FINAL PROJECT: EVAPORATOR FAN DESIGN

ChristopherPhaneuf

BrianTovarME407:ComputationFluidDynamics

ProfessorScottBondi05May2008

TABLEOFCONTENTS

1INTRODUCTION 1

1.1Objective 1

1.2HeatExchangerDesign 1

1.3FanTypes 22DESIGN 3 2.1Assumptions 3

2.2DesignConcept 3 2.3HeatExchangerRedesign 7 2.4DesignIntegration 153FANSIMULATION 16 3.1Preprocessing 17

3.1a)Full‐sizemodel 17

3.1b)Periodicmodel 18

3.1c)Shroud/manifold 23

3.2Solution 244RESULTS 25 4.1Postprocessing 25

4.1a)Fullfansimulation 25

4.1b)Periodicfansimualtion 29

4.1c)Shroud/manifoldsimulation 32

4.2FanPerformance 355DISCUSSION 36 5.1Generalresults 36 5.2CostEstimate 36 5.3Designbenefits 37APPENDIXI:DimensionedFanSchematics 38

APPENDIXII:HandCalculations 40

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1 INTRODUCTION

1.1 Objective

Norefrigerationcyclewouldbecompletewithoutanevaporator;noevaporatorwouldbecompletewithoutaheatexchangerandafan.Previousdesignandcomputationalfluiddynamicsanalysisaimedatthedevelopmentofanevaporatoryieldedanovelsingle‐passshell‐and‐tubeheatexchangerfeaturingannularfinsandperpendicularductingfortheinletandoutlet.Thisdevicecoolsairfrom100°Fto50°Fwhileminimizingthephysicalsizeandpressuredrop.Forallpastsimulations,flowthroughtheheatexchangerwassetatamid‐rangevelocityof250ft/min.Thesourceofthemovingairwasirrelevantuntilnow.

Todrivetheairthroughtheheatexchanger,acompact,energy‐efficientfanshallbedesignedtooperateatthreespeedscorrespondingtomeanflowvelocitiesof200,350,and500ft/min.Inaddition,thefanmustbecoupledtotheheatexchangerwithashroudoralternativeconnector.

1.2 HeatExchangerDesign

Thedesignofthefanandshroudbeganwherethefinaldesignofasimpleshell‐and‐tubeheatexchangerleftoff.Bydirectingairflowaroundaseriesofannularbaffles,moreturbulentmixingpromotesincreasedconvectivecoolingfromthelow‐temperaturerefrigerantpipe,whichisassumedtostayataconstanttemperaturealongitsentirelength.Plotsbelowdepicttheuniquegeometryandillustratetheeffectivecooling.

Figure1 Streamlinesfororiginalshell‐and‐tubeheatexchanger

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Figure2 Temperaturecontoursfororiginalshell‐and‐tubeheatexchanger

Theoriginalheatexchangerdesignperformedinaccordancewithrelativelylooserequirements.Thispromptedaneedtotesttheheatexchangerundernewlypresentedconditions,includingthevariableflowrateanddirectionofflow.

1.3 FanTypes

Mechanicalfanstylesandconfigurationsaremultifariousandsuitavarietyofneeds.Thetwobasicstypesareaxialandcentrifugalfans.Centrifugalfansareruggedmachinescommonlyusedforlarge,industrialapplications.Thedemandforacompactdesigneliminatesthisoptionandleavestheaxialfanstyleopentoadaptation.Elementsofthefansuchasmotortype,bladegeometry,andhousingpermitflexibilityofperformanceandallowspecificapplicationmatching.

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Figure3 Fantypes:centrifugal(left)1andaxial(right)2

2 DESIGN

2.1 Assumptions

Likemostengineeringproblems,acriticalsetofassumptionsisrequiredtorealizeandtesttheintegrationofafanforourevaporator.Detailsofpartswell,acousticeffects,bearingimplementation,inducedvibration,andotherphysicalnuancesencounteredintheoperationofafanareneglectedtosimplifythedesignandshiftemphasisonachievingbulkflowrequirements.AnothersimplificationisencounteredintheapproachtosolvingthegoverningfluidmechanicsequationsusingFLUENT.Thetwomainoptionswithinthesoftwareare1)steadystateanalysisusingmultiplereferenceframes,includingarotatingframeand2)unsteadyanalysisusingaslidingmesh.Duetoinexperiencewithunsteadysimulationsandalackofneedforthebehaviorcapturedbythistypeofsolution,asteadystateapproachwastaken.

