大学精品课件：chapter 8(Heat Transfer.J.P.Holman ).ppt

1、Chapter 8 Radiation heat transfer,8-1 Introduction,1,Thermal radiation is that electromagnetic radiation emitted by a body as a result of its temperature.,Three thermal transfer modes Conduction Convection Radiation Think about the differences between radiation heat transfer and conduction ,convecti

2、on heat transfer?,2,8-2 physical mechanism,Cosmic rays up to 410-7 m Gamma rays 410-7 to 1.4 10-4 m X-rays 110-5 to 2 10-2 m Ultraviolet rays 110-2 to 3.9 10-4 m Visible light 0.38 to 0.76 m Infrared rays 0.76 to 1000 m Radio and Hertzian waves 1000 to 21010 m Heat rays 0.1 to 100 m,3,4,Propagation

3、of thermal radiation takes places in the form of discrete quanta, the energy of each quantum can be calculated by the formula shown down there,5,Blackbody : body absorbs all radiation incident upon it,Energy density of radiation per volume per wavelength as,6,8-3 Radiation properties,7,8,9,10,11,Gra

4、y body,However there is no ideal body. We can construct a blackbody enclosure.,12,13,Plancks law Eb : spectral emissive power (the distribution of radiation energy on the wavelength ) W/m3 ；,According to the law , we know E=f(,T ) E has maximum T max ,14,Wien s displacement law. peak of the curve is

5、 shifted to the shorter wavelengths for higher temperature.,That explains the Phenomenon changes in color of a body as its heated,15,16,an angle formed by three or more planes intersecting at a common point,17,18,19,20,21,Blackbody radiation laws,8-4 Radiation shape factors,22,Two black surfaces(sho

6、wn below), we wish to obtain a general expression for energy exchange between them,Radiation shape factors(view factors, angle factors, configuration factors),now we get rid of the temperature in our expression.,23,24,25,More figures show the plane angle and solid angle,26,27,precondition：diffuse su

7、rface and uniform distribution; blackbody and gray body satisfy the precondition, real bodies are approximately taken as gray bodies.,28,Some special situations of Radiation shape factors,29,30,Real-surface behavior,31,not perfectly diffuse do not follow Lamberts Law :Intensity of emitted radiation

8、varies with direction E =f(,T ) selective,32,As the following figures are shown ,you may find the conductors and nonconductors have great difference,conductors,33,Nonconductors the three figures shown is at some Certain Temperature !,34,35,36,8-5 relation between shape factors,37,38,These are the pr

9、operties of shape factors. The reciprocal property The enclosure property Decomposability,Calculation of the shape factors Based on the properties of shape factors , we can use algebra method and cross-string method ; or we simply use definition equation to integrate its value; we also can look up i

10、n handbook of radiation shape factors. In the following slides you will be shown examples of the first two methods mentioned above.,Calculation of the shape factors Based on the properties of shape factors , we can use algebra method and cross-string method ; or we simply use definition equation to

11、integrate its value; we also can look up in handbook of radiation shape factors. In the following slides you will be shown examples of the first two methods mentioned above.,39,Algebra,40,Solution,41,42,43,Cross-string Two surfaces are 2-D, infinite in extend in one direction and characterized by al

12、l cross sections normal to the infinite direction being identical. Calculate the shape factor F1-2 .,8-6 . Heat exchange between nonblackbodies,There we define two new terms,44,then,45,The radiosity,The net energy flow leaving the surface,46,Now take the shape factors into consideration consider sur

13、face 1,2,the amount of energy that arrives surface 1 from surface 2 is,Or from 2 to1,Net interexchange between 1 and 2,surface resistance space resistance For two surfaces problem( figure is shown below ),47,),48,To determine the heat flow in complicated networks ,the value of J must be calculated .

14、(Kirchhoffs law ),For network likes that(two surfaces exchange heat and connected by a third surface that doesnt exchange heat ) this is a simple series-parallel network. And the heat flow is,8-7.Infinite parallel planes,49,if we encounter such situation of concentric cylinders as shown left.,When T

15、his equation is applied to calculate the radiation-energy loss from a hot object in a large room.,50,8-8 Radiation shields,51,Reduce radiation heat transfer by adding resistance in the heat-flow path.,Analysis (three parallel infinite planes have the same area),52,If the emissivities of the planes a

16、re the same Then,For n shields,Radiation shield applications,53,8-9.Gas radiation,54,Usually gaseous media exists between solid surfaces,Gas may emit and absorb thermal radiation,Diatomic gases with nonpolar symmetrical molecular structure , such as O2, N2, H2, air, are essentially transparent,Gases

17、 with more complex molecules , such as CO, CO2, H2O,CH4, SO2,NH3，emit and absorb thermal radiation , usually in certain wavelength band.,Properties of gases radiation,55,For gases that can emit and absorb.,1 gases frequently radiate and absorb only in narrow wavelength bands. 2 spectral emissive pow

18、er of gases obeys Plancks law , but not continuous for all wavelength.,56,The emission and absorption of gases take place all over the volume of gases, the shape of gas affects the properties.,Beers law the decrease of spectral intensity of radiation( for absorption )is to be proportional to the thi

19、ckness of the layer and intensity at that point.,57,The same gas , temperature and pressure.,58,Mean beam length Effect of the shape of the shape For easy calculation,mean beam length : radius of a gas hemisphere such that it radiates a flux to the center of its base equal to the average flux radiat

20、ed to the area of interest by actual volume of gas,The mean beam length of some common gas shapes are given in handbook,solar radiation,59,Equivalent blackbody temperature of the sun according to Wiens displacement law Most of the solar radiation(99%) is in the wavelength range of 0.2-2um,60,Greenho

21、use effect The glass allows much more radiation to come in than can escape.,61,The glass is almost transparent for radiation with the short -wavelength (2um),but essentially opaque to long-wavelength radiation(above 3 or 4um).,Greenhouse gas Gases with strong absorption in short wavelength band and

22、emissive power in long wavelength band ,such as CO, CO2, H2O,CH4,SO2,NH3, what the green house gases to the earth just like the glass to the greenhouses.,62,Greenhouse effect,63,64,Effect of radiation on temperature measurement,65,self-learn,Radiation net work Radiation heat exchange with specular surface Radiation with transmitting reflecting and absorbing media Numerical solution Temperature measurement Heat transfer coefficient,66,summary,67,review questions,68,69,The end !,