RADIATION HEAT TRANSFER_HEAT TRANSFER.pptx

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1. RADIATION

2. Heat exchanged between two bodies or mediums, which are separated and are not in contact with each other, is called RADIATION HEAT TRANSFER . Radiation heat transfer does not require presence of an intervening medium between the two bodies and it takes place most effectively in a vacuum. Radiation heat transfer that involves emission of energy, emitted in the form of electromagnetic waves or electromagnetic radiation . The electromagnetic radiation that can be detected as heat is termed as thermal radiation emitted as a result of energy transitions of molecules, atoms, and electrons of a substance. Thermal radiation emitted by a body or substance is directly proportional to its temperature. There are many types of electromagnetic radiation; thermal radiation is only one. Energy transfer by radiation occurs at the speed of light and suffers no attenuation in vacuum. Radiation can occur between two bodies separated by a medium colder than both bodies. Examples The transmission of electromagnetic waves through the microwave oven. Solar ultraviolet radiation, precisely the process that determines the Earth’s temperature. The light emitted by an incandescent lamp.

3. Two theories have been postulated to explain the phenomenon of heat transfer by radiation. i ) Electromagnetic Wave Theory ii) Quantum Theory Electromagnetic Wave Theory postulated by James Clerk Maxwell in 1864. According to Maxwell theory , energy transfer takes place via electromagnetic waves in radiation. Electromagnetic waves transport energy like other waves and travel at the speed of light. Electromagnetic waves are characterized by their frequency ν (Hz) and wavelength λ (μ m), where: λ = c / ν where c is the speed of light in that medium; in a vacuum c = 2.99 x 10 8 m / s. Quantum Theory Max Planck postulated quantum theory of radiation in 1900. The propagation of thermal radiation takes place in the form of discrete quanta, each quantum having an energy of E = h ν where h is Planck’s constant and has the value h =6 . 625×10 −34 J.s The unit for λ may be centimetres, angstroms (1 Å=10 −8 cm), or micrometres (1 μm =10 −6 m)

4. THERMAL RADIATION The complete electromagnetic spectrum is delineated in Figure The electromagnetic radiation encountered in practice covers a wide range of wavelengths, varying from less than 10 - 10 μ m for cosmic rays to more than 10 10 μ m for electrical power waves. The spectrum measures range of frequencies of all electromagnetic radiation and it includes Radio, Microwaves, Infrared, Visible, Ultra-violet, X-rays, Gamma rays Thermal radiation is also defined as the portion of the electromagnetic spectrum that extends from about 0.1 to 100 μ m, since the radiation emitted by bodies due to their temperature falls almost entirely into this wavelength range. Thus, thermal radiation includes the entire visible and infrared (IR) radiation as well as a portion of the ultraviolet (UV) radiation

5. RADIATIVE PROPERTIES TOTAL EMISSIVE POWER (E) is defined as the total amount of radiation emitted by a body per unit area per unit time. It is expressed in W/m 2 MONOCHROMATIC EMISSIVE POWER(E λ ) is defined as the rate of energy radiated per unit area of surface per unit wave length. EMISSIVITY ( ε ) is defined as the ratio of emissive power of any body to the emissive power of black body at the same temperature. It varies between zero and one, i.e.,0 < ε <1 The radiation flux incident on a surface from all directions is called irradiation G ABSORPTIVITY, REFLECTIVITY, AND TRANSMISSIVITY Every body, is constantly bombarded by radiation coming from all directions over a range of wavelengths. When radiation energy strikes a material surface, part of the radiation is reflected, part is absorbed, and part is transmitted as shown in Figure

6. The fraction of irradiation absorbed by the surface is called the absorptivity α , the fraction reflected by the surface is called the reflectivity ρ , and the fraction transmitted is called the transmissivity τ . Absorptivity Reflectivity Transmissivity where G is the radiation flux incident on the surface, and G abs , G ref , and G tr are the absorbed, reflected, and transmitted portions of it, respectively. By conservation of energy principle G abs + G ref + G tr = G Dividing each term of this relation by G yields α + ρ + τ = 1   A medium that experiences no transmission ( τ =0) is opaque solids and liquids α + ρ = 1 For most gases the reflectance is absent, ρ = 0 α + τ = 1

7. SPECULAR RADIATION AND DIFFUSE RADIATION Two types of reflection phenomena may be observed when radiation strikes a surface. If the angle of incidence is equal to the angle of reflection, the reflection is called specular. On the other hand, when an incident beam is distributed uniformly in all directions after reflection, the reflection is called diffuse. These two types of reflection are depicted in Figure. Specular (φ1 =φ2) diffuse reflection RADIOSITY(J) This is defined as the total energy leaving a surface per unit time per unit area of the surface. The radiation energy incident on a surface from all directions per unit area per unit time is called irradiation, G

8. Black Body A perfect black body is one that absorbs all the thermal radiation, irrespective of wavelength, received by it. It does not reflect or transmit incident thermal radiation; therefore, absorptivity of such a body is 100%. α =1, ρ =0 , τ = Consider a hollow sphere with inside surface blackened and having a small hole at its surface. Thermal radiations entering the sphere through the hole are reflected repeatedly by the inner walls till they are completely absorbed. Therefore, the small hole acts as a black body absorber. A blackbody is an hypothetical body and has following properties. A black body absorbs all incident radiation irrespective of their wavelength and direction. At a given temperature and wavelength, energy emitted by a black body is the highest as compared to any other body. The radiation emitted by a black body depends upon wavelength and temperature, but it is independent of direction.

