Presentation on radiation in heat transfer.pptx

Presentation on radiation

1. Radiation

2. Radiation heat transfer Radiation heat transfer is the transfer of energy in the form of electromagnetic waves, or photons, from one object to another without the need for physical contact or a medium. In this process, heat is transferred through a vacuum or transparent media such as air or glass There are many types of electromagnetic radiation; thermal radiation is only one. Regardless of the type of radiation, we say that it is propagated at the speed of light, 3×108 m/s. This speed is equal to the product of the wavelength and frequency of the radiation. c= λν where c= speed of light λ= wavelength ν= frequency

3. The propagation of thermal radiation takes place in the form of discrete quanta, each quantum having an energy of E=h ν h is Planck’s constant and has the value h=6.625×10^− 34 J · s

4. Stefan-Boltzmann law the total amount of energy emitted by the black body at a given temperature is proportional to the fourth power of the temperature. blackbody radiation because materials that obey this law appear black to the eye; they appear black because they do not reflect any radiation. Thus a blackbody is also considered one that absorbs all radiation incident upon it. Eb is called the emissive power of a blackbody A surface coated with lampblack appears black to the eye and turns out to be black for the thermal-radiation spectrum . On the other hand, snow and ice appear quite bright to the eye but are essentially “black ” for long-wavelength thermal radiation. Many white paints are also essentially black for long-wavelength radiation.

5. When radiant energy strikes a material surface, part of the radiation is reflected, part is absorbed , and part is transmitted , reflectivity ρ as the fraction reflected, the absorptivity α as the fraction absorbed, and the transmissivity τ as the fraction transmitted ρ+α+τ = 1

6. Specular and diffuse reflection If the angle of incidence is equal to the angle of reflection, the reflection is called specular. when an incident beam is distributed uniformly in all directions after reflection, the reflection is called diffuse. A rough surface exhibits diffuse behaviour better than a highly polished surface .

7. At equilibrium, the energy absorbed by the body must be equal to the energy emitted ; E A=qi A α radiant flux arriving at some area in the enclosure be qi W/m2 . If we now replace the body in the enclosure with a blackbody of the same size and shape and allow it to come to equilibrium with the enclosure at the same temperature, Eb A=qi A(1) , the absorptivity of a blackbody is unity . E/ Eb = α

8. The ratio of the emissive power of a body to the emissive power of a blackbody at the same temperature is equal to the absorptivity of the body. This ratio is defined as the emissivity ε of the body , ε = E/ Eb so ε =a for black body is called Kirchhoff’s identity the emissivity of a material varies with temperature and the wavelength of the radiation . Real substances emit less radiation than ideal black surfaces as measured by the emissivity of the material.

9. The Gray Body A gray body is defined such that the monochromatic emissivity ε λ of the body is independent of wavelength. The monochromatic emissivity is defined as the ratio of the monochromatic emissive power of the body to the monochromatic emissive power of a blackbody at the same wavelength and temperature. E bλ was derived by Planck by introducing the quantum The concept for electromagnetic energy.

10. A plot of Ebλ as a function of temperature and wavelength. maximum points in the radiation curves are related by Wien’s displacement law λ max T =2897.6 μ m·K [5215.6 μ m· ◦R ]

11. the amount of energy radiated from a blackbody in a certain specified wavelength range. The fraction of the total energy radiated between 0 and λ is given by

12. A glass plate 30 cm square is used to view radiation from a furnace. The transmissivity of the glass is 0.5 from 0.2 to 3.5 μm . The emissivity may be assumed to be 0.3 up to 3.5 μm and 0.9 above that . The transmissivity of the glass is zero, except in the range from 0.2 to 3.5 μm . Assuming that the furnace is a blackbody at 2000◦C, calculate the energy absorbed in the glass and the energy transmitted.

13. RADIATION SHAPE FACTOR Consider two black surfaces A 1 and A 2 , for the energy exchange between these surfaces when they are maintained at different temperatures. The problem becomes essentially one of determining the amount of energy that leaves one surface and reaches the other. To solve this problem the radiation shape factors are defined as

14. Other names for the radiation shape factor are view factor, angle factor, and configuration factor . The energy leaving surface 1 and arriving at surface 2 is Eb1 A1 F12 and the energy leaving surface 2 and arriving at surface 1 is Eb2 A2 F21 net energy exchange is If both surfaces are at the same temperature, there can be no heat exchange, that is, Q 1 − 2 = 0. Also, for T 1 = T 2 Although the relation is derived for black surfaces, it holds for other surfaces also as long as diffuse radiation is involved

15. The projection o The projection of dA 1 on the line between centers is surfaces are diffuse, that is, that the intensity of the radiation is the same in all directions . The intensity is the radiation emitted per unit area and per unit of solid angle in a certain specified direction. So, in order to obtain the energy emitted by the element of area dA 1 in a certain direction, we must multiply the intensity by the projection of dA 1 in the specified direction. Thus the energy leaving dA 1 in the direction given by the angle φ 1 is

16. where dA n is constructed normal to the radius vector. The quantity dA n /r 2 represents the solid angle subtended by the area dA n . The intensity may be obtained in terms of the emissive power by integrating expression ( b ) over a hemisphere enclosing the element of area dA 1 The area element dA n is given by

17. Consider the radiation from the small area dA 1 to the flat disk A 2 . The element of area dA 2 is chosen as the circular ring of radius x . Thus dA 2 = 2 π x dx

18. Relations/ Theorems of Shape Factors

19. B) Principle of conservation: If the enclosure consists of n number of surfaces and if one surface out of them is radiating energy then,The summation of energy incidents on each surface is equal to the total energy radiated by that single surface.

20. HEAT EXCHANGE BETWEEN NONBLACKBODIES for all the energy striking a surface will not be absorbed; part will be reflected back to another heat-transfer surface, and part may be reflected out of the system entirely We shall assume that all surfaces considered in our analysis are diffuse, gray , and uniform in temperature and that the reflective and emissive properties are constant overall the surface. Two new terms may be defined: G = irradiation = total radiation incident upon a surface per unit time and per unit area J = radiosity = total radiation that leaves a surface per unit time and per unit area the radiosity is the sum of the energy emitted and the energy reflected when no energy is transmitted, or J = ε E b + ρ G

21. total radiation leaving surface 1, the amount that reaches surface 2 is J 1 A 1 F 12 and of that total energy leaving surface 2, the amount that reaches surface 1 is J 2 A 2 F 21

22. INFINITE PARALLEL SURFACES two long concentric cylinders area ratio A 1 /A 2 may be replaced by the diameter ratio d 1 /d 2 when cylindrical bodies are concerned

23. Radiation can have a significant effect on temperature measurement because it can alter the temperature of the object being measured and introduce measurement errors. When an object is exposed to radiation, it absorbs and emits thermal radiation, which can result in an increase or decrease in its temperature depending on the net radiation exchange. This effect is more significant for objects with low thermal conductivity or high radiation absorptivity/emissivity.