdownwashtheendwallmorestronglySoheattransfercoefficiemvalueishi甑eratfinbase
andattheendwalLOntheotherhandupperwallofthetallductcouldnothavemuchinfluence
ontheflowbehavior,sotheHowdidnotcontactthefinsurfaceandendwallasstronglyas
nalTowductAsaresultcomparativelylowerheattransfercoeffIciemvaluesareobservedatflat
platewithco-angularpattem、Sametotheco-angularpattemzigzagpattemexpenencesthe
effectofductheiglltonheattransfbrasviewedbytheinhFaredimagewithzigzagpattem、
CompanngvaluesofAJrbetweenthetwopatter、sshowsthatthehigherheattmnsferregions
increasesfbrzigzagpattern・Intheco-angularpattern,thehorseshoevortexisresponsiblefbrthe
enhancementoftheheattransfbraroundthefins・Theheattransferintheregionbetweenfinsin
thespanwisedirectionisalsoenhancedduetolateralmixing、Inthezigzagpattem,thehorseshoe
vortexaswellasthevortexgeneratedbythesidetopandCorneredgesstronglytouchesthe
altematelyangledfinsurfacesandendwaU,whichsignificantlyenhancestheheattransfer・Next
wewilldiscussmoredetailgraphicalpresentationoftheheattransfercoefficientfbrthedifferent
finpattem、
Figs、4.6(b)and47(b)showtheheattransferdistributionalongthecenterlineandmidlme,
30
AirHow
し
3mrow 4111 6t11Sth
h
ヴヴゲザググ
霞LPg-
」
F
〆
-Wh12K1-0I
140曰~
鴨田0.0 -120
-100
-I8IOl-1.0
Eニト60
ロ 2 4*6X/L
(a〕
00000
50505
2211
厩日彦]ゴ
ocente「line
●mIdllne
●
、 2 4*6X/L
(b)
X聯/L=00511.522533.54
1-0
鵜一OCN
-10
■百■■■ F百■■■ ■苗■■■ |■君■■■ ■■■■■ ■君■■■ l宙■■■■ JBm■■ 蛍■■■■
]MVWinzK]
(c)
100300
Fig46Detailedheattransfercoefficientdistributionsaroundtherepresentativeflnsofco-
angularpatternattbeendwallinaflatplateboundarylayerfbrReL=L27xlO4,Br=10mm.(a)Infiaredimage.(b)Centerlineandmidlinedistributionsand(c)Spanwisedistributions.
31
h
W1m2K-140
1-12o
割-100-80
--60
、P!グ
ダダ
Airflow
P
5lh3rdrow 4111 6t11
h
[w/m2K]=し140
一■一旬一■j
PlPP
b-■■ユ
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、10
、已囚鶉
0.0 120
100
80
60
1-0
ロ 2 4*X几
目
(a)
00000
50505
2211
口や日雇邑郵
centenine
midline
○⑥
ID 2 4 圖
X/L
(b〕
X7L 40.51 15 225=0
3’祠
3国旨ご皀冒冒冒ヨョョ剴冒〕皀冒冒冒則
5
1.0
曰一国十斗 0.0
10
anロ■ ■■■■■ H■■■■F■■■■ IF、■■ F■■■■
100300MWm2K]
(c)
Fig47Detailedheattransfiercoefficientdistributionsaroundtherepresentativefinsofzigzag
patternattheendwallinafIatplateboundarylayerfbrReL=LZ7xIO4,H「=10mm.(a)Infiaredimage.(b)Centerlineandmidlinedistributionand(c)Spanwisedistribution
32
1000
h肛料烟相即釦
要|叫一川二
P
■
鱒==唇
座、輿
、!
