Physics
Thermal Radiation
Thermal radiation refers to the emission of electromagnetic waves from the surface of an object due to its temperature. This type of radiation does not require a medium to propagate and can transfer heat energy from one object to another through electromagnetic waves. It plays a crucial role in the transfer of heat energy in various natural and industrial processes.
Written by Perlego with AI-assistance
Related key terms
1 of 5
11 Key excerpts on "Thermal Radiation"
- Allan D. Kraus, James R. Welty, Abdul Aziz(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
26 Radiation Heat Transfer Chapter Objectives • To consider electromagnetic waves and the electromagnetic spectrum and to show that Thermal Radiation is a form of electromagnetic radiation. • To consider the emission of radiant energy and the Stefan-Boltzmann and Wien displacement laws. • To define the terminology peculiar to radiation heat transfer such as emissivity, absorptivity, reflectivity, and transmissivity. • To describe what is meant by a blackbody and a gray body. • To examine the directional characteristics of surface radiation. • To provide a relationship for radiant heat interchange with perfect absorbers and emitters and for surfaces not in full view of each other. • To modify the relationship for radiant heat interchange with nonperfect absorbers and emitters and for surfaces that are not in full view of each other. • To present an electrothermal analog method for handling radiation inside enclo-sures. 26.1 The Electromagnetic Spectrum Radiation of thermal energy is believed to be a specific form of radiation within the gen-eral phenomenon of electromagnetic radiation. As but one of numerous electromagnetic phenomena, thermal radiant energy travels at the speed of light: 2 . 9979 × 10 8 m/s. The existence of radiation as a mode of heat transfer can be observed from everyday experience. Consider, for example, a warm body enclosed without physical contact inside a cooler enclosure under complete vacuum. The warm body will eventually attain the temperature of the surrounding enclosure without the aid of conduction or convection. This statement may appear intuitive, and we can easily imagine, as well, the approxi-mation of the warm body suspended by nonconducting cords in the evacuated cooler enclosure. All bodies continuously emit radiation. Figure 26.1 displays the electromagnetic spec-trum showing a range of electromagnetic waves from long radio waves to the shorter wavelengths.
- Greg F. Naterer(Author)
- 2002(Publication Date)
- CRC Press(Publisher)
4 Radiative Heat Transfer 4.1 Introduction Thermal Radiation is a form of energy emitted as electromagnetic waves (or photons) by all matter above a temperature of absolute zero. The emissions are due to changes in the electron configurations of the constituent atoms or molecules. In this chapter, the mechanisms and governing equations of Thermal Radiation will be described. Consider the cooling of a hot solid object in a vacuum chamber. The vacuum chamber is an evacuated space containing a very low pressure and resulting negligible mass. Despite the absence of conduction or convection modes of heat transfer, energy is still transferred from the object to the vacuum chamber walls. There are two main theories that explain how this heat transfer occurs, based on quantum theory (Planck) or electromagnetic theory (Maxwell). In the former case (Planck, 1959), it is known that the energy is transported by radiation in the form of photons (or energy packets), which travel at the speed of light. In terms of the photon energy, e , and the frequency of radiation, n ; e ˆ h n ( 4 : 1 ) where ˆ h 6 : 63 10 34 J × s refers to Planck’s constant. Alternatively (Max-well), radiation is interpreted to be transported in the form of electro-magnetic waves traveling at the speed of light. In this interpretation, the speed of light, c , is related to the wavelength, l ; in the following way: c ln ( 4 : 2 ) The speed of light is 3 / 10 8 m/sec in a vacuum. 171 4.2 Fundamental Processes and Equations Radiation occurs across an electromagnetic spectrum (see Figure 4.1), i.e., over a wide range of wavelengths and corresponding frequencies. From very small wavelengths, below 10 5 m m (or very high frequencies), to the longest wavelengths in the microwave region (above 10 2 m m), the spectrum identifies the characteristics of the transmitted radiation. Many common everyday experiences can be explained by the electromagnetic spectrum.
