2. 1 Teaching Innovation - Entrepreneurial - Global The Centre for Technology enabled Teaching & Learning D M I E T R, Wardha DTEL DTEL (Department for Technology Enhanced Learning)

3. DEPARTMENT OF MECHANICAL ENGINEERING V –SEMESTER HEAT TRANSFER 2 UNIT NO.5 RADIATION

4. CHAPTER 1:- SYLLABUSDTEL 3 1. Radiation, nature of thermal radiation, black body radiation, radiation intensity, 2. Laws of radiation-Kirchhoff’s, Planks, Wien’s displacement, 3. Stefan-Boltzmann and Lamberts Co-sine law, 4. Emissivity, absorptivity, transitivity, reflectivity, radiosity, emissive power, irradiation, 5. Radiation network, radiation exchange between surfaces 6. Idea of shape factor and reciprocity theorem 7. Radiation between parallel plates, Cylinder and sphere 8. Radiation shields, effect of Radiation on temperature measurement

5. DTEL INTRODUCTION TO RADIATION 4 LECTURE 1:- RADIATION Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. In the transfer of energy by radiation, the absorption of electromagnetic waves is just as important as their emission. The surface of an object plays a significant role in determining how much radiant energy the object will absorb or emit.

6. DTEL INTRODUCTION TO RADIATION 5 LECTURE 1:- RADIATION How does heat energy get from the Sun to the Earth? There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. ? RADIATION

7. DTEL INTRODUCTION TO RADIATION 6 LECTURE 1:- RADIATION Examples of RADIATION 1.Fire 2.Heat Lamps 3.Sun

8. DTEL INTRODUCTION TO RADIATION 7 LECTURE 1:- RADIATION Since the color black is associated with nearly complete absorption of visible light, the term perfect blackbody or, simply, blackbody is used when referring to an object that absorbs all the electromagnetic waves falling on it.

9. DTEL LAWS OF RADIATION 8 LECTURE 2:- RADIATION Basic Laws of Radiation 1)All objects emit radiant energy. 2)Hotter objects emit more energy than colder objects (per unit area). The amount of energy radiated is proportional to the temperature of the object. 3)The hotter the object, the shorter the wavelength ( ) of emitted energy.  This is Wien’s Law max  3000  m T(K)

10. DTEL LAWS OF RADIATION 9 LECTURE 2:- RADIATION Basic Laws of Radiation 1)All objects emit radiant energy. 2)Hotter objects emit more energy than colder objects. The amount of energy radiated is proportional to the Absolute Temperature of the object raised to the fourth power.  This is the Stefan Boltzmann Law Q =  T 4 Q = flux of energy (W/m 2 ) T = temperature (K)  = 5.67 x 10 -8 W/m 2 K 4 (a constant)

11. DTEL LAWS OF RADIATION 10 LECTURE 2:- RADIATION  Stefan Boltzmann Law. Q =  T 4 Q = flux of energy (W/m 2 ) T = temperature (K)  = 5.67 x 10 -8 W/m 2 K 4 (a constant)  Wien’s Law max  3000  m T(K)

12. DTEL BLACK BODY RADIATION 11 LECTURE 2:- RADIATION We can use these equations to calculate properties of energy radiating from the Sun and the Earth. 6,000 K 300 K

13. DTEL BLACK BODY RADIATION 12 LECTURE 2:- RADIATION Blackbody Radiation Blackbody radiation—radiation emitted by a body that emits (or absorbs) equally well at all wavelengths

14. DTEL BLACK BODY RADIATION 13 LECTURE 3:- RADIATION Blackbody radiation follows the Planck function The Planck Function

15. DTEL BLACK BODY RADIATION 14 LECTURE 3:- RADIATION Kirchoff’s Law If the body is at uniform temp with surround emissivity is equal to absorpivity » a = E

16. DTEL RADIATIVE PROPERTIES 15 LECTURE 3:- RADIATION Radiative Properties When radiation strikes a surface, a portion of it is reflected, and the rest enters the surface. Of the portion that enters the surface, some are absorbed by the material, and the remaining radiation is transmitted through. The ratio of reflected energy to the incident energy is called reflectivity, ρ. Transmissivity (τ) is defined as the fraction of the incident energy that is transmitted through the object. Absorptivity (α) is defined as the fraction of the incident energy that is absorbed by the object. The three radiative properties all have values between zero and 1. Furthermore, since the reflected, transmitted, and absorbed radiation must add up to equal the incident energy, the following can be said about the three properties: 

17. DTEL RADIATIVE PROPERTIES 16 LECTURE 3:- RADIATION Emissivity A black body is an ideal emitter. The energy emitted by any real surface is less than the energy emitted by a black body at the same temperature. At a defined temperature, a black body has the highest monochromatic emissive power at all wavelengths. The ratio of the emissive power of real body E  to the blackbody emissive power  b  at the same temperature is the hemispherical emissivity of the surface.

18. DTEL RADIATIVE PROPERTIES 17 LECTURE 3:- RADIATION The Emission Process For gases and semitransparent solids, emission is a volumetric phenomenon. In most solids and liquids the radiation emitted from interior molecules is strongly absorbed by adjoining molecules. Only the surface molecules can emit radiation.

