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JournalofMechanicalScienceandTechnology2612012235240www.springerlink.com/content/1738494xDOI10.1007/s122060110921yInvestigationofthermostructuralbehaviorsofdifferentventilationapplicationsonbrakediscs†MesutDuzgunAutomotiveEngineeringDepartment,FacultyofTechnology,GaziUniversity,06500,Ankara,TurkeyManuscriptReceivedNovember12,2010RevisedJuly20,2011AcceptedSeptember18,2011AbstractOneofthemostcommonproblemsrelatedtobrakediscsisoverheating,whichnegativelyaffectsbrakingperformanceespeciallyunderthecontinuousbrakingconditionsofvehicles.Ventilationapplicationsonbrakediscscansignificantlyimprovethebrakesystemperformancebyreducingtheheatingofthediscs.Inthisstudy,thethermalbehaviorsofventilatedbrakediscsusingthreedifferentconfigurationswereinvestigatedatcontinuousbrakeconditionsintermsofheatgenerationandthermalstresseswithfiniteelementanalysis.Theresultswerecomparedwithasoliddisc.Heatgenerationonsolidbrakediscsreducedtoamaximumof24withventilationapplications.Theexperimentalstudyindicatedfiniteelementtemperatureanalysisresultsintherangebetween1.13and10.87.However,thermalstressformationswerehigherwithventilatedbrakediscsincomparisontothosewithsoliddiscs.KeywordsBrakediscsHeatgenerationThermostructuralbehaviorsVentilationapplications1.IntroductionVentilatedbrakediscsorrotorsareknownashighperformancebrakes,andareproducedbymakinghollowsorslotsorbothofdifferentshapesondiscsurfacesandsideedges.Ventilatedbrakediscswereoriginallytestedonracingcarsinthe1960s,andtheyhavebeenemployedwidelyintheautomotiveandrailwayindustryusingdifferentdesigns1,2.Duringbraking,kineticenergyisconvertedtoheat.Around90ofthisenergyisabsorbedbythebrakediscandthentransferredtoambientair.Solidbrakediscsdissipateheatslowly.Therefore,ventilatedbrakediscsareusedtoimprovecoolingbyfacilitatingaircirculation3,4.Theygenerallyexhibitconvectiveheattransfercoefficientsapproximatelytwiceaslargeasthoseassociatedwithsoliddiscs5.Therearenumerousstudiesrelatedtoventilationapplicationsonbrakediscs.ZuberandHeidenreich6manufacturedthreedifferentventilatedbrakediscconstructionsfromcarbonfiberreinforcedceramicmatrixcompositesCMCandcomparedtheirstrengths.AntanaitisandRifici7provedthatthe90holecrossdrilledpatternimprovedheatrejectioncapabilityofthediscbetween8.8and20.1dependingonthevehiclespeed.Aleksendricetal.8showedtheabilityofaventilatedbrakediscrotorindissipatingthermalflowbyfiniteelementanalysisFEA.VenkitachalamandMaharudrappa9conductedflowandheattransferanalysisofsixdifferenttypesofdiscconfigurationsbycomputationalfluiddynamicsCFDandrecommendedventilatedbrakediscsforhighspeedvehicles.Parketal.10designedahelicalsurfaceinsidevanesforaventilatedbrakedisc.TheyoptimizedReynoldsRe,PrandtlPr,andNusseltNunumbersfortheirdesignandobtainedimprovementstoamaximumof44inheattransfer.Improvementinbrakefaderesistanceandhigherbrakingperformanceinwetconditionsaresomeotherusefulaspectsofventilatedbrakediscs.However,theyalsohavesomedisadvantages.Cracking,isoneofthemandthisaphenomenonthathasbeencorrelatedtostressesduringbraking11.Kimetal.12showedthemaximumvonMisesstressgenerationofactualfatiguecrackslocatedonventilatedbrakediscsofrailwayvehiclesbythermalstressanalysis.Similarly,Bagnolietal.13performedFEAtodeterminethetemperatureprofileandtoestimatethevonMisesstressdistributionthatarisesduringbrakingforfirefightingvehicles.HwangandWu14investigatedtemperatureandthermalstressinaventilatedbrakediscbasedonathermomechanicalcouplingmodel.Decreasingthebraketemperaturesand/orredesigningthehubrotorunitweresomeconsiderableconclusionsofMackinetal.15toeliminatecrackinginbrakerotors.Heatgenerationalsoaffectsthermomechanicalinstabilityofbrakediscs16.Previousliteraturefocusedonheatandstressformationsonventilateddiscs.Inthisstudy,FEAwasusedtoinvestigatethe†ThispaperwasrecommendedforpublicationinrevisedformbyAssociateEditorDaeHeeLeeCorrespondingauthor.Tel.903122028650,Fax.903122120059Emailaddressmduzgungazi.edu.tr©KSMESpringer2012236M.Duzgun/JournalofMechanicalScienceandTechnology2612012235240thermalbehaviorofthreedifferentventilatedbrakedesignscrossdrilledCD,crossslottedCS,andcrossslottedwithsidegrooveCSSGdiscs.TheresultswerethencomparedtoasolidSLdisc.AnexperimentalstudywasalsoperformedtoverifytheFEAresults.2.ThermostructuralFEAForFEA,threedimensional3Dconstructionsofbrakediscs,brakepads,andtheirassemblydesignsweremodeledwith1/1scaleinasoftwareprogram,andthenimportedintoanothersoftwareprogramfortheinteractivethermostructuralanalyses.BrakediscsandbrakepadsweremodeledbyQuadraticHexahedronmeshtypes.QuadraticHexahedronmeshgenerationsareknownfortheiraccuracyandcomputationalefficiency17.AfrictionalcontactpairhavingtheelementtypeofQuadraticQuadrilateralContactwasdefinedbetweendiscpadinterfaces.Fig.1showsthemeshmodelsofthediscpadsystems.Greycastiron,acommonlyuseddiscmaterial,wasusedforthebrakediscs.ThemechanicalandthermalpropertiesofthebrakediscsandpadsaregiveninTable1.Ventilatedbrakediscsweredesignedaccordingtothepropellershapedmethodologyfortheholeandslotlocations.FortheCDdisc,fiveholeswith5.2mmdiameterwerearrangedatequalintervalsonanarcwithalengthof60.54mm.Theseholeswereduplicatedingroupsof20onthediscsurface.Thus,atotalof100holesweredrilledonthediscsurfaceoftheCDdisc.FortheCSdisc,20channels6.9mmwideand67.3mmlongwereplacedonasoliddiscsurface.Finally,theCSSGdiscwasdesignedbymakingagroove4mmwideand15mmdeeponanotherCSdiscedge.Hence,apathopeningtotheouterdiscsideedgewasobtainedtoprovidebetteraircirculation.2.1ThermalanalysisForthethermalanalysis,theambienttemperaturewasassumedat22°Candthediscsurfacetemperaturewas100°Cpriortobraking,whichwasrelatedwithcoldbrakingperformance8.Thecurrentstudyassumedthatheatdissipationfromthebrakedisctotheatmosphereoccursviaconvection,alsoknownasNewtonslawofcooling.ConvectionisgovernedbyEq.1,whereQistherateofheattransferW,histheconvectionheattransfercoefficient,Aisthesurfaceareaoftherotorm2,Tsisthesurfacetemperatureofthebrakerotor°C,andT∞istheambientairtemperature°C.Theconvectionheattransfercoefficientisappliedtothebodyofthebrakediscsastheboundarycondition.Thus,toincreaseheattransferfromthebrakediscsandtoreducethediscsurfacetemperatureonthetotalsurfaceareaofthebrakediscs,theheattransfercoefficientsweregraduallyincreasedbyemployingventilationapplications...QhATTs−∞1TheheattransfercoefficientassociatedwithlaminarflowforsolidornonventilatedbrakediscswasderivedbyEq