2.2 DesignConcept

Duringtheearlierdesignoftheheatexchanger,severaldiscussionsaroseoverthemeansofairflowdelivery.Althoughthefinaldesignfeaturedperpendicularductsateachendofthedevice,themoreobviousandpotentiallymorecompactmethodofaxiallydirectedflowintotheheatexchangerwasconsidered.(Thiscorrespondedtotheoriginal2Daxisymmetricsimulationthatvalidatedthegeneralshell‐and‐tube/annularfinconcept.)Uponmorecriticalconsiderationofanaxialflowscheme,theideawasabandonedduetoquestionsoffeasibility.Itwasthoughtthatthepathoftherefrigerantpipeandthe

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fan/shroudsystemwouldconflict.Thepipewouldhavetobebenttoexitashroudandinturncreateabarrierforflowintotheheatexchanger.

Thetaskofdesigningafanfortheevaporatorbroughtustorevisittheideaofanaxiallyorientedflowsource.Callingonpastresearchexperienceinelectricmotortechnologies,muchofwhichiscenteredonfandesign,wedevisedanapproachofdirectlycouplingourheatexchangertoanaxialfanpoweredbyabrushless,permanentmagnetmotor.Theflexibilityofanouter,electricallycommutatedstatorandaninner,permanentmagnet‐equippedrotormakesthisconfigurationpossible.Onthesurface,theroughspecificationsprovidedforthefancomponentsresembleacommoncoolingfan.Thegeneralbladeshapeandsizecorrespondcloselywithmid‐sized,axialcoolingfans.Theprimarydifferenceslieintheslightlymorecomplexbladestyleandthedistributionofthepowercomponents.

Withmodelingandmeshinglimitationsinmind,thefanbladegeometrywaskeptsimple.Theprofileneededtobemoreinterestingandmorecapableofacceleratingfluidthanaflat,angledplatebutnotascomplexasthecarefullydesigned,low‐noisespecialtydesignsfoundinconsumerproducts.UsingSolidworks,asplineintheshapeofanairfoilwascreatedandoffsettogivethebladeathickness.Thetwoedgeswereclosedwithtangentarcsateachend.Thisouterprofilewascopiedontoanotherplane(whichwasoffsetbytheexpectedlengthoftheblade)andmodifiedtobesmallerandexhibitamoresevereangleofattackatthebase/hub.Thesetwoprofileswereloftedtogenerateasingle3‐dimentionalblade(Figure4)andexportedasaSTEPfiletobeimportedintopreprocessingsoftware.

Figure4 Fanbladegeometry

Themotivationbehindthemotordesigndepictedinthe2Dschematicbelowistheabilitytorunapipethroughthecenterwithoutaffectingtheoperationofthefan.Byplacingmagnetsofalternatingpolarityalongthecircumferenceofaretainingringontheoutsideofthefanbladesandbyguidingtherotationofthefanblades(rotor)withanouterbearing

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builtintothefanhousing(notshown),thecenterofthehubcanbehollowandallowthepassageoftherefrigerantpipe.Thismakesthepreviouslyabandonedideaofaxially‐drivenflowcompletelyconceivable.Theonlyothercomponentofthefanisthehousing/enclosure,whichisanordinaryplasticcasemodifiedtosupporttherotorwithabearingsupport.

Figure5 Fanmotorconcept:custominrunnerbrushlessPMmotor

Figure6 Fanassembly

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ABS(AcrylonitrileButadieneStyrene)plasticwasselectedforthebulkofthefanparts,excludingthesteelstatorlaminationsforthemotor,thecontrollingelectronicsandthemagnetsontherotor.Foruseinsimulations,keypropertiesweregathered:

Density=1080kg/m3(SG=1.08)

Specificheat=0.34Btu/lb°F

ThermalConductivity=0.1125Btu/hrft2°F

Maximumtemperature=180–200°F

UsingSolidworkstodeterminethevolumeoccupiedbyeachprimarycomponent,theapproximatemassofthefanwascalculatedfromthecorrespondingdensities.