19. DTEL RADIATIVE PROPERTIES 18 LECTURE 4:- RADIATION Spherical Geometry q f dA

20. DTEL RADIATIVE PROPERTIES 19 LECTURE 4:- RADIATION Vectors in Spherical Geometry Zenith Angle :   zimuthal Angle :  ( ,r)

21. DTEL RADIATIVE PROPERTIES 20 LECTURE 4:- RADIATION Hemispherical Black Surface Emission Black body Emissive Intensity

22. DTEL RADIATIVE PROPERTIES 21 LECTURE 4:- RADIATION Hemispherical Black Surface Emission Black body Emissive Intensity

23. DTEL EMISSION 22 LECTURE 4:- RADIATION Real Surface emission The radiation emitted by a real surface is spatially distributed: Directional Emissive Intensity: Directional Emissivity:

24. DTEL EMISSION 23 LECTURE 5:- RADIATION Directional Emissivity

25. DTEL EMISSION 24 LECTURE 5:- RADIATION Planck Radiation Law The primary law governing blackbody radiation is the Planck Radiation Law. This law governs the intensity of radiation emitted by unit surface area into a fixed direction (solid angle) from the blackbody as a function of wavelength for a fixed temperature. The Planck Law can be expressed through the following equation. h = 6.625 X 10 -27 erg-sec (Planck Constant) K = 1.38 X 10 -16 erg/K (Boltzmann Constant) C = Speed of light in vacuum

26. DTEL EMISSION 25 LECTURE 5:- RADIATION Real Surface Monochromatic emission The radiation emitted by a real surface is spatially distributed: Directional MonochromaticEmissive Intensity: Directional Monochromatic Emissivity:

27. DTEL EMISSION 26 LECTURE 5:- RADIATION Monochromatic emissive power E All surfaces emit radiation in many wavelengths and some, including black bodies, over all wavelengths. The monochromatic emissive power is defined by: dE = emissive power in the wave band in the infinitesimal wave band between l and l+ d l. The monochromatic emissive power of a blackbody is given by:

28. DTEL EMISSION 27 LECTURE 5:- RADIATION Shifting Peak Nature of Radiation

29. DTEL WEINS LAW 28 LECTURE 6:- RADIATION Wein’s Displacement Law: At any given wavelength, the black body monochromatic emissive power increases with temperature. The wavelength max at which is a maximum decreases as the temperature increases. The wavelength at which the monochromatic emissive power is a maximum is found by setting the derivative of previous Equation with respect to 

30. DTEL WEINS LAW 29 LECTURE 6:- RADIATION Wien law for three different stars

31. DTEL STEFAN-BOLTZMANN LAW 30 LECTURE 6:- RADIATION Stefan-Boltzmann Law The maximum emissive power at a given temperature is the black body emissive power (Eb). Integrating this over all wavelengths gives E b.

32. DTEL ABSORPTIVITY, REFLECTIVITY, TRANSMISSIVITY 31 LECTURE 6:- RADIATION Absorptivity , Reflectivity  and Transmissivity  Consider a semi-transparent sheet that receives incident radiant energy flux, also known as irradiation, G. Let dG represent the irradiation in the waveband  to  d. Part of it may be absorbed, part of it reflected at the surface, and the rest transmitted through the sheet. We define monochromatic properties,

33. DTEL ABSORPTIVITY, REFLECTIVITY, TRANSMISSIVITY 32 LECTURE 6:- RADIATION Monochromatic Absorptivity : Monochromatic reflectivity : Monochromatic Transmissivity : Total Absorptivity : Total reflectivity : Total Transmissivity :

34. DTEL ABSORPTIVITY, REFLECTIVITY, TRANSMISSIVITY 33 LECTURE 6:- RADIATION Conservation of Irradiation The total Irradiation =

35. DTEL BLACK BODY RADIATION 34 LECTURE 7:- RADIATION Blackbody Radiation The characteristics of a blackbody are : It is a perfect emitter. At any prescribed temperature it has the highest monochromatic emissive power at all wave lengths. A blackbody absorbs all the incident energy and there fore   It is non reflective body (  It is opaque (  = 0). It is a diffuse emitter

36. DTEL RADIATIVE HEAT TRANSFER 35 LECTURE 7:- RADIATION Radiative Heat Transfer Consider the heat transfer between two surfaces, as shown in Figure. What is the rate of heat transfer into Surface B? To find this, we will first look at the emission from A to B. Surface A emits radiation as described in This radiation is emitted in all directions, and only a fraction of it will actually strike Surface B. This fraction is called the shape factor, F.

37. DTEL RADIATIVE HEAT TRANSFER 36 LECTURE 7:- RADIATION The amount of radiation striking Surface B is therefore: The only portion of the incident radiation contributing to heating Surface B is the absorbed portion, given by the absorptivity α B : Above equation is the amount of radiation gained by Surface B from Surface A. To find the net heat transfer rate at B, we must now subtract the amount of radiation emitted by B:

38. DTEL RADIATIVE HEAT TRANSFER 37 LECTURE 7:- RADIATION The net radiative heat transfer (gain) rate at Surface B is

39. DTEL SHAPE FACTOR 38 LECTURE 8:- RADIATION Shape Factors Shape factor, F, is a geometrical factor which is determined by the shapes and relative locations of two surfaces. Figure illustrates this for a simple case of cylindrical source and planar surface. Both the cylinder and the plate are infinite in length. In this case, it is easy to see that the shape factor is reduced as the distance between the source and plane increases. The shape factor for this simple geometry is simply the cone angle (θ) divided by 2π

40. DTEL SHAPE FACTOR 39 LECTURE 8:- RADIATION Shape factors for other simple geometries can be calculated using basic theory of geometry. For more complicated geometries, the following two rules must be applied to find shape factors based on simple geometries. The first is the summation rule. This rule says that the shape factor from a surface (1) to another (2) can be expressed as a sum of the shape factors from (1) to (2a), and (1) to (2b). The second rule is the reciprocity rule, which relates the shape factors from (1) to (2) and that from (2) to (1) as follows:

41. DTEL SHAPE FACTOR 40 LECTURE 8:- RADIATION Thus, if the shape factor from (1) to (2) is known, then the shape factor from (2) to (1) can be found by: If surface (2) totally encloses the surface 1: