Heat Chap03164.docx
- 文档编号:3310346
- 上传时间:2022-11-21
- 格式:DOCX
- 页数:19
- 大小:630.96KB
Heat Chap03164.docx
《Heat Chap03164.docx》由会员分享,可在线阅读,更多相关《Heat Chap03164.docx(19页珍藏版)》请在冰豆网上搜索。
HeatChap03164
3-164Acircuitboardhouseselectroniccomponentsononeside,dissipatingatotalof15Wthroughthebacksideoftheboardtothesurroundingmedium.Thetemperaturesonthetwosidesofthecircuitboardaretobedeterminedforthecasesofnofinsand20aluminumfinsofrectangularprofileonthebackside.
Assumptions1Steadyoperatingconditionsexist.2Thetemperatureintheboardandalongthefinsvariesinonedirectiononly(normaltotheboard).3Alltheheatgeneratedinthechipsisconductedacrossthecircuitboard,andisdissipatedfromthebacksideoftheboard.4Heattransferfromthefintipsisnegligible.5Theheattransfercoefficientisconstantanduniformovertheentirefinsurface.6Thethermalpropertiesofthefinsareconstant.7Theheattransfercoefficientaccountsfortheeffectofradiationfromthefins.
PropertiesThethermalconductivitiesaregiventobek=12W/m°Cforthecircuitboard,k=237W/m°Cforthealuminumplateandfins,andk=1.8W/m°Cfortheepoxyadhesive.
Analysis(a)Thethermalresistanceoftheboardandtheconvectionresistanceonthebacksideoftheboardare
Thensurfacetemperaturesonthetwosidesofthecircuitboardbecomes
(b)Notingthatthecross-sectionalareasofthefinsareconstant,theefficiencyoftheserectangularfinsisdeterminedtobe
Thefinnedandunfinnedsurfaceareasare
Then,
Substituting,thebasetemperatureofthefinnedsurfacesisdeterminedtobe
Thenthetemperaturesonbothsidesoftheboardaredeterminedusingthethermalresistancenetworktobe
3-165Acircuitboardhouseselectroniccomponentsononeside,dissipatingatotalof15Wthroughthebacksideoftheboardtothesurroundingmedium.Thetemperaturesonthetwosidesofthecircuitboardaretobedeterminedforthecasesofnofinsand20copperfinsofrectangularprofileonthebackside.
Assumptions1Steadyoperatingconditionsexist.2Thetemperatureintheboardandalongthefinsvariesinonedirectiononly(normaltotheboard).3Alltheheatgeneratedinthechipsisconductedacrossthecircuitboard,andisdissipatedfromthebacksideoftheboard.4Heattransferfromthefintipsisnegligible.5Theheattransfercoefficientisconstantanduniformovertheentirefinsurface.6Thethermalpropertiesofthefinsareconstant.7Theheattransfercoefficientaccountsfortheeffectofradiationfromthefins.
PropertiesThethermalconductivitiesaregiventobek=12W/m°Cforthecircuitboard,k=386W/m°Cforthecopperplateandfins,andk=1.8W/m°Cfortheepoxyadhesive.
Analysis(a)Thethermalresistanceoftheboardandtheconvectionresistanceonthebacksideoftheboardare
Thensurfacetemperaturesonthetwosidesofthecircuitboardbecomes
(b)Notingthatthecross-sectionalareasofthefinsareconstant,theefficiencyoftheserectangularfinsisdeterminedtobe
Thefinnedandunfinnedsurfaceareasare
Then,
Substituting,thebasetemperatureofthefinnedsurfacesdeterminetobe
Thenthetemperaturesonbothsidesoftheboardaredeterminedusingthethermalresistancenetworktobe
3-166Steampassesthrougharowof10parallelpipesplacedhorizontallyinaconcretefloorexposedtoroomairat25
withaheattransfercoefficientof12W/m2.C.Ifthesurfacetemperatureoftheconcretefloorisnottoexceed40
theminimumburialdepthofthesteampipesbelowthefloorsurfaceistobedetermined.
Assumptions1Steadyoperatingconditionsexist.2Heattransferistwo-dimensional(nochangeintheaxialdirection).3Thermalconductivityoftheconcreteisconstant.
PropertiesThethermalconductivityofconcreteisgiventobek=0.75W/m°C.
AnalysisInsteadyoperation,therateofheatlossfromthesteamthroughtheconcretefloorbyconductionmustbeequaltotherateofheattransferfromtheconcretefloortotheroombycombinedconvectionandradiation,whichisdeterminedtobe
ThenthedepththesteampipesshouldbeburiedcanbedeterminedwiththeaidofshapefactorforthisconfigurationfromTable3-5tobe
3-167Twopersonsarewearingdifferentclothesmadeofdifferentmaterialswithdifferentsurfaceareas.Thefractionsofheatlostfromeachperson’sbodybyrespirationaretobedetermined.
Assumptions1Heattransferissteady.2Heattransferisone-dimensional.3Thermalconductivitiesareconstant.4Heattransferbyradiationisaccountedforintheheattransfercoefficient.5Thehumanbodyisassumedtobecylindricalinshapeforheattransferpurposes.