eBook - PDFEngineering Calculations in Radiative Heat Transfer
International Series on Materials Science and Technology
- W. A. Gray, R. Müller, D. W. Hopkins(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
CHAPTER 1 Basic Principles of Thermal Radiation 1.1. NATURE OF RADIATION Radiation is the transmission of energy by electromagnetic waves; the energy transmitted is called radiant energy. However, the term radiation is also commonly used to describe the radiant energy itself. Electromagnetic waves are characterized by their wavelength or fre-quency, frequency being inversely proportional to wavelength. Wave-length is usually used in radiative heat transfer analysis. 1 Thermal . radiation | I km Im I mm I μπ IÂ I I I I I I I I I I I I I I I I I log, 0 λ 5 4 3 2 I 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10-11 ( λ metres) 1 I I I I I I I I I I I I I I I I Visible-^ /-UV Ι Ί I Radio I I Infra-red | | X rays I Gamma rays FIG. 1.1. Spectrum of electromagnetic waves. Figure 1.1 shows the electromagnetic spectrum and the names given to radiation transmitted in various ranges of wavelengths. The nature of radiation and its transport are not fully understood but they can be described satisfactorily either by wave or quantum theory. In simple terms, radiation travels in space with the velocity of light and does not require the presence of an intervening medium for its propagation. The velocity of light (c) is the constant of proportionality which relates wavelength ( ) and frequency (v): λ = φ. (1.1) 1 2 ENGINEERING CALCULATIONS IN RADIATIVE HEAT TRANSFER The frequency of radiation depends on the nature of the source. For example, a metal bombarded by high-energy electrons emits X-rays, high-frequency electric currents generate radio waves and a body emits Thermal Radiation by virtue of its temperature. Radiation in the wavelength range 0-1 to 100/xm (micrometre or micron), when incident upon a body, will heat it and, consequently, is called Thermal Radiation. In addition, since radiation within the wave-length band of 0-38 to 0-76 μτη affects the optic nerves, we can see Thermal Radiation within this band as light.
eBook - PDF- G. F. Nellis, S. A. Klein(Authors)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
Thermal Radiation is emitted by a surface due to its temperature. The magnitude of the radiation that is emitted by a surface at a given temperature may be a complicated function of wavelength (i.e., the radiation is distributed spectrally) and direction (i.e., the radiation is distributed directionally). A diffuse surface emits radiation uniformly in all directions. There is an upper limit on the rate that radiation can be emitted by any surface at a specified temperature defined by the blackbody. The behavior of any real surface is characterized by comparison with the limiting case of a blackbody. 10 –8 10 –6 10 –4 10 –2 10 0 10 2 10 4 10 6 10 8 10 10 Wavelength (mm) Thermal Radiation gamma rays cosmic rays ultraviolet long-wave radio short-wave radio visible (0.38–0.78) near infrared (0.78 –25) far infrared (25–1000) X-rays solar radar, TV, radio Figure 14.1 Electromagnetic spectrum (reproduced from Duffie and Beckman, 2013). 14.2 Emission of Radiation by a Blackbody 867 14.2.2 Blackbody Emission A blackbody has the following characteristics. 1. It absorbs all of the radiation that is incident upon it (regardless of the direction or wavelength of the incident radiation). 2. It emits radiation uniformly in all directions (i.e., it is a diffuse emitter). 3. It emits the maximum possible amount of radiation at a given temperature and wavelength. Planck’s Law The radiation that is emitted by a blackbody is referred to as the blackbody spectral emissive power, E b,λ . The blackbody spectral emissive power is a function of temperature and wavelength, as shown in Figure 14.2. A blackbody at a specific temperature will emit Thermal Radiation over a large range of wavelengths. As the temperature of the blackbody increases, the spectral emissive power increases at all wavelengths and a larger fraction of the emitted power occurs at lower wavelengths. The amount of Thermal Radiation emitted by a blackbody is an extremely strong function of temperature.