(mfanblades=0.02612kg)+(mfancover=0.131kg)+(mstator=0.198kg)+(mmagnets=0.02kg)

=0.375kg

Weight=3.675N=approx.0.8lbs.

Technicalspecificationsforthisfanarepotentiallylimitless.Brushlessmotor‐basedfansareversatiledevices,controlledwithvariousfeedbackschemesandcapableofeitherlow‐speed,high‐torqueorhigh‐speed,lowtorqueapplications.Forthisparticularfan,thestatorisdesignedforrelativelyhighspeedsandappropriatelylowtorquegeneration.Thestatorteethwillfeaturethinmagneticwireandahighnumberofwindings.Theoptimalmeansofpoweringandcontrollingthefanfromline‐fed(wall)electricityisacomplexsystemsummarizedbythefollowingflowofcomponents:apowerrectifierandregulatorconvertingACtomanageableDC,avariablefrequencyinvertertosupplythree‐phasealternatingsquarewavecurrentpulsestothreesetsofstatorwindingsdistributedovertwelveteeth.SteadyspeedcontrolisaccomplishedwitheitherhalleffectsensorspositionedaroundtherotororamethodsensorlesscontrolthatestimatesrotationalvelocitybasedonmeasurementsofbackEMFthroughnon‐energizedwindingphasesduringeachcycle.[NOTE:Thesedetailsareseeminglyexcessiveforthepurposesofthisassignmentbutrepresentacarefullyselectedsetofcomponentsthatareeasilymanufacturedandmeettheoperatingdemandsoftheevaporator.]

Whilemostheatexchangersrequireashroudtoconverttheroundgeometryoftheeffectivefanareatothesquareorrectangularfaceofthetypicalcross‐flowheatexchanger,ourshell‐and‐tubegeometryobviatestheuseofashroud.Withminormodification,theroundcylinderisdirectlycoupledtothefan.Inlieuofashroudanalysis,theintroductionofa3‐inchsectionofpipemanifold(tobeexplainedinthefollowingsection)necessitatesastudyofthissegmentoftransitionbetweenthefanandthebafflesoftheheatexchanger.Additionally,thesimulationsofthefangeometrycapturetheflowin3‐inchlong,5‐inchdiameterstraightshrouddirectlyaftertherotatingflowaroundthefanblades.

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2.3 HeatExchangerRedesign

Tovalidatethedirectcouplingapproachofincorporatingthefanintotheevaporatordesign,theheatexchangermustbealteredandtestedforadequateperformancewiththeaxialinlet.Sinceafandiameterof5incheswasselectedfromthestartofthedesign,priortotheinitialstepsofredesigningtheheatexchanger,theoutershelldiameteroftheexchangerwasadjustedaccordingly.ThegeometryofolddesignwasreproduceddirectlyinGAMBITwiththefollowingchanges:theeliminationoftheperpendicularducting,theincreaseofshelldiameterto5inches,andtheuseoffacestosplitthevolumefortheannularfinsinordertousecoupledthermalboundaryconditions.Quadmesheswereappliedtoallfacesandthevolumemeshwasgeneratedtoyieldedacleanlyrevolvedsetof640,549elements.Thegeneralboundaryconditions(forthissimulationandallsubsequentvalidationruns)wereaninletspeedof250ft/min,refrigeranttemperatureof10°F,andcoupledfins.Thek‐epsilonrealizableturbulencemodelwasusedandsolutionscontrolswerekeptattheirdefaultsforthesakeofintermediatetests.Theresultswerenotpromising,confirmingtheexpectedissueswiththealternativeflowscheme.Theaxialflowdidnotinducethesamedegreeofmixingandthearea‐weightedoutlettemperaturewasjustunder80°F.Thisnecessitatedaseriesofsimulationscheckingtheeffectofminorgeometricmodificationstowardabettercoolingshell‐and‐tubeheatexchanger;however,despitemeasuressuchaslengtheningtheheatexchangerandapplyingtighterfinspacing,thecoolingcapabilitiesofthesingle‐passexchangerwithannularfinswasnoteffectivewithinreasonableconstraintsofcompactness(Tout=57°F/∆P=00.28).Thefollowingfiguredemonstratestheinadequateheattransferoftheoriginaldesignexperiencingaxialinflow.

