Chapter 11 Fundamentals of Thermal Radiation 11-60 Water is observed to have frozen one night while the air temperature is above freezing temperature. The effective sky temperature is to be determined. T∞= 4°C Tsky= ? Water Ts= 0°C ε= 0.8 PropertiesThe emissivity of water is ε= 0.95 (Table A-21). AnalysisAssuming the water temperature to be 0°C, the value of the effective sky temperature is determined from an energy balance on water to be hTTTTairsurfacessky()(−=−εσ44)and [ ]K254.8=⎯→⎯−⋅×=°−°°⋅−skyskyTT444282K)273()KW/m1067.5(95.0)C0C4)(CW/m18(Therefore, the effective sky temperature must have been below 255 K. 11-61 The absorber plate of a solar collector is exposed to solar and sky radiation. The net rate of solar energy absorbed by the absorber plate is to be determined. PropertiesThe solar absorptivity and emissivity of the
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