eBook - PDF- Ethirajan Rathakrishnan(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
Chapter 6 Radiation Heat Transfer 6.1 Introduction We have seen that in contrast to the mechanism of conduction and convec-tion, where energy transfer through a material medium is involved, heat may also be transferred into regions where perfect vacuum exists. The mechanism in this case is electromagnetic radiation , which is propagated as a result of a temperature difference. This mode of energy transfer through electromag-netic radiation is called Thermal Radiation . Thus, Thermal Radiation is that electromagnetic radiation emitted by a body as a result of its temperature. 6.2 Radiation Mechanism There are many types of electromagnetic radiation, but the Thermal Radiation is only one. Irrespective of its type, a radiation is propagated at the speed of light c (3 × 10 10 cm/s), given by c = λν where λ is the wavelength and ν is the frequency. The wavelength is usually expressed in centimeters or angstroms (1 ˚ A= 10 − 8 cm) or micrometers. Ther-mal radiation lies in the range of wavelength from about 0.1 to 100 μ m, as shown in Figure 6.1, which shows a portion of electromagnetic spectrum. Thermal Radiation propagates in the form of discrete quanta with each quantum having an energy of E = hν where h is the Planck’s constant, and is equal to 6 . 625 × 10 − 34 J-s. To gain an insight into the process of radiation propagation, let us consider each quantum as a particle having mass, momentum and energy, as in the case of the molecules of a gas. Thus, the radiation might be regarded as a photon 297 298 Radiation Heat Transfer γ -rays 3 log λ , m 2 1 0 − 1 − 2 − 3 − 4 − 5 − 6 − 7 − 8 − 9 − 10 − 11 − 12 Radio waves Visible Ultra violet Infrared X-rays Thermal Radiation 1 ˚ A Figure 6.1 Electromagnetic spectrum.
eBook - PDFHeat Transfer, Volume 2
Radiation and Exchangers
- Yves Jannot, Christian Moyne, Alain Degiovanni(Authors)
- 2023(Publication Date)
- Wiley-ISTE(Publisher)
1 Heat Transfer by Radiation Between Surfaces 1.1. General: definitions 1.1.1. Type of radiation All bodies, whatever their state, solid, liquid or gaseous, emit electromagnetic radiations. This emission of energy is done at the expense of the internal energy of the emitting body. The radiation propagates in a straight line at the velocity of light; it consists of radiation of different wavelengths as demonstrated by William Herschel’s experiment shown in Figure 1.1. Figure 1.1. Principle of William Herschel’s experiment. For a color version of this figure, see www.iste.co.uk/jannot/heattransfer2.zip 2 Heat Transfer 2 Figure 1.2. Spectrum of electromagnetic waves (λ in m). For a color version of this figure, see www.iste.co.uk/jannot/heattransfer2.zip Heat Transfer by Radiation Between Surfaces 3 By passing through a prism, the radiation is more or less deflected depending on its wavelength. The radiation emitted by a source at temperature is sent to a prism and the deflected beam is projected onto an absorbing (blackened) screen, thus obtaining the decomposition of the total incident radiation into a spectrum of monochromatic radiation. If a thermometer is moved along the screen, the temperature is measured, characterizing the energy received by the screen in each wavelength. By constructing the curve = (), we obtain the spectral distribution of the radiated energy for the temperature of the source. We then see that: – the energy emitted is maximum for a certain wavelength variable with ; – the energy is only emitted over an interval [ ଵ , ଶ ] of wavelength characterizing Thermal Radiation. The different types of electromagnetic waves and their corresponding wavelengths are represented in Figure 1.2. It should be noted that the radiation emitted by bodies and having a thermal effect is between 0.1 and 100 μm.