Figure7 Post‐modification,single‐pass,15‐inch,9‐finheatexchanger

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Inanefforttoattainbetterperformancefromouraxially‐fedheatexchanger,thenextphaseoftheredesignborrowedfromthemorecommonstyleofshell‐and‐tubeheatexchangers,whichfeaturealargenumberpipeschanneledthroughtheinteriorandalternatingbafflesshapedliketruncatedcircles(Figure8).Byincorporatingamanifoldthatsplitsthesingle1‐inchpipeofrefrigerantinto7half‐inchpipes,morecoolingsurfaceareaisintroducedtothedesign.Additionally,bafflessimilartothosefoundwithinindustrialheatexchangersofthiskindwereincluded.

Thefirstattemptwiththistopologywasareturntotheoriginal12‐inchlengthandhad3baffles.Resultswerenotcompletelysuccessful(Tout=76.25°F/∆P=0.65inches‐water)butshowedroomforimprovement(Figure9).Alonger,15‐inchshellwith7bafflesat1.5‐inchspacingprovidedsufficientresultswithanoutlettemperatureof49.9°Fandapressuredropof2.2inchesofwater(Figures10‐15).Adrawbackofthisapproachistheinabilitytomatchthequalityofthemeshforthesingletube,axisymmetricdesign.The3DmeshwasmadewithTGridandconsistsof279,168elements.Also,whilethepressurelosswasgreaterthanthepreviousdesigns,themodelstillmettheprimaryrequirementofreducingtheairtemperature.Sincetheadditionofthemanifoldaffectstherefrigerantflow,asimulationofthedividingflowwasrunthoughameshof64,980elements(Figure16‐19).Thisconcludedtheheatexchangerredesignprocess.

Figure8 Commonshell‐and‐tubeheatexchanger3

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Figure9 7‐pipe,12‐inch,3‐baffleheatexchanger

Figure10 Residualshistoryforfinalheatexchangerredesign

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Figure11 Temperaturecontoursforfinalredesign

Figure12 Pressurecontoursforfinaldesign

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Figure13 Velocitycontoursforfinaldesign

Figure14 Pathlinescoloredbytemperature

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Figure15 Redesignedheatexchanger

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Figure16 Internalrefrigerantpipeflow‐‐manifoldmesh

Figure17 Pressurecontoursthroughmanifoldatmid‐plane

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Figure18 Velocitycontoursthroughmanifoldatmid‐plane

Figure19 Temperaturecontoursthroughmanifoldatmid‐plane

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2.4 DesignIntegration

Withthegeneralfanconceptdevelopedandmodeledandtheheatexchangeradaptedtothesizeofthefan,thetwopartswerecombined.Aflange,ormountingplate,wasaddedtotheendoftheheatexchangertoallowboltingtothefan.Figuresoftheassembled,in‐lineevaporatorweregeneratedwithSolidworks.

Figure20 Assembledevaporatorcomponents

Figure21 Evaporatorwithsectioncut

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Figure22 Explodedviewofevaporator

3 FanSimulation

Computationalfluiddynamicswasemployedtorefinethedetailsofourdesignandvalidatetheeffectivenessofthefan.Anaccuratesimulationcanprovideseveralmeasuresoffanperformancecriticaltotheoperationofanefficientandusableevaporator.Withthreedesiredsettingsforthefan,thesimulationprocesswasiterative.Addingtothenumberofrequirediterationwasthelackofexperienceandthereforenumerousissueswithmovingreferenceframes.

