Lecture notes on Radiation heat transfer 2018

lecture notes on radiation physics,what is thermal radiation and how does it change with temperature, what is thermal radiation physics, what is thermal radiation spectrum
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Published Date:21-07-2017
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ME 144: Heat Transfer Introduction to Radiation (v 1.0) J. M. MeyersInitial Concepts Heat transfer by conduction and convection requires the presence of a temperature gradient in some form of matter. Heat transfer by thermal radiation requires no matter. It is an extremely important process, and in the physical sense it is perhaps the most interesting of the heat transfer modes. It is relevant to many industrial heating, cooling, and drying processes, as well as to energy conversion methods that involve fossil fuel combustion and solar radiation Very important in high speed aerodynamics and reentry aerothermodynamics ME 144: Heat Transfer Radiation 2 J. M. MeyersInitial Concepts Consider a solid that is initially at a higher temperature than that of its surroundings ,   but around which there exists a vacuum The presence of the vacuum precludes energy loss from the surface of the solid by conduction or convection This cooling is associated with a reduction in the internal energy stored by the solid and is a direct consequence of the emission of thermal radiation from the surface. In turn, the surface will intercept and absorb radiation originating from the surroundings. However, if   the net heat transfer rate by   radiation  is from the surface, and the , surface will cool until reaches .   ME 144: Heat Transfer Radiation 3 J. M. MeyersInitial Concepts All forms of matter emit radiation. For gases and for semitransparent solids, such as glass and salt crystals at elevated temperatures, emission is a volumetric phenomenon we concentrate on situations for which radiation can be treated as a surface phenomenon. In most solids and liquids, radiation emitted from interior molecules is strongly absorbed by adjoining molecules. Accordingly, radiation that is emitted from a solid or a liquid originates from molecules that are within a distance of approximately ME 144: Heat Transfer Radiation 4 J. M. MeyersInitial Concepts We know that radiation originates due to emission by matter and that its subsequent transport does not require the presence of any matter. One theory views radiation as the propagation of a collection of particles termed photons or quanta. Alternatively, radiation may be viewed as the propagation of electromagnetic waves. Regardless, we will use the standard wave properties of frequency and wavelength when dealing with radiation exchanges.  These two properties are related by =  ≡ wavelength .  ≡ speed of light in a vacuum 2.998×10 m/s   ≡ frequency ME 144: Heat Transfer Radiation 5 J. M. MeyersInitial Concepts ELECTROMAGNETIC SPECTRUM A region containing a portion of the UV and all of the visible and infrared (IR) is termed thermal radiation because it is both caused by and affects the thermal state or temperature of matter… for this reason, thermal radiation is pertinent to heat transfer. ME 144: Heat Transfer Radiation 6 J. M. MeyersInitial Concepts Thermal radiation emitted by a surface encompasses a range of wavelengths The magnitude of the radiation varies with wavelength, and the term spectral is used to refer to the nature of this dependence. This spectral distribution will vary with the nature and temperature of the emitting surface A surface may emit preferentially in certain directions, creating a directional distribution of the emitted radiation. ME 144: Heat Transfer Radiation 7 J. M. MeyersRadiation Heat Fluxes Various types of heat fluxes are pertinent to the analysis of radiation heat transfer 6 Emissive power, 4 W/m , rate at which radiation is emitted from a surface per unit surface area, over all wavelengths and in all directions. Recall our treatment of radiation emission: 9 4 = 78 ME 144: Heat Transfer Radiation 8 J. M. MeyersRadiation Heat Fluxes 6 Irradiation, : W/m , rate at which radiation is incident upon the surface per unit surface area, over