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cen58933_ch11 - cen58933_ch11.qxd 9:38 AM Page 561 CHAPTER...

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FUNDAMENTALS OF THERMAL RADIATION S o far, we have considered the conduction and convection modes of heat transfer, which are related to the nature of the materials involved and the presence of fluid motion, among other things. We now turn our at- tention to the third mechanism of heat transfer: radiation, which is character- istically different from the other two. We start this chapter with a discussion of electromagnetic waves and the electromagnetic spectrum, with particular emphasis on thermal radiation. Then we introduce the idealized blackbody, blackbody radiation, and black- body radiation function, together with the Stefan–Boltzmann law, Planck’s law, and Wien’s displacement law. Radiation is emitted by every point on a plane surface in all directions into the hemisphere above the surface. The quantity that describes the magnitude of radiation emitted or incident in a specified direction in space is the ra- diation intensity. Various radiation fluxes such as emissive power, irradiation, and radiosity are expressed in terms of intensity. This is followed by a discus- sion of radiative properties of materials such as emissivity, absorptivity, re- flectivity, and transmissivity and their dependence on wavelength, direction, and temperature. The greenhouse effect is presented as an example of the consequences of the wavelength dependence of radiation properties. The last section is devoted to the discussions of atmospheric and solar radiation because of their importance. 561 CHAPTER 11 CONTENTS 11-1 Introduction 562 11-2 Thermal Radiation 563 11-3 Blackbody Radiation 565 11-4 Radiation Intensity 571 11-5 Radiative Properties 577 11-6 Atmospheric and Solar Radiation 586 Topic of Special Interest: Solar Heat Gain Through Windows 590 cen58933_ch11.qxd 9/9/2002 9:38 AM Page 561
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11–1 INTRODUCTION Consider a hot object that is suspended in an evacuated chamber whose walls are at room temperature (Fig. 11–1). The hot object will eventually cool down and reach thermal equilibrium with its surroundings. That is, it will lose heat until its temperature reaches the temperature of the walls of the chamber. Heat transfer between the object and the chamber could not have taken place by conduction or convection, because these two mechanisms cannot occur in a vacuum. Therefore, heat transfer must have occurred through another mecha- nism that involves the emission of the internal energy of the object. This mechanism is radiation. Radiation differs from the other two heat transfer mechanisms in that it does not require the presence of a material medium to take place. In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenua- tion in a vacuum. Also, radiation transfer occurs in solids as well as liquids and gases. In most practical applications, all three modes of heat transfer oc- cur concurrently at varying degrees. But heat transfer through an evacuated space can occur only by radiation. For example, the energy of the sun reaches the earth by radiation.
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