Sincethesymmetryoftheaxialfantypeslendsitselftoananalysisofonlyasectionofthefan,periodicboundarieswerefirstattempted.Althoughtheboundaryconditionsseemedtomakesense,resultspointedtomajorerrorsinthesetup.Followingmanypermutationsoflogicalboundaryconditions,thefailuretoyieldreasonableflowregimes,theperiodicconditionwasthesuspectedcause.Wetooktheapproachofmodelingtheentirefanstructure,whichintroducedmoreelementsbuteliminatedanypossibleproblemscausedbyanimproperperiodicconfiguration.Despitethesimplificationofthesimulation,resultsstillfailedtocapturetheintendedflowdirections.Lookingmorecloselyatthemixingplanetutorialthathadoriginallyguidedtheselectionofboundaryconditions,subtlesettingswerefoundtobedisparatewiththepastattemptsanduponadjustment,simulationseventuallyrevealedtheintendedflow.Byestimatingrotationalspeedsfromtheinitialresults,thecorrectvaluesforproducingthethreeoutletspeedsof200,350,and500ft/minwereeventuallydeterminedthroughtrialanderror.Toconfirmourabilitytouseperiodicconditions,wereturnedtomodelingonlyasinglebladeofthefan.Thefollowingsectionswillbrieflydepictthefull‐scalemeshandthentakeyouthroughthedetailedprocessesofcreatingthemeshandarrivingatasolutionfortheperiodicmodel.

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3.1 Preprocessing

3.1a)Full‐sizemodel

Figure23 Full‐sizemeshdisplayingallfacemeshes

Figure24 Close‐uponquad‐mappedfanbladewithboundarylayer

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3.1b)Periodicmodel

OncethegeneralfanconfigurationwasestablishedandthefanbladegeometrywasmodeledinSolidworks,preprocessingsoftwareknownasGAMBITwasusedtomodeltherelevantfluidvolume,createathree‐dimensionalmesh,andsetboundarytypestobefurtherdefinedinthesolver.

InGAMBIT,theimportofanytypeofnon‐nativefileleads,almostalways,toanexcessoflowergeometry.Forexample,somelinesorcurveshaveunnecessaryverticesalongtheedge.ThefirststepafterimportingtheSTEPfilegeneratedinSolidworksisaclean‐upoftheresultinggeometry,andforthiscasethatmeansacompletedismantlingoftheuppertopologiesandareconstructionoftheloftedfanbladefeature.Thishastobedonebeforetheairfoilvolumecanbesubtractedfromthepositiveairspace;otherwise,wewouldhavetoreconstructthatentiregeometryaltogetherandweprefertomakeourworkloadsimple.

Thefirstanomalyencounteredisthesingleloftededgesalongtheleadingandtrailingedgeoftheairfoilcross‐section.Afterboththevolumeandallthefaceshavebeenerasedfromtheimportedgeometry,thesetwoedgescanbedeleted.Whenonetakesacloserlookateachofthe(nowisolated)airfoils,youdiscovertheoverdefinedcurvesthatwereferredtoearlier.Theymustbereducedtoasinglecurveusingthesplit/mergeedgetoolbox.

Figure25 Exampleofsuperfluouslowergeometry

Thenwecanconnecttheairfoilstoeachotherthroughtheirfour(andonlyfour)existingvertices.Thiswillresultinthestraightloftthatwearelookingfor,butonlyafterthesixfaceshavebeencreatedandthevolumestitchedandsubtractedfromthecenterairspace.Itisnotimportanttosplitthisvolumewiththeairfoilvolumeandthenlaterdeleteit.Sinceweonlycareaboutthepositivespace,asimplesubtractionwillsuffice.Althoughsubtractionyieldsnoboundaryissues,theresultinggeometryrequiressomecleaningupsincetheSTEPfilegeometrysitsexactlytangenttothecylindricalvolume.Usingthesamemergeedgestoolfrombefore,thetwooutsideairfoiledges(specificallythosealongthe

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cylindersouterwall)needtobejoined.Thisjoiningdoesnotrequireadeconstructionoftheuppertopologicalentitieslikebefore;it'ssimplydoesn'thavethesameaim.

