C H A P T E R 13 The Transfer of Heat

CHAPTER13TheTransferofHeatTheTransferofHeatQ:Namethreemethodsofheattransfer?TheTransferofHeatQ:Namethreemethodsofheattransfer?A:Convection,Conduction,andRadiation.13.1ConvectionConvectionistheprocessinwhichheatiscarriedfromplacetoplacebythebulkmovementofafluid.Convectioncurrentsaresetupwhenapanofwaterisheated.BoilingWaterDuringavolcaniceruption,smokeatthetopoftheplumerisesthousandsofmetersbecauseofconvection.Q:Inthelivingroom,theheatingunitisplacedinthefloorbutthetherefrigeratorhasatop-mountedcoolingcoil.Why?Q:Inthelivingroom,theheatingunitisplacedinthefloorbutthetherefrigeratorhasatop-mountedcoolingcoil.Why?A:Airwarmedbythebaseboardheatingunitispushedtothetopoftheroombythecooleranddenserair.Aircooledbythecoolingcoilsinkstothebottomoftherefrigerator.13.2ConductionConductionistheprocesswherebyheatistransferreddirectlythroughamaterial,anybulkmotionofthematerialplayingnoroleinthetransfer.Thosematerialsthatconductheatwellarecalledthermalconductors,whilethosethatconductheatpoorlyareknownasthermalinsulators.Mostmetalsareexcellentthermalconductors,whilewood,glass,andmostplasticsarecommonthermalinsulators.Thefreeelectronsinmetalsareresponsiblefortheexcellentthermalconductivityofmetals.ConductionOfHeatThroughAMaterialRateofheattransferbyconduction,Q/tthroughthelength,Lacrossthecross-sectionalarea,Aisgivenbythefollowingequation,wherekisthethermalconductivityandΔTisthetemperaturedifferencebetweenthetwoends..LTkAtQSIUnitofThermalConductivity:J/(s·m·C°)TABLE13.1ThermalConductivitiesaaofSelectedMaterialsSubstanceThermalConductivity,k[J/(s·m·C°)]MetalsAluminum240Brass110Copper390Iron79Lead35Silver420Steel(stainless)14GasesAir0.0256Hydrogen(H2)0.180Nitrogen(N2)0.0258StyrofoamFruitgrowerssometimesspraywatertoprotecttheircropsagainstfreezing.Afterasubzeronight,theseberriesarevisibleintheirinsulatingjacketsofice.13.3RadiationRadiationistheprocessinwhichenergyistransferredbymeansofelectromagneticwaves.Heattransferbyradiationcantakeplacethroughvacuum.Thisisbecauseelectromagneticwavesareinvolvedinradiationandtheycanpropagatethroughemptyspace.SuntansSuntansareproducedbyradiation,ultravioletrays.BlackBodyBlackBodyAmaterialthatisagoodabsorber,likelampblack,isalsoagoodemitter,andamaterialthatisapoorabsorber,likepolishedsilver,isalsoapooremitter.SummerClothingQ:Peopleareuncomfortablewearingdarkclothesduringthesummer.Why?SummerClothingQ:Peopleareuncomfortablewearingdarkclothesduringthesummer.Why?A:Darkclothesabsorbalargefractionofthesun'sradiationandthenreemititinalldirections.Aboutone-halfoftheemittedradiationisdirectedinwardtowardthebodyandcreatesthesensationofwarmth.Light-coloredclothes,incontrast,arecoolertowear,sincetheyabsorbandreemitrelativelylittleoftheincidentradiation.AWhitesifakaLemurTowarmupinthemorning,theyturntheirdarkbelliestowardthesun.TheStefan–BOLTZMANNLawOfRadiationTherateatwhichanobjectemitsradiantenergyisproportionaltothefourthpowerofitsabsolutetemperature.ThisisknownasStefan’slawandisexpressedasfollows,whereσistheStefan-Boltzmannconstant,σ=5.6710-8W/m2.K4..4eATPtQThefactoreiscalledtheemissivity,whichisanumberbetween0and1.Perfectradiatorshaveavalueof1fore.AisthesurfaceareaandTisthetemperatureoftheradiatorinKelvin.Thenetpoweristhepowertheobjectemitsminusthepoweritabsorbs.ApplyingtheStefan-BoltzmannlawasinExample6leadstothefollowingexpressionforPnetwhenthetemperatureoftheobjectisTandthetemperatureoftheenvironmentisT0:13.4Applications1.RatingthermalinsulationbyRvalues.2.Regulatingthetemperatureofanorbitingsatellite.3.Athermosbottle.4.Ahalogencooktopstove.RValueThetermL/kinthedenominatoriscalledtheRvalueoftheinsulation.IntheUSR-valuesareexpressedinBritishunits,ft2.h.F0/Btu.Forexample,the6.0inchthickfiber-glassbattinghasanR-valueof19ft2.h.F0/Btu.OwensCorningOwensCorningRecommendedInsulationProducts:RaftersandCollarBeamsR-21FiberGlassInsulation51/2ThickR-19FiberGlassInsulation61/4ThickR-38C2x12CathedralCeilingFiberGlassInsulation101/4ThickR-30C2x10CathedralCeilingFiberGlassInsulation81/4ThickHighlyreflectivemetalfoilcoveringthissatelliteminimizesheattransferbyradiation.ThermosBottleAthermosbottleminimizesenergytransferduetoconvection,conduction,andradiation.Stopper-minimizeconduction.Double-walledglassvesselwiththespacebetweenthewallsisevacuatedtominimizeenergylossesduetoconductionandconvection.Thesilveredsurfacesreflectmostoftheradiantenergythatwouldotherwiseenterorleavetheliquidinthethermos.HalogenCooktopInahalogencooktop,quartz-iodinelampsemitalargeamountofelectromagneticenergythatisabsorbeddirectlybyapotorpan.