Physics
Heat Radiation
Heat radiation refers to the transfer of heat energy in the form of electromagnetic waves. It occurs through the emission and absorption of infrared radiation by objects. This process does not require a medium for heat transfer, making it distinct from conduction and convection.
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11 Key excerpts on "Heat Radiation"
- 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.
- 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.
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 - 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
Thermal Management of Electronics
- Younes Shabany(Author)
- 2009(Publication Date)
- CRC Press(Publisher)
319 13 Radiation Heat Transfer Radiation is the only mode of heat transfer between two objects that are at different temperatures if there is no material between them. However, it can also be present along, and in parallel, with other modes of heat transfer such as conduction and convection. For example, our bodies dissipate heat through convection and radiation and we feel warm because of the radiation heat received from a fire or from the sun. Every object radiates heat to other objects in its surroundings and in turn receives radiation energy from them. Net radiation heat transfer between any two objects depends on their temperatures, dimensions, radiation properties, and the relative ori-entation of those with respect to each other. Radiation heat transfer occurs through the emission of electromagnetic waves or photons from an object. An oscillating electric field generates an oscillating mag-netic field. The magnetic field in turn generates an oscillating electric field and so on. These oscillating fields form an electromagnetic wave. Electromagnetic waves are characterized by their frequency, ν , or wavelength, λ , that are related to each other through, c = λν , (13.1) where c is the speed of light. Energy of an electromagnetic wave with frequency ν is given by E h ν ν = , (13.2) where h = 6.626 × 10 –34 J.s is the Planck constant. Figure 13.1 shows the electromagnetic wave spectrum. It is seen that the wavelength of electromagnetic waves can be as low as 10 –9 µ m and as high as 10 10 µ m, and their properties and applications are significantly different. Large wavelength radio waves are generated by an electric current that alternates at a radio frequency in an antenna and are used for wireless transmission of information.
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 - PDF- Albert Ibarz, Gustavo V. Barbosa-Canovas(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
413 18 Heat Transfer by Radiation 18.1 INTRODUCTION Energy transfer by radiation is basically different from other energy transfer phenomena because it is not proportional to a temperature gradient, nor does it need a material medium to propagate. In addition, heat transfer by radiation is simultaneous with convective transfer. Any molecule possesses translational, vibrational, rotational, and electronic energy, and all of those are done on quantum states, that is, discrete values of energy. Passing from one energy level to another implies an energy absorption or emission. Passing to a higher energy state implies energy absorption by a molecule; on the contrary, a molecule emits energy as radiation when passing to a lower energy level. Since the energy levels are quantized, the absorption or emission of energy is in the form of photons where the duality wave particle becomes relevant. Any body at a temperature higher than absolute zero can emit radiant energy, and the amount of energy emitted depends on the temperature of the body. As the temperature of a body increases, energy levels are excited first, followed by the electronic level changes. A tem-perature increase implies that the radiation spectrum moves to shorter wavelengths or are more energetic. The corpuscle theory states that radiant energy is transported by photons and also that it is a function of its frequency ν , according to the expression E = h ν (18.1) 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 wave-length according to the following equation: ν λ = c (18.2) where λ is the wavelength of the radiation c is the value of light speed under vacuum (2.9979 × 10 8 m/s) So-called thermal radiation, which includes the ultraviolet, visible, and infrared spectra, corre-sponds to wavelengths of 10 −7 –10 −4 m.
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.
- James Welty, Gregory L. Rorrer, David G. Foster(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
▶ CHAPTER 23 Radiation Heat Transfer The mechanism of radiation heat transfer has no analogy in either momentum or mass transfer. Radiation heat transfer is extremely important in many phases of engineering design, such as boilers, home heating, and spacecraft. In this chapter, we will concern ourselves first with understanding the nature of thermal radiation. Next, we will discuss properties of surfaces and consider how system geometry influences radiant heat transfer. Finally, we will illustrate some techniques for solving relatively simple problems where surfaces and some gases participate in radiant-energy exchange. ▶ 23.1 NATURE OF RADIATION The transfer of energy by radiation has several unique characteristics when contrasted with conduction or convection. First, matter is not required for radiant heat transfer; indeed, the presence of a medium will impede radiation transfer between surfaces. Cloud cover is observed to reduce maximum daytime temperatures and to increase minimum evening temperatures, both of which are dependent upon radiant-energy transfer between Earth and space. A second unique aspect of radiation is that both the amount of radiation and the quality of the radiation depend upon temperature. In conduction and convection, the amount of heat transfer was found to depend upon the temperature difference; in radiation, the amount of heat transfer depends upon both the temperature difference between two bodies and their absolute temperatures. In addition, radiation from a hot object will be different in quality than radiation from a body at a lower temperature. The color of incandescent objects is observed to change as the temperature is changed. The changing optical properties of radiation with temperature are of paramount importance in determining the radiant-energy exchange between bodies. Radiation travels at the speed of light, having both wave properties and particle-like properties.
eBook - PDFWeather
A Concise Introduction
- Gregory J. Hakim, Jérôme Patoux(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
CHAPTER 4 Heat and Energy Transfer Energy enters the Earth system in the form of solar radiation, pref- erentially heating the ground and the tropical latitudes. The resulting temperature contrasts set the atmosphere in motion, as heat is transferred upward and poleward. Weather is largely the result of this transfer of heat by atmospheric motions. In this chapter, we will explore the nature of heat and radiation, the origin of temperature contrasts on Earth, the mechanisms by which energy is transferred, and the implications for weather. CONTENTS 4.1 Conduction 60 4.2 Convection 60 4.3 Radiation 61 4.4 Radiative Interactions 63 4.5 Radiation and Weather 73 Summary 80 Review Questions 81 Although we might tend to think about weather in terms of wind and rain, weather is in fact largely the result of the redistribution of heat in the atmosphere. A heat imbalance, i.e., a disequilibrium in the distribution of heat between different regions of Earth, causes a trans- fer and redistribution of heat in the atmosphere. The resulting movement of air masses creates wind, storms, clouds, and rain. Therefore, if we are to understand the development of weather systems, we need first to under- stand the source of the heat imbalance and the nature of heat transfer in the atmosphere. It is accepted in thermodynamics (the field of sci- ence that is concerned with the energy of systems) that heat does not normally flow from cold to warm objects (unless some other process is at work): it necessarily flows from warm to cold, i.e., in a direc- tion that will bring the system to a state of equilib- rium – technically, a state of lower potential energy. In the atmosphere, this transfer of heat can be achieved by three processes, conduction, convection, and radiation, which we will now explore in more detail. 4.1 Conduction Earlier we defined heat as the kinetic energy of atoms and molecules, or their energy of motion.
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
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