(5.8)--Chapter 8 Basic laws of thermal传热学传热学传热学.ppt

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1、Chapter 8 Basic laws of thermal radiation and radiation characteristics8-1 Basic concepts of thermal radiation8-2 Basic law of black body thermal radiation8-3 Radiation characteristics of solid and liquid8-4 Relationship between absorption and radiation of radiant energy for actual objectThermal rad

2、iation is one of three basic ways of heat transfer.It has important applications in many fields,and examples of thermal radiation are found everywhere in daily life.Meaning电磁炉好像不是热辐射？Terrestrial Heat PipeConcrete LayerFloor Decoration LayerAluminum FoilHeat Insulation LayerCement Mortar ScreedStruct

3、ural Layer8-1 Basic concepts of thermal radiation1.Characteristics of thermal radiation(1)Definition:Energy produced by thermal motion in the form of electromagnetic waves;(2)Characteristics a Any object,as long as its temperature is higher than 0 K,it will continuously emit thermal radiation to the

4、 surrounding space;b Can spread in vacuum;c With transformation of energy form;d Strong directionality;e Radiant energy is related to both temperature and wavelength;f Emitting radiation depends on the 4th power of the temperature.Electromagnetic radiation contains many forms,as shown in the figure.

5、The thermal radiation region in industry that we are interested in is general 0.1100m.2.Electromagnetic spectrum rayx rayUltravioletHeat rayInfraredRadio waveVisible light ray detectionx radiometerUV inspection lamp Infrared detectorVisible sunlightMicrowave ovenmWhere,and are absorptivity,reflectiv

6、ity and transmissivity,respectively.3.Absorption,reflection,and transmition of thermal radiation by an object11If incident radiation is the radiant energy Q of a certain wavelength,thenSpectral absorptivity Spectral reflectivity Spectral transmissivityRelationship between and :Reflection is divided

7、into specular reflection and diffuse reflection.Specular reflectionDiffuse reflectionFor most solids and liquids:For gases without particles:For block body:For mirror body or white body:For transparent body:1.黑体概念 黑黑体体：是指能吸收投入到其表面上的所有热辐射能的物体，是一种科学假想的物体，现实生活中是不存在的。但却可以人工制造出近似的人工黑体，如图所示。黑体模型8-2 黑体热辐射的

8、基本定律The absorptivity of cavity material itself has no effect on black body model in principle.The larger the absorptivity of material itself is,the larger the effective absorptivity of black body model is.The smaller the proportion of area of small hole of total inner wall area of cavity,the higher

9、the absorptivity of small hole is.If wall temperature of cavity is uniform,its radiation is uniform and isotropic.It is equal to black body radiation at the same temperature,regardless of radiation characteristics of cavity wall itself.Some explanations of black body modelSince thermal radiation has

10、 directional and spectral characteristics,energy is expressed more complex than heat conduction and convection.Such as following aspects:(1)Radiation energy of an object into hemispheric space is called hemispherical emissive power;(2)Radiation energy of an object in a certain direction is called di

11、rectional emissive power;(3)Radiation energy emitted by an object from a normal area in a certain direction is called directional radiation intensity；(4)When describing spectral characteristics,it is called spectral emissive power,spectral radiation intensity,etc.Only a few concepts related to the c

12、ontent of this book are given below.2Representation of thermal radiation energyEmissive power,E:The total energy of all wavelengths emitted by the unit surface area of an object into hemisphere space per unit time,also known as hemispheric total emissive power,(W/m2);Spectral emissive power E：The en

13、ergy emits per unit surface area of an object into hemisphere space per unit time,per unit wavelength range.Also known as hemispherical spectral emissive power,(W/m3);Obviously,there is a following relationship between E and E:Black body is generally indicated by subscript b.For example,emissive pow

14、er of black body is Eb,and spectral emissive power of black body is Eb.3Three basic laws and related properties of black body radiation(1)Stefan-Boltzmanns Law:gives the relationship between emissive power of black body E and thermodynamic temperature.where,=5.6710-8 W/(m2K4),is the black body radia

15、tionconstant;C0=5.67W/(m2K4),is the black body radiation coefficient.where，wavelength,m;T Black body temperature,K;c1 first radiation constant，3.741910-16 Wm2；c2 second radiation constant，1.438810-2 mK；(2)Plancks Law:reveals spectral characteristics of blackbody radiation energy,i.e.spectral emissiv

