INTRODUCTION Radiation differs from conduction and convection in that it does not require the presence of a material medium to take place.

Transcription

1 RADIATION

2 INTRODUCTION Radiation differs from conduction and convection in that it does not require the presence of a material medium to take place. Radiation: The energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules. The hot object in vacuum chamber will eventually cool down and reach thermal equilibrium with its surroundings by a heat transfer mechanism: radiation. Radiation transfer occurs in solids as well as liquids and gases. A hot object in a vacuum chamber loses heat by radiation only. Unlike conduction and convection, heat transfer by radiation can occur between two bodies even when they are separated by a medium colder than both. e.x. Radiation from Sun 2

3 INTRODUCTION Two theories have been proposed to explain phenomenon of radiation: 1. Wave theory 2. Quantum theory Quantum theory The energy values of the radiation so emitted are not continuous but in the form of successive and discrete quantities called quanta. The quantities are of different sizes. The energy E of a quantum may be represented by E = h γ Where, h = plank s constant γ = the frequency of the emitted quantity. The higher the frequency, the larger the energy of quantum. Also higher the temperature of emitter, the larger frequency of the quantum. The term photon is often used for the above quantum of radiation. A photon is believed to be particle having no mass, having energy hγ and momentum hγ /c where c is the velocity of the light.

4 THERMAL RADIATION The type of electromagnetic radiation that is pertinent to heat transfer is the thermal radiation emitted as a result of energy transitions of molecules, atoms and electrons of a substance. Temperature is a measure of the strength of these activities at the microscopic level and the rate of thermal radiation emission increases with increasing temperature. Thermal radiation is continuously emitted by all matter whose temperature is above absolute zero. Everything around us constantly emits thermal radiation. The electromagnetic wave spectrum. 4

5 Ludwig Boltzmann ( ) All objects above absolute zero emit radiant energy and the rate of emission increases and the peak wavelength decreases as the temperature of object increases 5

6 Light is simply the visible portion of the electromagnetic spectrum that lies between 0.40 and 0.76 µm. The wavelength ranges of different colors A body that emits some radiation in the visible range is called a light source. The sun is our primary light source. The electromagnetic radiation emitted by the sun is known as solar radiation, and nearly all of it falls into the wavelength band µm. Almost half of solar radiation is light (i.e., it falls into the visible range), with the remaining being ultraviolet and infrared. The radiation emitted by bodies at room temperature falls into the infrared region of the spectrum, which extends from 0.76 to 100 µm. The ultraviolet radiation includes the low-wavelength end of the thermal radiation spectrum and lies between the wavelengths 0.01 and 0.40 µm. Ultraviolet rays are to be avoided since they can kill microorganisms and cause serious damage to humans and other living beings. About 12 percent of solar radiation is in the ultraviolet range. The ozone (O 3 ) layer in the atmosphere acts as a protective blanket and absorbs most of this ultraviolet radiation. 6

7 In heat transfer studies, we are interested in the energy emitted by bodies because of their temperature only. Therefore, we limit our consideration to thermal radiation. Food is heated or cooked in a microwave oven by absorbing the electromagnetic radiation energy generated by the magnetron of the oven. The electrons, atoms and molecules of all solids, liquids and gases above absolute zero temperature are constantly in motion and thus radiation is constantly emitted, as well as being absorbed or transmitted throughout the entire volume of matter. That is, radiation is a volumetric phenomenon. Radiation in opaque solids is considered a surface phenomenon since the radiation emitted only by the molecules at the surface can escape the solid. 7

8 RADIATIVE PROPERTIES At any given temperature the quantity of radiation emitted per unit wavelength is different at different wavelengths. Monochromatic emissive power: The energy radiated per unit time per unit area of the radiating surface per unit wavelength range Radiation of energy at different wavelengths The curve shows that the major part of radiation is emitted within a narrow wavelength range on both sides of the wavelength at which the monochromatic emissive power is maximum. A body at C emits most of the radiation between 1 and 20 µ. Sun whose surface temperature is nearly C emits 90% of its radiation between 0.1 and 3 µ.

9 Absorptivity, Reflectivity, and Transmissivity Irradiation, : Radiation flux incident on a surface (amount of radiation per unit time per unit area of a surface) for opaque surfaces The absorption, reflection and transmission of incident radiation by a semitransparent material. 9

10 10 α α = ) ( ( ) α = = α α d d 0 0 α = = α α d 1 0 ρ = ρ d 1 0 τ = τ d 1 0 Monochromatic absorptivity α is the fraction of the incident energy in the wavelength, range to +d which is absorbed, Where = part of incident radiation which is in spectral range to +d ( ) α = amount absorbed in spectral range to +d Monochromatic absorptivity α : Monochromatic transmissivity : Monochromatic reflectivity ρ :

11 Transparent Body : A transparent body is one which transmits part of the radiation falling on its surface. Opaque Body : If it does not transmit any radiation at all it is called an opaque body. τ= 0 α + ρ = 1 Black body: A body which neither reflects nor transmits any part of the incident radiation but absorbs all of it is called a black body. ρ=0, τ=0 α = 1 a perfect black body does not exist. White Body : White body is one which reflects all the incident radiation and does not absorb or transmit any part of it. α = 0, τ = 0 ρ = 1 11

12 12 ray body: If the absorptivity of a surface does not vary with temperature and wavelength of the incident radiation, it is termed gray body For ray body: α = ( α) = constan Coloured body: If the absorptivity of a surface varies with the wavelength of radiation waves it is termed coloured body.