.2forRe2.4x1055,whereDistheouterdiameterofthediscsmm,ReistheReynoldsnumber,andkaisthethermalconductivityofairW/m°C.0.550.70/RehkDaR2Meanwhile,theheattransfercoefficientassociatedwithlaminarflowforventilatedbrakediscswasapproximatedbyEq.3forlaminarflowcondition,Re1045,wherePristhePrandtlnumber,dhisthehydraulicdiametermm,andlisthelengthofthecoolingvanemm.ThehydraulicdiameterdhisdefinedastheratiooffourtimesthecrosssectionalflowareawettedareaoftheholeandslotsinventilatedbrakediscsdividedbytheirwettedperimetersasillustratedinFig.2.1/30.331.86//hRePrdlkdaRhh3Inthiscondition,RenumberisassociatedwiththevelocityoftheairflowpresentintheholeandslotshapedvanesasdeterminedbyEq.4,whereρaisthedensityofairkg/mm3,maisthemassflowrateofairm3/sec,andVaverageistheaveragevelocitym/sec.Table1.Mechanicalandthermalpropertiesofbrakediscsandpads.MechanicalandthermalpropertiesDiscPadYoungsmodulusN/mm21100001500Poissonsratio0.280.25Densitykg/m372002595Thermalexpansion1/°C1.1e–0056.6e–005TensileultimatestrengthN/mm2240CompressiveultimatestrengthN/mm2820Coefficientoffriction0.350.35ThermalconductivityW/m°C521.212SpecificheatJ/kg°C4471465aSLdiscbCDdisccCSdiscdCSSGdiscFig.1.Finiteelementmeshmodels.M.Duzgun/JournalofMechanicalScienceandTechnology2612012235240237Re/dmVaaaveragehρ4TheaveragespeedcanbecalculatedbyEq.5,wherenTistherevolutionsperminute1/min,rpm,Distheouterdiameterofthediscmm,distheinnerdiameterofthediscmm,Aoutistheoutletareaoftheholeorslotshapedvanesmm2,andAinistheinletareaoftheholeorslotshapedvanesmm2.220.0158VnDdAAaverageoutoutinT−5Moreover,theairflowratemaisdeterminedbyEq.62230.00147sec.mnDdAmainT−62.1.1ExperimentalstudyTheexperimentalstudywasperformedtoexaminetemperaturechangesondiscsurfaces.Forthispurpose,theventilatedbrakediscsweremanufacturedfromtheAlfredTevesATEsoliddiscstoobtaintheirdesigncharacteristics.TheCDandCSdiscsweremanufacturedonathreeaxisCNCVerticalMachiningCentre.ThesidegroovefortheCSSGdiscwasmadeonaCNCTurningCentre.Thedisctemperatureoutputsweremeasuredonabraketestsystem.ThissystemconsistsofacalipermechanismasseeninFig.3,apiezocrystalforcemeasurementsystemforpedalandbrakeforcevariations,adrivingengine,agearbox,andindicatorsforbrake/pedalforcesandtemperature.Forthetemperaturemeasurement,athermocouplemountedonthecalipersystemwasused.Thepowerofthemotorwas4kW.Therotationofthediscswasonaclockwisedirection.ATE501FFbrakepadswereusedintheexperiments.Experimentswereconductedundercontinuousbrakingconditionsataconstantpedalforceof250N,andeightperiodicmeasurementsofbrakingtemperaturewereexecutedat30,60,90,120,150,180,210and240s.2.2StructuralanalysisDisctemperaturesobtainedbythermalanalyseswereimportedintothestructuralanalyticalmodelsasboundaryconditions.Inthestructuralanalyses,9689Nequivalentto250Npedalforcewasappliedoneachtopsurfaceofthebrakepadsatarevolutionspeedof60km/horangularvelocityof52.56rad/sasseeninFig.4.Therateofthediscwasassumedtobeconstantandthetimerequiredforacompletestopwas240s.ThevalueofthepedalforcewasobtainedbyEq.718,whereFfistheforceoneachfrontcylinderpiston,Fistheforceonthefootpedal,Afisthecrosssectionalareaofthefrontpistons,Amisthecrosssectionalareaofthemastercylinder,pnisthenumberofpistons,2.3isthepedalleverageratio,and2.75istheeffectofservounit....2,3.2,75fFApnfFAm7Thiscomputationwasdoneaccordingtothetestequipmentusedintheexperimentalstudy.Thepedalforceleverageratioandtheservouniteffectarethevaluesofthetestequipment.AnalysesweresimulatedaccordingtotheequivalentvonMisesstressdistributions.Fig.2.Wettedareashydraulicdiameter,dh,lengthofholeandslotshapedcoolingvanes,andinletandoutletareasforairflowofventilatedbrakediscsusedinthisstudy.aSLdiscbCDdisccCSdiscdCSSGdiscFig.3.Brakecaliperandbrakediscs.Fig.4.Structuralmodelforthebrakediscs.238M.Duzgun/JournalofMechanicalScienceandTechnology26120122352403.Resultsanddiscussion3.1HeatgenerationExperimentalandFEAresultsaregiveninTable2forthegenerationofheatonthediscsurfaces.Discsurfacetemperaturesincreasewithincreasingbrakingtimeforalldiscconfigurations.However,heatgenerationisremarkablyreducedbyventilationapplications.Thefrictioncoefficientbetweenbrakepadanddiscsurfacesdecreasesdependingonthetemperaturerise19,20.Hence,maintainingfrictionpropertiesofthepadsathightemperaturesispossiblebyselfventilationonthediscsundercontinuousbrakingconditions.Fig.5showsthetemperaturedistributionsonthediscsurfacesbyFEAattheendof240s.Themaximumtemperaturegenerationoccurredinthemiddleregionsofalldiscconfigurations,similarwithrelatedstudies12,14.However,thetemperatureregionmovesfromthemiddleregionofthedisctotheinnerregionduetotheadditionalsidecoolingintheCSSGdiscconfiguration.Themaximumheatgenerationonsoliddiscsurfacesisreducedaround4intheCDdiscdesign.Ontheotherhand,whilethemaximumheatgenerationonsoliddiscsurfacesisreducedaround19intheCSdesign,itisreducedaround24intheCSSGdesign.Thus,itisbeneficialtoinvestigatethethermalstressbehaviorofthesedesigns.3.2ThermalstressesFig.6showsthermalstressdistributionsonthediscsurfacesbyFEA.Ventilationapplicationsincreasethethermalstressesonthebrakediscs.Maximumthermalstressislocalizedonthecorneroftheinnerandouteredgesofthesoliddiscsurfaces.InthecaseoftheCDdiscconfiguration,themaximumstressislocatedintheinnersurfacesoftheholes.FortheCSandCSSGdiscconfigurations,maximumstressformationsaremainlylocalizedaroundtheslotsurfacesclosetotheinnerandouterpointsofthedisc.Hence,thespecificregions,wheremaximumstressesareseen,shouldbestrengthenedtopreventpotentialcrackandfatigueproblems.Oneofthepossiblesolutionsisthedesignofdifferentheatdissipationsurfacesforventilatedbrakediscs.Therefore,morehomogenousheatconvectionfromtherotorcanbeprovidedforthediscsurfaces.Table2.ExperimentalandFEAresultsfordisctemperatures.SLdiscCDdiscTimesec.Exp.temp.°CFEAtemp.°CExp.temp.°CFEAtemp.°C30124137.49156169.9960189174.97219200.0290238212.44254230.06120276249.92291260.09150318287.40321290.13180358324.87345320.16210378362.35355350.20240395399.82371380.23CSdiscCSSGdiscTimesec.Exp.temp.°CFEAtemp.°CExp.temp.°CFEAtemp.°C30128139.66125130.8960184165.68168155.1190213191.70201179.32120240217.72227203.52150273243.73253227.70180289269.74269251.87210314295.75285276.03240325321.76286300.18aSLdiscbCDdisccCSdiscdCSSGdiscFig.5.Heatgenerationondifferentkindofbrakediscs.aSLdiscbCDdisccCSdiscdCSSGdiscFig.6.Thermostructuralbehaviorsofdifferentkindsofventilatedandsolidbrakediscs.
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