16、e power of black body,EbFigure 1 illustrates the relationship between Plancks law and Stefan-Boltzmanns law.Figure 2 shows the relationship of spectral emissive power of black body as a function of wavelength and temperature as depicted by the above formula.It can be seen that Eb at a certain temper

17、ature has a maximum value,and the corresponding wavelength is m.The relationship between m and T is given by Wiens displacement law:(3)Black body radiation function In practical problems,it is often necessary to determine the radiation energy in a certain wavelength range,that is,the emissive power

18、emitted by black body in the wavelengths 1 and 2,as shown in right figure:Black body emissive power in a specific wavelength bandThe above formula is not conducive to engineering applications,so,divide by to form a black body radiation function,:Black body radiation fucntionTable(4)Solid angle What

19、was mentioned above is only spectral characteristics in hemispherical space.The spatial orientation characteristics of the black body emission radiation will be described below.Definition of solid angle:Spherical area divided by squared radius is called solid angle,unit:sr(space degree).As shown in

20、the two figures below,dAc=rdrsin d,is infinitesimal area between +d and +d,and infinitesimal solid angle is Definition of solid angle Calculate geometric relationship of infinitesimal solid angleThe energy of all wavelengths emitted in a unit solid angle per unit area in a vertical direction of emis

21、sion per unit time,see figure below.Definition of directional radiation intensity(5)Directional radiation intensity,I(6)Lamberts Law(the third basic law of black body radiation)Diffuse surface:radiation intensity is equal in all space directions.According to the definition of directional emissive po

22、wer,since the directional radiation intensity of black body or diffuse body is independent of direction,the above formula can be written as:This is the expression of Lamberts law.It shows that the directional emissive power of black body(distribution of radiation energy in different space directions

23、)changes with angle by cosine law,see right figure.Therefore,Lamberts law is also called cosine law.Schematic of Lamberts lawIntegrate directional emissive power E in the direction of hemisphere(2 solid angle),and hemispheric emissive power E is obtained:It can be seen that for the radiation that ob

24、eys Lambes law,its emissive power numerically equal to times of the directional radiation intensity.constant8-3 Radiation characteristics of solids and liquidsWavelengthDirection(angle from surface normal)1.EmissivityIn fact,the true surfaces emit ability is a function of direction and spectrum.Ther

25、e are several levels to consider when describe the radiation characteristics of an actual object:1)Directional spectral emissive power(radiation intensity)2)Hemispherical spectral emissive power3)Directional emissive power(radiation intensity)4)Total emissive powerEmissivity(also known as blackness)

26、:The ratio of total hemispherical emissive power of an actual object to the total hemispherical emissive power of black body at the same temperature:Directional spectral emissivity:For a given direction(,)and wavelength Directional total emissivity：Integrate the above formula over all wavelength ran

27、ges,i.e.the ratio of directional radiation intensity of actual object to the directional radiation intensity of black bodyHemispherical spectral emissivity:That is,the ratio of spectral emissive power of actual object to the spectral emissive power of black body:Hemispheric total emissivity：Hemisphe

28、ric total emissivity is average for all directions and all wavelengths.The concept of diffuse emission:The directional emissivity of surface()is independent of direction,that is,the directional radiation intensity is independent of direction.The surface that satisfies the above law is called diffuse

29、 emission surface(or diffuse body,Lambert body).For most actual surfaces,this is a good approximation.Directional emissivity ()of several metal conductors in different directions(t=150).Polished NiLustreless NiDirectional emissivity()of several non-conductor materials in different directions(t=093.3

30、)Wet iceClayWoodCopper oxideGlassAluminum oxidePaperSome points to note:1.The emissive power of actual object is not completely proportional to the fourth power of its thermodynamic temperature.2.Although the directional emissivity of an actual object does not completely with Lamberts law,it is stil

31、l considered that most engineering materials obey Lamberts law.3.The emissivity of object surface depends on the type of material,surface temperature and surface condition.This means that the emissivity is only related to the object itself that emits radiation,and does not involve external condition

32、s.4.Some uncertainty factors are attributed to correction factors.This is because thermal radiation is very complicated and difficult to determine theoretically.It is actually an expedient measure.8-4 Relationship between absorption and radiation of radiation energy by actual objectSemi-transparent

33、mediumThe previous section briefly introduced emission of actual object.Then,when external radiation reaches the object surface,what is the situation on absorption of irradiation?1)Irradiation:total radiation energy projected onto a unit surface area per unit time.2)Selective absorption:Irradiation

