1传热学-绪论[1]

传热学HeatTransfer北京工业大学环境与能源工程学院夏国栋Chapter1绪论(Introduction)Whatisheattransfer?•Heattransfer(orheat)isenergyintransitduetoatemperaturedifference.•Wheneverthereexistsatemperatureinamediumorbetweenmedia,heattransfermustoccur.1.1Heattransfermodes(传热方式)Therearethreeheattransfermodes.1.Conduction（热传导）2.Advection（热对流）3.Thermalradiation（热辐射）•Convection=Advection+Conduction•对流换热=热对流+热传导•Theconvectionheattransfermodeiscomprisedoftwomechanisms.HeattransfermodesMechanismsofheattransfermodesConductionDiffusionofenergyduetorandommolecularmotionConvectionDiffusionofenergyduetorandommolecularmotionplusenergytransferduetobulkmotion(advection)RadiationEnergytransferbyelectromagneticwavesHeatconduction•热流量：Heattransferrate（，W）•热流密度：Heatflux（q，W/m2）•导热系数：Thermalconductivity（，W/m·K）•热阻：Thermalresistance(R，K/W)•温度差：Temperaturedifference（t，K）•热流量方程：HeattransferrateequationBoundarylayer•受迫对流：Forcedconvection•自然对流：Freeornaturalconvection•沸腾：Boiling•凝结：Condensation•对流换热系数：Convectionheattransfercoefficient(h,W/m2·K)Heatconvection•辐射力：Emissivepower(E,W/m2)•投射辐射：Irradiation(G,W/m2)•吸收辐射：Absorbedradiation(Gabs,W/m2)•黑体：Blackbody(anidealradiator)•灰体：Greybody(anidealradiator)•发射率：Emissivity()•吸收率：Absorptivity()•辐射换热量：Radiationexchangerate•斯蒂芬-波尔茨曼：Stefan-BoltzmanLaw•斯蒂芬-波尔茨曼常数：Stefan-BoltzmanConstant(b)Heatradiation1.2Heattransferprocess(传热过程)图1为三种太阳能热水器的元件：图a为充满水的金属管；图b为在图a的管外加一玻璃罩，玻璃罩和金属管间有空气；图c为在图a的管外加一玻璃罩，但罩与金属管间抽真空。试分别用框图表示三元件的传热过程，并论述其效率由图a向图c逐步提高的原因。答案: