JournalofMechanicalScienceandTechnology26(1)(2012)235240/content/1738-494xDOI10.1007/s12206-011-0921-yInvestigationofthermo-structuralbehaviorsofdifferentventilationapplicationsonbrakediscsMesutDuzgun*AutomotiveEngineeringDepartment,FacultyofTechnology,GaziUniversity,06500,Ankara,Turkey(ManuscriptReceivedNovember12,2010；RevisedJuly20,2011；AcceptedSeptember18,2011)-AbstractOneofthemostcommonproblemsrelatedtobrakediscsisoverheating,whichnegativelyaffectsbrakingperformanceespeciallyun-derthecontinuousbrakingconditionsofvehicles.Ventilationapplicationsonbrakediscscansignificantlyimprovethebrakesystemperformancebyreducingtheheatingofthediscs.Inthisstudy,thethermalbehaviorsofventilatedbrakediscsusingthreedifferentcon-figurationswereinvestigatedatcontinuousbrakeconditionsintermsofheatgenerationandthermalstresseswithfiniteelementanalysis.Theresultswerecomparedwithasoliddisc.Heatgenerationonsolidbrakediscsreducedtoamaximumof24%withventilationappli-cations.Theexperimentalstudyindicatedfiniteelementtemperatureanalysisresultsintherangebetween1.13%and10.87%.However,thermalstressformationswerehigherwithventilatedbrakediscsincomparisontothosewithsoliddiscs.Keywords:Brakediscs；Heatgeneration；Thermo-structuralbehaviors；Ventilationapplications-1.IntroductionVentilatedbrakediscsorrotorsareknownashigh-performancebrakes,andareproducedbymakinghollowsorslots(orboth)ofdifferentshapesondiscsurfacesandsideedges.Ventilatedbrakediscswereoriginallytestedonracingcarsinthe1960s,andtheyhavebeenemployedwidelyintheautomotiveandrailwayindustryusingdifferentdesigns1,2.Duringbraking,kineticenergyisconvertedtoheat.Around90%ofthisenergyisabsorbedbythebrakediscandthentransferredtoambientair.Solidbrakediscsdissipateheatslowly.Therefore,ventilatedbrakediscsareusedtoimprovecoolingbyfacilitatingaircirculation3,4.Theygenerallyexhibitconvectiveheattransfercoefficientsapproximatelytwiceaslargeasthoseassociatedwithsoliddiscs5.Therearenumerousstudiesrelatedtoventilationapplica-tionsonbrakediscs.ZuberandHeidenreich6manufacturedthreedifferentventilatedbrakediscconstructionsfromcarbonfiber-reinforcedceramicmatrixcomposites(CMC)andcom-paredtheirstrengths.AntanaitisandRifici7provedthatthe90-holecross-drilledpatternimprovedheatrejectioncapabil-ityofthediscbetween8.8%and20.1%dependingonthevehiclespeed.Aleksendricetal.8showedtheabilityofaventilatedbrakediscrotorindissipatingthermalflowbyfiniteelementanalysis(FEA).VenkitachalamandMaharudrappa9conductedflowandheattransferanalysisofsixdifferenttypesofdiscconfigurationsbycomputationalfluiddynamics(CFD)andrecommendedventilatedbrakediscsforhigh-speedvehicles.Parketal.10designedahelicalsurfacein-sidevanesforaventilatedbrakedisc.TheyoptimizedRey-nolds(Re),Prandtl(Pr),andNusselt(Nu)numbersfortheirdesignandobtainedimprovementstoamaximumof44%inheattransfer.Improvementinbrakefaderesistanceandhigherbrakingperformanceinwetconditionsaresomeotherusefulaspectsofventilatedbrakediscs.However,theyalsohavesomedisadvantages.Cracking,isoneofthemandthisaphe-nomenonthathasbeencorrelatedtostressesduringbraking11.Kimetal.12showedthemaximumvon-Misesstressgenerationofactualfatiguecrackslocatedonventilatedbrakediscsofrailwayvehiclesbythermalstressanalysis.Similarly,Bagnolietal.13performedFEAtodeterminethetempera-tureprofileandtoestimatethevon-Misesstressdistributionthatarisesduringbrakingforfire-fightingvehicles.HwangandWu14investigatedtemperatureandthermalstressinaventilatedbrakediscbasedonathermo-mechanicalcouplingmodel.Decreasingthebraketemperaturesand/orre-designingthehub-rotorunitweresomeconsiderableconclusionsofMackinetal.15toeliminatecrackinginbrakerotors.Heatgenerationalsoaffectsthermo-mechanicalinstabilityofbrakediscs16.Previousliteraturefocusedonheatandstressformationsonventilateddiscs.Inthisstudy,FEAwasusedtoinvestigatetheThispaperwasrecommendedforpublicationinrevisedformbyAssociateEditorDaeHeeLee*Correspondingauthor.Tel.:+903122028650,Fax.:+903122120059E-mailaddress:.trKSME&Springer2012236M.Duzgun/JournalofMechanicalScienceandTechnology26(1)(2012)235240thermalbehaviorofthreedifferentventilatedbrakedesigns:cross-drilled(CD),cross-slotted(CS),andcross-slottedwithsidegroove(CS-SG)discs.Theresultswerethencomparedtoasolid(SL)disc.AnexperimentalstudywasalsoperformedtoverifytheFEAresults.2.Thermo-structuralFEAForFEA,three-dimensional(3D)constructionsofbrakediscs,brakepads,andtheirassemblydesignsweremodeledwith1/1scaleinasoftwareprogram,andthenimportedintoanothersoftwareprogramfortheinteractivethermo-structuralanalyses.BrakediscsandbrakepadsweremodeledbyQuad-raticHexahedronmeshtypes.QuadraticHexahedronmeshgenerationsareknownfortheiraccuracyandcomputationalefficiency17.AfrictionalcontactpairhavingtheelementtypeofQuadraticQuadrilat-eralContactwasdefinedbetweendisc-padinterfaces.Fig.1showsthemeshmodelsofthedisc-padsystems.Greycastiron,acommonlyuseddiscmaterial,wasusedforthebrakediscs.ThemechanicalandthermalpropertiesofthebrakediscsandpadsaregiveninTable1.Ventilatedbrakediscsweredesignedaccordingtothepro-peller-shapedmethodologyfortheholeandslotlocations.FortheCDdisc,fiveholeswith5.2mmdiameterwerearrangedatequalintervalsonanarcwithalengthof60.54mm.Theseholeswereduplicatedingroupsof20onthediscsurface.Thus,atotalof100holesweredrilledonthediscsurfaceoftheCDdisc.FortheCSdisc,20channels(6.9mmwideand67.3mmlong)wereplacedonasoliddiscsurface.Finally,theCS-SGdiscwasdesignedbymakingagroove(4mmwideand15mmdeep)onanotherCSdiscedge.Hence,apathopeningtotheouterdiscsideedgewasobtainedtoprovidebetteraircirculation.2.1ThermalanalysisForthethermalanalysis,theambienttemperaturewasas-sumedat22Candthediscsurfacetemperaturewas100Cpriortobraking,whichwasrelatedwithcoldbrakingper-formance8.Thecurrentstudyassumedthatheatdissipationfromthebrakedisctotheatmosphereoccursviaconvection,alsoknownasNewtonslawofcooling.Convectionisgov-ernedbyEq.(1),whereQistherateofheattransfer(W),histheconvectionheattransfercoefficient,Aisthesurfaceareaoftherotor(m2),Tsisthesurfacetemperatureofthebrakerotor(C),andTistheambientairtemperature(C).Theconvectionheattransfercoefficientisappliedtothebodyofthebrakediscsastheboundarycondition.Thus,toincreaseheattransferfromthebrakediscsandtoreducethediscsur-facetemperatureonthetotalsurfaceareaofthebrakediscs,theheattransfercoefficientsweregraduallyincreasedbyem-ployingventilationapplications.()QhATTs=(1)Theheattransfercoefficientassociatedwithlaminarflowforsolidornon-ventilatedbrakediscswasderivedbyEq.(2)(forRe2.4x105)5,whereDistheouterdiameterofthediscs(mm),ReistheReynoldsnumber,andkaisthethermalconductivityofair(W/mC).0.550.70(/)RehkDaR=(2)Meanwhile,theheattransfercoefficientassociatedwithlaminarflowforventilatedbrakediscswasapproximatedbyEq.(3)(forlaminarflowcondition,Re104)5,wherePristhePrandtlnumber,dhisthehydraulicdiameter(mm),andlisthelengthofthecoolingvane(mm).Thehydraulicdiameter(dh)isdefinedastheratiooffourtimesthecross-sectionalflowarea(wettedarea)oftheholeandslotsinventilatedbrakediscsdividedbytheirwettedperimetersasillustratedinFig.2.1/30.331.86()(/)(/)hRePrdlkdaRhh=(3)Inthiscondition,Renumberisassociatedwiththevelocityoftheairflowpresentintheholeandslot-shapedvanesasdeterminedbyEq.(4),whereaisthedensityofair(kg/mm3),maisthemassflowrateofair(m3/sec),andVaverageistheaver-agevelocity(m/sec).Table1.Mechanicalandthermalpropertiesofbrakediscsandpads.MechanicalandthermalpropertiesDiscPadYoungsmodulus(N/mm2)1100001500Poissonsratio0.280.25Density(kg/m3)72002595Thermalexpansion(1/C)1.1e0056.6e005Tensileultimatestrength(N/mm2)240-Compressiveultimatestrength(N/mm2)820-Coefficientoffriction0.350.35Thermalconductivity(W/mC)521.212Specificheat(J/kgC)4471465(a)SLdisc(b)CDdisc(c)CSdisc(d)CS-SGdiscFig.1.Finiteelementmeshmodels.M.Duzgun/JournalofMechanicalScienceandTechnology26(1)(2012)235240237Re(/)dmVaaaverageh=(4)TheaveragespeedcanbecalculatedbyEq.