文档介绍：Chapter8 NaturalConvection18-1PhysicalMechanismofNaturalConvection8-2NaturalConvectionoverSurfaces8-3NaturalConvectioninsideEnclosures8-4CombinedNaturalandForcedConvectionSummaryChapter8NaturalConvection Contents2DefinitionNaturalconvection::Thefluidvelocitiesassociatedwithnaturalconvectionarelow(notnoticeable),typicallyunder1m/:coolingofelectronicequipmentsuchaspowertransistors,TVs,andVCRs;heattransferfromelectricbaseboardheatersorsteamradiators;heattransferfromtherefrigerationcoilsandpowertransmissionlines;-1PhysicalMechanismofNaturalConvection4Notice:parablemagnitudeexceptforlow--1PhysicalMechanismofNaturalConvection5BuoyancyForceandForceanalysis:Thebuoyancyforceiscausedbythedensitydifferencebetweentheheated(orcooled)-1PhysicalMechanismofNaturalConvectionIntheabsenceofotherforces,verticalforceactingonabodyis(Archimedes’Principle)68-1PhysicalMechanismofNaturalConvectionInheattransferstudies,theprimaryvariableistemperature,8-1PhysicalMechanismofNaturalConvectionItcanalsobeexpressedapproximatelyreplacingdifferentialquantitiesbydifferences(atconstantP)Foranidealgas:8Discussion:Alargevalueofforafluidmeansalargechangeindensitywithtemperature;Theproduct∆Trepresentsthefractionofvolumechangeofafluidthatcorrespondstoatemperaturechange∆Tatconstantpressure;Thelargerthetemperaturedifferencebetweenthefluidadjacenttoahot(orcold)surfaceandthefluidawayfromit,thelargerthebuoyancyforceandthestrongerthenaturalconvectioncurrents,andthusthehighertheheattransferrate;8-1PhysicalMechanismofNaturalConvection9Them