傳熱Introduction

1、Heat TransferTextbooks and references: Heat Transfer , by J.P.Holman 傳熱學(xué)傳熱學(xué) 楊世銘、陶文銓編著，第三版楊世銘、陶文銓編著，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. 傳熱學(xué)是傳熱學(xué)是研究熱量傳遞規(guī)律的科學(xué)研究熱量傳遞規(guī)律的科學(xué). Explain how heat energy may be transferred; Predict the heat transfer rate. 研究熱量傳遞的方式以及傳熱速率研究熱量傳遞的方式以及傳熱

3、速率 Heat transfer force: temperature difference 熱量傳遞過程的推動力：溫差熱量傳遞過程的推動力：溫差 The second law of thermodynamics: heat flows readily from a region of high temperature to one of lower temperature. 熱力學(xué)第二定律：熱量可以自發(fā)地由高溫?zé)嵩磦鹘o低溫?zé)釤崃W(xué)第二定律：熱量可以自發(fā)地由高溫?zé)嵩磦鹘o低溫?zé)嵩丛? Where there is a temperature difference, there is heat t

4、ransfer. Temperature difference is the force of heat transfer. 有溫差就會有傳熱有溫差就會有傳熱 溫差是熱量傳遞的推動力溫差是熱量傳遞的推動力Difference between Thermodynamics and Heat transfer Thermodynamics: predict the amount of energy required to change a system from one equilibrium state to another. 工程熱力學(xué)：系統(tǒng)從一個平衡態(tài)到另一個平衡態(tài)的過程中傳遞工程熱力學(xué)：系

5、統(tǒng)從一個平衡態(tài)到另一個平衡態(tài)的過程中傳遞熱量的多少。熱量的多少。 Heat transfer：energy-transfer rate; the temperature of both the bar and the water as a function of time.傳熱學(xué)：關(guān)心的是熱量傳遞的過程傳熱學(xué)：關(guān)心的是熱量傳遞的過程即熱量傳遞的速率即熱量傳遞的速率; 以及物體的溫度分布以及物體的溫度分布Water, M220oCHot steel bar, M1300oC熱量傳遞的三種基本方式 heat transfer熱熱 量量 傳傳 遞遞熱熱 傳傳 導(dǎo)導(dǎo)（heat conduction）熱

6、熱 對對 流流 (heat convection)熱熱 輻輻 射射 (thermal radiation)Three modes of heat transferThree modes of heat transfer 1. Conduction heat transfer（導(dǎo)熱） 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. 定義：指溫度不同的物體各部分或溫度不同的兩物體間直接定義：指溫度不同的物體各部分或溫度不同的兩物體間直接接觸時，依靠分子、原子及自由電子等微觀粒子熱運(yùn)動而進(jìn)接觸時，依靠分子、原子及自由電子等微觀粒子熱運(yùn)動而進(jìn)行的熱量傳遞現(xiàn)象行的熱量傳遞現(xiàn)象Conduction heat transfer1.2 Conduction can take place in solids, liquids or gases. 物質(zhì)的屬性：可以在固體、液體、氣體中發(fā)生物質(zhì)的屬性：可以在固體、液體、氣體中發(fā)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.導(dǎo)熱的特點(diǎn)：導(dǎo)熱的特點(diǎn)：a 必須有溫差；必須有溫差；b 物體直接接觸；物體直接接觸；c依靠分子、原子及自由電子等微觀粒子熱運(yùn)動而傳遞熱量依靠分子、原子及自由電子等微觀粒子熱運(yùn)動而傳遞熱量Conduction heat transfer(W) ddxtA1.4 Fourierslaw of heat conduction（

9、傅立葉定律）（傅立葉定律）)(W/m dd2xtAqt0 x dxdtQq : the heat transfer rate (熱流率熱流率) , Wdt/dx: temperature gradient in the direction of heat flow(熱流方向上的溫度梯度）熱流方向上的溫度梯度）: the thermal conductivity of the material(導(dǎo)熱系數(shù)導(dǎo)熱系數(shù)), W/(mK)-: to satisfy the second principle of thermodynamics.（熱流密度（熱流密度與溫度梯度方向相反，從高溫流向低溫）與溫度梯

10、度方向相反，從高溫流向低溫）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(流場).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)(對流換熱系數(shù)，表面?zhèn)鳠嵯禂?shù)）對流換熱系數(shù)，表面?zhèn)鳠嵯禂?shù)）(W) )(tthAw2mW )( fwtthAqConvect

12、ion Heat Transfer2.3 convection heat-transfer coefficient Influencing factors: the nature of the fluid流體的物性流體的物性 viscosity黏度黏度, thermal conductivity導(dǎo)熱系數(shù)導(dǎo)熱系數(shù), specific heat比熱比熱, density密度密度 The nature of the surface and its geometry物物體表面的形狀尺寸體表面的形狀尺寸 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(有限溫度下的物質(zhì)發(fā)射出的能

14、量） radiation energy is transported by electromagnetic waves（輻射能通過電磁波傳遞）3.2 blackbody: the perfect absorber and emitter an ideal surface absorbs all incident radiation （吸收各種波長和方向的入射輻射） for a prescribed temperature and wavelength, no surface can emit more energy than a blackbody（對任一給定的波長和溫度，黑體輻射的能量均最多

15、） a diffuse emitter（黑體是漫射體）Radiation3.3 the Stefen-Boltzmann law of thermal radiation 4TAemitted4Tqemitted42T1T2Tradiant exchange between two surfaces41T)(424121TTq4TAreal surface: emissivity (發(fā)射率）發(fā)射率）Radiation3.4 Radiation in an enclosuresurface1: T1, A1surface 2:T2, A24241112, 1TTAb輻射強(qiáng)度與到輻射源距離的平方成

16、反比。例如，距離每增加1倍，輻射強(qiáng)度就減少到原來的14。 overall heat transfer process (傳熱過程）傳熱過程）radiationconvectionplane wallconvectionradiationsuperimposition of thermal resistances in series （串聯(lián)熱阻疊加原理）（串聯(lián)熱阻疊加原理）2121212111)()(AhAAhttRRRttffhhffAR 111AhRh221AhRhtAkttAkff)(21k: over-all heat transfer coefficient (傳熱系數(shù)）傳熱系數(shù)）Summary1.three modes of heat transfer: conduction, convection and radiation2.conduction: Fouriers law, th