(5),wherenTistherevolutionsperminute(1/min,rpm),Distheouterdiame-terofthedisc(mm),distheinnerdiameterofthedisc(mm),Aoutistheoutletareaoftheholeorslot-shapedvanes(mm2),andAinistheinletareaoftheholeorslot-shapedvanes(mm2).220.0158()VnDdAAaverageoutoutinT=+(5)Moreover,theairflowratemaisdeterminedbyEq.(6):2230.00147()(sec).mnDdAmainT=(6)2.1.1ExperimentalstudyTheexperimentalstudywasperformedtoexaminetem-peraturechangesondiscsurfaces.Forthispurpose,theventi-latedbrakediscsweremanufacturedfromtheAlfredTeves(ATE)soliddiscstoobtaintheirdesigncharacteristics.TheCDandCSdiscsweremanufacturedonathree-axisCNCVerticalMachiningCentre.ThesidegroovefortheCS-SGdiscwasmadeonaCNCTurningCentre.Thedisctempera-tureoutputsweremeasuredonabraketestsystem.Thissys-temconsistsofacalipermechanismasseeninFig.3,apiezocrystalforcemeasurementsystemforpedalandbrakeforcevariations,adrivingengine,agearbox,andindicatorsforbrake/pedalforcesandtemperature.Forthetemperaturemeasurement,athermocouplemountedonthecalipersystemwasused.Thepowerofthemotorwas4kW.Therotationofthediscswasonaclockwisedirection.ATE501FFbrakepadswereusedintheexperiments.Ex-perimentswereconductedundercontinuousbrakingcondi-tionsataconstantpedalforceof250N,andeightperiodicmeasurementsofbrakingtemperaturewereexecutedat30,60,90,120,150,180,210and240s.2.2StructuralanalysisDisctemperaturesobtainedbythermalanalyseswereim-portedintothestructuralanalyticalmodelsasboundarycondi-tions.Inthestructuralanalyses,9689Nequivalentto250Npedalforcewasappliedoneachtopsurfaceofthebrakepadsatarevolutionspeedof60km/h(orangularvelocityof52.56rad/s)asseeninFig.4.Therateofthediscwasassumedtobeconstantandthetimerequiredforacompletestopwas240s.ThevalueofthepedalforcewasobtainedbyEq.(7)18,whereFfistheforceoneachfrontcylinderpiston,Fistheforceonthefootpedal,Afisthecross-sectionalareaofthefrontpistons,Amisthecross-sectionalareaofthemastercyl-inder,pnisthenumberofpistons,2.3isthepedalleverageratio,and2.75istheeffectofservounit.2,3.2,75fFApnfFAm=(7)Thiscomputationwasdoneaccordingtothetestequipmentusedintheexperimentalstudy.Thepedalforceleverageratioandtheservouniteffectarethevaluesofthetestequipment.Analysesweresimulatedaccordingtotheequivalent(von-Mises)stressdistributions.Fig.2.Wettedareas(hydraulicdiameter,dh),lengthofholeandslot-shapedcoolingvanes,andinletandoutletareasforairflowofventi-latedbrakediscsusedinthisstudy.(a)SLdisc(b)CDdisc(c)CSdisc(d)CS-SGdiscFig.3.Brakecaliperandbrakediscs.Fig.4.Structuralmodelforthebrakediscs.238M.Duzgun/JournalofMechanicalScienceandTechnology26(1)(2012)2352403.Resultsanddiscussion3.1HeatgenerationExperimentalandFEAresultsaregiveninTable2forthegenerationofheatonthediscsurfaces.Discsurfacetempera-turesincreasewithincreasingbrakingtimeforalldisccon-figurations.However,heatgenerationisremarkablyreducedbyventilationapplications.Thefrictioncoefficientbetweenbrakepadanddiscsurfacesdecreasesdependingonthetem-peraturerise19,20.Hence,maintainingfrictionpropertiesofthepadsathightemperaturesispossiblebyself-ventilationonthediscsundercontinuousbrakingconditions.Fig.5showsthetemperaturedistributionsonthediscsur-facesbyFEAattheendof240s.Themaximumtemperaturegenerationoccurredinthemiddleregionsofalldiscconfigu-rations,similarwithrelatedstudies12,14.However,thetemperatureregionmovesfromthemiddleregionofthedisctotheinnerregionduetotheadditionalside-coolingintheCS-SGdiscconfiguration.Themaximumheatgenerationonsoliddiscsurfacesisreducedaround4%intheCDdiscdesign.Ontheotherhand,whilethemaximumheatgenerationonsoliddiscsurfacesisreducedaround19%intheCSdesign,itisreducedaround24%intheCS-SGdesign.Thus,itisbene-ficialtoinvestigatethethermalstressbehaviorofthesede-signs.3.2ThermalstressesFig.6showsthermalstressdistributionsonthediscsurfacesbyFEA.Ventilationapplicationsincreasethethermalstressesonthebrakediscs.Maximumthermalstressislocalizedonthecorne