all wavelengths and from all directions. All of the irradiation must be reflected, absorbed, or transmitted, it follows that ;+=+ = 1 A medium that experiences no transmission is termed opaque, in which case: ;+= = 1 ME 144: Heat Transfer Radiation 9 J. M. MeyersRadiation Heat Fluxes Radiosity, J (W/m2), of a surface accounts for all the radiant energy leaving the surface. For an opaque surface, it includes emission and the reflected portion of the irradiation, ? = 4+: = 4 +;:  ME 144: Heat Transfer Radiation 10 J. M. MeyersRadiation Heat Fluxes Net radiative flux from a surface, (W/m2), is the difference between the outgoing and incoming radiation " = ?−:  Combining previous relations: 9 " = 4+;: −: = 78 −=:   ME 144: Heat Transfer Radiation 11 J. M. MeyersRadiation Heat Fluxes ME 144: Heat Transfer Radiation 12 J. M. MeyersRadiation Intensity Radiation leaving a surface can propagate in all directions thus its directional distribution is important. Radiation incident upon a surface may come from different directions and the manner in which the surface responds to this radiation depends on the direction. These directional effects are quite important in determining the net radiative heat transfer rate and may be treated by introducing the concept of radiationintensity. Due to its nature, mathematical treatment of radiation heat transfer involves the extensive use of the spherical coordinate system. ME 144: Heat Transfer Radiation 13 J. M. MeyersRadiation Intensity Mathematical Definitions The differential solid angle CD is defined by a region between the rays of a sphere and is measured as the ratio of the areaCE on the sphere to the sphere’s radius squared: CE The unit of the solid angle is the steradian (sr), CD = 6 analogous to radians for plane angles. F ME 144: Heat Transfer Radiation 14 J. M. MeyersRadiation Intensity Mathematical Definitions ME 144: Heat Transfer Radiation 15 J. M. MeyersRadiation Intensity Radiation Intensity and Its Relation to Emission ME 144: Heat Transfer Radiation 16 J. M. MeyersRadiation Intensity Radiation Intensity and Its Relation to Emission 2 The total, hemispherical emissive power, 4 (W/m ), is the rate at which radiation is emitted per unit area at all possible wavelengths and in all possible directions. Although the directional distribution of surface emission varies according to the nature of the surface, there is a special case that provides a reasonable approximation for many surfaces. A diffuse emitter is a surface for which the intensity of the emitted radiation is independent of direction, in which case G ,I,J = G ( ,): H, H, ME 144: Heat Transfer Radiation 17 J. M. MeyersRadiation Intensity Radiation Intensity and Its Relation to Irradiance The intensity of the incident radiation may be related to the irradiation, which encompasses radiation incident from all directions. 2 The spectral irradiation :(W/m Mm) is defined as the rate at which radiation of wavelength is incident on a surface, per unit area of the surface and per unit wavelength intervalC about : Eq. 12.18 ME 144: Heat Transfer Radiation 18 J. M. MeyersBlackbody Radiation 1. A blackbody absorbs all incident radiation, regardless of wavelength and direction. 2. For a prescribed temperature and wavelength, no surface can emit more energy than a blackbody 3. Although the radiation emitted by a blackbody is a function of wavelength and temperature, it is independent of direction. That is, the blackbody is a diffuse emitter. As the perfect absorber and emitter, the blackbody serves as a standard against which the radiative properties of actual surfaces may be compared. ME 144: Heat Transfer Radiation 19 J. M. MeyersBlackbody Radiation PLANCK DISTRIBUTION Black body emission can be described by the well known Planck distribution: 6 2ℎ P G , = H,N Q exp ℎ / S  −1 P T VW9 ℎ = 6.626×10 J∙s Planck constant V6W S = 1.381×10 J/K Boltzmann constant T .  = 2.998×10 m/s Speed of light P 4 , = \G , = H,N H,N Q exp /  −1 6 6 . 9 6 First Radiation Constant: = 2\ℎ = 3.742×10 W∙μm /m P ℎ P 9 Second Radiation Constant: = = 1.439×10 μm∙K 6 S T ME 144: Heat Transfer Radiation 20 J. M. Meyers

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