9. A gray body is defined as a surface whose absorptivity is constant at all temperatures and throughout the entire range of wavelength. For a gray body emissivity and absorptivity are independent of wavelength. LAWS OF RADIATION STEFAN-BOLTZMAN LAW This law states that total energy emitted by a black body per unit area and per unit time is directly proportional to the fourth power of its absolute temperature and is expressed as: E b = σ T 4 E b - Energy emitted from a black body per unit area per unit time, W/m 2 σ – Stefan- Boltzman constant = 5.678 x 10 -8 W/(m 2 -K 4 ) T - Absolute temperature of the emitting surface, K.

10. KIRCHOFF’S LAW This law states that the emissivity of a surface is equal to its absorptivity when the surface is in thermal equilibrium with the surroundings. Consider a perfect black enclosure i.e. the one which absorbs all the incident radiation falling on it (see Fig). Now let the radiant flux in the enclosure arriving at some area be q i W/m 2 . Now suppose that a body is placed inside the enclosure and allowed to come to thermal equilibrium with it. At equilibrium, the energy absorbed by the body must be equal to the energy emitted; At thermal equilibrium we may write EA = q i A α -----------(1) If we now replace the body in the enclosure with a black body of the same size and shape and allow it to come to thermal equilibrium with the enclosure, E b A = q i A-----------(2) Since α = 1 for a blackbody If Eq.1 is divided by Eq.2we get E/ E b = α But by definition E/ E b = ε, the emissivity of the body, so that ε = α------(3) Eq (3) is called Kirchoff‟s law

11. PLANCK’S LAW we need to know the spectral blackbody emissive power, which is the amount of radiation energy emitted by a blackbody at a thermodynamic temperature T per unit time, per unit surface area, and per unit wavelength about the wavelength λ it is expressed in the units as W/m 2 . For example, we are more interested in the amount of radiation an incandescent lightbulb emits in the visible wavelength spectrum than we are in the total amount emitted. The relation for the spectral blackbody emissive power E b λ was developed by Max Planck in conjunction with his famous quantum theory. This relation is known as Planck’s law and is expressed as (W/m 2 – μ m) Where C 1 = 2 π h c 2 = 3.74177 × 10 8 W· μ m 4 /m 2 C 2 = hc /k = 1.43878 × 10 4 μ m·K T -absolute temperature of the surface, λ -wavelength of the radiation emitted, k is Boltzmann’s constant = 1.38065 × 10 -23 J/K. h- Planck's constant – 6.625×10 -34 j.s c- velocity of light - 3×10 8 m/s  

12. Above equation is of great importance as it provides quantitative results for radiation from a black body. Monochromatic emissive power is defined as the energy emitted by a black body in all directions at a given wavelength λ per unit area per unit time. The rate of energy emission in the interval d λ is equal to E b λ d λ . The total emissive power and monochromatic emissive power are related by equation  

13. Monochromatic emissive power of a black body is a function of wave length and its variation with wavelength at selected temperatures has been plotted as shown in Figure Several observations can be made from this figure: 1. The emitted radiation is a continuous function of wavelength. At any specified temperature, it increases with wavelength, reaches a peak, and then decreases with increasing wavelength. 2. At any wavelength, the amount of emitted radiation increases with increasing temperature. 3. As temperature increases, the curves shift to the left to the shorter wavelength region. Consequently, a larger fraction of the radiation is emitted at shorter wavelengths at higher temperatures.

14. WIEN’S DISPLACEMENT LAW As the temperature increases, the peak of the curve in Fig. shifts toward shorter wavelengths. The wavelength at which the peak occurs for a specified temperature is given by Wien’s displacement law. Wein’s displacement law states that product of absolute temperature and wavelength at which emissive power is maximum, is constant. It is known that monochromatic emissive power of a black body depends upon its temperature and wavelength of emitted radiations. For a given temperature, emissive power initially increases with increase in wavelength, attains a maximum value corresponding to particular wavelength and then decreases with further increases in wavelength of emitted radiations. With increase in temperature, maximum emissive power occurs at smaller wavelengths. Wein’s displacement law gives the value of the wavelength at which emissive power of a body is maximum for a given temperature and is expressed as λ max T = constant, here constant = 2898 μ mK