eBook - PDFHeat Transfer Basics
A Concise Approach to Problem Solving
- Jamil Ghojel(Author)
- 2023(Publication Date)
- Wiley(Publisher)
Electromagnetic wave propagation does not require a medium and can take place in a vacuum and the rate at which heat is radiated is proportional to the absolute temperature of the body raised to the fourth power. Thermal Radiation is strongly linked to: ● combustion processes in heat engines (internal combustion engines, gas turbines, rockets, etc.) ● furnace technology ● fires and fire engineering ● thermal control in space technology (space probes, satellites) ● cryogenic insulation ● studies of the energy balance of earth and global warming. 10 Thermal Radiation 328 10.2 Definitions and Radiation Properties When radiation falls on the surface of a solid or liquid medium, a part could be absorbed, part reflected, and part transmitted (Figure 10.2) ρ α τ + + = G G G G (10.1) In this equation, G is irradiation (incident Thermal Radiation energy), ρG is the reflected radiation, αG is the absorbed radiation, and τG is the transmitted radi- ation. Dividing both sides of Eq. (10.1) by G we obtain α ρ τ + + = 1 (10.2) where α is absorptivity, ρ is reflectivity, and τ is trans- missivity of the medium. Figure 10.1 Electromagnetic wavelength spectrum with the Thermal Radiation part highlighted. Figure 10.2 Radiation on the surface of a semi-transparent medium. 10.1 The Electromagnetic Spectrum Radiation is commonly viewed as propagation of electromagnetic waves at the speed of light c ( × m s 2.998 10 / 8 ), which can be written as λ = c v In this equation λ is wavelength (m ) and v is frequency ( − s 1 ). The micron or micrometre (µm), which is equal to − 10 6 m , is widely used as the unit of wavelength in engineering. Thermal Radiation is part of the electromagnetic spectrum within the wavelength range of 0.1 to 1000 µm, as shown by the highlighted portion in Figure 10.1. The range of wavelengths visible to the human eye ( µ − m 0.4 0.7 ) lies within the Thermal Radiation part as does the infrared range and parts of the ultraviolet and microwave ranges.
eBook - PDF- Albert Ibarz, Gustavo V. Barbosa-Canovas(Authors)
- 2002(Publication Date)
- CRC Press(Publisher)
467 14 Heat Transfer by Radiation 14.1 Introduction Energy transfer by radiation is basically different from other energy transfer phenomena because it is not proportional to a temperature gradient and does not need a natural medium to propagate. Also, its transfer is simulta-neous with convection. Any molecule possesses translation, vibrational, rotational, and electronic energy under quantum states. Passing from one energetic level to another implies an energy absorption or emission. Passing to a higher energy state implies energy absorption by a molecule; on the other hand, a molecule emits energy as radiation when passing to a lower energy level. Since the energy levels as quantums, the absorption or emission of energy is in the form of photons that have a double nature particle-wave. Any body at a temperature higher than absolute zero can emit radiant energy; the amount emitted depends on the temperature of the body. As the temperature of a body increases, energy levels are excited first, followed by electronic levels. A temperature increase implies that the radiation spectrum moves to shorter wavelengths or is more energetic. The corpuscle theory states that radiant energy is transported by photons and is a function of its frequency ν , according to the expression: (14.1) in which the proportionality constant h is the so-called Planck’s constant, whose value is h = 6.6262 × 10 –34 J·s. The wave theory considers radiation as an electromagnetic wave, relating frequency to wavelength according to the following equation: (14.2) where λ is the wavelength of the radiation and c is the value of light speed under vacuum conditions (2.9979 × 10 8 m/s). E h = ν ν λ = c 468 Unit Operations in Food Engineering So-called Thermal Radiation, which includes the ultraviolet, visible, and infrared spectrum, corresponds to wavelengths of 10 –7 to 10 –4 m.
- C.V. Heer(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Radiation 2.1 Introduction Planck's introduction of the concept of the quantization of electro-magnetic radiation to provide a satisfactory explanation of thermal or blackbody radiation has led to the description of Thermal Radiation as a gas composed of photons. The linear form of Maxwell's equations for electromagnetic phenomena indicates that photons do not interact with each other and the photon gas is an ideal gas. In many respects, the dis-cussion given for particles in Chapter I applies to photons. In Chapter I, the particles were treated in a classical manner, and quantum mechanics is needed to explain binary encounters and molecular forces. For photons, a quantum mechanical explanation is required at the beginning of the de-velopment. Although the historical approach to problems of radiant energy are of considerable interest, the study is greatly facilitated by the use of more recent developments. This latter approach is used here; therefore the elegant deductions made from experimental studies prior to 1900 will often appear as almost trivial. This is indeed not the case; the author does not wish to retrace this tortuous path of discovery, but wishes to devote the present chapter to problems to which this work is applicable. An excellent review is given in Planck's Wärmestrahlung [7]. The next section discusses some aspects of electromagnetic radiation that follow directly from Maxwell's equations. Measurable aspects of radia-tion from independent sources and the concept of the modes of the radia-36 II 2.2 ELECTROMAGNETIC RADIATION 37 tion field are introduced. This is then combined with the basic postulate of Planck to form the basis for a consideration of Thermal Radiation. Section 2.4 starts the discussion of the transfer of radiant energy and includes most of the simple and useful concepts in the transfer of Thermal Radiation. The next two sections can be omitted for the reader primarily interested in the transfer of radiant energy.