Figure26 Repairingsubtractedvolume

AperiodicboundaryconditionisusefulforthereductionoftheCPUtimerequired,andtoachieveaworkingmodelofthis,twocylindersweresubtractedfromoneanotherwhileapairofoppositelyrotatedbricksslicethegeometrydowntoaroundedwedgeshapedextrusion.Ifyoucanrecallfromthedesignsection,theradialdimensionsofthispart'scross‐sectionissuchthattheinnerdiameterisjustlargerthantheaxialpipecarryingrefrigerant;1.5inches,andtheouterdiameterissizedtofitexactlyalignedwiththeheatexchangerdiameter:aflatfiveinches.Thismeansthatthefanhassixbladesthatfitidenticallyontheinletfaceoftheheatexchangersothatnoarealconversionisrequired,reducingthenumberofpartsofthesystemandfurtherreducingcosts.

Figure27 Overallairspacewithvolumestobesubtractedfor60periodic

Whenweconsidertheflowofairthroughafansystem,weseethreesections.Thefirstistheairfromtheoutsidethattravelsthroughthefan,thesecondisthehighlyturbulentzonewherethebladessweepoutacylindricalvolume,andthelastistheairafterwards;theairintheheatexchanger.Forthisreasonwedivideourpositiveairspaceintothreesectionsofvaryingsize.Theseven‐inchheightoftheairspaceallowsacenteredvolumeofoneinchinheighttoencompassthefanbladesandresultsintwoequivalentthree‐inchsections;onebeforethefanandoneafterthefan.Aspecialboundaryconditionmustexisthereforfluidtoflowacrossitasifitwerenotaboundary.Toaccomplishthiswemustfirstsplitthe

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largervolumewithtworealfacesthatextendpastthelimitsofthevolume,justtobecertain.

Figure28 Splittingmainvolumearoundairfoil

Meshingbeginsfirstwithanunderstandingofperiodicboundaryconditions.Sinceeachperiodicboundarymustbelinkedformeshingbemeshingbegins,and,ifweconsiderourintentionstoconcentratethemeshingsizearoundtheairfoil,wearepresentedwithaproblem.Hereyoucansee,sincewehavetospecifyavertexforeachfaceanditmeshlinksthetwoadjacentedgestothecorrespondingoppositeface,whenwebegintoedgemeshonewitharatioalongtheaxialdirectionanduniformdensityedgemeshalongtheperpendiculartoflowedge,thereexistsaconflict;bothcan'texist.

Figure29 Edgemeshingalongperiodicboundarytypes

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Tosolvethisproblem,wetosplittheeightverticaledgesbyau­valuetocreateavertexinthecenterofeachsymmetricface.Nowwecanedgemeshthelongerofthetwoedgeswithanincreasingratio,andtheshorteredgewithaconstantspacingontheorderofthesmallest(thelastelement)fromthelongeredge,thirdlyandmostimportantofall,anindependentedgemeshisnowpossiblealongtheeightperpendicularedgesaswellasthefourairfoilloftedges.(Figure29).

Figure30 Methodofquad‐mappingtheinletandoutletvolumes

Forvolumemeshing,theordermatters.Asitis,thecentervolumewillnotmeshaHexMapscheme,simplybecausethehubsidefacewillresultinatoohighlyskewedmeshforittocomplete.Instead,acompromisemustbesetalongthesharedfacesoneithersideofthecentervolume;thevolumesbeforeandafterthefanwillhaveahexmapscheme,butthefanvolumeitselfwillbemeshedwithTGrid.Thisisunderstandable;itisdifficult(ifnotimpossible)topredicttheflowdirectionandtoconstructameshperpendiculartotheregionwiththemostexpectedturbulence.UsingaTGridschemeimpliesacertainamountoffreedom,andtorestrictthatfreedom,everyedgeofthevolumewasmeshedtoacertainqualitybeforethemeshingschemehadthefreedomtosetitsownconditions.Also,aboundarylayerwascreatedaroundtheairfoil.Theresultingmeshsizeis:372,160elements,938,893faces,226,643nodes.

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Figure31 Sectionofquad‐mappedfanbladeandsurrounding3Delements

Figure 32 Worst 25% of 3D elements according to equiskew

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A mesh analysis shows that the worst quarter of the tetrahedral shaped elements, based on their Equiskew feature, all reside in this center volume. This is completely acceptable since the flow direction in this region is difficult to predict. Most importantly, the airfoil itself has a quad-mapped face and the inlet and outlet interior faces of this volume are quad-mapped as well.