Forbothpattems,thecellterlinedistributionisperiodicandthepcakindicatesthehWalueatthe
fincenter、HeattransferdistributionsalongstI℃amwiseandspanwisewithvalyingvclocitywercalso
fbundperiodicalrBgardlessoffinpattemandveIocityintheprwiousexperimentalworkoflslamet
alPOO61AroundthethirdrowfinsthedistributionsshowhighervaluesofLfbllowedbya
subsequentdeclineinthedownsn℃amregion,ThcsIopeofthedeclincisgJ℃aterfbrtheco-anguIar
patternthanfbrthezigzagpattem・Themidlinedistribution,showingtheendwallheattrHnsfbr,is
aImostflatandisnearlyhalfthemaximumvalueltisaIsonotablethatthcslopcofthedeclineis
againhigherfbrtheco-angularpattem・T11clesserslopefbrthczigZagpattemmightbeduetothe
vortexthatstmmgIyattachestothefarthestrowfin
Figs、4.6(c)and4.7(c)showthespanwisedistributionfbrbothpattems・Thet1℃、dsofthe
distributionsaresimilar,butthevaluesofhzaredifYierent・Atr/Z=0,2and4,Le、atthecenterof
eachfinIDw,periodicdistributionsappear・Attheupsb『Bamsidefinsthelowerpeakandhigluerpeak
valuesarChighcrfbrtheco-angularpattem,butatthedownst1℃amsidefinsthezigzagpatbernexhibit
highervalues・Atr/Z=u5and25,thcdistributionstendtowardsbeingflatwhileatr7Z=land3,
theybecomenearlycompletelyHaLImmediatelybefbrethefins,i,eatr/Z=l5amd3、5,the
distributionsagainbecomeperiodic・Figs、4.8and49showthedistributionsfbrnarmwductHow,A
comparisonbetweenFigs、4.8(a)and4.9(a)identificsthediffe歴、tshapesofthehcattmnsferproflIe・
Imhecaseofthcco-angularpattern,thelowerheattransferrcgionsa1℃compamtiveIylargeand
hysterisisshaped,whilefbrthezigzagpattern,theIowerheattmnsfbr1℃gionsa1℃compamtively
smaUwithadeltashape・ThehoTseshoevortexesaroundthefinfbrbothpatternsobservedfbrtheoil
HImflowagTcewiththeobservedhigherheattmnsfbrr℃gionaroundthefinintheinfraMimages、
ThelevelofheattlnnsfeTincrcascsinanalTowductcomparedtoafIatplatebecausethcupperwaU
induccsthcflowvortcxtoattachtotheendwalIandfinsurfhcemo”strongly、Largehigherheat
transferregionsaroundeachfinrowareobservedfbrbothpattems・Figs、4.8(b)and4.9(b)showthe
distributionaloH1gthecenterlineandmidline,Thoughthepattcmsofthcdistributionsaresimilarto
thosefbrthefIatplatecase'んrisgrCaterandthercisnosignificantdecIine、
33
Airflow
傍
3rdrow 401 Stb 6t]]
hヴーグ
ゲ・餌
諄||伊
伊毎 [W/mzK]1-0
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(a)
Ocenterline
00000
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(b)
X率/L=00511.522.533.54
1.0
鵜已CO田
-1-0
100300
hz「Wm2K]
に)
Fig48DetailedheattransfbrcoefficientdistributionsaroundtherepresentativefInsofco-angularpattemattheendwallinaductflowfiorRc=2.34×104,舟=10mm.(a)Inffaredimage.(b)Centerlineandmidlinedistributionand(c)Spanwisedistribution.
34
Airflow
伊
3rdrow 4111 5111 6ul
h
,倭財p鷺倭■P■
ヂロ●弾!
10[Wノm2Iq言壽1-200
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場
(a)
ocenterline
00000
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midline
夢i`是凝汽
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(b)
X7L=00511.522533.54
1.0
驫己CO■
-1.0
qnu Huu F酌■■Ⅱ凹凹 ロ■■ Hmu ロ■■ uZuu ロ凹凹
100300hz[VWm2K]
(c〕
Fig.49DetailedheattransfercoefHcientdistrlbutionsaroundtherepresentativennsof
zigzagpatternattheendwallinaductflowfbrRc=2.34xlO4,Bi=10mm.(a)InfTaredimage.(b)Centerlineandmidlinedistributionand(c)Spanwisedistribution.
35
T11elowerpeakpmfilesareconvexfbrtheco-angularpattem,whileincontrasttheprofileisflat
fbrthezigzagpattemandthelowerpeaksarecomparativelyhigher・Themidlinedistributionis
neadystraightfbrbothpatternsbuttheAWaluesaregreaterfbrthezigzagpattern・Figs、48(c)
and49(c)showthespanwisedistributionfbrbothpatternsThcdistributionisalmostperiodic
exceptatr/Z=land3Boththelowerandhigherpeakprofilesarecomparativelymoreconvex
fbrtheco-angularpattern,whilefbrthezigzagpatterntheprofilesarehigherandsomewhatflat.
4.4.Area-averagedheattransfer
Thearea-averagedheattransfbrcoefficientsattheendwall,finbaseandtheoverall
surfacesoftherepresentativethird,fburth,fifihandsixthrowfinsweremeasuredfiDminffared
lmages・Heattransmissionfiomtheoverallsurfhceisthetotalheattransferf、mthefinsurface
andtheendwallwithoutthefins・Againheattransferratefromthefinsurfaceisequaltotheheat
transfbrfiomfinbase・Sotheareaaveragedheattransfercanbecalculatedusingthefbllowing
equation,Eq、4.3.
伽…F-4L恥瑞脇…AOM目,12〃(43)
where'4.…/listheoverallsurftlceareaincludingthefinbaseand蜘and鋤`wallarethesurface
areasofthefinbaseandendwall,respectively、Therelationshipbetweenthearea-averaged
NusseltnumberandtheReynoldsnumberisshowninFig4.10.Thearea-averagedNusselt
numberisrepresentedbythecurvefittedlines,A11thedataftl11smoothlyonthecurvefittedlmes
andcanbewellrePresentedbythefbllowingeqUation,Eq、4.4.