13 Monochromatic emissive power of black body, Plank s Distribution Law A black body does not emit the same amount of radiant energy at different temperatures. At a given wavelength the radiant energy emitted by a black body increase as temperature is increase. At a given temperature, the amount of radiation emitted by a black body varies with the wavelength. Monochromatic emissive power (E ) The energy emitted by a surface in all directions at a given wavelength is called the monochromatic emissive power of the surface. Monochromatic emissive power of black body (E ) b E and (E ) b increases with increase in temperature of the surface.

14 Monochromatic emissive power of black body, Plank s Distribution Law Wien proposed (short wavelength) (E 2 5 ) = 2πc h b ch exp( ) kt Rayleigh-jeans proposed (long wavelength) (E ) b = 2πkT 4 Plank Law (E ) b 5 2 = 2πc h ch exp 1 kt joule ( E ) b = 2 cm s cm Where c = Velocity of light = cm/sec h = Plank s quantum constant = joule-sec k = Boltzman constant = joule/ K = Wavelength, cm T = Asolute temperature, K (E ) b = C 1 5 C2 exp T 1 C 1 = 2πc 2 h = j m 2 /s and C 2 = ch/k = m K

15 Plank s Distribution Law 15 Close agreement with experiments. It correctly predicts the entire energy versus wavelength curve and the shift of the maximum towards shorter wavelengths at higher temperatures Plank s Distribution Law

16 Monochromatic emissive power of black body Plank s Distribution Law (i) if is small or 1; ch exp ff 1 kt Hence Plank equation 5 2 (E ) = 2πc h b ch exp 1 kt 2 5 Reduced to Wein 2πc h (E ) = b exp(ch / kt) 2 = 2πc h 5 exp( ch / kt) ch ch (ii) if is large or 1, 1 and exp can be expanded in a series kt kt ch exp(ch / kt) = 1+ kt 1+ ch / kt (E ) b 2 5 2πc h = ch 1+ 1 kt 2πckT c h k T Plank s equation reduce to Rayleigh-Jeans

17 Stefen Boltzmann Law Total Emissive Power: The total emissive power E of a surface is defined as the total radiant energy emitted by the surface in all directions over the entire wavelength range per unit time. E= cal/sec cm 2 J/m 2 Total emissive power of given radiating surface depends on temperature of the surface. Stefan using exp. data of Dulong and Petit and Tyndall, discovered in 1879 that the total emissive power of a radiating surface is proportional to the emissive power of the absolute temperature of the surface. Boltzmann furnished a theoretical proof of the empirical equation of Stefan. This proof is base on principles of thermodynamics. Boltzmann s proof was confined to black radiating surfaces. The result of the above work is called Stefan & Boltzmann law for the total emissive power of the black.

18 Stefan Boltzmann law 18 σ = W/m 2 K 4 Stefan Boltzmann constant Blackbody: The idealized surface that emits radiation at the maximum rate. Emissivity ε : A measure of how closely a surface approximates a blackbody for which ε = 1 of the surface. 0 ε 1. Radiation emitted by real surfaces Blackbody radiation represents the maximum amount of radiation that can be emitted from a surface at a specified temperature.

19 Emissivity The emissivity ε of a surface is defined as the ratio of the emissive power of the surface to the emissive power of a hypothetical black body at the same temperature. Emissivity of substance may depend both on temperature and wavelength. (a) monochromatic emissivity (b) total emissivity (c) normal total emissivity Monochromatic emissivity ε : monochromatic emissivity ε is the ratio of the monochromatic emissive power of a surface to the monochromatic power of a black body at the same wavelength and temperature. ε = ( E ) /(E ) b

20 Emissivity Total emissivity ε: Total emissivity ε is the ratio of the total emissive power of a surface to the total emissive power of a black body at the same temperature. ε = E / E h Normal total emissivity ε n : Normal total emissivity ε n is the ratio of the normal component of the total emissive power of a surface to the normal component of the total emissive power of the black body at the same temperature. ε = E /(E ) n n b n For body Black ( ε ( ε ( ε b n ) ) = 1 ) b b = 1 = 1 Emissivity of a surface is a property of the surface. It depends only on the nature or characteristic of the surface and is independent of the nature or wavelength of the impinging radiation waves. The absorptivity of a surface is not a property because it is depend on the nature of incident radiation.