(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)
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