ハノ1M=CRG0.7. (4.4)
ValuesfbrcfbrdifferentductheightsandfinpattemsarelistedinTableLItisfbundthatthe
zigzagpatternhashighercvalues`Fig.4.10(a)showsthevariationoftheNusseltnumberwith
theReynoldsnumberattheendwallandtheoverallsurfacefbrbothpatternsfbranarrOwduct,
36
3
2
1
10098765
'二
4
3
23456789
104 105Re
(a)Area-averagedNusseltnumberattheoverallsurfaceandtheendwa,,asafimctionoftheductReynoldsnumberfbrHb=20mm.
100
ざ函
104 ReL
(b)Area-averagedNusseltnumberattheoverallsurfaceandtheendwallasafimctionoftheReynoldsnumberbasedonfinlengthfbrHa=200mm.
Fig.4.10RelationshipbetweenNusseltnumberandReynoldsnumber
37
TablelCoefIicientofheattransfercorrelationattallductandnarrowductfbrco-angularandzigzagpattern
MノーcReo7FinPatternDuctHeight(nm) C
endwalloverall
Co-angularZigzag
Co-angular
ZigZag
20
20
200
200
0.151
0.17
0.096
0.100
0169
0.187
0.113
0.108
wheretheNusseltnumberisarrangedbyducthydraulicdiameter・Forallcases,theNusselt
numberincreaseswithReynoldsnumber・CO、sideringtheoverallheattransfbr,thezigzag
patternshowsahighervaluethantheco-angularpattem・Asnotedpreviously,thisisduetothe
factthatfbrthezigzagsettingthelongitudinalvortexstrikestheendwallandthefinsurfacemore
stronglyuptothefarthestfinrow.Comparingtheendwallheattransfer,thedifferenceintheheat
transfbrbetweentheco-angularandzigzagpattemsismorethanthedifferenceintheoverall
surfacearea,indicatingthattheendwallheattransferismoresigniflcamfbrthezigzagpattern
whiletheextendedsurface(fin)effectissignificantfbrtheco-angularpattern、Theoverallheat
transfbrisfbundtobeenhancedbymorethanfburtimesthesmoothsurfacevalue・
ThevariationintlleNusseltnumberwithReynoldsnumberattheflatplateboundary
layerisshowninFi94.10(b),withtheNusseltnumberalTangedbyfinlength、Theoverallheat
transferenhancememfbrbothfinpattemsisnearlythesamethoughthecoefficientcfbrthe
zigzagpatternissomewhathigher・Theendwallheattransferfbrthetwofinpattemsisfbund
similartoo.
Theheattransferffomasmoothsurfacewasestimatedasfbllows:Theheattransfer
CoefficientswerefirstcalculatedfromtheEq.(4.1)fbrtheHatplateboundarylayerwithinthe
rangeX=0.28m-0.36m,correspondingtothelengthbetweenthethirdandfIfihrows.
形。i`0。…ノw過ム“O28
X
(4.5)
38
TheseheattransfercoefflcientswerethenreplacedbythemeanNusseltnumberbased
onfinlengthandcanberepresemedbythefbllowingequation:
jVjJL=O0138ReLq8. (46)
Theoverallheattransferwasthencomparedandfbundtobemorethanthreetimesthesmooth
surfacevalue・Hence,itwasfbundthatheattransferisimprovedwiththeuseoffins,Azigzag
PatternfbrHli=20mmwasfbundtobemosteffectivefbrheattransferenhancement.
4.5FlowvisualizationbydyeUowinwaterChannel
Inordertodifferentiatetheflowbehavioroftheco-angularandzigzagpattern,dyeflow
inwaterchannelwasobservedatlowerReynoldsnumber・ThedyeHowpattemsfbrtheco-
angularandzigzagpatternsareshowninFig.4.11.Inthecaseoftheco-angularpattem,thedye
UowstagnatesinffontofthefIrstrowfInandfblmsahorseShoevortexsulToundingthefin,At
therearofthefin,thevortexrollsupandrisestojomtheflowseparatedfmmtheupstream
portionofthefin,whereitthenshowslongitudmalbehavior,Astllefinsaresetatanangle,edge
effbctsareobserved、Thecomeredgescauseturbulenceandenhancenowmixingintheinterfin
regionalongthestreamwisedirectionThemainvortexflowsabovethefinsasshowninFi9.
411(a),touchingthetopofthefinsandthesidewall,Thehorseshoevortexaroundtheco‐
angularpattemissostrongthatthelongitudmalvortexcannotcomacttheinnerportionofthe
horseshoevortexandthevicinityoftheendwallexceptfbrthebothsidewallsofthefins.It
causesheattransfbrenhancememfromthesurftlce、
InthecaseofthezigzagpattemshowninFi9.4.11(b),alongimdinalvortexgeneratedby
theHrstfInmwtouchestheffontofthesecondrowfinandattachestotheendwallaroundthefin
asthehorseshoevortexthatappearsisnotstrong・Duetothealtematedirectionsofthefins,the
flowdirectionischangedandthelongitudinalvortexstrikestheendwallbetweenthesecondand
39
thirdrowfIns、Theinterfinregioninthespanwisedirectionatthethirdrowiscontactedbythe
mainvortexBeyondthisrow,theHowadvancesandtouchesthefbllowingfinandtheendwall・
Wenotethattheflowattachesstronglytobothsidesofthefinibrtheco-angularpattern,while
化rthezigzagpatterntheffontsideistouchedbytheflowstronglybutthebacksideissolIly
touchedonly・Therefbre,heattransfiBrfiFomtheHI1shouldbesignificantintheco-angularcase,
whileheattransflerfiDmtheendwallshouldbehigberillthecaseofthezigzagpattern,aswillbe
describedlater
Waterflow
匡
Rolledupvortex
Horseshoevortex
I1U-IIlI1lIIIIIUIUII ’1111
0.O
Mainvortex(、.v、)
20 40 6.0
X几
(a〕
Waterflow
 ̄
可も一
鞭UP■
ylll…剰
戸<夛只 -==
=髭-1p
鰹露讓愛l學i學邑i雪ごi§霊露I1IUIIUIlllUU11 11
00 20 40 6.0
X/L
(b)
Fig.4.llFlowvisualizationinawaterchannelaroundshortrectangularfmof(a)co-angular,(b)zigzagpatternwithHi-lOmm,PR=ZandRe=1350.
40
4.6.Frictionfactor
lnordertocalculateftictionfactor,wefirstmeasuredthepressuredropdistribution・
Fig.4.12showsthehictionfMorobtainedffomEq.(3.1)inananDwduct、Inthetallduct,a
hydrodynamicboundarylayerdoesnotfUllydevelop・Thefiictionftuctorfbrlargescale
protrusionpositionedinsurfaceshowsalmostconstantregardlessoftheReynoldsnumberand
thehctionfactorvaluesfbrbothpattemsarelargerduetotllelargescalevortexfbrmedbythe
existingprotrusionsthantheskinfiictionfbrasmoothductwithoutfinsfbrMlydeveloped
mrbulentflow・Astheflowfbrthezigzagpattemhasmorecontactwiththefinsurfhceand
endwallandconseqUentlyalargerdragfbrceoccursthanfbrtheco-angularpattemAsaresult
thefiictionfhctor,/issomewhathigilerfbrthezigzagpatternthanthatofco-angularpattem.
2
01Cco-angular
●
恋zlgzag
864
、、
一一、一一一2
‐1/4
f=O3164Re
0.01
104 2 3456
Re
Fig.4.12Frictionfactorvs、ReynoldsnumberinananFowductfbrco-angularandzigzag
pattemofHイー10mm
41
4.7Summary
Afterperfbnningthisinvestigationwecanconcludeasfbllows:
LTheinclinationanglehasagreatinfluenceontheheattransferenhancement・Amongthe
inclinationanglesofO・'20.and25.,anangleof20・appearsoptimumfbrenhancementin
thisexpenment、
2.Thefifictionfactorsfbrbothpatternswerelargercomparedwiththeskinhdctiononthe
smoothsurfncefbrafUllydevelopedmrbulentnow・Thefrictionftlctorvaluewasfbundtobe
almostconstantandlargerfbrazigzagpatternofflnsthanfbraco-angularpattern、
3.Indyeflow,stronglongitudinalvortexesappearedthattouchtheendwallneartheinterfin
space・Inthecaseofthezigzagpattem,onesideofthefinwasalsocontactedbythevortex・
Thoughacornervortexalsotouchesthesidewallofthefin,thehorseshoevortexwasnotas
clearlyobservedIncontrast,alongimdinalvortexisalsoobservedfbrtheco-angularpattern,
whichtouchesthefinsurfnceaswellaslightlycontactingtheendwalLAhorseshoevortexis
alsoclearlyobservedinffontofthefins、
4.Companngarraysofco-angularandzigzagpattems,thelaterismoreeffectivefbrheat
transfbrenhancementinthecaseofnalTowductflow・Theaverageheattransfercoefficient
fbrazigzagpatternismorethanfburtimeshigherthanthatofwithoutfins・TheNusselt
numberincreaseswithReynoldsnumbertothe0.7uIpowerfbrbothpatternsinanaIrowduct
aswellasfbraflatplateboundarylayer.
42