eBook - PDF- Stephen Whitaker(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
As the wavelength decreases with increasing temperature, the rate of emission of electromagnetic energy increases and radiant energy transport becomes more important. The full spectrum of electromagnetic radiation is shown in Fig. 8.1.1. The radiation with wavelengths in the tA more general point of view considers the photon gas to be a form of matter in the traditional sense. 375 376 Thermal Radiation log (λ/λ 0 ) Ao = 10 8 cm 16[ 14 H c Q) 12 10 1 cm 8 1(T 2 cm 6 10 4 cm 4 1(T 6 cm 2 -2 radio waves infrared ultraviolet p visible x-rays gamma rays ■ 4 U Fig. 8.1.1 Spectrum of electromagnetic radiation. range of 10~ 2 cm to 5 x 10~ 5 cm is of the greatest concern to us for it is in this region that significant energy transfer usually takes place. Because of this, radiation ranging from the infrared to the ultraviolet is often referred to as Thermal Radiation in heat transfer texts. Thermal Radiation in the visible spectrum has been studied by the students in physics courses, and there it is referred to simply as light. We denote the velocity of propagation of electromagnetic waves in any medium by c. In a vacuum the value of c is given by c = 3 x 10 10 cm/sec = 186 000 miles/sec The wavelength of electromagnetic radiation will be denoted by λ and the frequency is related to c and λ by the equation v = c/λ (8.1-1) Throughout this chapter the symbol v will be used to denote the frequency; there should be no confusion with the prior use of v to indicate the kinematic viscosity, μ/ρ. A complete understanding of the physics of electromagnetic radiation is rather difficult to obtain; the subject currently being the domain of the quantum-electrodynamacist. The engineer is in no position to delve into the physics of electromagnetic radiation to an extent which would do the subject justice, and in fact we need not, for a reasonable picture of radiation can be obtained without doing so. However, it is still wise to know upon what ice one is skating, and before going on to the development of a simplified theory of Thermal Radiation we had best try to set down what we do, and do not, understand about radiation. From a previous course in physics the student has confronted the dual nature of light, i.e., it has both wave and particulate characteristics. We
eBook - PDF- K. Ya. Kondrat'Yev(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
C H A P T E R 1 Thermal Radiation. BASIC DEFINITIONS AND CONCEPTS 1. Basic quantitative characteristics in the field of Thermal Radiation The Thermal Radiation of the earth's surface and the atmosphere is of the same electromagnetic nature as light, radio waves and other electromagnetic waves. This means that certain generally accepted quantitative characteristics in the field of radiation can be used to describe the field of Thermal Radiation. No unified system of such quantitative characteristics is at present in existence and there are considerable differences in the terminology used to define various characteristics in either field. However, the system of quantitative characteristics developed by Kuznetsov 1,2 has been widely used in the investigation of problems concerning the transfer of radiant energy in the atmosphere, and in dynamical meteorology. We have utilized this system of quantitative characteristics in this book. We shall not repeat the relevant definitions 3 , but shall only point out that the main quantitative characteristic of the field of radiation is the intensity of radiation J. In the general case of a stable mono-chromatic radiation, propagating in a direction r, we shall denote the radiation intensity at a point Q by J* (Q, r). The second very important characteristic of the field of radiation is the radiative flux. In the following we shall consider primarily the radiative flux reduced to a hemisphere. For this special case the following relation exists between the flux and the intensity of mono-chromatic radiation: 2π π/2 F x = Jd
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