3.1 c) Shroud / manifold flow

Figure 33 Mesh for simulating flow around pipe manifold before entering exchanger

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3.2 Solution

ThemeshisreadintoFLUENTandcheckedforinconsistencies.Duringearlyattempts,gridcheckswouldoccasionallyfailduetoimproperlyconfiguredperiodicconditions.Oncehavinglearnedthecorrectmethodoflinkingfacemeshesandapplyingperiodicboundarytypes,thisnolongerpresentedanyproblems.ThemeshisscaledandunitsaresetforEnglishsysteminputs,includinglength,angularvelocity,pressure,temperature,andvelocity.Theenergyequationandk‐epsilonturbulencemodelareturnedon.BoundaryConditionsarelabeledinFigure34.Theproblemisinitializedanditerateduntilreachingconvergence.

Figure34 Modelboundaryconditions

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4 Results

4.1 Postprocessing

4.1a)Fullfansimulation

Figure35 Residualsforfull‐size,lowspeed

Figure36 Pathlinesthroughfull‐sizemodel

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Figure37 Pathlinesforlowspeedacrossmid‐plane(flowfromtoptobottom)

Figure38 Velocityvectorsforlowspeedthroughfanbladesection

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Figure39 Residualsforfull‐size,mediumspeed

Figure40 Pathlinesformediumspeedacrossmid‐plane,shroudandhub

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Figure41 Residualsforfull‐size,highspeed

Figure42 Pathlinesforhighspeedacrossmid‐plane,shroud,andhub

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4.1b)Periodicfansimulation

Figure43 Residualsforperiodic,lowspeed

Figure44 Pathlinesforlowspeedatmid‐plane

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Figure45 Residualsforperiodic,mediumspeed

Figure46 Pathlinesformediumspeedatmid‐plane

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Figure47 Residualsforperiodic,highspeed

Figure48 Pathlinesforhighspeedatmid‐plane

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Figure49 Pathlinesforhighspeedacrossfanblade,hub,andshroud

4.1c)Shroud/manifoldsimulation

Figure50 Residualsforshroudsimulationathighspeed

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Figure51 Pathlinesovermanifoldatmid‐plane

Figure52 Pressurecontoursatmid‐plane

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Figure53 Velocitycontoursatmid‐plane

Figure54 Temperaturecontoursatmid‐plane

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4.2 FanPerformance

Actualoutputvelocitiesofthefanwere:

Low‐speed(500rpm)=207ft/min

Medium‐speed(800rpm)=348ft/min

High‐speed(1000rpm)=444ft/min

Pressureincreases(head)throughthefanareminimalatabout0.015inchesofwater,notcompletelycompensatingforthepressuredropthroughtheheatexchanger.

Temperatureincreaseisnegligibleineachsimulation,reportinganincreaseontheorderof1·10‐3°F.

FLUENTprovedtobeextremelyusefulforreportingtheforcesandmomentsexperiencedbythefanblades,withoptionstowritethetotalforceduetobothpressureandviscousshearstresses,aswellasthemomentaroundeachaxiswithrespecttotheorigin(whichisconvenientlyatthecenterofthefanblades.Thefollowingvalueswerederivedfromtheperiodicmodel,allowingustodeterminetheforcesandtorquesonasinglebladeandsimplymultiplybysixfortheneteffect.Theforcesareusedforhandcalculationsofthestructuralstrengthofthefanbladesandthemomentsarehandingforcalculatedtherequiredpowerfromthemotor,sincebothtorqueandthesetrotationalspeedareknown.