34、itself has spectral characteristics.Therefore,the absorption capacity of actual object to irradiation also varies with its wavelength.This is called selective absorption.3）Absorptivity:The percentage of absorption to irradiation of an object,usually expressed in,i.e.1.absorptivity4）Spectral absorpti

35、vity:The percentage of an objects absorption of radiation energy at a particular wavelength,also known as monochromatic absorptivity.The spectral absorptivity versus with wavelength reflects the selective absorption characteristics of actual object.The following two figures show the relationship bet

36、ween spectral absorptivity and wavelength of several materials at room temperature.The relationship between spectral absorptivity wavelength of metal conductors.Polished aluminumAnodized aluminumPolished copperPowder wallWhite tileWhite fire mudThe relationship between spectral absorptivity waveleng

37、th of non-conductors.According to the previous definition,in addition to its surface properties and temperature,the absorptivity of an object is also related to energy distribution of irradiation by wavelength.Taking radiation heat transfer between two surfaces as an example,the subscripts 1 and 2 r

38、espectively represent the research object and the object that emits irradiation,and the absorptivity of the object 1 is:If irradiation comes from black body,due to the above formula can be modified to:Relationship between absorptivity and temperature of black body radiation on objects surfaceAsbesto

39、sWhite refractory soilWoodCorkCeramicConcreteRoof tileAluminumGraphite2.Engineering treatment method:gray body approximationGray body:Assume its spectral emissivity (or spectral blackness)and spectral absorption is independent of wavelength,for gray body:black bodygray bodyactual objectwhite body3.K

40、irchhoffs lawIn 1859,G.R.Kirchhoff revealed the relationship between ability of an object to emit radiation and to absorb radiation.The simplest derivation is the thermodynamic equilibrium method between two infinite plates.As shown on the right figure,Plate 1 is a black body,and plate 2 is any obje

41、ct.The parameters are Eb,T1 and E,T2.When the system is in thermal equilibrium,then,Assume that surface dA1 of an object is placed in black body cavity,and the two are in thermal equilibrium(T).In unit time,from a given direction,in the range of d wavelength,the energy projected from dA2 on the infi

42、nitesimal of the black body cavity to dA1 is:Solid angle:The energy absorbed by dA1 surface is:Monochromatic absorptivity of dA1 at temperature T and direction.where,I,T,represents monochromatic radiation intensity of surface dA1 at temperature T.The energy emitted by dA1 in unit time,in direction,a

43、nd in range of wavelength d is:Solid angle:The monochromatic radiation intensity I,T of surface dA1 can be expressed as:Under thermal equilibrium:It shows that under thermal equilibrium,the directional monochromatic emissivity(directional monochromatic blackness)of surface is equal to its directiona

44、l monochromatic absorptivity.Experiments show that directional monochromatic emissivity and directional monochromatic absorptivity of surface are radiation characteristics of surface of object,which only depend on their own temperature.For a non-thermal equilibrium relationship,the above formula is

45、still established.For diffuse radiation surfaces:For gray surfaces:For diffuse-gray surfaces:Kirchhoffs Law states that the stronger the ability to emit radiation energy,the stronger its ability to absorb radiation energy.For black body:Black body has the largest radiation and absorption capacity.Fo

46、r an engineering material which can be treated as gray body at industrial high temperature,its thermal radiation is mainly in the infrared range where does not change much and fraction of visible light is small.In calculation,for general engineering materials under industrial high temperature,it can

47、 be taken:For example,white paper absorbs only 0.27 for solar radiation while its blackness is as high as 0.95.However,solar radiation has about 43%in visible light range,and the surface with various colors has strong selectivity for absorption of visible light.That is,in visible light range,varies

48、greatly with wavelength.So,for solar radiation:Example 8-9 1.5 100.10.50.8It is known that the inner surface temperature of furnace wall is T1=500K and the spectral emissivity of furnace wall is shown in the above figure.The coal seam can be regarded as a black body with a temperature of T2=2000K.Fu

49、rnace wall is a diffuse reflection.Try to:(1)Calculate the emissivity of furnace wall(total hemispherical emissivity)(2)Absorptivity of radiation of furnace wall to coal seamChapter Summary1.Four ideal objects:black body,white body,transparent body,gray body2.Five Laws:Stefan-Boltzmanns Law,Plancks

50、Law,Wien s Displacement Law,Lambert s Law,Kirchhoff s Law3.Two approximations:gray surface,diffuse surface4.Radiation concepts:emissive power,spectral emissive power,directional emissive power,radiation intensity,incident radiation5.Some parameters：emissivity,spectral emissivity,directional emissivi