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Unformatted text preview: Chapter 11 Fundamentals of Thermal Radiation Atmospheric and Solar Radiation 11-50C The solar constant represents the rate at which solar energy is incident on a surface normal to sun's rays at the outer edge of the atmosphere when the earth is at its mean distance from the sun. It value is 2 W/m 1353 = s G . The solar constant is used to estimate the effective surface temperature of the sun from the requirement that 4 2 1 2 ) 4 ( ) 4 ( sun s T r G L σ π π = where L is the mean distance between the sun and the earth and r is the radius of the sun. If the distance between the earth and the sun doubled, the value of G s drops to one-fourth since 4 2 2 2 4 2 2 2 ) 4 ( 16 ) 4 ( ) 2 ( 4 sun s sun s T r G L T r G L σ π π σ π π = = 4 G G s1 s2 = → = 1 2 2 2 4 16 s s G L G L π π 11-51C The amount of solar radiation incident on earth will decrease by a factor of Reduction factor = = = σ σ T T sun sun new 4 4 4 4 5762 2000 68 9 , . (or to 1.5% of what it was). Also, the fraction of radiation in the visible range would be much smaller. 11-52C Air molecules scatter blue light much more than they do red light. This molecular scattering in all directions is what gives the sky its bluish color. At sunset, the light travels through a thicker layer of atmosphere, which removes much of the blue from the natural light, letting the red dominate. 11-53C The reason for different seasons is the tilt of the earth which causes the solar radiation to travel through a longer path in the atmosphere in winter, and a shorter path in summer. Therefore, the solar radiation is attenuated much more strongly in winter. 11-54C The gas molecules and the suspended particles in the atmosphere emit radiation as well as absorbing it. Although this emission is far from resembling the distribution of radiation from a blackbody, it is found convenient in radiation calculations to treat the atmosphere as a blackbody at some lower fictitious temperature that emits an equivalent amount of radiation energy. This fictitious temperature is called the effective sky temperature T sky . 11-55C There is heat loss from both sides of the bridge (top and bottom surfaces of the bridge) which reduces temperature of the bridge surface to very low values. The relatively warm earth under a highway supply heat to the surface continuously, making the water on it less likely to freeze. 11-24 SUN Earth L r (4 π L 2 ) G s 4 π r 2 ( σ T sun 4 ) Chapter 11 Fundamentals of Thermal Radiation 11-56C Due to its nearly horizontal orientation, windshield exchanges heat with the sky that is at very low temperature. Side windows on the other hand exchange heat with surrounding surfaces that are at relatively high temperature....
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This note was uploaded on 03/19/2008 for the course ME 410 taught by Professor Benard during the Spring '08 term at Michigan State University.
- Spring '08
- Heat Transfer