(5.1)--Chapter 1 Introduction传热学传热学传热学.ppt
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1、HEAT TRANSFERnTextbook:1.Heat Transfer Shiming Yang,Wenquan Tao,Higher Education Press,Forth Edtion 2.Fundamentals of Heat and Mass Transfer Theodore L.Bergman,Adrienne S.Lavine John Wiley&Sons Inc.Eighth EditionRequirements and AssessmentnRequirements:1.Listen carefully to class and follow classroo
2、m discipline;2 unexcused absentee in 3 spot checks considered failed this course.2.Complete the work on time,seriously,and independently.If you do not turn in more than half of the work,you lose the final exam.nAssessment method:mid-term exam+final exam+usual performance Chapter 1:IntroductionnResea
3、rch content of heat transfer and its application in science and technology and engineeringnThree basic modes of heat transfernHeat transfer process and heat transfer coefficientnA brief history and research methods for the development of heat transfer1.1 Research content of heat transfernThe science
4、 of studying the thermal energy(heat)transfer law caused by temperature differenceDriving force of heat transfer:temperature difference The second law of thermodynamics:thermal energy is spontaneously transmitted from a high-temperature heat source to a low-temperature heat source As long as there i
5、s a temperature difference,there is heat transfer.Heat:quantity of heat transferred by temperature difference.Temperature difference is almost ubiquitous,so heat transfer is a ubiquitous physical phenomenon in daily life and production practice.Law of heat transfer:rate equation of heat transfer Tem
6、perature DistributionRelationship between heat transfer and engineering thermodynamics(1)Thermodynamics+Heat Transfer=Thermal Science Heat transferred from one equilibrium state to another Focus on process of heat transfer,rate of heat transferWater,M220oCIron block,M1,300oCDifference between heat t
7、ransfer and thermodynamicsThermodynamics:Heat Transfer:(2)Heat transfer is based on the first law and the second law of thermodynamics,that is,is always transmitted from a high-temperature heat source to a low temperature heat source.If there is no conversion of energy form,then is always conserved.
8、Application examples of heat transfer-daily lifenRadiator inside northern housesnWindow insulation with double glassnsolar water heatersApplication examples of heat transfer-energy powerSteam turbineNuclear power plant unitPower plant cooling towerApplication examples of heat transfer-petrochemicalV
9、arious heat exchange devices in chemical plantsHeat pipes dotted in Oil refineryApplication examples of heat transfer-refrigeration and air conditioningVertical air conditionerAir-cooled refrigeration unitApplication examples of heat transfer-aerospacenIn aerospace industry,surface materials in spac
10、e shuttle require good thermal insulation;solar absorption devices installed on satellites can provide part of the energy needed for satellite work.Large passenger aircraftSpace shuttleApplication examples of heat transfer-electronic devicesSchematic diagram of heat transfer in the chip by air flowM
11、otherboardCPUApplication examples of heat transfer medical fieldnHeat transfer is widely used in laser surgery,high-temperature treatment of tumors,cryosurgery,frozen storage of transplanted organs,and thermal diagnosis of diseases.Cold airoutsideWarm and humid exhaled gasSchematic diagram of heat t
12、ransfer in human lung by air flowSchematic diagram of human head temperature(blue indicates low temperature)1.2 Modes of heat transfer1.2 Modes of heat transfer Three basic modes for heat transfer:Three basic modes for heat transfer:l Heat ConductionHeat Conductionl Heat Convection Heat Convectionl
13、Thermal Radiation Thermal Radiation1.2.1 Heat conduction1.2.1 Heat conductionHeat transfer inside or on the surface of an object due to the random motion of its constituent atoms,molecules and/or electrons.Heat conduction can occur in a solid or a stationary fluid(gas or liquid)This book does not di
14、scuss the microscopic mechanism of heat conduction,but the macroscopic law of heat transfer.Characteristics of heat conductionv There must be a temperature difference;v Direct contact between objects;vRelying on thermal motion of microscopic particles such as molecules,atoms and free electrons to tr
15、ansfer heat;vSimple heat conduction in gravitational field only occurs in dense solids.Physical mechanism of heat conductionDifferent materials have different mechanismsGas:collision of gas molecules with irregular thermal motionSolidConductor:Free electron movementInsulator:Vibration of lattice str
16、ucture-elastic waveLiquid:Have different opinionsOnly macroscopic law of heat conduction is studied in this book Fouriers Law:one-dimensional,one-dimensional,steady heat conduction steady heat conduction across a plane wallacross a plane wallFourier,French mathematician,1822Heat flux,W/m,W/m2 2Heat
17、rate,W,WTemperature Temperature gradient,K/mgradient,K/mThermal conductivity,Thermal conductivity,W/(m W/(m K)K)Area,mArea,m2 2Negative sign means direction of Negative sign means direction of heat flux is opposite to that of heat flux is opposite to that of temperature gradienttemperature gradientT
18、hermal ConductivityThermal Conductivity:A measure of a materials ability to transfer thermal energy by heat conduction.v Property:physical parameter,related to type of material and temperaturev Symbol:v Unit:W/(mK)vLaw of thermal conductivity for different materials:Heat Transfer through a Heat Tran
19、sfer through a Plane WallPlane Wall 0 xt tw2 tw1 Features:uniform and constant temperature on both surfaces and temperature inside plane wall does not change with time.The temperature inside the wall changes only in the direction perpendicular to the wall surface.Heat is only transmitted in the dire
20、ction perpendicular to the wall surface.Steady state Steady state const.const.FourierFourier s law is rewritten into:s law is rewritten into:Integrate on both sides:Integrate on both sides:Q Thermal resistance of plane wall,means the resistance of the object on heat conduction.unit is K/W Unit therm
21、al resistanceEquivalent thermal circuit1.2.2 Heat Convection1.2.2 Heat ConvectionA A heat heat transfer transfer phenomenon phenomenon caused caused by by relative relative movement movement due due to to macroscopic motion of fluids with different temperaturesmacroscopic motion of fluids with diffe
22、rent temperaturesFeaturesFeatures:only only occurs occurs in in fluid,fluid,accompanied accompanied with with heat heat conduction conduction from thermal motion of the microscopic particlesfrom thermal motion of the microscopic particlesConvection heat transfer:a phenomenon of heat transfer between
23、 a fluid and a solid surface in contact with each other and a combination of two basic heat transfer modes(conduction and convection).Features of convection heat transfer:Features of convection heat transfer:v Complex heat transfer process with conduction and convection Complex heat transfer process
24、 with conduction and convectionv Direct contact(fluid and wall)and bulk motion Direct contact(fluid and wall)and bulk motionv Boundary layer with a large velocity gradient formed near the wall Boundary layer with a large velocity gradient formed near the wallClassification of convection Classificati
25、on of convection heat transferheat transferCondensation heat transferCondensation heat transferFlow causeFlow causeForced convectionForced convectionFree convectionFree convectionPhase changePhase change NegativeNegativePositivePositiveBoiling heat transferBoiling heat transferNewtonNewton s Law of
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