SPEED Low Medium High

Fy(N) 0.001 0.0075 0.01217

Fz(N) 0.00115 0.00765 0.01234

Torque(N­m) 0.000069 0.0004675 0.000751

Angularspeed(rad/s) 52.36 99.5 125.664

Req.FanPower(W) 0.0216 0.279 0.566

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5 Discussion

5.1 GeneralResults

Throughcountlessiterationsofattemptedsimulations,attentiontothedetailsofsettingboundaryconditionsbecamethecriticalstepforyieldingreasonablenumbersforoutputflowandcorrespondingfanspeed.Forthesakeofthesizeandflowofthereport,thecoarserandfinermeshes,rangingfromabout200,000to1.8millionelementsfortheperiodicvolumes,runattheextremesofrotationalspeed,werenotshownbutservedasmeasuresofthelevelofdetailrequiredtocapturethefanflowconsistently.Thismeshsensitivitytestwassimilarlyperformedduringtheheatexchangerredesign,sincethemeshingprocesswasadverselyalteredbytheintroductionofmultiplepipesandasymmetricalbaffles.Thiswasatimesavingprocessthatensuredconsistentresultsovertherangeofspeeds.

5.2 CostEstimate

Thecostofthephysicalfanwaskeptlowforproductionconsiderations.BasedonrawmaterialspricingfromMcMaster‐Carr,thefanrequiresabout$15worthofplastic.Thepartsforprototypingthemotor,controller,andpowerconversioncouldbemanagedwithacombinationofoneofmanykitsfromaretailersuchasgobrushless.com(whichincludethematerialsforwindingacustomstatorandassemblingthepermanentmagnetrotor)andahome‐builtpowerandcontrolcircuit.Wehavepersonalexperienceinthisdepartment,havingbuiltathree‐phaseinverterforcontrollinganout‐runnerbrushlessmotorforahigh‐speed,rotaryvalveapplication.Althoughpricingwillbesubjecttoapersonalstockofbasicelectronicssuchaswiring,powertransistors(e.g.MOSFETS),diodes,atransformer,etc,thekitwillcostaround$30.Thistranslatestoasubtotalof$45fortheactualfan.

Laboristheotherfactorindeterminingthecostofdesigning,testing,andprototypingthefanconcept.Sincetheheatexchangerdesignimpressedtheprojectmanagers,ourhourlywageswereincreasedfrom$11/hrto$20/hr.Withapproximately70hoursputintothedesignandanalysisphases,andanestimated30hourstowardanexperimentalprototype,thelaborcostpeaksat$2,000.

Therefore,TOTALCOST=$2.045.00

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5.3 DesignBenefits

Beforediscussingthebenefitsofourdesign,weshouldaddressthedrawbacks.Thecomplexityofthemotorchosentopowerthefansacrificestheruggednessofotheroptions,suchasasquirrel‐cageinductionmotor.Theneedtoincreasethesizeoftheheatexchangertoaccommodatetheaxialfanisunfortunatebutstillkeepsthedesignwithinareasonablesizeenvelope.Also,fanperformancedoesnotmeeteverycriteria,mostnotableitsinabilitytocompletelyovercomethepressurelossthroughtheheatexchanger.

Therearemanyadvantagestothefandesignpresentedhere.ThechoiceofmaterialssuchasABSplasticyieldsalight‐weight,low‐costfanwiththestrengthtowithstandtheforcesofoperationatmultiplespeeds,asverifiedbyhandcalculations.Theuseofabrushless,permanentmagnetsynchronousmotorisanotherkeyappealtothedesign,embracingthetrendsofincreasedsophisticationandincreasedaffordabilityofmotortechnologywithongoingadvancesinthepower‐electronicsandsensorindustries.Alongwiththerelativelycomplexelectroniccontrolschemecomesnumerousbenefitsincludinghighpowerdensity(mostlycreditedtotheuseofpowerfulpermanentmagnets),lowheatgeneration,lownoiseproduction,andhighoverallefficiency.Omissionofanofficialshroudpromotesconvectivecoolingintheheatexchangerbydirectlyintroducingtheturbulentswirlingexitingthefanintotheshelloftheheatexchanger.Theresultingfan,coupledwiththeredesignedheatexchanger,presentsacompact,in‐linepackagethatwouldbeattractiveformanyevaporatorapplications.Thisisadesignmadetoblowawayallcompetingproposals.

1http://longbiao.win.mofcom.gov.cn/en/plate01/product.asp?id=36105

2http://longbiao.win.mofcom.gov.cn/en/plate01/index.asp

3http://commons.wikimedia.org/wiki/Image:Straight‐tube_heat_exchanger_1‐pass.PNG