PropertiesThethermalconductivitiesoftheleatherandsyntheticfabricaregiventobek=0.159W/m°Candk=0.13W/m°C,respectively.
AnalysisThesurfaceareaofeachbodyisfirstdeterminedfrom
Thesensibleheatlostfromthefirstperson’sbodyis
Thetotalsensibleheattransferisthesumofheattransferredthroughtheclothesandtheskin
Thenthefractionofheatlostbyrespirationbecomes
Repeatingsimilarcalculationsforthesecondperson’sbody
T1
3-168Awallconstructedofthreelayersisconsidered.Therateofhattransferthroughthewallandtemperaturedropsacrosstheplaster,brick,covering,andsurface-ambientairaretobedetermined.
Assumptions1Heattransferissteady.2Heattransferisone-dimensional.3Thermalconductivitiesareconstant.4Heattransferbyradiationisaccountedforintheheattransfercoefficient.
PropertiesThethermalconductivitiesoftheplaster,brick,andcoveringaregiventobek=0.72W/m°C,k=0.36W/m°C,k=1.40W/m°C,respectively.
AnalysisThesurfaceareaofthewallandtheindividualresistancesare
Thesteadyrateofheattransferthroughthewallthenbecomes
Thetemperaturedropsare
3-169Aninsulationistobeaddedtoawalltodecreasetheheatlossby85%.Thethicknessofinsulationandtheoutersurfacetemperatureofthewallaretobedeterminedfortwodifferentinsulatingmaterials.
Assumptions1Heattransferissteady.2Heattransferisone-dimensional.3Thermalconductivitiesareconstant.4Heattransferbyradiationisaccountedforintheheattransfercoefficient.
PropertiesThethermalconductivitiesoftheplaster,brick,covering,polyurethanefoam,andglassfiberaregiventobe0.72W/m°C,0.36W/m°C,1.40W/m°C,0.025W/m°C,0.036W/m°C,respectively.
AnalysisThesurfaceareaofthewallandtheindividualresistancesare
Therateofheatlosswithouttheinsulationis
(a)Therateofheattransferafterinsulationis
Thetotalthermalresistancewiththefoaminsulationis
Thethicknessofinsulationisdeterminedfrom
Theoutersurfacetemperatureofthewallisdeterminedfrom
(b)Thetotalthermalresistancewiththefiberglassinsulationis
Thethicknessofinsulationisdeterminedfrom
Theoutersurfacetemperatureofthewallisdeterminedfrom
DiscussionTheoutersurfacetemperatureissameforbothcasessincetherateofheattransferdoesnotchange.
3-170Coldconditionedairisflowinginsideaductofsquarecross-section.Themaximumlengthoftheductforaspecifiedtemperatureincreaseintheductistobedetermined.
Assumptions1Heattransferissteady.2Heattransferisone-dimensional.3Thermalconductivitiesareconstant.4Steadyone-dimensionalheatconductionrelationscanbeusedduetosmallthicknessoftheductwall.5Whencalculatingtheconductionthermalresistanceofaluminum,theaverageofinnerandoutersurfaceareaswillbeused.
PropertiesThethermalconductivityofaluminumisgiventobe237W/m°C.ThespecificheatofairatthegiventemperatureisCp=1006J/kg°C(TableA-15).
AnalysisTheinnerandtheoutersurfaceareasoftheductperunitlengthandtheindividualthermalresistancesare
Therateofheatlossfromtheairinsidetheductis
Foratemperatureriseof1°C,theairinsidetheductshouldgainheatatarateof
Thenthemaximumlengthoftheductbecomes
3-171Heattransferthroughawindowisconsidered.Thepercenterrorinvolvedinthecalculationofheatgainthroughthewindowassumingthewindowconsistofglassonlyistobedetermined.
Assumptions1Heattransferissteady.2Heattransferisone-dimensional.3Thermalconductivitiesareconstant.4Radiationisaccountedforinheattransfercoefficients.
PropertiesThethermalconductivitiesaregiventobe0.7W/m°Cforglassand0.12W/m°Cforpinewood.
AnalysisThesurfaceareasoftheglassandthewoodandtheindividualthermalresistancesare
Therateofheatgainthroughtheglassandthewoodandtheirtotalare
Ifthewindowconsistsofglassonlytheheatgainthroughthewindowis
Thenthepercentageerrorinvolvedinheatgainthroughthewindowassumingthewindowconsistofglassonlybecomes
3-172Steamisflowinginsideasteelpipe.Thethicknessoftheinsulationneededtoreducetheheatlossby95percentandthethicknessoftheinsulationneededtoreduceoutersurfacetemperatureto40Caretobedetermined.
Assumptions1Heattransferissteadysincethereisnoindicationofanychangewithtime.2Heattransferisone-dimensionalsincethereisthermalsymmetryaboutthecenterlineandnovariationintheaxialdirection.3Thermalconductivitiesareconstant.4Thethermalcontactresistanceattheinterfaceisnegligible.
PropertiesThethermalconductivitiesaregiventobek=61W/m°Cforsteelandk=0.038W/m°Cforinsulation.
Analysis(a)Consideringaunitlengthofthepipe,theinnerandtheoutersurfaceareasofthepipeandtheinsulationare
Theindividualthermalresistancesare
Thenthes
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- Heat Chap03164