传热Introduction

1、Heat TransferTextbooks and references: Heat Transfer , by J.P.Holman 传热学传热学 杨世铭、陶文铨编著，第三版杨世铭、陶文铨编著，第三版IntroductionDifference between Thermodynamics and Heat transferThree modes of heat transfer: conduction, convection and radiationoverall heat transfer processsuperimposition of thermal resistances i

2、n seriesHeat transfer is the science that seeks to predict the energy transfer that may take place between material bodies as a result of a temperature difference. 传热学是传热学是研究热量传递规律的科学研究热量传递规律的科学. Explain how heat energy may be transferred; Predict the heat transfer rate. 研究热量传递的方式以及传热速率研究热量传递的方式以及传热

3、速率 Heat transfer force: temperature difference 热量传递过程的推动力：温差热量传递过程的推动力：温差 The second law of thermodynamics: heat flows readily from a region of high temperature to one of lower temperature. 热力学第二定律：热量可以自发地由高温热源传给低温热热力学第二定律：热量可以自发地由高温热源传给低温热源源. Where there is a temperature difference, there is heat t

4、ransfer. Temperature difference is the force of heat transfer. 有温差就会有传热有温差就会有传热 温差是热量传递的推动力温差是热量传递的推动力Difference between Thermodynamics and Heat transfer Thermodynamics: predict the amount of energy required to change a system from one equilibrium state to another. 工程热力学：系统从一个平衡态到另一个平衡态的过程中传递工程热力学：系

5、统从一个平衡态到另一个平衡态的过程中传递热量的多少。热量的多少。 Heat transfer：energy-transfer rate; the temperature of both the bar and the water as a function of time.传热学：关心的是热量传递的过程传热学：关心的是热量传递的过程即热量传递的速率即热量传递的速率; 以及物体的温度分布以及物体的温度分布Water, M220oCHot steel bar, M1300oC热量传递的三种基本方式 heat transfer热热 量量 传传 递递热热 传传 导导（heat conduction）热

6、热 对对 流流 (heat convection)热热 辐辐 射射 (thermal radiation)Three modes of heat transferThree modes of heat transfer 1. Conduction heat transfer（导热） 1.1 Definition Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of intera

7、ctions between particles. 定义：指温度不同的物体各部分或温度不同的两物体间直接定义：指温度不同的物体各部分或温度不同的两物体间直接接触时，依靠分子、原子及自由电子等微观粒子热运动而进接触时，依靠分子、原子及自由电子等微观粒子热运动而进行的热量传递现象行的热量传递现象Conduction heat transfer1.2 Conduction can take place in solids, liquids or gases. 物质的属性：可以在固体、液体、气体中发生物质的属性：可以在固体、液体、气体中发生1.3 Characteristics of conducti

8、on(1) Temperature difference(2) Direct contact of bodies(3) Due to thermal motion of particles such as molecules, atoms and free electrons.导热的特点：导热的特点：a 必须有温差；必须有温差；b 物体直接接触；物体直接接触；c依靠分子、原子及自由电子等微观粒子热运动而传递热量依靠分子、原子及自由电子等微观粒子热运动而传递热量Conduction heat transfer(W) ddxtA1.4 Fourierslaw of heat conduction（

9、傅立叶定律）（傅立叶定律）)(W/m dd2xtAqt0 x dxdtQq : the heat transfer rate (热流率热流率) , Wdt/dx: temperature gradient in the direction of heat flow(热流方向上的温度梯度）热流方向上的温度梯度）: the thermal conductivity of the material(导热系数导热系数), W/(mK)-: to satisfy the second principle of thermodynamics.（热流密度（热流密度与温度梯度方向相反，从高温流向低温）与温度梯

10、度方向相反，从高温流向低温）2. Convection Heat Transfer2.1 Convection heat transfer refers to the heat exchanged between a surface and a fluid moving over the surface.the consequence of the moving of a fluid At the wall: a conduction process.The temperature gradient at the wall depends on the flow field(流场).high

11、velocity Large temperature gradientConvection Heat Transfer2.2 Newtons law of coolingA: the surface areatw-t: the overall temperature differencetw: surface temperaturet : fluid temperatureh: the convection heat-transfer coefficient， W/(m2)(对流换热系数，表面传热系数）对流换热系数，表面传热系数）(W) )(tthAw2mW )( fwtthAqConvect

12、ion Heat Transfer2.3 convection heat-transfer coefficient Influencing factors: the nature of the fluid流体的物性流体的物性 viscosity黏度黏度, thermal conductivity导热系数导热系数, specific heat比热比热, density密度密度 The nature of the surface and its geometry物物体表面的形状尺寸体表面的形状尺寸 Fluid velocity流速流速pclufh,Convection Heat Transfer2

13、.4 natural convection and forced convection(1)Natural convection temperature gradient density gradient buoyancy fluid motion(2) Forced convection external force field: by fans, pumps, etc.3. RadiationRadiation3.1 thermal radiationenergy emitted by matter that is at a finite temperature(有限温度下的物质发射出的能

14、量） radiation energy is transported by electromagnetic waves（辐射能通过电磁波传递）3.2 blackbody: the perfect absorber and emitter an ideal surface absorbs all incident radiation （吸收各种波长和方向的入射辐射） for a prescribed temperature and wavelength, no surface can emit more energy than a blackbody（对任一给定的波长和温度，黑体辐射的能量均最多

15、） a diffuse emitter（黑体是漫射体）Radiation3.3 the Stefen-Boltzmann law of thermal radiation 4TAemitted4Tqemitted42T1T2Tradiant exchange between two surfaces41T)(424121TTq4TAreal surface: emissivity (发射率）发射率）Radiation3.4 Radiation in an enclosuresurface1: T1, A1surface 2:T2, A24241112, 1TTAb辐射强度与到辐射源距离的平方成

16、反比。例如，距离每增加1倍，辐射强度就减少到原来的14。 overall heat transfer process (传热过程）传热过程）radiationconvectionplane wallconvectionradiationsuperimposition of thermal resistances in series （串联热阻叠加原理）（串联热阻叠加原理）2121212111)()(AhAAhttRRRttffhhffAR 111AhRh221AhRhtAkttAkff)(21k: over-all heat transfer coefficient (传热系数）传热系数）Summary1.three modes of heat transfer: conduction, convection and radiation2.conduction: Fouriers law, th